ABSTRACT
TALIAFERRO, CHELSEA MARIE. Photophysical Characterization and Ultrafast Dynamics of Diimine-Containing Metal Hydrides and Carbonyls (Under the direction of Prof. Felix N. Castellano).
Typically the center of photochemical reactivity, the metal hydride bond is instrumental in photocatalytic reactions involving transition metal hydrides. The ability to vary between photoacidic and photohydridic character is an extraordinary trait of some transition metal hydrides, such as [Ir(Cp*)(N^N)H]+, and allows for great tuning of photocatalytic behavior. However, this complicates the mechanistic study of these reactions. Time resolved
spectroscopies allow for direct observation of photoproduct generation, therefore better
elucidating the process(es) by which these reactions occur in different environments. And while transient absorption spectroscopy provides invaluable information on the underlying
photophysical and photochemical processes through visualization of electronic transitions of ground and excited states, specific bonds can be probed using transient infrared spectroscopy. While the Ir-H bond typically is in a clear window, far from diimine-ligand breathing modes, the low absorptivity of these modes makes probing them through time-resolved techniques difficult.
To aid in the effort of observing metal hydride stretching modes via transient infrared spectroscopy, and to better understand the underlying photophysics of Ir(III) diimine-containing hydrides, a robust dihydride with a short-lived metal-to-ligand charge transfer excited was selected for photophysical characterization. The stability of this complex allowed for direct interrogation of the Ir-H vibrational stretching modes using ultrafast transient infrared
The results presented in this document will hopefully lay the groundwork for future time-resolved infrared studies of more reactive metal hydrides.
Transient infrared spectroscopy has typically been reserved for complexes containing better IR tags than M-H bonds, such as CO or CN bonds. For example, a wealth of research has been performed on Re(I) tricarbonyl complexes in which the metal-carbonyl IR absorbances are examined after excitation. The backbonding nature of these ligands allows for easy
Photophysical Characterization and Ultrafast Dynamics of Diimine-Containing Metal Hydrides and Carbonyls.
by
Chelsea Marie Taliaferro
A dissertation submitted to the Graduate Faculty of North Carolina State University
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
Chemistry
Raleigh, North Carolina 2019
APPROVED BY:
_______________________________ _______________________________ Felix N. Castellano Elena Jakubikova
Committee Chair
_______________________________ _______________________________
BIOGRAPHY
Chelsea Taliaferro was born in Erie, Pennsylvania and grew up in Stafford, VA, where she attended Colonial Forge High School. She is the proud older sister to Kaitlin, Haley, Nic, Adele, and Ben. Grateful daughter to mother, Melissa, father, Joseph, and step-mother, Tara, she credits most of her personality and values to her large family. She received her Bachelor of Science in Chemistry from Virginia Tech in Blacksburg, VA where she met her husband, Ryland Taliaferro, and gained two more parents through Ellie and Jeff. It was at Virginia Tech where she fostered an interest in research after joining the lab of the late Prof. Karen Brewer, where she realized the beauty of synthetic and spectroscopic research of organometallic compounds. She then joined the Chemistry graduate program at North Carolina State University where she began her research under the direction of Prof. Felix Castellano, in which she was allowed the
ACKNOWLEDGMENTS
I would like to thank firstly my research advisor, Phil, for his guidance and support throughout my graduate career and for giving me the freedom to explore my scientific interests. I would also like to thank Evgeny for his infinite patience and guidance through the years as I learned ultrafast techniques. To Sofia, who not only was a great mentor, but also a good friend who gave me the outlets to vent on the bad days and celebrate the good days; I still have a collection of sticky notes you gave me to remind me what is important in life. To James, who was a great source of knowledge and provided me with confidence as a researcher, especially in my earlier years, and who was a formidable opponent on board game nights. To Joe Deaton, thank you for passing along your wealth of knowledge, especially in relation to air-free
chemistry and glovebox maintenance. To the rest of the Castellano group, past and present, who were an immense source of support and knowledge (also thank you for humoring my Halloween obsession). I would also like to thank the lab of Prof. Alex Miller for the insightful discussions and collaboration.
TABLE OF CONTENTS
LIST OF TABLES ... ix
LIST OF FIGURES AND SCHEMES ...x
CHAPTER 1: Fundamentals of Photophysical Processes and Spectroscopy ...1
1.1. Photophysics vs. Photochemistry ...1
1.2. Photophysical Processes ...1
1.2.1. Light Absorption ...1
1.2.2. Non-radiative Decay ...4
1.2.3. Radiative Decay ...5
1.3. Spectroscopic Instrumentation ...6
1.3.1. UV-vis Absorption (Electronic) Spectroscopy ...6
1.3.2. Transient Absorption (TA) Spectroscopy ...10
1.3.3. FT-IR Spectroscopy ...15
1.3.4. Ultrafast Transient Infrared (TRIR) Spectroscopy ...18
1.4. References ...22
CHAPTER 2: Photochemistry and Photophysics of [IrCp*(N^N)H]+ ...24
2.1. Background ...24
2.2. Experimental ...27
2.2.1. Synthesis of [IrCp*Cl2]2 ...27
2.2.2. Synthesis of [IrCp*(N^N)Cl]Cl [N^N = 2,2’-bipyridine (bpy), 1,10-phenanthroline (phen), 4,4’-di-tert-butyl-2,2’-bipyridine (dtbb)] ...28
2.2.3. Synthesis of [Ir(Cp*)(N^N)H]PF6 [N^N = 2,2’-bipyridine (bpy), 1,10-phenanthroline (phen), 4,4’-di-tert-butyl-2,2’-bipyridine (dtbb)] ...29
2.2.4. Synthesis of [RhCp*Cl2]2 ...30
2.2.5. Synthesis of [RhCp*(bpy)Cl]Cl ...31
2.2.6. Synthesis of [Rh(Cp*H)(bpy)]+ ...31
2.2.7. Nanosecond UV-VIS Transient Absorption (TA) Spectroscopy ...32
2.2.8. Ultrafast UV-VIS Transient Absorption (TA) Spectroscopy ...32
2.3. Spectroscopic Studies of [Ir(Cp*)(N^N)H]PF6 ...33
2.4.2. Photochemistry of [Ir(Cp*)(bpy)H]PF6 ...35
2.4.3. Photophysical Properties of [IrCp*(N^N)Cl]+ ...37
2.4. Electronic Structure Calculations of Cp*-containing Ir(III) Complexes ...39
2.5. Preliminary Spectroscopic and Computational Studies of Rh and Ru Cp* Diimine-containing Hydrides ...45
2.5.1. Preliminary UV-vis Spectroscopy and Computational Study of [Rh(Cp*H)bpy]+ ...45
2.5.2. Preliminary Ultrafast Dynamics of [Ru(C6Me6)bpyH]+ ...47
2.6. Acknowledgments ...51
2.7. References ...52
CHAPTER 3: Molecular Photophysics of [Ir(bpy)2H2]PF6 and [Ir(bpy)2D2]PF6 ...59
3.1. Research Summary ...59
3.2. Introduction ...60
3.3. Experimental ...62
3.3.1. Synthesis of cis-[Irbpy2Cl2]PF6 ...62
3.3.2. Synthesis of [Ir(bpy)2(CF3SO3)]CF3SO3 ...63
3.3.3. Synthesis of cis-[Irbpy2H2]PF6 ...63
3.3.4. Synthesis of cis-[Irbpy2D2]PF6 ...64
3.3.5. Ultrafast UV-VIS Transient Absorption (TA) Spectroscopy ...64
3.3.6. Nanosecond UV-VIS Transient Absorption (TA) Spectroscopy ...65
3.3.7. Ultrafast Mid-IR Transient Absorption (TRIR) Spectroscopy ...66
3.3.8. Electronic Structure Calculations using Density Functional Theory ...67
3.4. Results and Discussion ...67
3.4.1. Synthesis and Characterization ...68
3.4.2. FT-IR Spectroscopy and Solid-State Raman Spectroscopy ...68
3.4.3. Ultrafast Transient Absorption (TA) Spectroscopy ...71
3.4.4. Comparison to cis-[Irbpy2Cl2]+ ...73
3.4.5. Ultrafast Transient IR (TRIR) Spectroscopy of 1 and 2 ...74
3.4.6. DFT Calculated IR Difference Spectra ...77
3.5. Conclusions ...78
3.7. References ...79
CHAPTER 4: Ultrafast Dynamics of Re(I) Diimine-Containing Di- and Tricarbonyls ...86
4.1. Background and Scope ...86
4.2. Experimental ...89
4.2.1. Ultrafast UV-vis Transient Absorption (TA) Spectroscopy ...89
4.2.2. Ultrafast Mid-IR Transient Absorption (TRIR) Spectroscopy ...90
4.2.3. Electronic Structure Calculations ...91
4.3. Ultrafast TRIR Spectroscopy of Re(I) Tricarbonyls Containing NIBI-phen and an Ancillary Ligand ...91
4.3.1. Conclusions and Future Directions ...98
4.4. Ultrafast Dynamics of Re(I) Diimine-containing Dicarbonyls ...99
4.4.1. Conclusions and Future Directions ...103
4.5. References ...104
APPENDICES Appendix A ...112
Appendix B ...134
LIST OF TABLES
Table 2.1 Orbital contributions (%) to frontier MOs for [Ir(Cp*)(bpy)(CH3)]+ ... 41 Table 2.2 Orbital contributions (%) to frontier MOs for Ir(Cp*)(piq)(CH3) ... 42 Table 2.3 Orbital contributions (%) to frontier MOs for [Ir(Cp*)(piq)(CNAr)]+ ... 44 Table 4.1 Summary of FTIR results for the frequencies (ν) of metal-bound and
chromophoric ligand-bound CO vibrational modes in Re(I) tricarbonyls
in acetonitrile and tetrahydrofuran ... 92 Table A1 Summary of lifetimes measured via nsTA spectroscopy in methanol and
acetonitrile. *Indicates complex not stable in acetonitrile ... 121 Table A2 Summary of UFTA single-wavelength kinetics of [Ir(Cp*)(bpy)H]PF6 in
methanol after 400 nm excitation. (*Indicates dynamics continue past end
of delay line at 6.3 ns) ... 121 Table A3 Measured UFTA lifetimes of Ir(III) chlorides in methanol and water ... 124 Table A4 Summary of the TD-DFT results of the lowest energy transitions for
Rh(I) and Rh(III) complexes in three solvents.
