ABSTRACT TALIAFERRO, CHELSEA MARIE. Photophysical

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 spectroscopy despite the low extinction coefficients of such vibrations. The analogous deutero complex was also synthesized and photophysically characterized, producing similar conclusions. 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 determination of the nature of the excited states as changes in electron density around the metal center lead to large changes in C≡O frequency. The research presented here focuses on the transient infrared spectra and dynamics of a class of Re(I) tricarbonyls containing a highly- conjugated diimine ligand and different ancillary ligands. This structural motif allows for transient infrared spectroscopy to be performed on two separate windows, each visualizing the changes in electron density at the metal C≡O and ligand C=O regions. Majorly ligand localized excited states lead to large changes in the ultrafast spectra of the lower-energy ligand-based carbonyls while changes to the metal C≡O window are not as drastic, further supporting the largely ligand-centered assignment with small MLCT character. The ultrafast dynamics of several Re(I) dicarbonyl complexes containing simple diimine ligands are also presented, revealing minor changes in photophysical characteristics of these complexes upon changing the electronics of the diimine ligands. As these complexes are not complicated by low-lying LC- based excited states, the results are straightforward when compared to the investigated Re(I) tricarbonyls, with excitation into the low energy MLCT absorption band leading to marked increase in C≡O antisymmetric and symmetric stretching frequencies observed using transient infrared spectroscopy.

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

______David A. Shultz Thomas Theis

DEDICATION

To my loving husband

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 opportunity to explore the intricacies of time-resolved spectroscopies and photophysical research.

iii

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.

To my family, by both birth and marriage. You are the net that saved me from the rocks many times over the years. Thank you for the unending support and laughter. I am so grateful to each of you for the love you give and your advice to make sure I’m living my best life. To my friends, especially Kayley, Brittany, and Haley, all beautiful scientists who are going to do great things in the world. Occasional trips to DC were necessary for my sanity and I’m so happy to have such kind and intelligent friends who supported me over the past few years. Also thank you to Thomas for being my horror-movie bud and for siding with me every time I suggest burgers and milkshakes for dinner.

Finally, I would like to thank my husband, Ryland. You loved me the same on the days my experiments failed and I didn’t want to talk and the days I came home excitedly with new results and wanted to celebrate (usually with milkshakes). You listened to me rant about the things I’m passionate about and helped me foster all my new hobbies, no matter how silly. You listened to the same jokes over and over again because they worked the first time and have continued to pretend they land after the hundredth time. You are the most important thing to me, and I am so excited to start a new journey with you across the country!

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

2.3.1. Excited State Dynamics of [Ir(Cp*)(bpy)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 3.6. Acknowledgments ...... 78

vii

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 Appendix C ...... 145

viii

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 frequencies in 1-9 ...... 176

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 1.5 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 2.1 Schematic of the air-free apparatus used in the synthesis of metal hydrides. Constructed using A. 3-neck round bottom flask B. Stir bar C. Adapter connecting the Schlenk line to a side-neck D. Double-sided air-free medium porosity frit E. Schlenk flask attached to the Schlenk line ...... 30

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 performed under identical conditions ...... 72

+ 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 excitation of the [Re(N^N)2(CO)2]PF6 complexes 1-6 in

xii

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

1 Figure A2 400 MHz H NMR spectrum for [Ir(Cp*)(bpy)H]PF6 in acetone-d6 ...... 112

1 Figure A3 400 MHz H NMR spectrum for [Ir(Cp*)(phen)H]PF6 in acetone-d6 ...... 113

1 Figure A4 400 MHz H 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

1 Figure A10 H 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 A17 nsTA difference spectra of [Ir(Cp*)(dtbb)H]PF6 (left) and [Ir(Cp*)(phen)H]PF6 (right) in deaerated methanol (λex = 430 nm) ...... 119

xiii

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) from UFTA spectra of [Ru(C6Me6)(bpy)H]Cl measured in (a)

xiv

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

1 Figure B1 H NMR spectrum of cis-[Ir(bpy)2Cl2]PF6 in DMSO-d6 (400 MHz) ...... 134

1 Figure B2 H NMR spectrum of [Ir(bpy)2(CF3SO3)]CF3SO3 in DMSO-d6 (400 MHz) ...... 135

1 Figure B3 H 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

13 Figure B4 C 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 determined to be 24.4 ± 0.3 and 24.8 ± 0.5 ps, respectively ...... 139

+ 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 -1 (black trace) of 1 (left) and 2 (right) at 1490 cm 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

1 Figure C1 H NMR spectrum of cis-[Re(CO2)(3,4,7,8-Me4phen)2](CF3SO3) (1) in DCM-d2 (400 MHz) ...... 150

xvi

13 Figure C2 C NMR spectrum of cis-[Re(CO2)(3,4,7,8-Me4phen)2](CF3SO3) (1) in DCM-d2 (100 MHz) ...... 151

1 Figure C3 H NMR spectrum of cis-[Re(CO2)(4,7-Me2phen)2]PF6 (2) in ACN-d3 (400 MHz) ...... 151

13 Figure C4 C NMR spectrum of cis-[Re(CO2)(4,7-Me2phen)2]PF6 (2) in ACN-d3 (100 MHz) ...... 152

1 Figure C5 H NMR spectrum of cis-[Re(CO2)(5,6-Me2phen)2]PF6 (3) in ACN-d3 (400 MHz) ...... 153

13 Figure C6 C NMR specrum of cis-[Re(CO2)(5,6-Me2phen)2]PF6 (3) in ACN-d3 (100 MHz) ...... 154

1 Figure C7 H NMR spectrum of cis-[Re(CO2)(phen)2](CF3SO3) (4) in ACN-d3 (400 MHz) ...... 155

1 Figure C8 H NMR spectrum of cis-[Re(CO2)(4,7-Ph2phen)2]PF6 (5) in ACN-d3 (400 MHz) ...... 155

1 Figure C9 H NMR specrum of cis-[Re(CO2)(5,5’-Me2bpy)2]PF6 (6) in DCM-d2 (400 MHz) ...... 156

13 Figure C10 C NMR spectrum of cis-[Re(CO2)(5,5’-Me2bpy)2]PF6 (6) in DCM-d2 (100 MHz) ...... 157

1 Figure C11 H NMR spectrum of cis-[Re(CO2)(4,4’-Dtbbpy)2]PF6 (7) in ACN-d3 (400 MHz) ...... 157

13 Figure C12 C NMR spectrum of cis-[Re(CO2)(4,4’-Dtbbpy)2]PF6 (7) in ACN-d3 (100 MHz) ...... 158

1 Figure C13 H NMR spectrum of cis-[Re(CO2)(bpy)2]PF6 (8) in ACN-d3 (400 MHz) ...... 159

1 Figure C14 H NMR spectrum of cis-[Re(CO2)(4,4’-Me2bpy)2]PF6 (9) in ACN-d3 (400 MHz) ...... 159

13 Figure C15 C NMR spectrum of cis-[Re(CO2)(4,4’-Me2bpy)2]PF6 (9) in ACN-d3 (100 MHz) ...... 160

+ Figure C16 HRMS of cis-[Re(CO2)(3,4,7,8-Me4phen)2] (1) ...... 161

+ Figure C17 HRMS of cis-[Re(CO2)(4,7-Me2phen)2] (2) ...... 162

xvii

+ 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 C31 Simulated TRIR difference spectra of [Re(NIBI-phen)CH3CN] at early delay times. Performed at the PBE0-D3/Def2-SVP/SDD (PCM = acetonitrile) level of theory ...... 172

xviii

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 solvent=DCM) level of theory ...... 177

xix

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)

The absorption process is highlighted by the blue arrow in the generalized Jablonski diagram presented in Figure 1.1. Absorption to a higher excited state results in deactivation pathways to take place, which can include vibrational relaxation, internal conversion (IC), intersystem crossing (ISC), fluorescence, phosphorescence, and photochemical reactions.

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.

Singlet Excited States Triplet Excited States

Vibrational Relaxation Sn T2 IC

ISC S1

T1 Absorption Fluorescence Phosphorescence

S0 Figure 1.1 Generalized Jablonski diagram of common photophysical processes.

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

1,2 transition moment, defined as 〈Ψ�̂Ψ〉, where �̂ is the dipole moment operator. 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:

� = 4.315 × 10 ∫ �� = 〈Ψ �̂Ψ 〉 (3)

The transition moment (TM), originally defined as 〈Ψ�̂Ψ〉, 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

�� = ∫ ψ��̂ψ� �� (4)

At this point, the dipole moment operator, �̂, is recognized to be independent of nuclear coordinates according to the Condon principle so the integral can rewritten as:

�� = ∫ ψ�̂ψ �� ∫ � �� (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:

�� = ∫ ��̂�� ∫ SS �� ∫ � �� (6)

Electronic Spin Franck-Condon Transition Overlap Term Moment 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

Excited molecules can also undergo nonradiative transitions between excited states.

Internal conversion (IC) occurs between states of the same spin (e.g. singlet-singlet) while intersystem crossing (ISC) converts between states of different spin (e.g. singlet-triplet).

Although formally a “spin forbidden” process, ISC is more common in inorganic compounds where the heavy atom effect leads to large spin-orbit coupling (SOC). Spin orbit coupling is an

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> � >10-10 s) with respect to phosphorescence (�>10-7 s).

a) b) E E

Excited Electronic State Excited Electronic State

Ground Ground Electronic Electronic State State

E00 E00 Q Q

Figure 1.2 Effect of excited state distortion on the structure of emission bands. Figure was adapted from the literature.1

Important information can be gleaned from the experimental electronic absorption and fluorescence and/or phosphorescence emission spectra. In terms of the latter, usually most

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

Fortunately, light absorption, the fundamental process activating all excited state photophysics and photochemistry, requires relatively straightforward instrumentation for quantitative measurements. Ultraviolet-visible (UV-vis) spectroscopy gauges the relative absorption intensity (and allowed-ness) of photon absorption in the UV-vis region of the electromagnetic spectrum determined as a function of wavelength. It is commonly used to visualize the many electronic transitions present in a sample and can also be used analytically to determine the concentration of molecules present through the Beer-Lambert law discussed below.

