NITRILE-BASED CAROTENOID COMPLEXES OF RU(II), RE(I) AND PT(II) METALS:
COORDINATION SYNTHESIS, ELECTROCHEMICAL AND PHOTOPHYSICAL
CHARACTERIZATION
A Dissertation by
Arvin John Filoteo Cruz
B.S. Chemistry, De La Salle University, 1998
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 2009
© Copyright 2009 by Arvin John Filoteo Cruz
All Rights Reserved
NITRILE-BASED CAROTENOID COMPLEXES OF RU(II), RE(I) AND PT(II) METALS:
COORDINATION SYNTHESIS, ELECTROCHEMICAL AND PHOTOPHYSICAL
CHARACTERIZATION
The following faculty members 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
___________________________________
Donald Paul Rillema, Committee Chair
___________________________________
William Parcell, Committee Member
___________________________________
Erach Talaty, Committee Member
___________________________________
Michael VanStipdonk, Committee Member
___________________________________
Melvin Zandler, Committee Member
Accepted for the College of Liberal Arts and Sciences
________________________________
William D. Bischoff, Dean
Accepted for the Graduate School
________________________________
J. David McDonald, Dean
iii
DEDICATION
To my parents, Amado and Generosa Cruz
Para sa aking mga magulang, Amado at Generosa Cruz
iv
EPIGRAPH
Science without religion is lame. Religion without science is blind.
Albert Einstein
v
ACKNOWLEDGMENTS
As I close my graduate career in chemistry, it is important that I express my deepest
gratitude to all the people who have contributed to my endeavors. First, I thank my research
mentor, Dr. Paul Rillema for giving me the chance to work in his laboratory. His understanding,
support, supervision and guidance have helped me undertake the challenges in graduate research.
Throughout the years that I have worked in his laboratory, I have grown so much in my
understanding of modern chemistry not only in theory, but most importantly its practical
applications as evidenced by this dissertation. I also thank my committee members – Dr. Erach
Talaty, Dr. Michael Vanstipdonk, Dr. Melvin Zandler and Dr. William Parcell for their support
and helpful suggestions in the preparation of this research dissertation. I also thank Dr. Khamis
Siam (Pittsburgh State University) for the useful advice, support and encouragement. He has
truly served as my second mentor in our research laboratory and his continued support in my
academic journey is greatly appreciated. I also thank the Wichita State University Office of
Research Administration and High Performance Computing Center (John Matrow), Department
of Energy, National Science Foundation, RSEC Grant and Parker Fellowships. I also thank Dr.
David Eichhorn and Dr. Curtis Moore for the X-ray crystal structure determination and Manjula
Koralegedara for Mass Spectrometric analysis.
I am also very grateful to all my past and present colleagues in our research group
including Dr. Robert Kirgan, Dr. John Villegas, Dr. Stanislav Stoyanov, Dr. Yetta-Porter
Chapman, Dr. Loranelle Lockyear, Wei Huang and Venugopalreddy Komreddy. Our everyday
conversations have enlightened me on how to resolve research problems that I have encountered.
vi
I also like to extend my acknowledgments to the undergraduate students who helped out
with my laboratory work including Darnell Webb, Mohammad Rashedul Islam, J.R. Gillis,
Benjaman Hlad, Paul Heiland Monica Phommarath, Meagan Miller and Joshua Mayfield. I thank
dearly my family: my parents, Amado and Generosa Cruz whom this dissertation is dedicated to
as well as my sister Abigael Jean Cruz-Mariñas; brother-in-law, Michael Mariñas, nephew,
Micah Mariñas and guardian, Editha Nanlabis. I would also like to extend my gratitude to all my
friends in the US and the Philippines especially Irish Gibson, Janet de los Reyes, Anusha
Dissanayake, Roxanne Adeline Uy, Mark and Sherida Gray, Brenda Foster, Fernando Tan, Swee
Kean Tan, Gretchen Eick, Mike Poage, Dianne Rice (American mother), Megan (American
Sister) and Andrew (British Brother-in-Law) Hargrave, Monks of Conception Abbey and the
CCIF family. All your prayers and support mean a lot to me.
Most of all, I thank God, for all the blessings that he has shed upon me. I offer you in
thanksgiving this achievement of mine and all things that I had accomplished in my life. May
His graces dwell upon me, as I start a new chapter in my life.
vii
ABSTRACT
Nitrile-based carotenoid complexes of Ru(II), Re(I) and Pt(II) metal ions have been synthesized.
The photolability of two bis-nitrilo ruthenium(II) complexes formulated as [Ru(bpy)2(L)2](PF6)2,
where bpy is 2,2’-bipyridine and L = acetonitrile and succinonitrile have been investigated. The
UV/visible absorption spectrum of the sn derivative is dominated by an intense (εmax ~ 58700 M1
cm-1) absorption band at 287 nm assigned as a LC (π → π*) transition. The peak observed at
418 nm (ε ~ 10400 M-1 cm-1) is an MLCT band while the one at 244 nm (ε ~ 23600 M-1 cm-1) is
of LMLCT character. The AN derivative behaves similarly. These Ru(II) complexes undergo
photosubstitution of solvent with quantum efficiencies near one. Calculated and experimental
results support a stepwise replacement of the nitrile ligands. Based on DFT calculations, the
electron density of the HOMO lies on the metal center, the bipyridine ligands and the nitrile
ligands and electron density of the LUMO resides primarily on the bipyridine ligands. Ligand
pyridyl-cyano and dicyano derivatives of all-trans-retinal and β-apo-8’-carotenal have been
synthesized via Knoevenagel Condensation. Density Functional Theory and Time Dependent
Density Functional Theory calculations in all these ligand compounds reveal that the HOMO and
LUMO are located primarily on the polyene chain and the energy gap between them is consistent
with the observed optical spectrum. Spectroscopic treatments establish that their light-harvesting
properties are similar to those observed pigment-protein complexes in plants and purple bacteria
viii
These observed properties are due to the highly allowed population of the second-lying 1Bu
excited state. These compounds contain an electron pair donor and can be coordinated to
rhenium (I) and platinum (II) metal ion centers. These complexes are photosensitizer dyes in
dye-sensitized solar cells (DSSC’s) and also serve as photocatalysts and metallointercalators in
DNA.
ix
TABLE OF CONTENTS
Chapter
Page
1.
SCOPE OF STUDY
1
1.1 Nature of Photochemistry
1.2 Absorption and Emission of Radiation
1.3 Excited States
1.4 Fate of Excited State Species
1.5 Radiative Deactivation Process
1.6 Photosensitization Process
1.7 Carotenoids in Photosynthesis
1.8 Overview of the Carotenoid Photochemical Reactions
in Photosynthesis
1.9 General Structure and Nomenclature
1.10 Carotenoid Excited States
1.11 Geometrical Isomerization, Vision and Photosynthesis
1.12 Carotenoid Electrochemistry
1.13 Photophysical and Spectral Anomalies, Current Studies and Trends
1.14 Excited State Behavior of Ru(II), Re(I), Pt(II) Complexes
1.15 Solar Energy Conversion
1.16 References
1
2
3
5
7
9
10
11
PHOTOCHEMICAL AND PHOTOPHYSICAL PROPERTIES
OF RUTHENIUM(II) BIS-BIPYRIDINE BIS-NITRILE
COMPLEXES: PHOTOLABILITY
32
2.1 Introduction
2.2 Experimental Section
2.2.1 Materials
2.2.2 Measurements
2.2.3 Calculations
2.2.4 Preparation Procedure
2.2.4.1 Preparation of [Ru(bpy)2CO3]
2.2.4.2 Preparation of [Ru(bpy)2(AN)2](PF6)2
2.2.4.3 Preparation of [Ru(bpy)2(sn)2](PF6)2
2.3 Results and Discussion
2.3.1 Synthesis
2.3.2 Calculations (DFT/TDDFT)
2.3.3 Vibrational Spectroscopy
32
33
33
33
36
36
36
37
37
39
39
41
44
2.
x
13
14
17
19
19
20
24
26
TABLE OF CONTENTS (continued)
Chapter
Page
2.3.4 NMR Behavior
2.3.5 Cyclic Voltammograms
2.3.6 Electronic Spectra
2.3.7 Emission Spectra
2.3.8 Chemical Actinometry and Photolysis Studies
2.4 Summary
2.5 References
47
51
53
58
60
67
68
3. DICYANO AND PYRIDINE DERIVATIVES OF RETINAL:
SYNTHESIS, VIBRONIC, ELECTRONIC AND PHOTOPHYSICAL
PROPERTIES
71
3.1 Introduction
3.2 Experimental Section
3.2.1 Materials
3.2.2 Preparation of 15,15’-cyanopyridyl retinal
3.2.3 Preparation of 15,15’-dicyano-retinal
3.2.4 Measurements
3.2.5 Calculations
3.2.6 X-ray Crystallography
3.3 Results and Discussion
3.3.1 Synthesis
3.3.2 X-ray Results
3.3.3 Calculations (DFT/TDDFT)
3.3.4 Population Analysis
3.3.5 Vibrational Analysis
3.3.6 Cyclic Voltammetry
3.3.7 Electronic Spectra
3.3.8 Emission Studies
3.4 Summary
3.5 References
4. LONG-CHAINED DICYANO AND PYRIDINE DERIVATIVES:
SYNTHESIS, VIBRONIC, ELECTRONIC AND PHOTOPHYSICAL
PROPERTIES
4.1 Introduction
71
72
72
73
74
74
75
76
79
79
81
85
87
90
92
94
99
101
102
105
105
xi
TABLE OF CONTENTS (continued)
Chapter
Page
4.2 Experimental Section
4.2.1 Materials
4.2.2 Measurements
4.2.3 Calculations
4.2.4 Preparation of all-trans-7’-cyano-7’-pyridyl-7’-apo-β-carotenal
4.2.5 Preparation of all-trans-7’,7’-dicyano-7’-apo-β-carotenal
4.3 Results and Discussion
4.3.1 Synthesis
4.3.2 Vibrational Properties
4.3.3 Cyclic Voltammetry
4.3.4 Calculations (DFT/TDDFT)
4.3.5 Population Analysis
4.3.6 Electronic Spectra
4.4 Summary
4.5 References
5. SYNTHESIS, STRUCTURAL ELUCIDATION, ELECTROCHEMICAL AND
PHOTOPHYSICAL STUDIES OF CAROTENOID DERIVATIVE LIGANDS
TO RHENIUM (I) AND PLATINUM (II)
5.1 Introduction
5.2 Experimental Section
5.2.1 Materials
5.2.2 Preparation of [Re(CO)3(bpy)(L)],
L = 15,15’-cyanopyridyl retinal
5.2.3 Preparation of [Re(CO)3(bpy)(L)],
L = all-trans-7’-cyano-7’-pyridyl-7’-apo-β-carotenal
5.2.4 Preparation of [Pt(bph)(L)2]
L = 15,15’-cyanopyridyl retinal
5.2.5 Preparation of [Pt(bph)(L)2]
L = all-trans-7’-cyano-7’pyridyl-7’-apo-β-carotenal
5.2.6 Measurements
5.2.7 Calculations
5.3 Results and Discussion
5.3.1 Synthesis
5.3.2 Calculations (DFT/TDDFT)
5.3.3 Vibrational Properties
5.3.4 Cyclic Voltammetry
xii
107
107
107
109
109
110
111
111
113
117
120
123
125
130
131
135
135
136
136
137
138
138
139
139
140
141
141
144
160
162
TABLE OF CONTENTS (continued)
Chapter
Page
5.3.5 Electronic Spectra
5.4 Summary
5.5 References
165
177
178
APPENDICES
182
A. Character Table for C2h Point Group
B. Excited-State and Electrochemical Properties of Ru(II)-polypyridyl Complex
C. Crystallographic Data for 15,15’-cyanopyridyl retinal
D. Crystallographic Data for 15,15’-dicyano-retinal
E. DFT Optimized Structures of the Long-Chained Carotene Derivatives
F. DFT Optimized Structures of the Retinoid/Carotenoid Metal Complexes
G. Geometry Optimization Results for [Ru(sn)2(bpy)2]2+
H. Diagram of Gratzel Dye-Sensitized Solar Cell
xiii
183
184
185
199
211
212
215
232
LIST OF TABLES
Table
Page
2.1 Selected Bond Lengths (Å) in [Ru(bpy)2(sn)2](PF6)2
41
2.2 Percent Contributions of Each Group in HOMO and LUMO
in [Ru(bpy)2(sn)2]2+
42
2.3 Vibrational Frequencies of Metal Bound CN Groups (cm-1)
45
2.4 Calculated and Experimental 1H NMR Chemical Shifts (ppm)
of bpy and sn Ligands in [Ru(bpy)2(sn)2]2+
48
2.5 Experimental 13C NMR Chemical Shifts (ppm) of bpy and sn
Ligands in [Ru(bpy)2(sn)]2+
50
2.6 Electrochemical Data of the [Ru(bpy)2(AN)2]2+ and [Ru(bpy)2(sn)2]2+
51
2.7 Experimental Electronic Transitions and Calculated Excited-States of
[Ru(bpy)2(sn)2]2+ and [Ru(AN)2]2+
54
2.8 Calculated Singlet Energy State Transitions for [Ru(bpy)2(sn)2]2+
56
2.9 Low-Temperature (77K) Emission Spectral Data for [Ru(bpy)2(sn)2]2+
59
2.10 Quantum Yields of Photolysis in [Ru(bpy)2(sn)2](PF6)2
62
3.1 Crystal Data and Structure Refinements for 2 and 3
78
3.2 Selected Bond Lengths (Å)and Angles (deg) for 2 and 3
82
3.3 Electron Distribution (%) in 2 and 3
88
3.4 Vibrational Stretching Frequencies (cm-1) of CN and Polyene Groups
92
3.5 Oxidation Electrode Potentials (V) of Retinoids
93
3.6 UV/Visible Data of S0 1Ag → S2 1Bu Transition in 1, 2 and 3
95
3.7 Calculated UV/Visible Data for 2 and 3
97
xiv
LIST OF TABLES (continued)
Table
Page
4.1 Vibrational Stretching Frequencies (cm-1) of CN and Polyene Groups
114
4.2 Oxidation Electrode Potentials (V) of Carotenoids
119
4.3 Electron Distribution (%) in 5 and 6
124
4.4 UV/Visible Data of S0 1Ag → S2 1Bu Transition in 4, 5 and 6
125
4.5 Calculated UV/Visible Data 5 and 6
128
5.1 Vibrational Stretching Frequencies (cm-1) of CN and CO Groups in
Ligands and Metal Complexes
162
5.2 Electrode Potentials (V)
164
5.3 Calculate UV/Visible Data for 7 and 9
169
C1 Atomic Coordinates ( x 104) and Equivalent Isotropic Displacement Parameters
(Å2 x 103) for 15,15’-cyanopyridyl retinal
187
C2 Anisotropic Displacement Parameters (Å 2 x 103) for 15,15’-cyanopyridyl retinal
189
C3 Bond Lengths [Å] for 15,15’-cyanopyridyl retinal
191
C4 Bond Angles [deg] for 15,15’-cyanopyridyl retinal
194
D1 Atomic Coordinates ( x 104) and Equivalent Isotropic Displacement Parameters
(Å2 x 103) for 15’,15’-dicyano retinal
201
D2 Anisotropic Displacement Parameters (Å 2 x 103) for 15’,15’-dicyano retinal
202
D3 Bond lengths [Å] for 15’,15’-dicyano-retinal
205
D4 Bond Angles [deg] for 15’,15’-dicyano retinal
207
G1 Cartesian Coordinates from DFT Calculation
215
xv
LIST OF TABLES
Table
Page
G2 Mulliken Atomic Charges
217
G3 Selected TDDFT Singlet States for [Ru(sn)2(bpy)2]2+
219
G4 Selected TDDFT Triplet States for [Ru(sn)2(bpy)2]2+
231
xvi
LIST OF FIGURES
Figure
Page
1.1 Direct Absorption Process
3
1.2 Spin Multiplicities in Ground and Excited States
5
1.3 Deactivation Pathways of Excited States
6
1.4 Jablonski Diagram Illustrating Various
Excitation/Deactivation Processes
8
1.5 General Structure of β-carotene Molecule, C2h Point-Group
14
1.6 Electronic Energy State Diagram for Polyene Systems
15
1.7 Schematic diagram of [Ru(bpy)3]2+ , fac-[Re(CO)3(bpy)(L)]+
and [Pt(µ-SEt2)2(bph)2]
23
2.1 Preparation of [Ru(bpy)2(sn)2](PF6)2 from [Ru(bpy)2CO3]
39
2.2 Optimized Structure of [Ru(bpy)2(sn)2]2+
40
2.3 Molecular Orbital Picture of [Ru(bpy)2(sn)2]2+:
(a) Highest Occupied Molecular Orbital (HOMO) and
(b) Lowest Unoccupied Molecular Orbital (LUMO)
42
2.4 Molecular Orbital Energy Diagram in [Ru(bpy)2(sn)2](PF6)2:
(a) Singlet States and (b) Triplet States
43
2.5 Vibrational Spectra of [Ru(bpy)2(sn)2](PF6)2:
(a) IR and (b) Raman
44
2.6 Calculated DFT (a) Infrared spectrum and (b) Raman Spectrum
of [Ru(bpy)2(sn)2](PF6)2
46
2.7 Scheme for 1H and 13C Assignments
47
2.8 Cyclic Voltammogram for the [Ru(bpy)2(sn)2]2+, (a) Reduction Potential and
(b) Oxidation Potential
52
xvii
LIST OF FIGURES (continued)
Figure
Page
2.9 An Overlay of the Experimental and Calculated UV/VIS Absorption Spectrum of
[Ru(bpy)2(sn)2]2+
55
2.10 Calculated UV/Visible Spectrum and MO Pictures Involved in Electronic
Transitions in [Ru(bpy)2(sn)2](PF6)2
57
2.11 Low-Temperature (77 K) Emission Spectrum of [Ru(bpy)2(sn)2]2+ at Excitation
Wavelength, λex = 418 nm and Excitation Spectrum of [Ru(bpy)2(sn)2]2+ at
Emission Wavelength, λem = 537 nm
58
2.12 UV/Visible Spectrum of the MLCT Band in [Ru(bpy)2(sn)2]2+ During Photolysis
for t = 20 minutes in Methanol, MeOH and N,N-Dimethyl Formamide, DMF
61
2.13 Calculated Absorption Changes of [Ru(bpy)2(sn)2]2+ in Methanol
64
2.14 1H-NMR Spectra: a. Free Succinonitrile Ligand and b. Unphotolyzed
[Ru(bpy)2(sn)2](PF6)2 in CD3OD
65
2.15 1H-NMR Spectra of Photolyzed [Ru(bpy)2(sn)2](PF6)2 in CD3OD at Various
Photolysis Times: a. 30 mins, b. 1.5 hours and c. 2.5 hours
66
3.1 Knoevenagel Condensation of All-Trans-Retinal with (2) 4-PyridylAcetonitrile
Monohydrate and (3) Malononitrile
79
3.2 Mechanism for the Acid & Base Catalyzed Knoevenagel Condensation of
All-Trans-Retinal with 4-PyridylAcetonitrile Monohydrate
80
3.3 Crystal Structure in 2: a. ORTEP Diagram, b. Crystal Packing, Space-Group P21/n
83
3.4 Crystal Structure in 3: a. ORTEP Diagram, b. Crystal Packing, Space Group P-1
84
3.5 HOMO/LUMO Pictures in 2 and 3 Determined by DFT
85
3.6 Orbital Pictures of Selected States involved in Optical Transition in 2 and 3
86
3.7 Molecular Orbital Energy Diagrams of 2 and 3
89
xviii
LIST OF FIGURES (continued)
Figure
Page
3.8 Raman Spectra of 2 and 3
91
3.9 Cyclic Voltammogram of 2 and 3
93
3.10 Experimental and Calculated UV/Visible Data for 2 and 3
95
3.11 Energy Diagram for 2 and 3
98
3.12 Low-Temperature (77 K) Emission Spectra of 2 and Low-Temperature (77 K)
Excitation and Emission Spectrum of 3
99
4.1 Knoevenagel Condensation of β-Apo-8’-Carotenal 4-PyridylAcetonitrile
Monohydrate and Malononitrile
111
4.2 Mechanism for the Acid & Base Catalyzed Knoevenagel Condensation of
β-Apo-8’-Carotenal with 4-PyridylAcetonitrile Monohydrate
112
4.3 Raman Spectra of 4, 5 and 6
115
4.4 Infrared Spectrum of 5 and 6
116
4.5 Cyclic Voltammogram for 5 and 6
119
4.6 HOMO/LUMO Pictures in 5
120
4.7 HOMO/LUMO Pictures in 6
121
4.8 Orbital Pictures of Selected States involved in Optical Transition in 5 and 6
122
4.9 Experimental and Calculated UV/Visible Data for 5 and 6
126
5.1 Preparation of 7 and 8
142
5.2 Preparation of 9 and 10
143
5.3 HOMO/LUMO in 7 and 9
145
xix
LIST OF FIGURES (continued)
Figure
Page
5.4 HOMO/LUMO in 8 and 10
146
5.5 Surface Maps of Other Occupied Molecular Orbitals involved in
Electronic Transitions of 7
148
5.6 Surface Maps of Other Unoccupied Molecular Orbitals involved in
Electronic Transitions of 7
149
5.7 Surface Maps of Other Occupied Molecular Orbitals involved in
Electronic Transitions of 9
150
5.8 Surface Maps of Other Occupied Molecular Orbitals involved in
Electronic Transitions of 9
151
5.9 Surface Maps of Other Unoccupied Molecular Orbitals involved in
Electronic Transitions of 9
152
5.10 Surface Maps of Other Occupied Molecular Orbitals involved in
Electronic Transitions of 8
154
5.11 Surface Maps of Other Unoccupied Molecular Orbitals involved in
Electronic Transitions of 8
155
5.12 Surface Maps of Other Occupied Molecular Orbitals involved in
Electronic Transitions of 10
157
5.13 Surface Maps of Other Occupied Molecular Orbitals involved in
Electronic Transitions of 10
158
5.14 Surface Maps of Other Unoccupied Molecular Orbitals involved in
Electronic Transitions of 10
159
5.15 Overlayed IR Spectra of the Ligands with the Complex: (a) [Re(CO)3(bpy)(L)]
and (b) [Pt(bph)(L)2] where L = 15,15’-CyanoPyridyl Retinal
160
5.16 Overlayed IR spectra of the Ligands with the Complex: (a) [Re(CO)3(bpy)(L)]
161
and (b) [Pt(bph)(L)2 where L = All-Trans-7’-Cyano-7’-Pyridyl,7’-Apo-β-Carotenal
xx
LIST OF FIGURES (continued)
Figure
Page
5.17 Cyclic Voltammograms of Compound 7: 7a. Negative Potential Window
7b. Positive Potential Window
163
5.18 Cyclic Voltammograms of Compound 8: 8a. Negative Potential Window
8b. Positive Potential Window
163
5.19 Experimental and Calculated UV/Visible Data for 7
165
5.20 Some Pertinent Transitions in the UV/Visible Spectrum for 7
166
5.21 Experimental and Calculated UV/Visible Data for 8
167
5.22 Some Pertinent Transitions in the UV/Visible Spectrum of 8
168
5.23 Experimental and Calculated UV/Visible Data for 9
170
5.24 Some Pertinent Transitions in the UV/Visible Spectrum of 9
172
5.25 Experimental and Calculated UV/Visible Data for 10
173
5.26 Some Pertinent Transitions in the UV/Visible Spectrum of 10
174
A Character Table for C2h Point Group
183
B Excited-State and Electrochemical Properties of Ru(II)-polypyridyl Complex
184
C1 Optimized Structure of 15,15’-cyanopyridyl retinal
185
C2 ORTEP Diagram of 15,15’-cyanopyridyl retinal
186
D1 Optimized Structure of 15,15’-dicyano-retinal
199
D2 ORTEP Diagram of 15,15’-dicyano-retinal
200
E1 Optimized Structures of all-trans-7’-cyano,7’-pyridyl-7’-apo-β-carotenal
211
E2 Optimized Structures of all-trans-7’,7’-dicyano-7’-apo-β-carotenal
211
xxi
LIST OF FIGURES (continued)
Figure
Page
F1 Optimized structure of [Re(CO)3(bpy)(L)],
where L = 15,15’-cyanopyridyl retinal
212
F2 Optimized structure of [Re(CO)3(bpy)(L)]
where L = all-trans-7’-cyano,7’-pyridyl-7’-apo-β-carotenal
212
F3 Optimized structure of [Pt(bph)(L)2 ],
where L = 15,15’-cyanopyridyl retinal
213
F4 Optimized structure of [Pt(bph)(L)2
where L = all-trans-7’-cyano,7’-pyridyl-7’-apo-β-carotenal
214
H Diagram of Grätzel Dye-Sensitized Solar Cell
232
xxii
LIST OF SCHEMES
Scheme
Page
1.1 Photosensitization Mechanism
9
1.2 Photochemical Reactions Involving Carotenoid Protection in Photoxidative Killing
12
1.3 Photochemical Reaction Involving Carotenoid Light-Harvesting in Photosystems
13
1.4 Energy Transfer and Redox Reactions of [Ru(bpy)3]2+
21
2.1 Solvent Photosubstitution in Ru(II) Coordination Sphere
63
2.2 Stepwise Photosubstitution of MeOH in [Ru(bpy)2(sn)2]2+
64
3.1 Compounds 2 and 3 Derived from 1
72
4.1 Chemical Structure of Naturally Occurring β-apo-8’-carotenal (4) and its
Derivatives
106
xxiii
ABBREVIATIONS
AN/CH3CN
Acetonitrile
ATP
Adenosine Triphosphate
B3LYP
Becke- 3-Parameter-Lee-Yang-Parr
Exchange-Correlation Functional
bpy
2,2’-Bipyridine
[(bpy)Re(CO)3L]+
Rhenium(I) Tricarbonyl-Bipyridine-Ligand
CD3OD
Deuterated Methanol
CH3OH, MeOH
Methanol
Chl
Ground State Chlorophyll
Car
Carotenoid
1
Car*
Singlet Excited State Carotenoid
3
Car*
Triplet Excited State Carotenoid
1
Chl*
Single Excited State Chlorophyll
3
Chl*
Triple Excited State Chlorophyll
COSY
Correlation Spectroscopy
CV
Cyclic Voltammetry
dd
D-Orbital to D-Orbital Optical Transtion
DFT
Density-Functional Theory
DMF
N,N-Dimethyl Formamide
DMSO
Methyl Sulfoxide
DSSC
Dye-Sensitized Solar Cell
xxiv
ABBREVIATIONS (continued)
EPR
Electron Paramagnetic Resonance Spectroscopy
FT
Fourier Transform
Fe(C2O4)33-
Ferrioxalate Ion
HETCOR
Heteronuclear Chemical Shift Correlation
HOMO
Highest Occupied Molecular Orbital
HPLC
High-Performance Liquid Chromatography
IC
Internal Conversion
ILCT
Intraligand Charge-Transfer Optical Transition
IPCE
Incident Photon to Current Efficiency
IR
Infrared Radiation
ISC
Intersystem Crossing
LC
Ligand-Centered Optical Transition
LHC
Light-Harvesting Complex
LMLCT
Ligand-to-Metal-Ligand-Charge-Transfer Optical
Transition
LUMO
Lowest Unoccupied Molecular Orbital
MC
Metal-Centered Optical Transition
MLLCT
Metal-Ligand-to-Ligand-Charge-Transfer Optical
Transition
MMLCT
Metal-to-Metal-Ligand-Charge-Transfer Optical
Transition
MO
Molecular Orbital
xxv
ABBREVIATIONS (continued)
NADPH
Protonated Nicotinamide Adenine Diphospate
NH4PF6
Ammonium Hexafluorophosphate
NMR
Nuclear Magnetic Resonance
ORTEP
Orbital Representation Thermal Ellipsoid Plot
PSI (P700)
Photosystem I
PSII (P680)
Photosystem II
PSB (P870)
Photosystem in Purple-Bacteria
[Pt2(µ-SEt2)2(bph)2]
Platinum(II) biphenyl -µ-dithioether
RC
Reaction Center
[Ru(bpy)2(AN)2](PF6)2
Ruthenium(II) bis-2,2’-bipyridine
bisacetonitrile hexafluorophosphate
[Ru(bpy)2(sn)2](PF6)2
Ruthenium(II) bis-2,2’-Bipyridine
Bissuccinonitrile Hexafluorophosphate
[Ru(bpy)2CO3]
Ruthenium(II) bis-2,2’-Bipyridine Carbonate
[Ru(bpy)2Cl2·xH2O]
Ruthenium(II) bis-2,2’-Bipyridine Dichloride
[Ru(bpy)3]2+
Ruthenium(II) Trisbipyridine
SDD
Stuttgart-Dresden
sn
Succinonitrile
TDDFT
Time Dependent Density-Functional Theory
TiO2
Titanium Dioxide
TLC
Thin-Layer Chromatography
xxvi
ABBREVIATIONS (continued)
TMS
Tetramethyl Silane
tta
Triplet-Triplet Absorption
1
All-Trans-Retinal
2
15’,15’-Cyanopyridyl Retinal
3
15’,15’-Dicyano-Retinal
4
β-apo-8’-carotenal
5
all-trans-7’-cyano-7’-pyridyl-7’-apo-β-carotenal
6
all-trans-7’,7’-dicyano-7’-apo-β-carotenal
7
[Re(CO)3(bpy)(L)]
(L = 15,15’-cyanopyridyl retinal)
8
[Re(CO)3(bpy)(L)],
(L = all-trans-7’-cyano,7’-pyridyl-7’-apo-β-carotenal )
9
[Pt(bph)(L)2]
(L = 15,15’-cyanopyridyl retinal )
10
Preparation of [Pt(bph)(L)2]
(L = all-trans-7’-cyano,7’-pyridyl-7’-apo-β-carotenal)
xxvii
SYMBOLS
A
absorbance
∆A
absorbance change
a
absorption process
Dn (n = 0, 1, 2…..)
doublet electronic state
EC
electrochemical-chemical process
E
electrode potential
e-
electron
τem
emission lifetime decay
λem
emission wavelength
λex
excitation wavelength
M*
excited molecule
If
fluorescence emission intensity
f
fluorescence emission process
φf
fluorescence emission quantum yield
kf
fluorescence rate constant
M
ground state molecule
E1/2
half-wave electrode potential
I0
intensity of incident radiation
Ip
intensity of radiation used in photolysis
Ε
molar extinction coefficient
xxviii
SYMBOLS (continued)
nB
number of atoms/molecules/products
f
oscillator strength
p
phosphorescence emission process
kp
phosphorescence rate constant
hυ
photon
Φp
quantum yield of photolysis
ΦB
quantum yield of Fe2+ in the K3Fe(C2O4)3 Actinometer
Q*
quencher excited state
Q
quencher ground state
Sn (n = 0, 1, 2…..)
singlet electronic state
1
singlet oxygen
∆gO2
t
time
Tn (n = 0, 1, 2…..)
triplet electronic state
v
vibronic level
λ
wavelength
λmax
wavelength maximum
xxix
CHAPTER 1
SCOPE OF STUDY
1.1 Nature of Photochemistry
Radiant energy plays an important role in processes essential to the earth’s biosphere.