(B3LYP-D3/6-311G**/LANL2DZ, PCM=Solvent) ... 126 Table A5 Summary of UFTA single wavelength kinetic fits of
[Ru(C6Me6)(bpy)H]PF6 in different solvent environments with 500 nm
excitation ... 129 Table A6 Summary of UFTA single wavelength kinetic fits of
[Ru(C6Me6)(bpy)H]Cl in different solvent environments with 500 nm
excitation ... 131 Table C1 Summary of lifetimes obtained at select vibrational frequencies in
acetonitrile and tetrahydrofuran ... 168 Table C2 Ultrafast lifetimes of high energy excited state features of Re dicarbonyls
1-9. (*Indicates lifetimes were collected using time-resolved PL and
nsTA performed by Hala Atallah) ... 176 Table C3 Summary of the solid-state FTIR-ATR metal carbonyl stretching
LIST OF FIGURES AND SCHEMES
Figure 1.1 Generalized Jablonski diagram of common photophysical processes ... 2
Figure 1.2 Effect of excited state distortion on the structure of emission bands. Figure was adapted from the literature ... 5
Figure 1.3 Typical Czerny-Turner monochromator configuration ... 8
Figure 1.4 Common electronic transitions observed in organometallic UV-vis absorption spectra. LMCT (red), MLCT (blue), d-d (yellow), LC (green). The d-orbital splitting assumes an octahedral or pseudo- octahedral geometry ... 9
Figure 1.5 Simplified schematic diagram of the nsTA apparatus ... 11
Figure 1.6 Simplified schematic of the ultrafast transient absorption apparatus ... 12
Figure 1.7 Simplified schematic of a typical FT-IR spectrometer ... 15
Figure 1.8 Types of vibrational modes ... 17
Figure 1.9 Schematic of the Castellano group ultrafast TRIR instrument. This diagram was provided by Dr. Sofia Garakyaraghi and used with permission ... 18
Figure 1.10 Schematic of the TRIR sample holder used in TRIR experiments. The casing and o-rings were omitted from the diagram. ... 19
Figure 1.11 IR transmission characteristics of various solvents ... 20
Scheme 2.1 Summary of the excited state deprotonation of [IrCp*(bpy)H]+ in methanol. The proposed mechanistic scheme was reproduced from the literature ... 25
Scheme 2.2 Proposed mechanism for photochemical H2 release in the presence of acetonitrile and weak acid. This proposed mechanistic scheme was reproduced from literature ... 27
Figure 2.2 (left) UFTA difference spectra of [Ir(Cp*)(bpy)H]PF6 measured in
deaerated acetonitrile excited at 400 nm (right) nsTA difference spectra of
[Ir(Cp*)(bpy)H]PF6 in deaerated acetonitrile excited at 430 nm ... 34 Figure 2.3 nsTA (λex = 430 nm) (left) and UFTA (λex = 400 nm) (right) difference
spectra of [Ir(Cp*)(bpy)H]PF6 in deaerated methanol ... 36 Figure 2.4 UFTA difference spectra of [IrCp*(N^N)Cl]Cl in water after 400 nm
excitation. [N^N = (a) bpy (b) phen (c) dtbb (d) dcb] ... 38 Scheme 2.3 Chemical structures of the three series of molecules investigated and
of the monodentate ancillary ligands, L ... 39 Figure 2.5 Depictions of the frontier orbitals for [Ir(Cp*)(bpy)(CH3)]+ (left) and
Ir(Cp*)(piq)(CH3) (right) at the optimized S0 and at the T1 geometries ... 41 Figure 2.6 Depictions of the frontier orbitals for [Ir(Cp*)(piq)(CNAr)]+ at the
optimized S0 and T1 geometries ... 43 Scheme 2.4 Molecular structure of [Rh(Cp*H)bpy]+ ... 44 Figure 2.7 UV-vis spectra of [Rh(Cp*H)(bpy)]Cl in acetonitrile (left) and
methanol (right) measured under inert atmosphere and after being
opened to air ... 46 Scheme 2.5 Molecular structure of [Ru(C6Me6)(bpy)H]+ ... 47 Figure 2.8 UFTA difference spectra of [Ru(C6Me6)(bpy)H]PF6 in (a) acetonitrile
(b) methanol (c) tetrahydrofuran following excitation at 500 nm ... 49 Figure 3.1 Molecular structures of cis-[Ir(bpy)2H2]PF6 (1) and cis-[Ir(bpy)2D2]PF6
(2) ... 67 Figure 3.2 (a) Solid-state FT-IR (ATR) and (b) off-resonance Raman spectra (λex
= 785 nm) of 1 (red) and 2 (blue). The insets expand the Ir-H and Ir-D
band region ... 68 Figure 3.3 FTIR spectra of various blank solvents as indicated in the legend (top)
and 1 (bottom) at the window chosen for possible TRIR studies (350 μm spacer width; *630 μm spacer used for dichloromethane case to
demonstrate the effect of spacer width on Ir-H stretching absorption ... 70 Figure 3.4 Sub-picosecond transient absorption difference spectra of 1 measured
in acetonitrile following excitation by 480 nm laser pulses (0.436 μJ/pulse, 100 fs fwhm). UV-vis (left) and NIR (right) experiments
Figure 3.5 Ultrafast TA difference spectra of [Ir(bpy)2Cl2]+ in acetonitrile (left) after excitation at 400 nm and kinetic fit of transient feature at 550 nm
(right). The lifetime of [Ir(bpy)2Cl2]+ was determined as 12.4 ± 0.5 ps ... 73 Figure 3.6 Experimental ultrafast TRIR difference spectrum (left) of 1 measured
in acetonitrile following excitation at 480 nm (5 μJ/pulse, 100 fs fwhm). The right spectrum depicts the DFT calculated IR difference spectrum of 1 over the same spectral window (A indicates an
antisymmetric stretch and S labels the symmetric stretch) ... 75 Figure 4.1 Comparison of potential energy surfaces of Re(I) tri- and dicarbonyls.
This figure was produced by Ms. Hala Attalah, and it was used with
permission ... 86 Scheme 4.1 Structures of the Re(I) tricarbonyl molecules investigated here ... 88 Scheme 4.2 Structures of the Re(I) dicarbonyls examined in this chapter. Labels
indicate distinguishing diimine ligands ... 88 Figure 4.2 Ultrafast TRIR difference spectra of [Re(NIBI-phen)(CO)3CH3CN] in
acetonitrile (top) and tetrahydrofuran (bottom). (λex = 470 nm, spacer
width = 390 μm) ... 93 Figure 4.3 Ultrafast TRIR difference spectra of [Re(NIBI-phen)(CO)3PPh3]PF6
following excitation at 470 nm in acetonitrile (top) and tetrahydrofuran
(bottom). (spacer width = 390 μm) ... 95 Figure 4.4 Ultrafast TRIR difference spectra of [Re(NIBI-phen)(CO)3DMAP]PF6
following excitation at 470 nm in acetonitrile. (spacer width = 390 μm) ... 96 Figure 4.5 Simulated TRIR difference spectra of Re(I) tricarbonyl complexes.