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

A C

A) Light B) Entrance slit C) Concave mirror D) Diffraction grating D E) Concave mirror F) Exit slit F

Figure 1.3 Typical Czerny-Turner monochromator configuration.6

The general theory of measuring absorbance involves comparing the intensity of light passing through the reference (I0) and the sample (I) respectively:

� = � (7) �

%� = 100% × � (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:

� = − log(�) = − log � (9) �

� = �� (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

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.

π* (Ligand)

e (Metal) g

t2g (Metal)

π (Ligand)

σ (Ligand)

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. 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

a number of possible electronic transitions including ligand field (LF) d-d transitions, metal-to- ligand charge transfer (MLCT) transitions, 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

A time-resolved version of UV-vis absorption, transient absorption (TA) spectroscopy measures the absorbance of the excited states of a given sample, generated after a short pulse of light promotes the molecule under investigation into the excited state. This method is used to monitor the absorption profiles of excited states as a function of time after the laser pulse and is also used to measure the kinetics of excited state decay processes (such as lifetime) as well as

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.

B A

A) Nd: YAG F) Focusing I Laser Lens B) Pump G) Slit H Beam H) Diffraction F G D J C) Xenon Grating C E Lamp I) PMT D) White (kinetics) Light J) CCD E) Sample (spectra) Monochromator

Figure 1.5 Simplified schematic diagram of the nsTA apparatus.

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

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

H A) Coherent Libra G) Polarizer D G B) Beamsplitter H) Chopper C) OPerA Solo I) WLG I D) Probe Pulse J) White Light E E) Pump Pulse K) Sample ~6.3 ns J F) Delay Line L) Detector

A C F L

B 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

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.

∆�� = �� − �� (11)

∆�� = − log (12)

The spectrum obtained without the pump contains no excited state molecules and as such is treated as a baseline in which the ground state is the only absorbing species, aside from any experimental artifacts.8 However, upon excitation of the sample by the pump, only a fraction of

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.

/ / �(�) = � + �� + �� … (13)

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.

H E

G B F

A) IR Source E) Reflected Beam B) Beamsplitter F) Scanning Mirror C) Transmitted Beam G) Sample A D) Fixed Mirror H) Detector

Figure 1.7 Simplified schematic of a typical FT-IR spectrometer.

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 �̅ 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.

�̅ = (14)

� = (15)

There are several types of vibrational modes that consist of stretches and bends (Figure

1.8). For a molecule made up on N atoms, there are 3N degrees of freedom consisting of six rotations and translations of the molecule and 3N-6 (for non-linear molecules) or 3N-5 (for linear molecules) fundamental vibrations.

Symmetric Stretch Antisymmetric Stretch

Near Near Near Far

Twisting Wagging Scissoring Rocking

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

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.

Figure 1.9 Schematic of the Castellano group ultrafast TRIR instrument. This diagram was provided by Dr. Sofia Garakyaraghi and used with permission.

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

13 resistant to water damage than CaF2. 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.

BaF2 BaF2 window window (front) (back)

TeflonSpacerSpacers (0.150.15 – 1 mmmm)

Figure 1.10 Schematic of the TRIR sample holder used in TRIR experiments. The casing and o-rings were

omitted from the diagram.

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.

Figure 1.11 IR transmission characteristics of various solvents.

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

strongly absorbing CO and CN stretches, a smaller spacer may be used. This balancing act of solvent and sample absorbance is an important aspect to successful TRIR experimental design.

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.

(10) Michelson, A.; Morley, E. On the Relative Motion of the Earth and the Luminiferous

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.

(13) Thorlabs. Barium Fluoride Windows. 2019.

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

2,3 the chemical industry. 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

CO(g) + H2O(g) CO2(g) + H2(g) (1)

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

6 and H2. 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

+ + *

N hv Ir N N Ir CH3OH N H H

H+

Ir N N

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

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

In 2016, a very interesting mechanism was proposed by the Miller group at UNC wherein

[IrCp*(bpy)H]+ undergoes a bimetallic self-quenching mechanism in the presence of acetonitrile

9 and organic acid to produce H2 (Scheme 2.2). In this mechanism, the triplet excited state reacts with a ground state molecule in a diffusion controlled bimetallic self-quenching mechanism to yield two ground state intermediates, IrCp*(bpy)H and [IrCp*(bpy)H]2+. In the presence of acetonitrile, H2 is released from the reaction between the two intermediates, also yielding the 3-

2+ coordinate Ir(I) complex described earlier and [Cp*Ir(bpy)(NCCH3)] . The weak acid is then employed to regenerate the starting material.

+ * 2+

Self Quenching N - N Ir N N Ir Ir N N H H H

NCCH3

2+ +

Ir + N N H N Ir Ir N N N NCCH H 3

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. 9

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

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,

1 60%, H NMR 400 MHz (CDCl3): � 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)]

These molecules were synthesized according to literature procedures.11 The dimer

[IrCp*Cl2]2 (0.25 mol) and polypyridyl ligand of choice were placed in a 2-neck round bottom flask fitted with a septum and attached to a Schlenk line. The flask was purged with high purity

N2 while 25 mL of reagent grade methanol was bubble degassed. The methanol was then added to the reaction flask via syringe and the yellow mixture was stirred at room temperature for 30 min. Solid impurities were removed using a fine-fritted glass filter and the yellow filtrate was reduced in volume to approximately 10 mL using a rotary evaporator. The remaining liquid was then added dropwise to 100 mL of stirring diethyl ether at RT, in which a bright yellow solid immediately precipitated. This yellow solid was then vacuum filtered on a frit and dried in

1 vacuo. Yield: 70-80%, H NMR 400 MHz (acetone-d6): � 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 thoroughly. The apparatus depicted in Figure 2.1 was constructed and completely deaerated on a

N2 Schlenk line. The formic acid solution (10 mL) was then added to the round bottom flask via cannula. [Ir(Cp*)(N^N)Cl]Cl (0.36 mmol) was then added quickly to the solution and the reaction was protected from light and stirred at room temperature under nitrogen for 5 hours.

Excess KPF6 was then quickly added to the yellow solution, yielding a solid yellow precipitate.

The apparatus was then carefully inverted to the attached medium fritted filter and a slight vacuum was used to filter the solid product. Aluminum foil was wrapped around the filter portion of this apparatus to continue to protect the solid from ambient light. Deaerated deionized water was added by syringe through the septum to wash the yellow solid. The now-filled Schlenk flask and original reaction round bottom flask were then sequentially replaced under positive N2 pressure with clean glassware. The entire apparatus was then left under vacuum until the air- sensitive solid was fully dried before transferring the entire apparatus to a N2-filled dry glovebox to safely remove the air-sensitive solid from the filter in an air-free environment. If necessary, the solid was redissolved in acetone and recrystallized in diethylether. Yield: 65% bpy, 53%

1 phen, 62% (dtbb). H NMR 400 MHz (acetone-d6): � 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.

Figure 2.1 Schematic of the air-free apparatus used in the synthesis of metal hydrides. Constructed

using A. 3-neck round bottom flask B. Stir bar C. Adapter connecting the Schlenk line to a side- neck D. Double-sided air-free medium porosity frit E. Schlenk flask attached to the Schlenk line.

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 pentamethylcyclopentadiene was added via syringe at RT. The mixture was gently refluxed for

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

1 yellow solid which was vacuum dried. H NMR 400 MHz (CDCl3): � 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)]+

This synthetic procedure was adapted from the literature.12 A pH 5 solution of 3 M sodium formate was prepared, degassed, and added (10 mL) to the same apparatus as depicted in

Figure 2.1 that had been thoroughly degassed prior to the addition of any reagents. 68 mg of

[RhCp*(bpy)Cl]Cl was added quickly to the solution and the mixture was stirred under N2 at room temperature for 1 hour while being protected from light. The solution quickly changed from yellow to dark red. KPF6 was added to the final solution, ultimately yielding a green solid.

The green solid was filtered by inverting the apparatus and was washed thoroughly with degassed water added via syringe. The two round bottom flasks were replaced with clean, dry glassware and the solid was dried under vacuum before being stored in a N2-filled glovebox.

2.2.7. Nanosecond UV-VIS Transient Absorption (TA) Spectroscopy

Nanosecond transient absorption measurements were collected with a LP920 laser flash photolysis system (Edinburgh Instruments). Briefly, a tunable Vibrant 355 Nd:YAG/OPO system

(OPOTEK) was used for pulsed laser excitation. To collect transient difference spectra in the visible portion of the spectrum, an iStar ICCD camera (Andor Technology), controlled by the

LP900 software program (Edinburgh Instruments), was used. Samples were prepared in a glovebox prior to measurement in 1 cm path length quartz optical cells, prepared to have optical densities around 0.4 at the excitation wavelength (420 nm, ∼ 2.2 mJ/pulse). Ground state UV− vis absorbance measurements were taken before and after experiments to ensure sample stability.

Transient absorption decay kinetics were fit and analyzed with IGOR Pro.

2.2.8. Ultrafast UV-VIS Transient Absorption (TA) Spectroscopy

Sub-picosecond transient absorption (TA) measurements were performed at the NCSU

Imaging and Kinetic Spectroscopy (IMAKS) Laboratory in the Department of Chemistry and used a Ti:sapphire laser system described previously.23 Briefly, a portion of the output from a 1 kHz Ti:sapphire Coherent Libra regenerative amplifier (4 mJ, 100 fs (fwhm) at 800 nm) was split into the pump and probe beams. The pump beam was directed into an optical parametric amplifier (Coherent OPerA Solo) to generate the pump pulse used in these experiments, while the probe beam was delayed in a 6.6 ns optical delay stage. The probe beam was focused into a

CaF2 crystal to generate white light continuum (WLC) spanning 340−750 nm or into the NIR

with a proprietary crystal. The two beams were focused and spatially and temporally overlapped into a spot on the sample, with the relative polarizations of the pump and probe beams set at the magic angle. All solvents were fresh and spectrophotometric grade, and the ground-state absorption spectra were taken before and after each experiment using an Agilent 8453

UV−visible spectrophotometer to ensure there was no sample decomposition. Samples were prepared in 2 mm path length quartz cuvettes. The transient spectra and kinetics were obtained using a commercially available transient absorption spectrometer (Helios, Ultrafast Systems), averaging at least three scans and using 2 s of averaging at every given delay. Transient kinetics were evaluated and analyzed using Igor Pro 7.