Different life forms have developed a mechanism to utilize and convert this powerful source of
energy into other forms like chemical (i.e. photosynthesis) energy. 1-2 Photosynthesis in plants, for
example, is one of the widely studied mechanisms and yet its complexity deters one from having
a full understanding of how many light particles are being converted into energy that can be
applied to perform useful work.1-4 More and more studies are on-going to develop a deeper
understanding about how this vital link between solar energy and life survival takes place.5-12
Among others, it is the interaction between light and matter that makes up the interesting
field of photochemistry. 13-14 In nature, as will be discussed in the next sections, plants undergo
photosynthesis which uses light to convert solar energy into high energy molecules (chemical
energy) such as ATP’s for survival.1,4 Both in nature and in the laboratory, energy released in
chemical reactions can also produce light in processes called luminescence. Chemical response
that triggers fanciful light emissions in fireflies is an exemplary example of the
chemiluminescence phenomenon that involves energetic reactions.13 The chemical changes that
follow the absorption of light and the chemistry that produces light are examples of
photochemical processes.14 Fluorescence and phosphorescence luminescence occur as a result of
direct absorption and their phenomena is an example of a photochemical process that involves
the non-alteration of chemical identity of a compound. 15,17 Processes involving intermolecular
energy transfer (i.e. charge transfer, proton transfer) from one species to another that leads to the
1
excitation of a species other than the absorbing species are known to occur in nature.14-16 This
can be used to promote a chemical change that may not be possible through direct absorption.
Processes of this sort are called photosensitization.13-14, 16-17 In laboratories, dye-sensitized solar
cells (DSSCs)18-23 have been built to reduce our reliance on petrochemical fuels.24 Photolysis,
photoisomerization and photodissociation reactions are other examples of photochemical
processes.13, 17
For years, the photochemistry research avenue has to continue to be developed, in that
more and more investigations are being conducted in this field. This dissertation contributes in
this field. In Chapter 2, the photolysis of Ru(II)-bisbipyridine bis-cyano complexes was studied
to support photolability in cyano complexes of this type. The vibrational and photophysical
studies on synthetic retinoid and carotenoid derivatives are described in Chapter 3 and 4. In
chapter 5, some polypyridyl metal complexes are examined as possible candidates for solar
energy conversion. But first, some basics are needed to be examined to help one understand the
nature of the individual study in this dissertation.
1.2 Absorption and Emission of Radiation
Energy levels in matter are quantized. A transition between two states of specific energy
must associate with it a definite energy. Therefore, a species can only absorb specific energies
from an incident light source. When a species absorbs a quantum of radiation, it becomes
excited, and how the species assimilates that energy depends on the wavelength of the incident
radiation being absorbed. Low energy radiation (i.e. IR) results in rotational and vibrational
modes while high energy radiation (i.e. UV/Visible) results in electronic modes. In order to
discuss absorption and emission processes, the accepted model shown in Figure 1.1 will be used.
In this model a chemical species possesses two quantized states, l and m. Here, the energy of l
2
(lower energy) is different from m (higher energy). In its ground state, m level is occupied by
electron while the l state remains unoccupied. In order for the electron to reach the l level, the
species must gain energy upon absorption of electromagnetic radiation. Thus, absorption process
involves a light-initiated transition from lower to a higher energy level. On the contrary, when
the species undergoes a transition from a higher energy level to lower energy level, which means
that it goes from state m to state l, it is possible for the species to radiate light. This process is
called emission. 13-17
state l
hυ
state m
Figure 1.1 Direct absorption process
1.3 Excited States
The quantitative treatment of electronic excitation that results in absorption and emission
of radiation by a chemical species is governed by the complex mathematical theory of wave
mechanics that replaces classical concepts of position and trajectory with amplitude and
wavefunction. This wavefunction is a mathematical function that carries quantitative information
including energies and angular momentum associated with the energy levels described
previously. This treatment gives rise to quantum numbers that define the electronic states of
3
species.
Individual electrons possess both spin angular momentum and orbital angular
momentum. These momenta are vector quantities and they couple. The process of coupling
results from the vector addition of the spin and orbital angular momentum. One electron will
have a spin moment of 1/2. Vector addition of the spin moments of each electron in a particular
species gives rise to a resultant spin, S. In practice, the spin multiplicity, 2S + 1 rule is used
rather than the spin itself because it shows the number of energy sub-levels that can be present.
In species where all electrons are paired, S = 0 therefore 2S + 1 = 1, are called singlet states. This
is given by the following notation in order of increasing energy, S 0, S1, S2….. In cases where
there are two unpaired electrons, the multiplicity can either be a singlet or a triplet. It is singlet (S
= 0, 2S + 1 = 1) if the two unpaired electrons have opposite spins. However, when the unpaired
electrons have parallel spins (S = 1, 2S + 1 = 3), then the molecule is said to be in its triplet state,
T1, T2.... There are cases where there is one unpaired electron, such as those found in radicals,
where the spin multiplicity is a doublet (S = ½ =, 2S + 1 = 2). In most molecules, the ground
state has a singlet configuration. Few molecules like oxygen, for instance, have a triplet ground
state. Figure 1.2 illustrates spin multiplicities.
In the same way, transitions involving vibrational and rotational energy levels can also be
described by their own quantum numbers. Sometimes, a defined structure arises in the electronic
absorption spectrum due to these transitions, a phenomenon called vibronic coupling. For
simplicity however, it is assumed that these transitions are independent of each other. The basis
for this assumption is the fact that there is a large difference in the energy (frequencies) involved
in these transitions (vibrational, rotational, electronic). This is known as the Born-Oppenheimer
approximation. A further consequence of this assumption forms the basis of the Franck-Condon
principle. The idea behind this is that the nuclei of the molecule are fixed during the process of
4
electronic transition. As a result, the electronic transition can be represented using a potential
well diagram as a vertical transition between the lower and the upper states. In theory, a
transition is allowed only when the spin multiplicity does not change, ∆S = 0.13-14, 25-27
state l
state m
Ground State
S=1
Singlet Excited State
Triplet Excited State
S=1
S=3
Figure 1.2 Spin multiplicities in ground and excited states
1.4 Fates of Excited Species
The absorption of radiation by an atom or molecule, as mentioned earlier, can lead to the
production of an electronically excited species. This excited species can exhibit new chemistry
that may be different from the parent ground state species. There are several ways in which the
excited species can react or otherwise use or lose its excess energy. Some possible routes are
illustrated in Figure 1.3. It is noteworthy that some of the routes associated with absorption of
radiation leads to fragmentation in a molecular species. The energy of the photon is sufficient
enough to rupture a bond or enhance charge/electron transfer and therefore allows the chemically
excited species to undergo chemical reaction or conversion to another species (ionization,
dissociation, direct reaction). In chapter 2, it can be seen that this happens in Ru(II)-polypyridyl
5
complexes containing cyano ligands. Intramolecular processes are also well-known in which one
part of an excited molecule attacks another part of the same molecule. This kind of process then
leads to structural isomerizations and rearrangements.
Physical Quenching
AB + Q*
Luminescence
AB + hv
AB+ + eIonization
+Q
Intramolecular
energy transfer
(radiationless transition)
A+B
AB*
AB**
Dissociation
+E
AB + CD*
Intermolecular
energy transfer
BA
Isomerization
AE + B
Direct Reaction
AB
+
E
Charge Transfer
Figure 1.3 Deactivation pathways of excited states
This is expected in the retinoid/carotenoid chemistry and will be discussed later in this
section. Intermolecular pathways give rise to collisional intermolecular energy transfer
mechanism. Physical quenching occurs when the excited state species transfers the energy
gained by direct absorption to another molecule of the same energetics. Radiative loss of
excitation energy leads to emission as mentioned above. Sometimes it is termed luminescence.
This process can be classified into three types depending on the nature of the excited state being
6
populated after direct absorption. No matter what the fate of the excited species entails, overall it
can be stated that: for every one photon of light absorbed by a molecule, one photochemical
change occurs.13-16
1.5 Radiative Deactivation Process
As mentioned earlier, the excited species generated from direct absorption of radiation
can deactivate through radiative decay known as luminescence.13-14 Fluorescence luminescence is
a radiative decay where the spin multiplicity remains unchanged. It can be referred to as the
converse of absorption. Most fluorescence decay involves singlet-singlet transitions.
Fluorescence is a rapid process which obeys first-order kinetics with forward rate constants, kf
values ranging from 106 – 109 s-1. Phosphorescence, on the other hand is a radiative decay that is
accompanied by a change in spin multiplicity between states. This is typically a T1 → S0 process.
This process, just like other radiative decay, obeys first-order kinetics. Due to the nature of the
spin states involved, they are forbidden transitions and are much slower than its fluorescence
counterpart with phosphorescence rate constants kp ≈ 102 – 104 s-1.A third radiative decay
process, known as delayed fluorescence has also been reported. The only difference here is that
its decay rate is much slower than regular fluorescence but still faster than phosphorescence. 17, 27
These processes can be summarized using a Jablonski Diagram depicted in Figure 4. In this case,
a molecule in its ground singlet state, S0 undergoes absorption to a singlet excited state, S1. This
absorption may lead to the population of the higher vibrational level of this excited state (FranckCondon Principle) which can relax via internal conversion, IC to its lowest vibrational (v0) level
where fluorescence emission can take place back to its ground state.
7
T2v
T1v
T2
IC
Triplet Absorption
S1v
IC
S0v
ISC
S1
ISC
-
e
T1
ISC
S0v
(Donor)
(Acceptor)
Phosphorescence
IC
Absorption
Fluorescence
S0v
S0
S0
Figure 1.4 Jablonski diagram illustrating various excitation/deactivation processes
8
Another possible pathway is for the S1 state to undergo intersystem crossing, ISC to
higher vibrational level of the low-lying triplet (T1) state followed be relaxation to its v0 where
phosphorescence emission can take place back to its singlet ground state. It is also possible for it
to undergo triplet absorption to a higher triplet state (i.e. T2) and the process continues with a
series of deactivation.
Here, collisional deactivation is also illustrated via donor-acceptor
collision which involves electron transfer.13, 17, 27-28
1.6 Photosensitization Process
As mentioned in the opening section of this chapter, photosensitization is one of the most
important photochemistry in biological systems. This is crucial in photosynthetic process in
plants and purple bacteria which will be discussed later. Photosensitization involves
intermolecular energy transfer from one species to another. Photosensitization process can either
involve radiative or nonradiative energy transfer. It is a form of quenching where the initial
excited species, M*, transfers the energy absorbed to another molecule, Q, with the same
energetics. This Q molecule then becomes excited, Q*, say by energy pooling and can undergo
other decay pathway mechanism. This can be illustrated in t he scheme below:
M* → M + hυ
(1)
Q + hυ → Q*
(2)
M* + Q → M + Q*
(3)
Scheme 1.1 Photosensitization mechanism
9
Photosensitization employing nonradiative energy transfer mechanisms requires a
specific interaction between the photoexcited molecule, M* and the quencher, Q. This is
depicted in the energy diagram shown in Figure 1.6. Here, Q must have an unoccupied orbital
lower than the now occupied LUMO of M. Under these conditions, an electron can transfer to
this unoccupied orbital of Q. Another electron swap is then necessary between M* and Q to
retain the same number of electrons in the ground state, M. With this mechanism, photochemists
can exploit more mechanisms important to their research interests like singlet-singlet and singlettriplet energy transfers.13-14 This will be encountered later in the topic of carotenoid
photochemistry. Now that the basics necessary in this research study have been established, the
focus can now be turned towards the author’s research interests.
1.7 Carotenoids and Photosynthesis
Chapters 2 and 3 focus on the synthesis and photophysical studies of short-chained
(retinoid) and long-chained (carotenoid) polyene systems. Here, the basic foundations regarding
this research study will be laid down. The brilliant colors of nature always fascinate the scientific
community. 29-30 Scientists of all sorts of disciplines collaborate with one another to better
understand, at least within molecular levels, how these fecund colors came about. In plants for
instance, pigments are extensively studied to better understand the processes and mechanisms
involved in complex biosystems. An example mentioned above is in photosynthesis.1-12, 29-30 This
process alone constitutes a diverse set of mechanisms involving isomerization29-31, lightharvesting31-38, electron-energy-transfer reactions7,
29-30, 39-42
, photoprotection7,
43-44
and radical
quenching interactions7, 45-48. In recent years, photosystems (PSI & PSII)1-12, 29-30 have been used as
models for studying these mechanisms in terms of light-harvesting complexes’ (LHC’s)
composition49-55. An integral component of LHC photosynthetic systems in higher plant tissues
10
are carotenoids and carotenoid derivatives. Carotenoids function as accessory pigments for lightharvesting. Due to their general structure having conjugated double bonds, they serve as an
antenna for absorbing light in spectral (visible) regions where chlorophylls do not absorb. The
energy absorbed by these molecules is then transferred to chlorophylls to initiate photosynthesis.
Carotenoids are also known to act as photoprotectors43-44 involved in dissipating excess energy3942
and scavenging singlet oxygen31,33. The latter is due to the fact that these compounds also
behave electrochemically. Carotenoids are known for their roles as antioxidants49, 69. The latter is
due to the fact that these compounds also behave electrochemically; and is due to the fact that
under appropriate conditions, they form cation radicals in photosynthetic systems, particularly
their triplet states. It has been reported previously that in photosystem II (P680) for example,
carotenoids undergo cation radical formation upon optical excitation of chloroplasts.29-30
Carotenoids are also known for their important roles in vision chemistry. This research project
was designed to develop modified carotenoids to function as photosynthetic ligands and mimic
these functions.
1.8 Overview of the Carotenoid Photochemical Reactions in Photosynthesis
In photosynthetic systems, carotenoids are known to be bound to pigment-protein
complex reaction centers (RC) in plants and purple bacteria49-55. These reaction centers are
known as photosystems. In purple bacteria, the RC involves a membrane-bound
bacteriochlorophyll complex that undergoes photooxidation when illuminated with red light.
This photosystem involves a ground P870 complex where upon excitation generates an excited
P870* state that initiates a photooxidation cycle where the final electron acceptor is a
cytochrome bc1 complex which returns back the electrons to P870 and the cycle continues. In
this process, a proton gradient is generated (4 - H + transfer). These protons enter the periplasmic
11
space where ATP synthesis occurs. This process is known as photophosphorylation. A similar
mechanism can be extended to higher forms of life that depends on light-driven mechanisms
such as plants. In the case of the latter, an acyclic process occurs that involves photooxidation of
H2O to produce NADPH necessary to produce ATP’s. In plants and cyanobacteria, two
photosystems are involved at these processes known as PSI (P700) and PSII (P680). The
author’s interests lie on the role of carotenoids in these photosystems as light-harvesters and
photoprotectors. These functions involve energy-transfer mechanisms involving singlet and
triplet states of both carotenoids and chlorophyll (bacteriochlorophyll in purple bacteria)2,4,6-7.
In preventing photooxidative killing (Scheme 1.2, eqn. 4-9), first, a photon excites the ground
state chlorophyll (Chl) to its singlet excited state (1Chl*). This is known as chlorophyll
excitation. This is followed by the formation of the triplet chlorophyll excited state (3Chl*). This
triplet excited state of chlorophyll, detrimental to plants, is a precursor to singlet oxygen
formation. It is the latter that is toxic to plant life. To prevent photooxidative killing, carotenoids
serve dual functions.
Chl + hυ → 1Chl* (chlorophyll excitation)
1
Chl* → 3Chl* (triplet state formation)
3
Chl* + Car → 3Car* + Chl (triplet-triplet energy transfer)
3
(4)
(5)
(6)
Car* → Car + heat (carotenoid deactivation)
(7)
Chl* + O2 → Chl + 1∆gO2* (singlet oxygen formation)
(8)
3
1
∆gO2* + Car → O2 + 3Car* (singlet oxygen scavenging)
(9)
Scheme 1.2 Photochemical reactions involving carotenoid protection in photoxidative killing
12
First, a carotenoid in its ground state quenches 3Chl via triplet-triplet energy transfer
where the latter is reverted back to its singlet ground state. The triplet carotenoid excited state
generated in this process undergoes exothermic self-deactivation to its ground state and the cycle
continues. The second function of carotenoids in preventing photooxidative killing is in the
direct quenching of singlet oxygen (singlet oxygen scavenging). In the light-harvesting complex
as shown in scheme 1.3 (eqn. 10-11), ground state carotenoid absorbs a particular wavelength in
the visible region, 680 nm in PS II and 700 nm in PS I. Here, the generated carotenoid excited
state is singlet (1Car*) in nature. This in turn helps with the excitation of ground state chlorophyll
to its singlet excited state which participates in the electron transport chain necessary to generate
ATP’s in plants29-30. These photochemical processes are summarized in the schemes 1.2 and 1.3:
Car + hυ → 1Car* (carotenoid excitation)
1
Car* + Chl → Car + 1Chl* (singlet-singlet energy transfer)
(10)
(11)
Scheme 1.3 Photochemical reaction involving carotenoid light-harvesting in photosystems
1.9 General Structure and Nomenclature
Carotenoids, in general, have a symmetrical tetraterpene (4 isoprene units) backbone and
are made up of conjugated double bonds all the way from the center to the tail-groups. Variation
of these compounds depends on the length of its skeleton and the type of tail groups attached to
the skeleton. It is the conjugation that serves as an important chromophore for absorption of
light. Longer hydrocarbon chains are collectively known as carotenoids while shorter ones are
13
termed retinoids. The numbering of carbons in the structure of β-carotene shown below will be
used during discussions. As one can see, the molecule is centered at the 15,15’ position where
there is an inversion center. Two perpendicular C2-symmetry axes can be assigned at this point.
If one assumes that the molecule is flat, then a σ h-operation can also be applied towards the plane
of this paper. These symmetry operations lead to the assignment of a C2h point group29-30.
Since the chromophore depends on the double bond conjugation of the molecule, this point
group symmetry assignment is extended to its derivative compounds including xanthophylls and
non-hydrocarbon carotenoids. This extension is useful in comparing the states that are involved
in optical transitions and emissions of the starting and emitting materials. Isomers are labeled by
conventional nomenclature and include the position of the isomerization. Figure 5 shows the
general structure of β-carotene29-30.
18'
17
19
16
20
11
7
2
3
1
4
5'
15
13
9
6
8
10
12
14
14'
13'
15'
12'
10'
8'
9'
20'
1'
3'
2'
7'
11'
5
18
6'
4'
19'
16'
17'
Figure 1.5 General structure of β-carotene molecule, C2h point-group
1.10 Carotenoid Excited States
Figure 1.6 shows the Jablonski Diagram depicting the photochemical processes known
about carotenoids. The light-harvesting property of these compounds are based on the vertical
absorption from its singlet ground state (S0) 1 1Ag to a second-lying singlet excited state (S2) 1 1Bu
14
Internal conversion leads to the population of its lowest vibrational state wherein direct
fluorescence occur immediately31-37. A nonradiative decay can also occur further by internal
conversion to the lowest vibrational state of its first lying excited state (S1) 1 1Ag wherein
fluorescence emission can also be expected. Intersystem crossing, on the other hand, is
responsible for the population of the first lying triplet state T1. Nonradiative internal conversion
once again leads to the population of its lowest vibrational state which leads to phosphorescence
emission. Triplet-triplet absorption is rarely reported as well31-37, 29-30. This photochemistry was
utilized in synthetic carotenoid derivatives in the course of this study where these properties were
applied to transition metal complexes for solar energy conversion.
S0, S1, S2, S3: singlet energy states
T1, TE: first triplet and excited triplet states
a: absorption
f: fluorescence emission
p: phosphorescence emission
tta: triplet-triplet absorption
IC: internal conversion
ISC: intersysterm crossing
Figure 1.6 Electronic energy state diagram for polyene systems
15
The novel photophysical properties of carotenoids are influenced mainly by their lowlying energy states. Owing to their general structures, the energy gap between these states lies in
the UV/Visible spectrum. Molecular Orbital and Group Theory conjoined with experimental
studies show that three states are attributed to the observed optical transitions and emissions.
Linear combination of atomic orbitals (LCAO) using a group theoretical approach resulted in the
following symmetry orbital labels in order of decreasing energies: (S2) 1 1Bu > (S1) 2 1Ag > (S0) 1
1
Ag. In general, all carotenoids are compounds that have been given a C2h point group assignment
that are deduced from the properties of β-carotene 29-30.
As mentioned earlier, the pronounced absorption in carotenoids is distinctive of an
allowed transition from its ground state (S0) 1 1Ag to the second lying excited state (S2) 1 1Bu.
Calculations of its oscillator strength reveals that it is close to unity and that its intensity
increases with the extent of π-bond conjugation. Consequently, studies have also revealed that
the first lying excited state in carotenoids have the same symmetry type as the ground state
energy level. This argument is strongly supported by some spectral anomalies obtained
experimentally. These include the following: first, very large energy separations between the
spectral origins of absorptions and emissions, multiple emissions (room and low temperature
conditions) and long fluorescence lifetimes observed for polyenes. The lowest lying triplet state
(T1) energies have been determined for almost all naturally occurring carotenoids ranging from
1-13 doubly-bonded conjugation. The existence of carotenoid triplet states is important in
corollary to its role in scavenging singlet oxygen and quenching of chlorophyll triplet states as
shown in schemes 1.1 and 1.2. The efficiency of triplet absorption/emission greatly depends on
the extent of intersystem crossing. Studies of these triplet states are feasible only through the use
of energy donors during photosensitization process56-63.
16
1.11 Geometrical Isomerization, Vision and Photosynthesis
The presence of long and doubly-conjugated bonds in carotenoids presumes the existence
of geometrical isomers. Light-harvesting complexes (LHC’s) for instance have developed a
natural selection of carotenoid configurations in photosynthetic systems29-30. This correlation has
been established by examining the configurations of naturally occurring carotenoids in pigmentprotein complexes of various photosynthetic organisms. For example, all-trans carotenoids are
the major component in the light-harvesting complex II of spinach while the 15-cis configuration
of carotenoids have been identified as the major isomer in purple bacteria.
Koyama et al using electronic absorption spectroscopy have documented a low intensity
peak observed at higher energy wavelength for β-carotene65. An initial symmetry-based
transition band assignment given to this observed peak is that of an A g- → Ag+ owing to its
transition moment being perpendicular to the long axis. Studies suggested that this band arises
due to the trans-to-cis isomerization of the compound. It is sometimes called the cis peak. Data
deduction showed that the intensity ratio of the trans-to-cis (Ag- → Ag+) peak to the main
absorption band (Ag- → Ag+) peak to the main absorption band (Ag- → Bu+) increases in the order
from all-trans < peripheral-cis isomer. It was also observed that the energy difference between
the cis peak and that of the main absorption band increased in the same order. Another important
observation is that only the v0 → v0’ of the main absorption band is blue shifted while the cis
peak remained unchanged. The trans-to-cis isomerization in carotenoids is crucial in
understanding carotenoid’s role as light-harvesters particularly in the chemistry involved in
vision and even to its role as a natural photosensitizer in plants. This phenomenon is closely
associated with chlorophyll in PSII and is light-driven as mentioned earlier. The oxidation of
naturally occurring carotenoids has been previously studied by photochemical, radiolytic and
17
chemical analysis. Photochemical and radiochemical measurements showed that carotenoid
cations are formed in non-aqueous solvents such as hexane. Matthis et al have reported
previously that light excitation of chloroplasts lead to absorption spectral changes 67. These
occurrences are attributed to the formation of carotenoid cation radical intermediates at the
photosystem II reaction center. Chemical oxidation methods have also been employed using
electron-acceptor molecules such as I2, TCNQ and DDQ which in turn yielded fragments of ionic
pair complexes whose structures have not been determined yet due to their short lifetimes that
falls on the order of microseconds.
In PSII for example, carotenoid-mediated-redox reactions have been sought leading to the
better understanding of the light-driven electron-transport processes across membranes. These
processes however, paved the way for developing artificial photosynthetic systems that would
convert solar energy into chemical energy like those observed in carotenoid-porphyrin-quinone
complexes.
The measurement of electrode potentials is vital in understanding the role of
carotenoids in photooxidation and radical formation reactions. The reduction properties of βcarotene in an aprotic solvent mixture have also been reported previously. Mairanovsky et al
showed that the first cathodic wave initially observed in β-carotene is a reversible, one-electron
transfer process66. Using controlled potential electrolysis coupled with electron paramagnetic
resonance (EPR) spectroscopy, their study later on confirmed the electrochemical formation of
β-carotene anion radicals.
18
1.12 Carotenoid Electrochemistry
Carotenoids are also important in electron-transfer processes in plants. Their extended
conjugated double bonds allow them to generate cation radicals in photosynthetic processes 68-71.
Few electrochemical methods entailing potential sweep methods have been adopted to study the
behavior of carotenoids upon oxidation. Cyclic voltammetry has been previously employed to
measure these potentials. Mairanovsky et al found out that cyclic voltammograms support the
notion that electrochemical oxidation of carotenoid species are not straightforward and yet still
support the conclusions previously deduced through other methods of characterization 66. A
separate study for example, showed that the oxidation of β-carotene in tetrahydrofuran involved
an irreversible, two-electron process.
Grant et al have investigated the generation of carotenoid cation radicals. Results show
successful generation of cation radicals that are produced electrochemically (CV) and their
stability detected using EPR spectroscopy in THF, CH2Cl2 and C2H4Cl268. They proposed
mechanisms that explains the formation and stability of these cation radicals in β-carotene,
canthaxanthin and β-apo-8’-carotenal in non-aqueous solvents.
1.13 Photophysical and Spectral Anomalies, Current Studies and Trends
To date, an elementary understanding of the backbone theory behind photoelectron
transport in nature, particularly that of photosynthesis, has been deduced. Though many answers
has been sought through advanced scientific methods and technology, more studies are needed to
fully understand the dynamics of photosensitization involving carotenoids. Answers to the
queries regarding the molecular orbital nature of the first lying excited state (S1) 2 1Ag are still in
need of determination. Its symmetry forbidden character prevents detection of this state.
Compounds that give rise to electronic absorption that involve the first lying excited state (S1) 2
19
1
Ag are needed to help understand its nature. The common understanding of this ambiguous state
comes only from emission studies at low-temperature conditions (77K)72-74. To date, only
naturally occurring carotenoids have been explored and a venture into creating more synthetic
retinoid and carotenoid derivatives may be necessary to establish this chemistry. All the more,
their photophysical behavior can be taken advantage of by attaching its derivatives into TiO2
semiconductor for light-harvesting and measurements of incident photon to current efficiencies
(IPCE)19. They may also be used as powerful antioxidants in cancer treatments attributed to their
electrochemistry. Advances in our research have led to chemical modifications to these systems
through the use of its pyridyl and nitrile derivatives (Chapter 3-4). All the more, their
coordination to metal centers (i.e. Re(I), Pt(II), etc.) containing polypyridyl complexes76 must be
explored. These complexes have been known to be potential photocatalysts, photosensitizers and
photooxidants with various applications76-94. Reports on dual emissions have been also done and
needed to be explored as well75.
1.14 Excited State Behavior of Ru(II), Re(I) and Pt(II) complexes
The advent of Ru(II) polypyridine complexes have gained research interests in recent
decades76. This is due to the versatile luminescent and photoredox properties of these systems 7677
. Their powerful photosensitization properties for energy transfer and electron transfer
processes made these coordination compounds primary candidates in photoluminescence,
photolysis, photocatalysis (heterogeneous and homogeneous) and electrochemistry76. Among
others, ruthenium (II)-trisbipyridine, Ru(bpy) 3 received much attention and its chemistry is very
well studied among other complexes of ruthenium76-81. This polydipyridyl complex (d6 MC
configuration), has an octahedral configuration. These systems contains σ-donor orbitals
localized on the nitrogen atoms of dipyridyl as well as π-donor and π*-acceptor orbitals that are
20
delocalized on the aromatic rings. Promotions of electrons from dπM metal orbitals to πL* leads to
MLCT excited states observed in the visible region of electromagnetic spectrum. Ligandcentered (LC) πL to πL* transitions are also observed in these systems that are usually observed in
the UV region of the spectrum. Metal-centered (MC) transitions involving π M to σM* can also be
observed with low extinction coefficients76, 78-80. An important feature of these complexes is the
possibility of changing the various ligands gradually around the metal center, altering the nature
of the various ground and excited states to achieve the chemistry desired. This can be done by
using combination of ligands having different π-accepting and σ-donating abilities 76, 82-85. In the
next chapter, density-functional theory was used to gain insights on the nature of the backbonding involved between the Ru(II)-NC center in cyano derivatives of Ru(II)-bisbipyridine
complex. This alteration leads to its photolability and will be fully discussed in that section of the
dissertation. Another interesting feature of Ru(bpy)32+complexes is in its 3MLCT state. The
energy and redox potentials involved in this chemistry make it possible to serve as an electron
donor, electron acceptor and energy donor76, 79. This is shown in scheme 1.4.
Ru(bpy)32+* + Q → Ru(bpy)32+ + Q* (energy transfer)
(11)
Ru(bpy)32+* + Q → Ru(bpy)33+ + Q- (electron transfer)
(12)
Ru(bpy)32+* + Q → Ru(bpy)3+ + Q+ (electron transfer)
(13)
Scheme 1.4 Energy transfer and redox reactions of [Ru(bpy)3]2+
21
In this research, we take advantage of an organometallic derivative of rhenium (I) in a
similar fashion as mentioned in the chemistry of ruthenium (II) complexes 76, 86-94. Organometallic
rhenium complexes have received much attention in years. Most of these systems involve mixed
CO-L complexes, where L stands for aromatic diimines such as 1,10-phenanthroline and 2,2’bipyridine. Most of the studies are are focused on the MLCT behavior of these systems just like
their Ru(II) counterpart76, 86-89. Here, we take advantage of the [(bpy)Re(CO) 3L]+ system, where L
stands for any ligand of valued interests 76, 86 . In our case, we attempted to coordinate CN-pyridyl
retinoid and carotenoid ligand to metal center (chapter 5). In these systems, low-temperature
emission and lifetime measurements indicated intramolecular electron transfer quenching
causing a rather short lifetime of the emission of the MLCT excited state. In [Re(CO)3(L)Cl]+, it
was found that it undergoes facile electron transfer reactions with a series of electron donor and
acceptor molecules. For synthetic purposes, we use [fac-(bpy)Re(CO)3Cl]+, which are proven to
be efficient precursor in introducing monodentate ligands into the metal center17, 76, 86-94. Early
reports establishes that the lowest excited state in these complexes is based on the rhenium π M to
π* of dipyridyl (MLCT)76.
22
2+
N
N
N
Ru
N
+
N
L
N
OC
N
Re
N
OC
[Ru(bpy)3]
2+
CO
fac-[Re(CO)3(bpy)(L)]+
S
Pt
Pt
S
[Pt2(μ-SEt2)2(bph)2]
Figure 1.7 Schematic diagram of [Ru(bpy)3]2+ , fac-[Re(CO)3(bpy)(L)]+ and [Pt(μ-SEt2)2(bph)2]
23
The photoreactivity of platinum complexes have also been established76,
95-100
. Several
works have also been established regarding its photoredox properties especially involving Pt(II)
complexes. Square planar and pseudo square planar complexes of Pt(II) have been investigated.
Although their coordination chemistry is not as novel as its Ru and Re counterparts, its MLCT
excited states may play an important role in the photochemistry of bridged and unbridged
dithiolene and thioester ligands. Depending on the nature of the ligands attached, there is a
potential for it to participate in photocatalysis. In our case, we used an organometallic system,
Pt(II)-bph systems, where bph is biphenyl, to coordinate cyano-pyridyl derivatives of
retinoid/carotenoid. The structures of the lead compounds in our study are shown in Figure 1.7.
