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OXOPORPHYRINOGENS: ELECTROCHEMISTRY, ANION BINDING, AND LIGHT
INDUCED ELECTRON TRANSFER STUDIES
A Dissertation by
Amy Lea Schumacher
Bachelor of Science, Wilmington College, 2002
Submitted to the Department of Chemistry
and the faculty of the Graduate School of
Wichita State University
in partial fulfillment of
the requirements for the degree of
Doctor of Philosophy
December 2007
© Copyright 2007 by Amy Lea Schumacher
All Rights Reserved
OXOPORPHYRINOGENS: ELECTROCHEMISTRY, ANION BINDING, AND LIGHT
INDUCED ELECTRON TRANSFER STUDIES
I have examined the final copy of this dissertation for form and content and recommend that it be
accepted in partial fulfillment of the requirement for the degree of Doctor of Philosophy with a
major in Chemistry.
__________________________________
Francis D’Souza, Committee Chair
__________________________________
Melvin E. Zandler, Committee Member
__________________________________
Erach Talaty, Committee Member
__________________________________
D. Paul Rillema, Committee Member
__________________________________
Krishna Krishnan, Committee Member
Accepted for the College of Liberal Arts and Sciences
__________________________________________
William D. Bischoff, Dean
Accepted for Graduate School
__________________________________________
Susan Kovar, Dean
iii
For Lori
iv
ACKNOWLEDGEMENTS
I would like to express my gratitude to my research advisor Dr. Francis D’Souza for his
support and guidance. I would also like to thank a couple of my close collaborators; Dr. Melvin
E. Zandler and Dr. Paul A. Karr for their efforts performing computational calculations. Also,
Dr. Osamu Ito and coworkers at Tohoku University performed the time-resolved photochemistry
experiments, which provided valuable data in this work. I also want to express my utmost thanks
to Dr Jonathan P. Hill and coworkers at the International Center for Young Scientists at the
National Institute for Nationals Science in Ibaraki Japan, for synthesizing the fantastic
compounds that I have studied for the last five years.
I would like to thank the members of my graduate committee; Dr. Francis D’Souza, Dr.
Melvin E. Zandler, Dr. Erach R. Talaty, Dr. D. Paul Rillema, and Dr. Krishna Krishnan for their
time and support.
I would also like to thank the faculty and staff of Wichita State University
Chemistry Department who have been there when I needed them.
I am appreciative for
receiving a Department of Education GAANN fellowship. Also, I am grateful to all of the
graduate students of which I got to know during my stay at the department. I would like to give
thanks to my lab mates past and present; Dr. Phil Smith, Dr. Suresh Gadde, Dr. Raghu Chitta,
Eranda Maligaspe, Aravinda Wijesinghe, and Navaneetha Krishnan and James Blakemore. I
would also like thank Robert Kirgan for being my friend.
I would also like to thank everyone who supported me during my sister’s illness and after
her tragic passing. I will never be able to repay the people who took time out of their schedules
to teach mine and Curtis’ labs and recitations. For this, I want to thank Eranda, Rama, John,
v
Travis, and Arvin. I would also like to thank Dr. Eichhorn for allowing Curtis to leave school to
be with me when I needed him the most. I want to thank everyone that prayed for my family.
Thank you everyone!!
The last few months have been very rough for our family and it has shown me that time
with your loved ones is always precious. I would like to now thank the people that mean the
world to me. I most certainly want to thank my parents for their wonderful support. Without
their love and guidance, I would not be the person I am today.
I also want to thank my other
half, Curtis Moore, for being there for me when I needed him the most and always supporting
and loving me. I am very blessed to have you in my life.
Finally, I want to acknowledge my sister and best friend, Lori, to whom I have dedicated
this dissertation. I couldn’t wait to graduate because I knew then that I would get to spend more
time with my best friend and sister. I know that our time was cut short, but Lori, I can dedicate
the last five and a half years of my life to you. I am finishing school for you!!! I miss you,
Lori!!!
vi
ABSTRACT
The research presented in this dissertation deals with the electrochemical, anion binding
and photochemical studies of various oxoporphyrinogen systems. The first chapter provides a
brief introduction to the material discussed in subsequent chapters. The second chapter discusses
the
electrochemical,
spectroelectrochemical,
computational
(ab
inito),
characterization of an extended family of N-substituted oxoporphyrinogens.
and
structural
The effect of
increasing N-substitution on the electrochemical and spectroelectrochemical properties was
systematically investigated.
Chapter three focuses on the anion binding properties of the
oxoporphyrinogens. The compounds were studied by optical and electrochemical methods to
determine the response of the oxoporphyrinogens in the presence of anions. Chapter four deals
with the electron/energy transfer processes occurring in supramolecular systems composed of
oxoporphyrinogens and zinc porphyrin and the effect of functionalized fullerene when selfassembled to the systems. The photochemical measurements revealed energy transfer in nonpolar solvents, while in polar solvents electron transfer was possible and upon coordination to
fullerene there was a higher degree of charge stabilization.
The compounds discussed in this thesis were studied by optical absorbance and emission,
electrochemical, and time-resolved photochemical methods. These compounds were mainly
characterized by 1H NMR, UV-vis absorbance, and ESI-mass.
Binding constants for the
supramolecular complexes were calculated using UV-vis spectroscopic methods.
Density
functional theory (DFT) calculations were performed to gain insight concerning structural and
orientation of the donor-acceptor groups in these supramolecular complexes. Electrochemical
studies were performed to obtain free energy changes for charge separation and charge
vii
recombination and to find trends in the redox potentials with increasing N-substitution in the
oxoporphyrinogens.
Spectroelectrochemical measurements were carried out to find the peak
positions of anion and cation radicals for the oxoporphyrinogens with the various substitutions.
Steady state and time resolved fluorescence emission studies and transient absorption studies
were employed to obtain charge separation and charge recombination rates and the lifetimes of
the photo-induced electron transfer events.
viii
TABLE OF CONTENTS
Chapter
1
2
Page
INTRODUCTION
1
1.1
5
ELECTROCHEMICAL, SPECTROELECTROCHEMICAL, COMPUTATIONAL,
AND STRUCTURAL STUDIES OF OXOPORPHYRINOGENS
9
2.1
2.2
2.3
2.4
3
4
Introduction
Experimental Section
Results and Discussion
Summary
9
15
17
49
ANION BINDING PROPERTIES OF OXOPORPHYRINOGENS
53
3.1
3.2
3.3
3.4
53
60
62
95
Introduction
Summary
ELECTRON/ENERGY TRANSFER PROCESSES IN SUPRAMOLECULAR
DONOR/ACCEPTOR SYSTEMS COMPOSED OF OXOPORPHYRINOGEN,
ZINC PORPHYRIN(S) AND FULLERENE(S)
97
4.1
4.2
4.3
4.4
5
Scope of the Present Work
Introduction
Summary
97
103
105
128
SUMMARY
131
LIST OF REFERENCES
136
APPENDIX
145
A. List of Publications
146
ix
LIST OF TABLES
Table
……………………………………………………………………………
Page
2.1
Ab initio B3LYP/3-21G(*) Calculated Parameters for the Investigated
Compounds. All Values are in electron volts (eV)
30
2.2
Electrochemical Redox Potentials (V vs. Fc/Fc+) of the
Oxoporphyrinogens in DCB, 0.1 M (TBA)ClO4. Scan rate = 100 mV/s.
35
2.3
Comparison Between the HOMO-LUMO Gap Calculated from
Electrochemical and Computational (B3LYP/3-21G(*)) Methods.
36
2.4
UV-visible spectral data (λ nm) for the neutral, mono-anion and monocation species of the porphyrinogens (2, 5-8) in DCB.
44
3.1
Comparison of the radii of isoelectronic cation and anions in an
octahedral environment.
54
3.2
Donor Numbers and β values of the Employed Solvents and Optical
Absorption Spectral Data for the Investigated Porphyrinogens in
Various Solvents
62
3.3
Anion binding constants for 2 and 3 receptors determined from
absorption titration method in CH2Cl2 at room temperature.
71
3.4
Maximum cathodic shift, ∆E, observed for the first oxidation potential
(mV)
86
3.5
B3LYP/3-21G(*)/SCRF/PCM Calculated Energy of Anion Binding and
Energy Levels of the Frontier Orbitals of receptors 2 and 3.
89
3.6
B3LYP/3-21G(*) Calculated Bond Lengths for Receptor –Anion
Complexes in the Gas Phase.
90
4.1
120
Fluorescence lifetime (τf), quenching rate-constant (kq), quenching
quantum-yield (ΦCS) of 1ZnP* and 1C60*, free energy of charge-separation
(∆GSCS), charge-separation rate-constant (kCS), lifetime of radical ion pair
(τRIP) and of (C60Im:ZnP)n-OxP (n = 1 or 2).
x
LIST OF FIGURES
Figure ……………………………………………………………………………
Page
1.1
Structure of porphyrin macrocycle (M=H2 or metal).
1
1.2
Structures of a porphyrinogen, porphyrin, and calix[4]pyrrole.
3
1.3
Examples of conjugated porphyrinogens by Inhoffen and coworkers20
and Otto and coworkers.
3
1.4
Structure of conjugated tetrakis (3,5-di-t-butyl-4-oxo-cyclohexa-2,5dienylidene) porphyrinogen first studied by Milgrom and coworkers.
4
1.5
X-ray crystal structure of tetrakis (3,5-di-t-butyl-4-oxo-cyclohexa-2,5dienylidene) porphyrinogen.
5
2.1
Figures of different porphyrin macrocycles.
11
2.2
Structures of the studied meso-tetrakis(3,5-di-t-butyl-4hydroxyphenyl)porphyrin),1, tetrakis (3,5-di-t-butyl-4-oxo-cyclohexa2,5-dienylidene) porphyrinogen ,2.
13
2.3
Structures of benzylated family (3 and 4) of studied oxoporphyrinogens.
14
2.4
Structures of naphtylated family (5-8) of studied oxoporphyrinogens.
14
2.5
Structures of pyrenyl family (9-12) of studied oxoporphyrinogens.
15
2.6
Optical absorption spectra of (i) 1, (ii) 2, (iii) 3, and (iv) 4 in DCB.
19
2.7
Optical absorption spectra of (i) 2, (ii) 5, (iii) 6, (iv) 7, (v) 8, and (vi) 1,
in DCB.
20
2.8
Optical absorption spectra of compounds (i) 9, (ii) 10, (iii) 11, and (iv)
12 in DCB.
20
2.9
Comparison of the skeletal deformations upon N-alkylation in the X-ray
crystal structures of (a) 2, (b) 3, and (c) 4.
22
2.10
Molecular structure and packing arrangements in crystals of (a) 6, (b) 8,
(c) dimeric species by stacking interactions in 6, (d) 1-dimensional array
of 8 formed by stacking interactions (runs parallel with crystallographic
a-axis), and (e) binding of water and secondary guest, methanol, through
hydrogen bonding in 6. (Hydrogen atoms and solvent molecules omitted
for clarity in (a)- (d).)
24
xi
LIST OF FIGURES (continued)
Figure ……………………………………………………………………………
Page
2.11
Plots of the edge-on view of the porphyrin ring atoms plotted by vertical
distance from the least squares plane of the 20 porphyrin atoms for both
calculated (black line) and X-ray (red line) for compounds 1-4.
28
2.12
Coefficients of the first HOMO and the first LUMO for the B3LYP/321G(*) optimized 1, 2, 3, and 4.
29
2.13
Calculated structures of 9-12, and the HOMO and the LUMO 12 at the
B3LYP/3-21G* level.
32
2.14
Cyclic voltammograms of (a) 2, (b) 3, (c) 4, and (d) 1 in DCB, 0.1 M
(TBA)ClO4. Scan rate = 100 mV/s. The asterisk indicate the Fc/Fc+
redox couple used as an internal standard.
34
2.15
Cyclic voltammograms of (a) 2, (b) 5, (c) 6, (d) 7, (e) 8, and (f) 1 in
DCB, 0.1 M (TBA)ClO4. Scan rate = 100 mV/s. The asterisk indicate
the Fc/Fc+ redox couple used as an internal standard.
37
2.16
Reversible interconversion between 1 and 2 involving two-electron/two
proton processes.
39
2.17
Cyclic voltammograms of compounds 9–12 in DCB, 0.1M (TBA)ClO4
Scan rate= 100 mV/s.
40
2.18
Spectral changes observed during (a) first oxidation and (b) first
reduction of 3 in DCB, 0.1 M (TBA)ClO4.
42
2.19
.
42
2.20
43
2.21
reduction of 5, 6, 7, and 8 in DCB containing 0.1 M (TBA)ClO4.
45
2.22
Fluorescence spectra of compounds 9-12 and 2 in DCB with excitation
at (a) pyrene moiety (λex = 350 nm) and (b) porphyrinogen moiety (λex=
510 nm).
47
2.23
Optical absorption and fluorescence spectra of 10 in dioxane or DMSO
illustrating the solvent dependence of emission maximum wavelength
(λex= 500 and 533nm in dioxane and DMSO, respectively)
47
xii
Figure ……………………………………………………………………………
Page
2.24
Quenching of fluorescence of 12 (λex = 497 nm) caused by the addition
of fullerene, C60 (10 eq. each addition) in DCB.
49
3.1
Structure of calix[4]pyrrole studied by Anzenbacher and coworkers.
56
3.2
Structures of TTF appended calix[4]pyrroles investigated by Becher and
coworkers along with Sessler and coworkers .
57
3.3
Example of porphyrin employed in anion binding studies.
58
3.4
Structures of calix[4]arene compounds for anion recognition studies.
59
3.5
Structures of compounds used for anion binding.
60
3.6
Solvatochromism in 2 and 3: Optical absorption spectra of (a) 2 and (b)
3 in solvents an assortment of polarities.
64
3.7
Spectral and color variation upon changes in the ratio of CH2Cl2:DMF
solutions of 3.
66
3.8
Molecular structure of the acetone solvate of 3.
67
3.9
Plots of donor number versus absorption peak maxima (a and b), and β
versus absorption peak maxima (c and d) for 2 (plots a and c) and 3
(plots b and d). Solvents: 1 (hexane); 2 (benzene); 3 (benzonitrile); 4
(acetonitrile); 5 (1,4-dioxane); 6 (acetone); 7 (ethyl acetate); 8 (ethanol);
9 (diethyl ether); 10 (tetrahydrofuran); 11 (dimethylformamide); 12
(dimethyl sulfoxide); 13 (pyridine).
68
3.10
Spectral changes observed during the titration of 3 with
tetrabutylammonium fluoride in dichloromethane. The inset figure
shows Benesi-Hildebrand plot constructed for determination of the
binding constants.
70
3.11
Spectral changes observed during the titration of (TBA)F to a solution of
2. (a) The first set of spectral changes and (b) second set of spectral
changes. The inset plots in (a) and (b) reveal the respective BenesiHildebrand plots constructed for evaluation of binding constants.
72
3.12
Plots of binding constant, K (M-1), of different anions versus spectral
shift, ∆λ (cm-1), for (a) 2 and (b) 3
73
xiii
Figure ……………………………………………………………………………
Page
3.13
1
H-NMR spectra of 2 in CDCl3 in the absence and presence of excess
tetra-n-butylammonium chloride. (a) Structures of the two tautomers of
2 (Tautomer 1: porphyrinogen form, Tautomer 2: porphodimethene
form). (b) 1H-NMR spectrum of 2 in the presence of excess (TBA)Cl
revealing binding of chloride by the porphyrinogen form. (c) 1H-NMR
spectrum of 2 in the absence of excess (TBA)Cl indicating its existence
as the porphodimethene form.
75
3.14
Low temperature NMR data for 3 in CDCl3 solution at −70°C and in the
presence an various increments of (TBA)F.
77
3.15
Titration of 3 with Fluoride ions performed in CD2Cl2 solution at room
temperature.
77
3.16
(a) Colors of solutions of 2 in the presence of various anions and in the
solvents indicated. (b) The optical spectra of 2 in dichloromethane and
in the presence of the indicated anions.
79
3.17
Optical spectra of 3 in (a) dichloromethane and (b) ethanol in the
presence of the indicated anions.
81
3.18
Tile representation of the colors of dilute (~10−6 M) solutions of 3 in
several solvents in the presence of the anions studied.
81
3.19
Cyclic voltammograms of 2 with (i) 0 eq. (ii) 1.0 eq., and (iii) 3.0 eq
addition of (TBA)F in DCB containing 0.1 M (TBA)PF6. Both
oxidation and reduction waves are shown. Scan Rate= 0.1V/s
83
3.20
Cyclic voltammograms of 3 with (i) 0 eq. (ii) 1.0 eq., and (iii)2.0 eq
addition of (TBA)F in DCB containing 0.1 M (TBA)PF6. Both
oxidation and reduction waves are shown. Scan Rate= 0.1V/s.
84
3.21
Cyclic voltammograms of receptor, 3 in DCB containing 0.1 M
(TBA)PF6 on (i) 0, (ii) 1, (iii) 2 and (iv) 3 equivalent addition of
(TBA)C2H3O2. Both oxidation and reduction waves are shown. Scan
rate = 0.1 V/s.
Cyclic voltammograms of receptor, 3 in DCB containing 0.1 M
(TBA)PF6 on (i) 0, (ii) 1, (iii) 2 and (iv) 3 equivalent addition of
(TBA)NO3. Both reduction and oxidation waves are shown. Scan rate
= 0.1 V/s.
85
B3LYP/3-21G(*) optimized structures for 3 in the (a) absence and in the
presence of (b) F-, (c) Cl- (d) C2H3O2-, (e) NO3-, and (f) ClO4-.
88
3.22
3.23
xiv
85
Figure ……………………………………………………………………………
Page
3.24
B3LYP/3-21G(*) optimized structures 2 in the a) absence and in the
presence of b). F-, c). Cl- d). C2H3O2-, e) ClO4- and f) NO3- ,
91
3.25
Molecular electrostatic potential (MEP) maps for 3 (a) in the absence
and (b) presence of F-. The MEPs are shown in two different
orientations.
Plot of HOMO vs. oxidation potential (a and b), spectral shift vs.
HOMO (c and d), and K vs. HOMO (e and f) for receptor, 2(plots a, c,
and e) and receptor, 3 (plots b, d, and f)
Schematic representation of (a) excitation (b) electron transfer and (c)
energy transfer processes.
92
3.26
4.1
94
98
4.2
Structure of porphyryin-quinone compound studied by Milgrom and
coworkers.
99
4.3
Covalently linked ferrocene-zinc porphyrin-free base porphyrin-C60
tetrad, J, developed by Imahori and coworkers.
100
4.4
Porphyrin-fullerene dyads constructed via metal ligand axial
coordination by D’Souza and coworkers.
101
4.5
Structures of studied compounds.
102
4.6
Structures of supramolecular triad, 17, and pentad, 18.
103
4.7
Optical absorption spectrum of (i) 16, (ii) 13, and (iii) 2 in DCB.
106
4.8
UV-visible spectral changes observed during the titration of 15 (1.3 µM
each addition) with 13 (2.0 µM) in DCB. The figure inset shows
Benesi-Hildebrand plot constructed for evaluation of the binding
constant; Ao (intensity in the absence of added C60Im) and ∆A (changes
of absorption on addition of C60Im).
108
4.9
UV-visible spectral changes observed during the titration of 16 (1.3 µM
each addition) with 14 (2.0 µM) in DCB. The inset figure shows
Benesi-Hildebrand plot constructed for evaluation of the binding
constant; Ao (intensity in the absence of added C60Im) and ∆A (changes
of absorption on addition of C60Im).
109
4.10
Cyclic voltammograms of (a) 16, (b) 15, and (c) 2, and (d) 14 in DCB
containing 0.1 M (TBA)ClO4. Scan rate = 100 mV s-1.
111
4.11
(a) B3LYP/3-21G(*) optimized structure of triad, 16 and (b) the frontier
HOMO and (c) LUMO of the 16.
113
xv
Figure ……………………………………………………………………………
4.12
4.13
4.14
B3LYP/3-21G(*) optimized structures of (a) 17 and (b) 18.
(a) Fluorescence spectrum of (i) ZnP (13), (ii) (ZnP)-OxP, (15) and (iii)
(ZnP)2-OxP, (16) (iv) OxP (2) in DCB. The samples were excited at the
Soret band position of ZnP and the concentrations of the compounds
were held at 10 µM.
Fluorescence spectral changes observed on increasing addition of 14 (5
µM each addition) to a solution of 16 in DCB. (λex=424 nm).
Page
115
116
117
4.15
Fluorescence decay profiles of 13 and 16 (0.1 mM) collected in the (a)
600-640 nm corresponding to zinc porphyrin emission and (b) the 700750 nm range. λex = 410 nm. Black line 13 in toluene, red line 16 in
toluene, blue line 16 in DCB, and green line 16 in PhCN.
118
4.16
Fluorescence decay profiles of 17 ([component] = 0.07 mM) collected in
the 600-640 nm corresponding to zinc porphyrin emission, λex = 410
nm: ······· , 13 in toluene; —— 17 in toluene; -----, 17 in DCB.
Fluorescence decay profiles of 18 ([component] = 0.07 mM) collected in
the 700-750 nm corresponding to C60 emission in DCB, λex = 410 nm.
······· , 14 in toluene; —— 18 in toluene; -----, 18 in DCB.
(a) Nanosecond transient absorption spectra of 16 (0.1 mM) observed by
425 nm laser irradiation in at 0.1 µs (●) and 1.0 µs (○) in toluene. Inset:
Absorption-time profiles at 860 nm. (b) Nanosecond transient
absorption spectra of 16 (0.1 mM) observed by 425 nm laser irradiation
in at 0.1 µs (●) and 1.0 µs (○) in PhCN. Inset: Absorption-time profiles
at 880 nm.
119
4.17
4.18
121
122
4.19
Nanosecond transient absorption spectra of 3 (0.1 mM) observed by 550
nm laser irradiation in at 0.1 µs (●) and 1.0 µs (○) in toluene. Inset:
Absorption-time profiles at 920 nm in toluene.
123
4.20
Energy level diagram showing the different photochemical events of
triad 16.
124
4.21
(a) Nanosecond transient absorption spectra of 18 ([component] = 0.07
mM) observed by 532 nm laser irradiation in at 30 ns (o), 0.1 µs (●) and
1.0 µs (○) in DCB. (b) Absorption-time profiles for the peaks in figure
(b) at the indicated wavelengths.
126
4.22
Energy level diagram showing the different photochemical events of
(C60Im:ZnP)-OxP in DCB.
128
5.1
Structure of the proposed OxP-C60 Dyad
135
xvi
LIST OF ABBREVIATIONS
C60
Fullerene
LUMO
Lowest unoccupied molecular orbital
HOMO
Highest occupied molecular orbital
DFT
Density functional theory
NMR
Nuclear magnetic resonance spectroscopy
ESI-mass
Electrospray ionization mass spectrometry
FAB-mass
Fast atom bombardment spectrometry
CHCl3
Chloroform
CH2Cl2
Methylene chloride
THF
Tetrahydrofuran
MeOH
Methanol
DMF
N,N-dimethylformamide
m/z
Mass/charge
π
Pi
DCB
o-dichorobenzene
DMSO
Dimethylsulphoxide
TN
Toluene
PhCN
Benzonitrile
CH3CN
Acetonitrile
C60Im
N-methyl-2-(4’-N-imidazolylphenyl)- 3,4-fulleropyrrolidine
xvii
LIST OF ABBREVIATIONS (continued)
TPP
Tetraphenylporphyrin
ZnP
Zinc tetraphehylporphyrin
(TBA)ClO4
Tetrabutylammonium perchlorate
(TBA)PF6
Tetrabutylammonium hexafluorophosphate
SCE
Saturated calomel electrode
Ag / AgCl
Silver / Silver Chloride reference electrode
Fc / Fc+
Ferrocene / Ferrocenium internal reference electrode
E1/2
Electrochemical half wave potential
Epa
Anodic peak potential
Epc
Cathodic peak potential
ipa
Anodic peak current
ipc
Cathodic peak current
n
Number of electrons
c
Concentration
A
Electrode area
D
Diffusion coefficient
mV/s
millivolts per second
V
Volts
∆H
Enthalpy
∆S
Entropy
∆G
Gibbs free energy
K
Formation constant
xviii
LIST OF ABBREVIATIONS (continued)
o
Degree Celsius
φf
Fluorescence quantum yield
ket
Rate of electron transfer
Eo-o
Singlet excitation energy
λex
Excitation wavelength
λem
Wavelength at which emission is maximum
Å
Angstrom (1Å = 1 x 10-10 meters)
nm
nanometer (1nm = 1 x 10-9 meters)
µM
micromolar
M
molar
M-1
per mole
s-1
per second
C
xix
xx
CHAPTER 1
INTRODUCTION
Tetrapyrrole macrocycles comprise a class of compounds that are vital in nature.1 One of
the most well-known tetrapyrrole compounds is porphyrin, which is composed of four pyrrole
rings linked by a single carbon atom between each ring to form a fully conjugated macrocycle
(Figure 1.1).2,3 Porphyrins are easily oxidized and strongly absorb light in the visible region of
the electromagnetic spectrum.2
These highly-colored, redox-active pigments are critical in
chemistry and biology, ranging in roles from electron transfer1, anion binding1,4-6, oxygen
transport1, photosynthetic processes1,7-8, catalytic materials9, and photo-dynamic therapy.10,11
β−pyrrole positions
N
N
meso position
M
N
N
Figure 1.1: Structure of porphyrin macrocycle (M=H2 or metal).
In the traditional porphyrin structure, each of the nitrogen atoms is within the inner core
of the ring and thus is able to chelate a variety of metal cations.1 In general, the carbons on the
outer segment of the pyrrole rings are termed the β-pyrrole carbons while the carbons linking the
pyrrole rings are termed the meso carbons (figure 1.1). Synthetic modifications of the porphyrin
macrocycle are often carried out at these positions. The β-carbon substituted porphyrins closely
resemble naturally-occurring porphyrins. However, the meso- carbon substituted porphyrins have
1
no direct biological counterparts, but are widely used in material chemistry and biomimetic
models. Additionally, modifications at the meso position are preferred due to their relative
synthetic ease.1
There are many structural isomers of the porphyrin macrocycle including porphycene,
corrphycene, hemiporphycene, C-fused, core-modified, and N-confused porphyrins.1-3,13-14
Additionally, tetrapyrrole chemistry has been expanded by the use of various synthetic
procedures to produce remarkable compounds. One such class of compounds is called
porphyrinogens which are composed of four pyrrole units linked at the α-position via sp3hybridized carbon atoms. The chemistry of porphyrinogens with hydrogen atoms at the meso
positions is well-known given that the species will readily oxidize to a porphyrin. However,
fully meso-substituted porphyrinogens, which can not be oxidized to porphyrins, or otherwise
known as calix[4]pyrroles are also known and recently have attracted plenty of attention
especially for their anion binding properties.15-19
Figure 1.2 shows the structure of the
calix[4]pyrrole, porphyrinogen, and porphyrin for comparision. Interestingly, there are a few
porphyrinogens in which the meso substituents maintain conjugation of the tetrapyrrole system.