Computed by taking the difference of the simulated IR spectra calculated at the optimized S0 and T1 geometries. Performed at the
PBE0-D3/Def2-SVP/SDD (PCM = acetonitrile) level of theory ... 97 Figure 4.6 Ultrafast transient absorption difference spectra of phen-containing
complexes 1-5 in dichloromethane (λex = 500, 100 fs fwhm). The laser
scatter at 500 nm removed for clarity ... 100 Figure 4.7 Ultrafast transient absorption difference spectra of bpy-containing
complexes 6-9 in dichloromethane (λex = 500, 100 fs fwhm). The laser
scatter at 500 nm removed for clarity ... 101 Figure 4.8 Ultrafast transient infrared difference spectra following 500 nm
dichloromethane, where N^N is phen-based (top) or bpy-based
(bottom) ... 102
Figure A1 Absorption spectrum of [Ir(Cp*)(N^N)Cl]Cl measured in methanol ... 112
Figure A2 400 MHz 1H NMR spectrum for [Ir(Cp*)(bpy)H]PF6 in acetone-d6 ... 112
Figure A3 400 MHz 1H NMR spectrum for [Ir(Cp*)(phen)H]PF6 in acetone-d6 ... 113
Figure A4 400 MHz 1H NMR spectrum for [Ir(Cp*)(dtbb)H]PF6 in acetone-d6 ... 113
Figure A5 IR spectrum of [Ir(Cp*)(bpy)H]PF6 ... 114
Figure A6 IR spectrum of [Ir(Cp*)(phen)H]PF6 ... 114
Figure A7 IR spectrum of [Ir(Cp*)(dtbb)H]PF6 ... 114
Figure A8 HRESIMS of [Ir(Cp*)(bpy)H]PF6 in methanol ... 115
Figure A9 HRESIMS of [Ir(Cp*)(dtbb)H]PF6 in methanol ... 115
Figure A10 1H NMR spectrum of [RhCp*(bpy)Cl]Cl in CDCl3 ... 116
Figure A11 Absorbance of [Ir(Cp*)(N^N)H]PF6 in methanol ... 116
Figure A12 Representative kinetic trace and single exponential fit at 480 nm of nsTA of [Ir(Cp*)(bpy)H]PF6 in acetonitrile after excitation at 430 nm; τ = 82.7±0.7 ns ... 117
Figure A13 nsTA difference spectrum? of [Ir(Cp*)(dtbb)H]PF6 in acetonitrile following 430 nm excitation (left). Kinetic trace and singlet exponential fit at 500 nm; τ = 52.0 ± 0.4 ns (right) ... 117
Figure A14 nsTA difference spectrum of [Ir(Cp*)(phen)H]PF6 at time 0 in acetonitrile following excitation at 430 nm. Complex determined to be unstable under these conditions ... 118
Figure A15 Room temperature photoluminescence and excitation spectra of (a) [Ir(Cp*)(bpy)H]PF6 (b) [Ir(Cp*)(phen)H]PF6 (c) [Ir(Cp*)(dtbb)H]PF6 in deaerated acetonitrile ... 118
Figure A16 Photoluminescence of [Ir(Cp*)(N^N)H]+ at 77 K ... 119
Figure A18 nsTA kinetic fits of [Ir(Cp*)(N^N)H]+ (bpy = red, dtbb = green, phen
= yellow) at 600 nm after excitation at 430 nm in methanol ... 120 Figure A19 Kinetic traces of [Ir(Cp*)(bpy)H]PF6 at select wavelengths following
400 nm excitation in deaerated methanol ... 122 Figure A20 UFTA difference spectra at time 0 of Ir(III) chlorides (a) bpy (b) phen
(c) dtbb (d) dcb in methanol after 400 nm excitation ... 123 Figure A21 Kinetics and single exponential fits of the Ir(III) chlorides (a) bpy (b)
phen (c) dtbb (d) dcb in water (top) and methanol (bottom) at 450 nm ... 124 Figure A22 Absorbance change in [RhCp*bpyCl]Cl in a 3.0 M pH 5 sodium
formate solution (deaerated). The feature at 380 nm was a result of
baseline artifact ... 125 Figure A23 Simulated (B3LYP-D3/6-311G**/LANL2DZ, PCM=solvent) UV-vis
electronic spectra of [RhCp*bpyH]+ (red) and [Rh(Cp*H)bpy]+ (black)
in different solvents ... 125 Figure A24 HOMO and LUMO of [RhCp*bpyH]+ (top) and [Rh(Cp*H)bpy]+
(bottom; frontview and sideview depicted).
(B3LYP-D3/6-31G**/LANL2DZ) ... 126 Figure A25 Simulated IR spectrum of [RhCp*(6,6'-Me-bpy)H]+ in acetonitrile ... 127 Figure A26 Simulated IR spectrum of [RhCp*(bpy)H]+ in acetonitrile ... 127 Figure A27 UV-vis absorption spectra of [Ru(C6Me6)(bpy)H]PF6 in (a) acetonitrile
(b) methanol (c) tetrahydrofuran before and after UFTA measurements and after opening to air. (d) UV-vis spectra of [Ru(C6Me6)(bpy)H]Cl
in tetrahydrofuran before and after UFTA measurements ... 128 Figure A28 Steady-state photoluminescence emission of [Ru(C6Me6)(bpy)H]PF6
in acetonitrile. The artifact at 550 nm is the result of solvent baseline?
subtraction ... 128 Figure A29 Select kinetic traces, biexponential fits (black), and residuals (top)
measured at 380 nm for [Ru(C6Me6)(bpy)H]PF6 in (a) acetonitrile (b)
methanol and (c) tetrahydrofuran ... 129 Figure A30 UFTA difference spectra of [Ru(C6Me6)(bpy)H]Cl in (a) acetonitrile
(b) methanol (c) tetrahydrofuran after 500 nm excitation ... 130 Figure A31 Select kinetic traces, biexponential fits (black), and residuals (top)
acetonitrile (360 nm) (b) methanol (360 nm) and (c) tetrahydrofuran
(460 nm) ... 131 Figure A32 nsTA difference spectrum of [Ru(C6Me6)(bpy)H]Cl in tetrahydrofuran
after excitation at 500 nm ... 132 Figure A33 HOMO and LUMO of [Ru(C6Me6)(bpy)H]+ at the
B3LYP/ECP28MWB/6-31G** (PCM = acetonitrile) level of theory
using Gaussian09 ... 132 Figure A34 Simulated IR difference spectrum of [Ru(C6Me6)(bpy)H]PF6 ground
state and lowest triplet state at the B3LYP/ECP28MWB/6-31G**
(PCM = acetonitrile) level of theory ... 133 Figure B1 1H NMR spectrum of cis-[Ir(bpy)2Cl2]PF6 in DMSO-d6 (400 MHz) ... 134 Figure B2 1H NMR spectrum of [Ir(bpy)2(CF3SO3)]CF3SO3 in DMSO-d6 (400
MHz) ... 135 Figure B3 1H NMR spectrum of cis-[Ir(bpy)2H2]PF6 in DMSO-d6. Inset shows
the highly shielded hydride resonance at -17.92 ppm. The spectrum of cis-[Ir(bpy)2D2]PF6 was exactly the same, except for the undetected
hydride peak resulting from isotopic substitution ... 136 Figure B4 13C NMR spectrum of cis-[Ir(bpy)2D2]PF6 in DMSO-d6 (100 MHz) ... 137 Figure B5 ESI mass spectrum showing the theoretical (top) and measured
(bottom) isotope pattern for the [M]+ ion of cis-[Ir(bpy)2H2]+ ... 137 Figure B6 ESI mass spectrum showing the theoretical (top) and measured
(bottom) isotope pattern for the [M]+ ion of cis-[Ir(bpy)2D2]+ ... 138 Figure B7 Solid-state ATR FT-IR of synthesized product mixture after the
reaction of [Ir(bpy)2(CF3SO3)]CF3SO3 with NaBD4 in a H2O:EtOH
mixture ... 138 Figure B8 Ultrafast TA difference spectra of 2 in acetonitrile (λex = 480 nm). NIR
difference spectra are also shown on the right. Laser scatter at the
excitation wavelength was removed for clarity ... 139 Figure B9 Representative transient absorption kinetics of 1 (red) and 2 (blue) in
acetonitrile monitored at 510 nm. Kinetic traces display data within 400 ps following 480 nm pulsed laser excitation with single
exponential fit lines shown in black. The lifetimes of 1 and 2 were
Figure B10 Nanosecond TA of [Ir(bpy)2Cl2]+ in acetonitrile (left) after excitation at 420 nm and biexponential fit of feature at 470 nm (right). The