1.15 Solar Energy Conversion
Interests in dye-sensitized solar cells (DSSC) have increased in the past decades. Much
progress in this field was realized when Grätzel and O’Regan reported significant progress in
solar energy-to-electrical power conversion efficiency in DSSC’s. To date, conversion
efficiencies of about 10% have been reported. Some significant advances in this research include
replacing planar electrode materials with high surface-area, mesoporous, nanoncrystalline
semiconductor films. Mechanisms on how Grätzel cells work can be seen in many reviews on
this area. Here, the author attempts to summarize some of the key processes known to this date
to be involved in these complex cell systems18-23.
First, light must be absorbed by a sensitizer in the visible region of the spectrum to form a
molecular excited state. Second, the excited state must be able to inject an electron into a
semiconductor like TiO2 that maximized charge separation over charge recombination. Third, the
oxidized sensitizer must be able to regenerate itself by utilizing an electron donor. Once the
electron has performed useful work in the external circuit, it returns to a counter electrode where
24
it reduces the oxidized electron donor. In this manner, the cell is considered ‘regenerative’. A
fourth process is also important to consider wherein charge recombination of unwanted TiO 2
electrons can be returned back to the oxidized sensitizer19. One of the goals of our research group
is the design and synthesis of effective photoactive supramolecular systems that can be used as
dye-sensitizers that can be attached in TiO2 semiconductor in dye-sensitized Grätzel cells.
Four criteria are important in the design and synthesis of these systems. First, the
complex must be able to absorb light in the visible region of the spectrum. Second, it must be
stable to photodecomposition. Third, it must have long excited state lifetimes and good quantum
yields. A fourth criterion is that the position of the sensitizer conduction band should match that
of the semiconductor17.
Perhaps, the most well known and well studied photosensitizer that was initially applied
to Grätzel cells is [Ru(bpy) 3]2+. It was perceived that this complex has a unique combination of
chemical stability, acceptable redox potentials for effective charge separation/recombination,
emission properties, excited state reactivity and long-excited state lifetimes 17. In this dissertation,
rhenium(I) derivatives were employed since it has similar chemistry to ruthenium(II) as
mentioned above. Here, we connect a derivatized retinoid/carotenoid moiety, known for their
effective light-harvesting properties in photosynthetic purple bacteria and plants, into the metal
center to mimic light-driven processes found in nature into DSSC’s.
25
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31
CHAPTER 2
PHOTOCHEMICAL AND PHOTOPHYSICAL PROPERTIES OF RUTHENIUM(II)
BIS-BIPYRIDINE BIS-NITRILE COMPLEXES: PHOTOLABILITY
2.1 Introduction
Many reports have been written about ruthenium(II) tris-bipyridine complexes and
derivatives due to interest in their photophysical properties related to solar energy conversion,
catalysis and photoinduced chemical reactions1-3. The primary driving force in these studies has
been the exploitation of excited state redox processes, although interest in photochemical and
thermal chelate exchange reactions involving Ru(bpy) 32+, where bpy is 2,2’-bipyridine, and
derivatives have been reported where one moiety is expelled from the primary coordination
sphere and replaced by another4-9. The extent of these photosubstitution reactions depends on the
lability of these ligands under light conditions. For example, ruthenium(II) complexes involving
diimine chelates have been known to undergo photosubstitution reactions and have been
investigated using electrospray ionization mass spectrometry10. Brown has reported thermal and
light-induced decomposition of Ru(bpy)2(N3)2+ in CH3CN11. Bonneson et al. demonstrated lightdriven nitrile substitution in complexes involving 1,10-phenanthroline (phen) chelating ligands 12.
Baranoff et al. have also reported ruthenium complexes with photolabile ligands based on
bipyridine derivatives9.
In this chapter, we report an investigation of the photolability of [Ru(bpy)2(AN)2]2+ and
[Ru(bpy)2(sn)2]2+, where AN = CH3CN and sn = NC-CH2-CH2-CN, in various solvents.
[Ru(bpy)2(AN)2]2+ has been known as an intermediate in coordination synthesis but to our
knowledge has not been investigated regarding its photolability1,4,11-20. This study gives new
insight into solvent photosubstitution reactions in bis-nitrilo complexes of ruthenium(II) which
32
can be useful as intermediates in coordination synthesis as well as in the growing field of
“molecular machines”5-8.
2.2 Experimental Section
2.2.1 Materials
Ruthenium(II) bis-2,2’-bipyridine dichloride hydrate, [Ru(bpy)2Cl2∙xH2O], ruthenium(II)
bis-2,2’-bipyridine carbonate, [Ru(bpy)2CO3]21-24, and ruthenium(II) bis-2,2’-bipyridine bisacetonitrile hexafluorophosphate, [Ru(bpy)2(CH3CN)2](PF6)2, were synthesized using published
procedures18. The succinonitrile ligand was purchased from Aldrich and used as received.
Diethyl ether, HPLC grade methanol and optima grade acetonitrile were purchased from Fisher
Scientific. Ammonium hexafluorophosphate (NH4PF6), IR grade potassium bromide, deuterated
dimethyl sulfoxide (DMSO) and trifluoromethanesulfonic acid (99%) in a vacuum sealed glass
container were obtained from Aldrich. Deuterated methanol used in photolysis studies were
purchased from Cambridge Isotope Laboratories, Inc. Sulfur powder was provided by Thermo
Nicolet.
Electrochemical grade tetrabutylammonium perchlorate was purchased from
Southwestern Analytical. Ferrocene standard and dry acetonitrile in a seal tight bottle were
purchased from Aldrich. Butyronitrile used for emission and lifetime studies was obtained from
Acros; it was fractionally distilled prior to usage. Potassium ferrioxalate used in chemical
actinometry was synthesized using published procedures25-30.
2.2.2 Measurements
Infrared spectra were obtained using Perkin-Elmer Model 1600 FT-IR and Nicolet Avatar
Model FT-IR Spectrophotometers. All samples were prepared as potassium bromide pellets.
Raman spectra were obtained using a Thermo Nicolet Nexus Spectrophotometer with a FTRaman Module which was calibrated with sulfur. The Nicolet instruments were accompanied by
33
Omni software programs. Proton 1H-, 13C-NMR spectra and HETCOR spectra were obtained
using Varian Mercury 300 MHz and Varian Inova 400 MHz Fourier Transform-NMR
spectrometers (internal standard TMS). Ultraviolet spectra were obtained using a HewlettPackard Model 8452A diode array spectrophotometer interfaced with an OLIS software
program. Elemental (C, H, & N) analysis was performed by MHW Laboratories. An EG&G
PAR Model 263A potentiostat/galvanostat was used to obtain the cyclic voltammograms. All
CV measurements were carried out in a typical H-electrochemical cell using a platinum disk
working electrode, polished every run, and a platinum wire counter electrode. A Ag/AgNO 3
electrode freshly made from AgNO3 in dry CH3CN served as the reference for this study. The
supporting electrolyte was 0.1 M tetrabutylammonium perchlorate. Ferrocene was added as an
internal reference. The sample preparation for emission studies involved dissolving a small
amount of sample in distilled butyronitrile and then measuring the absorbance of the solution.
The concentration of the solution was altered in order to achieve an approximate absorbance of
0.10 at 418 nm of the sample. The sample solution was placed in a fluorescent quartz tube and
was degassed (minimum of five times) prior to actual measurements. Emission quantum yields
were calculated using equation 2.1 31, where φf is the fluorescence emission quantum yield, If is
the fluorescence emission intensity and A is the absorbance of the sample at the excitation
frequency.
(2.1)
34
All samples were degassed using the freeze-pump-thaw method a minimum of five times
prior to measurements. Sample tubes for 77 K measurements were immersed in a quartz finger
Dewar containing liquid N2. The Dewar was mounted in the center of the sample compartment
of a SPEX 212 Fluorolog Fluorometer which was modified to allow dry N2 gas to flow over the
finger of the Dewar in order to minimize condensation of moisture from the air. The sample was
held in place by a series of rubber rings located near the top and middle of the sample tube. This
ensured that the tube was centered in all directions. Accurate results were obtained with the
sample centered in the excitation beam and with a frost-free finger. Great efforts were made to
ensure the maximum emission intensity was obtained from the sample.
The excited state lifetimes were determined by exciting the sample at the third harmonic
of a Continuum Surlite Nd:YAG laser run at ~ 20 mJ/10 ns pulse. Instrumentation for steady
state photolysis consisted of a 100 watt xenon lamp (Oriel) and a monochrometer. The light was
directed into the cavity of the HP8452A Diode Array spectrophotometer.
The solution
containing the compound of interest was irradiated in a 1 cm cuvette at 405 nm, magnetically
stirred, and its absorbance was observed at right angles to the irradiating light. The intensity of
the light source at the wavelength of excitation was determined using potassium ferrioxalate as
the actinometer following published procedures25. Photolysis progress was monitored using UVVisible spectroscopy and the absolute quantum yields of photolysis were determined in various
solvents. Photosubstitution of methanol (CD3OD) was monitored using UV-Visible and 1H-NMR
spectroscopy.
35
2.2.3 Calculations
Gaussian ’03 (Rev. B.03) software32 for UNIX was used for calculations. The molecules
were optimized using Becke's three-parameter hybrid functional B3LYP33 with the local term of
Vosko, Wilk, and Nassiar. The basis set SDD34 was chosen for all atoms and the geometry
optimizations were all ran in the gas phase. TDDFT35 calculations were employed to produce a
number of singlet excited states in the gas phase based on the optimized geometry. All oscillator
values and singlet and triplet excited state values are presented in the supporting information.
All vibrational analyses revealed no negative frequencies and were run in the gas phase only.
2.2.4 Preparation Procedure
2.2.4.1 Preparation of [Ru(bpy)2CO3]
Ru(bpy)2Cl2·H2O (2.00 g, 3.70 mmol) was suspended in 150.0 ml of deaerated water in a
100-mL round-bottom flask. It was heated at reflux under Ar for 15 minutes. After reflux, excess
amount of sodium carbonate monohydrate (7.8 g, 63 mmol) was added into solution and the
mixture was allowed to reflux for another 2 hrs. The reaction was completed; the solution was
allowed to cool down in an ice bath. The black solid was vacuum filtered and oven-dried at 60ºC
overnight. Color: Black. Yield 74%. IR (KBr pellet, cm-1): 3411 br m, 1558 s, 1643 m, 1462 m,
1425 m, 1255 w, 766 m 1H-NMR (d6-DMSO): δ ppm 7.150 (t, 2H), 7.478 (d, 2H), 7.749 (t, 2H),
7.881 (t, 2H), 8.151 (t, 2H), 8.627 (d, 2H), 8.789 (d, 2H), 9.198 (d, 2H).
36
2.2.4.2 Preparation of [Ru(bpy)2(AN)2](PF6)2
Ru(bpy)2CO3 (102.00 mg, 0.237 mmol) was dissolved in 20.0 mL of dried CH3CN and
was placed in a 100-mL round-bottom flask. The mixture was allowed to stir at room
temperature
for
about
15
minutes
under
Ar
conditions.
Then,
0.10
mL
of
trifluoromethanesulfonic acid was added into the mixture. Evolution of gas was observed and the
color of the solution immediately changed into a yellow-orange tint. The reaction was set to
reflux overnight and the solvent was allowed to evaporate to about 2 mL. The remaining solution
was then added dropwise into a saturated aqueous solution of ammonium hexafluorophosphate
(NH4PF6). The orange precipitate was collected by vacuum filtration and was dried inside the
vacuum oven (40º) for 1 day.
Color: Orange. Yield 85%. UV/Visible: λmax = 420 nm. IR (KBr pellet, cm-1): 1605 w, 1467 w,
1448 w, 1266 s, 1224 m, 1147 m, 1029 m, 763 m, 729 w, 637 m, 572 w, 517 w. 1H-NMR (d6DMSO): 2.95 (s, 4H), 7.30 (t, 2H), 7.59 (d, 2H), 7.93 (t, 2H), 8.02 (t, 2H), 8.38 (t, 2H), 8.64 (d,
2H), 8.82 (d, 2H), 9.34 (d, 2H).
2.2.4.3 Preparation of [Ru(bpy)2(sn)2](PF6)2
Ru(bpy)2CO3 (105.00 mg, 0.230 mmol) was placed in a 100-mL round-bottom flask and
dissolved in 20.0 mL of methanol. The purple solution was allowed to stir under argon at room
temperature for another 30 minutes. Then 0.10 mL of trifluoromethanesulfonic acid was added.
The solution turned into a burnt-orange color and evolution of carbon dioxide gas was observed.
While the solution was allowed to stir for 3 h, the reaction was monitored using UV-visible
spectroscopy until no spectral shift was observed.
After the reaction was complete,
succinonitrile (60.2 mg, 0.750 mmol) was added and the solution was allowed to stir under reflux
overnight. The solution was evaporated to approximately 2 mL and the desired complex was
37
allowed to precipitate from a saturated aqueous ammonium hexafluorophosphate (NH4PF6)
solution. It was vacuum filtered to isolate the compound. The orange complex was washed with
diethyl ether to remove excess succinonitrile. The complex was placed inside the vacuum oven
(45 °C) and was allowed to dry overnight. Color: Yellow-orange. Yield: 90%.
Anal. Calcd for RuC24H20N6PF6: C, 38.92; H, 2.80; N, 12.97. Found: C, 39.13; H, 2.61; N, 12.75.
IR (KBr pellet, cm-1): 3088 m, 2967 m, 2277 m, 2252 m, 1737 m, 1606 s, 1468 s, 1448 s, 1426 s,
1315 m, 1262 br s, 1160 s, 1031 s, 965 w, 836 br s, 762 s, 730 s, 638 s, 557 s, 517 w, 421 w.
Raman (Solid, cm-1): 3091 w, 2948 w, 2273 s, 1603 m, 1561 w, 1489 w, 1313 w, 765 w, 717 w,
648 w, 450 w. 1H-NMR (d6-DMSO): δ ppm 2.84 (t, 4H, J = 5.7 Hz), 3.21 (t, 4H, J = 6.0 Hz),
7.37 (ddd, 2H, J = 0.90 Hz), 7.58 (d, 2H, J = 4.5 Hz), 7.90 (ddd, 2H, J = 1.5 Hz), 8.04 (ddd,
2H, J = 1.5 Hz), 8.35 (ddd, 2H, J = 1.0 Hz), 8.68 (d, 2H, J = 7.8 Hz), 8.82 (d, 2H, J = 7.8 Hz ),
9.35 (d, 2H, J = 5.1 )
13
C-NMR (d6-DMSO): δ ppm 14.4, 16.7, 117.7, 124.0, 124.5, 126.8, 127.4, 128.2, 138.6, 139.1,
152.5, 153.8, 157.4, 158.3
38
2.3 Results and Discussion
2.3.1 Synthesis
The reaction scheme for the preparation of [Ru(bpy)2(sn)2](PF6)2 is shown in Figure 2.1.
N
N
Ru
N
Cl
.
H2O
Cl
N
N
Na2CO3, MeOH
reflux (overnight), Ar
+
O
N
2 NaCl (aq)
O
N
N
(I)
(II)
N
N
Ru
N
N
O
O
O
triflic acid, MeOH
reflux (3 hrs), Ar
N
Ru
+
CO2(
)
+ H2O
N
(II)
(III)
N
Ru
SO3CF3
2 ( NC
CN )
SO3CF3 reflux (overnight), Ar
N
N
N
Ru
N
N
(III)
SO3CF3
SO3CF3
N
N
N
O
Ru
NC
NC
CN
+2
CN
N
(IV)
Figure 2.1 Preparation of [Ru(bpy)2(sn)2](PF6)2 from [Ru(bpy)2CO3]
39
CF3SO3- (aq)
The first step involved preparation of [Ru(bpy) 2CO3] from Ru(bpy)2Cl2∙H2O.
The
carbonato complex was reacted with equivalent amounts of trifluoromethanesulfonic (triflic) acid
to form the bis-triflate complex and carbon dioxide. Step three involved the replacement of the
triflate ligands with succinonitrile in the dark.
The optimized, calculated structure of [Ru(bpy)2(sn)2](PF6)2 is shown in Figure 2.2 and
the Cartesian coordinates are located in the appendix. The atoms in blue are nitrogen atoms, six
of which are coordinated to the metal center.
The calculated Ru-N bond lengths in
[Ru(bpy)2(sn)2](PF6)2 are listed in Table 2.1 and are compared to experimental and calculated
bond lengths determined for [Ru(bpy) 2(AN)2](PF6)2. Both of the calculated Ru-N (-N≡C-) bond
distances were the same with values of 2.045 Å and both bipyridine rings are equivalent in the
optimized structure with average calculated Ru-N (bpy) bond distances of 2.078 Å and 2.098 Å.
The shorter Ru-N (bpy) bond distance, 2.078 Å, was from Ru to the pyridine unit cis to the
nitrile ligand; the long Ru-N (bpy) bond distance, 2.098 Å, was from Ru to the pyridine unit
trans to the nitrile ligand. Shorter bond distances were found between the metal center and
nitrile groups compared to the bond distance between the metal and the bipyridyl groups. The
optimized structure showed slightly twisted bipyridyl rings,
1°, with one nitrogen atom closer to the ruthenium center
than the other.
Experimental bond distances36 in the
acetonitrile adduct were shorter than the ones calculated as
reported for most optimized structures which are evaluated in
the gas phase.
Figure 2.2 Optimized Structure of [Ru(bpy)2(sn)2]2+
40
TABLE 2.1
SELECETED BOND LENGTHS (Å) IN [Ru(bpy)2(sn)2](PF6)2
Bond
Ru-N (bpy)b
Ru-N (bpy)c
Ru-N (bpy)c
Ru-N (bpy)b
Ru-N (-N≡C-)
Ru-N (-N≡C-)
calc. (Parent)
2.095
2.078
2.077
2.095
2.049
2.049
calc.(sn)
2.098
2.079
2.078
2.098
2.045
2.045
exp. (Parent)a
2.061
2.052
2.051
2.059
2.040
2.045
a Ref 36, Parent = [Ru(bpy)2(AN)2]2+ . b Ru-N trans to nitrile. c Ru-N cis to nitrile.
2.3.2 Calculations (DFT/TDDFT)
Figure 2.3 shows pictures of the highest-occupied molecular orbital (HOMO) and lowestunoccupied molecular orbital (LUMO) of [Ru(bpy)2(sn)2](PF6)2. The HOMO is more metallic in
character compared to the LUMO which has more ligand-centered character. As listed in Table
2, the HOMO contains 75% metal, 14% bpy and 10 % sn character while the LUMO contains
2% metal, 98% bpy and 1% sn character. The electron density of the HOMO is concentrated
primarily on Ru with a small amount on bpy and CN while the electron density on the LUMO
dominantly resides on the two bipyridine ligands. The HOMO-LUMO transition occurs from the
metal to the ligand (bpy).
41
(a)
(b)
Figure 2.3 Molecular Orbital Picture of [Ru(bpy)2(sn)2]2+: (a) Highest Occupied Molecular
Orbital (HOMO) and (b) Lowest Unoccupied Molecular Orbital (LUMO)
TABLE 2.2
PERCENT CONTRIBUTIONS OF EACH GROUP IN HOMO AND LUMO IN
[Ru(bpy)2(sn)2]2+
Energy Level
HOMO
LUMO
Ru
75
2
bpy1
7
51
bpy2
7
47
sn
10
1
The molecular orbital energy diagrams for the singlet and triplet states of
[Ru(bpy)2(sn)2]2+ are shown in Figure 2.4 and energies are listed in the appendix calculations
part. Singlet-energy calculations results showed that the low-lying unoccupied energy states are
more of bipyridyl type.
The succinonitrile orbitals lie at much higher energies.
HOMO,
HOMO-1and HOMO-2 energy levels have most of the electron density lying on the ruthenium.
Lower HOMOs, on the other hand, have electron density concentrated on the bipyridyl and
succinonitrile ligands. DFT calculations of the triplet state reveal that the low-lying triplet states
are of 3MLLCT and 3LC character (Figure 2.4).
42
-40
(+8) Sn
-45
-50
-1
Wavenumbers (cm )
-55
(+6) Sn
(+3) Bpy
(+5) Bpy
(+2) Bpy
(+7) Sn
(+4) Bpy
-60
-65
Gap = 28.9
-70
-95
(+1) Bpy
(L) Bpy
(H) Ru
(-2) Ru
(-1) Ru
-100
(-3) Bpy
(-4) Bpy
-105
-110
(-6) Sn
(-5) Sn (-8) Sn
(-7) Sn
-115
(a)
32.5
30.0
-1
3
Wavenumbers (x10 cm )
22.5
LMLCT
ILCT
3
LMLCT
27.5
25.0
3
3
3
dd
3
dd
MLLCT
3
ILCT
3
20.0
4
3
2
1
1
G.S.
0
(b)
Figure 2.4 Molecular Orbital Energy Diagram in [Ru(bpy)2(sn)2](PF6)2: (a) Singlet States and
(b) Triplet States
43
2.3.3 Vibrational Spectroscopy
Both infrared and Raman data were obtained where interest was focused on the CN
group. Data are listed in Table 2.3 and the experimental spectra are shown in Figure 2.5. The
CN vibration of the free succinonitrile ligand in the infrared region was observed at 2254 cm-1
whereas two CN stretching vibrations were observed for the sn complex. None were found for
[Ru(bpy)2(AN)2]2+.
1.0
100
Raman Intensity
% Transmittance
80
60
40
0.5
20
0
2500
2400
2300
2200
2100
0.0
2500
2000
-1
2000
-1
Wavenumbers (cm )
Raman Shift (cm )
(a)
(b)
Figure 2.5 Vibrational Spectra of [Ru(bpy)2(sn)2](PF6)2: (a) IR and (b) Raman
The vibrational frequency found at 2277 cm-1 for the sn complex was assigned to the
metal bound nitrile group, while the one observed at 2252 cm-1 was attributed to the
uncoordinated nitrile group by comparison to the free ligand. The vibrational frequency of the
CN group in the Raman spectrum for [Ru(bpy)2(sn)2]2+ was observed at 2273 cm-1 and not
resolved into two components; none was found for [Ru(bpy)2(AN)2]2+ due to decomposition of
the complex in the laser beam. The number of scans was lowered due to sample decomposition
upon prolonged exposure to laser source. As a result, the observed intensity of Raman scattering
was weak and the aperture was opened completely giving rise to a less resolved peak.
44
TABLE 2.3
VIBRATIONAL FREQUENCIES OF METAL BOUND CN GROUPS (cm-1)
Compound
Infrared
Calc
[Ru(bpy)2(sn)2]2+
Raman
Exp
Exp
Calc
Free
Ligand
Ligand
2277a (Ru-NC)
2296
2252 (C-CN)
2254
2296
[Ru(bpy)2(AN)2]2
2325
NA
2200
2325
+
2318
2311
2318
KBr Pellet
Solid in tube
c
Solution (CCl4)
b
45
Exp
Free
2311
a
Exp
2273b (Ru-NC)
2189c
Decomposition
NA
1.0
0.0
0.2
Ru-NC Stretch
2311
2303
C-CN Stretch
2296
2296
0.8
0.4
0.6
0.6
0.4
0.8
0.2
1.0
3000
2500
0.0
3000
2000
Ru-NC Stretch
2311
2303
C-CN Stretch
2296
2296
2500
2000
Wavenumbers
Wavenumbers
(a)
(b)
Figure 2.6 Calculated DFT (a) Infrared spectrum and (b) Raman spectrum of
[Ru(bpy)2(sn)2](PF6)2
DFT calculations were used to simulate the infrared and Raman spectra of both the sn
and AN derivatives. The simulated spectra for the sn derivative are shown in Figure 2.6.
The nitrile stretches are doublets of doublets, and although the splittings of each doublet are
small, the two sn ligands occupy two different sites in the molecule which may account for the
origin of the doublet. These splittings were not observed in the experimental spectrum, perhaps
due to line broadening in the solid state. The high energy absorption at 2311 cm -1 and the low
energy one located at 2296 cm-1 are assigned to bound and unbound portion of the nitrile groups,
respectively, as found experimentally. Two very weak infrared vibrations located at 2325 and
2318 cm-1 were calculated for the AN complex and the calculated Raman vibration was located
at the same frequency as found for the sn derivative. The weak vibrations of the AN complex
are consistent with the lack of observing a vibration experimentally. The phase calculations are
46
20 – 30 cm-1 higher in energy than found experimentally for both the infrared and Raman results.
Calculations in the gas phase may account for this observation.
2.3.4 NMR Behavior
The 1H-NMR spectrum of the metal complex showed all eight chemically distinct
bipyridyl protons compared to four for the free bipyridyl ligands.
The chemical shift
assignments are tabulated in Table 2.4 according to the proton designations given in Figure 2.7.
All chemical shift assignments were based on calculations, experiments (coupling constants and
multiplicity) and chemical environment. Results show that the two coordinated bipyridyl ligands
have distinct pyridine rings on each ligand. While Ha and Hg experience downfield chemical
shifts due to ring current effects on the bipyridyl rings, H a undergoes more deshielding compared
to Hg since it resides perpendicular to the CN triple bond which has its magnetic current parallel
to the bonding axis. Thus, the chemical shift observed furthest downfield at around δ = 9.35
ppm was assigned to the Ha (Ha’) protons
while the one observed furthest upfield at
around δ = 7.37 ppm was assigned to Hg
(Hg’) protons. The succinonitrile methylene
proton signals (triplet) were observed at
around δ = 3.81 before and at 2.81 ppm after
coordination.
Figure 2.7 Scheme for 1H and 13C Assignments
47
TABLE 2.4
CALCULATED AND EXPERIMENTAL 1H NMR CHEMICAL SHIFTS (ppm) OF bpy AND
sn LIGANDS IN [Ru(bpy)2(sn)2]2+
Proton
Chemical
Chemical Shift
Type
Shift
(experimental)b
(calculated)a
Ha
Ha’
9.03
9.04
9.35(2H)
Hb
Hb’
7.82
7.82
7.90 (2H)
Hc
Hc’
8.27
8.29
8.35 (2H)
Hd
Hd’
8.23
8.20
8.68 (2H)
He
He’
8.06
8.03
8.82 (2H)
Hf
Hf’
7.91
7.90
8.04 (2H)
Hg
Hg’
7.17
7.19
7.37 (2H)
a
Gas-phase
Solvent: DMSO
b
48
TABLE 2.4 (continued)
Hh
Hh’
7.52
7.82
7.58 (2H)
Hi
2.29(2.10)
3.21 (4H)
Hj
1.73(1.46)
8.04 (2H)
a
Gas-phase
b
Solvent: DMSO
Heteronuclear chemical shift correlation spectroscopy was performed to support the
carbon nuclei chemical shift assignments listed in Table 2.5 following the numbering system in
Figure 2.7. The 13C peak for the uncoordinated CN peak was located at δ = 117 ppm while peak
for the metal-bound CN group was found at 126 ppm. Two peaks (C5/C6) observed furthest
downfield were assigned to the quaternary carbons of the bipyridyl rings. Upon coordination,
the two methylene carbon atoms of the succinonitrile ligand are now distinct with two C13 peaks
located at δ = 14 and 16 ppm. One-dimensional (1-D) and Correlation (COSY) spectra of the
complex further establishes the chemical structure of the complex. As mentioned earlier, 1HNMR spectrum of the two dipyridyl ligands gave rise to eight distinct protons which supports a
cis-bissuccinonitrile isomer over its trans- counterpart. This is attributed to the fact that the cisbissuccinonitrile configuration lowers the symmetry of the rings giving rise to the eight distinct
protons observed while the trans- configuration do not.
49
TABLE 2.5
EXPERIMENTAL 13C NMR CHEMICAL SHIFTS (ppm) OF bpy AND sn LIGANDS IN
[Ru(bpy)2(sn)]2+
Carbon Type
Chemical Shifta
C1/C10
153.82
C2
128.19
C3
136.69
C4
124.44
C5/C6
157.35
C6/C5
158.35
C7
124.10
C8
139.05
C9
127.38
C10/C1
152.47
C11
126.82
C12
16.70
C13
14.35
C14
117.65
a
Solvent: DMSO
50
The rings closest to the succinonitrile experiences a much more electron deshielded
nature owed to the electron withdrawing properties of the nitrile groups. The succinonitrile
proton closest to the metal center (Hi) is given an assignment of 3.21 ppm while the one observed
at 2.81 ppm was given to the proton next to the free nitrile (Hj).
2.3.5 Cyclic Voltammograms
The CV electrochemical data are listed in Table 2.6 and the cyclic voltammograms for
the [Ru(bpy)2(sn)2]2+ complex are shown in Figure 2.8.
ruthenium(II) bis-bipyridine complexes.
The CV profile is typical of
Two reversible peaks were observed at negative
potentials for the succinonitrile derivative. The first peak was observed at half-wave potential
(E1/2) of -1.33 V while the second one was located at -1.53 V. One reversible peak was
prominent at positive potentials. It was observed at half-wave potential (E1/2) of +1.54 V.
TABLE 2.6
ELECTROCHEMICAL DATA OF THE [Ru(bpy)2(AN)2]2+ AND [Ru(bpy)2(sn)2]2+ COMPLEX
Compound
E1/2 (V)oxa
[Ru(bpy)2(sn)2]2+
1.54
-1.33
-1.53
[Ru(bpy)2(AN)2]2+
1.45
-1.36
-1.57
a
E1/2 (V)red
Electrode Potentials in Volts vs. Ag/AgNO3, corrected with the
ferrocene/ferrocinium couple. Solvent: Dried acetonitrile
51
30
5
25
0
20
15
-5
I
I
10
5
-10
0
-15
-5
-10
-20
-0.5
-1.0
-1.5
1.5
-2.0
1.0
0.5
E
E
(a)
(b)
Figure 2.8 Cyclic voltammogram for the [Ru(bpy)2(sn)2]2+, (a) reduction potential and
(b) oxidation potential
Chattopadhyay et al. studied the electrochemical behavior of [Ru(bpy)2(AN)2]2+ using
cyclic and differential pulse voltammetry36.
Their studies showed that the each bipyridine
ligands underwent one-electron reduction in an irreversible manner at -1.45 V and -1.6 V while
two oxidation processes were observed, one irreversible process at 1.23 V and a quasi-reversible
wave at +1.45 V.
The CV data of [Ru(bpy)2(AN)2]2+ in our lab showed two reversible bipyridine ligand
reductions at E1/2 = -1.36 and -1.57 V and a reversible oxidation of the [RuIII/RuII] couple at +1.45
V consistent with a previous report11. Compared to [Ru(bpy)2(AN)2]2+, the ligand reductions of
[Ru(bpy)2(sn)2]2+ occurred at similar potentials, but its oxidation was more positive by 0.09 V.
52
2.3.6 Electronic Spectra
Figure 2.9 shows an overlay of the experimental and calculated UV/vis absorption
spectrum of [Ru(bpy)2(sn)2]2+ in acetonitrile. UV/vis absorption spectra were weakly solvent
dependent with the low energy maximum shifting as follows: acetonitrile, 418 nm; butyronitrile,
418 nm; methanol, 422nm; i-propanol; 424 nm; DMF, 427 nm and DMSO, 427 nm. The
absorption coefficients of the transitions for the experimental spectrum obtained in acetonitrile
were determined from Beer’s Law studies using at least five dilution points and are listed in
Table 2.7. The probable assignments of the experimental bands were based on computational
assignments of the singlet excited states and related reports of similar type of complexes 37-39.