This allows the electronic structures to vary substantially from non-conjugated systems. Some
early examples of this class of compounds include meso-oxo β-octaethylporphyinogen, A, and
meso-methylene β-octaethylporphyinogen, B, of Inhoffen20 and Otto21 and coworkers,
respectively (Figure 1.3).
2
H
R
R
N
H
NH
R
N
H
oxidation
HN
H
N
H
R
R
H
N
N
H
N
R
R
H
R
Porphyrinogen
Porphyrin
R
R
R
R
N
H
HN
NH
H
N
R
R
R
R
calix[4]pyrrole
Figure 1.2: Structures of a porphyrinogen, porphyrin, and calix[4]pyrrole.
O
H3CH2C
CH2CH3
H3CH2C
N
O
H
H
H
H2C
O
H
H
H
CH2
N
CH2CH3
H3CH2C
H3CH2C
O
N
H
N
CH2CH3
H3CH2C
CH2CH3
N
N
N
CH2CH3
H3CH2C
N
H
CH2
H3CH2C
CH2CH3
H3CH2C
CH2CH3
CH2
CH2CH3
B
A
Figure 1.3: Examples of conjugated porphyrinogens by Inhoffen and coworkers20 and Otto and
coworkers.21
3
Another interesting conjugated porphyrinogen of this type, reported first by Milgrom and
coworkers22, is conjugated through unsaturated meso-4-oxocyclohexadienylidene substituents
(figure 1.4). As a result of this extensive conjugation, an intense electronic absorption spectrum
in the visible region was observed.
This porphyrinogen was synthesized by using the meso-
tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin, TDtBHPP, (its porphyrin precursor) by a
facile two-electron aerial oxidation in basic solution.22 The crystal structure for this compound
was reported in 1989, and it revealed a highly puckered structure as shown in figure 1.5.23
Milgrom and coworkers further modified the oxoporphyrinogen by multi N-alkylation using
bulky substituents, such as benzyl groups.24 However, further studies on these compounds were
halted due to lack of X-ray structural analysis and the uncertainty of the isomeric identity of the
compounds.
O
NH
HN
NH
NH
O
O
O
Figure 1.4: Structure of conjugated tetrakis (3,5-di-t-butyl-4-oxo-cyclohexa-2,5-dienylidene)
porphyrinogen first studied by Milgrom and coworkers.22
4
Figure 1.5: X-ray crystal structure of tetrakis (3,5-di-t-butyl-4-oxo-cyclohexa-2,5-dienylidene)
porphyrinogen.23
1.1
Scope of the present work
As discussed in the previous section, tetrapyrrole macrocycles have been extensively
studied in order to understand their spectral, electrochemical, anion binding, and electron/energy
transfer properties. One exception for this, is the oxoporphyrinogen shown in figure 1.4. Apart
from basic synthesis and X-ray structure, no further studies have been reported in literature on
this compound.
This has been explored as part of this thesis work and reports various
physico/chemical, anion binding, and electron acceptor properties of oxoporphyrinogens.
The first section of this thesis work deals with the spectral and electrochemical
characterization of various oxoporphyrinogens.
The extended conjugation of the
oxoporphyrinogens predicts red-shifted optical transitions.
The presences of the four
hemiquinonoid structures on the oxoporphyrinogen macrocycle predict that they will be electron
deficient.
These predictions have been experimentally verified by performing systematic
spectral, electrochemical, and spectroelectrochemical studies.
Further, structural and
computational studies have been performed in collaboration with other research groups to
complement the experimental findings. These results have been summarized in Chapter 2.
5
In Chapter 3, anion binding properties of the oxoporphyrinogens have been explored.
The ruffled structure of the oxoporphyrinogen, as observed from its crystal structure, lends itself
to possible anion binding studies through hydrogen-bonding interactions.
In addition, the
oxoporphyrinogen is related structurally to calix[4]pyrrole, which has been investigated by
numerous research groups for its anion binding capabilities.
One drawback of the
calix[4]pyrrole is that it is a colorless and redox-inactive molecule and can not be probed by
either optical or electrochemical methods. Research groups have overcome this dilemma by
attaching various chromophores and redox-active moieties to these compounds.
The
oxoporphyrinogen is both optically and electrochemically active, making it possible to probe
anion binding properties of this molecule by optical and electrochemical methods without further
chemical modifications. Furthermore, solvatochromatic effects have been explored to determine
the interactions, if any that various solvents have on the oxoporphyrinogen. In addition, by
combining solvatochromism and anion binding induced spectral changes, an attempt has been
made to achieve anion selectivity.
As pointed out earlier, the presence of four hemiquinonoids on the macrocycle periphery
is expected to make the molecule electron deficient, hence, making them suitable candidates as
electron-acceptors in donor-acceptor systems. This has been verified by building donor-acceptor
dyads comprised of zinc porphyrin, a known good electron donor, and oxoporphyrinogen, as an
electron acceptor. To add further appeal, fullerene has been utilized to build supramolecular
triads comprised of zinc porphyrin, oxoporphyrinogen and fullerene. Light-induced electron and
energy transfer processes in these novel molecules has been studied using sophisticated spectral,
electrochemical, and photochemical techniques. The results of these studies are discussed in
Chapter 4.
6
The results of all of the studies completed to-date on oxoporphyrinogens are summarized
in Chapter 5, along with future directions of the research involving these tetrapyrrole molecules.
7
8
CHAPTER 2
ELECTROCHEMICAL AND SPECTROELECTROCHEMICAL
CHARACTERIZATION OF OXOPORPHYRINOGENS
2.1
Introduction
Tetrapyrrole macrocycles embrace a class of compounds that are vital in biological
systems.
In nature, they lie at the heart of many indispensable electrochemical and
photochemical systems.1,2,25 Synthetic porphyrin chemistry is a burgeoning area of chemistry not
only for its importance in modeling biological systems but for its use in catalytic9,26,
electronic,27,28,29 and optical materials.30,31 One of the synthetic porphyrins that have gained
popularity over the past few years is the porphyrinogen.15-19 The conventional porphyrinogen
has outstanding synthetic and coordination chemistries along with the potential for employment
as anion and cation binding agents. Sessler and co-workers have utilized simple non-conjugated
porphyrinogens for their anion binding and sensing capabilities.17
However much of the recent chemistry has utilized peripheral modification of the
porphyrin skeleton or expansion of the oligopyrrole core.32-34 Little effort has been placed on
modification of the nitrogen atoms except for heteroatom replacement.35
This is because
substitution at the porphyrin N atoms requires the loss of some of the features that make the
compound porphyrinic such as planarity or aromaticity.32-34
Porphyrinogens are much more accessible than porphyrins for chemistry involving the
pyrrolic nitrogen atoms.35-38 In addition, there are several porphyrinogens known where the meso
substituents maintain conjugation of the tetrapyrrole system so that their electronic structures
vary substantially from the non-conjugated porphyrinogens. The most notable examples of these
are the meso-oxo-OEP and meso-methylene-OEP (OEP = β-octaethylporphyrinogen) of
9
Inhoffen20 and Otto et al.,21 respectively shown in figure 1.3., and the meso-tetrakis-(4-oxo-3,5di-tert-butyl-2,5-cyclohexadienylidene) porphyrinogen, 2, first reported by Milgrom22 shown in
figure 1.4.
Conventional porphyrinogens due to their non-conjugated structure are not electro- or
photo-active.
The oxoporphyrinogen reported by Milgrom and coworkers bears a highly,
electronically conjugated structure and intense electronic absorption spectrum in the visible
region.22 Furthermore variation of the multiplicity and nature of the N-substituents should allow
tuning of the redox potentials as desired. This tuning of the redox processes has been observed
in the numerous electrochemical studies of porphyrins and metalloporphyrins.
The electrochemical studies of tetrapyrroles especially porphyrins and metalloporphyrins
have been extensively studied for the past sixty years.39 Many laboratories have studied the
electrochemistry of synthetic and natural tetrapyrroles and thousands of published works are in
the literature.39
The electrochemical properties of tetrapyrroles are greatly influenced by
structural factors including the number and type of substituents attached to the macrocycle, the
central metal ion, the number and type of axial ligands bound to the central metal ion, and the
type and planarity of the macrocycle. Also, the electrochemistry is influenced by experimental
conditions such as the solvent and supporting electrolyte used in the experiment.39
Redox potentials for π-anion and π-cation radical formation will vary substantially
depending on the central metal ion. For example, Kadish and coworkers studied the effect of
metal ion on the π-anion radical for (OEP)AgII and (OEP)Ca.40
The E1/2 values for the
compounds were -1.29 V and -1.68 V for the (OEP)AgII and (OEP)Ca, respectively.
By
modifying the central metal ion, a change of 390 mV was observed in the π-anion radical for this
porphyrin.
10
Large shifts in redox potentials of porphyrins are also observed from changes in the
porphyrin macrocycle or changes in the substituents to the macrocycle. For instance, when
electrochemistry of H2TPP and H2OEP (figure 2.1) is performed in DMSO with (TBA)ClO4 as
the supporting electrolyte, the first reduction of H2TPP is at -1.04 V versus SCE41 while the
reduction of H2OEP take place at -1.46V versus SCE.42
Et
Et
Et
Et
N
NH
N
N
NH
HN
N
HN
Et
Et
Et
Et
H2TPP
H2OEP
Figure 2.1: Figures of different porphyrin macrocycles.
Another factor that affects the reduction and oxidation potentials is the experimental
conditions which includes solvent used to perform the experiment. For example, the
electrochemistry of ZnTPP has been performed in numerous non-aqueous solvents and the effect
of the redox potential has been observed. In CH2Cl2, the first reduction of ZnTPP has an E1/2
value of -1.85 V versus Fc/Fc+.43 While in DMF and CH3CN, under the same conditions, the
reduction potentials are -1.80 V and -1.74 V, respectively, versus Fc/Fc+.43
Also changes in the nature of the axial ligand may change the redox potentials to a large
or small extent depending on the ligand. When 1-methyl imadizole coordinated to ZnTPP in
11
CH2Cl2 using TBAClO4 as the supporting electrolyte, the reduction potential is -1.49 V versus
SCE44, while ZnTPP under the same condition the reduction potential is -1.35 V versus SCE.44-45
Many research groups have added to the study of tetrapyrroles by combining
electrochemistry and spectroscopy. Spectroelectrochemistry is a very valuable tool for the study
of a chemical system46 and was first utilized in 1964 by Kuwana and coworkers.47 Kadish and
coworkers have published many articles on the subject.46,48 One example of this group using
spectroelectrochemistry was to determine the site of electron transfer in two different porphyrin
compounds, (TPP)Ru(CO) and (OEP)Ru(CO).48
Although electrochemical measurements
revealed that (OEP)Ru(CO) reductions were at more negative potentials, the site of electron
transfer was not clearly deduced. When spectroelectrochemical data was executed on both
porphyrins, it revealed the site of electron transfer for (TPP)Ru(CO) was a porphyrin π-anion
radical, while for (OEP)Ru(CO) the site of electron transfer was a metal centered reduction.48
In this Chapter the electrochemical, spectroelectrochemical, ab inito computational, and
structural characterization of an extended family of N-substituted oxoporphyrinogens is
summarized. For the purposes of this study, meso-tetrakis (3,5-di-t-butyl-4-hydroxyphenyl)
porphyrin ,1, shown in figure 2.2, was selected because of the perceived ease of crystallization of
its derivatives and because O-alkylation is somewhat hindered by the proximity of tertiary butyl
groups to the phenol hydroxyl group. Oxidation of 1 in basic solution gives the respective
oxocyclohexadienylidene porphyrinogen, 2, shown in figure 2.2,22 whose structure has already
been reported.23 In the present study three different groups were chosen for N-alkylation to see
the effect that various groups had on the structural, electrochemical, and spectroelectrochemical
properties of the oxoporphyrinogens as shown in figures 2.2, 2.3, 2.4, and 2.5. The naphthyl
group was chosen in an attempt to synthesize mono and tris derivatives. The pyren-1-ylmethyl
12
group was chosen because it is larger than the naphthyl group in terms of size and delocalized
electronic system along with added attraction of some unusual effects in its fluorescence
behavior. Furthermore, the variation of multiplicity and nature of N-substituents allowed the
tuning of the redox potentials of the redox processes as desired.
O
OH
N
NH
OH
HO
N
Structures
hydroxyphenyl)porphyrin),1,
NH
NH
O
O
1
2.2:
HN
O
HN
OH
Figure
NH
2
of
tetrakis
the
studied
meso-tetrakis(3,5-di-t-butyl-4-
(3,5-di-t-butyl-4-oxo-cyclohexa-2,5-dienylidene)
porphyrinogen ,2.
13
O
NR1 R2N
O
O
NR4 R3N
Y=
O
3: R1=R3=Y; R2=R4=H
4: R1=R2=R3=R4=Y
Figure 2.3: Structures of benzylated family (3 and 4) of studied oxoporphyrinogens
O
NR1 R2N
O
O
NR4 R3N
Y=
O
5: R1=Y; R2=R3=R4=H;
6: R1=R3=Y; R2=R4=H
7: R1=R2=R3=Y; R4=H
8: R1=R2=R3=R4=Y
Figure 2.4: Structures of naphtylated family (5-8) of studied oxoporphyrinogens.
14
O
NR1 R2N
O
O
NR4 R3N
X=
O
9:
10:
11:
12:
R1=X R2=R3=R4=H
R1=R2=X R3=R4=H
R1= R2=R3=X R4=H
R1= R2=R3=R4=X
Figure 2.5: Structures of pyrenyl family (9-12) of studied oxoporphyrinogens.
2.2
Chemicals
o-Dichlorobenzene (DCB) for electrochemical studies was dried over CaH2 and distilled
under vacuum prior to the experiments. The (TBA)ClO4 was recrystallized from ethanol and
dried in a vacuum oven at 35 oC for 10 days. Buckminsterfullerene, C60, (+99.95%) was from
SES Research (Houston, TX). DMSO and dioxane were purchased from Aldrich Chemical Co.
meso-Tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin) (1),
tetrakis (3,5-di-t-butyl-4-oxo-
cyclohexa-2,5-dienylidene) porphyrinogen (2), N21,N23-dibenzyl-5,10,15,20-(3,5-di-t-butyl-4oxo-cyclohexa-2,5-dienylidene) porphyrinogen (3), and N21,N22,N23,N24-tetrabenzyl-5,10,15,20(3,5-di-t-butyl-4-oxo-cyclohexa-2,5-dienylidene) porphyrinogen (4,) N21-(2-methylenenaphthyl)5,10,15,20-(3,5-di-t-butyl-4-oxocyclohexa-2,5-dienylidene)porphyrinogen, (5), N21,N23-bis-(2methylenenaphthyl) -5,10,15,20-(3,5-di-t-butyl-4-oxo-cyclohexa-2,5-dienylidene)porphyrinogen
15
(6),
N21,N23,N23-
tris-(2-methylenenaphthyl)-5,10,15,20-(3,5-di-t-butyl-4-oxocyclohexa-2,5-
ienylidene)porphyrinogen (7), N21,N22,N23,N24-tetrakis-(2-methylenenaphthyl)-5,10,15,20-(3,5di-t-butyl-4-oxocyclohexa-
2,5-dienylidene)porphyrinogen
(8),
N21-(Pyren-1-ylmethyl)-
5,10,15,20-tetrakis(3,5-di-tert-butyl-4-oxocyclohexadien-2,5-yl)porphyrinogen (9), N21,N23-Di(pyren-1-ylmethyl)-5,10,15,20-tetrakis(3,5-di-tert-butyl-4-oxocyclohexadien-2,5yl)porphyrinogen (10), N21,N22,N23-Tri-(pyren-1-ylmethyl)-5,10,15,20-tetrakis(3,5-di-tert-butyl4-oxocyclohexadien-2,5
-yl)porphyrinogen (11), N21,N22,N23,N24-Tetra-(pyren-1-ylmethyl)-
5,10,15,20-tetrakis(3,5-di-tert-butyl-4-oxocyclohexadien-2,5-yl)porphyrinogen
(12)
were
synthesized and supplied by our collaborator, Dr. Jonathan P. Hill.49,50,51
Instrumentation
The 1H NMR spectra were obtained using a Bruker AC 400Mhz spectrometer using
residual chloroform as the internal standard (δ = 7.24 ppm). FAB-MS spectra were obtained
with a VG Trio-2 spectrometer using 3-nitrobenzyl alcohol as matrix and chloroform co-solvent.
UV/Visible spectra were recorded on a Shimadzu Model 1600 UV-visible spectrophotometer.
Mass spectra were obtained using a Kratos-Shimadzu Axima+ MALDI-TOF mass spectrometer
with dithranol as matrix.
Electrochemistry
Cyclic voltammograms were recorded on an EG&G Model 263A potentiostat using a
three electrode system. A platinum button electrode was used as the working electrode. A
platinum wire served as the counter electrode and an Ag/AgCl electrode was used as the
reference.
Ferrocene/ferrocenium redox couple was used as an internal standard.
All the
solutions were purged prior to electrochemical and spectral measurements using argon gas.
16
Spectroelectrochemistry
Spectroelectrochemical measurements were performed either on a Princeton Applied
Research (PAR) diode array rapid scanning spectrometer or a Shimadzu UV-visible
spectrophotometer using a homemade cell with optically transparent (platinum mesh) electrodes.
All the experiments were carried out at 23 + 1°C.
Computational Calculations
The computational calculations were performed in collaboration with Dr Melvin E.
Zandler, Wichita State University and Dr. Paul A. Karr, Wayne State College.
These
calculations were performed by DFT B3LYP/3-21G(*) methods with GAUSSIAN 98 or 0352
software packages on various high speed PCs or at the Wichita State University Supercomputer
facility. The graphics of HOMO and LUMO coefficients were generated with the help of
GaussView software.
2.3
Optical Absorption Studies
The optical absorption spectra of compounds 1-4 are presented in figure 2.6.
The
porphyrin 1, exhibited an intense Soret band and weak Q-bands consistent of meso-substituted
porphyrins.
As expected for extended conjugated derivatives, the oxoporphyrinogens 2-4
exhibited red shifted absorption bands compared to 1. However, the bands were found to be
broad with one or more shoulder bands instead of the usual Q-bands of porphyrins. The
absorption maxima of the Soret-type band were located at 518, 509, and 504 nm for 2, 3, and 4,
respectively. That is, upon N-substitution the porphyrinogens exhibited a small blue shift in their
absorption maxima. When the optical studies for the N-naphthyl family of compounds (5-8) was
investigated, similar results were obtained when compared to compounds 3 and 4. All of the
17
porphyrinogens 5-8 exhibited strong spectral features encompassing the entire visible range as
shown in figure 2.7. The peak position of the Soret band revealed a 2-5 nm blue shift upon
successive addition of N-substituents at the porphyrinogen macrocycle.
When the optical studies of the N-pyrenyl compounds were studied a blue shift similar to
the other N-substituted oxoporphyrinogens was observed. In addition, for compounds 9-12 there
existed an absorbance due to the pyren-1-ylmethyl groups whose intensity increased relative to
the tetrapyrrole chromophore passing from 9 to 12. Two further lower intensity bands existed at
around 330 and 270nm. In the case of pyrene-substituted derivatives, these low intensity bands
are obscured by those due to the pyrene groups, which increase in intensity with increasing
multiplicity of substitution. Compounds 9-12 are soluble in DCB such that increasing pyrene
substitution gave spectra as shown in figure 2.8.
18
(iii)
1.2
(i)
(iv)
Absorbance
1.0
0.8
0.6
(ii)
0.4
0.2
0.0
400
600
800
1000
Wavelength (nm)
Figure 2.6: Optical absorption spectra of (i) 1, (ii) 2, (iii) 3, and (iv) 4 in DCB.
19
1 .2
1 .0
Absorbance
( i)
( ii)
0 .8
( iii)
( iv )
0 .6
(v )
( v i)
0 .4
0 .2
0 .0
400
600
800
1000
W a v e le n g th (n m )
Figure 2.7: Optical absorption spectra of (i) 2, (ii) 5, (iii) 6, (iv) 7, (v) 8, and (vi) 1, in DCB.
1.0
(iv)
Absorbance
0.8
(iii)
0.6
(ii)
0.4
(i)
0.2
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 2.8: Optical absorption spectra of compounds (i) 9, (ii) 10, (iii) 11, and (iv) 12 in DCB.
20
X-ray Structural Characterization
Structural studies were completed by our collaborator Jonathan P. Hill (NIMS) to solve
the uncertainty over the isomeric identity of the N-alkylated compounds.
The molecular
structure of 2 has already been reported in the literature.23 The molecular structures of 3 and 4,
determined by X-ray crystallography, are shown in figure 2.9.
There were several important
structural features of 3. The studies revealed that the benzyl groups are substituted at two of the
central nitrogen atoms. Also, an analysis of the meso substituent C-O bonds have indicated that
they are all double bonds (~1.24 Å) revealing that the two remaining exchangeable protons rest
at the other central nitrogen atoms. This is further confirmed in the structure by the presence of a
water molecule that is strongly hydrogen-bonded via these N-H protons (Npyrrole-Owater 3.02Å)
and the N-H resonance at low field in the proton NMR spectrum of 3. Further bond length
analyses confirmed that the meso-substituents are 4-oxocyclohexa-2,5-diene moieties and that
these are bonded at the meso positions via double bonds. The meso-substituents can similarly be
assigned as 4-oxocyclohexa-2,5-dienes in 4.
21
Figure 2.9: Comparison of the skeletal deformations upon N-alkylation in the X-ray crystal
structures of (a) 2,23 (b) 3, and (c) 4.
The molecular structures of 6 and 8 determined by X-ray crystallography are shown in
figure 2.10. The orientations of N-substituents and conformations of the tetrapyrrole are similar
to those already reported.39,49 An interesting feature of these crystal structures lies in the packing
of the molecules, which is strongly influenced by a dipole-dipole interaction between the N-
22
substituents. In fact, molecules of 6 contained within the structure are dimerized through this π-π
stacking interaction while the same effect operating in 8 gives a long range ordering of the
molecules into 1-dimensional arrays, which run parallel with the crystallographic a-axis. The π-π
stackings in both 6 and 8 have intermolecular interplane distances of approx. 3.7Å, which is
similar to that observed in the solid state structures of other polyaromatic molecules.53
Previously, the π-π stacking effect has been used to introduce long range ordering allowing
observation of phenomena related to the close contact between aromatic systems54 and it can also
be used to efficiently assemble discrete nanoscale objects.55 Figure 2.10 a and c shows the
molecular structure of 6 and its π-stacked dimeric unit. Figure 2.10b and d illustrates the
molecular structure of 8 and how these molecules are forced to compose a 1D array through a
similar π-stacking interaction. It is noteworthy that not all N-substituents are involved in stacking
interactions. Thus in 6, while one of the naphthyl groups is involved in intermolecular
interactions, the other remains passive, ensuring formation of the dimeric unit. The same is true
for 8 except that two naphthyl groups per molecule are submissive in terms of the π-π stacking
effect and a linear arrangement of the molecules results. The intermolecular stacking is mediated
by the groups substituted on adjacent rather than opposing nitrogen atoms, i.e. N21 and N22.
23
(a)
(c)
(b)
(d)
(e)
Figure 2.10: Molecular structure and packing arrangements in crystals of (a) 6, (b) 8, (c)
dimeric species by stacking interactions in 6, (d) 1-dimensional array of 8 formed by stacking
interactions (runs parallel with crystallographic a-axis), and (e) binding of water and secondary
guest, methanol, through hydrogen bonding in 6. (Hydrogen atoms and solvent molecules
omitted for clarity in (a)- (d).)
24
Figure 2.10e illustrates the binding of solvent molecules by hydrogen bonding to the free
NH protons in 6. The binding of water in this way is a general feature of the N21,N23disubstituted derivatives and allows capture of a secondary guest when that guest contains an
alcohol group. The primary guest water molecule also apparently plays a role in fixing the
conformation of the macrocycle.
Distortions of the tetrapyrrole framework can have important effects on the properties of
these macrocycles. If we consider the dihedral angles between the least squares mean plane of
the macrocycle and those of each pyrrole group, some dependence of these angles on substituent
identity and multiplicity emerges. In the case of 2, all pyrrole groups are essentially equivalent
with a dihedral angle of 48°. In the N-substituted derivatives, there is some deviation from this
arrangement so that in 6 the respective dihedral angles for unsubstituted and N-substituted
pyrroles are 45° and 56°. This represents a less puckered conformation than for the benzyl
analogue where the angles 47°, 52.4° (NH pyrroles) and 60.8°, 68.0° (N-benzyl pyrroles) were
observed.49 These changes observed in the dihedral angle are likely due to the presence of
different packing arrangements within the crystals. However, they illustrate that there is still
some degree of flexibility in the disubstituted species despite a substantial crowding at the
macrocyclic core. For 8, the average dihedral angle subtended between macrocyclic mean plane
and pyrrole planes is 53°, which is the same as in the benzylic analogue.49 Thus, the difference in
packing between the 2-methylenenaphthyl derivative 8 and its benzyl analogue has little effect
on the macrocyclic conformation and the small degree of flexibility available to the di-Nsubstituted compounds is effectively removed by complete N-alkylation.
25
DFT B3LYP/3-21G(*) Computational Studies
To gain insight into their geometry and electronic structure, computational studies were
performed on compounds 1-4 by using density functional methods (DFT) at the level B3LYP/321G(*) level by our collaborators Dr. Melvin E. Zandler and Dr. Paul A. Karr. The DFT
methods were chosen over the Hartree-Fock or semiempirical approach since recent studies have
shown that DFT methods at the 3-21G(*) level predict the geometry and electronic structure
more accurately.56
In all of the calculations, all of the compounds were fully optimized to a stationary point
on the Born-Oppenheimer potential energy surface. To gain more confidence in the computed
structures, especially the non-planarity factors of the oxoporphyrinogens, the computed
structures were compared to the X-ray structures. The results of the comparison are portrayed in
figure 2.11. The flat-scale projections were generated by least squares fitting a plane to the 24
heavy atoms of the porphyrin ring, establishing the origin of the plane at the centroid of the 24
atoms, defining the positive X-axis in the direction of one of the meso-carbons, the positive Yaxis orthogonal to X in the direction of another one of the meso-carbons, and the positive Z axis
orthogonal to the XY plane. The original atomic coordinates were transformed to the newly
defined coordinate system, and the Z coordinate plotted against the angle of the arctangent of the
XY coordinates. For compound 2 bearing no N-substituents, an excellent agreement between the
computed and X-ray structure was observed. Here, the β-pyrrole carbons were displaced as
much as 1.7 Å from the least squares plane. The nonplanarity of the porphyrinogen macrocycle
is comparable to that reported previously for highly substituted porphyrins.57 Additionally, it is
interesting to note that the X-ray and computed structures are so similar suggesting that the
extensive H-bonding in the crystals of 2 has little effect on its molecular conformation.
26
Compounds 3 and 4, bearing two and four N-benzyl substituents, respectively, exhibited
macrocyclic distortions similar to compound 2 (the β-pyrrole carbons were displaced by around
1.7 Å). The position of the N-substituted benzyl groups could not exactly be matched with the
X-ray structure since crystal packing makes these flexible substituents occupy less symmetric
positions. However, the excellent agreement of the porphyrin ring atoms between the computed
and the X-ray structures gave us enough confidence to investigate the electronic structure of
these molecules.