2.3. Spectroscopic Studies of [Ir(Cp*)(N^N)H]PF6

2.3.1. Excited State Dynamics of [Ir(Cp*)(bpy)H]PF6

In order to comprehend the excited state dynamics and potential photochemical bond cleavage pathways of [Ir(Cp*)(bpy)H]PF6, it was initially examined using time-resolved methods in dry acetonitrile. Upon excitation into its low energy MLCT band in the visible

(Figure A11) two transient absorption features are immediately observed in UFTA experiments, located at 350 and 500 nm, respectively, Figure 2.2. The broad absorption band at lower energy forms a double-top feature within 50 ps, which remains until the end of the optical delay line is reached at 6.3 ns. These identical excited state absorption features persist into the longer time regime measured using conventional laser flash photolysis and were determined to decay with a single exponential lifetime of 83 ns, Figure A12. The general absorption shape observed in each of these techniques was indicative of the formation of the radical bpy anion, consistent with the

formation of an MLCT excited state in which the Ir(III) center is correspondingly oxidized to

Ir(IV).

6 0 ns 0.12 -1 ps 50 ns 5 0 ps 100 ns 0.10 150 ns 0.5 ps 200 ns 1.0 ps 4 250 ns 10.4 ps 300 ns 21 ps 0.08

350 ns

-3 500 ps 3 400 ns 6.3 ns A 450 ns

∆ 0.06 Ax10 ∆ 2 0.04

1 0.02

0 0.00

350 400 450 500 550 600 650 350 400 450 500 550 600 650 700 Wavelength (nm) Wavelength (nm)

Previous DFT calculations also support this MLCT assignment.7 The 83 ns lifetime also agrees with the experimentally observed photoluminescence emission lifetime, previously assigned to

3 5 originate from the MLCT excited state. [Ir(Cp*)(dtbb)H]PF6 and [Ir(Cp*)(phen)H]PF6 were also examined using the same techniques (Figures A13 and A14) and produced similar findings, although the phen analog proved to be rather unstable in acetonitrile with prolonged visible light exposure, whereas the dtbb analog featured a shorter excited state single exponential lifetime of

52 ns.

The aforementioned photoluminescence spectra of the Ir(III) hydrides are plotted in

Figure A15 along with their representative excitation spectra. An obvious Stokes shift was observed, with very little overlap between the absorption and emission profiles, indicative of

large changes in geometry between ground and emissive excited state. Low temperature (77 K) photoluminescence spectra are all hypsochromically shifted from the room temperature photoluminescence spectra by about 100 nm [2,422 cm-1 (bpy); 2,426 cm-1 (phen); 2,450 cm-1

(dtbb)] (Figure A16) This large thermally induced Stokes shift is consistent with a 3CT luminescence manifold.

2.3.2. Photochemistry of [Ir(Cp*)(bpy)H]PF6

Transient absorption experiments were also performed on the Ir(III) hydride complexes dissolved in methanol. The research performed by Fukuzumi et al. led to the proposed photoacid behavior mechanism as presented in Scheme 2.1. The nsTA difference spectra reveal a highly structured set of absorption bands spanning across the visible region, quantitatively matching previous results from the Fukuzumi and Guldi laboratories (Figure 2.3).7 These absorption features have been primarily assigned to the deprotonated Ir(I) intermediate, based on previous transient absorption experiments and direct comparison to the ground state absorption spectrum of the isolated 3-coordinate Ir(I) compound.7,13 This intermediate survives on the milliseconds time scale, which ultimately reverts back to the original ground state molecule through protonation by the methanol solvent. The dtbb analog exhibits similar absorption features albeit with a shorter lifetime, while the phen complex possesses a lifetime similar to that observed in the bpy hydride with a much less structured absorption profile (Figures A17 and A18 and Table

A1).

Interested in gleaning insight into this excited state deprotonation, UFTA was also performed on [IrCp*(bpy)H]PF6 in methanol. However, the results were more complicated than originally anticipated based on previous results.7 Two absorption features were observed

8 1.2 0Time ps 0 - 2 ps 0 ps 150 µs 10 ps 1.0 1 ms 30 ps 100 ps 2 ms 304 ps 0.9 4 ms 6 505 ps 750 ps 10 ms 1 ns 2 ns

0.7 3 ns -1 -3 4 ns 4 5 ns 0.6 6 ns

Ax10 6.3 ns Ax10 ∆ ∆ 0.4 0.3 2

0.1 0.0 0

400 500 600 700 350 400 450 500 550 600 650 700 WavelengthWavelength (nm) (nm) Wavelength (nm)

Figure 2.3 nsTA (λex = 430 nm) (left) and UFTA (λex = 400 nm) (right) difference spectra of [Ir(Cp*)(bpy)H]PF 6 in deaerated methanol.

promptly at time 0 at both 350 and 500 nm, with rapid kinetic growth to the double-top feature observed in acetonitrile. These features were once again assigned to the formation of the relaxed

3MLCT excited state. Skipping ahead to the end of the delay line at 6.3 ns, the highly structured absorption bands spanning most of the visible region are once again observed, suggesting the deprotonation of the excited complex to form the 3-coordinate Ir(I) ground state intermediate.

However, between these delay times, there appears to be other dynamic processes occurring that were not discussed or presented in the original published report.7 This result suggests that there lies an intermediate between the 3MLCT state and the 3-coordinate Ir(I) product. Single wavelength kinetics are reported in Table A2 with corresponding kinetic transients presented in

Figure A19. These data reveal multiple time constants further supporting the likelihood of the

formation of an intermediate prior to the formation of the Ir(I) complex. This intermediate could possibly be the result of proton transfer followed by electron transfer to the metal center, rather than a concerted mechanism. If correct, this mechanism would suggest the presence of a short- lived deprotonated Ir(II) complex, Ir(II)Cp*(bpy•-). However, herein lies one of the major drawbacks of electronic spectroscopy, in that very broad non-descript absorbance features make structural characterization difficult and hence represents why TRIR spectroscopy was pursued for possibly confirming whether the 3-coordinate Ir(I) product is formed in a stepwise or concerted mechanism. However, as discussed in Chapter 1, solvent absorbance is incredibly important in observing sample stretching modes, with methanol absorbing a large amount of light at the window of interest. As M-H stretching modes are weak and had never been observed using TRIR techniques, the experiment would require a more suitable solvent environment.

Experiments in dry acetonitrile also proved difficult. To confirm whether M-H stretching modes could be observed with our UF-TRIR setup, an unreactive Ir(III) dihydride was selected for analysis, as described in the following chapter. Using the information obtained from those results, future attempts at observing photochemistry with [IrCp*(bpy)H]+ will be performed under different experimental conditions such as mixed solvent environments and through the use of a flow cell.

2.3.3. Photophysical Properties of [IrCp*(N^N)Cl]+

As a comparison to the photoreactive Ir(III)-hydrides, the analogous Ir(III)-chlorides were also examined. An additional molecule was added to this particular series, the 2,2’- bipyridine-4,4’-dicarboxylic acid (dcb) complex [Ir(Cp*)(dcb)Cl]Cl. Firstly, the UV-Vis absorbance spectra of the Ir(III)-chlorides (Figure A1) exhibit significantly reduced oscillator

strengths in their low energy MLCT transitions with respect to those measured for the analogous

Ir(III)-hydrides.

a b

c d

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)

No absorption difference features were observed in any of the Ir(III)-chlorides in conventional laser flash photolysis experiments exciting with nanosecond laser pulses. To test whether these molecules exhibited any photochemical reactivity on shorter timescales, UFTA experiments were executed in methanol (Figure A20) and water, Figure 2.4. The obtained difference spectra were weak and broad, spanning most of the visible portion of the spectrum.

All four Ir(III)-chloride complexes exhibited a short (tens of picoseconds) transient growth before decaying completely by 1.5 ns (Figure A21 and Table A3). The short inherent lifetime of these complexes, the lack of any significant excited state dynamics, and no changes measured in the ground state absorption spectra before and after the transient absorbance experiments all suggest that the chlorides are not undergoing any photochemistry under these reaction conditions. These data agrees with previous research in which [IrCp*bpyCl]Cl was used in dark reactions before being converted to the hydride, in which the light-assisted reactions could then take place. 2,14 For example, in the presence of formic acid, the chloride ligand is replaced with formate followed by decarboxylation to the hydride. Upon light illumination of the hydride,

14 photochemical H2 release occurs, with no such chemistry occurring in the dark.

2.4. Electronic Structure Calculations of Cp*-containing Ir(III) Complexes

THIS PARTICULAR SECTION HAS BEEN PUBLISHED - Inorganic Chemistry, 2018, 57 (24), 15445-15461. DOI: 10.1021/acs.inorgchem.8b02753

n+ n+ m+

Ir Ir Ir N L N L N L N

[Cp*Ir(tpy)L]n+ [Cp*Ir(piq)L]n+ [Cp*Ir(bpy)L]m+ L = L = L = CNAr CNAr CNAr NHC NHC NHC – – – CH3 CH3 CH3 H– N C N N

CNAr NHC

Scheme 2.3 Chemical structures of the three series of molecules investigated and of the monodentate ancillary ligands, L.