These assignments are listed in Table 2.8.
There is a close match between the electronic absorption of the [Ru(bpy) 2(sn)2]2+ with the
one for [Ru(bpy)2(AN)2]2+. Both have a MLCT transition located near 420 nm and a LC
transition near 288 nm. The absorption coefficients at ~ 410 nm differ with a slightly lower ε
value of 9,200 M-1 cm-1 for [Ru(bpy)2(sn)2]2+ compared to 10,400 M-1 cm-1 for [Ru(bpy)2(AN)2]2+.
Calculated singlet energy state transitions for [Ru(bpy) 2(sn)2](PF6)2 using TDDFT
calculations are tabulated in Table 2.8 and the calculated absorption spectrum is shown in Figure
2.10.
53
TABLE 2.7
EXPERIMENTAL ELECTRONIC TRANSITIONS AND CALCULATED EXCITED-STATES
OF [Ru(bpy)2(sn)2]2+ AND [Ru(AN)2]2+
Compound
Eexp, (λ , x 103 cm-1)a
ε (M-1 cm-1)
Ecalc, x 103 cm-1
Assignments
[Ru(bpy)2(sn)2]2+
244 nm (41.0)
23565
41.3
LC (π → π*)
287 nm (34.8)
58709
36.6
LC (π → π*)
418 nm (23.9)
10420
24.9
MLCT
242 nm
18900
41.5
LC (π → π*)
283 nm
58700
36.6
LC (π → π*)
420 nm
9200
24.4
MLCT
[Ru(bpy)2(AN)2]2+
a
Solvent: Acetonitrile
54
1.0
Rusn calculated
Rusn experimental
Absorbance
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
Wavelength (nm)
Figure 2.9 An overlay of the experimental and calculated UV/VIS absorption spectrum of
[Ru(bpy)2(sn)2]2+
The band found at 309 nm is a combination of contributions from the calculated 310 and
308 nm bands which have oscillator strengths of 0.066 and 0.043, respectively. The 310 and 308
nm bands consist of several transitions primarily MLCT in character, although one of the
contributors at 308 nm is a dd transition. The 276 nm band is composed of three components
which have oscillator strengths of 0.202 while the one at 243 nm contains only one component
which has oscillator strength of 0.069. These transitions mainly involve the bipyridine ligands
corresponding to intraligand π → π* transitions (ILCT) although one of the contributors at 276
nm is a dd transition. The succinonitrile ligands make little contribution to these transitions.
55
TABLE 2.8
CALCULATED SINGLET ENERGY STATE TRANSITIONS FOR [Ru(bpy)2(sn)2]2+
Wavelength
λ (nm)
Ψ0 → ΨE
405
H-2 → L
80
MLCT
Osc.
Strength
f
0.112
390
H-2 → L+1
70
MLCT
0.044
Ru(80) → bpy(100)
310
H → L+5
32
H-1 → L+3 15
H-2 → L+13 13
MLCT
MLCT
MMLCT
0.066
“
“
Ru(80) → bpy(100)
Ru(80) → bpy(100)
Ru(76) → Ru(54)
bpy(31)
sn(15)
308
H
MLCT
MLCT
dd
MMLCT
0.043
“
“
“
Ru(75) → bpy(100)
Ru(80) → bpy(100)
Ru(80) → Ru(70)
Ru(76) → Ru(54)
bpy(31)
sn(15)
→ L+5
H-1 → L+3
Contribution Type
(%)
14
27
21
11
H-1 → L+14
Nature of Transition
Ru(80) → bpy(100)
H-2 → L+13
276
H → L+14 22
H-1 → L+13 28
H-4 → L+1 4
dd
ILCT
ILCT
0.202
“
“
Ru(75) → Ru(70)
bpy(100) → bpy(100)
bpy(100) → bpy(100)
243
H-4 → L+2
ILCT
0.069
bpy(100)→bpy(98)
73
56
L
H-2
H
A
B
MLCT
Extinction Coefficient
MLCT
B
MLCT
50000
H-4
40000
C
D
B
L+2
H-4
A
10000
0
200
L+1
p ? p*
30000
20000
L+3
H-1
A
A == 402
402 nm
nm (3.08
(3.08 eV)
eV)
B
=
309
nm
(4.01
B = 309 nm (4.01 eV)
eV)
C
C == 273
273 nm
nm (4.54
(4.54 eV)
eV)
D
=
242
nm
(5.12
D = 242 nm (5.12 eV)
eV)
C
60000
L+5
D
250
300
350
400
450
500
LMLCT
Wavelength (nm)
Figure 2.10 Calculated UV/Visible spectrum and MO pictures involved in electronic transitions
in [Ru(bpy)2(sn)2](PF6)2
Experimentally, only three major bands were observed. The band at 244 nm (41.0 x 10 3
cm-1) was given an ILCT assignment. Calculations showed that the 244 nm transition observed
experimentally occurs upon exciting an electron from the HOMO-4 to LUMO+2 (Figure 2.10).
This ILCT transition is predominantly ligand-centered (LC) with some metal orbitals involved in
the LUMO. The ligand character is centered on the bipyridine ligands with minor contributions
from the succinonitrile ligand. The intense band observed experimentally at 287 nm (34.8 x 103
cm-1) is primarily ILCT derived from the calculated 273 nm bands with a contribution from the
calculated 309 nm band which appears in the simulated spectra as a shoulder on the ligand-
57
centered transition. The band located at 418 nm (23.9 x 10 3 cm-1) was assigned as a MLCT band
which occurs from the HOMO-2 to LUMO, HOMO to LUMO+5 and HOMO-1 to LUMO+3.
These HOMO’s are primarily metal based and the LUMO is ligand based. The majority of
HOMO-2 excited state electron density lies on the dz2 orbital.
1800000
1000000
1600000
1400000
800000
Intensity (cps)
Intensity (cps)
1200000
600000
400000
1000000
800000
600000
400000
200000
200000
0
0
-200000
500
600
700
250
Wavelength (nm)
300
350
400
450
500
Wavelength (nm)
(a)
(b)
Figure 2.11 (a) Low-temperature (77 K) emission spectrum of [Ru(bpy)2(sn)2]2+ at excitation
wavelength, λex = 418 nm. and (b) Excitation spectrum of [Ru(bpy)2(sn)2]2+ at
emission wavelength, λem = 537 nm.
2.3.7 Emission Spectra
The emission properties and excited state lifetimes (τem) of [Ru(bpy)2(sn)2]2+ were
determined at 77 K in freshly distilled butyronitrile. The values of the emission lifetimes were
determined by curve-fitting analysis. The low-temperature (77K) emission data are summarized
in Table 2.9 and the spectrum is shown in Figure 2.11a. The emission band of the complex had
its peak at 537 nm with vibronic peaks located at 576 nm and 625 nm, respectively. The
58
emission lifetime of the compound was 10 μs at 77 K. The excitation spectrum was determined
from the emission maximum located at 537 nm and is shown as an inset in Figure 2.11b. It
mirrors the MLCT assignments obtained by DFT and TDDFT calculations.
TABLE 2.9
LOW-TEMPERATURE (77K) EMISSION SPECTRAL DATA FOR [Ru(bpy)2(sn)2]2+
Compound
Eexp (λ , x 103 cm-1)
Ecalc
τem, μs
Φemission
[Ru(bpy)2(sn)2]2+
537 nm (18.62)
576 nm (17.36)
625 nm (16.00)
538 nm
579 nm
631 nm
21.3
10.0
0.0158
Na
9.9
0.0148
[Ru(bpy)2(AN)2]2+
DFT calculations of the triplet excited states relative to the ground state were performed.
Figure 2.4 shows the energy diagram of the first seven low-lying triplet states of the complex.
The energy difference between the singlet ground state ( 1G.S.) and the triplet metal-to-ligand
charge-transfer excited state (3MLCT) is about 2.63 eV (475 nm). The experimentally observed
maximum is at 538 nm, Figure 18a - a difference of ≈ 63 nm. The calculation most likely is
inaccurate since the role of solvent has not been taken into account. The 77 K lifetime of the
luminescence was observed at around 10 μs which is almost twice as that of Ru(bpy) 32+
counterpart 40-43. The emission quantum yield of the [Ru(bpy)2(sn)2](PF6)2 at 77 K was 0.0158.
59
2.3.8 Chemical Actinometry and Photolysis Studies
Photolysis of the [Ru(bpy) 2(sn)2](PF6)2 was carried-out at an excitation wavelength of 405
nm. This is a shoulder in the actual UV-Visible spectrum of the compound. This selection was
made because the quantum yield of the Fe2+ generated from the photolysis of the Fe(C2O4)3- at
this wavelength was reported and is useful in determining the absolute intensity of the incident
radiation25. This intensity can be calculated using the equation25 given below.
nB
I0 =
(2.2)
ФB x t
( 1 – 10 –A)
Here, the nB is the number of products generated after photolyzing the actinometer (in
this case, Fe2+), ФB is the quantum yield of photolysis of the actinometer (ferrioxalate ion) at
certain λex, t is the time of photolyte irradiation and A is the initial absorbance of the photolyte.
Once I0 was determined, the absolute quantum yields of photolysis Фp can now be evaluated
using the equation 3
25
where all the variables have the same assignments as previous and Ip
being the intensity of the radiation required in photolysis.
Ip
Фp =
(2.3)
I0 x ( 1 – 10 –A )
60
In this case, Ip is determined from the absorbance change ΔA of the photolyte at certain time t
using equation 4 25.
I0
ΔA = log
(2.4)
Ip
[Ru(bpy)2(sn)2](PF6)2 and [Ru(bpy)2(AN)2](PF6)2 were dissolved in methanol and DMF and
irradiated with monochromatic radiation at 405 nm.
Figure 19 shows the changes in the
UV/visible spectra of the complex as a function of time. In all cases, the MLCT band at 420 nm
1.0
0.5
Absorbance
Absorbance
1.0
0.5
0.0
300
400
500
600
0.0
300
Wavelength (nm)
400
500
600
Wavelength (nm)
(a)
(b)
Figure 2.12 UV/Visible spectrum of the MLCT band in [Ru(bpy)2(sn)2]2+ during photolysis for
t = 20 minutes in (a) Methanol, MeOH and (b) N,N-dimethyl formamide, DMF
61
in methanol and at 428 nm in DMF decreased in absorbance while a new band appeared at 450
nm in methanol and at 460 nm in DMF. Three isosbestic points located at 320, 365 and 434 nm
were found in methanol. In N,N-dimethyl formamide, three isosbestic points were observed
centered at 322, 375 and 443 nm. The quantum yields of photolysis Фp in the two solvents are
listed in Table 2.10. The value in methanol was 0.778 and in N,N-dimethyl formamide it was
0.829.
Shown in Figure 2.13 is the calculated stepwise change in the absorption spectrum of
[Ru(bpy)2(sn)2]2+ with incorporation of methanol into the coordination sphere with loss of
succinonitrile.
The calculated result suggests that the complexes undergo stepwise solvent
substitution of the nitrile ligands as given by equations 2.2 and 2.3. This is not discerned in
methanol as the solvent where the second reaction is faster than the first, but both steps are noted
in DMF (Figure 2.12 ).
TABLE 2.10
QUANTUM YIELDS OF PHOTOLYSIS IN [Ru(bpy)2(sn)2](PF6)2
Compound
Eex (λ)
Φphotolysis (solvent)
[Ru(bpy)2(sn)2]2+
405 nm
[Ru(bpy)2(AN)2]2+
405 nm
0.778 (methanol)
0.829 (DMF)
0.819 (methanol)
0.859 (DMF)
0.908 (methanol)
305 nm
I0 = 1.79 x 1011 quanta / s (Es)
62
[Ru(bpy)2(R-CN)2]2+
+
S
[Ru(bpy)2(R-CN)(S)]2+
+
S
S = solvent
S = solvent
[Ru(bpy)2(R-CN)(S)]2+
[Ru(bpy)2(S)2]2+
+
+
R-CN
R-CN
(2)
(3)
Scheme 2.1 Solvent photosubstitution in Ru(II) coordination sphere
Labilization of the coordination sphere is well documented for ruthenium(II) diimine
complexes in the literature and is due to thermal population of the 3dd by intersystem crossing
from the 3MLCT state.44,45 However, the magnitude of the effect is much greater for solvent
substitution in these bis-nitrile complexes than found for other systems. 1-9, 11, 13-19
Photolysis studies of [Ru(bpy)2(sn)2]2+ in methanol were further investigated using 1HNMR (Figure 2.14-2.15) spectroscopy to monitor the detachment of the succinonitrile ligands
from the coordination sphere both in the forward and reverse reaction. The mechanism occurs as
follows: first, in the presence of a photon, the complex undergoes mono-substitution of methanol
with one succinonitrile being displaced outside the coordination sphere. This is clearly shown by
the presence of an isosbestic point in the UV spectra during photolysis. Second, as one
succinonitrile gets displaced, the second succinonitrile undergoes fast photosubstitution with
methanol. This is supported by the appearance of the free succinonitrile peak as seen in the 1HNMR spectrum (Figure 2.14-2.15). Third, the reaction reverses back in the presence of heat
(45ºC) under dark conditions. Here one succinonitrile goes back inside the coordination sphere.
This mechanism is shown in the scheme 2.2 as follows:
63
MeOH
[Ru(bpy)2(sn)2]
→
2+
[Ru(bpy)2(MeOH)(sn)]2+ + sn
(slow)
(4)
[Ru(bpy)2(MeOH)2]2+ + sn
(fast)
(5)
[Ru(bpy)2(MeOH)(sn)]2+ + MeOH
(slow)
(6)
hυ
MeOH
[Ru(bpy)2(MeOH)(sn)]
→
2+
hυ
dark
[Ru(bpy)2(MeOH)2]2+ + sn
→
Δ
Scheme 2.2 Stepwise photosubstitution of MeOH in [Ru(bpy)2(sn)2]2+
Ru(bpy)2(sn)2
Ru(bpy)2(sn)(sol)
Ru(bpy)2(sol)2
Extinction Coefficient
60000
50000
402 272
428 273
454 275
black: [Ru(bpy)2(sn)2]2+;
red: [Ru(bpy)2(sn)(MeOH)]2+;
blue: [Ru(bpy)2(MeOH)2]2+
40000
30000
20000
10000
0
200
Figure 2.13 Calculated absorption
changes of [Ru(bpy)2(sn)2]2+ in
methanol.
300
400
500
Wavelength (nm)
64
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
3
Chemical Shift (ppm)
(a)
2000
1000
0
3
Chemical Shift (ppm)
(b)
Figure 2.14 1H-NMR spectra: a. Free Succinonitrile Ligand and b. Unphotolyzed
[Ru(bpy)2(sn)2](PF6)2 in CD3OD
65
2000
1800
1600
1400
1200
1000
800
600
400
200
0
3
Chemical Shift (ppm)
(a)
2000
1800
1600
1400
1200
1000
800
600
400
200
0
3
Chemical Shift (ppm)
(b)
2000
1800
1600
1400
1200
1000
800
600
400
200
0
3
Chemical Shift (ppm)
(c)
Figure 2.15 1H-NMR spectra of photolyzed [Ru(bpy)2(sn)2](PF6)2 in CD3OD at various
photolysis times: a. 30 mins, b. 1.5 hours and c. 2.5 hours
66
2.4 Summary
The electrochemical and photophysical properties of two bis-nitrilo ruthenium (II)
complexes formulated as [Ru(bpy)2(L)2](PF6)2, where bpy is 2,2’-bipyridine and L is AN =
CH3CN and sn = NC-CH2CH2-CN, have been investigated. Electrochemical data were typical of
Ru-bpy complexes with two reversible reduction peaks located near -1.3 V and -1.6V assigned to
each bipyridine ligand and one RuII/RuIII oxidation were centered at approximately +1.5 V. The
vibrational spectrum of the sn derivative were both IR and Raman active and agreed with
calculated results. It’s coordinated CN stretch appearing at 2277 cm-1 and 2273 cm-1,
respectively. The UV/Visible absorption spectrum of the sn derivative is dominated by an intense
(εmax ~ 58700 M-1 cm-1) absorption band at 287 nm assigned as a LC (π → π*) transition. The
peak observed at 418 nm (εmax ~ 10400 M-1 cm-1) is an MLCT band while the one at 244 nm
(εmax ~ 23600 M-1 cm-1) is of LMLCT character. The AN derivative behaved similarly. Both
complexes showed low-temperature emission at around 537 nm with a lifetime near 10.0 µs. 1H
and
13
C assignments were consistent with the formulation of the complexes. The complexes
underwent photosubstitution of solvent with quantum efficiencies near one. Calculated and
experimental (1H-NMR) results supported a stepwise replacement of the nitrile ligands. 1H-NMR
spectroscopy proved the removal of the nitrile groups from the coordination sphere. Based on
DFT calculations, the electron density of the HOMO lies on the metal center, the bipyridine
ligands and the nitrile ligands and electron density of the LUMO resides primarily on the
bipyridine ligands. The electronic spectra obtained from TDDFT calculations closely matched
the experimental ones.
67
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70
CHAPTER 3
DICYANO AND PYRIDINE DERIVATIVES OF RETINAL: SYNTHESIS, VIBRONIC,
ELECTRONIC AND PHOTOPHYSICAL PROPERTIES
3.1 Introduction
β-carotenes serve as antennas for absorbing light in spectral regions where chlorophylls
do not absorb; and thus initiate photosynthesis.1-8 According to group theoretical treatments
based on the C2h point group, the electronic ground state is 1Ag in character while the symmetryallowed transition found in the visible region which is responsible for pigment coloration is a 1Bu
type.1,9-22 Low-temperature (77K) emission studies have shown that another 1Ag excited state lies
between the ground state and the 1Bu excited state.13-15,
23-26
Triplet states have also been
previously investigated.27-28
In this study, we have synthesized two cyanine derivatives of all-trans-retinal, β-carotene
analogs (Scheme 3.1), as antennas for attachment to transition metal complexes using an acid
catalyzed approach. Crystal structures were determined for both compounds using single crystal
X-ray diffraction and their electronic and their photophysical properties have been characterized.
Density-functional theory (DFT) and Time Dependent Density Functional Theory (TDDFT)29
calculations have also been employed to ascertain their spectral properties with respect to those
determined experimentally.
71
CHO
All-trans-retinal (1)
CN
N
15’,15’-cyanopyridyl retinal (2)
CN
CN
15’,15’-dicyano-retinal (3)
Scheme 3.1 Compounds 2 and 3 derived from 1
3.2 Experimental Section
3.2.1 Materials
The compound all-trans-retinal (Vitamin-A aldehyde) was obtained from Sigma.
Malononitrile was obtained from Fluka while the compound 4-pyridyl acetonitrile
monohydrochloride was purchased from Acros Scientific. Butyronitrile used for emission and
72
lifetime studies was obtained from Acros Scientific; it was fractionally distilled prior to use.
Dried benzene, diethyl ether, HPLC grade methanol, chloroform, methylene chloride and optima
grade acetonitrile for UV/Visible spectral determination were purchased from Fisher Scientific.
Propionic acid and ammonium carbonate were purchased from Aldrich. IR grade potassium
bromide used as a pellet for the infrared spectra and deuterated dimethylsulfoxide (DMSO) for
NMR analysis were obtained from Aldrich. Ferrocene standard and dried acetonitrile (suresealed bottle) used as an electrochemical solvent were purchased from Aldrich. Sulfur powder
used as a standard to calibrate the FT-Raman Spectrometer was provided by Thermo Nicolet.
Electrochemical grade tetrabutylammonium perchlorate was purchased from Southwestern
Analytical.
3.2.2 Preparation of 15,15’-cyanopyridyl retinal, 2
All-trans-retinal (100.0 mg, 3.49 x 10-4 mol) was placed in a 10 mL round-bottom flask.
The compound was then dissolved in benzene and was allowed to stir for 5 minutes. After all
the solid dissolved, 4-pyridylacetonitrile monohydrochloride (58.3 mg, 5.30 x 10-4 mol) and
ammonium carbonate (98.6 mg, 1.03 mmol) were added to the solution and the reaction was
allowed to stir further for about 5 minutes. Then, 1.00 mL of propionic acid was added drop
wise into the solution. After all the reagents were added, the reaction was heated overnight with
its temperature regulated at approximately 70 ºC. The reaction was then allowed to cool in an
ice water bath and the solvent was removed by lyophilization.
The red-orange solid was
redissolved in chloroform and was subjected to column chromatography through an oven dried
neutral alumina (~10.0 g) stationary phase.
The mobile phase used was (7:1 chloroform:
methylene chloride). The first and final fractions were discarded while the middle fractions were
saved and examined for purity by TLC. The solids were recovered after combining the pure
73
fractions. These fractions were freeze-dried to remove the solvent. The orange compound was
dried in a vacuum oven and stored in a desiccator.
Color: brick-red Yield. 80%
Anal. Calcd. for C27H32N2: C, 84.31; H, 8.39; N, 7.29. Found. C, 84.05; H, 7.27; N, 7.27
IR (KBr pellet, cm-1): (CN) 2208 w, (-C=C-) 1542 sh, 3745 m, 2924 m, 1097 m, 1023 m.
Raman (Solid, cm-1): (CN) 2207 w, (-C=C-) 1534 sh, (C=C-H) 1211 sh.
13
C-NMR (CDCl3): δ ppm 150.7 (pyr), 140.7, 140.2, 137.5, 135.6, 131.5, 130.3, 129.6, 127.1,
119.6 (CN), 39.9, 34.5, 33.4, 29.2 (β-ring), 22.0, 19.4, 13.9.
3.2.3 Preparation of 15,15’-dicyano-retinal, 3
Compound 3 was prepared analogous to compound 2 except malononitrile (58.3 mg, 5.30
x 10-4 mol) was substituted for 4-pyridylacetonitrile monohydrochloride (58.3 mg, 5.30 x 10-4
mol). The dark-red compound was dried in a vacuum oven and stored in a desiccator.
Color: burgundy, 85%
Anal. Calcd. for C23H28N2: C, 83.07; H, 8.50; N; 8.43. Found. C, 82.98; H, 7.76; N, 8.45.
IR (KBr pellet, cm-1): (CN) 2219 m, (-C=C-) 1544 sh, 3746 w, 2926 w, 1244 w, 1188 w.
Raman (Solid, cm-1): (CN) 2215, (-C=C-) 1528, (C=C-H) 1171 sh.
3.2.4 Measurements
Infrared spectra were obtained using Perkin-Elmer Model 1600 FT-IR and Nicolet Avatar
model FT-IR spectrophotometers. All samples were prepared as potassium bromide pellets.
Raman spectra were obtained using a Thermo Nicolet Nexus FT-Raman Module
Spectrophotometer. The Nicolet instruments were accompanied by Omni software programs.
Proton (1H) and 13C-NMR spectra were obtained using Varian Mercury 300 MHz and Varian
Inova 400 MHz Fourier Transform-NMR spectrometers (internal standard TMS). Ultraviolet
74
spectra were obtained using a Hewlett-Packard Model 8452A diode array spectrophotometer
interfaced with an OLIS software program. Elemental (C,H,N) analysis was performed by
MHW Laboratories, Phoenix, AZ. An EG&G PAR model 263A potentiostat/galvanostat was
used to obtain cyclic voltammograms. All measurements were carried out in a typical Helectrochemical cell using a platinum disk working electrode (polished every run) and a platinum
wire counter electrode. A Ag/AgNO3 reference electrode in dried CH3CN was freshly made and
used for this study. The supporting electrolyte was 0.1 M tetrabutylammonium perchlorate.
Ferrocene was added as a reference. The sample preparation for emission studies involved
dissolving a small amount of sample in dried butyronitrile and then measuring the absorbance of
the solution. The concentration of the solution was altered in order to achieve an approximate
absorbance of ~0.10 at the λmax of the sample. The sample solution was placed in a quartz tube
and was degassed a minimum of five times using the freeze-pump-thaw method prior to actual
measurements. Emission quantum yields were calculated using equation 1, where φf is the
emission quantum yield, If is the emission intensity and A is the absorbance of the sample at the
(3.1)
excitation frequency.30
Low temperature (77K) emissions were measured using a Spex
Fluorolog 212 Spectrofluorometer equipped with a homemade sample holder connected to an Ar
gas tank for continuous deaeration.
75
3.2.5 Calculations
Calculations were effected using Gaussian ’03 (Rev. B.03)29 for UNIX. The molecules
were optimized using Becke's three-parameter hybrid functional B3LYP31 with the local term32 of
Lee, Yang, and Parr and the nonlocal term 33 of Vosko, Wilk, and Nassiar. The basis set 6311G** was chosen for all atoms and the geometry optimizations were all ran in the gas phase.
TDDFT34-40 calculations were employed to produce a number of singlet excited states41 in the gas
phase based on the optimized geometry. All oscillator values and singlet and triplet excited state
values are presented in the supporting information.
3.2.6 X-ray Crystallography
15,15’-cyanopyridyl retinal, 2; 15,15’-dicyano-retinal, 3. Crystals were affixed to a nylon
cryoloop using oil (Paratone-n, Exxon) and mounted in the cold stream of a Bruker Kappa-Apex
II area-detector diffractometer. The temperature at the crystals was maintained at 150 K using a
Cryostream 700EX Cooler (Oxford Cryosystems). A scan with 0.3 degrees and time of 10
seconds was employed along with graphite mono-chromated Molybdenum Kα radiation (λ =
0.71073 Å) that was collimated to a 0.6 mm diameter. Data collection, reduction, structure
solution, and refinement were performed using the Bruker Apex2 suite (v2.0-2).
The unit cell of 2 was determined from the setting angles of 50 reflections collected in 36
frames of data. Data were measured with a redundancy of 7 using a CCD detector at a distance
of 50 mm from the crystal with a combination of phi and omega scans. All available reflections
to 2θmax = 54° were harvested (37745 reflections, 5276 unique) and corrected for Lorentz and
polarization factors with Bruker SAINT (v6.45). Reflections were then corrected for absorption
(numerical correction, μ = 0.064 mm-1), interframe scaling, and other systematic errors with
76
SADABS 2004/1 (combined transmission and other correction factors min./max. =
0.9731/0.9911).
The unit cell of 3 was determined from the setting angles of 74 reflections collected in 36
frames of data. Data were measured with a redundancy of 7.8 using a CCD detector at a distance
of 50 mm. from the crystal with combination of phi and omega scans. All available reflections
to 2θmax = 54° were harvested (37913 reflections, 4782 unique) and corrected for Lorentz and
polarization factors with Bruker SAINT (v6.45). Reflections were then corrected for absorption
(numerical correction, μ = 0.062 mm-1), interframe scaling and other systematic errors with
SADABS 2004/1 (combined transmission and other correction factors min./max. =
0.9796/0.9915).
The structures were solved (direct methods) and refined (full-matrix least squares against
F2) with the Bruker SHELXTL package (v6.14-1). All non-hydrogen atoms were refined using
anisotropic thermal parameters.
All hydrogen atoms were included at idealized positions;
hydrogen atoms were not refined. Pertinent crystal, data collection and refinement parameters
are given in Table 3.1.
77
TABLE 3.1
CRYSTAL DATA AND STRUCTURE REFINEMENT FOR (2) AND (3)
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
Unit cell dimensions
Volume
Z
Calculated density
Absorption coefficient
F(000)
Crystal size
Crystal habit
Crystal color
θ range for data collection
Limiting indices
Reflections collected / unique
Completeness to θ = 25.00°
Refinement method
Data / restraints / parameters
Refinement threshold
Data > threshold
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
R indices (all data)
Largest diff. peak and hole
C27H32N2 (2)
384.55
150 K
0.71073 Ǻ
Triclinic
P-1
a = 6.8351(7) Å
b = 7.4409(7) Å
c = 24.397(2) Å
α = 89.250(6)°
β = 85.685(6)°
γ = 69.736(5)°
1160.61(19) Å3
2
1.100 g/cm3
0.064 mm-1
416
0.43 x 0.32x 0.14 mm
Block
Dark Red
1.67 o to 25.00o
-8≤ h ≤ 7
-8 ≤ k ≤ 8
-28 ≤ l ≤ 28
33891 / 4069 [R(int) = 0.0862]
99.9 %
Full-matrix least-squares on F2
4069 / 0 / 268
I>2σ(I)]
2417
1.022
R1 = 0.0627, wR2 = 0.1455
R1 = 0.1182, wR2 = 0.1774
0.471 and -0.257 e- Ǻ-3
78
C23H28N2 (3)
332.47
150 K
0.71073 Ǻ
Monoclinic
P21/n
a = 10.2960(7) Å
b = 11.5493(7) Å
c = 17.6364(12) Å
α = 90°
β = 100.126(3)°
γ = 90°
2064.5(2) Å3
4
1.070 g/cm3
0.062 mm-1
720
0.39 x 0.31x 0.22 mm
Block
Dark Red
2.12o to 27.11o
-13≤ h ≤ 13
-14 ≤ k ≤ 14
-22 ≤ l ≤ 22
36637 / 4549 [R(int) = 0.1076]
99.6 %
Full-matrix least-squares on F2
4549 / 0 / 231
I>2σ(I)]
2506
1.048
R1 = 0.0681, wR2 = 0.1570
R1 = 0.1353, wR2 = 0.1968
0.468 and -0.172 e- Ǻ-3
3.3 Results and Discussion
3.3.1 Synthesis
The preparation of 2 and 3 followed the Knoevenagel reaction42-43 which involved the
condensation of the aldehyde, retinal, with substrates lacking α-hydrogen atoms adjacent to the
coupling site. This criterion is required to enhance the acidity of the methylene (-CH2-) protons.
CHO
(1)
CN
a.
a. CN-CH2-CN
N
b.
b. (NH4)2CO3
(NH4)2CO3
H
H+
+
CN
CN
CN
(2)
N
(3)
Figure 3.1 Knoevenagel condensation of all-trans-retinal with (2) 4-pyridylacetonitrile
monohydrate and (3) malononitrile
Traditionally, this condensation reaction is catalyzed using only a weak base such as
piperidine or NaNH2.42 In our case, we used simultaneous base and acid catalyzed condensation
reactions to enhance the yield. The base catalyst used was (NH4)2CO3 while the acid catalyst
chosen was propionic acid (CH3CH2COOH). The reaction scheme is shown in Figure 3.1 The
mechanism of formation of the pyridyl adduct is presented in Figure 3.2 which can be extended
to the dicyano derivative. In this reaction, the base catalyst was added first to the reaction
79
mixture to neutralize hydrochloride and to deprotonated one of the methylene (-CH 2-) protons.
Then, the weak acid-catalyst was added to protonate the carbonyl oxygen generating the
carbonyl carbocation necessary to initiate the condensation reaction.
H
CN
CO3-2
N
H+
H+
OH
O
H
H
CN
(1)
N
OH2+
CN
H
N
H2O
CN
(2)
N
Figure 3.2 Mechanism for the acid & base catalyzed Knoevenagel condensation of
all-trans-retinal with 4-pyridylacetonitrile monohydrate
80
3.3.2 X-ray Results
The crystal structures of 2 and 3 were determined. Selected bond lengths, bond angles
and torsion angles were tabulated in Table 11 and ORTEP diagrams together with the crystal
packings are shown in Figures 3.3 and 3.4. The space-group in 2 is P-1 while that in 3 is P21/n.
The retinal-pyridine adduct, in its crystal form, retained its all-trans configuration as can
be seen clearly in Figure 3.3. The experimental bond distance for the C≡N group was 1.14 Å.