27
1
2
3
4
Figure 2.11: Plots of the edge-on view of the porphyrin ring atoms plotted by vertical distance
from the least squares plane of the 20 porphyrin atoms for both calculated (black line) and X-ray
(red line) for compounds 1-4.
28
HOMO
LUMO
1
2
3
4
Figure 2.12: Coefficients of the first HOMO and the first LUMO for the B3LYP/3-21G(*)
optimized 1, 2, 3, and 4.
29
The first HOMO and LUMO of 1-4 are shown in figure 2.12, while table 2.1 lists the
energies of the first two HOMO’s and first two LUMOs along with the calculated HOMOLUMO gap. As expected for porphyrins, the HOMO’s and LUMO’s of compound 1-4 were all
π-orbitals. However, the symmetries of the HOMO orbitals of compounds 2-4 were different
(pseudo D2d, a2 HOMO, and b2 LUMO) from compound 1 (pseudo D4h, a1u, a2u HOMO, and eg
LUMO). The HOMO and LUMO orbitals for compound 1 were found to be mainly on the
porphyrin π-ring system, while for 2-4 these orbitals were spread to the oxocyclohexadienylidene
rings of the porphyrinogen macrocycle due to the extension of conjugation caused by oxidation.
Interestingly, there were only minor orbital contributions on the ring nitrogens (only for the
LUMO) and almost no contribution from the benzyl groups was observed.
Table 2.1: Ab initio B3LYP/3-21G(*) Calculated Parameters for the Investigated Compounds.
All Values are in electron volts (eV)
Compound
HOMO+1
HOMO
LUMO
LUMO +1
HOMO-LUMO gap
1
-5.147
-4.711
-2.029
-2.017
2.68
2
-5.831
-5.536
-3.625
-3.060
1.91
3
-5.754
-5.515
-3.501
-3.089
2.01
4
5-.826
-5.464
-3.420
-2.941
2.04
The calculated HOMO-LUMO gap followed the trend: 2 < 3 < 4 < 1. These results
suggest a decreased HOMO-LUMO gap for the porphyrinogens compared to the parent
30
porphyrin and this gap increases slightly with increasing number of N-substituents at the
porphyrinogen macrocycle.
In order to elicudate the structure and any potential interchromophore interactions in the
pyrenyl-substituted derivatives (9-12), the structures and energies of the HOMO and LUMO
were calculated. The compounds 9-12 calculated geometries are similar to the other deriviates of
2. Figure 2.13 shows the optimized structures for 9-12 along with the HOMO and LUMO for 12,
which are similar for all of the pyrenyl-1-ylmethyl compounds. Interchromophore interactions
are highly unlikely beause the HOMO and LUMO reside on the macrocycle only; this is perhaps
not surprising given the usual role of these moieties as electron acceptors. The energies of the
orbitals are also similar. One interesting feature arising from the computational data is that of the
HOMO-LUMO gap. Its value for evenly substituted compounds (i.e. 10 and 12) exceeds slightly
that of the unevenly substituted compounds being on average 2.07 eV and 2.00 eV, respectively.
This may reflect similarities in the structures of the two kinds of compound related to an uneven
puckering of the macrocycle.
31
10
9
12
11
12: HOMO
12: LUMO
Figure 2.13: Calculated structures of 9-12, and the HOMO and the LUMO 12 at the B3LYP/321G* level.
32
Electrochemical Studies
Electrochemical studies using cyclic voltammetric techniques were performed to evaluate
the redox potentials, to verify the predictions of computational studies on HOMO-LUMO gap,
and to seek structure reactivity relationships for the N-substituted oxoporphyrinogens. Figure
2.14 shows the cyclic voltammograms for the oxoporphyrinogens 2-4 along with the starting
material, 1. Table 2.2 lists the values of the redox potentials for the entire family of studied
oxoporphyrinogens (1-12).
The cyclic voltammogram of tetra-oxacyclohexadienylidene
porphyrinogen, 2, bearing no N-substituents, revealed two reversible oxidations located at E1/2 =
0.27 and 0.48 V vs. Fc/Fc+ and an irreversible reduction at Epc = -1.33 V vs. Fc/Fc+. The larger
current for this reduction process suggests involvement of one or more electrons. Variable scan
rate, multi-cyclic, and low-temperature (0 oC) voltammetric studies revealed no changes in the
shape of the voltammogram. Interestingly, compounds 3 and 4 exhibited a systematic anodic
shift in their oxidation potentials. The first oxidations of 3 and 4 (quasi-reversible) are located at
E1/2 = 0.48 and Epa = 0.73 V vs. Fc/Fc+. These values represent over 200 mV and 400 mV
anodic shifts in their oxidation potentials, respectively.
A similar but less pronounced trend was observed during the reduction of the oxocyclohexadienylidene porphyrinogens. While the reduction of 2 was fully irreversible, the cyclic
voltammograms corresponding to the reduction of 3 and 4 were found to be quasi-reversible to
reversible. Scanning the potential to more negative values revealed additional reductions (figure
2.14) for 3 and 4. The potentials corresponding to the first reduction of compounds 2, 3, and 4
were found to be close to each other (table 2.2). The cyclic voltammogram of 1 revealed two
irreversible oxidation peaks located at Epa = 0.31 V and 0.43 V vs. Fc/Fc+ and, as expected for
free-base porphyrins, two reversible reductions at E1/2 = -1.82 V and -2.19 V vs. Fc/Fc+,
33
respectively. The location and irreversible nature of the anodic waves suggest oxidation of the
porphyrin ring leading to the formation of oxo-cyclohexadienylidene porphyrinogens. It is
important to note that the oxidation behavior of 1 and the reduction behavior of 2 complement
each other, that is, the irreversible reduction of 2 involves the conversion of oxocyclohexadienylidene porphyrinogens to the tetrakis (4-hydroxyphenyl) porphyrin (in its
deprotonated form in the absence of protons). Importantly, the magnitude of the redox potentials
of compounds 2, 3, and 4 suggests that these compounds are electron deficient. That is to say that
they are nearly 500 mV easier to reduce than 1.
m
(a)
1 A
*
m
(b)
1 A
*
m
(c)
1 A
*
(d)
m
1 A
1000
500
*
0
-500
-1000 -1500 -2000 -2500
+
Potential mV vs. Fc/Fc
Figure 2.14: Cyclic voltammograms of (a) 2, (b) 3, (c) 4, and (d) 1 in DCB, 0.1 M (TBA)ClO4.
Scan rate = 100 mV/s. The asterisk indicates the Fc/Fc+ redox couple used as an internal
standard.
34
Table 2.2: Electrochemical Redox Potentials (E1/2) (V vs. Fc/Fc+) of the Oxoporphyrinogens in
DCB, 0.1 M (TBA)ClO4. Scan rate = 100 mV/s.
Compound 2nd Ox
a
1st Red
2nd Red
3rd Red
4th Red
1
0.43
0.31a
-1.82
-2.19
--
--
2
0.48
0.27
-1.33b
--
--
--
3
0.69
0.48
-1.38b
-1.45
-2.00
-2.08
4
--
0.73a
-1.34c
-1.38
-1.86
--
5
0.56
0.37
-1.29b
-1.44
-2.06b
--
6
0.69
0.47
-1.36b
-1.46
-1.95b
-2.04
7
0.81
0.54
-1.30b
-1.46
-1.75
-2.10
8
--
0.71
-1.32
-1.41
-1.85c
-1.85c
9
0.56
0.35
-1.33b
-1.44b
--
--
10
0.68
0.49
-1.36b
-1.46b
--
--
11
--
0.54
-1.31b
-1.47
--
--
12
--
0.76a
-1.35
-1.46
--
--
Epa at 0.1 V/s.
b
c
1st Ox
Epc at 0.1 V/s
overlap of two one-electron processes
The HOMO-LUMO gap calculated from the computational studies and that from the
electrochemical measurements (the difference between the first oxidation and first reduction
potentials) are listed in table 2.3. A surprisingly excellent trend for the HOMO-LUMO gap, 2 <
3 < 4 < 1, between the computed and experimental results is observed. These observations are
also consistent with the optical absorption data shown in figure 2.6, where the position of the
35
absorption bands also follows a similar trend. Previously, similar attempts have been made to
match the HOMO-LUMO gap from computational, electrochemical, and optical studies.58-61 The
results presented here serve as an excellent illustration that this is an attainable goal.
Table 2.3: Comparison Between the HOMO-LUMO Gap Calculated from Electrochemical and
Computational (B3LYP/3-21G(*)) Methods.
Compound
Electrochemical, V
Computational, eV
1
2.13
2.68
2
1.60
1.91
3
1.86
2.01
4
2.07
2.04
Figure 2.15 shows the cyclic voltammograms of the porphyrinogens 2, 5, 6, 7, and 8,
together with that of the porphyrin parent, 1.
Compounds 5 – 8 bear an increasing number of 2-methylenenaphthyl entities at the
macrocyclic nitrogens and exhibit reversible oxidation processes, while undergoing a systematic
anodic shift in their oxidation potentials (table 2.2).
Several interesting observations were made during the reduction of the N-substituted
porphyrinogens. While the reduction of 2 is an irreversible two-electron process, the reductions
of compounds 5 – 8 become increasingly reversible and the initial two-electron process is
resolved into two reversible one-electron processes for the tetra-N-substituted compound, 8.
Scanning the potential further in the cathodic direction revealed up to two more one-electron
reductions, which also become reversible with an increase of the multiplicity of N-substituents
36
on the macrocycle. For compound 8, this wave involves two one-electron processes. The
potential for the first reduction process is similar for all the derivatives.
Conversely, the
potentials of oxidation processes are anodically shifted by 70-130 mV for each additional Nsubstituent, indicating an increasing stabilization of the HOMO level upon N-substitution as
observed with the benzyl substituted compounds 3 and 4.49 Previous ab initio calculations also
suggest a stabilized HOMO level for these N-substituted compounds.49
(a)
(b)
(c)
(d)
(e)
(f)
*
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
+
Potential (V vs. Fc/Fc )
Figure 2.15: Cyclic voltammograms of (a) 2, (b) 5, (c) 6, (d) 7, (e) 8, and (f) 1 in DCB, 0.1 M
(TBA)ClO4. Scan rate = 100 mV/s. The asterisk indicate the Fc/Fc+ redox couple used as an
internal standard.
37
As a result, the experimental HOMO-LUMO gap, calculated from the potential
difference between the first oxidation and first reduction follow the trend: 2 < 5 < 6 < 7 < 8, as
predicted by the earlier computational studies for N-substituted porphyrinogens.49
The initial two-electron reduction of compound 2 and the two one-electron reductions of
compounds 5 – 8 can be rationalized based on the Scheme shown in figure 2.16.
The
transformation of 1 to 2 involves abstraction of two electrons and two protons. Similarly, one
could envisage reversible conversion of 2 to 1 by the addition of two electrons and two protons.
As a consequence, two facile and irreversible one-electron oxidations were observed for 1 (figure
2.15f) while a facile, irreversible two-electron reduction process was observed for 2 (figure
2.15a). The irreversible oxidation of 1 and irreversible reduction of 2, even in dry solvent
conditions, suggest conversion between them. The reduction potentials also suggest that 1 is
electron rich and that 2 is electron deficient. Assuming that the two-electron reduced product of 2
is doubly deprotonated 1, one could argue that the potentials for the 3rd and 4th reductions should
match that of compound 1. Indeed, an examination of the voltammogram of 2 in the potential
range of -1.5 to -2.3 V exhibits such reductions at potentials corresponding to the first and
second reductions of 1, although they are not fully developed. Substitution by N-substituents at
the porphyrinogen nitrogen atoms not only increases the already puckered non-planarity of the
macrocycle as revealed by the X-ray and computational structures49 but also causes resolution of
the initial irreversible two-electron process into two reversible one-electron processes and
improves the definition of the additional reductions.
38
- 2e- 2H+
O
OH
N
NH
N
HN
NH
O
O
OH
HO
NH
HN
HN
OH
O
1
2
+ 2e+
+ 2H
Figure 2.16: Reversible interconversion between 1 and 2 involving two-electron/two proton
processes.
Cyclic voltammograms for compounds 2 and 9-12 are shown in figure 2.17 and the data
are summarized in table 2.2. Compound 2 possesses two reversible oxidation steps with E1/2 =
0.27 and 0.48 V and an irreversible reduction at Epc = -1.33 V, all vs. Fc/Fc+. Deviations from
this behaviour upon N-alkylation have been previously discussed for the other families of
derivatives of 249,50 and compounds 9-12 do not significantly depart (Table 2.2). Compound 10
and 11 both stand out for their well-defined reversible anodic processes while undergoing a
systematic anodic shift in their oxidation potentials. It may be mentioned here that no oxidation
or reduction processes corresponding to the pyrene entities within the potential window of the
solvent were observed.
In general, N-substituents at the porphyrinogen nitrogen causes
39
resolution of the initial irreversible process of 2 into reversible one-electron processes.
Interestinlgy, 12 formed films at the working electrode surface during cyclic voltammetry. It is
currently unclear whether this is due to polymerization or crystallization processes.
12
11
10
9
2
1.0
0.5
0.0
-0.5
-1.0
-1.5
+
Potential (V) (vs Fc/Fc )
Figure 2.17: Cyclic voltammograms of compounds 9–12 in DCB, 0.1M (TBA)ClO4 Scan rate=
100 mV/s.
Spectroelectrochemical Studies
The anodic shift and reversibility of the oxidation processes clearly suggest stabilization
of the HOMO of tetra-oxo-cyclohexadienylidene porphyrinogens upon N-substitution. Although
40
the effect on the LUMO is lesser, the reductions of 3 and 4 are reversible compared to the
irreversible reduction of 2. The reversible redox processes of 3 and 4 suggest formation of stable
π-cation and π-anion radicals upon the first oxidation and first reduction processes. Spectral
characterization of such radicals is essential considering the extended π-conjugation of the tetraoxo-cyclohexadienylidene porphyrinogen macrocycles and their possible applications as
materials for building molecular electronic devices62 or electron transfer model compounds.63
With this in mind, we have performed spectroelectrochemical characterization of the oxidized
and reduced species of 3 and 4 and the results are discussed below.
Figures 2.18 and 2.19 present the spectral changes observed during the oxidation and
reduction of 3 and 4, the benzyl family of oxoporphyrinogens, in DCB containing 0.1 M
(TBA)ClO4.
The spectral changes recorded during the first oxidation of 3 revealed new
absorption bands at 588 and 750 nm with a decrease in the Soret band intensity (figure 2.20a).
Isosbestic points at 430 and 570 nm were also observed suggesting the occurrence of only one
equilibrium process in solution. Similar observations were also made during the first reduction
of 3 (figure 2.20b). New absorption bands at 744 and 572 nm were observed with a concurrent
decrease of the Soret band intensity of the neutral species. No new spectral bands were observed
during the oxidation of 4 with the spectra undergoing only a small intensity gain (figure 2.21a).
Interestingly, the reduction of 4 resulted in the formation of well-resolved bands at 571 and 589
nm with a concurrent decrease of the Soret band at 504 nm (figure 2.21b). Isosbestic points were
also observed at 454 and 571 nm.
41
(a)
Absorbance
0.6
0.4
0.2
0.0
300
400
500
600
700
800
900
Absorbance
0.6
1000
(b)
0.4
0.2
0.0
300
400
500
600
700
800
900
1000
Wavelength (nm)
Figure 2.18: Spectral changes observed during (a) first oxidation and (b) first reduction of
3 in DCB, 0.1 M (TBA)ClO4.
0.6
(a)
Absorbance
0.5
0.4
0.3
0.2
0.1
0.0
300
400
500
600
700
800
Absorbance
0.4
900
1000
(b)
0.3
0.2
0.1
0.0
300
400
500
600
700
800
900
1000
Wavelength (nm)
Figure 2.19: Spectral changes observed during (a) first oxidation and (b) first reduction of 4 in
DCB, 0.1 M (TBA)ClO4.
42
The observation of the nicely formed π -cation radical spectra and, especially the π -anion
radical spectra of 3 and 4 clearly suggest their higher stability as predicted from the reversible
redox processes. It should be mentioned here that spectroelectrochemical investigations of the
oxoporphyrinogen derivative without N-substitution, 2, revealed irreversible spectral features
without the presence of clear isosbestic points as shown in figure 2.20.
The present
electrochemical and spectroelectrochemical studies collectively indicate that the investigated
tetra oxoporphyrinogens are electron deficient and become stable towards the formation of cation
and anion species upon N-substitution.
0.5
(a)
Absorbance
0.4
0.3
0.2
0.1
0.0
-0.1
300
400
500
600
700
800
Absorbance
0.5
900
1000
(b)
0.4
0.3
0.2
0.1
0.0
300
400
500
600
700
800
900
1000
Wavelength (nm)
Figure 2.20: Spectral changes observed during (a) first oxidation and (b) first reduction of 2 in
DCB, 0.1 M (TBA)ClO4.
Figure 2.21 presents the spectral changes observed during the oxidation and reduction of
5, 6, 7, and 8 in DCB containing 0.1 M (TBA)ClO4, while the peak positions of the neutral,
43
mono-cation and mono-anion species are listed in table 2.4. The spectral changes recorded
during the first oxidation of compounds 5 – 8 revealed a lowering of the intensity of the most
intense peak located at ~500 nm with the appearance of new bands in the visible or near-IR
region characteristic of the formation of the π-cation radical species. One or more isosbestic
points were also observed indicating the presence of two species at equilibrium in solution. As
expected, the spectral changes were also found to be reversible upon stepping the potential back
to 0.0 V. The position of the new band corresponding to the formation of the π-cation radical
depends upon the number of N-substituents on the porphyrinogen macrocycle. There is a
systematic red shift of the newly developed low intensity absorption band. For compound 8, this
new absorption appears to be split into two bands located at 717 and 910 nm, respectively.
Table 2.4: UV-visible spectral data (λ nm) for the neutral, mono-anion and mono-cation
speciesa of the porphyrinogens (2, 5-8) in DCB.
Compound
a
Neutral
Mono cation
Mono anion
1b
427, 523, 561, 665
429, 464, 560, 721
428, 464, 557, 729
2b
351, 518, 598(s)
456, 598(s), 705
476, 552, 614(s), 769
5b
340, 513, 579(s)
483, 525, 749
480, 564, 606, 745
6
344, 511, 579(s)
523, 600(s), 752
531(s), 575, 739
7
333, 507, 578(s)
425, 507, 603, 793
421, 507, 578(s), 742,
821
8
331, 505, 568(s)
505, 717, 910
506, 722, 916
- the mono-anion and mono-cation were obtained by a spectroelectrochemical method.
b
- irreversible spectral behaviour during oxidation and reduction.
44
(a) 1st Oxidation
(b) 1st Reduction
Absorbance
0.3
0.3
0.2
5
0.1
0.1
0.0
0.0
400
Absorbance
0.2
500
600
700
800
900
400
1000
0.6
0.6
0.5
0.5
0.4
500
600
700
800
900
1000
0.4
6
0.3
0.3
0.2
0.2
0.1
0.1
0.0
300
400
500
600
700
800
900
0.0
300
1000
400
500
600
700
800
900
1000
0.5
0.4
0.4
Absorbance
0.3
0.3
7
0.2
0.2
0.1
0.1
0.0
0.0
400
500
600
700
800
900
400
1000
500
600
700
800
900
1000
0.6
0.5
0.5
0.4
Absorbance
0.4
8
0.3
0.3
0.2
0.2
0.1
0.1
0.0
0.0
400
500
600
700
800
900
1000
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Wavelength (nm)
Figure 2.21: Spectral changes observed during (a) first oxidation and (b) first reduction of 5, 6,
7, and 8 in DCB containing 0.1 M (TBA)ClO4.
In accordance with the cyclic voltammetric observations for the naphthyl family of
compounds (5-8), the spectral changes observed during the first reduction of compounds 2 and 5
are irreversible although new absorption peaks were observed during this process.
For
compounds 6 – 8, the spectral changes are almost reversible and contain one or more isosbestic
45
points. Since the first reduction is either overlapped or located close to the second reduction
process for the compounds studied, a potential of 130 mV past the first/second reduction process
was applied for spectroelectrochemical measurements. Under these conditions, two sets of
spectral changes were observed, presumably corresponding to the one- and two-electron reduced
products. The spectral changes shown in figure 5.21 correspond to the initial spectral changes
(i.e. first reduction). Under these conditions, the peak position of the emergent band, which
corresponds to the formation of the porphyrinogen π-anion radical was found to depend on the
number of N-substituents at the porphyrinogen macrocycle, an observation similar to that
observed for the π-cation radical species.
Fluorescence Emission Studies
The oxoporphyrinogen macrocycle weakly fluoresces at 700 nm upon excitation around
500 nm. For compounds 9-12, there is an additional feature of their fluorescence due to both 2
and the pyrene moieties. Pyrene demonstrates strong fluorescence emission bands in the range
of 380-500 nm upon excitation at its absorption maximum.
For compounds 9-12 both
chromophores revealed fluorescence emission almost independently even though fluorescence
intensity around 700 nm increased upon increasing pyrene substitution indicating some
communication between the distinct chromophores. Emission spectra are shown in figure 2.22
which exemplifies an interesting feature of the oxoporphyrinogen is that the fluorescence
quantum yield increases with increasing substitution, perhaps this reflects the reducing number
of secondary amine groups passing down the series. Another trait of compounds 9-12 is their
inability under the conditions employed here, to produce excimer emission by intra- or
intermolecular interactions between pyrene moieties.
46
(a)
150
(b)
18
9
10
11
12
120
9
10
11
12
2
16
14
Intensity
Intensity
12
90
60
10
8
6
4
30
2
0
0
400
500
600
800
550
900
600
650
700
750
800
850
900
Wavelength (nm)
Wavelength (nm)
Figure 2.22: Fluorescence spectra of compounds 9-12 and 2 in DCB with excitation at (a)
pyrene moiety (λex = 350 nm) and (b) porphyrinogen moiety (λex= 510 nm).
Figure 2.23: Optical absorption and fluorescence spectra of 10 in dioxane or DMSO illustrating
the solvent dependence of emission maximum wavelength (λex= 500 and 533nm in dioxane and
DMSO, respectively)
47
Compound 10, owing to the polar site at the core of the macrocycle due to the presence of
two pyrrolic NH groups, showed some solvent dependence in its emission properties. The
solvent-dependent emission studies are illustrated in figure 2.23, where solutions of 10 are
dissolved in dioxane or DMSO. Polar solvents are bound though hydrogen bonding at the
pyrrolic NH groups which in turn alters the conformation of the macrocycle and its electronic
properties. The resulting forty nanometer shift was brought about by variation of the solvent
polarity. This presents a potentially useful method for the fine tuning of emission properties of
the compounds.
Finally, 12 was considered of interest for its potential to interact with fullerenes. Figure
2.24 shows the effect C60 has on the fluorescence emission of 12. Emission intensity was
quenched to a measurable extent, indicating that some type of intermolecular interaction
occurred, probably through binding of the fullerene by the pyrene moieties in the absence of
other potential interactions. The suspected π- π interaction was deemed weak, but it was still
strong enough to bring 12 into close proximity for energetic and electrostatic processes to occur.
48
Fl. Intensity (a.u.)
1.0
0.8
0.6
0.4
0.2
0.0
550
600
650
700
750
800
850
900
Wavelength (nm)
Figure 2.24: Quenching of fluorescence of 12 (λex = 497 nm) caused by the addition of fullerene,
C60 (10 eq. each addition) in DCB.
2.4
Summary
The structural and electrochemical characterization of an interesting family of N-
alkylated tetrapyrrole macrocycles has been presented. The N-alkylation has been shown to
occur at the pyrrolic nitrogen atoms without altering the conformation of the macrocycle. This is
a significant point since it allows us to predict the three-dimensional structure of other similar
derivatives where structural data is not available. Moreover, this system avoids the purported
difficulties of tautomerism in phenolic porphyrin systems while retaining the prospect of
porphyrin cation radical states that are available through electrolytic or chemical reductions. The
X-ray structural analyses along with the ab initio computational studies revealed highly nonplanar macrocycles in compounds 3 and 4.
The electrochemical studies revealed that the
porphyrinogen ring is electron deficient by nearly 500 mV and the redox processes of N-
49
substituted 3 and 4 are better defined than those of 2. The computationally predicted and
electrochemically determined HOMO-LUMO gap followed the trend: 2 < 3 < 4 < 1.
The observation of the well formed π -cation radical and π -anion radical spectra for the
highly N-substituted porphyrinogens indicate their higher stability as predicted from the
reversible redox processes and the spectral features extending into the near-IR region.
Introduction of the N-substituents allows us greater access to discrete oxidized and reduced
forms of the tetra-oxoporphyrinogen molecule. Additionally, variation of the multiplicity of
substitution permits tuning of the redox properties of this system. Thus, when passing from 5
through 6 and 7 to 8, not only are the reduced or oxidized states stabilized but there is a shift in
the relative ease of oxidation and reduction processes so that increasing substitution results in
higher oxidation potentials but lower reduction potentials the same holds true when going from 3
to 4.
This is related to the enhanced stability of the intermediate oxidation states of the
compounds upon substitution at the macrocyclic nitrogen atoms. The new absorption bands that
appear at longer wavelength during redox processes add to the appeal of this system. The
combination of oxidation state accessibility, redox potential ‘tunability’ and oxidation statedependent absorption spectra is a useful tool in the design of multichromophoric and sensing
systems. These features also imply potential applications in molecular memory devices or as
electron transfer mediators in organic photovoltaic cells.
Various properties of the pyrenyl N-substituted derivatives of 2 have been presented. The
results help to further our understanding of this unusual class of tetrapyrrole macrocycle. While
the electrochemical properties of the compounds here are complementary the other families of
porphryinogens reported, there are several substituent specific properties that illustrated the
scope of this system. The somewhat unusual fluorescence properties and lack of excimer
50
emission from the pyrenyl substituted porphyrinogens make this system attractive for further
study. The interaction of these pyrene-substituted compounds with other aromatic chromophores
is a valid subject given the affinity of compound 12 for C60 fullerene.
51
52
CHAPTER 3
ANION BINDING BEHAVIOR OF OXOPORPHYRINOGENS
3.1
Introduction
The study of anion binding is a growing area of research in chemistry because of the
importance anions play in biology and in the environment.64-67 A majority of enzyme substrates
and co-factors are anions.61
DNA, the carrier of genetic information, is a polyanion.64-67
Phosphate and nitrate ions used in agricultural fertilizers cause the eutrophication of aquatic
systems.67 Fluoride is utilized in the treatment of osteoporosis, however, overuse may lead to
fluorosis, a type of fluoride toxicity.68-69
While the importance of anion sensing is present, the design of anion receptors is
particularly challenging compared to cation receptors for several reasons.64-71 Anions are larger
than isoelectronic cations (Table 1.1) and, consequently, have a lower charge to radius ratio,
meaning electrostatic binding interactions are less effective for anions compared to cations.