Work was also completed on three series of pentamethylcyclopentadienyl (Cp*) Ir(III) complexes with different bidentate ligands (Scheme 2.3), [Cp*Ir(tpy)L]n+ (tpy = 2- tolylpyridinato; n = 0 and 1), [Cp*Ir(piq)L]n+ (piq = 1-phenylisoquinolinato; n = 0 and 1), and

[Cp*Ir(bpy)L]n+ (bpy = 2,2'-bipyridine; n = 1 and 2), featuring a range of monodentate carbon- donor ligands within each series (L = 2,6-dimethylphenylisocyanide (CNAr); 3,5- dimethylimidazol-2-ylidene (NHC); methyl)).15 The spectroscopic and photophysical properties of these molecules and those of the photocatalyst [Cp*Ir(bpy)H]+ were examined to establish electronic structure-photophysical property relationships that engender productive photochemical reactivity of this hydride and its methyl analogue. The Ir(III) chromophores containing ancillary

CNAr ligands exhibited features anticipated for predominantly ligand-centered (LC) excited states, and analogues bearing the NHC ancillary exhibited properties consistent with LC excited states containing a small admixture of metal-to-ligand charge transfer (MLCT) character.

However, the molecules featuring anionic and strongly σ-donating methyl or hydride ligands exhibited photophysical properties consistent with a high degree of charge transfer (CT) character. The high degree of CT character in the triplet excited states of methyliridium complexes bearing C^N-cyclometalated ligands offer a striking contrast to the photophysical properties of pseudo-octahedral structures fac-Ir(C^N)3 or Ir(C^N)2(acac) that have lowest- energy triplet excited states characterized as primarily LC character with more moderate MLCT admixture.

DFT calculations at the B3LYP+D3/LANL2DZ+f,6-311G** level of theory16-22 in

23 dichloromethane (SMD) were performed in order to obtain the optimized ground state (S0) and

+ lowest triplet excited state (T1) for [Cp*Ir(bpy)(CH3)] and Cp*Ir(piq)(CH3). The HOMO and

LUMO calculated at the S0 geometry, and the singly occupied natural orbitals (SONOs) at the T1

+ geometry are sketched in Figure 2.5. In [Cp*Ir(bpy)(CH3)] , the LUMO at the S0 geometry is clearly localized on the bpy ligand, whereas the HOMO appears to be spread over the Ir(III) ion and the Cp* ligand, and to a lesser extent on the other ligands. Similarly, for Ir(Cp*)(piq)(CH3), the LUMO appears localized on the piq ligand while the HOMO is mainly comprised of the metal center and the Cp* ligand. For both complexes, the SONOs at the T1 geometry closely resemble the LUMO and HOMO at S0 geometry, likely indicating that the T1 state arises mainly from a HOMO to LUMO transition.

LUMO (S0) SONO2 (T1) LUMO (S0) SONO2 (T1)

HOMO (S0) SONO1 (T1) HOMO (S0) SONO1 (T1)

Figure 2.5 Depictions of the frontier orbitals for [Ir(Cp*)(bpy)(CH3)]+ (left) and Ir(Cp*)(piq)(CH3) (right) at the S0 and at the T1 geometries.

+ Table 2.1 Orbital contributions (%) to frontier MOs for [Ir(Cp*)(bpy)(CH3)] .

Atom or group HOMO (S0) LUMO (S0) SONO1 (T1) SONO2 (T1) Ir 37.53 4.18 47.67 1.54 bpy 13.15 92.86 7.07 98.2 Cp* 35.83 2.18 37.00 0.24 CH3 13.49 0.78 8.27 0.02

Table 2.2 Orbital contributions (%) to frontier MOs for Ir(Cp*)(piq)(CH3).

Atom or group HOMO (S0) LUMO (S0) SONO1 (T1) SONO2 (T1) Ir 39.12 3.99 48.20 4.04 piq 23.31 93.50 17.14 93.47 Cp* 27.48 1.73 28.73 2.05 CH3 10.08 0.78 5.92 0.44

Orbital composition analysis for both molecules confirmed that the singly occupied orbitals of T1 have very similar orbital compositions as the respective HOMOs and LUMOs of

+ S0. These results are summarized in Table 2.1 for [Ir(Cp*)(bpy)(CH3)] , and in Table 2.2 for

+ Ir(Cp*)(piq)(CH3). In particular, 98% of the T1 SONO2 of [Ir(Cp*)(bpy)(CH3)] was found to comprise orbitals from the bpy ligand, while the T1 SONO1 was comprised of very little (7%) from the bpy orbitals but about 48% from the Ir and 37% from the Cp* group, and a small contribution from the methyl (8%). Calculations reported7 for [Ir(Cp*)(bpy)H]+ at the

B3LYP/STO-2G,STO-4G level of theory similarly show a highly delocalized HOMO, and a

LUMO predominantly located on the bpy.

The T1 excited state is therefore best described as having predominantly CT character, with very little LC character. This CT character comprises both MLCT and LLCT components, particularly involving Cp*. Perusal of Table 2.2 reveals that the T1 state of Ir(Cp*)(piq)(CH3) has an increased LC character as shown by the 17% piq contribution to SONO1, but that the T1 is still mostly comprised of MLCT/LLCT character. The computational results suggest that the lowest energy triplet states (T1) for the above two methyl bearing molecules are highly CT in nature, consistent with the experimentally determined spectroscopic observations. Qualitative experimental support for the more specific description of mixed MLCT + LLCT character was suggested by the temperature dependence of the photoluminescence intensity decay time

+ constants (measured between 1.7 to 100 K) for [Ir(Cp*)(bpy)(CH3)] that indicated that the zero

field splitting (ZFS) in the emissive state is not larger than that in Ir(ppy)3 (EII =13.5 and EIII =

83 cm–1 in frozen THF),24 whereas it would have been predicted to be larger if the MLCT

25-27 28 character was much greater. Previous TD-DFT calculations on Ir(ppy)3 at the

B3PW91/LANL2DZ,D95 level of theory predicted majority LC character (58%), whereas the

LC content was only calculated as 17% in the cyclometalating complex Ir(Cp*)(piq)(CH3). The degree of MLCT character (40%) in Ir(ppy)3 was estimated to be similar to that in the two methyl complexes reported here (about 48%), but the latter also have substantial LLCT

+ character. The established literature complex [Ir(ppy)2(bpy)] appears to be much more similar in excited state character to the present methyl-containing complexes, as evidenced by calculations at the B3LYP/ SBKJC-VDZ,3-21G* level of theory; these data suggest 43.7%

MLCT character and 51.3% LLCT character, and only 5% LC character of the chromophoric bpy ligand in the lowest energy excited state.29

LUMO (S0) SONO2 (T1)

HOMO (S0) SONO1 (T1)

Figure 2.6 Depictions of the frontier orbitals for [Ir(Cp*)(piq)(CNAr)]+

at the optimized S0 and T1 geometries

Table 2.3. Orbital contributions (%) to frontier MOs for [Ir(Cp*)(piq)(CNAr)]+.

Atom or group HOMO (S0) LUMO (S0) SONO1 (T1) SONO2 (T1) Ir 23.28 2.47 1.94 3.97 piq 49.72 95.89 97.37 94.51 Cp* 18.22 0.91 0.50 0.55 CNAr 8.78 0.74 0.21 0.97

DFT calculations were also performed on the [Ir(Cp*)(piq)(CNAr)]+ chromophore. In this instance, the LUMO was found once again to be highly localized on the piq ligand, and the

HOMO was highly mixed with contributions from Ir and the different resident ligands (Figure

2.6 and Table 2.3). The S1 state arising from the HOMO to LUMO transition is therefore best described as mixed MLCT/LLCT/LC character. The T1 excited state was found to be dissimilar, being highly LC in character, in agreement with the conclusions drawn from the spectroscopic and photophysical properties. This situation arises when the 1CT states are relatively close in energy to the 1LC states. The S-T splitting for the LC-based π-π* states are generally larger than the S-T splitting for CT states,30 making it possible for the 3LC to emerge as the lowest excited state. An example of this has been found in a study on Ir(ppy)2(CO)Cl, where the S1 was assigned experimentally as 1MLCT, but the emissive triplet excited state was characterized as highly triplet ligand centered.31 The HOMO of the “theoretical” model complex

+ Ir(tpy)2(CNCH3)2 was calculated to be a mixture of Ir 5d and ligand π orbitals, while the LUMO was predominantly ligand π*,32 corresponding to a mixed MLCT/LC transition. Experimentally, the emissive state of the CN-t-Bu analogue was found to be highly 3LC with relatively small

MLCT admixture.32

2.5. Preliminary Spectroscopic and Computational Studies of Rh and Ru Cp* Diimine- containing Hydrides

2.5.1. Preliminary UV-vis Spectroscopy and Computational Study of [Rh(Cp*H)bpy]+

Moving forward, it would be interesting to perform a comparative study between the discussed Ir(III) hydrides and hydrides containing different metal centers. Preliminary research

+ + has begun on two metal hydrides in particular, [Rh(Cp*H)bpy] and [Ru(C6Me6)bpyH] . First, the complex [Rh(Cp*H)bpy]+ was chosen for its very interesting Cp* non-innocence, a term used to describe ligands that bind multiple ways making the determination of oxidation state difficult.33-35 The ligand non-innocence manifests itself through the hydride thermodynamically preferring to bond to the Cp* ligand (Scheme 2.4) rather than the Rh center, as demonstrated in previous literature.12,36-40

Scheme 2.4 Molecular structure of [Rh(Cp*H)bpy]+.

UV-vis absorption was used to compare [Rh(Cp*H)bpy]+ in acetonitrile and methanol (Figure

2.7). Upon dissolving in deaerated acetonitrile, the once dark green solid produces a pink solution. UV-vis spectroscopy reveals a broad absorbance covering the visible region. The

general shape of this band is very similar to that of the isolated Cp*Rhbpy complex.13 Upon opening the sample to air, the solution turns yellow. The absorbance profile mirrors that of the precursor chloride. In methanol, the complex retains its green color under nitrogen, suggesting higher stability in this solvent. A broad absorbance is present from 650 to 850 nm, which matches the absorbance profile obtained from a UV-vis absorbance study in which the precursor chloride was monitored in the presence of sodium formate (Figure A22). Upon opening to air, the yellow chloride is once again generated as suggested through UV-vis.