For the polyene chain, localized C-C single bonds in both cases averaged 1.44 Å while double CC bonds averaged 1.36 Å. The new C-C double bond that resulted between the methylene (CH2-) and carbonyl (CO) carbon after condensation gave an average bond distance of 1.36 Å but
the bond angles at the condensation point were not the same. The C(22)-C(16)-C(17) angle was
117.9º for 2 while the C(17)-C(18)-C(22) angle of 3 was 121.1º. For 2 the C(14)-C(15)-C(16)C(17) torsion angle was 177.8º while the C(21)-C(17)-C(16)-C(22) angle was 11.9º. For 3, the
C(16)-C(17)-C(18)-C(22) torsion angle was 178.0º. The optimized structure of 2 and 3 were
also calculated using density functional theory (DFT) and the corresponding theoretical bond
lengths and angles were correlated with the experimental data. Results showed good correlation
with alternating single-double bonds present in the polyene chain. Both the actual crystal and
DFT optimized structures showed gradual bending in the polyene chain with the methyl groups
pointed away from the curvature.
From Figures 3.3 and 3.4, one also notes the lowering in symmetry of the polyene
derivatives from C2h suggested for β-carotenes to possibly Cs due to substituent effects and to
twisting of the polyene chain.
81
TABLE 3.2
SELECTED BOND LENGTHS (Å) AND ANGLES (deg) FOR 2 AND 3
Atoms 2
Exp.
Calcd.
Atoms 3
Exp.
Calcd.
C(22)-N(2) (nitrile)
C(16)-C(22)
1.142
1.442
1.155
1.430
C(22)-N(1) (nitrile)
C(22)-N(2) (nitrile)
1.143
1.142
1.154
1.154
C(19)-N(1) (pyridyl)
C(20)-N(1) (pyridyl)
1.329
1.324
1.334
1.335
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
C(9)-C(10)
C(10)-C(11)
C(11)-C(12)
C(12)-C(13)
C(13)-C(14)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
1.479
1.328
1.458
1.351
1.440
1.346
1.443
1.357
1.424
1.355
1.481
1.549
1.342
1.457
1.361
1.432
1.357
1.442
1.368
1.427
1.366
1.481
C(5)-C(9)
C(9)-C(10)
C(10)-C(11)
C(11)-C(12)
C(12)-C(13)
C(13)-C(14)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
C(17)-C(18)
C(18)-C(22)
C(18)-C(23)
1.469
1.343
1.451
1.364
1.423
1.353
1.428
1.386
1.395
1.370
1.425
1.430
1.471
1.349
1.449
1.365
1.428
1.360
1.437
1.372
1.416
1.371
1.426
1.427
C(22)-C(16)-C(17)
C(22)-C(16)-C(15)
117.9
116.1
116.0
117.2
C(17)-C(18)-C(22)
C(17)-C(18)-C(23)
121.1
122.9
120.7
122.0
N(1)-C(22)-C(18)
N(2)-C(23)-C(18)
176.3
178.4
179.2
178.7
C(16)-C(17)-C(18)C(22)
178.0
178.6
C(14)-C(15)-C(16)C(17)
C(21)-C(17)-C(16)C(22)
177.8
11.9
173.7
82
(a)
(b)
Figure 3.3 Crystal Structure in 2: a. ORTEP diagram, b. Crystal Packing, space-group P21/n.
83
(a)
(b)
Figure 3.4 Crystal Structure in 3: a. ORTEP diagram, b. Crystal Packing, space group P-1
84
3.3.3 Calculations (DFT/TDDFT)
The HOMO and LUMO orbitals determined by DFT calculations are shown in Figure
3.5; lower energy HOMO and higher energy LUMO orbitals are shown in Figure 3.6.
HOMO
LUMO
2
2
Figure 3.5
3
3
HOMO/LUMO pictures in 2 and 3 determined
by DFT
85
HOMO – 1 (2)
HOMO – 1 (3)
HOMO – 3 (2)
HOMO – 2 (3)
LUMO + 1 (2)
LUMO + 1 (3)
LUMO + 3 (2)
Figure 3.6 Orbital pictures of selected states involved in optical transition in 2 and 3
86
3.3.4 Population Analysis:
Table 3.3 lists the population analysis data and the percentage electronic distribution of
some selected states in 2 and 3. The HOMO is predominantly polyene in character with 87%
occupancy in 2 and 67% in 3. The most remarkable difference is that the HOMO of 3 also
showed some important contributions from the β-ring (19% occupancy) and the CN group (15%
occupancy). Although, the LUMO character lies on the chain of 2 and 3, the electron density of
the LUMO in 2 is distributed throughout the molecule with a 10% occupancy coming out from
the pyridyl group.
The pyridyl group in 2 plays an important role in hosting the excited electron. In some
cases, the methyl groups of the polyene chain have minor contributions in one of the states such
as that observed in H – 1. In the latter case, a rather different π-bonding interaction was
observed with a nodal plane arising every third atom of the polyene chain. In H – 3 the electron
density is localized only on the pyridyl ring with no contributions coming from the polyene
chain.
The electronic distributions of the orbitals in 3 are much more intriguing than the pyridyl
derivative. The electron density in H – 1 state is almost divided equally into three segments
throughout the molecule. One segment is localized in the β-ring region; another segment is
located in the middle of the polyene chain, while the last one is concentrated in the dicyano
group. H – 2, on the other hand, contains “banana-shaped” electron density lobes extended
throughout the molecule. It is also important to note that in all transitions, the dicyano groups
are involved, particularly in the L + 1 state.
87
TABLE 3.3
ELECTRON DISTRIBUTION (%) IN 2 AND 3
States:
LUMO + 1
LUMO
HOMO
HOMO – 1
HOMO – 2
2
60 (chain)
2 (β-ring)
1 (CN)
37 (pyridyl)
82 (chain)
1 (β-ring)
7 (CN)
10 (pyridyl)
87 (chain)
3 (β-ring)
5 (CN)
5 (pyridyl)
77 (chain)
5 (β-ring)
6 (CN)
11 (pyridyl)
1 (chain)
0 (β-ring)
0 (CN)
99 (pyridyl)
3
77 (chain)
12 (β-ring)
11 (CN)
72 (chain)
3 (β-ring)
25 (CN)
67 (chain)
19 (β-ring)
15 (CN)
35 (chain)
49 (β-ring)
15 (CN)
57 (chain)
24 (β-ring)
19 (CN)
Figure 3.7 shows the molecular orbital energy diagrams of 2 and 3. The HOMO-LUMO
gaps are consistent with the observed transition energy as discussed in the electronic absorption
section (see Table 3.6).
88
10000
L+4 (chain)
0
L+3 (pyr)
Wavenumbers
-10000
L+2 (pyr)
L+1 (chain)
-20000
LUMO (chain)
-30000
-40000
HOMO (chain)
-50000
HOMO-1 (chain)
-60000
HOMO-2 (pyr)
HOMO-3 (pyr)
HOMO-4 (chain)
-70000
2
10000
L+4 (chain)
0
-10000
L+3 (CN)
L+2 (chain)
L+1 (chain)
Wavenumbers
-20000
LUMO (chain)
-30000
-40000
HOMO (chain)
-50000
HOMO-1 (β -ring)
HOMO-2 (chain)
-60000
HOMO-3 (β -ring)
HOMO-4 (β -ring)
-70000
3
Figure 3.7 Molecular Orbital energy diagrams of 2 and 3
89
3.3.5 Vibrational Properties
The Infrared and Raman Spectroscopy data are given in Table 3.4 while the Raman
spectra of 2 and 3 are shown in Figure 3.8. Although the intensity of the Raman scattering in 2 is
a lot weaker than that of 3, it is evident that the polyene mode of vibration in all cases remains IR
and Raman active. The main points of interest in structural elucidation are the nitrile and polyene
groups before and after condensation. As shown in Table 3.4, the carbonyl stretching frequencies
of all-trans-retinal appeared at around 1655 cm-1 (IR & Raman). The IR frequency of the C≡N
vibration for malononitrile was located at 2273 cm-1 while it’s Raman frequencies were found at
2295 cm-1 (sym) and 2265 cm-1 (asym). After condensation, the carbonyl peak of retinal
disappeared and peaks for the C≡N vibration appeared. The IR C≡N frequency for 2 was located
at 2208 cm-1 in close agreement with its Raman frequency at 2207 cm-1. For 3, the related
frequencies were 2219 cm-1 in the IR and 2215 cm-1 in the Raman. It is notable that the CN
stretching frequency in the pyridyl adduct is weaker than the malononitrile counterpart. This
behavior may be due to the effect of pyridine ring that allows the CN to vibrate freely in the
conjugation. Retention of the polyene groups after the condensation was best monitored using
Raman spectroscopy. These polyene group frequencies are also listed in Table 4. The frequency
of alternating carbon-carbon double bond backbone in 1 was observed at 1574 cm-1 in the IR and
at 1568 cm-1 in the Raman. For 2, the frequency of the polyene backbone was observed at 1542
cm-1 in the IR and at 1534 cm-1 in the Raman; while for 3 it was found at 1544 cm-1 in the IR and
at 1528 cm-1 in the Raman.
90
Alkene protons in conjugated systems are known to be Raman active modes. These
proton frequencies of the methyl groups and the polyene backbone protons are also listed in
Table 3.4.
1.0
Relative Raman Intensity
Relative Raman Intensity
1.0
0.8
0.6
0.4
0.8
0.6
0.4
0.2
0.2
0.0
0.0
2400
2200
2000
1800
1600
1400
2400
1200
2200
2000
1800
1600
-1
-1
Raman Shift (cm )
3
Raman Shift (cm )
2
Figure 3.8 Raman spectra of 2 and 3
91
1400
1200
TABLE 3.4
VIBRATIONAL STRETCHING FREQUENCIES (cm-1) OF CN AND POLYENE GROUPS
Compound:
Functional Grp.
C=O stretch
Experimental:
(Infrared)
1656
Experimental:
(Raman)
1654
1
-C=C- skeletal
1574
1568
=C-H in-plane
bending
2
1268
CH3 stretch
2934
CN stretch
2208
2207
-C=C- skeletal
1542
1534
=C-H in-plane
bending
3
1211
CH3 stretch
2924
CN stretch
2219
2215
-C=C- skeletal
1544
1528
=C-H in-plane
bending
1171
CH3 stretch
2925
92
3.3.6 Cyclic Voltammetry
Oxidations for 1, 2 and 3 were examined and the data are summarized in Table 3.5.
Figure 3.9 shows the cyclic voltammograms of 2 and 3. Electrochemical results showed
irreversible oxidation waves for all three which is typical in most retinoids and carotenoids44-46
supporting their antioxidant properties; no reversible reduction was observed. For 2, the
irreversible peak was observed at 1.08 V while for 3 it was observed at a higher potential of 1.28
V. Both were shifted to a lower electrode potential with respect to all-trans-retinal (1.40 V). The
much larger shift for the 2 derivative may be related to additional π conjugation with the pyridine
ring.
TABLE 3.5
OXIDATION ELECTRODE POTENTIALS (V) OF RETINOIDSa
Compound:
All-trans-retinal
2
3
E (V)
1.40
1.08
1.28
a. O.1 M TBAClO4 in acetonitrile
0.5
0
0.0
-5
-0.5
-10
-1.5
I
I
-1.0
-15
-2.0
-2.5
-20
-3.0
-25
-3.5
1400
1200
1000
800
600
400
200
0
1600
E (mV)
1400
1200
1000
800
E (mV)
3
2
Figure 3.9 Cyclic voltammogram of 2 and 3
93
600
400
200
0
Some naturally occurring carotenoids (i.e. β-carotene) undergo cation radical formation.
The irreversible oxidation observed in the CV of 2 resembles that of β-carotene.45 It is possible
for the compound to undergo a two-electron oxidation reaction at the electrode surface to
generate a dication intermediate. This EC mechanism is well-established in most carotenoids. In
the case of the dicyano adduct, another irreversible peak appears after the initial one. This
behavior has been observed in naturally occurring compounds such as β-apo-8’-carotenal
wherein a two or three electron-transfer processes are involved.46
3.3.7 Electronic Spectra
The UV/Visible spectral data of the compounds are given in Table 3.6 and shown in
Figure 3.10. Relative to 1, compounds 2 and 3 showed bathochromic shifts. For 2, λmax was
shifted by 64 nm to 446 nm; for 3 it was shifted by about 82 nm to 464 nm. This bathochromic
shift was due to the lengthening of the polyene chain (1-D box). After the condensation, a new
double bond was generated between the carbonyl carbon of the retinal and the methylene groups
of pyridine-CN and malononitrile. Simulated UV/Visible absorption spectra of 1, 2 and 3 were
determined using TDDFT calculations and are overlaid with the actual electronic spectra (Figure
3.10). Results showed good correlation between experimental and calculated spectra. In the
case of the 2, the calculated spectrum underwent a hypsochromic shift from 446 to 441 nm in its
lowest energy π → π* transition while 3 underwent a bathochromic shift from 464 to 487 nm.
Calculations revealed that the lowest energy transition occurred from the HOMO to LUMO in
both 2 and 3 with an energy gap of 441 nm for the 2 and 489 nm for 3. Experimentally, two
other major bands were observed in the spectra of both 2 and 3. For 2, a weak absorption band
was also observed near 290 nm while a similar band was observed in the spectrum of 3 centered
94
at 285 nm. These bands (Figure 3.10) correlated with the calculated 299 and 284 nm bands,
respectively.
TABLE 3.6
UV/VISIBLE DATA OF S0 1Ag → S2 1Bu TRANSITION IN (1), (2) AND (3)
Compound:
E (λmax)
(Experimental)
ε
(M-1 cm-1)
E (λmax)
(Calculated)
1
382
44000
408
2
446
37673
442
3
464
86800
487
1.0
1.0
0.8
Absorbance
Absorbance
0.8
0.6
0.4
0.6
0.4
0.2
0.2
0.0
-0.2
300
400
500
0.0
600
300
Wavelength (nm)
400
Wavelength (nm)
2
3
Figure 3.10 Experimental and calculated UV/Visible data for 2 and 3
95
500
600
As expected, the polyene chain is the major contributor to the lowest-energy transitions.
The HOMO of 2 (Figure 3.5) shows the electron density as located on the polyene chain while
the LUMO shows some electron density has shifted to the pyridyl side increasing the conjugative
(π→ π*) behavior between the polyene and the pyridyl group. The HOMO of 3 is spread out
over the polyene chain, the dicyano group and the β-ring. Some p-orbital contribution in the case
of methylene carbon is also observed. The LUMO on the other hand, had a more distorted
conjugated π-bonding system with its electron density shifted more towards the dicyano group.
As shown in Figure 3.10, the simulated UV/Visible spectra obtained using TD-DFT
calculations gave rise to four distinct theoretical bands in 2 while three bands were calculated for
3. Table 3.7 lists selected transitions, their corresponding oscillator strengths and the major
molecular orbitals that contribute in each transition.
96
TABLE 3.7
CALCULATED UV/VISIBLE DATAFOR 2 AND 3
2
3
λ (nm)
442
F
1.8130
Major Contributors
HOMO → LUMO
λ (nm)
487
f
1.4752
Major Contributors
HOMO → LUMO
337
0.2685
H - 1 → LUMO
HOMO → L + 1
374
0.6686
H - 1 → LUMO
327
0.0014
H - 2 → LUMO
330
0.0547
H - 2 → LUMO
HOMO → L + 1
305
0.0069
H - 3 → LUMO
284
0.1392
HOMO → L + 1
H - 2 → LUMO
H-1 →L+1
299
0.2312
HOMO → L + 1
HOMO → L + 3
H - 1 → LUMO
265
0.0058
H - 3 → LUMO
H-1 →L+1
279
0.0029
HOMO → L + 2
HOMO → L + 3
259
0.0184
H-1 →L+1
HOMO → L + 2
H - 3 → LUMO
265
0.1569
H - 4 → LUMO
H - 6 → LUMO
HOMO → L + 3
255
0.0005
H-4 → LUMO
HOMO → L + 2
The HOMO to LUMO transition of 2 has high oscillator strength in agreement with its
high absorption coefficient and is the predominant peak calculated in the UV-Visible spectrum
with its maximum centered at 442 nm. The other calculated peaks merge pair-wise into the
peaks shown in Figure 3.10. For example, the peaks at 305 nm and 299 nm appear as one peak.
The calculated peak at 299 nm gave oscillator strength of 0.23. Under this band lie the following
transitions: HOMO → L + 1, HOMO → L + 3, and H – 1 → LUMO.
97
The calculated UV/Visible data of 3 are also listed in Table 3.7. The HOMO – LUMO
transition dominates with high oscillator strength 1.5 times greater than the others. Two peaks
also are observed located near the other calculated bands of high oscillator strength. The
theoretical peak observed at around 284 nm has oscillator strength of about 0.14 and involves the
following electronic transitions: HOMO → L + 1, H – 2 → LUMO and H – 1 → L + 1.
Group theory has been used in recent years to generate symmetry adapted orbital labels
of naturally occurring retinoids and carotenoid. In general, the method uses A g and Bu symmetry
orbital labels deduced when symmetry operations in C2h point group was applied to β-carotene
rather than treating it with π (π*) labels. Previous studies also showed that this approach can be
extended to non-symmetric polyene chains.1,9-22,47-52
The electronic energy level diagram proposed for
β-carotenes based on C2h symmetry is shown in Figure
Internal Conversion
1
3.11. Direct absorption occurs from the S0 (1 1Ag) → S2
(1 1Bu)2 state. Vibrational relaxation and internal energy
Bu
conversion occurs to the (2 1Ag) state from which
Direct Absorption
emission takes place to the (1
2 1Ag
1
Ag).
Previous
calculations (semi-empirical) have shown that the 1Ag →
1
Bu transition in polyenes have high molar extinction
Fluorescence Emission
coefficients.2 The HOMO → LUMO transitions of 2
1 1Ag
Fig. 3.11 Energy Diagram for 2 and 3
and 3 located at 442 nm (f = 1.8)and 487 nm (f = 1.5) ,
respectively, correspond to this criterion.
98
3.3. 8 Emission Studies
The emission properties of 2 and 3 were collected at 77 K; room temperature emission
for both was weak. Population of this first excited (2 1Ag) state at 77 K by internal conversion
resulted in an emission band at lower energy (λem for 2 = 583 nm and λem for 3 = 500 nm) as
shown in Figure 3.12. One interesting feature valid for both compounds is that more intense
emission occurred upon excitation at higher wavelengths (λex = 290 nm in 2 and λex = 280 nm in
3) rather than at the highly intense visible band. This data can be compared to the emission
behavior of all-trans retinal determined in our laboratories. Excitation at 380 nm resulted in
emission at 570 nm. The emission intensity increased 50-fold at 77 K. The quantum yields of
emission for all-trans-retinal determined in the lab using equation 1 at room temperature and 77
K are 0.001096 and 0.04465, respectively.
The quantum yield compares favorably to 2
(0.005465) and 3 (0.005371) at 77K.
1.0
1.0
Relative Intensity
Relative Intensity
0.8
0.8
0.6
0.4
0.6
0.4
0.2
0.2
0.0
0.0
550
600
650
300
Wavenumber (nm)
350
400
450
500
550
600
650
Wavelength (nm)
(b)
(a)
Figure 3.12 (a) Low-temperature (77 K) emission spectra of 2 and (b) Low-temperature (77 K)
excitation (___) and emission (___) spectrum in 3
99
The excitation spectra peaked at 355 nm for 1, 445 nm in 2 and 410 nm for 3. However,
according to the Franc-Condon Principle, excitation occurs vertically to higher vibronic states of
the 1Bu state followed by internal conversion to the 2 1Ag excited state which then leads to
vibrational relaxation (IC) followed by fluorescence emission back to the ground (1 1Ag) state.
The excitation at higher energy (290 and 280 nm) compared to their lowest-energy absorption is
attributed as its ‘cis-peak’ absorption band. This have been shown in the photochemistry of βcarotene and were extended to its apo-derivatives.1-2, 9 This approach is useful in describing the
emission data obtained above. Here, the ‘cis-peak’ is described as the forbidden trans- to cisisomerization (S3 1Ag+ ← S0 1Ag-) absorption peak that lies perpendicular to the highly allowed (S2
1 1Bu ← S0 1 1Ag) transition.2
Emission lifetimes in all three cases were more rapid than 5 ns
which is the pulse width of our nanosecond laser.
100
3.4 Summary
Two new derivatives of all-trans retinal, one containing a pyridine and a cyano
substituents (15,15’-cyanopyridyl retinal, 2) and the other containing two cyano substituents
(15,15’-dicyano-retinal, 3) were synthesized by an acid-catalyzed Knoevenagel condensation
reaction. X-ray diffraction data showed that compound 2 crystallized in the space group P-1
while compound 3 crystallized in the space group P21/n. The polyene chains were bowed. The
compounds exhibited CN vibrations in the infra-red and Raman regions. CV data showed that
both compounds undergo irreversible oxidations. Compounds 2 and 3 displayed electronic
absorptions with maxima located at 442 nm and 487 nm, and emission peaks located at 583 nm
and 500 nm, respectively. Density Functional Theory and Time Dependent Density Functional
Theory calculations reveal that the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) were located primarily on the polyene chain and the
energy gap between them was consistent with the observed optical spectrum. All optical data
were interpreted based on group theoretical approach with the lowest energy absorption in the
visible region occurring from the ground state 1 1Ag → 1 1Bu excited state while its lowtemperature (77K) fluorescence luminescence originated from the 2 1Ag state to its 1 1Ag ground
state.
101
3.5 References
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Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Hitao, O.; Nakai, H.; Klene, M.; Li, X.;
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R.; Stratmann, R.E.; Yazyev, O.; Austin, A.J.; Cammi, R.; Pommelli, C.; Ochterski, J.W.;
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Dapprich, S.; Daniels, A.D.; Strain, M.C.; Farkas, O.; Malick, D.K.; Rabuck, A.D.;
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104
CHAPTER 4
LONG-CHAINED DICYANO AND PYRIDINE DERIVATIVES: SYNTHESIS,
VIBRONIC, ELECTROCHEMICAL AND ELECTRONIC PROPERTIES
4.1 Introduction
β-carotenes serve as antennas for absorbing light in spectral regions where chlorophylls
do not absorb; and thus initiate photosynthesis.1-9 Group theoretical treatments based on the C2h
point group reveals that the electronic ground state is 1Ag in character while the symmetryallowed transition found in the visible region, responsible for pigment coloration in plant leaf
tissues and light-harvesting properties in photosystems, is due to the population of 1Bu-state.1-2, 9-25
Low-temperature (77K) emission supported studies which have shown that a second (2 1Ag)
excited state lies between the ground state and the 1Bu excited state.16-18, 26-29 Triplet states have
also been previously investigated by flash photolysis and pulse radiolysis and are known to be
responsible in photo protection in plants by quenching triplet chlorophyll.30-31
In this study, we have investigated the chemistry of two cyanine derivatives of β-apo-8’carotenal, as antennas for attachment to transition metal complexes using an acid-catalyzed
approach. Their photophysical properties have been characterized. Density Functional Theory
(DFT) and Time Dependent Density Functional Theory (TD-DFT)32 calculations have also been
employed to ascertain their spectral properties with respect to those determined experimentally.
Scheme 4.1 shows the structures of the β-carotene analogs.
105
O
H
β-apo-8’-carotenal (4)
CN
N
all-trans-7’-cyano-7’-pyridyl-7’-apo-β-carotenal (5)
CN
CN
all-trans-7’,7’-dicyano-7’-apo-β-carotenal (6)
Scheme 4.1 Chemical structure of naturally occurring β-apo-8’-carotenal (4) and its
derivatives
106
4.2 Experimental Section
4.2.1 Materials
The compound all-trans-β-apo-8’-carotenal (4) was obtained from Sigma. Malononitrile
was obtained from Fluka while the compound 4-pyridyl acetonitrile monohydrochloride was
purchased from Acros Scientific. Propionic acid was purchased from Aldrich. Dried benzene was
purchased from Fisher Scientific and was used as received. Ammonium carbonate was purchased
from Aldrich. Diethyl ether, HPLC grade methanol, chloroform, methylene chloride and optima
grade acetonitrile for UV/Visible spectral determination were purchased from Fisher Scientific.
IR grade potassium bromide salt used as a pellet for the infrared spectra and deuterated
dimethylsulfoxide (DMSO) for NMR analysis were obtained from Aldrich. Sulfur powder used
as standard to calibrate the FT-Raman Spectrometer was provided by Thermo Nicolet.
Electrochemical grade tetrabutyl ammonium perchlorate was purchased from Southwestern
Analytical. Ferrocene standard and dried acetonitrile used as an electrochemical solvent in a seal
tight bottle were purchased from Aldrich. Butyronitrile used for emission and lifetime studies
was obtained from Acros Scientific; it was fractionally redistilled prior to usage.
4.2.2 Measurements
Infrared spectra were obtained using Perkin-Elmer Model 1600 FT-IR and Nicolet Avatar
model FT-IR Spectrophotometers. All samples were prepared as potassium bromide pellets.
Raman spectra were obtained using a Thermo Nicolet FT-Raman Module Spectrophotometer.
The Nicolet instruments were accompanied by Omni software programs. Proton (1H) and 13CNMR spectra were obtained using Varian Mercury 300 MHz and Varian Inova 400 MHz Fourier
Transform-NMR spectrometers (internal standard TMS).
Ultraviolet spectra were obtained
using a Hewlett-Packard Model 8452A diode array spectrophotometer interfaced with an OLIS
107
software program. Elemental (C,H,N) analysis was performed by MHW Laboratories. An
EG&G PAR model 263A potentiostat/galvanostat was used to obtain the cyclic voltammograms.
All measurements were carried out in a typical H-electrochemical cell using a platinum disk
working electrode, polished every run, and a platinum wire counter electrode. A Ag/AgNO 3
reference electrode in dried CH3CN was freshly made and used for this study. The supporting
electrolyte was 0.1 M tetrabutylammonium perchlorate. Ferrocene was added as a reference.
The sample preparation for emission studies involved dissolving a small amount of sample in
dried butyronitrile and then measuring the absorbance of the solution. The concentration of the
solution was altered in order to achieve an approximated absorbance of 0.10 at the λ max of the
sample.
The sample solution was placed in a fluorescent quartz tube and was degassed
(minimum of five times) prior to actual measurements.
Emission quantum yields were
calculated using equation 1, where φf is the emission quantum yield, If is the emission intensity
and A is the absorbance of the sample at the excitation frequency. 33 All samples were degassed a
minimum of five times prior to actual measurements using the
(4.1)
freeze-pump-thaw method. Low temperature (77K) emissions were measured using a Spex
Fluorolog 212 Spectrofluorometer equipped with a homemade sample holder connected to an Ar
gas tank for continuous de-aeration.
108
4.2.3 Calculations
Calculations were effected using Gaussian ’03 (Rev. B.03)32 for UNIX. The molecules
were optimized using Becke's three-parameter hybrid functional B3LYP34 with the local term of
Lee, Yang, and Parr and the nonlocal 35 term of Vosko, Wilk, and Nassiar. The basis set 6311G** was chosen for all atoms and the geometry optimizations were all ran in the gas phase.
TD-DFT37-43 calculations were employed to produce a number of singlet excited states 44 in the
gas phase based on the optimized geometry. All oscillator strength values and singlet and triplet
excited state values are presented in the supporting information.
4.2.4 Preparation of all-trans-7’-cyano-7’-pyridyl-7’-apo-β-carotenal, 5
(103.3 mg, 2.23 x 10-4 mol) was placed in a 10-ml round bottom flask. The compound
was then dissolved in benzene and was allowed to stir for 5 mins. After all the solid dissolved, 4pyridylacetonitrile monohydrochloride (58.3 mg, 5.30 x 10-4 mol) and ammonium carbonate
(98.6 mg, 1.03 mol) was added to the solution and the reaction was allowed to stir further for
about 5 mins. Then, 1.00 ml of propionic acid was added dropwise into the solution. After all the
reagents were added, the reaction was set on reflux overnight with its temperature regulated at
approximately 70ºC. Upon completion, the reaction was allowed to cool down in an ice water
bath and the solvent was removed by lyophilization. The red-orange solid was re-dissolved in
chloroform and was subjected to column chromatography through an oven dried neutral alumina
(~10.0 g) stationary phase. The mobile phase used was (7:1 chloroform/methylene chloride) ref.
The first and final fractions were discarded while the middle fractions were introduced on a TLC
plate. The solids were recovered after combining the pure fractions. These fractions were freezedried to remove the solvent. The orange compound was placed in a vacuum oven and was stored
in a dessicator.
109
Color: Purple, 82%
Anal. Calcd. for C37H44N2: C, 85.98; H, 8.50; N, 5.40. Found. C, 86.01; H, 8.19; N, 4.25
IR (KBr pellet, cm-1): (CN) 2201 m, (-C=C-) 1503 sh, 2924 w
Raman (Solid, cm-1): (CN) 2266, (C=C-H) 1232
4.2.5 Preparation of all-trans-7’,7’-dicyano-7’-apo-β-carotenal, 6
All-trans-retinal (101.2 mg, 2.18 x 10-4 mol) was placed in a 10-ml round bottom flask.
The compound was then dissolved in benzene and was allowed to stir for 5 mins. After
dissolution, malononitrile (58.3 mg, 5.30 x 10-4 mol) and ammonium carbonate (98.6 mg, 1.03
mol) was added to the solution and the reaction was allowed to stir further for about 5 mins.
Then, 1.00 ml of propionic acid was added dropwise into the solution. After all the reagents were
added, the reaction was set on reflux overnight with its temperature regulated at approximately
70ºC. Upon completion, the reaction was allowed to cool down in an ice water bath and the
solvent was removed by lyophilization. The red-orange solid was re-dissolved in chloroform and
was subjected to column chromatography through an oven dried neutral alumina (~10.0 g)
stationary phase. The mobile phase used was (7:1 chloroform/methylene chloride). 47 The first
and final fractions were discarded while the middle fractions were introduced on a TLC plate. T
The solids were recovered after combining the pure fractions. These fractions were freeze-dried
to remove the solvent. The orange compound he dark-red compound was placed in a vacuum
oven.
Color: Black, 85%
Anal. Calcd. for C33H40N2: C, 85.28; H, 8.68; N, 6.03. Found. C, 85.09; H, 8.41, N, 5.86
IR (KBr pellet, cm-1): (CN) 2205 m, (-C=C-) 1501 sh, 2962 w
Raman (Solid, cm-1): (-C=C-) 1506, (C=C-H) 1164
110
4.3 Results and Discussion
4.3.1 Synthesis
The preparation of 5 and 6 followed the Knoevenagel condensation reaction46-48 which
involved the condensation of the aldehyde, β-apo-8’-carotenal, with substrates lacking αhydrogen atoms adjacent to the coupling site. This criterion is required to enhance the acidity of
the methylene (-CH2-) protons.
(4)
(5)
(6)
Figure 4.1 Knoevenagel condensation of β-apo-8’-carotenal 4-pyridylacetonitrile monohydrate
and malononitrile
Traditionally, this condensation reaction is catalyzed using only a weak base such as
piperidine or NaNH2.46 In our case, we used simultaneous base and acid catalyzed condensation
111
reactions to enhance the yield. The base catalyst used was NH 4CO3 while the acid catalyst
chosen was propionic acid (CH3CH2COOH). The reaction scheme is shown in Figure 4.1.