Anions are also pH sensitive by becoming protonated at low pH. As anions appear in a wide
range of geometries; a lot of design may be needed to make selective anion receptors. Solvent
effects also have an important role in controlling anion binding strength and selectivity. A
receptor must compete with the solvent environment where the anion recognition occurs.
53
Table 3.1: Comparison of the radii of isoelectronic cation and anions in an octahedral
environment.72
Cation
+
r, Å
Anion
-
r, Å
Na
1.16
F
K+
1.52
Cl-
1.67
1.66
-
Br
1.82
1.81
-
2.06
Rb
Cs
+
+
I
1.19
A plethora of compounds has been utilized in anion recognition studies to selectively
complex a target analyte. Some of the compounds employed in anion complexation applications
are polyamide macrocycles,73-76 ureas/thioureas,78-79 calixarenes77-80, and the calix[4]pyrroles.8186
Some of these molecules have been found to bind specific anions with greater selectivity. In
these neutral anion receptors, hydrogen-bonding interactions seem to be the primary mode of
anion binding.64,65,70,71
Many techniques1,2,70,71,87 have been employed in anion recognition studies some of them
include 1H NMR spectroscopy,85-86 optical spectroscopy,77 and electrochemistry.4,85 Optical and
electrochemical methods of detection are the popular technique for signal transduction.1,2,70,71
There has been a great interest in the development of an anion sensor showing an optical signal
output.70,71,87
In the presence of a target anion, these receptors respond by a change in
absorbance or fluorescence.
Similarly for electrochemical detection, a current or potential
perturbations of the redox-active host upon complexation with a guest are observed.1,70,71 In this
method, the receptor has a binding site and a redox-active group in close proximity.1,70,71,85 The
redox-active group has well defined electrochemical properties which will be perturbed upon
guest complexation. To achieve significant changes in the redox potentials, the receptor must be
able to couple its complexation to the redox reaction through one or several pathways. That is
54
the receptor must have a structure upon guest binding, and the electron transfer to or from the
redox center is carried out at a significantly different potential than that of the free receptor.
When cations are the target guests for electrochemical sensors, an anodic shift of the redox
processes will be observed, since the complex should be harder to oxidize or easier to reduce
than the free receptor.
Similarly, receptors for anions are expected to exhibit cathodic shifts in
their redox-processes when complexed to an anion, since they are easier to oxidize or harder to
reduce than the anion-free redox-active receptor. 1,70,71,88
The calix[4]pyrroles have been investigated intensively from the point of view of anion
binding.17,65,80,83,85,86,89 They are ideal for this purpose being available in high yield from a
relatively simple synthesis. Their appeal is furthered by the availability of various postmacrocyclization modifications.90-91 Anion binding properties of the calix[4]pyrroles depend on
hydrogen bonding interactions between the pyrrole NH groups and the analyte anion. Also, the
inherent flexibility of the porphyrinogen skeleton allows the calix[4]pyrroles to exist in
conformations somewhat analogous with the calixarenes, hence, the term calix[4]pyrroles.
Although a major shortcoming of using the calix[4]pyrrole is the difficulty involved in detecting
their absorption spectra, the bands fall at energies that are too high to make them helpful in the
binding process.89 Hence, various research groups have created functionalized calixpyrroles to
overcome this problem.4,84-86
Different electro-active species have been attached to the
calix[4]pyrrole to utilize them for electrochemical anion recognition.84-85
Recently Anzenbacher and coworkers, studied the anion binding properties of a
chromogenic octamethylcalix[4]pyrrole, C, shown in figure 3.1.86
Compound C was of
particular interest due to its strong anion binding affinity and selectivity as well as its ability to
act as a colorimetric sensor. Absorption spectra of compound C in the presence of the fluoride
55
ion showed a moderate spectral shift in DMSO. The color of the solution changed from pink to
orange upon addition of F- while no detectable color change was observed upon addition of Cl-.
The absorption spectra after addition of Cl- noted very minor changes. Upon comparison the
binding affinities for compound C for fluoride and acetate were higher than that of chloride. 86
N
H
NH
CN
HN
H
N
CN
CN
C
Figure 3.1: Structure of calix[4]pyrrole studied by Anzenbacher and coworkers.86
Becher and coworkers along with Sessler and coworkers have incorporated redox-active
tetrathiafulvalene (TTF) units onto calix[4]pyrrole macrocycles, compounds D and E, shown in
figure 3.2.84 TTF units were chosen for its ability to exist in three stable redox states (TTFo,
TTF+., TTF2+). When using compound D, upon addition of Br- and Cl-, cathodic perturbations of
the first oxidation potential were 34 mV and 43 mV, respectively. While for compound E, shifts
of 70 mV were obtained upon addition of Br- and Cl- for the first oxidation. The shifts of the
oxidation potential revealed that the compounds D and E can be employed as potential
electrochemical sensors. 84
56
N
H
NH
s
s
s
s
s
HN
H
N
s
D
PrS
SPr
S
S
S
S
Me
Me
RPS
S
Me
Me
N
H
S
NH
PrS
S
S
HN
H
N
Me
Me
Me
Me
E
Figure 3.2: Structures of TTF appended calix[4]pyrroles investigated by Becher and coworkers
along with Sessler and coworkers .84
Porphyrins have also been employed as anion sensors.1,4 As a result of its excellent
optical and redox properties, porphyrin is an attractive framework to be modified with anion
recognition moieties.4 Beer and co-workers have reported a tetra-imidazolium zinc
metalloporphyrin receptor that was studied as an optical and electrochemical sensor shown in
figure 3.3.4 Compound F binds Cl-, H2PO4-, and HSO4- with higher association constants in
DMSO.
The electrochemistry was investigated in CH3CN by using cyclic voltammetry
technique. The first oxidation of the receptor was monitored since it was the easier to examine.
57
Significant cathodic shifts were observed upon addition of anions. Compound, F, exhibited the
largest cathodic shifts of 175 mV and 140 mV upon addition of Cl- and HSO4-, respectively. The
electrochemical results correlated well with the observed association constants.4
F
Figure 3.3: Example of porphyrin employed in anion binding studies.4
Beer and coworkers have incorporated a ferrocene electro-active groups onto the upper
rim of a calix[4]arene through urea hydrogen-bonding units to be able to sense anions
electrochemically.77 The two urea compounds were utilized in this study (H and G) as shown in
figure 3.4. On progressive addition of an anionic guest, a significant cathodic shift of the
ferrocenes’s oxidation potential was observed. A 180 mV shift in the ferrocene oxidation to for
compound H binding to H2PO4- was observed, while, G revealed a shift of 220 mV for H2PO4binding.
The other anions used revealed meager shifts ranging from 40-70 mV.
Good
correlation was observed upon comparison of electrochemical data with the stability constants
calculated from 1H NMR data.
The studies revealed that H binds anions weakly while G
confirmed stronger binding, perhaps due to the higher number of anion binding urea units.77
58
H
G
Figure 3.4: Structures of calix[4]arene compounds for anion recognition studies.92
A species related to the calix[4]pyrroles is the oxoporphyrinogen 222-24,39,49-51which,
although bears essential similarities in its structure with the calix[4]pyrroles, can be
distinguished from them by an increased macrocyclic rigidity as a result of its conjugated
electronic system. Other novel features of the extended π-electronic system are an intense color
of its derivatives and rich redox chemistry. 22-24,38,49-51
In the present investigation, anion binding of oxoporphyrinogens using optical and
electrochemical methods was probed, along with solvatochromism effects. Figure 3.5 shows the
structures of the compounds utilized for the studies. Since these compounds possess intense
electronic absorptions, we hoped to obtain an anion-dependent chromogenic response.92 Further
to this, these molecules bear a moderate solvatochromism, which we believed could improve
anion differentiation. Additionally, the four fused semi-quinonoid substituents at the mesopositions make the macrocyclic ring electron deficient and redox active, hence, electrochemical
studies could be utilized to probe anion binding.49-51
59
O
NR1 R2N
O
O
NR4 R3N
Y=
O
2: R1=R2=R3=R4=H
3: R1=R3=Y; R2=R4=H
Figure 3.5: Structures of compounds used for anion binding.
3.2
Chemicals
Tetra-n-butylammonium salts and solvents used for spectroscopic and electrochemical
measurements were obtained from Wako Chemical Company or Aldrich Chemical Company.
Tetrakis (3,5-di-t-butyl-4-oxo-cyclohexa-2,5-dienylidene) porphyrinogen (2), N21,N23-dibenzyl5,10,15,20-(3,5-di-t-butyl-4-oxo-cyclohexa-2,5-dienylidene) porphyrinogen (3) was synthesized
and supplied by our collaborator Dr Jonathan P. Hill at NIMS. 23,24,38,49-51
Instrumentation
Electronic absorption spectra were measured using JASCO V-570 UV/Vis/NIR
spectrophotometer or a Shimadzu UV/Visible 1600 spectrophotometer.
obtained using JEOL AL300BX or Bruker AM400 spectrometers.
60
1
H-NMR spectra were
Variable temperature
measurements were performed using either instrument equipped with the appropriate liquid
nitrogen cooling apparatus. An Accumet XL25 pH meter was used from Fisher Scientific.
Electrochemistry
reference. Ferrocene/ferrocenium redox couple was used as an internal standard. All of the
solutions were purged prior to electrochemical and spectral measurements using argon gas. All
the experiments were carried out at 23 + 1°C unless noted.
Computational Calculations
The computational calculations were performed in collaboration with Dr. Melvin E.
Zandler, Wichita State University and Dr. Paul A. Karr, Wayne State College.
These
calculations were performed by DFT B3LYP/3-21G(*) methods with GAUSSIAN 98 or 0352
software packages on various PC’s. The graphics of HOMO and LUMO coefficients were
generated with the help of GaussView software.
X-ray Crystallography
Structure of the acetone solvate of compound 3 was solved in our collaborators lab. The
crystals were grown by refrigeration (T = −20°C) of a solution of 3 in anhydrous acetone. Very
thin purple plates, appeared after two-three weeks, and were subjected to X-ray crystallographic
analysis using a rotating anode MoKα source. A suitable crystal was selected and mounted on a
glass fiber using perfluoropolyether oil. This was placed in the cold stream of the diffractometer
(Bruker-Nonius Kappa CCD) at 120K prior to data collection.
61
Crystal data for 3-acetone: C93H110N4O5, M = 1363.85, triclinic, P1, a = 12.1693(11), b
= 17.5849(13), c = 21.0965(17) Å, α = 69.870(4), β = 78.140(4), γ = 82.865(5) °, V = 4141.3(6)
Å3, Z = 2, Dc = 1.094 g cm-3, µ(Mo-Kα) = 0.067 mm-1, crystal size 0.12 × 0.10 × 0.02 mm3,
56176 reflections measured of which 10799 were independent, Rint = 0.193, data corrected for
absorption on the basis of symmetry equivalent and repeated data (min and max transmission
factors: 0.992, 0.999) and for Lp effects, structure solved by direct methods, F2 refinement, R1=
0.117 for 6377 data with F2> 2σ(F2), wR2 = 0.180 for all data, 946 parameters, largest difference
map features ±0.23 e Å-3.
3.3
Solvatochromism
Solvatochromism is the term for changes in electronic spectra of compounds when
dissolved in different media and is related to changes in the electronic structure of the subject
molecules caused by variation of solvent polarity or other interactions, especially hydrogen
bonding.92 For compounds 2 and 3, absorption maxima in different solvents appeared in the
range 400-900 nm and is illustrated by the spectra shown in figure 3.6 while the positions of the
absorption maxima in different solvents are summarized in table 3.2. Low solubility of 2 in
several of the solvents used did not thwart observation of trends in its electronic absorption
spectrum. However, 3 proved to be soluble in a greater assortment of solvents. Hexane was the
least polar of the solvents employed and gives λmax for 3 at 486 nm. Increasing the polarity of the
solvent resulted in a red shift so that λmax for 1,4-dioxane occurs at 504 nm. Other non-hydrogen
bonding solvents with comparable polarities, such as chloroform and dichloromethane, had λmax
of similar values. When solvents capable of H-bonding were used new absorption bands
62
appeared at 600 and 750 nm, the relative intensity of which varies with the solvent polarity and
may be a measure of how strongly the hydrogen bonding interaction occurs with the
oxoporphyrinogens, 2 and 3. The original ‘Soret’-type band at 500 nm reduced in intensity with
increasing solvent polarity. Variation of solvent polarity could also be accomplished using a
binary mixture. Spectra were measured in CH2Cl2-DMF mixtures of various ratios. Even at high
CH2Cl2-DMF ratios there was a noticeable color change from pink to violet with a gradual
attainment of the blue color as shown in figure 3.7.
63
(a)
(b)
Figure 3.6: Solvatochromism in 2 and 3: Optical absorption spectra of (a) 2 and (b) 3 in
solvents an assortment of polarities.
64
Table 3.2: Donor Numbers93 and β94 values of the Employed Solvents and Optical Absorption
Spectral Data for the Investigated Porphyrinogens in Various Solvents
Solvent
DNa βb
Hexane
0.0
0.00 330, 364,538(s),660 322, 486, 552(s)
Benzene
0.1
0.10 343, 514, 583(s)
329, 503, 69(s),
Nitrobenzene
4.4
0.30 530, 592(s)
521, 585(s)
Benzonitrile
11.9 0.41 342, 531, 594(s)
339, 522, 589(s)
Acetonitrile
14.1 0.31 367, 543, 682
344, 519, 586
Dioxane
14.8 0.37 350,512.5,584(s)
338,504, 574(s)
Acetone
17.0 0.48 360, 524, 590(s)
352,516, 580(s)
Ethyl acetate
17.1 0.45 356, 521, 602(s)
348,510, 583(s)
Ethanol
19.2 0.77 360, 535, 604(s)
353,517, 588(s)
Diethyl ether
19.2 0.47 343, 500, 577(s)
344, 497
Tetrahydrofuran
20.0 0.55 356, 521, 598(s)
349,507, 575(s)
2
3
Dimethylformamide 26.6 0.69 372, 535, 637
360,534, 586, 757
Dimethylsulphoxide 29.8 0.76 371, 544, 625
365,536, 606(s)
Pyridine
374,532(s), 601, 679
33.1 0.64 389, 537, 631
(s)- denotes shoulder
65
Figure 3.7: Spectral and color variation upon changes in the ratio of CH2Cl2:DMF solutions of
3.
To emphasize the possibility of hydrogen bonding involving the pyrrolic NH groups and
solvent molecules, the crystal structure of the acetone solvate of 3 was solved. Figure 3.8 shows
the insertion of acetone at the binding site of 3 in the compound 3⋅acetone. Notably, crystals of
3⋅acetone are purple, which signifies a departure from the usual green/red dichroism in crystals
of these compounds. However, the purple color of the crystals was consistent with the observed
solvatochromism of 3. When compared with the known structures of di-N-alkylated 3,38,49-51 the
most obvious difference was the displacement of the water molecule from the binding site
between the pyrrolic NH groups. The N22-Oacetone distance in 3⋅acetone was 2.90 Å as compared
to 3.02 Å when water was hydrogen bonded as shown in figure 2.9.23 This change in bond
lengths is one of the factors that affected the dihedral angles subtended between the pyrrole
groups and the macrocyclic least squares plane so that while the angle between benzyl-
66
substituted pyrroles is similar in both water and acetone solvates, the angle subtended by the
pyrroles involved in the H-bonding interaction is lower in the acetone solvate (41° and 43°
against 47° and 52.4° for the water solvate38,49).
Figure 3.8: Molecular structure of the acetone solvate of 3.
Qualitatively, the solvatochromism of 2 and 3 resulted in a color change from red/pink in
non-polar solvents through violet to blue for the most polar solvents used (DMF, DMSO). For
the fully N-substituted derivatives of 2, otherwise known as 4, there was predictably no
substantial change in the electronic spectrum upon variation of solvent polarity since the
possibilities for hydrogen bonding have been negated by complete N-alkylation of the
oxoporhyrinogen.
Several models have been developed in the literature to visualize the linear solvation
energy relationships.92-94 A comprehensive collection of solvatochromic parameters and methods
for simplifying the generalized solvatochromic equations has been reported by Kamlet et al.49 In
the present study, we utilized solvent donor number (quantitative measurement of Lewis
basicity) (DN) and the beta scale of HBA (hydrogen-bond acceptor) (β) basicities to seek linear
solvation energy relationships. The beta scale provides a measure of the solvent’s ability to
67
accept a proton (donate an electron pair) in a solute-to-solvent hydrogen bond. Figure 3.9
illustrates the relationships between the solvent donor number, the β values, and the spectral
shifts of compounds 2 and 3. The correlation coefficients for these plots were found to be 0.85,
0.97, 0.73, and 0.90, respectively. The linear trend in the plots suggested that solute-solvent
hydrogen bonding was primarily responsible for the observed solvatochromic effect of
oxoporphyrinogens.
(a)
(a)
13
30
(b)
(b)
14
13
30
12
12
Donor Number
11
8
20
4
10
5
10
0
0.8
1
28000
0.8
(c)
(c)
0.6
13
10
6
0.4
β
(d)
(d)
11
13
10
9
3
7
7
0.4
5
4
1
30000
8
12
11
0.6
2
0
26000
30000
8
12
6
4
10
2
28000
10
8
5
3
26000
11
9
7
7
6
20
9
9
6
3
5
4
0.2
0.2
2
0.0
2
1
0.0
-0.2
26000
28000
30000
26000
-1
1
28000
30000
-1
λ (cm )
λ (cm )
Figure 3.9: Plots of donor number versus absorption peak maxima (a and b), and β versus
absorption peak maxima (c and d) for 2 (plots a and c) and 3 (plots b and d). Solvents: 1
(hexane); 2 (benzene); 3 (benzonitrile); 4 (acetonitrile); 5 (1,4-dioxane); 6 (acetone); 7 (ethyl
acetate); 8 (ethanol); 9 (diethyl ether); 10 (tetrahydrofuran); 11 (dimethylformamide); 12
(dimethyl sulfoxide); 13 (pyridine).
68
UV/Visible Spectrophotometry of Anion Binding to 2 and 3
The potential of polytopic-hydrogen binding sites can be exploited by evaluating their
anion binding properties. Previously, it was observed that the optical absorption spectra of the
oxoporphyrinogens were modified by the presence of perchlorate when carrying out
spectroelectrochemistry measurements. Therefore, optical absorption spectroscopy was used to
quantify the binding of halides and several other anions.
Figure 3.10 illustrates the spectral changes observed during the titration of
tetrabutylammonium fluoride against a solution of 3 in dichloromethane. During the titration,
absorption bands located at 333 and 509 nm underwent a red shift to 374 and 604 nm,
respectively. In addition, a new intense band was detected at 753 nm. An isosbestic point was
monitored at 420 nm suggesting the presence of one equilibrium process in solution. Similar
spectral changes were observed for all of the anions investigated. Job’s plot of method of
continuous variation yielded a 1:1 stoichiometry for the porphyrinogen (3):anion complexes.
The anion binding constants were evaluated using the spectral data by construction of BenesiHildebrand plots95 (figure 3.10 inset). Linear plots were observed for all of the studied anions
and the calculated binding constants are summarized in table 3.3.
69
1.0
o
A /∆A
300
0.8
200
Absorbance
100
0.6
0
0
5
5x10
6
1x10
6
2x10
-
1/[F ]
0.4
0.2
0.0
400
600
800
1000
Wavelength (nm)
Figure 3.10: Spectral changes observed during the titration of 3 with tetrabutylammonium
fluoride in dichloromethane. The inset figure shows Benesi-Hildebrand plot95 constructed for
determination of the binding constants.
70
Table 3.3: Aniona binding constantsc for 2 and 3 receptors determined from absorption titration
method in CH2Cl2 at room temperature.
Anion
OxP(Bz)2,3
OxP, 2
∆λ, cm-1, b
K1, M-1
K2 M-1
∆λ, cm-1
K, M-1
F-
821
1.0 x 105
2.7 x 104
854
1.2 x 105
Cl-
647
8.3 x 104
3.2 x 104
521
5.1 x 104
Br-
534
8.2 x 104
2.4 x 104
247
1.9 x 104
I-
83
4.8 x 104
1.6 x 103
58
3.5 x 103
PF6-
15
1.2 x 104
1.5 x 103
8
3.2 x 103
ClO4-
19
1.4 x 104
2.1 x 103
62
7.9 x 103
NO3-
383
9.7 x 104
2.7 x 104
548
8.2 x 104
C2H3O2-
554
6.6 x 104
3.3 x 104
526
6.1 x 104
H2PO4-
520
3.3 x 104
2.3 x 104
570
6.1 x 104
abc-
tetrabutylammonium salts were utilized.
For the first set of spectral changes
Error < 15%
Interestingly, since compound 2 has the ability to bind two anions, two sets of spectral
changes were observed during anion titrations as shown in figure 3.11. Using this spectral data,
the binding constants were evaluated and are also given in table 3.3. The magnitudes of K values
suggested stable anion binding by both porphyrinogens, which for halides follows the trend: F>> Cl- > Br- > I-. However, for polyatomic anions, no clear tread could be observed perhaps due
to different modes of binding of these anions to the porphyrinogen pyrrolic NH groups. In
general, 2 revealed higher K values compared to 3 for a given anion (K1 was utilized for
comparison). The spectral shifts of the UV band were correlated with the K values as shown in
figure 3.12. Respectable trends were observed for both of the investigated oxoporphyrinogen
derivatives. Based on the results of solvatochromic studies in figure 3.6, and the results of figure
3.12 one can conclude that hydrogen bonding interactions were primarily responsible for these
71
observations.
Further detailed 1H NMR studies were performed to confirm the hydrogen
bonding interactions for solvent and anions by the investigated porphyrinogens.
1.0
120
(a)
Ao/∆ A
100
0.8
80
Absorbance
60
0.6
40
20
0
0
0.4
5
2x10
5
4x10
5
6x10
5
8x10
6
1x10
-
1/[F ]
0.2
0.0
400
600
800
1000
Wavelength (nm)
4.0
(b)
Ao/∆ A
0.7
Absorbance
0.6
0.5
3.5
3.0
2.5
0.4
2.0
4
1x10
0.3
4
2x10
4
3x10
4
4x10
4
5x10
-
1/[F ]
0.2
0.1
0.0
400
600
800
1000
Wavelength (nm)
Figure 3.11: Spectral changes observed during the titration of (TBA)F to a solution of 2. (a)
The first set of spectral changes and (b) second set of spectral changes. The inset plots in (a) and
(b) reveal the respective Benesi-Hildebrand plots95 constructed for evaluation of binding
constants.
72
-
5
1x10
F
(a) 2
(a)
-
NO3
4
K
8x10
-
Cl
-
Br
4
-
6x10
BH3CN
C2H3O2
-
4
4x10
-
H2PO4
-
4
2x10
-
ClO4
PF6
0
0
5
1x10
I
200
400
800
-
F
(b)
(b)2a
4
-
9x10
NO3
K
C2H3O2
4
6x10
-
Cl
-
H2PO4
-
BH3CN
4
3x10
-
-
0
600
PF6
ClO4
0
I
Br
-
-
200
400
600
800
1000
∆λ, cm-1
Figure 3.12: Plots of binding constant, K (M-1), of different anions versus spectral shift, ∆λ
(cm-1), for (a) 2 and (b) 3.
1
H NMR Studies
Proton NMR was carried out by our collaborator Jonathan P. Hill and is presented to
complete the story of anion binding. In general, it is possible to detect hydrogen bonding
interactions during binding of an anion by a small molecule using proton NMR spectroscopy.
Observation of the interaction is preferred in a solvent where formation of an ion pair by the
73
anion salt (e.g. n-tetrabutylammonium fluoride) is not favored, such as DMSO or CH3CN. For
derivatives of 2, which are poorly soluble in these solvents, this presented a problem. However,
in the presence of excesses of the analyte anion the compounds are somewhat solubilized
permitting measurement of their NMR spectra. The increase in solubility was caused by anion
complexation.
Proton NMR spectra of 2 in the absence and presence of chloride ions in CDCl3 are
shown in figure 3.13. The spectra were interpreted with reference to the known protic
tautomerization of 2. The tautomerism is illustrated together with the 1H-NMR spectrum of a
CDCl3 solution of the tautomers in admixture. The NH and OH peaks assigned to the
porphyrinogen (figure 3.13a) and porphodimethene (figure 3.13 b) forms, respectively, were
present. In the aromatic region, the peak due to residual chloroform obscured one of the peaks.
Increasing the measurement temperature shifted the tautomeric equilibrium to the porphyrinogen
form. Stabilization of the porphyrinogen form can also be achieved introducing a hydrogen
bonded guest, such as a halide or other anion. In the presence of anions the peaks due to the
porphodimethene form are reduced as illustrated in figure 3.13.
74
a)
Tautomer 1
Tautomer 2
b)
c)
Figure 3.13: 1H-NMR spectra of 2 in CDCl3 in the absence and presence of excess tetra-nbutylammonium chloride. (a) Structures of the two tautomers of 2 (Tautomer 1: porphyrinogen
form, Tautomer 2: porphodimethene form). (b) 1H-NMR spectrum of 2 in the presence of excess
(TBA)Cl revealing binding of chloride by the porphyrinogen form. (c) 1H-NMR spectrum of 2 in
the absence of excess (TBA)Cl indicating its existence as the porphodimethene form.
In the case of 3 in CDCl3, the complexation of an anion is again obviated not only by a
downfield shift of the pyrrolic NH protons but by variations in the chemical shifts of the pyrrolic
β-proton resonances. The downfield shift of the NH proton peak is a result of hydrogen bonding
with the substrate anion. For the more strongly interacting fluoride, cyanide and acetate anions,
the resonance due to the NH protons gradually broadens with a slight downfield shift. Shifts in
75
the positions of the β-proton resonances are also more pronounced than for the other anions
studied because of larger macrocyclic perturbations. For instance, complexation by the smaller
fluoride ion must have caused more serious macrocyclic deformations and this was seen by the
substantial red shift in the electronic spectrum. The potential coupling between proton and
fluorine nuclei could not be initially observed in 3 although NMR spectra at lower temperatures,
as shown in figure 3.14, indicated the presence of more than one species in solution which were
assigned to complexed and non-complexed 3. A third peak observed during the low temperature
measurements could be due to complexation of one fluoride ion by two molecules of 3. When
CH2Cl2 was used as solvent the complexation by fluoride ion can be more easily observed and a
titration of 3 against fluoride ions in CD2Cl2 is shown in figure 3.15. During the titration and for
low proportions of fluoride ion, there appear two distinct NH resonances As fluoride ion
concentration is increased the NH peak at lower field (assigned to the 3⋅F− complex) increases
then diminishes until at proportions above 1 equivalent it disappears from the spectrum
altogether, presumably because of anionic exchange. In the initial stages of the titration
involving very low concentrations of fluoride ion the NH resonance becomes significantly
broadened although the NH peak(s) reappear at higher fluoride concentrations. The peak at
higher field is due to the anion-free 3.
76
Figure 3.14: Low temperature NMR data for 3 in CDCl3 solution at −70°C and in the presence
an various increments of (TBA)F.
Figure 3.15: Titration of 3 with Fluoride ions performed in CD2Cl2 solution at room
temperature.