lifetimes were found to be 14.9 ± 3.5 ns and 331 ± 3.2 ns ... 140 Figure B11 UV-vis spectrum of 1 and [Ir(bpy)2Cl2]PF6 ... 140 Figure B12 Room temperature time-resolved photoluminescence spectra in
acetonitrile (left) and 77 K steady-state emission in 4:1 ethanol:
methanol (right) spectra of [Ir(bpy)2Cl2]PF6 ... 141 Figure B13 Low temperature (77 K) time-resolved photoluminescence of 1 and 2
in 4:1 ethanol:methanol ... 141 Figure B14 Ultrafast TRIR difference spectra of (a) 1 and (b) 2 measured in
acetonitrile-d3 following 480 nm excitation ... 142 Figure B15 TRIR difference spectra of 1 in acetonitrile following 480 nm
excitation. The spectral window was purposely shifted to lower energy in order to illustrate the broad excited state feature located in that
region ... 142 Figure B16 Excited state kinetics and single exponential fits of 1 at 2025 cm-1
(left) and 2130 cm-1 (right) in acetonitrile after excitation at 480 nm. Excited state lifetime determined to be 21.6 ± 1.3 ps (left). The growth and decay time constants were measured to be 1.3 ± 0.4 ps and 24.1 ± 0.9 ps, respectively. The growth in the negative polarity signal at early delay times is most likely the result of a superposition of absorptions
and bleaches at this particular frequency ... 143 Figure B17 Select excited state kinetics (colored traces) and single exponential fits
(black trace) of 1 (left) and 2 (right) at 1490 cm-1 in acetonitrile-d3 after excitation with 480 nm laser pulses. The lifetimes were
determined to be 28.9 ± 1.7 and 28.2 ± 2, respectively ... 143 Figure B18 Calculated IR spectra of the ground states of 1 (red) and 2 (blue) and
their respective 3MLCT excited states (black) ... 144 Figure B19 Calculated IR difference spectra of 1 (red) and 2 (blue) using the
simulated spectra from Figure B18 ... 144
Scheme C1 Generalized synthetic scheme for rhenium(I) dicarbonyl complexes
(1-9) ... 149 Figure C1 1H NMR spectrum of cis-[Re(CO2)(3,4,7,8-Me4phen)2](CF3SO3) (1) in
Figure C2 13C NMR spectrum of cis-[Re(CO2)(3,4,7,8-Me4phen)2](CF3SO3) (1)
in DCM-d2 (100 MHz) ... 151 Figure C3 1H NMR spectrum of cis-[Re(CO2)(4,7-Me2phen)2]PF6 (2) in ACN-d3
(400 MHz) ... 151 Figure C4 13C NMR spectrum of cis-[Re(CO2)(4,7-Me2phen)2]PF6 (2) in ACN-d3
(100 MHz) ... 152 Figure C5 1H NMR spectrum of cis-[Re(CO2)(5,6-Me2phen)2]PF6 (3) in ACN-d3
(400 MHz) ... 153 Figure C6 13C NMR specrum of cis-[Re(CO2)(5,6-Me2phen)2]PF6 (3) in ACN-d3
(100 MHz) ... 154 Figure C7 1H NMR spectrum of cis-[Re(CO2)(phen)2](CF3SO3) (4) in ACN-d
3
(400 MHz) ... 155 Figure C8 1H NMR spectrum of cis-[Re(CO2)(4,7-Ph2phen)2]PF6 (5) in ACN-d3
(400 MHz) ... 155 Figure C9 1H NMR specrum of cis-[Re(CO2)(5,5’-Me2bpy)2]PF6 (6) in DCM-d2
(400 MHz) ... 156 Figure C10 13C NMR spectrum of cis-[Re(CO2)(5,5’-Me2bpy)2]PF6 (6) in DCM-d2
(100 MHz) ... 157 Figure C11 1H NMR spectrum of cis-[Re(CO2)(4,4’-Dtbbpy)2]PF6 (7) in ACN-d3
(400 MHz) ... 157 Figure C12 13C NMR spectrum of cis-[Re(CO2)(4,4’-Dtbbpy)2]PF6 (7) in ACN-d3
(100 MHz) ... 158 Figure C13 1H NMR spectrum of cis-[Re(CO2)(bpy)2]PF6 (8) in ACN-d3 (400
MHz) ... 159 Figure C14 1H NMR spectrum of cis-[Re(CO2)(4,4’-Me2bpy)2]PF6 (9) in ACN-d3
(400 MHz) ... 159 Figure C15 13C NMR spectrum of cis-[Re(CO2)(4,4’-Me2bpy)2]PF6 (9) in ACN-d3
Figure C18 HRMS of cis-[Re(CO2)(5,6-Me2phen)2]+ (3) ... 163
Figure C19 HRMS of cis-[Re(CO2)(5,5’-Me2bpy)2]+ (6) ... 164
Figure C20 HRMS of cis-[Re(CO2)(4,4’-Dtbbpy)2]+ (7) ... 165
Figure C21 HRMS of cis-[Re(CO2)(4,4’-Me2bpy)2]+ (9) ... 166
Figure C22 FTIR spectra of [Re(NIBI-phen)(CO)3L]PF6 in acetonitrile and tetrahydrofuran ... 167
Figure C23 DFT-calculated IR spectra of [Re(NIBI-phen)(CO)3L]PF6. Performed at the PBE0-D3/Def2-SVP/SDD PCM=THF level of theory ... 167
Figure C24 Ultrafast TRIR difference spectra of [Re(NIBI-phen)(CO)3CH3CN]PF6 in CH3CN separated into "short" (< 10 ps) and "long" (> 2 ns) components. (λex = 470 nm, spacer = 390 μm). Note: color scheme is not identical to that seen in Figure 2 ... 168
Figure C25 Kinetic fits of [Re(NIBI-phen)(CO)3CH3CN]PF6 at select vibrational frequencies in ultrafast TRIR experiments ... 169
Figure C26 Ultrafast TRIR difference spectra of [Re(NIBI-phen)(CO)3PPh3]PF6 in CH3CN separated into "short" (< 10 ps) and "long" (> 2 ns) components. (λex = 470 nm, spacer = 390 μm) ... 170
Figure C27 Kinetic fits of [Re(NIBI-phen)(CO)3PPh3]PF6 at select vibrational frequencies in ultrafast TRIR experiments ... 170
Figure C28 Ultrafast TRIR difference spectra of [Re(NIBI-phen)(CO)3DMAP]PF6 in CH3CN separated into "short" (< 10 ps) and "long" (> 2 ns) components. (λex = 470 nm, spacer = 390 μm) ... 171
Figure C29 Kinetic fits of [Re(NIBI-phen)(CO)3DMAP]PF6 at select vibrational frequencies in ultrafast TRIR experiments ... 171
Figure C30 Calculated IR spectra of optimized S0 and T1 states of Re(I) tricarbonyl complexes. Performed at the PBE0-D3/Def2-SVP/SDD (PCM = acetonitrile) level of theory ... 172
Figure C32 nsTA difference spectra measured in deaerated dichloromethane with 500 nm pulsed excitation (2 mJ/pulse) (a) for complexes 1- 5 and (b)
for complexes 6-9. These data were collected by Ms. Hala Atallah ... 173 Figure C33 UFTA single wavelength kinetics of the Re(I) dicarbonyls 1-5
measured at 360 nm ... 174 Figure C34 UFTA single wavelength kinetics of the Re(I) dicarbonyls 6-9 at select
wavelengths ... 175 Figure C35 Simulated transient infrared difference spectra of complexes 1-9.
Symmetric (S) and antisymmetric (A) modes are labeled on the figure. Calculated at the B3LYP/D3/6-31G*/LANL2DZ (PCM
CHAPTER 1:Fundamentals of Photophysical Processes and Spectroscopy 1.1. Photophysics vs. Photochemistry
When comparing the terms “photophysics” and “photochemistry” we are drawn to the prefix “photo”, indicating the study of how light interacts with matter. However, the differences between the subjects lies in what processes they describe. It should make sense that photophysics describes the physical processes that occur in the presence of light such as absorption and emission processes. Photochemistry is used to describe reactions, or the chemistry, that takes place following light excitation. Another important distinction to be made is that while not every photophysically active compound exhibits photochemical behavior, a photochemical reaction is always preceded by photophysical processes generating the reactive excited state. Therefore, it is incredibly important to not only recognize the overall photochemical reactions taking place but also to probe the excited states responsible for such chemical reactions.