(1)
(5)
Figure 4.2 Mechanism for the acid & base catalyzed Knoevenagel condensation of
β-apo-8’-carotenal with 4-pyridylacetonitrile monohydrate
112
The mechanism of formation of the pyridyl adduct is presented in Figure 4.2 which can
be extended to the dicyano derivative. In this reaction, the base catalyst was added first to the
reaction mixture to neutralize hydrochloride and to deprotonate one of the methylene (-CH 2-)
protons. Then, the weak acid-catalyst was added to protonate the carbonyl oxygen generating the
carbonyl carbocation necessary to initiate the condensation reaction.
4.3.2 Vibrational Properties
The infrared and Raman spectral data of the carotene reactant and products are presented in
Table 4.1. The Raman spectra are shown in Figure 4.2 while the IR spectra are shown in Figure
4.3. As seen in Table 1, the carbonyl (C=O) stretch of β-apo-8’-carotenal is IR and not Raman
active. This stretching frequency occurs at 1669 cm-1. Results show that the Raman behavior of
the long-chained derivatives follows the same trend as their short-chained counterpart. The
Raman scattering in 5 still holds to be a lot weaker than that of 6. In case of the pyridine adduct,
the CN stretching frequency is both IR and Raman active with the latter having a higher energy.
Their energies are found at 2201 and 2266 cm-1 respectively. As expected, the -C=C- skeletal
stretching frequencies of the cyano products are lowered compared to carotenal. The -C=C- IR
stretch in 5 can be found at 1503 cm-1. In case of the dicyano, this stretching frequency is both IR
and Raman active found at 1501 and 1506 cm-1 respectively. The polyene –CH3 stretch of the
pyridine and dicyano adduct are found at 2924 and 2962 cm-1, respectively.
113
TABLE 4.1
VIBRATIONAL STRETCHING FREQUENCIES (cm-1) OF CN AND POLYENE GROUPS
Compound:
Functional Group:
4
C=O stretch
5
Experimental:
(Infrared)
1669
-C=C- skeletal
1521
=C-H in-plane
bending
1148
CN stretch
2201
-C=C- skeletal
1503
=C-H in-plane
bending
6
Experimental:
(Raman)
1232
Polyene –CH3
stretch
2924
CN stretch
2205
-C=C- skeletal
1501
=C-H in-plane
bending
Polyene –CH3
stretch
2266
1506
1164
2962
114
1.0
Raman Intensity
0.8
0.6
0.4
0.2
0.0
3500
3000
2500
2000
1500
1000
500
0
-1
Raman Shift (cm )
4
Relative Raman Intensity
1.0
0.8
0.6
0.4
0.2
0.0
2400
2200
2000
1800
1600
1400
1200
1000
-1
Raman Shift (cm )
5
Relative Raman Intensity
1.0
0.8
0.6
0.4
0.2
0.0
3000
2500
2000
1500
1000
-1
Raman Shift (cm )
6
Figure 4.3 Raman Spectra of 4, 5 and 6
115
500
0
% Transmittance
1.0
2500
2000
-1
Wavenumber (cm )
5
% Transmittance
1.0
0.8
0.6
2500
2000
-1
Wavenumber (cm )
6
Figure 4.4 Infrared Spectrum of 5 and 6
116
4.3.3 Cyclic Voltammetry
Oxidation potentials for 5 and 6 were examined and the data are summarized in Table
4.2. Carotenoids in general are known to undergo oxidation processes.50-55 Most of these
processes are irreversible in nature. For instance, in case of β-apo-8’-carotenal, Kispert et al have
previously reported its cyclic voltammogram in tetrahydrofuran where two irreversible peaks
have been observed suggesting a two-electron process followed by a chemical reaction (EEC
character).51-54 The first one was observed at 0.910 V while the second one observed at 1.100 V.
For our purpose, CV data for the starting β-apo-8’-carotenal was obtained in methylene chloride
and only one irreversible peak was observed at 0.936 V.
The cyclic voltammograms of the condensed products are presented in Figure 4.5. In all
cases, no reversible reductions were observed, typical in most carotenoids which support their
antioxidant properties in-vivo. Results showed three irreversible oxidation wave potentials
(Table 4.2) are observed in the case of the pyridyl-cyano compound. The first one is observed at
0.817 V, the second at 1.28 V and the last one at 1.399 V. It can be suggested that the second
oxidation involves a two-electron process. In the case of the dicyano counter part, the oxidation
potentials are observed at 0.935 V, 1.372 V and 1.549 V. Just like their retinoid counterparts,
mentioned in the previous chapter, the oxidation waves in both condensed products were shifted
to lower electrode potentials with respected to their starting material, β-apo-8’-carotenal. Once
again, this much larger shift (0.817 V vs 0.935 V) may be attributed to the additional πconjugation arising from the pyridine ring.
117
The nature of the intermediates arising from the electrochemistry of carotenoids is still
unclear to this point. Previous studies however, involving other naturally occurring carotenoids
(i.e. β-carotene) suggests that these systems undergo cation radical formation involving either
electrochemical-chemical (EC) behavior or electrochemical-chemical-electrochemical (ECE)
behavior. In the case of the dicyano, ECE is more likely the case where the intermediate is
generated after and irreversible oxidation observed at 0.935 V.51-55 This new species then undergo
a different electrochemistry giving rise to the irreversible oxidation peaks found at 1.372 V and
1.549 V respectively. One plausible explanation found in the literature is that the
electrochemically generated intermediate can diffuse away from the electrode surface and then
captures a neutral compound generating a monocationic radical species.51,53 This species gets
further oxidized to form its dication counterpart, Car + Car2+ → 2 Car+. .
118
TABLE 4.2
OXIDATION ELECTRODE POTENTIALS (V) OF CAROTENOIDS
E(V)1, 2
0.910
0.936
Compound:
β-apo-8’-carotenal
1.100
2
0.817
1.280a
1.499
0.935
3
1.372
1.549
Observed potentials are irreversible
2
Reference Electrode: Ag/AgCl in CH3CN
Working Electrode: Pt disk electrode
Solvent: Dried CH2Cl2 obtained at Room Temperature
a
Two-electron process
1
0
0
-5
-5
-10
-15
I
I
-10
-20
-15
-25
-20
-30
1400
1200
1000
800
600
400
200
0
1400
E (mV)
1200
1000
800
E (mV)
2
3
Figure 4.5 Cyclic voltammogram for 5 and 6
119
600
400
200
0
4.3.4 Calculations (DFT / TDDFT)
The HOMO and LUMO orbitals of the pyridyl-cyano adduct determined by Density
Functional Theory calculations are shown in Figure 4.6. The pictures clearly illustrates the π →
π* nature of the polyene A1g → B1u transitions. One interesting factor is that electron density is
somewhat shifted towards the pyridyl ring. This trend is similar to the short-chained counterpart,
as previously discussed in the previous chapter.
HOMO Picture of 5
LUMO Picture of 5
Figure 4.6 HOMO/LUMO pictures in 5
120
HOMO Picture of 6
LUMO Picture of 6
Figure 4.7 HOMO/LUMO pictures in 6
A similar trend can also be observed in the nature of the HOMO/LUMO of the dicyanocarotenoid derivative. This is shown in Figure 4.7. Here the shift in the electron density is even
farthest from the β-ring and moves towards the dinitrile groups. Although, the electron densities
where shifted more towards the dinitrile groups, the π* electron density in the LUMO of the
long-chained derivative is more defined in the functional groups involved and a much less orbital
interaction was observed towards the dinitrile terminus of the chain.
121
HOMO - 2 (5)
HOMO -2 (6)
HOMO - 1 (5)
HOMO - 1 (6)
LUMO + 1 (5)
LUMO + 1 (6)
LUMO + 3 (5)
LUMO + 2 (6)
Figure 4.8 Orbital pictures of selected states involved in optical transition in 5 and 6
122
4.3.5 Population Analysis
Table 4.3 lists the population analysis data and the percentage electronic distribution of
some selected states in 5 and 6 as shown in Figure 4.7. All calculations are effected using
TDDFT. Results show similar trends as their short-chained counterparts. The HOMO is
predominantly polyene in character with 90% occupancy in both 5 and 6. The most remarkable
result is that both have equal electron densities in the conjugated chain, where in dicyano’s case,
the β-ring and CN groups do not play an important contribution. The same trend was observed in
the case of the LUMO in 5 and 6. Although, the LUMO character lies on the polyene chain, the
electron density of the LUMO in 5 is distributed throughout the molecule and is shifted to the
newly condensed group, 4% pyridyl group contribution.
This pyridyl group plays an important role in hosting the excited electron. In some cases
the methyl groups of the polyene chain have minor contributions in one of the states such as that
observed in H – 2. In the latter case, the π-bonding interactions observed in retinoids are arrange
in a conjugated pattern. All low-lying LUMO orbitals in 5 have major contributions from the
pyridyl ring. The electronic distributions of other low-lying HOMO and LUMO orbitals are
pictured in Figure 4.7. The higher LUMO’s in 6 are consistent in chain electron density
contributions. As this density shifts toward the dicyano group, the β-ring electronic density
depletes as one go from the LUMO to the higher LUMO orbitals. For instance, the β-ring has 1%
contribution in the electron density of its LUMO and 18% contribution in the LUMO + 3.
123
TABLE 4.3
ELECTRON DISTRIBUTION (%) IN 5 AND 6
States:
LUMO + 3
LUMO + 2
LUMO + 1
LUMO
HOMO
HOMO – 1
HOMO – 2
HOMO – 3
5
5 (chain)
0 (β-ring)
90 (CN)
5 (pyridyl)
77 (chain)
7 (β-ring)
1 (CN)
15 (pyridyl)
83 (chain)
3 (β-ring)
10 (CN)
3 (pyridyl)
88 (chain)
1 (β-ring)
7 (CN)
4 (pyridyl)
90 (chain)
6 (β-ring)
2 (CN)
2 (pyridyl)
76 (chain)
16 (β-ring)
5 (CN)
3 (pyridyl)
57 (chain)
35 (β-ring)
6 (CN)
2 (pyridyl)
62 (chain)
29 (β-ring)
8 (CN)
2 (pyridyl)
124
6
78 (chain)
18 (β-ring)
4 (CN)
87 (chain)
8 (β-ring)
5 (CN)
89 (chain)
3 (β-ring)
7 (CN)
91 (chain)
1 (β-ring)
8 (CN)
90 (chain)
7 (β-ring)
3 (CN)
74 (chain)
20 (β-ring)
6 (pyridyl)
56 (chain)
38 (β-ring)
6 (pyridyl)
70 (chain)
24 (β-ring)
7 (pyridyl)
4.3.6 Electronic Spectra
The UV/Visible spectral data of the compounds are given in Table 4.4. Both
experimental and calculated data showed bathochromic shifts in the visible absorption spectrum,
as expected, with respect to the starting β-apo-carotenal derivative. For the pyridyl-cyano
derivative, λmax was shifted by 69 nm from 448 nm to 517 nm. Unlike the short-chained version,
the shifts in the λmax of the dicyano derivative did not shift much relative to the pyridyl adduct,
which is observed at 519 nm. However, the same argument can be used to account for the
bathochromic shift observed. This is due to the lengthening of the polyene chain (1-D box). After
condensation, a new double bond was generated between the carbonyl carbon of the retinal and
the methylene groups of pyridine-CN (for 2) and malononitrile (for 3). The experimental molar
extinction coefficients are also tabulated in table 3. Results gave a value of 8.00 x 104 M-1 cm-1
for 2 and 6.33 x 104 M-1 cm-1 in 3. The UV/Visible spectra of 2 and 3 are shown in Figure 4.9.
TABLE 4.4
UV/VISIBLE DATA OF S0 1Ag → S2 1Bu TRANSITION in (4), (5) and (6)
Compound:
E (λmax)
(Experimental)
ε
(M-1 cm-1)
E (λmax)
(Calculated)
4
448
7.11 x 105
541
5
517
8.00 x 104
609
6
519
6.33 x 104
625
125
1.0
1.0
Experimental
Calculated
Experimental
Calculated
0.8
Absorbance
Absorbance
0.8
0.6
0.4
0.6
0.4
0.2
0.2
0.0
0.0
300
400
500
600
700
300
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
3
2
Figure 4.9 Experimental and calculated UV/Visible data for 5 and 6
The simulated UV/Visible absorption spectra of 4, 5 and 6 were determined using
TDDFT calculations and are overlaid with the actual electronic spectra (Figure 4.8). The
calculated λmax are presented in Table 4.4. Like the retinoid derivatives, a hypsochromic shift was
observed in the actual electronic spectra of the carotenoid derivatives compared to the simulated
ones. However, in the latter case, the absorption spectra in gas-phase appear approximately 100
nm more red-shifted compared to the ones in liquid solution. The lowest energy π → π*
transitions in 5 underwent an actual blue-shift from 609 nm in gas-phase to 517 nm in
acetonitrile solution. For 6, this transition is actually observed at 519 from 625 nm. Calculations
revealed that the lowest energy transition occurred from the HOMO to LUMO in both 5 and 6,
similar to the ones observed in their retinoid counterparts. Experimentally, a weak absorption
band was also seen in both cases appearing in the UV region (Figure 4.9).
As expected, the π → π* located on the polyene chain is the major contributor to the
lowest-energy transitions. The HOMO of 5 (Figure 4.6) shows the electron density as located on
126
the polyene chain and a little contribution from the β-ring while the LUMO shows some electron
density of the polyene chain extending towards the pyridyl group.
Just like 15,15’-dicyano-retinal, the HOMO of 6 is spread out over the polyene chain, the
dicyano group and the β-ring. Some p-orbital contribution in the case of methylene carbon is also
observed. Its LUMO on the other hand is less distorted compared to its retinoid derivative and is
very distinctive of polyene chain π*-orbital with a little bit of contribution coming from the
dicyano groups.
As also shown in Figure 4.9, the simulated UV/Visible spectra obtained using TD-DFT
calculations gave rise to four distinct theoretical bands in 5 while three bands were calculated for
6. Table 4.5 lists selected transitions, their corresponding oscillator strengths and the major
molecular orbitals that contribute in each transition. As expected, the HOMO to LUMO
transition of 5 has highest oscillator strength (f = 3.2580) in agreement with its high molar
extinction coefficient and is the predominant peak calculated in the UV-Visible spectrum with its
maximum centered at 609 nm. A theoretical peak located at 486 nm with an oscillator strength of
0.75 embellishes the following transitions: H – 1 → LUMO and HOMO → L + 1. The other
calculated peaks merge pair-wise into the peaks shown in Figure 4.8. For example, the peaks at
372 nm and 296 nm appear as one peak. The calculated peak at 296 nm and 294 nm, gave
average oscillator strength of 0.13. Under this band lie the following transitions: H – 2 → L + 1
and HOMO → L + 3. The molecular orbital pictures showing these transitions are shown in
Figure 4.8.
127
TABLE 4.5
CALCULATED UV/VISIBLE DATE FOR 5 AND 6
5
6
λ (nm)
612
f
3.2580
Major Contributors
HOMO → LUMO
486
0.7496
H – 1 → LUMO
λ (nm)
622
f
2.9117
Major Contributors
HOMO → LUMO
483
1.0436
H – 1 → LUMO
HOMO → L + 1
372
309
0.1123
0.0851
H – 2 → LUMO
HOMO → L + 1
369
0.0422
H – 2 → LUMO
H–1→L+1
H–1→L+1
HOMO → L + 1
HOMO → L + 1
H – 3 → LUMO
352
0.0132
H–2→L+1
H – 3 → LUMO
H – 2 → LUMO
HOMO → L + 2
H–1→L+1
HOMO → L + 4
296
0.1509
H–2→L+1
306
0.0653
HOMO → L + 3
H–2→L+1
HOMO → L +2
294
0.1115
HOMO → L + 3
294
0.0152
H – 4 → LUMO
287
0.0399
H–1→L+2
287
0.2894
H–2→L+1
HOMO → L + 4
264
0.0266
H – 7 → LUMO
H–1→L+2
277
H–3→L+1
0.0262
HOMO → L + 2
H–1→L+2
HOMO → L + 3
128
The calculated UV/Visible data of 6 are also listed in Table 4.5. One again, the HOMO –
LUMO transition dominates with high oscillator strength 2.5 times greater than the others. Two
peaks are also observed located near the other calculated bands of high oscillator strength. The
theoretical peak observed at 483 nm ( f = 1.04) embellishes a similar type of electronic
transitions as the pyridyl adduct being H – 1 → LUMO and HOMO → L + 1. Both of these
peaks may lie under the λmax of both 5 and 6. Once again, a theoretical peak in the UV region was
observed, this time at 287 nm. It has an oscillator strength of 0.29 involving the following
electronic transitions: H – 2 → L + 1, H – 1 → L + 2 and HOMO → L + 2.
Group theoretical treatments have also been useful in absorption process involved in
these systems.1, 12-25, 56-61 Just like the corresponding retinoids, the Ag and Bu symmetry orbital
labels deduced when symmetry operation in C2h point group was to the cyano carotenoid
derivatives can be applied here to describe the nature of colorful the π → π* band. 1-2 As
expected, the intense color of these synthetic carotenoids, affirmed by calculations to occur
between HOMO-LUMO band gap can be assigned to the S0 (1 1Ag) → S2 (1 1Bu) state (Figure
3.11).
129
4.4 Summary
Two cyanine derivatives of β-carotene were synthesized in this laboratory using acid-catalyzed
Knoevenagel condensation similar to its short-chained counterparts. Vibrational spectroscopy
resulted to infra-red IR CN stretch at 2201 cm-1 while its Raman counterpart was observed at
2266 cm-1. The Raman spectrum of the dicyano derivative showed fluorescence signal. The CN
stretch was not observed in this case however, its infra-red stretch was located at 2205 cm-1.
Electrochemical (CV) data also showed irreversible oxidation wave potentials, similar to its
short-chained counterparts. UV/Visible absorption spectroscopy showed a bathochromic shift in
its λmax in its actual spectrum compared to its parent β-apo-8’-carotenal. Density Functional
Theory (DFT) and Time Dependednt Density Functional Theory (TDDFT) calculations were
performed to establist the nature of the transitions involved in the optical spectra. These
calculations revealed that the HOMO and LUMO were involved in the low-energy absorption
process and were located primarily on the on polyene chain. The energy gap between them was
consistent with the observed absorption spectrum. Once again, group theory was useful to
accound for the optical behavior of these carotenoid derivatives. The point-group assignment
given to the molecules was C2h, similar to its short-chained counterparts.
130
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57. Snyder, R.; Arvidson, E.; Foote, C.; Harrigan, L.; Christensen, R.L. J. Am. Chem. Soc.
1985, 107, 4117
58. Young, A.K; Phillip D.; Ruban, A.V.; Horton, P.; Frank, H.A. Pure Appl. Chem. 1997,
69, 2125
59. Andersson, P.O.; Gillbro, T.; Asato, A.E.; Liu, R.S.H. J. Lumin. 1992, 51, 11
60. Moroni, L., Gellini, C.; Salvi, P.R.; Schettino, V. J. Phys. Chem A 2000, 104, 11063
61. O’Neil, M.P.; Wasielewski, M.; Khaled, M.M.l Kispert, L.D. J. Chem. Phys. 1991, 95,
7212
134
CHAPTER 5
SYNTHESIS, STRUCTURAL ELUCIDATION, ELECTROCHEMICAL AND
PHOTOPHYSICAL STUDIES OF CAROTENOID DERIVATIVE LIGANDS TO
RHENIUM (I) AND PLATINUM (II)
5.1 Introduction
There is currently a considerable interest to find alternative sources of energy other than
fossil fuels1. This is triggered by the current economic situation and crisis being experienced
world wide regarding the use and consumption of this limiting resource 2. This has led scientists
to continually explore the most powerful source of energy for our planet – the sun 1-2. This
massive object is the source of the most abundant form and free energy in our ecosystem 3-4.
Plants and purple bacteria, for instance, have their own way of converting this source of energy
into its chemical form which sustains their daily existence5-11. As mentioned earlier, this energy
transformation is illustrated in PSI and PSII in LHC’s of plant’s chloroplasts12-26. Plants are
known to utilize solar energy to approximately ten times our current energy needs4.
It is the same principle of harnessing solar energy that led scientists to develop solar cells.
One intriguing type of solar cell is the Grätzel cell which is a form of dye-sensitized solar cells
(DSSC’s)27-32. These cells require photosensitizer dyes that can be attached to semiconductors
such as TiO2; where, upon excitation of the dye, photooxidation processes occur in its excited
state and the ejected electron enters the conduction band of the semiconductor. This
semiconductor can then be connected into an electrochemical cell and a cycle (i.e. I3-/I- (aq)
medium) be developed for enhance conversion efficiency28. To date, these cells are known to
have incident photon-current conversion efficiencies28 (IPCE) of about 11%. Complexes
involving [Ru(bpy)3]2+ and its derivatives33-46 have been well-studied as photosensitizers in this
process due to their stability33, redox34-35, excited state chemisty43,33-46 and luminescence33,58-59
135
properties. In this research, we are interested in developing more efficient dyes based on the
combined properties of retinoids/carotenoids’ light-harvesting properties23-26 in nature with the
chemistry of Re(I)-tricarbonyl-bipyridine4,33,43-51
and Pt(II) biphenyl 33,52-57 complexes. Other
applications may include photocatalysis, DNA-intercalation and antioxidants.
5.2 Experimental Section
5.2.1 Materials
The compound all-trans-retinal (Vitamin A aldehyde) was obtained from Sigma.
Malononitrile was obtained from Fluka while the compound 4-pyridyl acetonitrile
monohydrochloride was purchased from Acros Scientific. Rhenium (I) pentacarbonyl chloride
was purchased from Acros Scientific. Silver trifluoromethane sulfonate was obtained from
Aldrich Chemical Company. USP grade dehydrated ethanol was purchased from McCormick
Distilling Company, Incorporated. Butyronitrile used for emission and lifetime studies was
obtained from Acros Scientific; it was fractionally distilled prior to use. Dried benzene, diethyl
ether, HPLC grade methanol, chloroform, methylene chloride and optima grade acetonitrile for
UV/Visible spectral determination were purchased from Fisher Scientific. Propionic acid and
ammonium carbonate were purchased from Aldrich Chemical Company. IR grade potassium
bromide used as a pellet for the infrared spectra and deuterated dimethysulfoxide (DMSO) for
the NMR analysis were obtained from Aldrich. Ferrocene standard and dried acetonitrile (suresealed bottle) used as an electrochemical solvent were purchased from Aldrich. Sulfur powder
used as a standard to calibrate the FT-Raman Spectrometer was provided by Thermo Nicolet.
Electrochemical grade tetrabutylammonium perchlorate was purchased from Southwestern
Analytical.
136
5.2.2 Preparation of [Re(CO)3(bpy)(L)], (L = 15,15’-cyanopyridyl retinal), 7
The compound, 15,15’-cyanopyridyl retinal (2) was synthesized using Knoevenagel
Condensation60 mentioned in the procedure in chapter 3. [Re(CO)5Cl] (100.0 mg) was placed in a
25-mL round bottom flask and was dissolved in 15.0 mL of absolute ethanol. The flask was
wrapped with aluminum foil and silver trifluorosulfonate (71.0 mg) was added in the solution.
The reaction was allowed to stir under reflux overnight. After the reaction, white precipitate of
AgCl was filtered off from the solution. The collected filtrate was transferred into an empty
round bottom flask and 2,2’-dipyridyl (113 mg) was added into the solution and the reaction was
allowed to set on reflux for another 30 minutes in the dark. After the reaction, 15,15’cyanopyridyl (100 mg) was added into the mixture and was set on reflux under Ar conditions
overnight. After completion of the reaction, the solvent was evaporated until dryness and the
solid was allowed to precipitate in a saturated solution of aqueous NH4PF6. The solid was
collected and dried inside the vacuum oven and was stored inside the freezer.
Color: purple-black Yield. 70%
Anal. Calcd. for ReC40H40N4O3PF6: C, 50.21; H, 4.22; N, 5.86. Found. C, 51.08; H, 4.19; N, 5.79
IR (KBr pellet, cm-1): (CN) 2211 w, (CO) 2032 s, 1916 s
MS (ESI): m/z 811
137
5.2.3 Preparation of [Re(CO)3(bpy)(L)], (L = all-trans-7’-cyano,7’-pyridyl-7’-apo-βcarotenal ), 8
The compound, all-trans-7’-cyano-7’-pyridyl,7’-apo-β-carotenal (5) was prepared using
the procedure60 mentioned in chapter 4. Compound 8 was prepared analogous to compound 7
except that the L = all-trans-7’-cyano-7’-pyridyl,7’-apo-β-carotenal. The black compound was
dried in a vacuum oven and stored inside the freezer.
Color: black Yield. 70%
Anal Calcd. for ReC50H52N4O3PF6: C, 55.16; H, 4.82; N, 5.15. Found. C, 55.20; H, 4.90; N, 5.05
IR (KBr pellet, cm-1): (CN) 2210 w, (CO) 2034 s, (CO) 1925 s
MS (FAB): m/z 943
5.2.4 Preparation of [Pt(bph)(L)2], (L = 15,15’-cyanopyridyl retinal ), 9
The compound, 15,15’-cyanopyridyl retinal (2) was synthesized using the procedure60
mentioned in chapter 3. The compound [Pt2(μ-SEt2)2(bph)2] was synthesized using published
procedures61-62. The compound [Pt2(μ-SEt2)2(bph)2] was dissolved in absolute ethanol and was
allowed to stir for 5 mins. Then, four equivalents of compound 2 were added to the mixture and
was set on reflux for 5 hours under Ar conditions. The insoluble precipitate was filtered
immediately after cooling the reaction vessel and was placed inside the vacuum oven for drying
and was stored inside the freezer.
Color: brick-red Yield. 80%
Anal.Calcd. for PtC66H72N4: C, 70.99; H,6.50; N, 5.02. Found. C, 68.92; H, 5.75; N, 5.05
IR (KBr pellet, cm-1): (CN) 2212 w
138
5.2.5 Preparation of [Pt(bph)(L)2],
(L = all-trans-7’-cyano,7’-pyridyl-7’-apo-β-carotenal), 10
The compound, all-trans-7’-cyano-7’-pyridyl-7’-apo-β-carotenal using the procedure60
mentioned in chapter 4. Compound 10 was prepared analogous to compound 9 except that L =
all-trans-7’-cyano-7’-pyridyl-7’-apo-β-carotenal. The reddish black compound was placed inside
the vacuum oven for drying and was stored inside the freezer.
Color: reddish black Yield. 78%
Anal. Calcd. for PtC86H96N4: C, 74.79; H, 7.01; N, 4.06. Found: C, 74.00; H, 6.91; N, 3.95
IR (KBr pellet, cm-1): (CN) 2206 w
5.2.6 Measurements
Infrared spectra were obtained using Perkin-Elmer Model 1600 FT-IR and Nicolet Avatar
model FT-IR spectrophotometers. All samples were prepared as potassium bromide pellets.
Raman spectra were obtained using a Thermo Nicolet Nexus FT-Raman Module
Spectrophotometer. The Nicolet instruments were accompanied by Omni software programs.
Proton (1H)-NMR spectra were obtained using Varian Mercury 300 MHz and Varian Inova 400
MHz Fourier Transform-NMR spectrometers (internal standard TMS). Ultraviolet spectra were
obtained using a Hewlett-Packard Model 8452A diode array spectrophotometer interfaced with
an OLIS software program. Elemental (C,H,N) analysis was performed by MHW Laboratories,
Phoenix, AZ. Mass spectrometric analysis was performed using Varian 1200L Quadrupole
instrument. An EG&G PAR model 263A potentiostat/galvanostat was used to obtain cyclic
voltammograms. All measurements were carried out in a typical H-electrochemical cell using a
platinum disk working electrode (polished every run) and a platinum wire counter electrode. A
Ag/AgNO3 reference electrode in dried CH3CN was freshly made and used for this study. The
supporting electrolyte was 0.1 M tetrabutylammonium perchlorate. Ferrocene was added as a
139
reference. The sample preparation for emission studies involved dissolving a small amount of
sample in dried butyronitrile and then measuring the absorbance of the solution. The
concentration of the solution was altered in order to achieve an approximate absorbance of ~
0.01 at the λmax of the sample. The sample solution was placed in a quartz tube and was degassed
a minimum of five time using the freeze-pump-thaw method prior to actual measurements.
Emission quantum yields were calculated using equation 1, where φf is the emission quantum
yield, If is the emission intensity and A is the absorbance of the sample at the excitation
frequency63.
(5.1)
Low temperature (77K) emissions were measured using a Spex Fluorolog 212
Spectrofluorometer equipped with a homemade sample holder connected to an Ar gas tank for
continuous deaeration.
5.2.7 Calculations
Calculations were affected using Gaussian ’03 (Rev. B. 03) for UNIX64. The molecules
were optimized using Becke’s three-parameter hybrid functional B3LYP65 with the local term66
of Lee, Young, Parr and the nonlocal term67 of Vosko, Wilk, and Nassiar. The basis set 6311G** was chosen for all atoms and the geometry optimizations were all ran in the gas phase.
TDDFT68-73 calculations were employed to produce a number of singlet excited states41 in the gas
phase based on the optimized geometry. All oscillator strength values and singlet and triplet
excited state values are presented in the supporting information.
140
5.3 Results and Discussion
5.3.1 Synthesis
The preparation of compounds 2 and 5 followed the Knoevenagel condensation
previously discussed. The pyridyl ring will serve as the electron pair donor and can be
coordinated to the metal centers of rhenium (I) and platinum (II). The rhenium (I) complexes
were prepared according to the scheme presented in Figure 1. In this process, two solvent
molecules of ethanol displace two carbonyl groups from the coordination sphere. After the
displacement proceeded, addition of bipyridine can further displace the two solvent molecules
since they are weakly bound to the metal center. Lastly, the ligand of interest (compound 2 or 5)
can be introduced into the metal center to form the desired compounds 7 and 8. In this
preparation, the chloride was removed from the coordination sphere with Ag + forming AgCl(s)
and replaced with CF3SO3- anion. This is followed by displacement of the CF 3SO3- moiety with
the ligand of interest. Precipitation in aqueous ammonium hexafluorophosphate generates the
complex salt.
Figure 5.2 outlines the preparation of the Pt-biphenyl adducts. In this process, dibromobiphenyl is reacted with two moles of n-butyl lithium. This results to the formation of the LiBr
and generation of the biphenyl dianion intermediate. On a separate reaction, potassium
tetrarchloro platinate (II) can be reacted with aqueous diethyl sulfide to generate the bischloro,
bis-diethyl sulfide platinate (II) complex. This complex can then be reacted with the biphenyl
dianion intermediate to generate the bridged [Pt2(μ-SEt2)2(bph)2] complex. This intermediate can
then be reacted with four equivalents of retinoid (carotenoid) ligand to generate compounds 9
and 10.
141
[Re(CO)5Cl]
AgCl(s)
a. EtOH
b.AgCF3SO3
c. Dark
[Re(CO)3(EtOH)2(CF3SO3)]
Bpy
reflux (30 mins)
[Re(CO)3(bpy)(CF3SO3)]
CN
CN
N
OC
OC
N
CO N
OC
Re
OC
N N
CO N
Re
N
N
NC
NC
7
8
Figure 5.1 Preparation of 7 and 8
142
Br
2 moles of n-BuLi
Br
S
Pt
Pt
S
Cl
Cl
Pt
Cl
Cl
[
[
K2
S
Cl
S
Pt
H2O
Cl
S
NC
N
Pt
CN
N
N
NC
9
S
Pt
Pt
S
NC
N
CN
Pt
N
N
NC
10
Figure 5.2 Preparation of 9 and 10
143
5.3.2 Calculations (DFT/TDDFT)
The HOMO and LUMO surface maps of the retinoid-metal complexes 7 and 9,
determined using DFT calculations are shown in Figure 5.3. In case of the Re(I)-cyanopyridyl
derivative, the HOMO is located primarily on the polyene chain with a contribution from the
pyridyl group of the retinoid ligand. Its LUMO on the other hand is concentrated around the
metal center coordination sphere with some minor contributions from the dipyridyl ring and the
tricarbonyl groups. It can be noted also that the polyene chain of the HOMO has a π (π*)
structure. A similar argument is also true in case of the dipyridyl rings.