77
Anion Selectivity by a Combination of Anion Binding and Solvatochromism Effects
The effect of the solvatochromism of 2 and 3 on their optical spectra in the presence of
complexing anions was observed. This is intended as a qualitative assessment of the colorimetric
responses of 2 and 3 to anions in various solvents. For 2, solvents used for this study were
dichloromethane, chloroform, tetrahydrofuran, and pyridine. Compound 2 was virtually
insoluble in other solvents unless in the presence of excess of strongly interacting anions.
Electronic absorption spectra indicate the effect of anion binding and solvent identity on the
color of the solutions. A beautiful example of this for 2 was in a CH2Cl2 solution where changing
the anion gave relatively well defined changes in the UV/Vis spectrum shown in figure 3.16b.
Tetrafluoroborate anions interacted weakly and did not change the spectrum significantly.
Binding of the halides gradually increased the intensity of a new band at 665 nm passing from
iodide to chloride. However, when fluoride was present the absorption spectrum deviated greatly
from that of the parent. A broad band appeared at 758 nm as was noted previously. This band
was noteworthy in that it would allow monitoring for the presence of fluoride at around 860 nm
without interference from the other anions studied in CH2Cl2. In acetone, the response of 2 to
different anions was similar to that in dichloromethane. In acetone, however, cyanide and acetate
anions give a response almost identical with that of fluoride ions. For pyridine and
tetrahydrofuran, the spectra were overlapping although iodide ions in tetrahydrofuran introduced
a new absorption band at 358 nm. The effect of the presence of particular anions on the color of
solutions of 2 is summarized in figure 3.16. It is interesting to note in figure 3.16 that an apparent
isosbestic point appears in the spectra containing non-identical anions. This is because the mode
of binding of the anion is similar for those participating in this phenomenon.
78
Figure 3.16: (a) Colors of solutions of 2 in the presence of various anions and in the solvents
indicated. (b) The optical spectra of 2 in dichloromethane and in the presence of the indicated
anions.
For 3, fluoride ions could be readily distinguished from the other halides in most solvents
while they could also be distinguished from acetate and cyanide through variation of the solvent
medium. In solvents of higher polarity, differentiation of fluoride became increasingly unlikely
because the indicative color for fluoride ions coincides with the native color of 3 in those
solvents. Generally, the ions acetate, cyanide and fluoride can be differentiated from the other
ions by the blue color of their solutions. The presence of other anions usually results in violet
solutions. The two prominent color differences were for fluoride in pyridine and cyanide in
79
chloroform. Acetone was a typical case where the spectra of 3 in the presence of fluoride,
cyanide or acetate are similar and contained a substantial absorption band at 728 nm. For the
other ions, the spectra were similar to the parent except for the halides where a shoulder at 600
nm increases in intensity passing from iodide to chloride. Optical absorption spectra of solutions
containing individual anions, 3 and in selected solvents are shown in figure 3.17. The most
notable separation of anion response comes in dichloromethane where fluoride can be
distinguished from all the other ions studied by virtue of the absorption band at 750 nm. Also,
there was an interesting response to cyanide which results in an intense new absorption band at
300 nm. This response may be due to some chemical reaction. Cyanide ions may be
distinguished from the others studied anions by using ethanol as solvent as shown in figure 3.17
b. In that case, it was the lack of response from other anions which enables almost unobstructed
observation of an absorption band at 756 nm. Finally, iodide ions gave a variety of responses
depending on solvent. Often a new absorption band(s) is observed in the UV around 380 nm.
Where propionic acid is the solvent, iodide was the only analyte to expose a distinct response
from 3. In the UV/Vis spectrum, this manifests itself as intense new absorption bands at 350 nm
and 440 nm with a less intense band at 785 nm which does not overlap with the absorption bands
in the spectra of solutions containing other anions. The tile representation in figure 3.18
illustrates the colors of dilute solutions (~10−6 M) of 3 in various solvents and in the presence of
excess of the respective anions. The solvents are arranged in order of their polarities while the
anions are arranged approximately by the apparent strengths of interaction with the anion
complexing agent according to the color changes.
80
Figure 3.17: Optical spectra of 3 in (a) dichloromethane and (b) ethanol in the presence of the
indicated anions.
Figure 3.18: Tile representation of the colors of dilute (~10−6 M) solutions of 3 in several
solvents in the presence of the anions studied.
81
In order to evaluate receptors 2 and 3 as potential electrochemical anion sensors their
voltammetric behavior was investigated in the absence and presence of anions in DCB using
tetrabutylammonium hexafluorophosphate as the supporting electrolyte.
This choice of the
supporting electrolyte was due to its weak binding to the receptors 2 and 3 as shown in optical
studies.96 Titrations of 2 and 3 were undertaken by progressive addition of F-, Cl-, C2H3O2-, NO3H2PO4-, or ClO4- in solution in the form of the tetrabutylammonium salt. These anions were
employed due to the lack of a redox response to these anions in the employed potential window
of DCB.
For the purpose of electrochemical recognition studies, the first two reversible
oxidations and the first two reductions of the oxoporphyrinogens were probed.
Upon
complexation of anions to the receptor, a cathodic shift in the potential of the oxidation and the
reduction are expected. That is, anion binding is expected to make the oxidation process easy
and the reduction process difficult, compared to the potentials in the absence of added anions
(except for the supporting electrolyte).70-71 As discussed below, this expected trend was observed
for the oxidations of 2 and 3 but for reduction of 2, although smaller in magnitude, a reverse
trend was observed.
Figure 3.19 illustrates the effect of F- addition on the cyclic voltammograms of 2 in DCB.
The first two oxidations of 2 were located at E1/2 = 0.27 and 0.48 V vs. Fc/Fc+. Progressive
addition of F- revealed a cathodic shift of first oxidation with a maximum cathodic shift of 566
mV upon addition of 3 equivalents of F-. Although 2 is known to bind two anions, three
equivalents of anions were used to ensure complete binding of F- to 2. The first reduction of 2
was located at Epc = -1.33 V vs. Fc/Fc+. Interestingly, addition of one equivalent of F- resulted in
a cathodic shift of ~50 mV, however, the second and third additions of F- reversed the direction
82
of the potential shift to the anodic side. That is, the F- bound 2 was easier to reduce by ~60 mV
compared to pristine 2 in solution. A similar trend was observed for 3 during the titration of
(TBA)F in DCB as shown in figure 3.20. The first two reversible oxidations of 3 located at E1/2
= 0.48 and 0.69 V vs. Fc/Fc+ experienced cathodic shift up to 600 mV. Since 3 binds only one
equivalent of a given anion, a maximum of 2 equivalents of F- was enough to visualize the
optimum potential shift. During reduction, as predicted, a systematic cathodic shift of the
potentials were observed upon increasing addition of F- to the solution containing 3 as shown in
figure 3.20. Unlike the reversible oxidations and their large anion binding induced cathodic
shifts, the reductions of 2 and 3 were irreversible and the observed potential shifts were marginal
without a systematic trend in the potential shifts. Therefore, emphasis was placed on the anion
binding induced changes in the oxidation potentials of 2 and 3.
(iii)
(a)
(i)
250 nA
(ii)
(iii)
(i)
(ii)
0.5
0.0
-0.5
-1.0
-1.5
+
Potential (V) vs Fc/Fc
Figure 3.19: Cyclic voltammograms of 2 with (i) 0 eq. (ii) 1.0 eq., and (iii) 3.0 eq addition of
(TBA)F in DCB containing 0.1 M (TBA)PF6. Both oxidation and reduction waves are shown.
Scan Rate= 0.1V/s
83
250 nA
(i)
(iii)
(ii)
(ii)
(iii)
(i)
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
+
Figure 3.20: Cyclic voltammograms of 3 with (i) 0 eq. (ii) 1.0 eq., and (iii) 2.0 eq addition of
(TBA)F in DCB containing 0.1 M (TBA)PF6. Both oxidation and reduction waves are shown.
Scan Rate= 0.1V/s.
The anion binding induced potentials shifts for 2 and 3 with rest of the anions followed a
similar trend but with potential shifts smaller than that observed for F- binding. Figure 3.21
illustrates the effect of C2H3O2- on 3 in DCB. Upon addition of C2H3O2-, a cathodic shift of 553
mV was observed for the first oxidation potential. Table 3.4 lists the maximum cathodic shift
(∆E) of the first oxidation potential of the porphyrinogen receptors 2 and 3 upon binding of
various anions. The following trend was observed irrespective of the nature of the receptor 2 or
3: ClO4- < NO3- < Cl- < H2PO4- < C2H3O2- < F-. The magnitude of the potential shifts for a given
anion was slightly more for 3 than for 2, although the former bound to only one anion. These
results suggest 3 being a better anion redox sensor compared to 2. Additionally, for most of the
anions, the potential shift values tracked that of the binding constants and shift in peak maxima
upon anion binding to the receptors, as listed in table 3.3. It is clear from the data that the
magnitude of the binding constant, spectral shift and potential shift correlate well for the
employed series of anions and the receptors.
84
(iv)
250 nA
(ii)
(i)
(i)
(ii)
(iv)
(iii)
(iii)
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
+
Figure 3.21: Cyclic voltammograms of receptor, 3 in DCB containing 0.1 M (TBA)PF6 on (i) 0,
(ii) 1, (iii) 2 and (iv) 3 equivalent addition of (TBA)C2H3O2. Both oxidation and reduction
waves are shown. Scan rate = 0.1 V/s.
(i)
(ii)
250 nA
(iv)
(iii)
(i)
(ii)
(iii)
(iv)
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
+
Figure 3.22: Cyclic voltammograms of receptor, 3 in DCB containing 0.1 M (TBA)PF6 on (i) 0,
(ii) 1, (iii) 2 and (iv) 3 equivalent addition of (TBA)NO3. Both reduction and oxidation waves
are shown. Scan rate = 0.1 V/s.
85
Table 3.4: Maximum cathodic shift, ∆E, observed for the first oxidation potential (mV)b
2, ∆E a
3, ∆Ea
F-
566
600
Cl-
205
118
C2H3O2-
503
553
NO3-
67
102
ClO4-
32
48
H2PO4-
466
522
Anion
a
- DCB containing 0.1 M (TBA)PF6. Scan rate = 0.1 V/s.
b
- Tetrabutylammonium salts were utilized as anion carriers.
Computational Modeling of Anion Binding to Oxoporphyrinogens, 2 and 3
Computational studies were performed to shed light on the structures of the receptoranion complexes. These studies were also used to compare the electrochemical and optical
results to supplement the findings of the experimental results. The structures of 2 and 3 were
optimized using density functional methods (DFT) at the B3LYP/3-21G(*) level52 in the gas
phase. The DFT method were chosen over Hartree-Fock or semiempirical methods since recent
studies on oxoporphyrinogens at the DFT method and 3-21G(*) level predicted the geometry and
electronic structure much more accurately.56 All of the structures were completely optimized to
a stationary point on a Born-Oppenheimer potential energy surface. Once the structures of 2 and
3 were optimized, solvation methods were employed to see how the receptors behaved in the
presence of the anions in a solvent medium. The “gas-phase” optimized structure was used as
86
input to run a single point energy calculation in the solution phase. The method that was
employed in this investigation was the Self-Consistent Reaction Field (SCRF) - Polarizable
Continuum (PCM) model. In this model the solvent is treated as a continuum of uniform
dielectric constant in which the solute is placed in a cavity created via a series of overlapping
spheres, initially devised by Tomasi and coworkers.97 The solvent used in the solvation methods
was dichloromethane since it had the closest dielectric constant to DCB used in the
electrochemical studies.
Figure 3.23 shows the optimized structures of 3 in the presence various anions while
table 3.5 lists the calculated energy of anion binding, ∆U, along with the energies of the frontier
orbitals. As mentioned earlier, compound 3 is only able to bind one anion since two of the
nitrogens are carrying benzyl substituents.96 The optimized 3•F- structure is shown in figure 3.23
where the fluoride is bridged between the two N-H moieties. Table 3.6 lists the calculated bond
lengths for the receptor-anion complex for compounds 2 and 3. The computed bond length of NH bond is lengthened significantly from 1.01 Å in 3 to 1.18 Å in 3•F-. The NH•••F bond length is
1.29 Å. The lengthening of the N-H bond and the relatively short NH•••F bond length show that
the hydrogen bond is quite strong. When the chloride complex of 3 was investigated, the N-H
bond length was 1.06 Å which is significantly shortened when compared to the 3•F- complex. In
this case the NH•••Cl bond length is 2.05 Å which is 0.76 Å longer than that of the 3•F- complex.
The longer the NH•••X bond length, the weaker the hydrogen bond. When comparing the two
halide ions the predicted order of affinity to 3 was F- >> Cl-.
87
a).
b).
c).
d).
e).
f).
Figure 3.23: B3LYP/3-21G(*) optimized structures for 3 in the (a) absence and in the presence
of (b) F-, (c) Cl- (d) C2H3O2-, (e) NO3-, and (f) ClO4-.
88
Table 3.5: B3LYP/3-21G(*)/SCRF/PCM Calculated Energy of Anion Binding and Energy
Levels of the Frontier Orbitals of receptors 2 and 3.
HOMO-LUMO
Receptor•Anion
2
2•[F-]2
2•[Cl-]2
-
2•[NO3 ]2
∆U, kJ/mol
HOMO, eV
LUMO, eV
gap, eV
-
-5.351
-3.374
1.977
-808.78
-4.696
-3.118
1.578
-111.43
-4.835
-3.013
1.822
-326.19
-4.818
-2.984
1.834
-
2•[H2PO4 ]2
-398.22
-4.813
-2.954
1.859
2•[ClO4-]2
-202.69
-4.876
-3.004
1.872
2•[C2H3O2-]2
-376.61
-4.758
-2.921
1.837
-
-5.396
-3.323
2.073
-429.29
-4.702
-2.804
1.897
3•[Cl ]
-66.65
-4.827
-2.895
1.932
3•[NO3-]
-151.96
-4.859
-2.887
1.972
3•[H2PO4-]
-215.23
-4.811
-2.831
1.980
3•[ClO4-]
-112.42
-4.894
-2.891
2.003
3•[C2H3O2-]
-208.73
-4.744
-2.782
1.962
3
3•[F-]
-
When polyatomic anion binding were compared to the N-H bond lengths, good
agreement was seen for C2H3O2- and H2PO4- having similar binding constants and bond lengths.
For ClO4- with a tetrahedral structure, binding to 3 with the smallest binding constant, the
NH•••O bond was found to be quite lengthy. Nitrate, with a trigonal planar shape, had the two
oxygen atoms binding to two imino-H. It is important to point out that for polyatomic anions,
generally two oxygen atoms were bound to the two available imino-H, although their NH•••O
distance was dependent upon the nature of the anion.
89
Table 3.6: B3LYP/3-21G(*) Calculated Bond Lengths for Receptor –Anion Complexes in the
Gas Phase.
d(N-H), Åa
d(N-H), Åa
d(NH•••X), Åb
d(NH•••X), Åb
1.012
1.012
-
-
1.053
1.053
-
-
1.094
1.099
1.456
1.484
1.100
1.092
1.478
1.462
1.050
1.050
2.116
2.116
1.050
1.050
2.116
2.116
1.047
1.048
1.727
1.736
1.047
1.048
1.735
1.728
1.065
1.073
1.525
1.557
1.065
1.071
1.529
1.556
1.033
1.034
1.525
1.557
1.033
1.034
1.529
1.556
1.076
1.076
1.545
1.545
1.076
1.076
1.545
1.545
1.014
1.014
-
-
3•[F-]
1.176
1.176
1.292
1.292
-
3•[Cl ]
1.063
1.063
2.049
2.048
3•[NO3-]
1.073
1.077
1.538
1.549
3•[H2PO4-]
1.114
1.122
1.402
1.407
3•[ClO4-]
1.052
1.055
1.622
1.65
3•[C2H3O2-]
1.119
1.117
1.432
1.438
Receptor•Anion
2
2•[F-]2
-
2•[Cl ]2
2•[NO3-]2
2•[H2PO4-]2
-
2•[ClO4 ]2
2•[C2H3O2-]2
3
a
The four N-H distances are given in the corresponding two columns and two rows.
b
The four NH•••X distances are given in the corresponding two columns and two rows.
90
Similar results were obtained for 2 binding to two equivalents of anions. Figure 3.24
shows the optimized structures, while table 3.5 shows the energetics. For halides the NH•••X
followed the trend: F- << Cl- similar to that observed for 3. Based on the NH•••O bond distances
the predicted affinity for polyatomic anions binding to 3 was C2H3O2- > H2PO4- > NO3- > ClO4-.
The calculated bond lengths for 2 and 3 binding to various anions are given in table 3.6. The
data points out that the NH•••X and NH•••O bond lengths are smaller for 3 binding to a particular
anion over 2 binding to the same anions. A similar trend was also observed for N-H bond
lengths upon anion binding. That is, the increase in N-H bond length upon 3 binding to a given
anion is larger than 2 binding to the same anions.
a).
b).
c).
d).
e).
f).
Figure 3.24: B3LYP/3-21G(*) optimized structures 2 in the a) absence and in the presence of
b). F-, c). Cl- d). C2H3O2-, e) ClO4- and f) NO3- ,
91
Hydrogen bonding interactions have widely been studied by using molecular electrostatic
potential maps (MEP).98 Figure 3.25 shows the molecular electrostatic potential map of 3 before
and after binding to F-. The red areas on map indicate negative values of electrostatic potential
while the blue areas indicate positive values of electrostatic potential. Before anion binding the
pyrrole N-H area was electron deficient, shown in blue. After the hydrogen bonding with F-, the
area is now electron rich and is shown in red. These results clearly point out the anion/solvent
binding site of oxoporphyrinogens. The earlier discussed X-ray structure of acetone bound to 3
unanimously proved such a structure and the binding mode as shown in figure 3.496
(a)
(b)
Figure 3.25: Molecular electrostatic potential (MEP) maps for 3 (a) in the absence and (b)
presence of F-. The MEPs are shown in two different orientations.
The B3LYP/3-21G(*)/SCRF/PCM calculated HOMO and LUMO energies, and the
HOMO-LUMO gap point out the following: (i) In agreement with the experimental results,23 the
HOMO-LUMO gap for 3 was slightly larger than that for 2. (ii) The HOMO-LUMO gap shrinks
92
in energy upon anion binding and this effect was larger for anions with higher binding constants
illustrated in table 3.5. (iii) Both HOMO and LUMO revealed changes in their energy levels
upon anion binding, however, such changes were much more pronounced for HOMO, an
observation that readily agreed with the experimental findings where the oxidation potentials of
2 and 3 were affected more than the respective reduction potentials. (iv) The ∆U value and the
HOMO-LUMO gap exhibited a reverse trend, that is, higher the energy of association, the
smaller the HOMO-LUMO gap for either of the receptors.
Figure 3.26 summarizes the relationship that exists between the different computed and
experimental findings for receptors 2 and 3. Figures 3.26a and b show plots of B3LYP/321G(*)/SCRF/PCM solvated HOMO energy level to that of the first oxidation potential for
receptors 2 and 3 in the presence of various anions. Linear plots with correlation coefficients of
0.79 and 0.87 were observed. Similar correlations were also obtained for the change in spectral
shift versus the HOMO energy level (Figure 3.26c and d). Correlation coefficients of 0.85 and
0.81 were observed, respectively, for 2 and 3 indicating excellent agreement between the
experiment and theory. Finally, the binding constant, K were also plotted against the HOMO
energy level (Figure 3.26e and f). Although good correlation coefficients of 0.62 and 0.90 were
observed, respectively, for 2 and 3, the points for NO3- seem to be slightly off scale. The reasons
for this observation were not fully clear. It is important to note that although the absolute values
of the correlation coefficients departed from the theoretical value of 1.0, such agreement between
theory and experimental findings using different techniques (optical and electrochemical) for
different type of anions (monoatomic, polyatomic, etc.) was especially encouraging.
93
21
-4.68
-
(a)
F
F
-4.72
(b)
-
C2H3O2
-4.75
HOMO (eV)
32
-
-4.70
-
-4.76
C2H3O2
-4.80
-4.80
-
-4.84
-
H2PO4
-
NO3
-
ClO4
Cl
-4.85
-
-
NO3
Cl
-4.88
-0.2
0.0
-4.90
-
-
PF6
H2PO4
0.2
-
-
ClO4 PF6
0.0
0.4
0.1
0.2
0.3
0.4
0.5
0.6
+
+
First Oxidation Potential (V) vs Fc/Fc
First Oxidation Potential (V) vs Fc/Fc
-
F
800
800
Cl
600
-
H2PO4
-
C2H3O2
-
-
NO3
600
-
-
Cl
H2PO4
-
C2H3O2
-1
(∆λ) (cm )
F
-
400
-
400
NO3
200
200
-
0
-3.30
ClO4
-
ClO4
(c)
-3.25
-3.20
-3.15
-3.10
(d)
0
-3.05
-4.90
-4.85
-4.80
HOMO (eV)
-4.75
-4.70
-
-
F 1.2x105
-
5
NO3
1.0x10
-4.65
HOMO (eV)
F
5
1.0x10
-
Cl
4
8.0x10
-
C2H3O2
-
NO3
4
8.0x10
-
H2PO4
-1
K (M )
4
6.0x10
-
C2H3O2
4
6.0x10
-
Cl
4
4
4.0x10
4.0x10
-
H2PO4
4
2.0x10
4
2.0x10
(e)
-
ClO4
0.0
-3.30
-3.25
-3.20
-3.15
-3.10
-3.05
HOMO (eV)
(f)
-
0.0
ClO4
-4.90
-4.85
-4.80
-4.75
-4.70
-4.65
HOMO (eV)
Figure 3.26: Plot of HOMO vs. oxidation potential (a and b), spectral shift vs. HOMO (c and
d), and K vs. HOMO (e and f) for receptor, 2 (plots a, c, and e) and receptor, 3 (plots b, d, and f).
94
3.4
Summary
In conclusion, the oxoporphyrinogens 2 and 3 are capable of binding anionic species such
as halides, and other complex polyatomic anions. Further there was some specificity in the
chromogenic response observed by the oxoporphyrinogens in the presence of miscellaneous
anions. Fluoride ions were distinguished from the halides by the substantial color change in a
solution. This variation was also observed to a great degree in spectrophotometric terms with
fluoride binding resulting in a strong absorption band at longer wavelengths.
Both the
solvatochromic and anion binding effects were greatly influenced by the hydrogen-bonding
ability of the pyrrolic NH groups.
When increased hydrogen bonding between the
oxoporphyrinogens and the solvent are observed, this obscures the effect of anion binding.
The investigated oxoporphyrinogens behaved as good redox sensors for anion recognition
as revealed by the anion-dependent electrochemical response for F-, Cl-, NO3-, C2H3O2-, H2PO4-,
ClO4- and PF6- in an organic solvent like DCB. The magnitude of the potential shift was found
to depend upon the nature of the anion and correlated well with the anion binding constant and
shift in absorption peak maxima. That is, anions with higher binding constants resulted in
pronounced cathodic shifts of the oxidation potentials of the oxoporphyrinogens. The DFT
B3LYP/3-21G(*) calculations utilizing the Self-Consistent Reaction Field (SCRF) -Polarizable
Continuum (PCM) solvation model yielded plausible geometries of the anion bound receptors.
These studies also showed that the energy of anion binding and energies of the HOMO orbital
follows the experimental anion induced spectral and electrochemical shifts.
95
96
Chapter 4
ELECTRON AND ENERGY TRANSFER PROCESSES IN SUPRAMOLECULAR
DONOR/ACCEPTOR SYSTEMS COMPOSED OF OXOPORPHYRINOGEN, ZINC
PORPHYRIN(S) AND FULLERENE(S)
4.1
Introduction
Energy and electron-transfer processes in molecular and supramolecular donor-acceptor
systems are of great interest (1) to address mechanistic details of electron transfer in chemistry
and biology,99-100 (2) to develop light energy harvesting systems,101-105 and (3) to build
optoelectronic devices106. To achieve these goals, covalently-linked donor-acceptor systems are
frequently employed along with self-assembled donor-acceptor systems.107 To construct these
systems porphyrins, phthalocyanines, or ruthenium-(II) tris(bipyridine) are often utilized as
primary electron donors,108-109 while the role of electron acceptor is often played by fullerene,
quinone, or nitroaromatic compounds.
108-109
Various research groups have utilized these
compounds to construct donor/acceptor molecular and supramolecular systems.
There are two main mechanisms by which an excited molecule can transfer its energy to
another molecule: electron transfer and energy transfer. A schematic representation of these
processes using energy level diagrams is presented in Figure 4.1. Electron transfer occurs when
the donor molecule transfers an electron to the vacant LUMO energy level of the acceptor,
creating a charge separated donor-acceptor pair. Energy transfer occurs when the energy of the
excited donor molecule is donated to the acceptor and this energy, in turn, excites the acceptor
molecule, leaving the donor molecule in the ground state and the acceptor in the singlet excited
state.
97
(a) excitation
hν
Ground-state
chromophore
Excited-state
ch romophore
(b) electron
transf er
Excited-state
chromophore
Acceptor
Oxidized
chrom ophore
R educed
Acceptor
(c) energy transf er
Excited-state
chromoph ore
Acceptor with
accessible
excited state
Ground-state
chromophore
Excited-state
Acceptor
Figure 4.1: Schematic representation of (a) excitation (b) electron transfer and (c) energy
transfer processes.
The concept of quinone-linked porphyrin was launched to model the photochemical
processes occurring in photosynthetic systems.110-111 Models of the photosynthetic reaction
center sheds light on the structural and chemical features that govern electron-transfer processes
essential for charge separation. Many groups have developed synthetic models which consist of
a porphyrin covalently bound to quinones by flexible and rigid linkers to study the dependence
of variables including, but not limited to, distance and orientation in electron-transfer
processes.110-111
Milgrom and Dalton studied a compound that had benzoquinone units at
98
the meso positions of the porphyrin ring (figure 4.2).110 The absorbance and emission spectra
suggested extensive intramolecular charge transfer.
Ph
N
NH
P
h
R
N
HN
Ph
O
R=
O
I
Figure 4.2: Structure of porphyryin-quinone compound studied by Milgrom and coworkers.110
Imahori and coworkers studied several donor-acceptor systems including a molecular
tetrad J (figure 4.3) where photoinduced energy transfer was followed by multistep electron
transfer to achieve a long lived charge separated state in frozen media and solution.112 Upon
excitation, singlet excited energy transfer occured from the zinc porphyrin, 1ZnP*, to the free
base porphyrin followed by electron transfer from the singlet excited free base porphyrin, 1H2P*,
to the fullerene generating an initial charge separated state, Fc-ZnP-H2P•+-C60•- and subsequently
intermediate charge separated state, Fc-ZnP•+-H2P-C60•-. Then ferrocene separated the charges
further by being oxidized to yield a long-range charge separated state, Fc+-ZnP-H2P-C60•consisting of a ferrocenium ion (Fc+)-C60 radical anion (C60•-) pair. The lifetime of the resulting
charge separated species was determined to be 0.38 s, which is more than one order of magnitude
longer than any other intramolecular charge recombination processes of synthetic systems.