1.2 1.0 Chloride in ACN Chloride in MeOH "Hydride" in ACN N "Hydride" in MeOH N2 2 "Hydride" in MeOH air 1.0 "Hydride" in ACN air 0.8

0.8 0.6 0.6 0.4 0.4

0.2 Normalized Absorbance Normalized

Absorbance Normalized 0.2

0.0 0.0 300 400 500 600 700 800 300 400 500 600 700 800 Wavelength (nm) Wavelength (nm) 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.

TD-DFT calculations were also performed at the B3LYP-D3/6-311G**/LANL2DZ level of theory to gauge differences in electronic transitions between the discussed endo complex and the thermodynamically unfavorable Rh(III)-H complex in three different solvents (Figure A23).

The two simulated absorbance profiles are fairly similar, with an increase in oscillator strength at the 400 nm band and the addition of transitions around 560 nm. The character of the lowest energy transitions for both complexes are dominated by HOMO to LUMO transitions (Table

A4). In both molecules, the transition is largely metal-to-ligand with somewhat more ligand-to- ligand character present in the metal-coordinated hydride complex (Figure A24). DFT calculations were also performed on the methyl-substituted bpy complex, [RhCp*(6,6'-Me- bpy)H]+, which was reported to be isolated,12,41 supposedly because of added steric bulk, in preparation for possible TRIR experiments. The simulated IR spectra for the complex in acetonitrile reveal an estimated Rh-H stretching frequency of 1893 cm-1, with a clear window ideal for possible TRIR experiments (Figure A25). Similar IR simulations were performed on the unsubstituted Rh-H complex, revealing a possible Rh-H stretching frequency of 1957 cm-1

(Figure A26). The decrease in stretching frequency after substitution with the two methyl groups indicates a weakening of the Rh-H bond because of increased electron donating character of the diimine ligand.

+ 2.5.2 Preliminary Ultrafast Dynamics of [Ru(C6Me6)bpyH]

N Ru N

+ Scheme 2.5 Molecular structure of [Ru(C6Me6)(bpy)H] .

A Ru(II) complex (Scheme 2.5) was also selected for photophysical study as previous research have shown it is more hydridic than the Ir(III) complex discussed earlier and would provide insight into the hydridic character of metal hydrides in different solvents.42 Three

solvents (methanol, tetrahydrofuran, and acetonitrile) were chosen to study the photophysical properties of this complex based on previous research that suggested intense solvatochromism based on the reversible conversion between the deprotonated Ru0 complex and RuII-hydride

43 complex. According to this study, the complex quickly converts to Ru(C6Me6)(bpy) in the presence of tetrahydrofuran and can then protonate once again with the addition of methanol.

UV-vis absorption studies were first conducted to gauge the dark reactions that may

- - occur prior to photoexcitation. The hydride complex was supplied as the PF6 and Cl salts, though the [Ru(C6Me6)(bpy)H]Cl sample was determined to be impure and any spectroscopic results using this sample are purely qualitative in terms of comparison with the PF6 complex.

Interestingly, the complex appeared to remain intact across all three solvents when the counter

- ion is PF6 . Only in the case of the chloride salt was the reported solvatochromism observed

(Figure A27). Upon addition to tetrahydrofuran, the chloride salt solution turns pink in color, signifying the conversion to the Ru(0) complex. The absorbance profile of this sample is highly structured and matches the reported absorbance of the isolated compound.43,44 The samples are

45,46 all CO2-sensitive, as demonstrated by the absorbance spectra recorded after opening to air

(Figure A27), and were prepped in a N2-filled glovebox. It should be mentioned that the referenced study generated the hydride complex through addition of methanol to the Ru(0) complex and was not isolated as a salt.43 To examine this apparent counterion effect, transient absorption studies were undertaken.

No long-lived absorption features were observed using nsTA in acetonitrile and methanol, suggesting short-lived excited states and little chance for photoreactivity in these solvents. UFTA spectroscopy confirms the short-lived nature of the excited complex with the ground state being regenerated within 50 ps (Table A5 and Figure A28). No changes in dynamics

were observed across the three solvent environments, with several absorption features present, seen in Figure 2.8. At time 0, the window is dominated by four major features, a broad absorbance centered around 380 nm, a strong bleach at 450 nm, a small positive feature at 550 nm, and an unexpected short-lived negative feature around 630 nm. This low-energy negative feature cannot be assigned as a ground state bleach as the complex does not absorb at this wavelength. This limits the possibilities to photodegradation or impurities and stimulated emission. No photodegradation was visible using ground-state UV-vis absorbance before and after the experiment. Steady-state photoluminescence studies revealed a broad emission with a

λmax of 610 nm, suggesting the negative feature could be assigned as stimulated emission (Figure

A29).

-2 ps -3 -1 ps -3 a 0 ps 4 x10 b 0 ps 4 x10 0.15 ps 0.5 ps 0.4 ps 1.5 ps 0.9 ps 2.8 ps 3 2.3 ps 5.5 ps 5 ps 10.9 ps 9.9 ps 2 25 ps 2 25.2 ps 50 ps 50.3 ps 109 ps A A ∆ ∆ 1 0 0 -1 -2

-2

350 400 450 500 550 600 650 700 350 400 450 500 550 600 650 700 Wavelength (nm) Wavelength (nm)

-3 4 x10 -2 ps c 0 ps 3 0.5 ps 1 ps 2.3 ps 5.3 ps 2 10.6 ps 27.4 ps 109 ps 235 ps 1 A ∆ 0

-1

-2

-3 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 2.8 UFTA difference spectra of [Ru(C6Me6)(bpy)H]PF6 in (a) acetonitrile (b) methanol (c) tetrahydrofuran following excitation at 500 nm.

Within 2 ps, new features become visible in which the broad absorbance at low energy reveals a maximum at 360 nm and a broad, structured absorbance appears at lower energy. These

“long-lived” features last for about 15 ps. The general shape of the three features agrees well with the formation of the bpy radical anion after MLCT excitation. The chloride salt was also examined using UFTA in the same solvents, revealing similar dynamics as the PF6 cases in acetonitrile and methanol (Figure A30). The spectrum in tetrahydrofuran is dominated by a structured ground state bleach, matching the absorbance of the Ru(0) complex. Weak positive absorbances are also visible at the edges of the window. The lifetimes of all features are close to the values collected in the other solvents (Table A6 and Figure A31), with a weak, long-lived feature appearing around 450 nm. This feature is also present in the experiments performed in acetonitrile, though much weaker.

As mentioned previously, nsTA experiments in acetonitrile and methanol did not reveal any resolvable features but a very weak TA spectrum was obtained for the chloride in THF,

2+ which is similar in shape to the TA spectrum of standard [Ru(bpy)3] . Although a confident assignment cannot be made at this time, the feature is present in larger quantity in a sample consisting of mostly Ru(0); therefore it seems likely the feature belongs to an excited state of this deprotonated complex. In the experiments in which the majority of the sample is the Ru(II) hydride, there may be a small amount of Ru(0) formed in solution, but not enough to be seen in ground state UV-vis spectra. This explanation also agrees with the lack of evidence of this feature in both methanol experiments. The protic nature of methanol may allow for greater stability, as also evidenced by the apparent increase in stability after opening the samples to air

Figure A27).

The lack of significant reactivity of [Ru(C6Me6)(bpy)H]PF6 in all three solvents suggests it may be a better option for possible TRIR experiments probing the M-H stretching mode than

[IrCp*(bpy)H]PF6 proved to be. Preliminary DFT calculations at the B3LYP/ECP28MWB/6-

31G** level of theory suggest the HOMO and LUMO of the Ru(II) complex are very similar to those of the Ir complex, with high metal-hydride character in the HOMO and almost exclusive

7 bpy character in the LUMO (Figure A33). Simulated IR spectra of the S0 and T1 states indicate a strengthening of the M-H bond in the T1 state, leading to an increase in stretching frequency at the 2100 cm-1 window (Figure A34).

2.6. Acknowledgments

Special thanks to Prof. Alex Miller’s group (UNC Chapel Hill) for their collaboration and for providing the samples of [Ru(C6Me6)(bpy)H]PF6 and [Ru(C6Me6)(bpy)H]Cl. I also acknowledge the use of the computing resources of the High-Performance Computing Center at

NCSU.

2.7. References

(1) Youinou, M.-T.; Ziessel, R. Synthesis and Molecular Structure of a New Family of

+ Iridium-(III) and Rhodium(III) Complexes: [(Η5-Me5C5)Ir(LL)X] and [(Η5-

+ Me5C5)Rh(LL)Cl] ; LL = 2,2′-Bipyridine or 1,10-Phenanthroline; X = Cl or H. Single

Crystal Structures of [(Η5-Me5C5)Ir(Bpy)Cl]Cl. J. Organomet. Chem. 1989, 363 (1), 197–

208.

(2) Ziessel, R. Photocatalysis of the Homogeneous Water-Gas Shift Reaction under Ambient

Conditions by Cationic Iridium (III) Complexes. Angew. Chemie Int. Ed. English 1991, 30

(7), 844–847.

(3) Ziessel, R. Photocatalysis: Reduction of Carbon Dioxide and Water-Gas-Shift Reaction

Photocatalyzed by 2,2′-Bipyridine or 1,10-Phenanthroline Cobalt(II), Ruthenium(II),

Rhenium(I) and Iridium(III) Complexes. In Photosensitization and Photocatalysis Using

Inorganic and Organometallic Compounds. Catalysis by Metal Complexes;

Kalyanasundaram, K., Grätzel, M., Eds.; Springer: Dordrecht, The Netherlands, 1993.

(4) Greener Fischer-Tropsch Processes; Maitlis, P. M., de Klerk, A., Eds.; Wiley: Weinheim,

Germany, 2013.