The opposite is true in case of the Pt(II)-biphenyl cyanopyridyl complex 9 at which there
is a significant electron density contribution that arises from both the metal center as well as the
biphenyl group. This is clearly observed in the HOMO picture of the Pt-cyanopyridyl retinal
complex in Figure 5.3. Another interesting feature is the fact that the d-orbital involved around
the metal center changed its planar coordinate, more likely from a dyz configuration in the
HOMO to a dxy orbital in the LUMO. The LUMO picture in this case is located on the polyene
chain where it depicts a π (π*) structure.
DFT surface maps of the long-chained pyridyl carotenoid Re(I) and Pt(II) metal
complexes are similar to its short-chained derivatives and are shown in Figure 5.4. As expected
with the Re(I) complex, majority of the electron density in the HOMO is located on the polyene
chain of the ligand with some minor contributions on the β-ring and the pyridyl groups. Once
again, the polyene π (π*) structure is located on the HOMO. The electron density from the
HOMO to the LUMO is shifted from one end of the complex to the other. Here, majority of the
electron density is in the dipyridyl ring with some minor contributions arising at the tricarbonyl
144
groups. Also, the type of molecular orbitals involve are the same as those with its short-chained
derivatives.
HOMO of 7
LUMO of 7
HOMO of 9
LUMO of 9
Figure 5.3 HOMO/LUMO in 7 and 9
145
HOMO of 8
LUMO of 8
HOMO of 10
LUMO of 10
Figure 5.4 HOMO/LUMO in 8 and 10
146
Just like the Re(I) complex derivative, the HOMO/LUMO of the Pt(II) metal complex of
the long-chained ligand also behave similarly as its short-chained derivatives. The electron
density of the HOMO of compound 10 is similar to the electron density observed in the HOMO
of compound 9 as pictured earlier. Here, the electron density is located on the biphenyl ring and
some minor contributions arising from the Pt(II) metal center. The same is true with its resulting
LUMO surface map. Although the electron density of the LUMO in 10 is still localized on the
polyene group, the major difference it has with respect to its short-chained counterpart depends
on the way its electron density is distributed. As clearly seen in Figure 5.4, the electron density is
split between the two polyene chains instead of only one. Another interesting feature is that
there is zero electron density on the metal center in the LUMO.
All other HOMO/LUMO’s involved in the electronic transitions of the complexes have
been selected and their corresponding surface maps are presented in the succeeding figures
located in the electronic section. Other significant low-lying occupied orbitals in 7 are illustrated
in Figure 5.5. This includes H – 1, H – 2, H – 4 and H – 11. It is interesting to note that with H –
4, the electron density is pretty much scattered over the whole molecule. The involvement of the
dz2 orbital of the rhenium can also be noted on its surface map. The other LUMO’s are presented
in Figure 5.6 which includes L+ 1, L + 2, L + 4, L + 5 and L + 10. In this case, all but L+ 2 have
electron densities distributed between the polyene chain and the metal coordination sphere.
147
H-1
H-2
H-4
H – 11
Figure 5.5 Surface maps of other occupied molecular orbitals involved in electronic transitions
of 7
148
L+1
L+2
L+4
L+5
L + 10
Figure 5.6 Surface maps of other unoccupied molecular orbitals involved in electronic transitions
of 7
149
H–1
H–2
H–3
H–4
H–5
Figure 5.7 Surface maps of other occupied molecular orbitals involved in electronic transitions
of 9
150
H–7
H–8
H–9
H – 10
Figure 5.8 Surface maps of other occupied molecular orbitals involved in electronic transitions
of 9
151
L+1
L+2
L+3
L+5
L+7
Figure 5.9 Surface maps of other unoccupied molecular orbitals involved in electronic transitions
of 9
152
The surface maps of the other HOMO’s in 9 shown in Figure 5.7 and 5.8 are H – 1, H –
2, H – 3, H – 4, H – 5, H – 7, H – 8, H – 9 and H – 10. Here there is no trend observed as to
where the electron densities are located but one thing is for sure in terms of the electron
occupancy in the molecule, the electrons are distributed throughout the molecule. For instance,
the electron density in H – 1 is located on the polyene with minor contributions on the biphenyl
ring ligand and the metal center. The electron densities of the H – 3 and H – 4, on the other hand
are contained at the metal-biphenyl groups. The surface maps of all the other LUMO’s in 9 also
gave a similar result as the HOMO’s and are presented in Figure 9. They are L + 1, L + 2, L + 3,
L + 5 and L + 7.
The surface maps of the other HOMO’s in 8 presented in Figure 5.10 are H – 1, H – 2, H
– 3 and H – 6. In all cases, the electron densities are located predominantly on the polyene chain
of the complex with some minor contributions on the metal center. A similar trend that is
significant in 8 can be seen on the surface maps presented in Figure 5.11 for the other LUMO’s.
It consists of L + 1, L + 4, L + 8 and L + 11. As their HOMO counterparts, the electron densities
are mostly polyene in character with very minimal contributions involving the metal center. This
entails that for the long-chained metal complexes, the carotene π (π*) structure has mostly been
retained. In L + 8, there is a slight contribution arising from the biphenyl-metal center.
153
H–1
H–2
H–3
H–6
Figure 5.10 Surface maps of other occupied molecular orbitals involved in electronic transitions
of 8
154
L+1
L+4
L+8
L + 11
Figure 5.11 Surface maps of other unoccupied molecular orbitals involved in electronic
transitions of 8
155
The surface maps of the other low-lying HOMO’s in 10 presented in Figures 5.12 and
5.13 are H – 1, H – 2, H – 3, H – 5 , H – 6, H – 7 and H – 8. It can be seen clearly that in the case
of H – 3 and H – 5, the electron densities are located on the metal-biphenyl coordination sphere.
The opposite is true with H – 7 where the electron density is localized on the polyene chain. All
the other significant HOMO’s have electron densities spread out slightly throughout the
molecule. In case H – 1, the electron density is distributed almost equally over the polyene chain,
biphenyl and the metal d-orbitals. In the case of H – 2, although majority of its electron density is
located on the polyene π (π*) structure, a contribution from the d x2- y2 of the Pt(II) metal center is
also found. The other low-lying LUMO’s of 10 are presented Figure 5.14 include L + 1, L + 2, L
+ 3 and L + 11. All of which have electron densities localized on its π (π*) structure of the
polyene chain. In some cases, the electron density is split between the two chains attached to the
metal center while in others it is only contained on one chain only. Subsequently, these orbitals
will be related to optical transitions in these molecules.
156
H–1
H–2
H–3
H–5
Figure 5.12 Surface maps of other occupied molecular orbitals involved in electronic transitions
of 10
157
H–6
H–7
H–8
Figure 5.13 Surface maps of other occupied molecular orbitals involved in electronic transitions
of 10
158
L+1
L+2
L+3
L + 11
Figure 5.14 Surface maps of other unoccupied molecular orbitals involved in electronic
transitions of 10
159
5.3.3 Vibrational Properties
Table 5.1 summarizes the infrared frequencies in the complex and the ligand. Figure
5.15a shows the overlayed infrared spectra of the retinal ligand (red) with the Re(I) complex
(blue) while Figure 5.15b shows the overlayed spectrum of the ligand (red) with Pt(II) complex
(purple). As can be seen clearly, the CN peak of the ligand is present in the IR spectrum of all the
complexes. It is observed at 2211, 2212, 2210 and 2206 cm-1 for complex 7, 8, 9 and 10
respectively versus 2208 cm-1 for the free ligand. The unbound nature of the CN peak is clearly
seen since there is no significant shift in its absorption band. For the Re(I)-retinal species, the CN
stretch is a weak band (2211 cm-1) while the Pt(II)-retinal species contains a weak CN frequency
observed at 2212 cm-1. It is also noteworthy to mention the well-defined CO peaks in the Re(I)
complex. Experimental IR vibrational modes of the carbonyl groups trans to the bipyridine
ligand were observed as a sharp stretch at 1916 cm-1 while the carbonyl group trans to the
pyridine ring of the retinoid was found at 2032 cm-1. This is accompanied by DFT calculations.
100
% Transmittance
% Transmittance
100
80
80
2000
2000
-1
-1
Wavenumber (cm )
Wavenumber (cm )
(a)
(b)
Figure 5.15 Overlayed IR Spectra of the Ligands with the Complex: (a) [Re(CO)3(bpy)(L)]
and (b) [Pt(bph)(L)2] where L = 15,15’-cyanopyridyl retinal
160
A similar discussion can be made with the long-chained derivative. Figure 5.16a shows
the overlayed IR spectrum of the carotenal ligand (wine) with that of the Re(I) complex
(magenta) and Figure 5.16b illustrates the overlayed spectrum of the carotenal ligand with that
of the Pt(II) complex (orange). Here, the CN stretch in Re(I) is found at 2210 cm -1 while that of
Pt(II) falls at a frequency of 2206 cm-1. The carbonyl groups trans to bipyridine group in the
Re(I) complex showed some significant blue shift in its stretching frequency located at 1925 cm 1
. However, the CO stretching frequency of the carbonyl group trans to the pyridine of the
carotene ligand did not change too much as one goes from the short-chained to the long-chained
derivative. In case of the latter, the vibrational mode was observed at 2034 cm-1.
100
% Transmittance
% Transmittance
100
80
80
60
2000
2000
-1
-1
Wavenumber (cm )
Wavenumber (cm )
(a)
(b)
Figure 5.16 Overlayed IR spectra of the ligands with the complex: (a) [Re(CO)3(bpy)(L)]
and (b) [Pt(bph)(L)2 where L = all-trans-7’-cyano-7’-pyridyl,7’-apo-β-carotenal
161
TABLE 5.1
VIBRATIONAL STRETCHING FREQUENCIES (cm-1) OF CN AND CO GROUPS IN
LIGANDS AND METAL COMPLEXES
Compound
2
Functional Group
CN
Frequencies
2208
5
CN
2208
7
CN
2211
CO (axial)
2032
8
CO (equatorial)
CN
1916
2212
9
CN
2210
CO (axial)
2034
CO (equatorial)
1925
CN
2206
10
5.3.4 Cyclic Voltammetry
Figures 5.17 and 5.18 illustrate the cyclic voltammograms of compounds 7 and 8,
respectively - in different potential windows. The electrochemical data are presented in Table
5.2. Results show one irreversible reduction peak at around -0.814 V and two irreversible
oxidation peaks at 1.192 and 1.580 V in case of the Re(I)-retinal derivative. In case of the Re(I)carotenal derivative, irreversible oxidations were observed at 0.840 and 1.367 V. A reversible
162
peak is also seen at E1/2 = -1.079 V. This reversible peak observed in the long-chained derivative
can be assigned to the reduction of dipyridyl ligand which is typical. The reduction peak
observed at -0.814 V maybe due to the reduction of pyridine. As expected, the Pt(II) complex did
not show worthwhile electrochemistry.
7
2
6
0
5
-2
-4
I(µ Α )
I(µ Α )
4
3
2
-6
1
-8
0
-10
-1
-400
-600
-800
-1000
-1200
-1400
1600
E(mV
)
1400
1200
1000
800
600
400
200
0
-200
E(mV
)
(7a)
(7b)
Figure 5.17 Cyclic voltammograms of compound 7: 7a. Negative potential window
7b. Positive potential window
1
5
0
4
-1
-2
I(µ Α )
I(µ Α )
3
2
-3
-4
1
-5
0
-400
-6
-600
-800
-1000
-1200
1600
-1400
1400
1200
1000
800
600
400
200
E(mV
)
E(mV
)
(8a)
(8b)
Figure 5.18 Cyclic voltammograms of compound 8: 8a. Negative potential window
8b. Positive potential window
163
0
-200
TABLE 5.2
ELECTRODE POTENTIALS (V)a
Compound:
E(V)
2
1.080
5
0.817
1.280
1.399
7
-0.814
1.192
1.58
-1.079 (E1/2)
8
0..840
9
1.367
0.945
10
0.953
a. 0.1 M TBAClO4 in acetonitrile
164
5.3.5 Electronic Spectra
The overlayed experimental (black) and calculated (red) UV/Visible spectra of the Re(I)tricarbonyl bipyridine- 15,15’-cyanopyridyl retinal compound is shown in Figure 5.19. In all
cases, the band maxima in gas-phase TDDFT calculations are bathochromically shifted by
approximately 100 nm from the experimental data. Experimental results show three major bands
in the short-chained Re(I) complex. The calculated band in the visible region centered at 566 nm
originates from transitions from the HOMO to the L + 1 and L + 2 levels. The oscillator strength,
f determined for this band is 1.2653. This can be correlated with the high absorptivity band in its
experimental spectrum that is centered at 507 nm. The nature of the HOMO → L + 1 is a π to π*
type of transition located on its polyene chain and the HOMO → L + 2, observed at 545 nm
transition is charge transfer from the polyene chain to the metal center (Figure 5.20).
1.0
Abso rbance
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
W
avelength(nm)
Figure 5.19 Experimental and calculated UV/Visible data for 7
165
→
HOMO
L+1
} 566 nm
f = 1.2633
→
HOMO
L+2
→
H–1
L+1
→
} 392 nm
f = 0.4345
HOMO
L+4
→
HOMO
L+5
Figure 5.20 Some pertinent transitions in the UV/Visible spectrum for 7
166
The bands observed in the ultraviolet region of the actual spectrum are due to the ligandcentered (LC) transitions of the complex. These LC band assignments can be correlated to the
calculated bands observed at 392, 367 and 294 nm (Table 5.3, Figure 5.3, 5.5 and 5.6). Although
the nature of some of the states involved in electronic transitions involves both a metal-ligand
linear combination, it can be concluded that all of the states involved are predominantly ligand in
nature, thus affirming our assignments.They can also be correlated to the extinction coefficients
observed in the actual spectrum. Some of the pertinent optical transitions responsible for its
absorption process are shown in Figure 5.20 (Table 5.3).
1.0
Absorbance
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength (nm)
Figure 5.21 Experimental and calculated UV/Visible data for 8
The overlayed experimental (black) and calculated (red) UV/Visible data for Re(I)tricarbonyl bipyridine – 7’-cyano-7’-pyridyl-7’-apo-β-carotenal derivative is shown in Figure
5.21. Calculation results gave rise to a huge absorption band located in the Near-IR. Population
analysis showed that this optical transition is HOMO/LUMO in nature. This band assignment
will not be correlated to its actual Near-IR spectrum, in this dissertation, due to instrumental
167
limitations. However, the calculated band that is centered around 597 nm, f = 1.0501 is due to H
– 1 → L + 1 and HOMO → L + 4.
→
H–1
L+1
} 597 nm
f = 1.0501
→
HOMO
L+4
→
H–3
L+1
→
} 412 nm
f = 0.2012
H–2
L+1
→
H–1
L+4
Figure 5.22 Some pertinent transitions in the UV/Visible spectrum of 8
168
TABLE 5.3
CALCULATED UV/VISIBLE DATA FOR 7 AND 9
7
9
λ (nm)
566
f
1.2653
Major Contributors
HOMO → L + 1
HOMO → L + 2
λ (nm)
621
F
0.1304
Major Contributors
H–2→L+1
H–1→L+1
545
0.3428
HOMO → L + 2
569
0.3159
H–4→L+1
H–1→L+1
392
0.4345
H–1→L+1
HOMO → L + 4
HOMO → L + 5
553
1.5900
H – 5 → LUMO
H–4→L+1
367
0.1105
H – 4 → LUMO
H–1→L+1
542
0.9814
H – 5 → LUMO
H–2→L+1
H–1→L+1
366
0.2976
H – 4 → LUMO
H–1→L+1
HOMO → L + 4
388
0.3395
H – 10 → LUMO
H–7→L+1
H–5→L+2
294
0.1506
H–1→L+4
HOMO → L + 10
386
0.4391
H – 10 → LUMO
H–8→L+1
H–7→L+1
H–1→L+3
H–5→L+2
281
0.2423
H – 11 → LUMO
379
0.3195
H–8→L+1
H–4→L+3
H–3→L+5
H–1→L+3
374
0.1196
H–9→L+1
H–7→L+1
H–4→L+3
H–1→L+3
H–1→L+7
169
This band may shed light on the nature of the transition observed in the experimental
absorption spectrum of the long-chained derivative centered at 510 nm. This results to a
transition occurring from the H – 1 → L + 1 and HOMO → L + 4 as seen in Figure 5.22. The
calculated band at 412 nm maybe correlated to the nature of the transition observed in the lowenergy UV region of the absorption spectrum. Results show that the transitions are: H – 3 → L
+ 1, H – 2 → L + 1 and H – 1 → L + 4. It can be seen in Figure 5.22 that all transitions are
predominantly located on the polyene chain of the carotene ligand. The other calculated band is
centered at 329 nm (Table 5.4, Figure 5.10 and 5.11).
1.0
Absorbance
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 5.23 Experimental and calculated UV/Visible data for 9
170
The overlayed experimental and calculated UV/Visible spectrum in the Pt(II)-biphenylcyanopyridyl retinoid derivative is pictured in Figure 5.23. Some prominent transitions are
shown in Figure 5.24. The major peak centered at 553 nm in the calculated spectrum can be
correlated to the high absorptivity band located at 464 nm in its actual UV/Visible spectrum
(Figure 5.23, Table 5.4). The nature of the transition is metal-to-ligand charge transfer (MLCT)
(Figure 5.24).
The band in the UV region, λ = 386 nm can be attributed as ligand-centered (LC) in
nature as depicted in Figure 5.24 and Table 5.4. Although there are some contributions on the
metal, the transitions are predominantly polyene nature giving rise to the said transition. Some
transitions giving rise to this peak include H – 10 → LUMO and H – 8 → L + 1. Other
transitions involve H – 7 to L + 1, H – 1 to L + 3 and H – 5 to L + 2. It is noteworthy to mention
that in some transitions, the electron density originates from the Pt(II)-biphenyl group towards
the polyene chain of the retinoid ligand. Vertical transitions located at other wavelengths, 621,
569, 542, 388, 379 and 374 nm also contribute to the overall feature of the bands in the
calculated and experimental absorption spectra. The major contributors in these transitions are
presented in Table 5.3.
171
→
H–5
LUMO
}
553 nm
f = 1.5900
}
386 nm
f = 0.4391
→
H–4
L+1
→
H – 10
LUMO
→
H–8
L+1
Figure 5.24 Some pertinent transitions in the UV/Visible spectrum of 9
172
1.0
Absorbance
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 5.25 Experimental and calculated UV/Visible data for 10
Once again, a similar argument can be extended to the long-chained metal complex of
Pt(II)-biphenyl. The overlayed experimental (black) and calculated (red) is shown in Figure 5.25.
Some of its pertinent transitions are pictured in Figure 5.26 and all major contributors including
theoreticl oscillator strengths are presented in Table 5.4. The electronic states under the
calculated band observed around 711 nm in the simulated spectrum (Figure 5.25 and 5.26) are
responsible for the experimental band observed at 546 nm. The electronic transitions include H –
5 → LUMO, H – 3 → L + 1, H – 1 → L + 1 and HOMO → L + 2. It can be noted that the
electron density of H – 5 state for example, is located on the metal-biphenyl group.
173
H–5
LUMO
→
H–3
L+1
→
HOMO
} 711 nm
f = 0.4735
L+2
→
H–7
LUMO
→
508 nm
f = 0.5980
Figure 5.26 Some pertinent transitions in the UV/Visible spectrum of 10
174
TABLE 5.4
CALCULATED UV/VISIBLE DATA FOR 8 AND 10
8
λ (nm)
597
f
1.0501
10
Major Contributors
H–1→L+1
HOMO → L + 4
λ (nm)
793
F
0.2278
788
0.1117
Major Contributors
H–3→L+1
H–1→L+1
H – 3 → LUMO
H – 1 → LUMO
412
0.2012
H–3→L+1
H–2→L+1
H–1→L+4
740
728
0.4350
0.1632
H – 2 → LUMO
H – 3 → LUMO
H–2→L+1
329
0.1018
H–6→L+1
H–1→L+8
HOMO → L + 11
711
0.4735
701
0.1189
H – 5 → LUMO
H–3→L+1
H–1→L+1
HOMO → L + 2
H–5→L+1
HOMO → L + 3
692
1.0722
684
0.5207
659
3.3160
H – 3 → LUMO
H–2→L+1
H – 1 → LUMO
544
532
0.3320
0.5340
H – 6 → LUMO
H–6→L+1
508
0.5980
506
0.5135
H – 7 → LUMO
H – 7 → L + 11
H – 7 → LUMO
476
0.1055
467
0.1793
175
H – 5 → LUMO
H–3→L+1
HOMO → L + 2
H–8→L+1
H–3→L+2
H–8→L+1
H–3→L+2
H–2→L+3
The molecular orbitals of the other states are a combination of polyene, biphenyl and Pt
(II) orbitals. In case of H – 7, the electron density seems to lie on the polyene chain groups of the
metal complex. Other electronic states giving rise to the peaks are tabulated in Table 4 under
793, 788, 740, 728, 701, 692, 684 and 659 nm (Figure 5.4, 5.12, 5.13, 5.14 and 5.26). The peaks
in the UV region are based on the calculated states under 476 and 467 nm (Table 5.4, Figure 5.4,
5.12, 5.13, 5.14 and 5.26).
176
5.4 Summary
[Re(CO)3(bpy)L] and [Pt(bph)2(L)2] complex derivatives, where L = 15,15’-cyanopyridyl
retinal and all-trans-7’-cyano,7’-pyridyl-7’-apo-β-carotenal were synthesized. Infra-red spectral
analysis showed CO stretching frequencies at 2032 cm-1 for the Re(I)-cyanopyridyl retinal and
2034 cm-1 for the Re(I)-all-trans-7’-cyano,7’-pyridyl-7’-apo-β-carotenal. While another CO
stretch was found at 1916 and 1925 cm-1. ESI-MS gave a m/z of 811 for the Re(I) retinal and m/z
of 943 (MS-FAB) for the Re(I) carotenal. IR, elemental analysis and NMR spectroscopy
supported structures. Electrochemical (CV) data showed irreversible oxidation wave potentials.
UV/Visible data showed low-energy absorption, λmax in the visible region typical of polyene.
Ligand-centered (LC) and metal-to-ligand (MLCT) band assignments were based on Density
Functional Theory and Time Dependent Density Functional Theory Calculations (TDDFT).