99
R
R
O
N
H
Fe
N
N
Zn
N
H
N
N N
H H
N N
N
O
R
R
O
N
H
N
CH3
J
Figure 4.3:
Covalently linked ferrocene-zinc porphyrin-free base porphyrin-C60 tetrad, J,
developed by Imahori and coworkers.112
Among the supramolecular donor-acceptor systems, axial coordination seems to be a
popular choice. The first report was presented in the form of ZnP binding to C60-pyridine and
C60- imidazole complexes (K and L) as shown in figure 4.4.113 These complexes revealed
reversible coordination of pyridine or imidazole appended fullerene to the zinc center of the
porphyrin which constituted labile but measurable binding. Increasing addition of fullerene
derivatives resulted in hypochromic and bathochromic shifts in the Soret and visible bands in the
UV/Visible spectra, followed by efficient quenching of porphyrin emission revealed by the
formation of binding constants of 7.0 X 103 M-1 and 1.1X 104 M-1 in the case of C60-pyridine and
C60- imidazole binding to ZnP, respectively.
100
NH
NH
N
N
N
N
N
N
Zn
N
N
N
L
K
Figure 4.4:
N
Zn
N
Porphyrin-fullerene dyads constructed via metal ligand axial coordination by
D’Souza and coworkers.113
The oxoporphyrinogen and its N-alkylated derivatives discussed in chapter 2 are electron
deficient and can exist in various redox states and, hence, are good candidates to build donoracceptor systems.49-51 In the present investigation, this has been verified by constructing, (zinc
porphyrin)-oxoporphyrinogens (15 and 16) dyads and supramolecular triads, 17, and pentad 18,
by coordinating functionalized fullerenes C60- imidazole to ZnP. The geometry and electronic
structures of the compounds have been deduced by DFT B3LYP 3-21G(*) calculations and
spectroscopic and electrochemical techniques. Photochemical studies have been carried out to
determine electron or energy transfer quenching mechanisms.
101
N
H3C N
N
Zn
N
N
N
N
ZnP,
N
13
C 60Im,
14
O
N
Zn
N
N
HN
N
O
O
NH
N
O
(ZnP)-OxP,
N
15
O
N
Zn
N
N
HN
N
O
O
NH
N
N
N Zn
O
(ZnP)2-OxP,
Figure 4.5: Structures of studied compounds.
102
N
16
N
H3C N
H3C N
N
N
N
N
N
N
N
N
O
O
Zn N
N
Zn N
N
HN
N
O
O
NH
HN
N
O
N
O
NH
N
O
N
N Zn N
N
N
O
N
(C60Im:ZnP)-OxP, 17
N CH3
(C60Im:ZnP)2-OxP, 18
Figure 4.6: Structures of supramolecular triad, 17, and pentad, 18.
4.2
Chemicals
DCB for electrochemical studies was dried over CaH2 and distilled under vacuum prior to
the experiments. PhCN and toluene were purchased from Aldrich Chemical Co and was used as
received. The (TBA)ClO4 was recrystallized from ethanol and dried in a vacuum oven at 35 oC
for 10 days. N21,N23-bis-(methylphenyl)10, 15, 20-triphenylporphinato zinc) -5,10,15,20-(3,5-dit-butyl-4-oxo-cyclohexa-2,5-dienylidene)porphyrinogen
(15)
and
N21,N22,N23,N24-tetrakis-(
methylphenyl)10, 15, 20-triphenylporphinato zinc)-5,10,15,20-(3,5-di-t-butyl-4-oxocyclohexa2,5-dienylidene)porphyrinogen (16) was synthesized by reported literature methods by our
103
collaborator, Dr Jonathan P. Hill as shown in figure 4.5.114-115. For the triad, the (ZnP)-OxP, 15,
dyad was coordinated axially to one equivalent of C60Im, 14, yielding (C60Im:ZnP)-OxP, 17 as
shown in figure 4.6. For the pentad, two equivalents of an imidazole functionalized fullerene,
C60Im, 14, was coordinated axially to the zinc center of the covalently linked (ZnP)2-OxP, 16,
yielding the pentad, (C60Im:ZnP)2-OxP, 18 also shown in figure 4.6.
N-methyl-2-(4’-N-
imidazolylphenyl)- 3,4-fulleropyrrolidine, C60Im, was a gift from Dr. Suresh Gadde, my labmate,
and was synthesized by reported literature methods shown in figure 4.5.116 5,10,15,20tetraphenylporphyrinatozinc(II), ZnP, was also synthesized by reported methods117 as shown in
figure 4.5. For comparative purposes, the dyad, C60Im:ZnP was also constructed using literature
methods.113
Instrumentation
The UV-visible spectral measurements were carried out with a Shimadzu Model 1600
UV-visible spectrophotometer. The fluorescence emission was monitored by using a Spex
Fluorolog-tau or Varian spectrometers. A right angle detection method was used.
Electrochemistry
reference. Ferrocene/ferrocenium redox couple was used as an internal standard. All of the
solutions were purged prior to electrochemical and spectral measurements using argon gas. All
the experiments were carried out at 23 + 1°C unless noted.
Time-resolved Emission and Transient Absorption Measurements
The time-resolved emission and transient absorption studies were performed in
104
collaboration with Dr. Osamu Ito, Tohoku University, Sendai, Japan. The picosecond timeresolved fluorescence spectra were measured using an argon-ion pumped Ti:sapphire laser
(Tsunami) and a streak scope (Hamamatsu Photonics). The details of the experimental setup
were followed from literature.118 The subpicosecond transient absorption spectra were recorded
by the pump and probe method. The samples were excited with a second harmonic generation
(SHG, 388 nm) output from a femtosecond Ti:sapphire regenerative amplifier seeded by SHG of
a Er-dropped fiber (Clark-MXRCPA-2001 plus, 1 kHz, fwhm 150 fs). The excitation light was
depolarized. The monitor white light was generated by focusing the fundamental of laser light
on flowing D2O/H2O cell.
The transmitted monitor light was detected with a dual MOS linear image sensor
(Hamamatsu Photonics, C6140) or InGaAs photodiode array (Hamamatsu Photonics, C5890128). Nanosecond transient absorption spectra in the NIR region were measured by means of
laser-flash photolysis; 532 nm light from a Nd:YAG laser was used as the exciting source and a
Ge-avalanche-photodiode module was used for detecting the monitoring light from a pulsed Xelamp.
4.3
Optical Absorption Studies
The electronic spectra of 2, 15, and 16 are shown in figure 4.7. Compound 2 exhibits a
broad absorption maximum at 518 nm, occupying most of the UV and visible region of the
spectrum. This broad red shifted spectrum of 2 relative to 1, the parent porphyrin, has been
attributed to the extended conjugation of the macrocycle to include its meso-substituents. As a
consequence of N-substitution at N21 and N23 in 16, there is a nearly 10 nm blue shift of the
oxoporphyrinogen absorption maximum with the loss of a tail in the longer wavelength than 650
105
nm. This is due to the increasing steric bulk at the oxoporphyrinogen core and the consequent slight
reduction in overlap between unsaturated groups. There is no noticeable shift in the Soret band
position of the ZnP entity of 16, when compared to pristine 13, indicating that there is little or no
electronic interchromophore interaction between identical or non-identical moieties within 16.
Intensity of the 420 nm band of 16 is more than that of 13, which is only a sign of the weak
interaction between 13 unit and 2 component in 16. Thus, the electronic absorption spectrum is
essentially the superimposition of the spectra due to 13 and an N21,N23-dibenzyloxoporphyrinogen
such as 3. This lack of intramolecular interaction between N-substituents is expected because of the
restricted rotation at the methylene group adjoining the two different components of 16. This has
been noted in the N-naphth-2-ylmethyl derivatives where intermolecular interactions are preferred.50
2.5
(i)
Absorbance
2.0
(ii)
1.5
1.0
0.5
(iii)
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 4.7: Optical absorption spectrum of (i) 16, (ii) 13, and (iii) 2 in DCB.
106
Coordinative supramolecular assembly with C60Im was monitored using optical absorption
methods.113 Figure 4.8 shows the spectral changes observed upon addition of C60Im (14) to a
solution of (ZnP)-OxP (15) in DCB. The spectral changes include a red shift of ZnP Soret band
accompanied by its diminishing intensity. Isosbestic points at 419 and 427 nm were observed
suggesting the existence of one equilibrium process in solution. During the titration, the band at 511
nm corresponding to 2 revealed no changes in the absorption maxima indicating a lack of interaction
between 2 and 14 entities.
Job’s plot constructed using the spectral data confirmed 1:1
complexation, confirming formation of the (C60Im:ZnP)-OxP (17) triad (see figure 4.8).
The
formation constant calculated by construction of a Benesi-Hildebrand plot95 was found to be 1.26 x
104 M–1 in DCB, which is comparable with a value of 1.16 x 104 M–1 for the C60Im:ZnP in DCB113
and suggests reasonably stable complex formation. Titration of (ZnP)2-OxP (16) with C60Im (14)
revealed spectral changes similar to those observed for the triad as shown in figure 4.9. That is, a
red shifted ZnP Soret band and one or more isosbestic points were observed. Job’s plot confirmed
1:2 stoichiometry establishing the formation of the pentad, (C60Im:ZnP)2-OxP (18) in solution. The
formation constant calculated by construction of a Benesi-Hildebrand plot95 was found to be 2.4 x
104 M–1 in DCB.
107
25
Ao/∆A
0.6
20
Absorbance
15
0.4
10
1.0x10
5
1.5x10
5
2.0x10
5
1/[Im C 60 ]
0.2
0.0
400
450
500
550
600
650
W avelength/nm
Figure 4.8: UV-visible spectral changes observed during the titration of 15 (1.3 µM each addition)
with 13 (2.0 µM) in DCB.
The figure inset shows Benesi-Hildebrand plot95 constructed for
evaluation of the binding constant; Ao (intensity in the absence of added C60Im) and ∆A (changes of
absorption on addition of C60Im).
108
16
14
0.8
Ao/dA
Absorbance
12
0.6
10
8
6
0.4
4
2.0x10
4
4.0x10
4
6.0x10
4
8.0x10
5
1.0x10
5
1.2x10
1/[C60 im]
0.2
0.0
400
450
500
550
600
650
Wavelength/nm
Figure 4.9: UV-visible spectral changes observed during the titration of 16 (1.3 µM each addition)
with 14 (2.0 µM) in DCB.
The inset figure shows Benesi-Hildebrand plot95 constructed for
evaluation of the binding constant; Ao (intensity in the absence of added C60Im) and ∆A (changes of
absorption on addition of C60Im).
Electrochemical studies were performed to evaluate the redox potentials of the individual
entities and energetics of different photochemical processes. Figure 4.10 shows the cyclic
voltammograms of 16, 15, 2, and 14 in DCB containing 0.1 M TBAClO4. Owing to the
presence of multiple redox-active entities, the voltammograms of 15 and 16 were rather
complex.
However, accurate analysis of the site of electron transfer corresponding to the
individual redox entities was possible by a comparison of the voltammograms of zinc
109
tetraphenylporphyrin, ZnP and mono and bis methylenenaphthyl N-substituted porphyrinogens,
5 and 6, which was discussed in Chapter 2.50 The site of electron transfer thus obtained is
indicated at the top of each redox couple. In the case of 15 and 16, the redox potentials
corresponding to ZnP entities are close to that of the reference compound, ZnP (two one-electron
oxidations at 0.28 ad 0.62 V vs. Fc/Fc+, and two one-electron reduction at −1.92 and −2.23 V vs.
Fc/Fc+) suggesting absence of interactions between ZnP and OxP units. The redox potentials
corresponding to the oxidation and reduction of the OxP entity are close to their respective Nalkylated analogues previously studied in Chapter 2. That is, the first reversible oxidation of
OxP is located at 0.37 V and 0.47 V, while the first reduction is located at Epc = −1.29 V and Epc
= −1.36 V, respectively, for 15 and 16. The more facile reduction of the OxP entity (close to
quinones) in the 15 and 16 suggests that it should act as an electron acceptor. The HOMOLUMO gap, measured as the potential difference between the first oxidation of the donor, ZnP
entity and the first reduction of acceptor, OxP entity, were found to be 1.57 and 1.61 V,
respectively, for 15 and 16.
The first three one-electron reductions of C60Im (14) used in the formation of the
supramolecular assemblies (figure 4.6) were located at E1/2 = −1.13 V, −1.51 V, and −2.05 V vs.
Fc/Fc+ as shown in figure 4.10. There is no significant shift (< 10 mV) in the potentials upon
coordination of C60Im to the ZnP moieties of 15 and 16. The electrochemically measured
HOMO (ZnP)-LUMO (C60Im) gap for structures (C60Im:ZnP)n-OxP was found to be 1.40 V,
which is similar to that (1.41 V) of C60Im:ZnP. Thus, the smaller HOMO-LUMO gap of
(C60Im:ZnP)-OxP indicates that the charge-separated state (ZnP•+: C60Im •−)-OxP is more stable
than (ZnP•+:ImC60)-OxP•−, and this situation is the same for the pentad, 18, studied. The freeenergy changes of charge separation, ∆GCS, were calculated according to the Rehm and Weller
110
method119 and employing the first oxidation potential of ZnP, the first reduction potential of OxP
or C60, the singlet excitation energy of ZnP, and the estimated Coulomb energy.
The ∆GCSZ
values for generating radical ion-pairs, ZnP•+-OxP•− and (ZnP)(ZnP•+)-OxP•− were found to be
−0.63 and −0.58 eV, respectively, in DCB, indicating the possibility of photoinduced charge
separation from singlet excited ZnP to the OxP entity. Similar calculations performed on 17 and
18 revealed a ∆GCSZ value of −0.73 for both compounds indicating a more exothermic charge
separation process from the singlet excited ZnP to the C60 entity in DCB.
OxP OxP
ZnP
ZnP
ZnP OxP ZnP
(a)
ZnP OxP
OxP OxP
ZnP
ZnPOxP
ZnP
(b)
OxP
OxP
OxP
(c)
C60im
C60im
C60im
(d)
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
+
Potential (V vs Fc/Fc )
Figure 4.10: Cyclic voltammograms of (a) 16, (b) 15, and (c) 2, and (d) 14 in DCB containing
0.1 M (TBA)ClO4. Scan rate = 100 mV s-1.
111
Computational Studies
To understand the geometry and electronic structures of 16, 17, and 18, computational
studies using B3LYP/3-21G(*) method
120
were performed. Figure 4.11a shows the optimized
structure while figure 4.11 (b and c) shows the frontier HOMO and LUMO orbitals. The structure
of the oxoporphyrinogen in 16 was found to be highly ruffled so that the β-pyrrole carbon
displacement is as much as 1.7 Å from the macrocyclic least squares plane and this is in agreement
with earlier X-ray and computational analyses of di-N-benzyl-substituted oxoporphyrinogens.49-51
In the calculated structure, the intramolecular Zn-Zn distance is 18.6 Å while that between the Zn
and the center of oxoporphyrinogen is 11.8 Å and there is no apparent association between the two
zinc porphyrins of 16, a feature confirmed by the optical absorption data. The frontier HOMO was
found located on a single zinc porphyrin, as was predicted from the electrochemical data, and
suggests a non-symmetric arrangement of the two ZnP moieties. On the other hand, the frontier
LUMO was found localized on the oxoporphyrinogen entity so that the requirement of zinc
porphyrin as electron donor and oxoporphyrinogen as electron acceptor is fulfilled in the triad 5.
The B3LYP/3-21G(*)-computed HOMO-LUMO gap was found to be 1.83 eV, which is comparable
with that deduced from electrochemical measurements.
112
(a)
(b)
(c)
Figure 4.11: (a) B3LYP/3-21G(*) optimized structure of triad, 16 and (b) the frontier HOMO
and (c) LUMO of the 16.
To build the supramolecular structures, 17 and 18, the starting molecules, C60Im (14) and
(ZnP)n-OxP (15 and 16) were fully optimized to a stationary point on the Born Oppenheimer
potential energy surface and allowed to interact. Figure 4.12 shows the optimized structures of
the supramolecular complexes. It should be mentioned here that owing to the very flexible
nature of the systems, several structures are plausible, but those shown in figure 4.12 are
apparently the least energetic structures. In agreement with the earlier X-ray and computational
analyses of bis N-alkyl substituted oxoporphyrinogens, 3,49 the oxoporphyrinogens in the
optimized (C60Im:ZnP)n-OxP (17 and 18) have a highly ruffled structure, in which the β-pyrrole
113
carbon displacement is as much as 1.7 Å. Also, based on the 1H NMR spectral data, the two zinc
porphyrin units in (ZnP)2-OxP (18) were assumed to be in the cis-isomer form, that is, pointing
in the same direction with respect to the plane of OxP macrocycle. In the optimized structure
(C60Im:ZnP)2-OxP (18), the Zn-Zn distance between two ZnP moieties was found to be ~19 Å
while the distance between the Zn and the center of OxP was found to be ~12 Å. The two
fullerene entities were separated by a distance as great as 29 Å while the closet center-to-center
distance between the ZnP and C60 was ~13 Å. In the case of (C60Im:ZnP)n-OxP, the center-tocenter distance between ZnP and C60 was ~13.7 Å while this distance between OxP and C60 was
about 15 Å. Thus, no apparent association between the different entities was observed in the
case of both 17 and 18 in agreement with the optical absorption data discussed earlier.
The HOMO(ZnP)-LUMO(C60) values for (C60Im:ZnP)n-OxP (n=1 and 2) (17 and 18)
calculated using B3LYP/3-21G(*) in gas phase were found to be 1.02 and 0.99 eV, respectively.
These values are lower than those obtained using electrochemical data and such a trend has also
been reported for, C60Im:ZnP.113
114
Figure 4.12: B3LYP/3-21G(*) optimized structures of (a) 17 and (b) 18.
Steady State Fluorescence
The emission behavior of the supramolecular systems was investigated initially using the
steady-state fluorescence technique. Figure 4.13 shows the emission spectrum of 2, 13, 15, and
16 in DCB. The intensities of ZnP emission bands located at 600 and 646 nm were quenched by
>85% for (ZnP)-OxP (15) and by >95% for (ZnP)2-OxP (16). In addition to the ZnP emission
bands, there was also a weak emission band at 720 nm corresponding to the N-substituted
porphyrinogen for both derivatives as confirmed by independent experiments performed using
simple N-alkylated porphyrinogens.
115
Fl. Intensity (a.u.)
1.00
0.75
(i)
0.50
(ii)
0.25
0.00
550
(iii)
(iv)
600
650
700
750
800
Wavelength (nm)
Figure 4.13: (a) Fluorescence spectrum of (i) ZnP (13), (ii) (ZnP)-OxP, (15) and (iii) (ZnP)2OxP, (16) (iv) OxP (2) in DCB. The samples were excited at the Soret band position of ZnP and
the concentrations of the compounds were held at 10 µM.
As shown in figure 4.14, addition of C60Im to a DCB solution containing (ZnP)n-OxP
caused additional quenching of the ZnP emission bands, which confirms interaction between
C60Im and ZnP due to the formation of a supramolecular pentad, 18, or triad, 17, as shown in
figure 4.6. Because of the observed decrease in ZnP emission intensities, the occurrence of one
or both of the following processes is suggested: (i) charge-separation quenching from the excited
singlet state of ZnP to the fullerene entity and (ii) energy transfer from ZnP to the fullerene
entity. In the following sections the time-resolved emission spectral results are discussed in order
to verify the different quenching pathways.
116
Fluorescence Intensity (a.u.)
1.0
0.8
0.6
0.4
0.2
600
650
700
750
800
Wavelength/nm
Figure 4.14: Fluorescence spectral changes observed on increasing addition of 14 (5 µM each
addition) to a solution of 16 in DCB. (λex=424 nm).
Pico-second Time-Resolved Emission Studies
Emission from pristine ZnP, 13, shows monoexponential decay when dissolved in either
toluene or DCB solvents. The ZnP emissions in 15 and 16 were found to be quenched and follow
biexponential decay with major short and minor long components.
Figure 4.15 shows the
fluorescence decay time profiles of 16. The lifetime of the minor long component is comparable
with that of unbound ZnP (see table 4.1). From the major short decay component of 1ZnP* (lifetime
is defined as τfZ), the quenching rate and quantum yield calculated for the dyad and triad were found
to be ~ 1.0 x 1010 s-1 and > 0.95, respectively (see table 4.1 footnote for relevant equations). In
nonpolar solvents, such as toluene, the fluorescence quenching process can be attributed mainly to
energy transfer, because of an insufficient driving force for the charge-separation process (∆GCSZ =
−0.05 eV in table 4.1). In slightly polar DCB, the results are indicative of the occurrence of
photoinduced charge separation from 1ZnP* to OxP.
117
1.0
1.0
(a)
(b)
(a)
ZnTPP in toluene
5 in toluene
5 in DCB
5 in PhCN
0.6
0.4
0.6
0.4
0.2
0.2
0.0
5 in DCB
5 in PhCN
0.8
Fluo. Int.
0.8
Fluo. Int.
(b)
5 in toluene
0
1
2
3
Time / ns
0.0
4
0.0
0.5
1.0
Time / ns
1.5
2.0
Figure 4.15: Fluorescence decay profiles of 13 and 16 (0.1 mM) collected in the (a) 600-640
nm corresponding to zinc porphyrin emission and (b) the 700-750 nm range. λex = 410 nm.
Black line 13 in toluene, red line 16 in toluene, blue line 16 in DCB, and green line 16 in PhCN.
Addition of C60Im (14) to (ZnP)n-OxP (15 or 16) forming the supramolecular structure
(C60Im:ZnP)n-OxP (17 or 18) caused additional quenching of the already quenched ZnP emission
(figure 4.12). In case of 18, the lifetime of ZnP was <10 ps close to the time-resolution of the
instrument. The lifetime of the quenched component of ZnP in 17 was 79 ps in DCB. That is,
highly efficient quenching was observed in both the supramolecular 17 and 18. The photo-physical
data are given in table 4.1. Increasing the polarity of the solvent revealed acceleration of the excited
singlet state ZnP quenching based on the increase in the negative free-energy change (∆GCSZ =
−0.73 eV in DCB), this observation has been interpreted in terms of the occurrence of a competitive
charge-separation process from 1ZnP* to the C60 moiety.
118
1.0
toluene, ZnTPP
toluene, C60Im:ZnP -OxP
DCB, C60Im:ZnP -OxP
Fluo. Int.
0.8
0.6
0.4
0.2
0.0
0
1
2
Time / ns
3
4
Figure 4.16: Fluorescence decay profiles of 17 ([component] = 0.07 mM) collected in the 600640 nm corresponding to zinc porphyrin emission, λex = 410 nm: ······· , 13 in toluene; —— 17 in
toluene; -----, 17 in DCB.
119
Table 4.1: Fluorescence lifetime (τf), quenching rate-constant (kq), quenching quantum-yield
(ΦCS) of 1ZnP* and 1C60*, free energy of charge-separation (∆GSCS), charge-separation rateconstant (kCS), lifetime of radical ion pair (τRIP) of (C60Im:ZnP)n-OxP (n = 1 or 2).
Compd
Solvent τfZ/[ps] [a] kqZ [a]/[s-1]
1
15
17
ZnP*
1
ZnP*
ФqZ[a] -∆GCSZ [b] τfC,O/[ps] [a] kqC,O [a]/ s-1 ФqC,O [a] -∆GCS [b]
1
ZnP*
eV
1
C60*
DCB
102
9.3 x 109
0.95
0.65d
50 (τfO)
TN
147
6.3 x 109
0.93
---
56 (τfO)
DCB
79
1.2 x 1010
0.96
0.73d
100 (τfC)
(0.66)
TN
92
1.0 x 1010
0.95
1
C60*
9.3 x 109
1
kCS [c,d]/
Τrip /[c]
C60*
/eV
s-1
[ns]
0.93
0.34e
9.5 x 106
110
e
(1020nm)
120 (τfC)
---
7.6 x 109
0.91
---
8.6 x 106
120
(1020nm)
16
18
DCB
80
1.2 x 1010
0.96
0.58
83 (τfO)
TN
104
1.0 x 1010
0.95
----
118 (τfO)
DCB
<10
>1.0 x 1011
0.99
0.73d
160 (τfC)
(0.66)
5.5 x 109
0.89
0.35 e
e
1.1 x 107
(1020nm)
90
70
1.4 x 107
(840nm)
TN
<10
>1.0 x 1011
0.99
190 (τfC)
---
4.6 x 109
0.86
---
8.7 x 106
120
(1020nm)
a
Lifetime (τf)sample for major short component, and (τf)ref of the reference compound ZnP was
evaluated to be 1900 ps in DCB and 2000 ps in toluene. The kq and Фq were calculated from
eqns. 1 and 2;
kq = (1/τf )sample - (1/τf )ref
(1)
Φq = [(1/τf )sample - (1/τf )ref ] / (1/τf )sample
(2)
b
Calculated from eqs. 3 and 4, employing ∆E0-0 = 2.07 eV for 1ZnP* and 1.75 eV for 1C60*, EOx
= 0.28 V for ZnP, and ERed = -1.29 V for OxP vs. Fc/Fc+ in DCB. RCC = 11.8 Å, for (ZnP)2-OxP.
Permittivities of toluene (TN) and DCB are 2.38 and 9.93, respectively.
∆GCS = EOx- ERed - ∆E0-0 + ∆GS
(3)
∆GS = (e2/(4πε0εR RCC)
(4)
c
From the decay rate at 1000 nm.
d
For (ZnP•+)-OxP•−and (ZnP•+:ImC60)-OxP•− etc.
e
For (ZnP•+:ImC60•−)-OxP etc.
120
The fluorescence time profiles in the 700 – 750 nm region in toluene and DCB were also
monitored for 17 and 18. In this wavelength region both C60 and OxP emit. Figure 4.17 shows
fluorescence decay profile of 18 in this range. The fluorescence lifetimes were evaluated by the
curve-fitting method and the τfC,O values are listed in table 4.1. In DCB, the τfC,O values are 160 and
100 ps for 17 and 18, respectively. From these τfC,O values, the rate constants (kqC,O) and quantum
yields (ΦqC,O) were evaluated as summarized in table 4.1. The kqC,O values are in the range of (4.0 –
8.0) x 109 s–1 and the ΦqC,O values are in the range of 0.85 – 0.95 (table 4.1). Both kqC,O and ΦqC,O
values tend to increase from 18 to 17. Further, these values increase with the solvent polarity,
suggesting the occurrence of competitive charge-separation and energy-transfer processes. In order
to distinguish between these two quenching mechanisms, nanosecond transient absorption spectral
measurements on the triad and the pentad were performed to characterize the charge-separation
products.
1.0
toluene, C60Im
toluene, (C60Im:ZnP)2 -OxP
DCB, (C60Im:ZnP)2 -OxP
Fluo. Int.
0.8
0.6
0.4
0.2
0.0
0
1
2
3
Time / ns
4
Figure 4.17: Fluorescence decay profiles of 18 ([component] = 0.07 mM) collected in the 700750 nm corresponding to C60 emission in DCB, λex = 410 nm. ······· , 14 in toluene; —— 18 in
toluene; -----, 18 in DCB.