(5) Sandrini, D.; Maestri, M.; Ziessel, R. Spectroscopic Behavior of a New Family of Mixed-

Ligand Iridium(III) Complexes. Inorganica Chim. Acta 1989, 163 (2), 177–180.

(6) Watson, K. J.; Ziessel, R. Photochemical Production of Hydrogen and Carbon Dioxide

from Formate Using Mixed-Ligand Iridium Complexes as Catalysts. Inorganica Chim.

Acta 1992, 197 (2), 125–127.

(7) Suenobu, T.; Guldi, D. M.; Ogo, S.; Fukuzumi, S. Excited-State Deprotonation and H/D

Exchange of an Iridium Hydride Complex. Angew. Chemie - Int. Ed. 2003, 42 (44), 5492–

5495.

(8) Barrett, S. M.; Pitman, C. L.; Walden, A. G.; Miller, A. J. M. Photoswitchable Hydride

Transfer from Iridium to 1-Methylnicotinamide Rationalized by Thermochemical Cycles.

J. Am. Chem. Soc. 2014, 136 (42), 14718–14721.

(9) Chambers, M. B.; Kurtz, D. A.; Pitman, C. L.; Brennaman, M. K.; Miller, A. J. M.

Efficient Photochemical Dihydrogen Generation Initiated by a Bimetallic Self-Quenching

Mechanism. J. Am. Chem. Soc. 2016, 138 (41), 13509–13512.

(10) White, C.; Yates, A.; Maitlis, P. M.; Heinekey, D. M. (Η5-

Pentamethylcyclopentadienyl)Rhodium and -Iridium Compounds. In Inorganic Syntheses;

Grimes, R. N., Ed.; Hoboken, NJ, 1992; pp 228–234.

(11) Brewster, T. P.; Miller, A. J. M.; Heinekey, D. M.; Goldberg, K. I. Hydrogenation of

Carboxylic Acids Catalyzed by Half-Sandwich Complexes of Iridium and Rhodium. J.

Am. Chem. Soc. 2013, 135 (43), 16022–16025.

(12) Pitman, C. L.; Finster, O. N. L.; Miller, A. J. M. Cyclopentadiene-Mediated Hydride

Transfer from Rhodium Complexes. Chem. Commun. 2016.

(13) Ladwig, M.; Kaim, W. Electronic Structure of Catalytic Intermediates for Production of

H2: (C5Me5)Ir(Bpy) and Its Conjugated Acid. J. Organomet. Chem. 1992, 439 (1), 79–90.

(14) Barrett, S. M.; Slattery, S. A.; Miller, A. J. M. Photochemical Formic Acid

Dehydrogenation by Iridium Complexes: Understanding Mechanism and Overcoming

Deactivation. ACS Catal. 2015, 5 (11), 6320–6327.

(15) Deaton, J. C.; Taliaferro, C. M.; Pitman, C. L.; Czerwieniec, R.; Jakubikova, E.; Miller, A.

J. M.; Castellano, F. N. Excited-State Switching between Ligand-Centered and Charge

Transfer Modulated by Metal–Carbon Bonds in Cyclopentadienyl Iridium Complexes.

Inorg. Chem. 2018, 57 (24), 15445–15461.

(16) Becke, A. D. Density Functional Thermochemistry, 3. The Role of Exact Exchange, J.

Chem. Phys. 1993, 98, 5648–5652.

(17) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy

Formula into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater.

Phys. 1988, 37, 785–789.

(18) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio

Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 elements

H-Pu J. Chem. Phys. 2010, 132, 154104.

(19) Hay, P. J.; Wadt, W. R. J. Chem. Phys. Ab Initio Effective Core Potentials for Molecular

Calculations. Potentials for K to Au Including the Outermost Core Orbitals. 1985, 82, 299.

(20) Ehlers, A. W.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth, A.; Jonas, V.; Kohler, K.

F.; Stegmann, R.; Veldkamp, A.; Frenking, G. A Set of f-Polarization Functions for

Pseudo-Potential Basis Sets of the Transition Metals Sc-Cu, Y-Ag and La-Au. Chem. Phys.

Lett. 1993, 208, 111-114.

(21) Hehre, W. J.; Stewart, R. F.; Pople, J. A. Self-Consistent Molecular-Orbital Methods. i.

Use of Gaussian Expansions of Slater-Type Atomic Orbitals J. Chem. Phys. 1969, 51,

2657-2664.

(22) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-Consistent Molecular Orbital

Methods. XX. A Basis Set for Correlated Wave Functions J. Chem. Phys. 1980, 72, 650-

654.

(23) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal Solvation Model Based on Solute

Electron Density and a Continuum Model of the Solvent Defined by the Bulk Dielectric

Constant and Atomic Surface Tensions J. Phys. Chem. B 2009, 113, 6378-96.

(24) Finkenzeller, W. J.; Yersin, H. Emission of Ir(Ppy) 3. Temperature Dependence, Decay

Dynamics, and Magnetic Field Properties. Chem. Phys. Lett. 2003, 377, 299–305.

(25) Yersin, H.; Finkenzeller, W. Triplet Emitters for Organic Light-Emitting Diodes: Basic

Properties. In Highly Efficient OLEDs with Phosphorescent Materials; Yersin, H., Ed.;

Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2008.

(26) Yersin, H.; Rausch, A. F.; Czerwieniec, R. Organometallic Emitters for OLEDs: Triplet

Harvesting, Singlet Harvesting, Case Structures, and Trends. In Physics of Organic

Semiconductors; Brutting, W., Adachi, C., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA:

Weinheim, 2012.

(27) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Al., E. The Triplet State of Organotransition

Compounds. Triplet Harvesting and Singlet Harvesting for Efficient OLEDs. Coord.

Chem. Rev. 2011, 255, 2622–2652.

(28) Nozaki, K. Theoretical Studies on Ir(Ppy)3. J. Chinese Chem. Soc. 2004, 53, 101–112.

(29) Ladouceur, S.; Fortin, D.; Zysman-Colman, E. Role of Substitution on the Photophysical

Properties of 5,5′-Diaryl-2,2′-Bipyridine (Bpy*) in [Ir(Ppy)2(Bpy*)]PF6 Complexes: A

Combined Experimental and Theoretical Study. Inorg. Chem. 2010, 49 (12), 5625–5641.

(30) Colombo, M.; Hauser, A.; Güdel, H. U. Competition Between Ligand Centered and

Charge Transfer Lowest Excited States in Bis Cyclometalated Rh3+ and Ir3+ Complexes.

Top. Curr. Chem. 1994, 171, 144–171.

(31) Finkenzeller, W. J.; Stössel, P.; Yersin, H. Emission and Absorption of Ir(Ppy)2(CO)Cl:

Temperature Dependence, Phosphorescence Decay Dynamics, and Assignment of Excited

States. Chem. Phys. Lett. 2004, 397, 289–295.

(32) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters,

J. C.; Bau, R.; Thompson, M. E. Synthetic Control of Excited-State Properties in

Cyclometalated Ir(III) Complexes Using Ancillary Ligands. Inorg. Chem. 2005, 44 (6),

1713–1727.

(33) Jørgensen, C. K. Differences between the Four Halide Ligands, and Discussion Remarks

on Trigonal-Bipyramidal Complexes, on Oxidation States, and on Diagonal Elements of

One-Electron Energy. Coord. Chem. Rev. 1966, 1 (1), 164–178.

(34) Lyaskovskyy, V.; de Bruin, B. Redox Non-Innocent Ligands: Versatile New Tools to

Control Catalytic Reactions. ACS Catal. 2012, 2 (2), 270–279.

(35) Saund, S. S.; Goldschmid, S. L.; Ng, K.; Stewart, V.; Siegler, M. A.; Thoi, V. S.

Exploring Ligand Non-Innocence of Coordinatively-Versatile Diamidodipyrrinato Cobalt

Complexes. Chem. Commun. 2019, 55 (12), 1825–1828.

(36) Pal, S.; Kusumoto, S.; Nozaki, K. Dehydrogenation of Dimethylamine–Borane Catalyzed

by Half-Sandwich Ir and Rh Complexes: Mechanism and the Role of Cp* Noninnocence.

Organometallics 2018, 37 (6), 906–914.

(37) Ganesan, V.; Sivanesan, D.; Yoon, S. Correlation between the Structure and Catalytic

Activity of [Cp*Rh(Substituted Bipyridine)] Complexes for NADH Regeneration. Inorg.

Chem. 2017, 56 (3), 1366–1374.

(38) Quintana, L. M. A.; Johnson, S. I.; Corona, S. L.; Villatoro, W.; Goddard, W. A.; Takase,

M. K.; VanderVelde, D. G.; Winkler, J. R.; Gray, H. B.; Blakemore, J. D. Proton–Hydride

Tautomerism in Hydrogen Evolution Catalysis. Proc. Natl. Acad. Sci. 2016, 113 (23),

6409 LP-6414.

(39) Johnson, S. I.; Gray, H. B.; Blakemore, J. D.; Goddard, W. A. Role of Ligand Protonation

in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst. Inorg.

Chem. 2017, 56 (18), 11375–11386.

(40) Leiva, C.; Lo, H. C.; Fish, R. H. Aqueous Organometallic Chemistry Catalytic Hydride

∗ Transfer Reactions with Ketones and Aldehydes Using [Cp Rh(Bpy)(H2O)](OTf)2 as the

Precatalyst and Sodium Formate as the Hydride Source: Kinetic and Activation

Parameters, and the Significance of Steric and Electronic Effects. J. Organomet. Chem.

2010, 695 (2), 145–150.

(41) Steckhan, E.; Herrmann, S.; Ruppert, R.; Dietz, E.; Frede, M.; Spika, E. Analytical Study

of a Series of Substituted (2,2’-Bipyridyl)(Pentamethylcyclopentadienyl)Rhodium and -

Iridium Complexes with Regard to Their Effectiveness as Redox Catalysts for the Indirect

Electrochemical and Chemical Reduction of NAD(P)+. Organometallics 1991, 10 (5),

1568–1577.