1.2. Photophysical Processes 1.2.1. Light Absorption
The initial photophysical process to occurring in a given molecular system is the
absorption of light. In particular, the absorption of one photon by a single molecule (A) leading to an excited state that lies at higher energy (*A), proportional to the photon energy, with respect to the ground state, Eq. 1.
A + hν → *A (1)
When determining if an absorption process will occur and with what intensity, one must refer to the relationship between the frequency of light and the energy change in the molecule. This is described in the Bohr equation, Eq. 21:
hν = Ef - Ei (2)
Where Ef and Ei are the energies of the excited state Ψf and ground state Ψi, respectively.
Absorption of different energies of light will result in different types of transitions. For instance, UV and visible light irradiation results in electronic transitions while mid-IR excitation results in transitions between ground state vibrational energy levels. This changes the spectroscopic methods necessary to probe these respective processes, which are described in section 1.3.
S
0S
1S
nT
1T
2 A b so rp ti o n F lu o re sce n ce P h o sp h o re sce n ce ISC Vibrational Relaxation ICSinglet Excited States Triplet Excited States
The second criteria that needs to be met for light absorption to occur is that there must be a finite probability for the transition. This probability is proportional to the square of the
transition moment, defined as 〈Ψ#𝜇̂Ψ&〉, where 𝜇̂ is the dipole moment operator.1,2 It can be experimentally obtained from the absorption spectra through its relationship with the oscillator strength, f, of a given transition and the integrated absorption intensity of the whole band. The oscillator strength is a dimensionless quantity with a value between 0 and 1 and is described by the Equation 31,2:
𝑓 = 4.315 × 1012∫ 𝜀𝑑𝑣 =789:;<=>
?@AB 〈Ψ#𝜇̂Ψ&〉C (3)
The transition moment (TM), originally defined as 〈Ψ#𝜇̂Ψ&〉, can be simplified through
several approximations. The first being the Born-Oppenheimer approximation, which assumes that nuclear motions associated with molecular vibrations are much slower than light absorption processes related to electronic motion. This allows the total wave function to be separated into electronic (ψ) and nuclear (θ) parts.1,2
𝑇𝑀 = ∫ ψ&𝜃&𝜇̂ψ#𝜃#𝑑𝜏 (4)
At this point, the dipole moment operator, 𝜇̂, is recognized to be independent of nuclear coordinates according to the Condon principle so the integral can rewritten as:
𝑇𝑀 = ∫ ψ#𝜇̂ψ&𝑑𝜏A∫ 𝜃#𝜃&𝑑𝜏I (5)
The wavefunction is then split into three parts through a final approximation in which ψ is factorized into the product of one-electron wavefunctions (orbitals), Φ, and spin functions, S. The final transition moment equation is therefore described as:
𝑇𝑀 =∫ 𝜙#𝜇̂𝜙&𝑑𝜏A∫ S#S&𝑑𝜏L∫ 𝜃#𝜃&𝑑𝜏I (6) Electronic
Transition Moment
Spin Overlap Integral
As depicted in Eq. 6, each integral represents a different selection rule for an electronic transition to occur. For the electronic transition moment, sufficient orbital overlap and proper symmetry is required for the transition to be considered electronically allowed. The aptly-named spin overlap integral describes spin allowed transitions as occurring between initial and final states of the same spin (e.g. singlet-singlet or triplet-triplet) while a spin forbidden transition describes transitions occurring between different spins (e.g. singlet-triplet or triplet-singlet). The last integral term is the quantum mechanical basis of the Franck-Condon principle.1,2
1.2.2. Non-radiative Decay Processes
Typically, light absorption initiates from the ground vibrational state (ν= 0) and vertical transitions terminate in higher vibrational levels in the excited state, known as a “vibrationally hot state”. This energy can be dissipated through thermal equilibrium with the surroundings in a process known as vibrational relaxation, depicted as short, purple lines in Figure 1.1. Within this definition, there lies different types of vibrational relaxation. Vibrational cooling (VC) is the result of vibrational coupling between the molecule and its solvent environment.3 Intramolecular vibrational redistribution (IVR) involves energy from the initially populated vibrational mode being dissipated among other vibrational modes in the molecule itself. Balzani and coworkers describe this as a large molecule acting as its own “heat bath”.1,3
interaction between the electron’s spin angular momentum and the angular momentum of orbitals that increases substantially with increasing atomic number.
1.2.3. Radiative Decay Processes
Once an excited molecule has reached the lowest excited singlet (S1) or triplet (T1) state through nonradiative transitions on the excited state potential energy surface, radiative transitions may then occur (Kasha’ rule). If the transition from the excited state to the ground state is spin-allowed (S1-S0), it is referred to as fluorescence. The spin forbidden (T1-S0) case is referred to as phosphorescence. The spin allowed-ness of the transition also determines the rate at which the process occurs. As such, fluorescence occurs much faster (~10-7> 𝛕 >10-10 s) with respect to phosphorescence (𝛕>10-7 s).
Important information can be gleaned from the experimental electronic absorption and fluorescence and/or phosphorescence emission spectra. In terms of the latter, usually most
E
Q Q
E
E00 E00
Ground Electronic
State Excited Electronic
State
Ground Electronic
State Excited Electronic
State
a) b)
obvious is the shape/structure and broadness of the band(s), which yields qualitative information on the type of excited state from which the emission is occurring and the extent of excited state distortion, as illustrated in Figure 1.2. Figure 1.2a represents the case featuring little excited state distortion, which will result in a sharp band at E00, the electronic transitions occurring between the lowest energy vibrational levels in the excited and ground states. Figure 1.2b shows the effect of significant distortion in the excited state geometry, which leads to an emission maximum Stokes shifted away from E00 and features a much broader, more Gaussian-shaped band. Important to note is the relationship between the absorption and emission spectra. Excited state displacement (distortion) leads to what is known as a Stokes shift, which is represented by the energy shift between the absorption and emission spectra. A smaller Stokes shift indicate smaller changes in size, shape, and solvation between the excited state and ground state, indicating little excited state distortion.4
1.3. Spectroscopic Instrumentation
1.3.1. UV-vis Absorption (Electronic) Spectroscopy
UV-vis spectrophotometers are commercially available in two primary instrument
formats – single beam and dual beam. In single beam spectrophotometers, the reference standard, typically the blank solvent, is measured prior to taking measurements on the actual sample dissolved in the same solvent. The dual beam apparatus’ splits the probe light into two separate beams that are directed into a sample and a reference simultaneously. In general, single beam spectrophotometers are more compact while dual beam spectrophotometers provide more reproducible and faster results as the reference does not need to be recorded separately.5
UV-vis spectrophotometers contain three major components: a light source, a
monochromator, and a detector. The light source is typically a deuterium lamp (190-400 nm), a tungsten filament (300-2500 nm) or a xenon arc lamp (160-2,000 nm). The monochromator contains a diffraction grating to separate the wavelengths of light. The Czerny-Turner
monochromator is a configuration of optics commonly used in spectrophotometers (Figure 1.3).6 It consists of an entrance slit, which is at the effective focus of a concave mirror that collimates the light before it is diffracted at the diffraction grating. The diffracted light then reaches another curved mirror that focuses the light to the exit slit. By rotating the diffraction grating, the
wavelength range reaching the exit slit changes. There are also multiple kinds of detectors that can be employed. Scanning spectrometers contain either a photomultiplier or photodiode detector. However, faster data acquisition can be obtained by non-scanning spectrometers in which array detectors such as photodiode arrays or charge coupled devices (CCDs) are used. In these spectrophotometers the diffraction grating is immobile, and all of the diffracted light is recorded simultaneously as these devices contain multiple detectors grouped into arrays.7
The general theory of measuring absorbance involves comparing the intensity of light passing through the reference (I0) and the sample (I) respectively:
𝑇 = 𝐼 𝐼QP (7)
%𝑇 = 100% × 𝑇 (8)
Equation 7 is referred to as the transmittance (T) of a sample. Essentially, transmittance is a measure of how much light passes through a sample. It is typically reported as %T, described in Equation 8. If no light is absorbed from a sample and it is perfectly transparent, then %T would be 100 while a totally opaque material would have a %T of 0. Absorbance is then calculated using the relationship in Equation 9:
𝐴 = − log(𝑇) = − log Z𝐼 𝐼Q [ P (9)
𝐴 = 𝜀𝑏𝑐 (10)
Equation 10 illustrates the relationship between sample concentration and its absorption
properties at a particular wavelength. This relationship is known as the Beer-Lambert Law where
A
B
C
D
E A) Light
B) Entrance slit C) Concave mirror D) Diffraction grating E) Concave mirror
F) Exit slit F
ε is the wavelength-dependent molar absorptivity of a sample reported in units of M-1cm-1, b is the path length of the sample (in cm), and c is the analyte concentration in units of molarity (M = moles/L).