121
Nanosecond Transient Absorption Studies
Compounds 2 and 16 were subjected to nanosecond transient absorption spectral
measurements for characterization of the electron transfer products. Nanosecond transient
absorption spectra are shown in figure 4.18a for the triad 16 in toluene, which were obtained by
the predominant excitation of the ZnTPP entity. Broad absorption bands were observed in the
600 – 1100 nm region with peak maxima around 830 nm. These absorptions are characteristic of
the triplet states of ZnTPP103 and 3, and this assignment is supported by their rapid decay in the
presence of O2. An initial increase in intensity at 860 nm in the time profile (inset figure 4.17)
may be due to the triplet energy transfer from oxoporphyrinogen to ZnTPP.121
0.16
Ar
0.012
Ar
0.010
0.10
0.04
0.02
O2
0.08
0.00
0.06
0.0
0.5
1.0
Time / µs
1.5
0.020
∆A
0.008
∆A
∆Absorbance
0.12
0.014
(b)
0.025
0.06
∆Absorbance
0.14
0.030
0.08
(a)
0.006
0.004
0.015
O2
0.002
0.000
0.0
0.5
1.0
Time / µs
1.5
0.010
0.04
0.005
0.02
0.00
600
800
1000
1200
Wavelength / nm
0.000
600
800
1000
Wavelength / nm
1200
Figure 4.18: (a) Nanosecond transient absorption spectra of 16 (0.1 mM) observed by 425 nm
laser irradiation in at 0.1 µs (●) and 1.0 µs (○) in toluene. Inset: Absorption-time profiles at 860
nm. (b) Nanosecond transient absorption spectra of 16 (0.1 mM) observed by 425 nm laser
irradiation in at 0.1 µs (●) and 1.0 µs (○) in PhCN. Inset: Absorption-time profiles at 880 nm.
122
In a more polar solvent (PhCN), transient absorption spectra observed by the
predominant excitation of the ZnP groups, revealed a weak absorption in the 600 – 1000 nm
region (figure 4.18b), and suggests the occurrence of electron transfer as a competitive process
with intersystem crossing for attenuation of generation of the triplet state of ZnP. It is likely that
absorption bands corresponding to the radical ion-pair are obscured by the strong triplet
absorptions, probably because of the fast charge-recombination rates.
Similar transient
absorption spectral behaviour was also observed in DCB. Figure 4.19 shows the nanosecond
transient absorption spectra of 2 in toluene, which shows the absorptions that are characteristic of
2.
0.08
0.06
0.05
∆A
0.04
∆Absorbance
0.06
0.03
0.02
0.01
0.00
0.04
0.0
0.5
1.0
Time / µs
1.5
0.02
0.00
600
800
1000
Wavelength / nm
1200
Figure 4.19: Nanosecond transient absorption spectra of 3 (0.1 mM) observed by 550 nm laser
irradiation in at 0.1 µs (●) and 1.0 µs (○) in toluene. Inset: Absorption-time profiles at 920 nm in
toluene.
Figure 4.20 shows the energy level diagram for the different photochemical events
occurring in 16. Emission and transient absorption studies demonstrate that upon excitation of
the ZnTPP entity of 16 in toluene, energy-transfer is the predominant photochemical process and
123
the anticipated electron-transfer process is not appreciable because of the lack of a driving force.
However, in polar solvents such as DCB and PhCN, an electron-transfer process is possible to
occur competitively with the energy-transfer process.
Contribution of the electron transfer
increases with solvent polarity by increasing the rate of electron transfer.
1
(ZnP)* (ZnP)-O xP
toluene (2.02 eV )
(2.07 eV)
(ZnP)2 - 1OxP *
k EN
k ISC
3
k S CS I
k SCS II
(ZnP)*(ZnP)-OxP
DCB (1.50 eV)
(1.52 eV)
PhCN (1.40 eV )
(1.90 eV)
k ISC
(ZnP)2 - 3 OxP *
(ZnP)(ZnP .+ )-OxP .-
hν
(425 nm )
k CR
(ZnP)2 -OxP
Figure 4:20: Energy level diagram showing the different photochemical events of triad 16.
The effect of solvent polarity on the formation and stability of charge-separated states is
relatively well appreciated both in biochemical and synthetic systems.122 It is exciting to note
that, in the case of 16, formation of a charge-separated state involves donation of an electron
from an N-substituent (giving a zinc porphyrin cation radical) to the oxoporphyrinogen. The
resulting state of the oxoporphyrinogen moiety can be best described as an anion radical
although it could be also formally seen as a porphyrinic cation radical
124
Nanosecond transient absorption spectra are shown in figure 4.21 for 18 in DCB, which were
obtained by excitation with 532 nm laser light. Broad absorption bands were observed in the 600 –
1200 nm region with a band in the 600 – 700 nm region attributable to the ZnP•+ moiety, a band at
700 – 900 nm due to the OxP•−, and a band around 1020 nm due to C60•−. Thus, these absorptions
may
include
two
radical
ion
pairs
(RIP),
(C60Im:ZnP)(ZnP•+:ImC60•−)-OxP
and
(C60Im:ZnP)(ZnP•+:ImC60)-OxP•-, suggesting the occurrence of charge separation between ZnP and
C60, and a competitive charge separation process between ZnP and OxP.
A similar transient
absorption spectral behaviour was observed for the triad, 17. From the decay time profiles in figure
4.17b, the charge-recombination rate constants (kCR) were evaluated. The decay of C60•− at 1020 nm
was found to be faster than that of OxP•- at 840 nm, resulting into two kCR values of 1.4 x 107 s-1 and
1.1 x 107 s-1, respectively. For (C60Im:ZnP)-OxP in DCB, only clear decay was observed at 1020
nm. In toluene, the transient absorption band appeared at 1020 nm as a shoulder of the main triplet
absorption bands in the 650 – 800 nm region for both (C60Im:ZnP)n-OxP complexes, while no near
IR band was observed for the (ZnP)n-OxP compounds suggesting that charge-separation takes place
generating
(ZnP•+:ImC60•−)-OxP
in
case
of
the
supramolecular
triad,
and
(C60Im:ZnP)(ZnP•+:ImC60•−)-OxP in case of the supramolecular pentad. Using the kCS and kCR thus
evaluated, the ratios kCS/kCR were evaluated as a measure of the extent of charge stabilization in the
photoinduced charge-separation process.
These values were approximately 100 demonstrating
charge stabilization to some extent in the supramolecular triad and pentad.
125
0.20
(a)
∆ Absorbance
(b)
0.15
0.3
0.2
30 ns
100 ns
1000 ns
∆A
∆ ∆Absorbance
Absorbance
0.4
0.1
0.0
600
0.10
640 nm
840 nm
1020 nm
0.05
800
1000
Wavelength / nm
1200
0.00
0.0
0.5
1.0
Time / µs
1.5
Figure 4.21: (a) Nanosecond transient absorption spectra of 18 ([component] = 0.07 mM) observed
by 532 nm laser irradiation in at 30 ns (o), 0.1 µs (●) and 1.0 µs (○) in DCB. (b) Absorption-time
profiles for the peaks in figure (b) at the indicated wavelengths.
Figure 4.22 shows the energy level diagram for the different photochemical events
occurring in the pentad. The energy levels of the charge-separated states were evaluated using
the earlier discussed Rehm-Weller approach (table 4.1 footnote). Although the photochemical
events of the supramolecular complexes in figure 4.22 seem complex, it has been possible to
dissect the different photochemical paths and evaluate the rate constants with reasonably good
estimates. As demonstrated by the emission and transient absorption studies, upon excitation of
the ZnP entity of (C60Im:ZnP)-OxP in toluene, energy-transfer from singlet excited ZnP to OxP is
the predominant photoinduced process (kENI in Figure 4.22) and the anticipated charge separation
process is not appreciable. The kENI values were evaluated to be 6.3 x 109 s–1 from the kqZ of
(ZnP)-OxP in toluene (in Table 4.1). From the difference, kqZ (= 1.0 x 1010 s-1) of (C60Im:ZnP)OxP, sum of the energy-transfer and charge-separation from singlet excited ZnP to C60 (kENII +
126
kCSII ) is evaluated to be 3.7 x 109 s–1 in toluene. However, in polar DCB, a charge-separation
process predominantly takes place with the kCSI value for the generation of (C60Im:ZnP•+)-OxP•−
being 3.0 x 109 s–1, as estimated from the difference between the kqZ values of (ZnP)-OxP and
kENI value. The kCSII value for (C60•−Im:ZnP•+)-OxP via 1ZnP* is 2.0 x 109 s–1, as estimated from
the difference in the kqZ values between (C60Im:ZnP)-OxP and (ZnP)-OxP in DCB.
In toluene for (ZnP)n-OxP, the kqO values can be attributed to the energy transfer from
1
OxP* to ZnP (kENIII), while in DCB, they are ascribed to charge-separation via 1OxP* to
generate (ZnP•+)-OxP•− (kCSIV). For (C60Im:ZnP)n-OxP in toluene, the kqC,O value is longer than
that for (ZnP)n-OxP, the kqC,O value must be kqC, which can be assigned to the energy transfer
from 1OxP* to C60 (kENIII). In DCB, the kqC,O value of (C60Im:ZnP)-OxP must be kqC, which can
be attributed to kCSIII value via 1C60* for the generation of (C60•−Im:ZnP•+)-OxP.
For the charge-recombination processes for (C60Im:ZnP)2-OxP in DCB, the kCR (1.1 x 107
s-1) evaluated from the decay of OxP•− can be assigned to kCRI, while the kCR (1.4 x 107 s-1)
evaluated from the decay of C60•− can be assigned to kCRII. Intramolecular electron mediation
from OxP•− to C60 is considered to be slow due to the long distance (~15 Å) between these
entities.
127
(C60Im:1ZnP*)-OxP
k EN I
(2.07 eV)
k ENII
k CSI
(C 60Im:ZnP)-1OxP*
(1.90 eV)
k ENIII
(1C60*Im:ZnP)-OxP
(1.75 eV)
k CSIV
k CS
k CS
III
II
(1.60 eV)
(1.40 eV)
.
.
.
.
(C60Im:ZnP +)-OxP -
(C60 -Im:ZnP +)-OxP
hν
k CRII
k CRI
(C60Im:ZnP)-OxP
Figure 4.22: Energy level diagram showing the different photochemical events of (C60Im:ZnP)OxP in DCB.
4.4
Summary
In conclusion, we have prepared and studied an unprecedented example of a porphyrin-
quinonoid featuring tetraphenylporphinatozinc(II) moieties covalently attached to an
oxoporphyrinogen through the macrocyclic nitrogen atoms of the latter moiety.
Optical
absorption and computational studies revealed an absence of π-π type interactions between the
different chromophores of the triad. Free-energy calculations based on redox and emission data
revealed electron transfer from the singlet excited zinc porphyrin to the oxoporphyrinogen is
exothermic in polar solvents. The arrangement of chromophores in the triad results in an
interesting interplay between the energy/electron-donating ZnP group(s) and the energy/electronaccepting tetrapyrrole-quinonoid as a function of solvent polarity.
Successful assembly of novel supramolecular pentad and triad composed of zinc
porphyrin(s), fullerene(s), and oxoporphyrinogen donor-acceptor entities by use of the ‘covalent-
128
coordinate’ bonding approach was accompolished. The supramolecular structures were fully
characterized by spectral, computational, and electrochemical techniques. As predicted from the
free-energy calculations, photoinduced electron transfer from singlet excited porphyrin to the
fullerene entity in DCB was demonstrated from time-resolved emission and transient absorption
studies. The experimentally measured charge-separation rates (kCS) were higher for the pentad,
(C60Im:ZnP)2-OxP (18) than that of the corresponding triad, C60Im:ZnP-OxP (17). The lifetimes
of the radical ion-pair (τRIP) were found to be about 100 ns indicating some degree of charge
stabilization in the supramolecular systems studied here. The results reveal that some modulation
of the porphyrin-fullerene energy/electron transfer processes can be achieved by careful design
of the polychromophoric system.
129
130
CHAPTER 5
SUMMARY
The work presented in this dissertation describes the spectral, electrochemical,
spectroelectrochemical,
computational,
and
X-ray
structural
properties
of
various
oxoporphyrinogens and explore their potential applications as anion sensors and light energy
harvesting pigments. Various N-substituted oxoporphyrinogens, with respect to the number and
type of substituents, were utilized in order to understand their structure-reactivity aspects.
As demonstrated in Chapter 2, N-substitution at the central nitrogens of the
oxoporphyrinogen does not alter the conformation of the macrocycles, maintaining their high
non-planar geometry. The electrochemical studies revealed that the oxoporphyrinogen ring is
electron deficient owing to the presence of four hemi-quinonoids on the macrocycle periphery.
Variation of the multiplicity of N-substitution permitted fine tuning of the redox properties in
these systems. Increasing the number of N-substitution resulted in higher oxidation potentials,
but lower reduction potentials. This observation is explained by the enhanced stability of the
intermediate oxidation states of the macrocycles upon N-substitution. Spectroelectrochemical
studies revealed well-formed π-cation and π-anion radical peaks for the N-substituted
oxoporphyrinogens at longer wavelengths. This indicates higher stability for the oxidized and
reduced species, a result that agrees well with the reversible redox processes. Interestingly, the
pyrenyl N-substituted oxoporphyrinogens lacked excimer emission indicating absence of intraor intermolecular aggregation. The combination of accessible oxidation states, redox potential
tunability, and oxidation state dependent absorption spectra of the oxoporphyrinogen qualify
them to be useful in designing multichromophoric and sensing systems.
131
X-ray structural studies of the pristine and N-substituted oxoporphyrinogen revealed
highly non-planar structures. When the X-ray structural studies were compared to the DFT
computational results, the porphyrinogen structures were very similar with regard to the
molecular conformation.
The excellent agreement between the computational and X-ray
structures allowed investigation of the electronic structure of these molecules. The calculated
HOMO, LUMO and the energy gap for the oxoporphyrinogens agreed well with the optical,
electrochemical, and spectroelectrochemical results.
A decreased HOMO-LUMO gap was
observed when compared to the parent porphyrin and upon N-substitution of oxoporphyrinogens
this gap increased slightly. The excellent agreement between theory and experimental results
prompted us to seek other applications of this fascinating series of compounds.
Anion binding studies of the oxoporphyrinogen and its bis N-benzylated derivative were
systematically studied in Chapter 3. The ruffled geometry of the oxoporphyrinogens projecting
the imino-H outward promoted hydrogen bonding to various anions. Optical absorption studies
revealed new bands at higher wavelengths for the anion bound species. Such data was also
utilized to construct Benesi-Hildebrand plots for determining the binding constants for the
studied compounds for different anions.
Job’s plots were constructed to determine the
stoichiometry between the oxoporphyrinogens and the anions, revealing 1:2 ratio for the pristine
oxoporphyrinogen and 1:1 ratio for the bis-alkyl N-substituted oxoporphyrinogen.
The
determined binding constants revealed that the oxoporphyrinogens were capable of binding
anionic species at varying affinities. Also, solvatochromatic effects were exploited to achieve
selectivity in the chromogenic response of the oxoporphyrinogens for a given anion.
The investigated oxoporphyrinogens behaved as electrochemical anion sensors by
revealing a large anion-dependent electrochemical response (up to 600 mV cathodic shifts) for
132
the anions studied. The magnitude of the cathodic shift of the oxoporphyrinogen was found to
be dependent on the nature of the anion and these results correlated well with the calculated
binding constants and the observed shift in the absorption peak maxima. Anions with larger
binding constants revealed the largest cathodic shifts of the oxidation potential of the
oxoporphyrinogen. The DFT B3LYP/3-21G(*) calculations revealed plausible geometries of the
anion-bound oxoporphyrinogens.
Furthermore, and much more interestingly, quantitative
relationships were found to exist between the experimental and computed data. Linear plots with
good correlation coefficients were obtained for all data indicating an excellent agreement
between the theory and experimental findings. This is especially important considering the
different techniques (optical and electrochemical) and different types of anions (monatomic and
polyatomic along with different geometries) employed in the present study.
In Chapter 4, the electron acceptor properties of oxoporphyrinogens was exploited by
constructing zinc porphyrin-oxoporphyrinogen and zinc porphyrin-oxoporphyrinogen-fullerene
donor-acceptor molecular and supramolecular systems.
Optical and computational studies for
the zinc porphyrin-oxoporphyrinogen systems revealed absence of π-π type interactions between
the different chromophores.
The supramolecular structures were characterized by spectral,
computational, and electrochemical techniques.
Formation of the zinc porphyrin-
oxoporphyrinogen-fullerene systems were monitored by using optical absorption methods.
Electrochemical studies revealed complex voltammograms due to the presence of different types
of redox entities. However, accurate analysis was achieved by comparing the voltammograms of
the individual ZnP, oxoporphyrinogen, and C60Im entities.
Free-energy calculations based on
redox and emission data revealed that electron transfer from the singlet-excited zinc porphyrin to
the oxoporphyrinogen in the zinc porphyrin-oxoporphyrinogen systems is exothermic in polar
133
solvents.
Photochemical studies using time-resolved emission and nanosecond transient
absorption techniques confirmed occurrence of electron transfer in competition with energy
transfer.
In the case of zinc porphyrin-oxoporphyrinogen-fullerene systems, free energy
calculations predicted competitive electron transfer from the singlet-excited zinc porphyrin to the
fullerene in addition to zinc porphyrin to oxoporphyrinogen in dichlorobenzene and this was
confirmed by time-resolved emission and nanosecond transient absorption studies.
The
experimentally measured charge-separation rates were higher for the zinc porphyrinoxoporphyrinogen-fullerene systems. The lifetimes of the radical ion-pair were found to be
about 100 ns, indicating some degree of charge stabilization in the supramolecular systems
studied.
The results of the present investigation suggest that oxoporphyrinogens are good anion
sensors as well as electron acceptors. Future applications of these novel compounds may be
envisioned to involve modification of column chromatography materials, perhaps by covalent
attachment of oxoporphyrinogen to silica, to separate anions of biological or abiological nature.
Another interesting application would be to construct molecular systems to probe anioninduced photoinduced electron transfer. An example so one such dyad is shown below in Figure
5.1.
134
hv
hv
CH3
N
N CH3
O
O
CH2
CH2
hv'
N
ET
NH
O
O
NH HN
F
-
O
N
N
F
O
O
HN
N
O
Figure 5.1: Structure of the proposed of OxP-C60 Dyad.
In this dyad, in the absence of any bound anion, excitation of oxoporphyrinogen may not
induce photoinduced electron transfer due to unfavorable reaction conditions. That is, the large
oxidation potential of the mono N-substituted oxoporphyrinogen makes electron transfer
thermodynamically not favorable. However in the presence of anions, the oxidation potential of
oxoporphyrinogen is expected to become easier (anion binding effect), that is, it would become a
great electron donor.
Hence, photoinduced electron transfer from the excited C60 to
oxoporphyrinogen is expected to occur. Development of such exotic molecular systems will
have potential in making molecular switches, in addition to photovoltatic applications.
135
REFERENCES
136
LIST OF REFERENCES
1.
Milgrom, L. R. The Colours of Life; Oxford University Press Inc.,: New York, 1997.
2.
The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: Burlington, MA, 2000; Vol. 1-10.
3.
Pushpan, S.K.; Chandrashekar, T.K. Pure Appl. Chem. 2002, 74, 2045-2055.
4.
Cormode, D.P.; Murray, S.S.; Cowley, A.R.; Beer, P.D. J. Chem. Soc. Dalton Trans.
2006, 5135-5140.
5.
Gunter, M.J.; Farquahar, S.M.; Mullen, K.M. New J. Chem. 2004, 28, 1443.
6.
Burns, D.H.; Calderon-Kawasaki, K.; Kularante, S. J. Org. Chem. 2005, 70, 2803-2807.
7.
Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198.
8.
Gust, D.; Moore, T. A. Science, 1989, 244, 35.
9.
Lee, C.; Lee, D. H.; Hong, J-I.; Tetrahedron Lett. 2001, 42, 8665.
10.
Takemura, T.; Ohta, N.; Nakajima, S.; Sakata, I. Photochem. Photobiol. 1991, 54, 683.
11.
Maziere, J.C.; morliere, P.; Santus, R.; Photochem. Photobiol. B: Biology 1991, 8, 351.
12.
Press: Burlington, MA, 2000; Vol. 1 pg 46.
13.
Furuta, H. ; Asano, T. ; Ogawa, T. J. Am. Chem. Soc. 1994, 116, 767.
14.
Anzenbacher Jr., P.; Nishiyabu, R.; Palacios, M.A.; Coord. Chem. Rev. 2006, 250, 29292938.
15.
Baeyer, A. Chem. Ber., 1886, 2184.
16.
Floriani, C. Chem. Commun. 1996, 1257-1263.
17.
Gale, A.; Sessler, J.L.; Kral, V. Chem. Commun. 1998, 1-8.
18.
Press: Burlington, MA, 2000; Vol. 6 pg 257-277.
19.
137
20.
Inhoffen, H. H., Fuhrhop, J.-H., van der Haar, F., Liebigs Ann. Chem., 1966, 700, 92–
105.
21.
Otto, C., Breitmaier, E., Liebigs Ann. Chem., 1991, 1347.
22.
Milgrom, L. R. Tetrahedron 1983, 39, 3895–3898.
23.
Golder, A. J.; Milgrom, L. R.; Nolan, K. B.; Povey, D. C. Chem. Commun. 1989, 1751–
1753.
24.
Milgrom, L. R.; Hill, J.P.; Yahioglu, G. J. Heterocyclic Chem. 1995, 32, 97–101.
25.
The Porphyrins; vols 1-7 (Ed: D Dolphin), Academic Press: New York, 1978.
26.
Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem. Soc. 1996, 118,
5708-5711 and references cited therein.
27.
Clausen, C.; Gryko, D. T.; Dabke, R. B.; Dontha, N.; Bocian, D. F.; Kuhr, W. G.;
Lindsey, J. S. J. Org. Chem. 2000, 65, 7363–7370.
28.
Li, J.; Gryko, D.; Dabke, R. B.; Diers, J. R.; Bocian, D. F.; Kuhr, W. G.; Lindsey, J. S. J.
Org. Chem. 2000, 65, 7379–7390.
29.
Morgado, J.; Cacialli, F.; Iqbal, R.;Moratti, S. C.; Holmes, A. B.; Yahioglu, G.; Milgrom,
L. R.; Friend, R. H. J. Mater. Chem. 2001, 11, 278-283.
30.
Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57–69.
31.
Screen, T.E.O.; Thorne, J.R.G.; Denning, R.G.; Bucknall, D.G.; Anderson, H.L. J. Am.
Chem. Soc. 2002, 124, 9712.
32.
Jackson, A.H., in The Porphyrins, Vol 1, D. Dolphin, Ed., Academic Press, New York,
1979, 341.
33.
Dearden, G.R., Jackson, A.H., Chem. Commun., 1970, 205–206.
34.
Lavallee, D.K. The Chemistry and Biochemistry of N-substituted Porphyrins, VCH
Publishers: New York: 1987.
35.
Floriani, C., Floriani-Moro, R. in The Porphyrin Handbook, K. M. Kadish, K. M. Smith
and R. Guilard, Eds.; Academic Press: Burlington, MA, 2000; Vol. 3, Chapters 24 and
25.
36.
Floriani, C.; Solari, E.; Solari, G.; Chiesi-Villa, A.; Rizzoli, C. Angew. Chem., Int. Ed.
Engl. 1998, 37, 2245–2248.
37.
Woods, C. J.; Camiolo, S.; Light, M. E.; Coles, S. J.; Hursthouse, M. B.; King, M. A.;
Gale, P. A.; Essex, J. W. J. Am. Chem. Soc. 2002, 124, 8644–8652.
138
38.
Dolušić, E., Toppet, S., Smeets, S., Meervelt, L. V., Tinant, B., Dehaen, W.,
Tetrahedron, 2003, 59, 395–400. (naph 18)
39.
40.
Fuhrhop, J.H.; Kadish, K.M.; Davis, D.G. J. Am. Chem. Soc., 1973, 95, 5140-5147.
41.
Constant, L.A.; Davis, D.G., Anal. Chem. 1975, 47, 2253-2260.
42.
Gong, L-C. ; Dolphin, D. J. Am. Chem. Soc. 1985, 63, 401-405.
43.
Kadish, K.M.; Cornillon, J.K.; Yao, C-L.; Malinski, T.; Gritzner, G.J. J Electroanal.
Chem. 1987, 235, 189-207.
44.
Kadish, K.M.; Shiue, L.R.; Rhodes, R.K.; Bottomley, L.A. Inorg. Chem. 1981, 20, 12741277.
45.
Hariprasad, G.; Dahal, S.; Maiya, B.G. J. Chem. Soc. Dalton Trans. 1996, 3429-3436.
46.
Toma, H.E.; Araki, K. Current Organic Chemistry 2002, 6, 21-34 and references therein.
47.
Kuwana, Y.; Darlington, R.K.; Leedy, D.W. Anal. Chem. 1964, 36, 2023.
48.
Kadish, K.M.; Mu, X. Pure and Appl. Chem. 1990, 62, 1051-1054.
49.
Hill, J. P., Hewitt, I. J., Anson, C. E., Powell, A. K., McCarthy, A. L., Karr, P. A.,
Zandler, M. E., D’Souza, F., J. Org. Chem., 2004, 69, 5861–5869.
50.
Hill, J. P., Schmitt, W., McCarty, A. L., Ariga, K., D’Souza, F. Eur. J. Org. Chem. 2005,
2893–2902.
51.
Hill, J. P.; Ariga. K.; Schumacher, A. L.; Karr, P. A.; D’Souza, F. J. Porphyrins
Phthalocyanines 2007, 11, 390–396.
52.
Gaussian 98 (Revision A.7), Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G.
E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R.
E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.;
Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.;
Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A.
Gaussian, Inc., Pittsburgh PA, 1998.
139
53.
Herwig, P.T., Enkelmann, V., Schmelz, O., Muellen, K., Chem. Eur. J., 2000, 6, 1834–
1839 and references cited therein.
54.
Van de Craats, A.M., Warman, J.M., Muellen, K., Geerts, Y., Brand, J.D., Adv. Mater.,
1998, 10, 36–38.
55.
Hill, J.P., Jin, W., Kosaka, A., Fukushima, T., Ichihara, H., Shimomura, T., Ito, K.,
Hashizume, T., Ishii, N., Aida, T., Science, 2004, 304, 1481–1483.
56.
(a) D’Souza, F.; Zandler, M. E.; Deviprasad, G. R.; Kutner, W. J. Phys. Chem. A. 2000,
104, 6887. (b) D’Souza, F.; Zandler, M. E.; Smith, P. M.; Deviprasad, G. R.; Arkady, K.;
Fujitsuka, M.; Ito, O J. Phys. Chem. A. 2002, 106, 649. (c) Zandler, M. E.; Smith, P. M.;
Fujitsuka, M.; Ito, O.; D’Souza, F. J. Org. Chem. 2002, 67, 9122. (d) Marczak, R.;
Hoang, V. T.; Noworyta, K.; Zandler, M. E.; Kutner, W.; D’Souza, F. J. Mater. Chem.
2002, 12, 2123.
57.