(42) Pitman, C. L.; Brereton, K. R.; Miller, A. J. M. Aqueous Hydricity of Late Metal Catalysts

as a Continuum Tuned by Ligands and the Medium. J. Am. Chem. Soc. 2016, 138 (7),

2252–2260.

(43) Jeong, K.; Nakamori, H.; Imai, S.; Matsumoto, T.; Ogo, S.; Nakai, H. A Neutral Five-

Coordinated Organoruthenium(0) Complex: X-Ray Structure and Unique

Solvatochromism. Chem. Lett. 2012, 41 (6), 650–651.

(44) Kaim, W.; Reinhardt, R.; Sieger, M. Chemical and Electrochemical Generation of

Hydride-Forming Catalytic Intermediates (Bpy)M(CnRn): M = Rh, Ir (n = 5); M = Ru, Os

(n = 6). Coordinatively Unsaturated Ground State Models of MLCT Excited States?

Inorg. Chem. 1994, 33 (20), 4453–4459.

(45) Creutz, C.; Chou, M. H.; Hou, H.; Muckerman, J. T. Hydride Ion Transfer from

Ruthenium(II) Complexes in Water: Kinetics and Mechanism. Inorg. Chem. 2010, 49

(21), 9809–9822.

(46) Matsubara, Y.; Fujita, E.; Doherty, M. D.; Muckerman, J. T.; Creutz, C. Thermodynamic

and Kinetic Hydricity of Ruthenium(II) Hydride Complexes. J. Am. Chem. Soc. 2012, 134

(38), 15743–15757.

CHAPTER 3: Molecular Photophysics of [Ir(bpy)2H2]PF6 and [Ir(bpy)2D2]PF6

Portions of this chapter have been previously published in Journal of Physical Chemistry A, 2018, 122 (18), 4430–4436. DOI: 10.1021/acs.jpca.8b02266

3.1. Research Summary

For decades, transition metal hydrides have been at the forefront of numerous photocatalytic reactions leveraging either photoacid or photohydride generation. Of upmost importance is the nature of the M-H bond itself, which is typically the major site of photochemical reactivity, particularly in Ir(III) hydrides featuring metal-to-ligand charge transfer

(MLCT) excited states. As a departure point for understanding the fundamental spectroscopy and

+ photophysics of the MLCT excited states of Ir(III) diimine hydrides, cis-[Ir(bpy)2H2] (bpy =

+ 2,2’-bipyridine) and its deuterated analog cis-[Ir(bpy)2D2] were prepared and investigated. The robust nature of these molecules enabled detailed solution-based photophysical studies using ultrafast transient absorption and infrared spectroscopy, executed without the generation of permanent photoproducts. Static FT-IR and Raman spectra (λex = 785 nm) of these two molecules revealed weak but measurable Ir-H and Ir-D stretching vibrations centered at 2120 cm-1 and 1510 cm-1, respectively. Short-lived (τ = 25 ps) MLCT excited states were observed for

+ + both cis-[Ir(bpy)2H2] and cis-[Ir(bpy)2D2] following femtosecond pulsed laser excitation at 480 nm in visible and near-IR transient absorption experiments. A similar time constant was measured for the in-phase and out-of-phase Ir-H stretching modes of the triplet excited state between 1900 and 2200 cm-1 using transient IR spectroscopy. The Ir-D stretching modes in the

MLCT excited state were masked by bpy-localized vibrations rendering quantitative evaluation of these modes difficult. The time-resolved infrared data were consistent with DFT-calculated mid-IR difference spectra in both of these molecules, yielding quantitative matches to the

measured IR difference spectra. The information presented here provides valuable insight for understanding the primary photophysical events and transient absorption and IR spectroscopic signatures likely to be encountered throughout metal hydride photochemistry.

3.2. Introduction

Despite their name, metal “hydrides” can be either hydridic or acidic, implying the nature of the metal hydride bond significantly affects its behavior in different solvent media. A popular example is the intensively investigated [Cp*Ir(bpy)H]+ (Cp* = pentamethylcyclopentadiene and bpy = 2,2’-bipyridine) which varies between hydridic and acidic character dependent upon solvent environment.1–6 Although there has been extensive research performed on the photochemical properties and product analysis of these Ir(III) hydrides, the photophysics and dynamics of these species and their highly reactive intermediates relevant to the Ir-H bond cleavage and/or formation remain largely unexplored. One example to the contrary was the observation of light-induced proton transfer from [Cp*Ir(bpy)H]+ to methanol, ultimately recombining on the millisecond scale, reforming the hydride species.4 More recently, Miller and coworkers probed excited state self-quenching reactions of this same molecule in CH3CN, suggesting that H2 generation directly resulted from a bimolecular reaction between an energized

Ir(III) chromophore and ground state [Cp*Ir(bpy)H]+ species.7 While supra-nanosecond timescales are valuable for observing the formation of photoproducts from such reactions, ultrafast time-scales are necessarily required to study excited state kinetics related to the initial Ir-H bond- breaking and/or bond-forming chemistry. However, the presence of highly reactive species formed in such photoreactions makes it difficult to study the pure photophysics and the primary photoactivation events of the isolated compounds. Taking a step back from the spectroscopic

observation of photochemical processes commonly observed in species such as [Cp*Ir(bpy)H]+, non-photoreactive Ir-H containing compounds become valuable for comprehending the spectroscopic signatures inherent to these Ir(III) hydrides.

Transient absorption spectroscopy can often times be nondescript and does not permit clear observation of bond breaking/formation following photoexcitation if the products do not absorb light in a distinctive manner. Alternatively, vibrational spectroscopy can serve as a most useful method for investigating transition metal hydrides as the M-H bond and changes therein can be directly monitored in a localized and characteristic manner. M-H bonds are easily identifiable as they typically lie outside of the highly congested fingerprint region and are usually

-1 8 fairly broad (△νfwhm = 10-30 cm ). Their intensities can vary, but they are almost always weaker than those characteristic of M-CO or M-CN bonds, which can also lie within the M-H window.1,8

Deuterium labeling provides another method of identifying terminal metal-hydride bonds. Upon replacing the hydride with a more massive deuterium atom, the frequency of the bond vibration drastically shifts to lower energy as a result of the increased reduced mass. Therefore, isotopic labeling is not only a useful characterization tool but can unequivocally identify the path of hydride or proton transfer photoprocesses in reactive transition metal hydrides. Additionally, this ability to directly monitor the M-H/D bond formation/breaking renders time-resolved infrared

(TRIR) spectroscopy most valuable in photophysical studies of transition metal hydride complexes. Typically, TRIR experiments of transition metal complexes take advantage of strongly allowed absorptions from C≡O or C≡N bonds.9–17 IR bands due to M-H bond stretching vibrations possess very low molar absorptivity for relevant excited state features to be nominally observed.1 However, improved sensitivity in these experimental apparatus’ now appear promising for observing such weaker but uniquely characteristic molecular vibrations.

Combining the benefits of photostable Ir(III) hydrides with modern ultrafast transient IR (TRIR) techniques, M-H stretching modes following visible pumping of the MLCT excited state were observed for the first time. The work presented here represents a key step towards understanding the underlying photophysics of Ir(III) polypyridyl hydrides and the nature of the M-H bond(s) in the MLCT excited state. Moving forward, we seek to apply these results to more reactive metal hydrides where the underlying photophysics are difficult to evaluate when short-lived intermediates and other photoproducts become abundant.

3.3. Experimental

All commercially available reagents were used as purchased, without further purification.

1H and 13C NMR spectra were recorded 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). Solid-state Raman spectra were collected at NCSU’s Analytical Instrumentation Facility (AIF) using a Horiba XploRA PLUS

Confocal Raman Microscope. All Ir(III) metal complexes were synthesized according to literature procedures, and structural characterization data matches previously reported values.18–

3.3.1. Synthesis of cis-[Irbpy2Cl2]PF6

Iridium trichloride hydrate (0.20 g, 0.67 mmol) was added to 2,2'-bipyridine (bpy) (0.21 g, 1.34 mmol) dissolved in ~2 mL of glycerol. The reaction was then refluxed (290°C) for 30 minutes.

The reaction color changed from deep red to yellow. The yellow solution was then cooled to room temperature and ~8 mL of DI water was added. The mixture was filtered then washed 3 times with diethyl ether. A saturated NaCl (aq) solution was then added to the aqueous layer to yield yellow [Irbpy2Cl2]Cl. The solid was then dissolved in excess water and saturated KPF6 was added, yielding [Irbpy2Cl2]PF6. The final product was then washed with DI water thoroughly and dried before dissolving in minimal acetone. The solution was added dropwise to diethyl ether to precipitate the yellow solid. This reprecipitation process was repeated 5 times. (Yield: 65 %). 1H

NMR (400 MHz, DMSO), δ (ppm): 7.50 (t), 7.82 (d), 8.12 (t), 8.20 (t), 8.52 (t), 8.82 (d), 8.92

(d), 9.64 (d) (Figure B1)

3.3.2. Synthesis of [Ir(bpy)2(CF3SO3)]CF3SO3

Orthodichlorobenzene (20 mL) was added to a 2-necked round bottom flask connected to a condenser and bubble degassed with N2 for 45 minutes. [Irbpy2Cl2]PF6 (0.10 g, 0.14 mmol) was then added and stirred for about 20 minutes. Triflic acid (0.4 mL) was then added via syringe. The mixture was then heated to 200°C and stirred for 3 hours then cooled to room temperature. The product mixture was then added to excess diethyl ether and left overnight in which time a fine, off-white precipitate was formed. The product was then filtered and washed thoroughly with diethyl ether. (Yield: 70%) 1H NMR (400 MHZ, DMSO) δ (ppm): 7.51 (t), 7.68

(d), 8.22 (t, 1H, H5), 8.33 (t), 8.65 (t), 8.82 (d), 9.01 (m) (Figure B2).