Because the energy levels of all matter are quantized, only light of a particular energy will promote electronic transitions between certain molecular energy levels. The larger the energy difference between these levels, the higher energy is required to make the transition and therefore a shorter wavelength of light is absorbed as a result of the inverse relationship between the frequency of a photon and its associated wavelength.
Inorganic coordination and organometallic compounds both feature ligands and metal-based orbitals that can participate in a variety of electronic transitions. As ligands approach the metal center, the five-fold d-orbital degeneracy characteristic of the free ion is broken, revealing
σ(Ligand)
π(Ligand)
π* (Ligand)
t2g (Metal) eg(Metal)
Figure 1.4 Common electronic transitions observed in
a number of possible electronic transitions including ligand field (LF) d-d transitions, metal-to-ligand charge transfer (MLCT) transitions, metal-to-ligand-to-metal charge transfer (LMCT) transitions, as well as ligand-localized or ligand-centered (LC) transitions as generically depicted for an octahedral coordination compound in Figure 1.4. The molar absorptivity and energy of the transitions observed in UV-vis spectroscopy can be used to roughly assign distinct transitions. Ligand-centered (LC) π-π* transitions appear at lower wavelengths and are very intense (ε ~ 105 M-1cm-1). Charge transfer (CT) transitions appear at lower energy and can also be intense (ε ~ 104 M-1cm-1) and are typically broad and featureless. They are defined based on the direction of the transfer, with MLCT signifying electrons move from the metal to the ligand and LMCT meaning electrons move from the ligand to the metal. Metal-centered (MC) d-d transitions are localized on the metal and are Laporte forbidden in octahedral complexes and are incredibly weak even in lower symmetry molecules. The Laporte Rule applies to centrosymmetric molecules and states that transitions between states of the same symmetry with respect to inversion are forbidden. Therefore, transitions within a given set of d- or p-orbitals are
considered forbidden. The general allowedness of each of these transitions follow the selection rules described in section 1.2.1.
1.3.2. Transient Absorption (TA) Spectroscopy
those of short-lived photoproducts. The general principle is that a sample is “pumped” at a particular wavelength to excite a fraction of the molecules in the pump beam volume to an electronic excited state. A broadband “probe” is then used to measure the absorbance of the sample at select time delays following excitation.
The two types of transient UV-vis transient absorption techniques are known as
nanosecond TA (nsTA) and ultrafast TA (UFTA). As implied by their names, the two differ in terms of time resolution. nsTA can monitor processes ranging from low ns up to hundreds of milliseconds while UFTA looks from less than a picosecond up to several ns. The block diagram of a typical nsTA apparatus is depicted in Figure 1.5.
A monochromatic light pulse exits the Nd:YAG laser and is directed onto the center of a sample in a cuvette. A xenon lamp acts as the probe and pulses white light onto the sample. The pump and probe are overlapped spatially onto the sample at the standard cross-beam sample geometry. The transmitted light is then directed into a Czerny-Turner monochromator with a
A
A) Nd: YAG Laser B) Pump
Beam C) Xenon
Lamp D) White
Light E) Sample
F) Focusing Lens G) Slit
H) Diffraction Grating
I) PMT
(kinetics) J) CCD
(spectra)
B
C D E
F G H
Monochromator
I
J
triple grating. The diffracted light is then directed into either a PMT detector for kinetic measurements or an iCCD camera to obtain spectra.
While the nsTA system is able to use electronics to control the delay between the pump and probe pulses, UFTA requires the use of a delay line to obtain time resolution on the order of less than a ps. A delay line takes advantage of the speed of light by changing the distance at which the pulse must travel before reaching the sample. Following the beam path in Figure 1.6, a Ti:Sapphire Coherent Libra regenerative amplifier produces pulses (100 fs, 1 kHz, 4 mJ) at 800 nm which are directed into a beamsplitter to produce the pump and probe beams. The pump is directed into an optical parametric amplifier (OPerA Solo), which generates the desired
Figure 1.6 Simplified schematic of the ultrafast transient absorption apparatus.
excitation wavelength. The pump is then directed through a chopper, which blocks every other pulse from reaching the sample by operating at 500 Hz. This allows for measurements to be obtained both with and without excitation in order to calculate the △OD spectrum at each delay time. The pump is then directed into the sample and finally dumped at the back of the box onto a nonreflective surface. Concurrently, the probe beam is directed into the delay line referenced previously. It is made up of mirrors on a motorized track that move forward and back to change
~6.3 ns
A C
B
D
E
F
G H
I
J
L
A) Coherent Libra B) Beamsplitter C) OPerA Solo D) Probe Pulse E) Pump Pulse F) Delay Line
G) Polarizer H) Chopper
I) WLG
the distance at which the light must travel before reaching the sample, creating a temporal delay between pump and probe. The probe is then passed through a polarizer that is situated at the magic angle (54.7°) to remove any polarization artifacts. It is now that the 800 nm probe is converted into white light through a white light generating (WLG) crystal. The crystal is on a moving stage so as to prevent any thermal damage during the experiment if necessary. The pump is then directed into the sample, which is held is a 2 mm pathlength quartz cuvette and is stirred with a micro stir bar to prevent photodecomposition. It is imperative that the pump and probe are overlapped spatially so the portion of sample that is excited is the area that is monitored. Finally, the transmitted light after the sample reaches the detector.
All TA experiments measure the change in absorbance between the ground and excited states as shown in Equation 11, where △OD is change in optical density with the pump (ODpump) and without (ODprobe). Keeping in mind that optical density is a measure of the absorbance, whose relationship with transmittance was described in Equation 9, the relationship between sample transmittance and OD can be constructed as Equation 12. Essentially, the △OD spectrum obtained at a given time delay is mathematically described as the difference between ground state and excited state absorptions.
∆𝑂𝐷 = 𝑂𝐷bc=b − 𝑂𝐷bdefA (11)
∆𝑂𝐷 = − log ghijki
hilmn>o (12)
molecules are excited, meaning the absorption spectrum at a given time delay nominally contains a mixture of ground state and excited state species. As a result, △OD spectrum can consist of positive and negative absorbance values. The sign (+ or -) of an absorbance feature reveals whether it belongs to an excited state or ground state. When the ground state absorbs more than the excited state at a particular wavelength, it produces a negative absorbance feature (as
expected according to Equation 11). This is referred to as a ground state bleach. Positive features are indicative of a transition originating from an excited state or a ground state photoproduct. However, it is imperative to emphasize that the detector is not only recording the transmittance from the sample but all light that reaches it. As such, photoluminescence from the excited sample (stimulated emission) and scattering from the pump can be observed as negative features in UFTA experiments. In both cases, more light is reaching the detector when the sample is excited. The detector translates this as more transmittance (or less absorbance) at this wavelength, which produces an apparent negative feature.9
To determine kinetics related to the interconversion of different excited state features or those from the excited to the ground state, the changes in absorption intensity (OD) at select wavelengths are plotted vs the relative delay time between pump and probe. This generates a kinetic transient which can be fit to a single or multiexponential decay (Equation 13), where An is the amplitude coefficient of the time component n and τn is the time constant of that event(s) occurring at that wavelength.
𝑦(𝑡) = 𝑦P+ 𝐴s𝑒1u/wx + 𝐴
1.3.3. FT-IR Spectroscopy
Similar to UV-vis absorption spectroscopy, infrared (IR) spectroscopy also involves measuring how light is absorbed by a molecule but in a different region of the electromagnetic spectrum. Rather than measuring electronic transitions in the UV-visible region, IR spectroscopy measures the absorption of vibrational quanta in the molecule revealing important details related to the bonding characteristics in this moiety. In a sense, this technique yields important structural information regarding molecules that otherwise are not discernable through exclusive reliance on electronic spectroscopy.
In IR spectroscopy, the sample is irradiated with many frequencies of infrared light and the transmittance is measured at each frequency. The most common instrument for obtaining IR spectra is a Fourier Transform Infrared (FT-IR) spectrometer. This technique allows for all frequencies to be scanned simultaneously and the data to be combined into an interferogram, which is then Fourier transformed by a computer to produce the IR spectrum. A basic FT-IR spectrometer is depicted in Figure 1.7.
Figure 1.7 Simplified schematic of a typical FT-IR spectrometer.