177
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181
APPENDICES
182
Appendix A: Character Table for C2h Point Group
C2h
E
C2
i
σh
Ag
1
1
1
Bg
1
-1
1
Au
1
1
-1
Bu
1
-1
-1
1 Rz
-1 Rx , Ry
-1 z
1 x,y
183
x2 , y2, z2, xy
xz , yz
Appendix B: Excited-State and Electrochemical Properties of Ru(II)-polypyridyl Complex
184
Appendix C: Crystallographic Data for 15,15’-cyanopyridyl retinal
Figure C1: Optimized Structure of 15,15’-cyanopyridyl retinal
185
Figure C2: ORTEP Diagram of 15,15’-cyanopyridyl retinal
186
TABLE C1
ATOMIC COORDINATES ( x 104) AND EQUIVALENT ISOTROPIC DISPLACEMENT
PARAMETERS(Å2 x 103) FOR 15,15’-CYANOPYRIDYL RETINAL
U(eq) is defined as one third of the trace of the orthogonalized Uij tensor
X
Y
Z
U(eq)
C(1)
11181(5)
4790(4)
6044(1)
40(1)
C(2)
12805(5)
5172(4)
5651(1)
52(1)
C(3)
11976(6)
6992(5)
5335(2)
76(1)
C(4)
10639(6)
8542(5)
5695(1)
69(1)
C(5)
8690(5)
8196(4)
5982(1)
43(1)
C(6)
9350(5)
6149(4)
6199(1)
38(1)
C(7)
7809(5)
5766(4)
6599(1)
40(1)
C(8)
8189(5)
5110(4)
7103(1)
40(1)
C(9)
6749(4)
4722(4)
7520(1)
38(1)
C(10)
7413(5)
4254(4)
8027(1)
41(1)
C(11)
6303(5)
3798(4)
8503(1)
40(1)
C(12)
7140(4)
3397(4)
8992(1)
39(1)
C(13)
6209(4)
2894(3)
9494(1)
34(1)
C(14)
7328(4)
2545(4)
9945(1)
35(1)
C(15)
6737(4)
2001(3)
10476(1)
34(1)
C(16)
7941(4)
1663(3)
10909(1)
32(1)
C(17)
7405(4)
1033(3)
11460(1)
32(1)
C(18)
5425(4)
991(4)
11618(1)
40(1)
C(19)
5046(5)
349(4)
12133(1)
47(1)
C(20)
8332(5)
-218(4)
12348(1)
49(1)
C(21)
8880(4)
415(4)
11844(1)
43(1)
C(22)
9922(4)
1956(4)
10835(1)
39(1)
C(23)
4063(4)
2772(4)
9490(1)
42(1)
C(24)
4616(5)
4885(4)
7364(1)
50(1)
187
TABLE C1 (continued)
X
Y
Z
U(eq)
C(25)
7899(5)
9635(4)
6455(1)
53(1)
C(26)
6974(6)
8532(5)
5592(1)
76(1)
C(27)
11826(5)
2738(4)
6225(1)
52(1)
N(1)
6454(4)
-268(3)
12504(1)
47(1)
N(2)
11469(4)
2227(4)
10775(1)
56(1)
188
TABLE C2
ANISOTROPIC DISPLACEMENT PARAMETERS (Ǻ2 x 103)
FOR 15,15’-CYANOPYRIDYL RETINAL
The anisotropic displacement factor exponent takes the form:
-2 π2 [ h2 a*2 U11 + ... + 2 h k a* b* U12 ]
U11
U22
U33
U23
U13
U12
C(1)
52(2)
44(2)
34(2)
3(1)
-6(1)
-28(2)
C(2)
57(2)
54(2)
51(2)
0(2)
4(2)
-30(2)
C(3)
79(3)
70(3)
78(3)
16(2)
22(2)
-32(2)
C(4)
100(3)
56(2)
58(2)
5(2)
27(2)
-42(2)
C(5)
59(2)
44(2)
32(2)
5(1)
3(1)
-25(2)
C(6)
50(2)
44(2)
29(2)
5(1)
-5(1)
-27(2)
C(7)
49(2)
44(2)
36(2)
6(1)
-3(1)
-26(1)
C(8)
49(2)
41(2)
37(2)
4(1)
-4(1)
-24(1)
C(9)
49(2)
35(2)
36(2)
2(1)
1(1)
-21(1)
C(10)
46(2)
41(2)
38(2)
5(1)
2(1)
-21(1)
C(11)
48(2)
37(2)
38(2)
2(1)
4(1)
-20(1)
C(12)
45(2)
40(2)
36(2)
2(1)
2(1)
-20(1)
C(13)
42(2)
30(2)
34(2)
1(1)
2(1)
-17(1)
C(14)
35(2)
38(2)
36(2)
2(1)
4(1)
-18(1)
C(15)
31(2)
35(2)
37(2)
0(1)
2(1)
-15(1)
C(16)
30(2)
32(2)
35(2)
0(1)
3(1)
-13(1)
C(17)
35(2)
30(2)
31(2)
-1(1)
0(1)
-11(1)
C(18)
40(2)
51(2)
34(2)
4(1)
-3(1)
-21(1)
C(19)
49(2)
62(2)
38(2)
6(2)
1(1)
-30(2)
C(20)
48(2)
57(2)
40(2)
11(2)
-11(1)
-14(2)
C(21)
35(2)
52(2)
43(2)
9(1)
-2(1)
-17(1)
C(22)
35(2)
47(2)
37(2)
6(1)
-2(1)
-16(1)
189
TABLE C2 (continued)
______________________________________________________________________________
U11
U22
U33
U23
U13
U12
C(23)
49(2)
48(2)
38(2)
3(1)
-2(1)
-29(1)
C(24)
60(2)
57(2)
43(2)
7(2)
-3(2)
-33(2)
C(25)
65(2)
47(2)
53(2)
-3(2)
8(2)
-28(2)
C(26)
108(3)
67(2)
55(2)
18(2)
-33(2)
-31(2)
C(27)
63(2)
45(2)
52(2)
4(2)
-2(2)
-23(2)
N(1)
51(2)
54(2)
37(1)
9(1)
-3(1)
-21(1)
N(2)
42(2)
84(2)
53(2)
17(1)
-5(1)
-34(2)
190
TABLE C3
BOND LENGTHS [Å] FOR 15,15’-CYANOPYRIDYL RETINAL
C(1)-C(6)
1.339(4)
C(1)-C(27)
1.506(4)
C(1)-C(2)
1.511(4)
C(2)-C(3)
1.503(4)
C(2)-H(2A)
0.9700
C(2)-H(2B)
0.9700
C(3)-C(4)
1.453(4)
C(3)-H(3A)
0.9700
C(3)-H(3B)
0.9700
C(4)-C(5)
1.557(4)
C(4)-H(4A)
0.9700
C(4)-H(4B)
0.9700
C(5)-C(26)
1.518(4)
C(5)-C(25)
1.519(4)
C(5)-C(6)
1.531(4)
C(6)-C(7)
1.479(4)
C(7)-C(8)
1.328(4)
C(7)-H(7)
0.9300
C(8)-C(9)
1.458(4)
C(8)-H(8)
0.9300
C(9)-C(10)
1.351(4)
C(9)-C(24)
1.498(4)
C(10)-C(11)
1.440(4)
C(10)-H(10) 0.9300
C(11)-C(12)
191
1.346(4)
TABLE C3 (continued)
C(11)-H(11) 0.9300
C(12)-C(13)
1.443(3)
C(12)-H(12) 0.9300
C(13)-C(14)
1.357(4)
C(13)-C(23)
1.502(4)
C(14)-C(15)
1.424(3)
C(14)-H(14) 0.9300
C(15)-C(16)
1.355(3)
C(15)-H(15) 0.9300
C(16)-C(22)
1.442(4)
C(16)-C(17)
1.481(3)
C(17)-C(21)
1.383(4)
C(17)-C(18)
1.390(4)
C(18)-C(19)
1.377(4)
C(18)-H(18) 0.9300
C(19)-N(1)
1.329(4)
C(19)-H(19) 0.9300
C(20)-N(1)
1.324(4)
C(20)-C(21)
1.382(4)
C(20)-H(20) 0.9300
C(21)-H(21) 0.9300
C(22)-N(2)
1.142(3)
C(23)-H(23A) 0.9600
C(23)-H(23B) 0.9600
C(23)-H(23C) 0.9600
C(24)-H(24A) 0.9600
C(24)-H(24B) 0.9600
192
TABLE C3 (continued)
C(24)-H(24C) 0.9600
C(25)-H(25A) 0.9600
C(25)-H(25B) 0.9600
C(25)-H(25C) 0.9600
C(26)-H(26A) 0.9600
C(26)-H(26B) 0.9600
C(26)-H(26C) 0.9600
C(27)-H(27A) 0.9600
C(27)-H(27B) 0.9600
C(27)-H(27C) 0.9600
193
TABLE C4
BOND ANGLES [deg] FOR 15,15’-CYANOPYRIDYL RETINAL
C(6)-C(1)-C(27)
124.5(2)
C(6)-C(1)-C(2)
122.5(3)
C(27)-C(1)-C(2)
112.9(3)
C(3)-C(2)-C(1)
113.8(3)
C(3)-C(2)-H(2A)
108.8
C(1)-C(2)-H(2A)
108.8
C(3)-C(2)-H(2B)
108.8
C(1)-C(2)-H(2B)
108.8
H(2A)-C(2)-H(2B)
107.7
C(4)-C(3)-C(2)
110.6(3)
C(4)-C(3)-H(3A)
109.5
C(2)-C(3)-H(3A)
109.5
C(4)-C(3)-H(3B)
109.5
C(2)-C(3)-H(3B)
109.5
H(3A)-C(3)-H(3B)
108.1
C(3)-C(4)-C(5)
115.0(3)
C(3)-C(4)-H(4A)
108.5
C(5)-C(4)-H(4A)
108.5
C(3)-C(4)-H(4B)
108.5
C(5)-C(4)-H(4B)
108.5
H(4A)-C(4)-H(4B)
107.5
C(26)-C(5)-C(25)
109.0(3)
C(26)-C(5)-C(6)
110.5(2)
C(25)-C(5)-C(6)
110.4(2)
194
TABLE C4 (continued)
C(26)-C(5)-C(4)
111.3(3)
C(25)-C(5)-C(4)
106.5(2)
C(6)-C(5)-C(4)
109.1(2)
C(1)-C(6)-C(7)
121.8(2)
C(1)-C(6)-C(5)
123.3(2)
C(7)-C(6)-C(5)
115.0(2)
C(8)-C(7)-C(6)
125.1(3)
C(8)-C(7)-H(7)
117.5
C(6)-C(7)-H(7)
117.5
C(7)-C(8)-C(9)
128.1(3)
C(7)-C(8)-H(8)
116.0
C(9)-C(8)-H(8)
116.0
C(10)-C(9)-C(8)
117.9(3)
C(10)-C(9)-C(24)
123.7(2)
C(8)-C(9)-C(24)
118.4(2)
C(9)-C(10)-C(11)
128.4(3)
C(9)-C(10)-H(10)
115.8
C(11)-C(10)-H(10)
115.8
C(12)-C(11)-C(10)
122.3(3)
C(12)-C(11)-H(11)
118.9
C(10)-C(11)-H(11)
118.9
C(11)-C(12)-C(13)
127.6(3)
C(11)-C(12)-H(12)
116.2
C(13)-C(12)-H(12)
116.2
C(14)-C(13)-C(12)
118.2(3)
C(14)-C(13)-C(23)
123.5(2)
C(12)-C(13)-C(23)
118.3(2)
195
TABLE C4 (continued)
C(13)-C(14)-H(14)
116.2
C(15)-C(14)-H(14)
116.2
C(16)-C(15)-C(14)
124.6(2)
C(16)-C(15)-H(15)
117.7
C(14)-C(15)-H(15)
117.7
C(15)-C(16)-C(22)
117.9(2)
C(15)-C(16)-C(17)
126.0(2)
C(22)-C(16)-C(17)
116.1(2)
C(21)-C(17)-C(18)
116.3(2)
C(21)-C(17)-H(16)
121.3(2)
C(18)-C(17)-C(16)
122.4(2)
C(19)-C(18)-H(17)
119.4(3)
C(19)-C(18)-H(18)
120.3
C(17)-C(18)-H(18)
120.3
N(1)-C(19)-C(18)
124.9(3)
N(1)-C(19)-H(19)
117.6
C(18)-C(19)-H(19)
117.6
N(1)-C(20)-C(21)
124.9(3)
N(1)-C(20)-H(20)
117.6
C(21)-C(20)-H(20)
117.6
C(20)-C(21)-C(17)
119.4(3)
C(20)-C(21)-H(21)
120.3
C(17)-C(21)-H(21)
120.3
N(2)-C(22)-C(16)
178.6(3)
C(13)-C(23)-H(23A)
109.5
C(13)-C(23)-H(13B)
109.5
196
TABLE C4 (continued)
H(23A)-C(23)-H(23B)
109.5
C(13)-C(23)-H(23C)
109.5
C(23A)-C(23)-H(23C)
109.5
H(23B)-C(23)-H(23C)
109.5
C(13)-C(23)-H(23C)
109.5
C(9)-C(24)-H(24A)
109.5
C(9)-C(24)-H(24B)
109.5
H(24A)-C(24)-H(24B)
109.5
C(9)-C(24)-H(24C)
109.5
H(24A)-C(24)-H(24C)
109.5
H(24B)-C(24)-H(24C)
109.5
C(5)-C(25)-H(25A)
109.5
C(5)-C(25)-H(25B)
109.5
H(25A)-C(25)-H(25B)
109.5
C(5)-C(25)-H(25C)
109.5
H(25A)-C(25)-H(25C )
109.5
H(25B)-C(25)-H(25C)
109.5
C(5)-C(26)-H(26A)
109.5
C(5)-C(26)-H(25B)
109.5
H(26A)-C(26)-H(26B)
109.5
C(5)-C(26)-H(26C)
109.5
H(26A)-C(26)-H(26C)
109.5
H(26B)-C(26)-H(26C)
109.5
C(1)-C(27)-H(27A)
109.5
C(1)-C(27)-H(27B)
109.5
H(27A)-C(27)-H(27B)
109.5
C(1)-C(27)-H(27C)
109.5
197
TABLE C4 (continued)
H(27A)-C(27)-H(27C)
109.5
H(27B)-C(27)-H(27C)
109.5
C(20)-N(1)-C(19)
109.5
198
Appendix D: Crystallographic Data for 15,15’-dicyano-retinal
Figure D1: Optimized Structure of 15,15’-dicyano-retinal
199
Figure D2: ORTEP Diagram of 15,15’-dicyano-retinal
200
TABLE D1
ATOMIC COORDINATES ( x 104) AND EQUIVALENT ISOTROPIC DISPLACEMENT
PARAMETERS (Å2 x 103) FOR 15,15’-DICYANO-RETINAL
U(eq) is defined as one third of the trace of the orthogonalized Uij tensor
X
Y
Z
U(eq)
C(1)
13328(3)
5061(2)
11666(2)
49(1)
C(2)
12847(3)
4268(2)
10989(2)
51(1)
C(3)
12635(3)
4975(2)
10256(2)
50(1)
C(4)
11902(2)
6088(2)
10316(1)
41(1)
C(5)
11725(2)
6546(2)
10992(1)
36(1)
C(6)
12319(2)
5994(2)
11769(1)
40(1)
C(7)
13027(3)
6911(2)
12327(2)
55(1)
C(8)
11196(3)
5462(2)
12127(2)
55(1)
C(9)
10968(2)
7616(2)
11034(1)
38(1)
C(10)
9780(2)
7852(2)
10612(1)
39(1)
C(11)
9037(2)
8922(2)
10589(1)
37(1)
C(12)
7925(2)
9035(2)
10045(1)
39(1)
C(13)
7078(2)
10010(2)
9876(1)
38(1)
C(14)
6046(2)
9994(2)
9284(1)
40(1)
C(15)
5108(2)
10892(2)
9050(1)
37(1)
C(16)
4152(2)
10664(2)
8410(1)
42(1)
C(17)
3180(2)
11432(2)
8064(1)
43(1)
C(18)
2292(2)
11179(2)
7411(1)
41(1)
C(19)
9552(3)
9876(2)
11136(1)
42(1)
C(20)
5216(2)
12010(2)
9486(1)
46(1)
201
TABLE D1 (continued)
X
Y
Z
U(eq)
C(21)
11487(3)
6661(3)
9541(1)
54(1)
C(22)
1346(3)
12011(2)
7070(1)
46(1)
C(23)
2257(2)
10085(2)
7028(2)
43(1)
N(1)
571(3)
12640(2)
6762(2)
68(1)
N(2)
2205(2)
9220(2)
6710(1)
58(1)
202
TABLE D2
ANISOTROPIC DISPLACEMENT PARAMETERS (Ǻ2 x 103)
FOR 15,15’-DICYANO-RETINAL
The anisotropic displacement factor exponent takes the form:
-2 π2 [ h2 a*2 U11 + … + 2 h k a* b* U12 ]
U11
U22
U33
U23
U13
U12
C(1)
44(2)
50(2)
50(2)
3(1)
0(1)
6(1)
C(2)
51(2)
46(2)
59(2)
-4(1)
6(1)
8(1)
C(3)
53(2)
50(2)
47(2)
-9(1)
10(1)
-1(1)
C(4)
37(2)
42(2)
42(1)
1(1)
5(1)
-6(1)
C(5)
29(1)
38(1)
41(1)
2(1)
5(1)
-6(1)
C(6)
39(2)
43(2)
38(1)
2(1)
3(1)
-1(1)
C(7)
64(2)
51(2)
44(2)
-1(1)
-8(1)
1(1)
C(8)
60(2)
58(2)
50(2)
11(1)
19(1)
1(2)
C(9)
39(2)
39(1)
36(1)
1(1)
7(1)
-7(1)
C(10)
37(2)
38(2)
42(1)
1(1)
4(1)
-7(1)
C(11)
34(1)
41(2)
36(1)
4(1)
7(1)
-7(1)
C(12)
35(1)
40(2)
42(1)
-1(1)
7(1)
-8(1)
C(13)
34(1)
44(2)
36(1)
4(1)
5(1)
-5(1)
C(14)
35(1)
47(2)
37(1)
1(1)
7(1)
-5(1)
C(15)
33(1)
46(2)
34(1)
7(1)
7(1)
-6(1)
C(16)
39(2)
43(2)
46(1)
7(1)
13(1)
2(1)
C(17)
42(2)
43(2)
45(1)
2(1)
11(1)
-2(1)
C(18)
39(2)
47(2)
36(1)
1(1)
5(1)
-10(1)
C(19)
41(2)
45(2)
39(1)
2(1)
3(1)
-2(1)
203
TABLE D2 (continued)
U11
U22
U33
U23
U13
U12
C(20)
35(2)
50(2)
51(2)
12(1)
3(1)
0(1)
C(21)
63(2)
60(2)
37(1)
-4(1)
5(1)
-1(1)
C(22)
48(2)
45(2)
42(1)
-1(1)
-4(1)
-2(1)
C(23)
42(2)
42(2)
45(1)
15(1)
2(1)
5(1)
N(1)
74(2)
52(2)
71(2)
-5(1)
-9(1)
10(1)
N(2)
66(2)
41(1)
64(2)
7(1)
2(1)
6(1)
204
TABLE D3
BOND LENGTHS [Å] FOR 15,15’-DICYANO-RETINAL
C(1)-C(2)
1.518(4)
C(1)-C(6)
1.529(3)
C(1)-H(1A)
0.9700
C(1)-H(1B)
0.9700
C(2)-C(3)
1.513(4)
C(2)-H(2A)
0.9700
C(2)-H(2B)
0.9700
C(3)-C(4)
1.503(4)
C(3)-H(3A)
0.9700
C(3)-H(3B)
0.9700
C(4)-C(5)
1.347(3)
C(4)-C(21)
1.511(3)
C(5)-C(9)
1.469(3)
C(5)-C(6)
1.537(3)
C(6)-C(7)
1.539(3)
C(6)-C(8)
1.540(4)
C(7)-H(7A)
0.9600
C(7)-H(7B)
0.9600
C(7)-H(7C)
0.9600
C(8)-H(8A)
0.9600
C(8)-H(8B)
0.9600
C(8)-H(8C)
0.9600
C(9)-C(10)
1.343(3)
C(9)-H(9)
0.9300
C(10)-C(11)
1.451(3)
C(10)-H(10) 0.9300
205
TABLE D3 (continued)
C(11)-C(12)
1.364(3)
C(11)-C(19)
1.498(3)
C(12)-C(13)
1.423(3)
C(12)-H(12) 0.9300
C(13)-C(14)
1.353(3)
C(13)-H(13) 0.9300
C(14)-C(15)
1.428(3)
C(14)-H(14) 0.9300
C(15)-C(16)
1.386(3)
C(15)-C(20)
1.497(3)
C(16)-C(17)
1.395(3)
C(16)-H(16) 0.9300
C(17)-C(18)
1.370(3)
C(17)-H(17) 0.9300
C(18)-C(22)
1.425(4)
C(18)-C(23)
1.430(4)
C(19)-H(19A) 0.9600
C(19)-H(19B) 0.9600
C(19)-H(19C) 0.9600
C(20)-H(20A) 0.9600
C(20)-H(20B) 0.9600
C(20)-H(20C) 0.9600
C(21)-H(21A) 0.9600
C(21)-H(21B) 0.9600
C(21)-H(21C) 0.9600
C(22)-N(1)
1.143(3)
C(23)-N(2)
1.142(3)
206
TABLE D4
BOND ANGLES [deg] FOR 15,15’-DICYANO-RETINAL
C(2)-C(1)-C(6)
112.6(2)
C(2)-C(1)-H(1A)
109.1
C(6)-C(1)-H(1A)
109.1
C(2)-C(1)-H(1B)
109.1
C(6)-C(1)-H(1B)
109.1
H(1A)-C(1)-H(1B)
107.8
C(3)-C(2)-C(1)
108.9(2)
C(3)-C(2)-H(2A)
109.9
C(1)-C(2)-H(2A)
109.9
C(3)-C(2)-H(2B)
109.9
C(1)-C(2)-H(2B)
109.9
H(2A)-C(2)-H(2B)
108.3
C(4)-C(3)-C(2)
113.6(2)
C(4)-C(3)-H(3A)
108.8
C(2)-C(3)-H(3A)
108.8
C(4)-C(3)-H(3B)
108.8
C(2)-C(3)-H(3B)
108.8
H(3A)-C(3)-H(3B)
107.7
C(5)-C(4)-C(3)
123.1(2)
C(5)-C(4)-C(21)
124.6(2)
C(3)-C(4)-C(21)
112.2(2)
C(4)-C(5)-C(6)
122.1(2)
C(9)-C(5)-C(6)
115.8(2)
C(1)-C(6)-C(5)
111.0(2)
C(1)-C(6)-C(7)
107.8(2)
207
TABLE D4 (continued)
C(5)-C(6)-C(7)
110.7(2)
C(1)-C(6)-C(8)
110.0(2)
C(5)-C(6)-C(8)
108.73(19)
C(7)-C(6)-C(8)
108.5(2)
C(6)-C(7)-H(7A)
109.5
C(6)-C(7)-H(7B)
109.5
H(7A)-C(7)-H(7B)
109.5
C(6)-C(7)-H(7C)
109.5
H(7A)-C(7)-H(7C)
109.5
H(7B)-C(7)-H(7C)
109.5
C(6)-C(8)-H(8A)
109.5
C(6)-C(8)-H(8B)
109.5
H(8A)-C(8)-H(8B)
109.5
C(6)-C(8)-H(8C)
109.5
H(8A)-C(8)-H(8C)
109.5
H(8B)-C(8)-H(8C)
109.5
C(10)-C(9)-C(5)
125.6(2)
C(10)-C(9)-H(9)
117.2
C(5)-C(9)-H(9)
117.2
C(9)-C(10)-C(11)
127.8(2)
C(9)-C(10)-H(10)
116.1
C(11)-C(10)-H(10)
116.1
C(12)-C(11)-C(10)
118.3(2)
C(12)-C(11)-C(19)
122.8(2)
C(10)-C(11)-C(19)
118.9(2)
C(11)-C(12)-C(13)
129.0(2)
C(11)-C(12)-H(12)
115.5
208
TABLE D4 (continued)
C(13)-C(12)-H(12)
115.5
C(14)-C(13)-C(12)
121.3(2)
C(14)-C(13)-H(13)
119.3
C(12)-C(13)-H(13)
119.3
C(13)-C(14)-C(15)
127.8(2)
C(13)-C(14)-H(14)
116.1
C(15)-C(14)-H(14)
116.1
C(16)-C(15)-C(14)
116.4(2)
C(16)-C(15)-C(20)
124.1(2)
C(14)-C(15)-C(20)
119.5(2)
C(15)-C(16)-C(17)
126.0(2)
C(15)-C(16)-H(16)
117.0
C(17)-C(16)-H(16)
117.0
C(18)-C(17)-C(16)
123.6(3)
C(18)-C(17)-H(17)
118.2
C(16)-C(17)-H(17)
118.2
C(17)-C(18)-C(22)
121.1(2)
C(17)-C(18)-C(23)
122.9(2)
C(22)-C(18)-C(23)
116.0(2)
C(11)-C(19)-H(19A)
109.5
C(11)-C(19)-H(19B)
109.5
H(19A)-C(19)-H(19B)
109.5
C(11)-C(19)-C(19C)
109.5
H(19A)-C(19)-H(19C)
109.5
H(19B)-C(19)-H(19C)
109.5
C(15)-C(20)-H(20A)
109.5
C(15)-C(20)-H(20B)
109.5
209
TABLE D4 (continued)
H(20A)-C(20)-H(20B)
109.5
C(15)-C(20)-H(20C)
109.5
C(15)-C(20)-H(20C)
109.5
H(20A)-C(20)-H(20C)
109.5
H(20B)-C(20)-H(20C)
109.5
C(4)-C(21)-H(21A)
109.5
C(4)-C(21)-H(21B)
109.5
C(4)-C(21)-H(21C)
109.5
H(21A)-C(21)-H(21C)
109.5
H(21B)-C(21)-H(21C)
109.5
N(1)-C(22)-C(18)
176.3(3)
N(2)-C(23)-C(18)
178.4(3)
210
Appendix E: DFT optimized structures of the long-chained carotene derivatives
Figure E1. Optimized structures of all-trans-7’-cyano,7’-pyridyl-7’-apo-β-carotenal
Figure E2. Optimized structures of all-trans-7’,7’-dicyano-7’-apo-β-carotenal
211
Appendix F: DFT optimized structures of the retinoid/carotenoid metal complexes
Figure F1. Optimized structure of [Re(CO)3(bpy)(L)]
where L = 15,15’-cyanopyridyl retinal
Figure F2. Optimized structure of [Re(CO)3(bpy)(L)]
where L = all-trans-7’-cyano,7’-pyridyl-7’-apo-β-carotenal
212
Figure F3. Optimized structure of [Pt(bph)(L)2 ]
where L = 15,15’-cyanopyridyl retinal
213
Figure F4. Optimized structure of [Pt(bph)(L)2
where L = all-trans-7’-cyano,7’-pyridyl-7’-apo-β-carotenal
214
Appendix G: Geometry Optimization Results for [Ru(sn)2(bpy)2]2+
TABLE G1
CARTESIAN COORDINATES FROM DFT CALCULATION
Atomic
Symbol
Ru
N
C
C
C
H
C
C
H
H
H
N
C
C
C
C
H
C
H
H
H
N
C
C
C
C
H
C
H
H
_________________________________________________________
X
Y
Z
0.000000
0.000000
1.111548
-1.242837
1.042844
2.065990
-1.370483
-0.220630
1.957358
-2.350135
-0.308536
0.207554
1.496953
-0.814323
1.763161
-0.606734
-1.810222
0.705716
2.778905
-1.453788
0.900838
-2.058119
-2.389920
-3.053528
-3.736294
-4.409926
-2.743256
-4.758735
-3.990859
-5.167248
0.000000
0.000000
0.000000
0.005909
0.001720
0.000092
0.006516
0.003259
0.001366
0.009051
0.002966
-2.068144
-2.538412
-2.961502
-3.919652
-4.346644
-2.549910
-4.836532
-4.282005
-5.018348
-5.903990
0.016302
0.019843
0.042842
0.041316
0.064417
0.044371
0.061872
0.043394
0.082884
0.000000
2.078488
2.864467
2.676144
4.263503
2.354505
4.077240
4.883026
4.846561
4.540156
5.964745
-0.014967
-0.150031
0.089927
-0.174403
0.070375
0.189218
-0.062554
-0.278768
0.157664
-0.079613
0.406658
1.745431
-0.522661
2.155401
-0.171129
-1.559749
1.193056
3.208624
-0.947347
215
TABLE G1 (continued)
H
N
C
C
C
H
C
C
H
H
H
N
C
N
C
C
H
H
C
H
H
C
H
H
C
H
H
C
N
C
N
-5.799802
2.076464
2.966505
2.538100
4.342150
2.555723
3.909637
4.823735
5.012589
4.266542
5.883314
-0.051689
-0.092675
-0.152817
-0.228004
-0.146600
-0.532831
0.223408
-0.317766
-0.755620
-1.004074
1.073992
1.513455
1.765719
-1.590307
-2.276382
-1.965897
-1.584951
-1.571740
0.926625
0.799393
0.077139
-0.198721
0.825452
-1.497586
0.611435
1.826269
-1.770748
-0.711021
1.459485
-2.793179
-0.911677
2.044533
3.215070
-0.012029
-0.024577
4.683175
5.058211
5.063372
-0.047080
-1.002958
0.743865
0.146052
1.107739
-0.646403
5.225635
4.874078
4.864790
6.696304
7.875520
0.108447
0.076506
216
1.499329
-0.224193
-0.342806
-0.271706
-0.502961
-0.304861
-0.431459
-0.546486
-0.590803
-0.467787
-0.668547
0.008324
0.014090
-2.039585
-3.208363
0.027597
0.803934
-0.933336
-4.674568
-4.990006
-5.003544
-5.361151
-5.070203
-5.050955
0.286569
-0.493403
1.251686
0.293972
0.298678
-6.823986
-7.995965
TABLE G2
MULLIKEN ATOMIC CHARGES
1 Ru
2 N
3 C
4 C
5 C
6 H
7 C
8 C
9 H
10 H
11 H
12 N
13 C
14 C
15 C
16 C
17 H
18 C
19 H
20 H
21 H
22 N
23 C
24 C
25 C
26 C
27 H
28 C
29 H
30 H
31 H
32 N
33 C
34 C
35 C
36 H
37 C
38 C
39 H
0.809635
-0.321616
-0.108118
0.274699
-0.237364
0.260565
-0.298596
-0.173197
0.268902
0.253567
0.279501
-0.321390
0.275115
-0.108385
-0.298155
-0.237129
0.260501
-0.173137
0.253558
0.268878
0.279488
-0.311159
0.250859
-0.154913
-0.290365
-0.213480
0.255767
-0.175207
0.254789
0.269634
0.280043
-0.310794
-0.155215
0.250603
-0.213579
0.255662
-0.290229
-0.175225
0.269578
217
TABLE G2 (continued)
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
H
H
N
C
N
C
C
H
H
C
H
H
C
H
H
C
H
H
C
N
C
N
0.254844
0.280032
0.327178
-0.310009
0.326370
-0.308707
-0.425494
0.287167
0.285731
-0.425925
0.288046
0.285289
-0.343905
0.256288
0.255093
-0.343986
0.256492
0.255154
-0.161824
0.059940
-0.161925
0.060057
Sum of Mulliken charges = 2.00000
218
TABLE G3
SELECTED TDDFT SINGLET STATES FOR [Ru(sn)2(bpy)2]2+
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 1:
129 ->132
131 ->132
2.7489
451.03
0.0070
Excited State 2:
129 ->133
131 ->133
2.7630
448.72
0.0033
Excited State 3:
129 ->132
130 ->133
2.8876
429.37
0.0014
Excited State 4:
129 ->133
130 ->132
2.9042
426.92
0.0004
Excited State 5:
129 ->132
130 ->133
131 ->132
3.0580
405.44
0.1120
Excited State 6:
129 ->133
130 ->132
131 ->133
131 ->136
3.1804
389.84
0.0440
Excited State 7:
131 ->134
3.6817
336.76
0.0038
219
TABLE G3 (continued)
Excitation (eV)
Wavelength, λ (nm)
Oscillator
Energies
Strengths, f
______________________________________________________________________________
Excited State 8:
130 ->134
131 ->135
131 ->136
3.8288
323.82
0.0008
Excited State 9:
130 ->134
131 ->135
131 ->136
3.8594
321.25
0.0052
Excited State 10:
130 ->135
130 ->141
130 ->146
131 ->137
131 ->138
131 ->144
131 ->145
3.8928
318.50
0.0009
Excited State 11:
129 ->134
130 ->135
131 ->137
3.9104
317.06
0.0032
Excited State 12:
127 ->132
130 ->135
130 ->136
131 ->137
131 ->145
3.9542
313.55
0.0038
220
TABLE G3 (continued)
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 13:
129 ->135
129 ->141
129 ->142
129 ->143
129 ->146
130 ->138
130 ->144
130 ->145
131 ->135
131 ->136
131 ->141
131 ->146
3.9649
312.71
0.0017
Excited State 14:
127 ->133
128 ->132
129 ->135
130 ->134
130 ->137
131 ->136
3.9853
311.10
0.0131
Excited State 15:
129 ->134
129 ->145
130 ->135
130 ->136
130 ->141
130 ->146
131 ->137
131 ->144
131 ->145
4.0014
309.85
0.0662
221
TABLE G3 (continued)
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 16:
129 ->144
129 ->145
130 ->135
130 ->141
130 ->146
131 ->137
131 ->145
4.0310
307.57
0.0431
Excited State 17:
129 ->135
129 ->136
129 ->146
130 ->137
130 ->145
4.0569
305.61
0.0005
Excited State 18:
129 ->145
130 ->135
130 ->136
4.0916
303.02
0.0004
Excited State 19:
128 ->132
129 ->136
130 ->137
131 ->136
4.1113
301.57
0.0173
Excited State 20:
127 ->132
128 ->133
129 ->137
130 ->136
4.1317
300.08
0.0026
222
TABLE G3 (continued)
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 21:
127 ->133
129 ->135
129 ->136
130 ->137
131 ->136
4.2286
293.20
0.0464
Excited State 22:
127 ->132
127 ->133
128 ->132
128 ->133
4.2759
289.96
0.0007
Excited State 23:
127 ->132
127 ->133
128 ->132
128 ->133
4.2808
289.63
0.0025
Excited State 24:
129 ->144
129 ->145
130 ->141
130 ->146
131 ->138
4.4745
277.09
0.0088
Excited State 25:
127 ->133
129 ->135
129 ->139
130 ->144
130 ->145
131 ->139
131 ->142
131 ->143
131 ->146
4.4850
276.44
0.0109
223
TABLE G3 (continued)
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 26:
127 ->133
128 ->132
128 ->134
129 ->136
130 ->137
130 ->145
131 ->136
131 ->146
4.4959
275.77
0.2018
Excited State 27:
129 ->139
129 ->142
129 ->143
129 ->146
130 ->138
130 ->145
131 ->139
131 ->141
131 ->146
4.5634
271.69
0.0096
Excited State 28:
127 ->132
127 ->134
128 ->133
129 ->137
130 ->135
4.5703
271.28
0.5917
224
TABLE G3 (continued)
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 29:
129 ->138
129 ->144
129 ->145
130 ->141
130 ->146
131 ->138
131 ->144
131 ->145
4.6578
266.19
0.0150
Excited State 30:
129 ->138
130 ->138
131 ->139
131 ->141
131 ->142
131 ->143
131 ->146
4.7555
260.72
0.0017
Excited State 31:
129 ->138
129 ->145
131 ->140
4.8254
256.94
0.0261
225
TABLE G3 (continued)
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 31:
129 ->138
4.8254
256.94
0.0261
Excited State 32:
129 ->139
129 ->141
129 ->142
129 ->143
129 ->146
130 ->138
130 ->144
130 ->145
131 ->139
131 ->141
131 ->142
131 ->146
4.8841
253.85
0.0143
Excited State 33:
129 ->138
130 ->139
131 ->140
4.9182
252.09
0.0014
Excited State 34:
129 ->139
129 ->141
129 ->142
130 ->138
130 ->140
131 ->141
131 ->142
5.0141
247.27
0.0007
226
TABLE G3 (continued)
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 35:
129 ->139
129 ->140
129 ->141
129 ->145
130 ->139
130 ->140
130 ->141
130 ->142
130 ->143
130 ->146
131 ->140
5.0454
245.74
0.0163
Excited State 36:
129 ->139
129 ->140
129 ->141
130 ->138
130 ->140
130 ->141
130 ->142
5.0625
244.90
0.0031
Excited State 37:
120 ->132
128 ->134
5.0753
244.29
0.0248
Excited State 38:
119 ->132
120 ->133
127 ->134
5.1128
242.50
0.0692
Excited State 39:
125 ->132
125 ->133
5.1184
242.23
0.0000
227
TABLE G3 (continued)
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 40:
126 ->132
126 ->133
5.1199
242.16
0.0000
Excited State 41:
120 ->133
123 ->132
127 ->137
128 ->135
128 ->136
129 ->140
130 ->139
130 ->140
130 ->141
130 ->142
131 ->140
131 ->144
5.1463
240.92
0.0184
Excited State 42:
117 ->132
127 ->135
127 ->136
128 ->137
129 ->140
129 ->141
129 ->142
130 ->140
131 ->142
131 ->143
5.1490
240.79
0.0148
228
TABLE G3 (continued)
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 43:
117 ->132
123 ->132
123 ->133
127 ->135
127 ->136
128 ->137
129 ->141
129 ->142
129 ->143
130 ->140
131 ->141
131 ->143
5.1759
239.54
0.0015
Excited State 44:
117 ->133
120 ->133
123 ->133
124 ->132
124 ->133
127 ->137
128 ->135
128 ->136
129 ->140
129 ->141
130 ->141
130 ->142
130 ->143
131 ->140
131 ->144
5.1783
239.43
0.0116
Excited State 45:
126 ->132
126 ->133
5.2074
238.09
0.0000
229
TABLE G3 (continued)
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 46:
125 ->132
125 ->133
5.2116
237.90
0.0000
Excited State 47:
123 ->132
123 ->133
124 ->132
124 ->133
131 ->142
131 ->143
5.2163
237.69
0.0016
Excited State 48:
123 ->132
123 ->133
124 ->132
124 ->133
128 ->135
128 ->136
131 ->144
5.2190
237.57
0.0059
Excited State 49:
120 ->133
127 ->134
128 ->135
128 ->136
5.2824
234.71
0.0283
Excited State 50:
123 ->132
124 ->132
124 ->133
127 ->135
128 ->137
131 ->142
131 ->143
5.2872
234.50
0.0116
230
TABLE G4
SELECTED TDDFT TRIPLET STATES FOR [Ru(sn)2(bpy)2]2+
Excitation (eV)
Energies
Wavelength, λ (nm)
Oscillator
Strengths, f
Excited State 1:
132A ->133A
0.0322
38546.90
0.0003
Excited State 2:
114B ->131B
116B ->131B
118B ->131B
127B ->131B
127B ->143B
130B ->131B
0.1793
6913.01
0.0002
Excited State 3:
132A ->138A
115B ->131B
128B ->131B
0.2560
4842.41
0.0000
Excited State 4:
132A ->134A
132A ->135A
132A ->140A
0.7196
1722.85
0.0004
Excited State 5:
132A ->134A
132A ->135A
129B ->131B
0.9632
1287.17
0.0003
Excited State 6:
132A ->136A
132A ->137A
0.9719
1275.67
0.0009
231
Appendix H: Diagram of Grätzel Dye-Sensitized Solar Cell
Titanium Dioxide
Photosensitizer-Dye (i.e. Ru(bpy)32+ Derivative)
232