Shelnutt, J. A.; Song, X.-Z.; Ma, J.-G.; Jia, S.-L.; Jentzen, W.; Medforth, C. J. Chem. Soc.
Rev. 1998, 27, 31 and references cited therein.
58.
Ghosh, A. Acc. Chem. Res. 1998, 31, 189.
59.
Ghosh, A. J. Am. Chem. Soc. 1995, 117, 4691.
60.
Ghosh, A. J. Phys. Chem. 1994, 98, 11004.
61.
D’Souza, F.; Zandler M. E.; Tagliatesta, P.; Ou, Z., Shao, J., Caemelbecke, E. V., Kadish,
K. M. Inorg. Chem. 1998, 37, 4567.
62.
Langford, S. J.; Yann, T. J. Am. Chem. Soc. 2003, 125, 11198. (b) Introduction to
Molecular Electronics, M. C. Petty, M. R. Bryce, D. Bloor, Oxford University Press,
New York, 1995.
63.
Sutin, N. Acc. Chem. Res. 1983, 15, 275. (b) Wasielewski, M. R. Chem. Rev. 1992, 92,
435. (c) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198. (d)
Paddon-Row, M. N. Acc. Chem. Res. 1994, 27, 18. (e) Hayashi, T.; Ogoshi, H. Chem.
Soc. Rev. 1997, 26, 355. (f) Piotrowiak, P. Chem. Soc. Rev. 1999, 28, 143. (g) Guldi, D.
M. Chem. Soc. Rev. 2002, 31, 22.
64.
Bianchi, A.; Bowman-James K.; Garcia-Espana, E. Eds. Supramolecular Chemistry of
Anions. Wiley-VCH: New York 1997.
65.
Anion Receptor Chemistry, Sessler, J. L.; Gale, P. A.; Cho, W.-S. Royal Society of
Chemistry: Cambridge, UK, 2006.
66.
Lang, L. G.; Riordan, J. F.; Vallee, B. L., Biochemistry, 1974 13, 4361.
67.
Harrison, R.M.; Pollution: Causes, Effects, and Control. RSC: London 1983.
140
68.
Cho, E. J.; Ryu, B.J.; Lee, Y.J.; Nam, K.C. Org, Lett. 2005, 7, 2607.
69.
Cho, E. J.; Moon, J.W.; Ko, S.W.; Lee, Y.J.; Kim, S.K.; Yoon, J.; Nam, K.C. J. Am.
Chem. Soc., 2003, 125, 12376.
70.
Beer P.D.; Gale, P.A. Angew. Chem., Int. Ed. 2001, 40, 486.
71.
Beer, P.D.; Gale, P.A.; Chen, G.Z. Coor. Chem. Rev. 1999, 185-186, 2-36.
72.
Shannon, R.D. Acta Crystallogr. Sect A 1976, 32, 751.
73.
Kang, S. O.; Llinares, J. M.; Powell, D.; VanderVelde, D.; Bowman-James, K. J. Am.
Chem. Soc. 2003, 125, 10152-10153.
74.
Kang, S. O.; Hossain, M. A.; Powell D.; Bowman-James K. Chem. Commun., 2005, 328–
330.
75.
Choi, K.; Hamilton, A. D. J. Am. Chem. Soc., 2003, 125, 10241-10249.
76.
Hossain, M. A.; Llinares, J. M.; Powell, D.; Bowman-James, K. Inorg. Chem. 2001, 40,
2936-2937.
77.
Evans, A.J.; Matthews, S.E.; Cowley, A.R.; Beer, P.D. J. Chem. Soc. Dalton Trans. 2003,
4644-4650.
78.
Arimori, S.; Davidson, M.G.; Fyles, T.M.; Hibbert, T.G.; James, T.D.; Kociok-Koehn,
G.I. Chem. Commun. 2004, 1640.
79.
Cooper, J. B.; Drew, M. G. B.; Beer P. D. J. Chem. Soc., Dalton Trans., 2000, 27212728.
80.
Gale, P. A.; Sessler, J. L.; Lynch V.; Sansom, P. I. Tetrahedron Lett. 1996, 37, 7881.
81.
The Porphyrin Handbook, vol. 6, ed. K. M. Kadish, K. M. Smith and R. Guilard,
Academic Press, San Diego, CA and Burlington, MA, 2000, pp. 257-278.
82.
Camiolo, S.; Gale P. A. Chem. Commun. 2000, 1129-1130.
83.
Gale, P. A.; Sessler, J. L.; Král, V.; Lynch, V. J. Am. Chem. Soc., 1996, 118, 5140.
84.
Nielsen, K.A.; Jeppesen, J.O. Levillan, E.; Becher, J. Angew. Chem. Int. Ed. 2003, 42,
187-191.
85.
Nielsen, K.A.; Cho, W-S.; Lyskwa, J.; Levillan, E.; Lynch, V.M.; Sessler, J.L.; Jeppsesn,
J.O. J. Am Chem. Soc. 2006, 128, 2444-2451.
86.
Nishiyabu, R.; Palacios, M.A.; Dehaen, W.; Anzenbacher Jr., P. J. Am Chem. Soc. 2006,
128, 11496-11504.
141
87.
Descalzo, A.B.; Jimenez, D.; Haskuori, J. E.; Beltran, D.; Amoros, P.; Marcos, M. D.;
Martinez-Manez, M.; Soto J. Chem. Commun. 2002, 562-563.
88.
Beer, P.D.; Gale, P. A., Chen, G. Z. J. Chem. Soc, Dalton Trans., 1999, 1987-1909.
89.
Shriver, J.A. Ph.D. Disseration, The Univeristy of Texas at Austin, 2002.
90.
Woods, C. J.; Camiolo, S.; Light, M. E.; Coles, S. J.; Hursthouse, M. B.; King, M. A.;
Gale, P. A.; Essex, J. W. J. Am. Chem. Soc., 2002, 124, 8644-8652.
91.
(a) Miyaji, H.; Kim, H.-K.; Sim, E.-K.; Lee, C.-K.; Cho, W.-S.; Sessler, J. L.; Lee, C.-H.
J. Am. Chem. Soc. 2005, 127, 12510-12512. (b) Nishiyabu, R.; Anzenbacher, P. J. Am.
Chem. Soc. 2005, 127, 8270-8271.
92.
(a) Dodsworth, E. S.; Hasegawa, M.; Bridge, M.; Linert, W. Comprehensive Coord.
Chem. II 2004, 2 351-365. (b) Karelson, M. Handbook of Solvents, 2001, 639-679. (c)
Nigam, S.; Rutan, S. Applied Spectroscopy 2001, 55(11), 362A-370A. (d) Carr, P. W.
Microchem. J. 1993, 48(1), 4-28. (e) Sone, K.; Fukuda, Y. Reviews Inorg. Chem. 1990,
11(2-4), 123-53. (f) Buncel, E.; Rajagopal, S. Acc. Chem. Res. 1990, 23(7), 226-31.
93.
Gutman, V. Coord. Chem. Rev. 1976, 18, 225-255.
94.
Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48,
2877-2887.
95.
Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703.
96.
Hill J.P.; Schumacher, A.L.; D’Souza, F.; Labuta, J.; Redshaw, C.;. Elsegood, M.R.J;
Aoyagi, M.; Nakanishi, T.; Ariga, K., Inorg. Chem. 2006, 45, 8288.
97.
(a) Miertus, S.; Scrocco, E.; and Tomasi, J. Chem. Phys. 1981, 55, 117 . (b) Mennucci,
B.;Tomasi, J. J. Chem. Phys.1997, 106, 5151.
98.
(a)Schottel, B.L.; Chifotide, H.T.; Shatruk, M.; Chouai, A., Perez, L.M., Bacsa, J.;
Dunbar, K.R. J. Am. Chem. Soc. 2006, 128, 5895-5912.; (b)A Laboratory Book of
Computational Organic Chemistry by Hehre, W. J.; Shusterman, A. J.; Huang, W. W.
Wavefunction Inc., Irvine, CA, 1996.
99.
Marcus, R.A.; Sutin, N. Biochim. Biophys. Acta, 1985, 811, 265.
100.
Marcus, R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111.
101.
(a)Schottel, B.L.; Chifotide, H.T.; Shatruk, M.; Chouai, A., Perez, L.M., Bacsa, J.;
Dunbar, K.R. J. Am. Chem. Soc. 2006, 128, 5895-5912.; (b)A Laboratory Book of
Computational Organic Chemistry by Hehre, W. J.; Shusterman, A. J.; Huang, W. W.
Wavefunction Inc., Irvine, CA, 1996.
142
102.
(a) Miller, J. R.; Calcaterra, L. T.; Closs, G.L. J. Am. Chem. Soc. 1984, 106, 3047; (b)
Closs, G. L.; Miller, J. R.; Science 1988, 240, 440; (c) Connolly, J. S.; Bolton, J. R. In
Photoinduced Electron Transfer; M. A. Fox, M. Chanon, Eds.; Elsevier: Amsterdam,
1988; Part D, pp 303-393.
103.
(a) Wasielewski, M. R. Chem. Rev. 1992, 92, 435; (b) Kurreck, H.; Huber, M. Angew.
Chem. 1995, 107, 929; Angew. Chem., Int. Ed. Engl. 1995, 34, 849; (c) Gust, D.; Moore,
T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40.
104.
(a) A. Harriman, J.-P. Sauvage, Chem. Soc. Rev. 1996, 26, 41; (b) M.-J. Blanco, M. C.
Jiménez, J.-C. Chambron, V. Heitz, M. Linke, J.-P. Sauvage, Chem. Soc. Rev. 1999, 28,
293; (c) V. Balzani, A. Juris, M. Venturi, S. Campagna, S. Serroni, Chem. Rev. 1996, 96,
759; (d) Electron Transfer in Chemistry; V. Balzani, Ed.; Wiley-VCH: Weinheim, 2001.
105.
(a) Paddon-Row, M. N. Acc. Chem. Res. 1994, 27, 18; (b) Verhoeven, J. W. Adv. Chem.
Phys. 1999, 106, 603; (c) Maruyama, K.; Osuka, A.; Mataga, N. Pure Appl. Chem. 1994,
66, 867; (d) Osuka, A.; Mataga, N.; Okada, T. Pure Appl. Chem. 1997, 69, 797; (e)
Molecular Catenanes, Rotaxanes and Knots; Sauvage, J.-P.; Dietrich-Buchecker, C.
Wiley-VCH; Weinheim, Germany, 1999.
106.
(a) Introduction of Molecular Electronics Eds. Petty, M. C. ; Bryce, M. R. ; Bloor, D.;
Oxford University Press, New York, 1995; (b) Molecular Switches, Feringa, B. L. Ed.,
Wiley-VCH GmbH, Weinheim, 2001.
107.
a) Connolly, J. S.; Bolton, J. R. In Photoinduced Electron Transfer; Fox, M. A. Chanon,
M. Eds.; Elsevier: Amsterdam, 1988; Part D, pp 303-393; b) Guldi, D. M.; Martin, N. J.
Mater. Chem. 2002, 12, 1978-1992; c) Sanchez, L.; Martin, N.; Guldi, D. M. Angew.
Chem. 2005, 117, 5508-5516; Angew. Chem. Int. Ed. 2005, 44, 5374-5382.
108.
Gust, D. and Moore, T. A. in The Porphyrin Handbook, K. M. Kadish, K. M. Smith and
R. Guilard, Eds.; Academic Press: Burlington, MA, 2000; Vol. 8, pp153-190.
109.
(a) Kadish, K. M. ; Ruoff, R. S. Fullerenes: Chemistry, Physics, and Technology, WileyVCH; Weinheim, 2000; b) Imahori, H.; Sakata, Y. Adv. Mater. 1997, 9, 537; c) Prato,
M. J. Mater. Chem. 1997, 7, 1097; d) Martín, N.; Sánchez, L.; Illescas, B.; Pérez, I.
Chem. Rev. 1998, 98, 2527; e) Diederich, F.; Gómez-López, M. Chem. Soc. Rev. 1999,
28, 263; f) Guldi, D. M. Chem. Commun. 2000, 321; g) Guldi, D. M.; Prato, M. Acc.
Chem. Res. 2000, 33, 695; h) Guldi, D. M. Chem. Soc. Rev. 2002, 31, 22; i) Meijer, M.
D.; M. van Klink, G. P.; van Koten, G. Coord. Chem. Rev. 2002, 230, 141; j) El-Khouly,
M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem. Photobiol. C. 2004, 5, 79; k)
Imahori, H. Fukuzmi, S. Adv. Funct. Mater. 2004, 14, 525; l) D’Souza, F.; Ito, O. Coord.
Chem. Rev. 2005, 249, 1410; m) Bouamaied, I.; Coskun, T.; Stulz, E. Structure and
Bonding, 2006, 121, 1-147.
110.
Dalton, J.; Milgrom, L.R. J. Chem Soc., Chem. Commun. 1979, 609-610.
111.
Joran, A.D.; Leland, B.A.; Geller, G.G.; Hopfield, J.J.; Dervan, A.D. J. Am. Chem. Soc.
1984, 106, 6090-9092.
143
112.
Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata, Y.; Fukuzumi, S. J.
Am. Chem. Soc. 2001, 123, 6617-6628.
113.
D’Souza, F.; Deviprasad, G. R.; Zandler. M. E.; Hoang, V. T.; Klykov, A.; Vanstipdonk,
M.; Perera, A.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O. J. Phys. Chem. A 2002, 106,
3243-3252.
114.
Hill, J. P., Sandanayaka, A. S. D., McCarty, A. L., Karr, P. A., Zandler, M. E.,
Charvet, R., Ariga, K., Araki, Y., Ito, O., D’Souza, F. Eur. J. of Org. Chem. 2006,
595-603.
115.
Schumacher, A.L.; Sandanayaka, A. S. D., Hill, J. P., Ariga, K., Karr, P. A., Araki, Y.,
Ito, O., D’Souza, F. Chem. Eur. J. 2007, 13, 4628-4635.
116.
Gadde, S Ph.D. Disseration, Wichita State University, 2006 and references therein.
117.
Chitta, R Ph.D. Disseration, Wichita State University, 2007 and references therein.
118.
(a) Imahori, H.; Tamaki, K.; Araki, Y.; Sekiguchi, Y.; Ito, O.; Sakata, Y.; Fukuzumi, S.
J. Am. Chem. Soc. 2002, 124, 5165-5174. (b) Imahori, H.; Sekiguchi, Y.; Kashiwagi, Y.;
Sato, T.; Araki, Y.; Ito, O.; Yamada, H.; Fukuzumi, S. Chem. Eur. J. 2004, 124, 31843196.
119.
Rehm, D. ; Weller, A. Isr. J. Chem. 1970, 7, 259.
120.
Gaussian 98 (Revision A.7), Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G.
E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R.
E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.;
Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.;
Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A.
Gaussian, Inc., Pittsburgh PA, 1998.
121.
(a) D’Souza, F.; Deviprasad, G. R.; El-Khouly, M. E. Fujitsuka, M. Ito, O. J. Am. Chem.
Soc. 2001, 123, 5277-5284. (b) D’Souza, F.; Deviprasad, G. R.; Zandler, M. E.; ElKhouly, M. E.; Fujitsuka, M.; Ito, O. J. Phys. Chem. B 2002, 106, 4952-4962.
122.
(a) Bixon, M.; Jortner, J. Adv. Chem. Phys. 1999, 106, 35-202. b) Photoinduced Electron
Transfer, Eds. Fox, M. A.; Chanon, M. Parts A-D, Elsevier, Amsterdam, 1988.
123.
Park, C.H.; Simmons, H.E. J. Am. Chem. Soc. 1968, 90, 2431.
124.
Bayer, A. Ber. Dtsch. Chem. Ges. 1886 19, 2184.
144
APPENDIX
145
APPENDIX A
LIST OF PUBLICATIONS
Thesis Related Work
1.
Highly Nonplanar, Electron Deficient, N-Substituted tetra-Oxocyclohexadienylidene
Porphyrinogens: Structural, Computational, and Electrochemical Investigations. Hill,
Jonathan P.; Hewitt, Ian J.; Anson, Christopher E.; Powell, Annie K.; McCarty, Amy
Lea; Karr, Paul A.; Zandler, Melvin E.; D'Souza, Francis J. Org. Chem, 2004, 69(18),
5861-5869.
2.
Structures, Spectral and Electrochemical Properties of N-(naphth-2-ylmethyl)-appended
Porphyrinogens.
Hill, Jonathan P.; Schmitt, Wolfgang; McCarty, Amy Lea; Ariga,
Katsuhiko; D'Souza, Francis. European Journal of Organic Chemistry 2005, (14),
2893-2902.
3.
Chromogenic Indicator for Anion Reporting Based on an N-Substituted
Oxoporphyrinogen.
Hill, Jonathan P.; Schumacher, Amy Lea; D'Souza, Francis;
Labuta, Jan; Redshaw, Carl; Elsegood, Mark R. J.; Aoyagi, Masaru; Nakanishi, Takashi;
Ariga, Katsuhiko. Inorg. Chem. 2006, 45(20), 8288-8296.
4.
A Novel Bis(zinc-porphyrin)-Oxoporphyrinogen Donor-Acceptor Triad: Synthesis,
Electrochemical, Computational, and Photochemical Studies.
Hill, Jonathan P.;
Sandanayaka, Atula S. D.; McCarty, Amy L.; Karr, Paul A.; Zandler, Melvin E.; Charvet,
Richard; Ariga, Katsuhiko; Araki, Yasuyuki; Ito, Osamu; D'Souza, Francis. European
Journal of Organic Chemistry 2006, (3), 595-603.
5.
Pyrenyl-1-ylmethyl N-substitued Oxoporphyrinogens.
Hill, Jonathan P.; Ariga,
Katsuhiko; Schumacher, Amy Lea; Karr, Paul. A.; D’Souza, Francis. Journal of
Porphyrins and Phthalocyanines 2007, 11, 390.
6.
Supramolecular triad and pentad composed of zinc-porphyrin(s), oxoporphyrinogen, and
fullerene(s): design and electron-transfer studies. Schumacher, Amy Lea; Sandanayaka,
Atula S. D.; Hill, Jonathan P.; Ariga, Katsuhiko; Karr, Paul A.; Araki, Yasuyuki; Ito,
Osamu; D'Souza, Francis. Chemistry--A European Journal 2007, 13(16), 4628-4635.
7.
Highly effective electrochemical anion sensing based on
Schumacher, Amy Lea; Hill, Jonathan P.; Ariga, Katsuhiko;
Electrochemistry Communications 2007, in press.
146
oxoporphyrinogen.
D’Souza, Francis.
Non-Thesis Related Work
1.
Bis-functionalized Fullerene-dibenzo[18]crown-6 Conjugate: Synthesis and Cationcomplexation dependent Redox Behavior. Smith, Phillip M.; McCarty, Amy Lea;
Nguyen, Nhu Yen; Zandler, Melvin E.; D'Souza, Francis. Chemical Commun. (2003),
(14), 1754-1755.
2.
Ab initio Density Functional Methods to probe the Site of Electron Transfer in Fullerene
bearing Dyads and Triads. Zandler, Melvin E.; Kullman, Michael J.; Gadde, Suresh;
McCarty, Amy Lea; Smith, Phillip M.; D'Souza, Francis. Proceedings - Electrochemical
Society 2003, 2003-15(Fullerenes--Volume 13: Fullerenes and Nanotubes), 31-39.
3.
Bis fulleropyrrolidine-dibenzo[18]crown-6 Conjugate: Cation-complexation induced
Reduction Potential Changes. Smith, Phillip M.; McCarty, Amy Lea; Nguyen, Nhu Yen;
Zandler, Melvin E.; D'Souza, Francis. Proceedings - Electrochemical Society 2003,
2003-15(Fullerenes--Volume 13: Fullerenes and Nanotubes), 40-46.
4.
Supramolecular Triads Formed by Axial Coordination of Fullerene to Covalently Linked
Zinc Porphyrin-Ferrocene(s): Design, Syntheses, Electrochemistry, and Photochemistry.
D'Souza, Francis; Smith, Phillip M.; Gadde, Suresh; McCarty, Amy L.; Kullman,
Michael J.; Zandler, Melvin E.; Itou, Mitsunari; Araki, Yasuyaki; Ito, Osamu. Journal of
Physical Chemistry B. 2004, 108(31), 11333-11343.
5.
Energy Transfer Followed by Electron Transfer in a Supramolecular Triad Composed of
Boron Dipyrrin, Zinc Porphyrin, and Fullerene: A Model for the Photosynthetic AntennaReaction Center Complex.
D'Souza, Francis; Smith, Phillip M.; Zandler, Melvin E.;
McCarty, Amy L.; Itou, Mitsunari; Araki, Yasuyuki; Ito, Osamu. J. Am. Chem. Soc.
2004, 126(25), 7898-7907.
6.
Preparation,
Surface
Characteristics
and
Electrochemical
Properties
of
Electrophoretically Deposited C60 Films.
Kutner, Wlodzimierz; Pieta, Piotr;
Nowakowski, Robert; Sobczak, Janusz W.; Kaszkur, Zbigniew; McCarty, Amy Lea;
D'Souza, Francis.
AIP Conference Proceedings 2005, 786(Electronic Properties of
Novel Nanostructures), 13-16.
7.
Composition, Structure, Surface Topography, and Electrochemical Properties of
Electrophoretically Deposited Nanostructured Fullerene Films. Kutner, Wlodzimierz;
Pieta, Piotr; Nowakowski, Robert; Sobczak, Janusz W.; Kaszkur, Zbigniew; McCarty,
Amy Lea; D'Souza, Francis. Chemistry of Materials 2005, 17(23), 5635-5645.
8.
Spectral, Electrochemical, and Photophysical Sstudies of a Magnesium PorphyrinFullerene Dyad. El-Khouly, Mohamed E.; Araki, Yasuyuki; Ito, Osamu; Gadde, Suresh;
McCarty, Amy L.; Karr, Paul A.; Zandler, Melvin E.; D'Souza, Francis. Physical
Chemistry Chemical Physics 2005, 7(17), 3163-3171.
147
9.
Effect of Axial Ligation or π -π -type Interactions on Photochemical Charge Stabilization
in "TwoPoint" Bound Supramolecular Porphyrin-Fullerene Conjugates. D'Souza,
Francis; Chitta, Raghu; Gadde, Suresh; Zandler, Melvin E.; McCarty, Amy L.;
Sandanayaka, Atula S. D.; Araki, Yasuyaki; Ito, Osamu.
Chemistry--A European
Journal 2005, 11(15), 4416-4428.
10.
Self-Assembled via Axial Coordination Magnesium Porphyrin-Imidazole Appended
Fullerene Dyad: Spectroscopic, Electrochemical, Computational, and Photochemical
Studies. D'Souza, Francis; El-Khouly, Mohamed E.; Gadde, Suresh; McCarty, Amy L.;
Karr, Paul A.; Zandler, Melvin E.; Araki, Yasuyaki; Ito, Osamu. J. Phys. Chem. B
2005, 109(20), 10107-10114.
11.
Design and Studies on Supramolecular Ferrocene-Porphyrin-Fullerene Constructs for
Generating Long-Lived Charge Separated States. D'Souza, Francis; Chitta, Raghu;
Gadde, Suresh; Islam, D.-M. Shafiqul; Schumacher, Amy L.; Zandler, Melvin E.; Araki,
Yasuyuki; Ito, Osamu. J. Phys. Chem. B 2006, 110(50), 25240-25250.
12.
Predicting the Site of Electron Transfer using DFT Frontier Orbitals: Studies on
Porphyrin attached either to Quinone or Hydroquinone, and Quinhydrone SelfAssembled Supramolecular Complexes.
Karr, Paul A.; Zandler, Melvin E.; Beck,
Michael; Jaeger, Jared D.; McCarty, Amy L.; Smith, Phillip M.; D'Souza, Francis.
THEOCHEM 2006, 765(1-3), 91-103.
13.
Regulating the Stability of 2D Crystal Structures using an Oxidation State-Dependent
Molecular Conformation. Hill, Jonathan P.; Wakayama, Yutaka; Schmitt, Wolfgang;
Tsuruoka, Tohru; Nakanishi, Takashi; Zandler, Melvin L.; McCarty, Amy L.; D'Souza,
Francis; Milgrom, Lionel R.; Ariga, Katsuhiko. Chem. Commun. 2006, (22), 23202322.
14.
Potassium Ion Controlled Switching of Intra- to Intermolecular Electron Transfer in
Crown Ether Appended Free-Base Porphyrin-Fullerene Donor-Acceptor Systems.
D'Souza, Francis; Chitta, Raghu; Gadde, Suresh; Zandler, Melvin E.; McCarty, Amy L.;
Sandanayaka, Atula S. D.; Araki, Yasuyaki; Ito, Osamu. J. Phys. Chem. A 2006,
110(13), 4338-4347.
15.
Design, Syntheses, and Studies of Supramolecular Porphyrin-Fullerene Conjugates,
Using Bis-18-crown-6 Appended Porphyrins and Pyridine or Alkyl Ammonium
Functionalized Fullerenes. D'Souza, Francis; Chitta, Raghu; Gadde, Suresh; McCarty,
Amy L.; Karr, Paul A.; Zandler, Melvin E.; Sandanayaka, Atula S. D.; Araki, Yasuyaki;
Ito, Osamu. J. Phys. Chem. B 2006, 110(12), 5905-5913.
16.
Supramolecular Triads bearing Porphyrin and Fullerene via 'Two-point' Binding
involving Coordination and Hydrogen Bonding. D'Souza, Francis; El-Khouly, Mohamed
E.; Gadde, Suresh; Zandler, Melvin E.; McCarty, Amy Lea; Araki, Yasuyaki; Ito, Osamu.
Tetrahedron 2006, 62(9), 1967-1978.
148
17.
Supramolecular Triads of Free-Base Porphyrin, Fullerene, and Ferric Porphyrins via the
"Covalent-Coordinate" Binding Approach: Formation, Sequential Electron Transfer, and
Charge Stabilization. D'Souza, Francis; Gadde, Suresh; Schumacher, Amy L.; Zandler,
Melvin E.; Sandanayaka, Atula S. D.; Araki, Yasuyuki; Ito, Osamu.
J. Phys. Chem. C
2007, 111(29), 11123-11130.
18.
Donor-Acceptor Nanohybrids of Zinc Naphthalocyanine or Zinc Porphyrin
Noncovalently Linked to Single-Wall Carbon Nanotubes for Photoinduced Electron
Transfer. Chitta, Raghu; Sandanayaka, Atula S. D.; Schumacher, Amy L.; D'Souza,
Lawrence; Araki, Yasuyuki; Ito, Osamu; D'Souza, Francis. J. Phys. Chem. C 2007,
111(19), 6947-6955.
19.
Photoinduced Electron Transfer in a Watson-Crick Base-paired, 2-aminopurine:uracil-C60
Hydrogen Bonding Conjugate.
D'Souza, Francis; Gadde, Suresh; Islam, D.-M.
Shafiqul; Pang, Siew-Cheng; Schumacher, Amy Lea; Zandler, Melvin E.; Horie,
Rumiko; Araki, Yasuyaki; Ito, Osamu. Chem. Commun. 2007, (5), 480-482.
149

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