H
A
B C
D
E
F G
A) IR Source B) Beamsplitter C) Transmitted Beam D) Fixed Mirror
The most important part of an FT-IR spectrometer is the Michelson Interferometer.10–12 It contains a beamsplitter, which directs the transmitted portion to a mirror in a fixed position and the reflected beam to a scanning mirror, whose relative distance moves during the
experiment. The two beams are then recombined at the beamsplitter where the light combines according to the superposition principle. When the mirrors are at the same distance from the beamsplitter, then the radiation undergoes constructive interference. If the relative mirror
distances are different, then the radiation is phase shifted and will undergo some constructive and some destructive interference. The recombined light is then passed through the sample, which can be suspended in solution or fused into a transparent disc of KBr or another pelleting agent, before reaching the detector. Finally, a computer performs a Fourier transform on the generated interferogram to extract the IR spectrum.
When the sample absorbs low-energy infrared light, it may induce vibrations in
covalently bonded atoms. Covalent bonds can be compared to stiff springs, that have the ability to move according to Hooke’s Law (Equation 14), where 𝜐̅ is the vibrational frequency of the bond, measured in wavenumbers (cm-1), k is the force constant which reflects the bond strength, and μ is the reduced mass (Equation 15), where m1 and m2 are the masses of the two bonded atoms. Based on this law, the stronger the bond and lighter the atoms, the higher the frequency.
𝜐̅ =C8s {|} (14)
𝜇 ===x=B
x~=B (15)
Figure 1.8 Types of vibrational modes.
In order for a molecule to be considered “IR active”, there must be an induced dipole change. Dipole moment is a vector quantity that depends on the electric vector of the IR
radiation and the orientation of the molecule, which changes as a bond moves. Essentially, dipole moment represents the polarity of a molecule and therefore a change in dipole induced by IR radiation would represent an induced charge separation in the molecule. As such, the symmetry of the molecule as well as the relative electronegativity between bonded atoms, and the
polarization of the radiation are incredibly important for interpreting vibrational spectra. For example, CO2 is a linear molecule and has two stretching modes. However, only the
antisymmetric stretch is IR active as the symmetric stretch does not involve a dipole moment change.
1.3.4 Ultrafast Transient Infrared (TRIR) Spectroscopy
As discussed previously, the ability to glean structural information of a molecule is a major benefit of IR spectroscopy over UV-vis absorbance. With this in mind, time-resolved
Symmetric Stretch Antisymmetric Stretch
Twisting Wagging Scissoring Rocking
Near Near Near
methods involving an IR probe will provide information on the vibrations of a molecule after light excitation by a visible pump. The experimental setup of an ultrafast TRIR apparatus is presented in Figure 1.9. The output from a Ti:Sapphire amplifier (4 mJ, 100 fs (fwhm) at 800 nm) is directed into a beamsplitter, which separates the light into two OPA’s. The visible pump beam is generated in one OPA before being sent to a computer-controlled delay line (4 ns maximum delay time) to generate time delays between pump and probe. The pulse is then directed through a chopper, polarizer, and variable neutral density filter before entering the sample chamber where it is centered on the sample holder and dumped on the back of the box.
The mid-IR probe is generated from the 2nd OPA before passing through a 2500 cm-1 long pass (LP) filter to remove residual signal and idler wavelengths. The pulse is then separated by a beamsplitter into probe and reference beams. The probe beam is overlapped with the pump while the reference beam is lowered in relation to the probe in the sample cell and does not interact with any excited portions of the sample. Both beams are both recollimated and travel in parallel into the entrance slit of an imaging spectrometer (Horiba Scientific iHR320). The signal is collected using a 64x2 dual array MCT (Mercury Cadmium Telluride) liquid N2–cooled detector (FPAS integrator and electronics from Infrared Systems Development Corporation). The
experiments are controlled by in-house composed LabVIEW software.
Samples are maintained in an optical cell consisting of two BaF2 or CaF2 windows separated by a Teflon spacer cut to allow flow of liquid sample through the top and bottom injection ports of the holder (Figure 1.10). BaF2 is has a greater transmission range but is less resistant to water damage than CaF2.13 The Teflon spacer can range in size between 150 μm to 950 μm to account for solvent absorbance and concentration of sample. The injection ports also allow for the holder to be used as a flow cell to account for possible sample heating effects.
Spacer 0.15 – 1 mm
BaF2
window (front)
BaF2
window (back)
Teflon Spacers (0.15 – 1 mm)
When preparing to conduct a TRIR experiment, solvent absorbance is one of the most important properties to account for. Compared to the UV-vis absorbance of most organic solvents, which typically exhibit absorbance cut-offs below 350 nm, IR backgrounds of these solvents reveal many vibrations that can make resolution of weaker sample stretching modes incredibly difficult. As seen in Figure 1.11, chloroform is a great solvent for TRIR experiments as it does not absorb much at all across a very wide window while ethanol absorbs very strongly across the whole window shown. However, solubility of the complex of interest and sample stability is incredibly important as well, limiting the available solvent choices as well.
To account for solubility, solvent absorbance, and strength of the IR signal for the sample, the Teflon spacer can be increased or decreased. Weak IR stretches will require much higher concentrations for detection, requiring not only a higher amount of sample but also an increased spacer width. However, this will also increase the absorbance of the solvent, which may overpower the sample signal. On the other hand, if the compound of interest contains
1.4. References
(1) Balzani, V.; Ceroni, P.; Juris, A. Photochemistry and Photophysics. Concepts, Research, Applications; Wiley-VCH: Weinheim, Germany, 2014.
(2) Horváth, O.; Stevenson, K. L. Charge Transfer Photochemistry of Coordination Compounds; John Wiley & Sons, Ltd: Weinheim, Germany, 1993.
(3) Rosspeintner, A.; Lang, B.; Vauthey, E. Ultrafast Photochemistry in Liquids. Annu. Rev. Phys. Chem. 2013, 64 (1), 247–271.
(4) Balzani, V.; Maestri, M. Intermolecular Energy and Electron Transfer Processes. In Photosensitization and Photocatalysis Using Inorganic and Organometallic Compounds; Kalyanasundaram, K., Grätzel, M., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1993; pp 15–49.
(5) Spectrophotometry Handbook.
(6) Czerny, M.; Turner, A. F. Über Den Astigmatismus Bei Spiegelspektrometern. Zeitschrift für Phys. 1930, 61 (11), 792–797.
(7) Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis, 7th ed.; Cengage Learning: Boston, MA, 2018.
(8) Foggi, P.; Bussotti, L.; Neuwahl, F. V. R. Photophysical and Photochemical Applications of Femtosecond Time-Resolved Transient Absorption Spectroscopy. Int. J. Photoenergy 2001, 3, 103–109.
(9) Berera, R.; van Grondelle, R.; Kennis, J. T. M. Ultrafast Transient Absorption
Spectroscopy: Principles and Application to Photosynthetic Systems. Photosynth. Res. 2009, 101 (2), 105–118.
Ether. Am. J. Sci. 1887, 34, 333–345.
(11) Hariharan, P. Two-Beam Interferometers. In Basics of Interferometry; Academic Press: Burlington, 2007; pp 13–22.
(12) Hariharan, P. Fourier Transform Spectroscopy. In Basics of Interferometry; Academic Press: Burlington, 2007; pp 145–151.
CHAPTER 2: Photochemistry and Photophysics of [IrCp*(N^N)H]+
A portion of this chapter has been previously published: Inorganic Chemistry, 2018, 57 (24), 15445-15461. DOI: 10.1021/acs.inorgchem.8b02753
2.1. Background
The piano stool IrCp*(N^N) chlorides and hydrides, where N^N is a diimine ligand, were first synthesized in the late 1980’s with the intention of catalyzing the water gas shift reaction (WGSR) under mild visible light initiating photochemical conditions.1 These molecules were designed with three targeted ligand components in mind: a bidentate diimine ligand (N^N) to produce MLCT excited states, pentamethylcyclopentadiene (Cp*) which serves as a stabilizing ligand for the myriad of oxidation states and coordination number changes these molecules would potentially undergo, and a labile chloride or hydride anion to promote catalytic reactivity with various substrates.
The WGSR generalized in Equation 1 below, has proven to be an important process in the chemical industry.2,3 For instance, it is an essential part of balancing H2/CO ratios in the Fischer–Tropsch process.4 Typically, catalysts used in this reaction require high temperatures and pressures but photochemical activation would potentially enable milder reaction conditions since molecular excited states would now represent the reactive species. Ziessel independently reported highly efficient catalysis of the WGSR using visible light excited Ir(III)Cp*(N^N) complexes at room temperature and atmospheric pressure. Also, by adding electron-withdrawing groups to the 4,4’ positions of the 2,2’-bipyridyl ligand (bpy), a significant increase in catalytic reactivity was observed. The opposite effect was true upon the addition of electron-donating groups in the identical substitution pattern on the bpy ligand.2
Following this initial report, there was a short period where the basic photophysical properties of these Ir(III) complexes in acetonitrile were studied.5 Static absorption and
photoluminescence emission properties were reported for both the chloride and hydride analogs containing either bpy or phen diimine fragment; the discussion of these results will be detailed later in this dissertation. Further photochemical studies were performed shortly after, revealing that these molecules were capable of accomplishing photochemical conversion of formate to CO2 and H2.6 Research on Ir(III)Cp*(N^N) molecular photophysics did not gain much traction until the 21st century, wherein the photophysical and photochemical properties of these molecules were examined using UV-Vis transient absorption techniques. Upon selective MLCT excitation in the visible region, [IrCp*(bpy)H]+ exhibited deprotonation to an IrI intermediate in methanol, suggesting a photoacidic excited state, Scheme 2.1.7 This was counterintuitive to the expectation of this molecule which was anticipated to exhibit photohydridic behavior. The photoacidic nature of this triplet excited state was further confirmed through comparison of the S0 and T1 pKa values, which were estimated to be 23.3 and -12 (in acetonitrile), respectively.8
Scheme 2.1 Summary of the excited state deprotonation of [IrCp*(bpy)H]+ in methanol. The proposed mechanistic scheme was reproduced from the literature.7
Ir
H N
N
+
Ir
H N
N
+ *
Ir N
N
H+
hv
Interestingly, later research suggested that the T1 MLCT excited state can also behave as a photohydride. This was determined through experimental hydride transfer reactions to various acids as well as through theoretical thermodynamic calculations. The relatively weak ground state hydricity of this molecule (△G°H− = 62 kcal/mol) is consistent with the lack of reactivity that [IrCp*(bpy)H]+ exhibits in the dark, even in strongly acidic environments. On the other hand, the hydricity of the excited state, △G°H−* = 14 kcal/mol, suggests an extremely hydridic excited state behavior to be expected. Although paradoxical, the ability for the T1 excited state to be both a stronger photoacid and photohydride was rationalized as MLCT excitation leads to the one-electron oxidation of the metal center, rendering the excited state more acidic, while also reducing the diimine ligand by one-electron, making a net transfer of H- (H+/2e-) now
thermodynamically favorable.8 Photochemical hydride transfer of [IrCp*(bpy)H]+ was evaluated with several organic acids, and H2 production was consistently observed for acids with pKa’s approaching 23.3 (acetic acid). When weaker acids were employed, decomposition of
[IrCp*(bpy)H]+ occurred, implying the molecule also serves as a sacrificial proton donor in those instances.8
2.2. Experimental
All commercially available reagents were used as received, without further purification. 1H and 13C NMR spectra were collected on a 400 MHz Varian Innova Spectrometer, and the resulting spectra were processed with the MestReNova 10.0.2 software package. Electrospray ionization (ESI) mass spectra were measured at the Michigan State University Mass
Spectrometry Core, East Lansing, MI. Solid-state ATR-FTIR was performed using a Bruker Alpha ATR-FTIR and OPUS Spectroscopy Software (v. 7.2). Air-free samples (hydrides) were prepared in a nitrogen-filled glovebox prior to their spectroscopic interrogation.
2.2.1. Synthesis of [IrCp*Cl2]2
This chloride-bridged dimer was synthesized according to a literature procedure.10
Iridium trichloride hydrate (1.2 g, 3.7 mmol) was dissolved in reagent grade methanol (35 mL) in
Ir H N N + Ir H N N + * Ir H N N Ir H N N 2+ Ir NCCH3 N N 2+ Ir N N H+ H2 NCCH3
Self-Quenching
a 3-neck 250-mL round bottom flask fitted with 2 septa and a condenser, which was connected to a Schlenk line. The solution was bubble-degassed with N2, and the entire apparatus purged with high purity N2. Pentamethylcyclopentadiene (Cp*) (0.6 mL, 3.8 mmol) that had been stored in a nitrogen-filled glovebox freezer was then added to the solution via syringe at RT and the mixture was refluxed under N2 for 48 hours with stirring. The resulting rusty-orange mixture was then cooled to room temperature and the orange solid was vacuum filtered in air using a medium frit. The collected filtrate was then dried on a rotary evaporator and dissolved in a minimal volume of chloroform before being recrystallized with hexane. This second fraction collected from the filtrate was combined with the first solid fraction and the two fractions were recrystallized together using chloroform and hexane, vacuum filtered in air, and dried in vacuo.Yield: 0.89 g, 60%, 1H NMR 400 MHz (CDCl3): 𝛿 ppm: 1.59 (s).
2.2.2. Synthesis of [IrCp*(N^N)Cl]Cl [N^N = 2,2’-bipyridine (bpy), 1,10-phenanthroline (phen), 4,4’-di-tert-butyl-2,2’-bipyridine (dtbb)]
vacuo. Yield: 70-80%, 1H NMR 400 MHz (acetone-d6): 𝛿 ppm: (bpy) 9.21 (d, 2H), 8.79 (d, 2H), 8.23 (t, 2H), 7.76 (t, 2H), 1.69 (s, 15H); (phen) 9.37 (d, 1H), 8.74 (d, 1H), 8.28 (t, 1H), 8.14 (s, 1H), 1.69 (s, 7.25H); (dtbb) 8.74 (d, 1H), 8.63 (s, 1H), 7.72 (d, 1H), 1.71 (s, 7.25H), 1.49 (s, 9H). UV-vis spectra are presented in Figure A1.
2.2.3. Synthesis of [Ir(Cp*)(N^N)H]PF6 [N^N = 2,2’-bipyridine (bpy), 1,10-phenanthroline
(phen), 4,4’-di-tert-butyl-2,2’-bipyridine (dtbb)]
These Ir(III) hydrides were synthesized according to literature procedures.9 A 3 M formic acid solution was prepared with pH adjusted to 5.0 using NaOH, and bubble degassed
phen, 62% (dtbb). 1H NMR 400 MHz (acetone-d6): 𝛿 ppm: (bpy) 9.14 (d, 1H), 8.70 (d, 1H), 8.25 (t, 1H), 7.78 (t, 1H), 1.94 (s, 7.5H), -11.44 (s, 0.5H); (phen) 9.69 (d, 1H), 9.04 (d, 1H), 8.53 (s, 1H), 2.21 (s, 7.5H), -11.15 (s, 0.5H); (dtbb) 8.98 (d, 1H), 8.73 (s, 1H), 7.76 (d, 1H), 1.92 (s, 7.5H), 1.46 (s, 9H), -11.50 (s, 0.5H) (Figures A2-A4) FT-IR (cm-1): (bpy) 2031(w), (phen) 2094(w), (dtbb) 2045(w) full IR spectra are shown in Figures A5-A7 ; HRESIMS spectra (bpy and dtbb) are presented in Figures A8-A9.
2.2.4. Synthesis of [RhCp*Cl2]2
1.0 g (4.78 mmol) of rhodium trichloride hydrate was added to 30 mL of methanol and the mixture was bubble degassed with N2 for 45 minutes. 0.75 mL (4.78 mmol) of
48 hours under N2, by which time a red-orange solid precipitate formed. The solid was vacuum filtered on a frit and washed with minimal methanol followed by ether and was then vacuum dried. The solid was then dissolved in a minimal amount of chloroform then added dropwise to stirring diethyl ether, precipitating the red-orange solid product. This isolated solid was used without further purification for the synthesis of [RhCp*(bpy)Cl]Cl .
2.2.5. Synthesis of [RhCp*(bpy)Cl]Cl
20 mL of methanol was bubble degassed for 45 minutes before 0.16 g (1 mmol) 2,2’-bipyridine was added as a solid. The dimer, [RhCp*Cl2]2 (0.32 g, 0.5 mmol), was then added to the solution and the mixture was stirred under N2 at room temperature for 2 hours. The reaction mixture quickly changed to a bright orange color after about 5 minutes and transitioned to a more yellow-orange appearance by the end of the 2 hour reaction time. The final solution was
evaporated to approximately 7 mL, which was added dropwise to diethyl ether, precipitating a yellow solid which was vacuum dried. 1H NMR 400 MHz (CDCl3): 𝛿 ppm: 8.95 (d, 2H), 8.85 (d, 2H), 8.22 (t, 2H), 7.82 (t, 2H) (Figure A10).
2.2.6. Synthesis of [Rh(Cp*H)(bpy)]+
![Figure 2.4 UFTA difference spectra of [IrCp*(N^N)Cl]Cl in water after 400 nm excitation](https://thumb-us.123doks.com/thumbv2/123dok_us/1638809.1204688/60.612.75.527.138.491/figure-ufta-difference-spectra-ircp-cl-water-excitation.webp)