3.3.3. Synthesis of cis-[Irbpy2H2]PF6

This molecule was synthesized using a slight modification of previously reported

18,19 procedures. [Irbpy2CF3SO3]CF3SO3 (150 mg) was added to 4:1 H2O:EtOH (20 mL). NaBH4

(800 mg) was added, resulting in a dark green mixture that was refluxed for 1 hour under nitrogen. The final yellow-orange solution was then cooled to room temperature and aqueous

KPF6 was added, producing a yellow-orange solid that was washed with water and dried. The solid was then dissolved in minimal acetone and recrystallized with excess diethyl ether. 67%

-1 1 yield. IR �̅max (cm ): 2125, 2093 (Ir-H); H NMR (DMSO-d6, 400 MHz): δ 9.36 (d, 2H), 8.70 (t,

4H), 8.20 (t, 2H), 8.11 (t, 2H), 7.77 (d, 2H), 7.59 (t, 2H), 7.49 (t, 2H), -17.92 (s, 2H) (Figure B3).

13 C NMR (DMSO-d6, 100 MHz) δ 158.22, 156.86, 155.85, 147.74, 138.11, 136.78, 128.54,

127.43, 124.27, 123.71; ESI-MS m/z: calcd for 507.1166; found 507.1161 (Figure B5).

3.3.4. Synthesis of cis-[Irbpy2D2]PF6

This complex was synthesized in the same manner as [Irbpy2H2]PF6; replaced the solvent environment with deuterated analogs, D2O and EtOD, and the reducing agent with NaBD4. 60%

-1 1 yield. IR �̅max (cm ): 1527, 1505 (Ir-D); H NMR (DMSO-d6, 400 MHz): δ 9.36 (d, 2H), 8.70 (t,

13 4H), 8.20 (t, 2H), 8.11 (t, 2H), 7.77 (d, 2H), 7.59 (t, 2H), 7.49 (t, 2H); C NMR (DMSO-d6, 100

MHz) δ 158.22, 156.87, 155.87, 147.77, 138.14, 136.81, 128.57, 127.47, 124.30, 123.74 (Figure

B4) ESI-MS m/z: calcd for 509.1287; found 509.1282 (Figure B6).

3.3.5. Ultrafast UV-VIS Transient Absorption (TA) Spectroscopy

Sub-picosecond transient absorption (TA) measurements were performed at the NCSU

amplifier (Coherent OPerA Solo) to generate the 480 nm pump pulse used in these experiments, while the probe beam was delayed in a 6.6 ns optical delay stage. The probe beam was focused into a CaF2 crystal to generate white light continuum (WLC) spanning 340−750 nm or into the

NIR with a proprietary crystal. The two beams were focused and spatially and temporally overlapped into a spot on the sample, with the relative polarizations of the pump and probe beams set at the magic angle. All solvents were fresh and spectrophotometric grade, and the ground-state absorption spectra were taken before and after each experiment using an Agilent

8453 UV−visible spectrophotometer to ensure there was no sample decomposition. Samples were prepared in 2 mm path length quartz cuvettes. The transient spectra and kinetics were obtained using a commercially available transient absorption spectrometer (Helios, Ultrafast

Systems), averaging at least three scans and using 2 s of averaging at every given delay.

Transient kinetics were evaluated using Igor Pro 7.

3.3.6. Nanosecond UV-VIS Transient Absorption (TA) Spectroscopy

Nanosecond transient absorption measurements were collected with a LP920 laser flash photolysis system (Edinburgh Instruments). Briefly, a tunable Vibrant 355 Nd:YAG/OPO system

(OPOTEK) was used for pulsed laser excitation. To collect transient difference spectra in the visible portion of the spectrum, an iStar ICCD camera (Andor Technology), controlled by the

Transient absorption decay kinetics were fit and analyzed with IGOR Pro.

3.3.7. Ultrafast Mid-IR Transient Absorption (TRIR) Spectroscopy

Sub-picosecond mid-infrared time-resolved measurements were performed using an in- house built pump-probe transient absorption spectrometer. The output from a 1 kHz Ti:Sapphire

Coherent Libra regenerative amplifier (4 mJ, 100 fs (fwhm) at 800 nm) was split into the pump and probe beams. The pump beam was directed into an optical parametric amplifier (Coherent

OPerA Solo, UV-VIS) to generate tunable excitation (480 nm), delayed in a 3.3 ns optical delay stage, and then focused into the sample. An optical chopper synchronized with the laser output at

500 Hz was placed in the pump beam for delta OD calculation, and a wavelength-appropriate half-wave plate rotated polarization to ensure the excitation at the magic angle.

The probe beam was directed into another optical parametric amplifier (Coherent OPerA Solo,

DFG) to generate the Mid IR probe. After entering a N2-purged sample compartment, the probe beam was split 50/50 into probe and reference beams which were both focused into the sample sealed in a demountable flow cell with round 25 mm Diam. x 1 mm BaF2 windows and a variable spacer (0.95 mm) but only the probe was overlapped with the pump. Both probe and reference were re-focused onto the entrance slit of a Horiba Scientific iHR320 imaging spectrometer. The signal was collected using a 64x2 dual array MCT liquid N2–cooled detector

(FPAS integrator and electronics from Infrared Systems Development Corporation). The experiment was controlled by in-house built LabVIEW software. Typically, 1600 laser pulses were averaged. To aid in sample stability, scanning was stopped shortly after signals returned to the baseline, though one full 3.3 ns scan was performed during setup to confirm no further features evolved after this point. The sample solutions were prepared to have FTIR absorbance values of ca. 0.3 at the metal hydride stretch. The ground state IR absorption spectra were taken before and after each experiment using a Bruker Vertex 80V FTIR spectrophotometer operating

with OPUS v.7.2 software to ensure there was no sample decomposition observable in the mid-

IR fingerprint regions.

3.3.8. Electronic Structure Calculations using Density Functional Theory (DFT)

The electronic structure calculations utilized in this study were performed using the

Gaussian 0924 software package and the computation resources of the North Carolina State

University High Performance Computing Center. Geometry optimizations were performed at the

B3LYP-D3/6-31G*/LANL2DZ level of theory.25–32 The polarizable continuum model (PCM) was used to simulate the acetonitrile solvent environment for all calculations.33 Frequency calculations were performed on all optimized structures and no imaginary frequencies were obtained.

3.4. Results and Discussion

+ +

N N N H N D

Ir Ir N H N D

N N

1 2

Figure 3.1 Molecular structures of cis-[Ir(bpy)2H2]PF6 (1) and cis-[Ir(bpy)2D2]PF6 (2).

3.4.1. Synthesis and Characterization

The proteo and deutero containing Ir(III) bipyridyl complexes 1 and 2 (Figure 3.1) were synthesized using a modified 3-step synthesis originally reported by Meyer and coworkers.18,19

Starting with commercially available 2,2’-bipyridine and IrCl3•xH2O, the dichloride,

[Ir(bpy)2Cl2]PF6 was prepared before replacing the chlorides with triflate groups using

18,19,34,22 CF3SO3H. The reducing agents NaBH4 and NaBD4 in either CH3OH or CH3OD, respectively, were then used to synthesize 1 and 2, in moderate yields.18,19 Each product was purified through multiple recrystallizations. The final synthesized molecules were structurally characterized by 1H and 13C NMR spectroscopy, solid-state FT-IR and Raman spectroscopy, and

ESI-MS.

3.4.2. FT-IR Spectroscopy and Solid-State Raman Spectroscopy

(a) (b)

2200 2100 2000 -1 2200 2100 2000 Wavenumber (cm ) -1 Raman Shift (cm )

NormalizedIntensity

Absorbance Normalized

1600 1500 1400 -1 1550 1500 1450 Wavenumber (cm ) -1 Raman Shift (cm )

3000 2500 2000 1500 1000 500 2200 2000 1800 1600 1400 1200 -1 Wavenumber (cm ) -1 Raman Shift (cm ) 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.

Figure 3.2 presents the solid-state FT-IR (ATR) and off-resonance Raman spectra (λex =

785 nm) measured for 1 and 2. Deuterium labeling resulted in red-shifting the metal hydride stretching frequencies between 1 and 2, making these techniques facile indicators of successful isotope substitution. The magnitude of the isotopically induced frequency shift can be approximated using Equations 1 and 2, where µ is the reduced mass calculated from respective masses of the two bonded atoms (Ir and H or D; ma and mb, respectively) and ν is the vibrational frequency value (in cm-1) calculated from the force constant (k), µ, and the speed of light c.

-1 Using Eq. 2 with the measured Ir-H stretching frequency (2120 cm ) and μIr-H (~1), k can be readily calculated and then substituted into the same equation with μIr-D (~2) to estimate the Ir-D frequency as 1500 cm-1, which is in quantitative agreement with experimental FT-IR and Raman data presented in Figure 3.2.

µ = (1)

ν = (2)

Upon inspection, the relevant IR and Raman spectra reveal symmetric and anti-symmetric H-M-

H stretches are only separated by a few wavenumbers. The higher frequency vibration has been assigned to the symmetric stretch and the anti-symmetric stretch to the lower frequency vibration using DFT calculations as described below. Interestingly, upon using H2O (rather than D2O) and

NaBD4 during the final synthetic step of 2, a mixture of 1, 2, and possibly a heteroleptic compound with both Ir-H and Ir-D bonds were synthesized, as suggested by solid-state ATR FT-

IR. The spectrum seen in Figure B7 depicts both features at 2100 and 1500 cm-1, suggesting the presence of both 1 and 2.

The FTIR results are useful indicators for the relevant spectroscopic windows to be used in transient IR studies where the Ir-H/D vibrations are probed. In particular, the lack of bpy-and

solvent-based vibrations near the characteristic Ir-H stretches near 2100 cm-1 will be shown to be paramount in evaluating the excited state features of 1. Unfortunately, the Ir-D stretches were shifted into a highly congested spectral region inhabited by many bpy-localized vibrations; a situation that was detrimental for quantitatively studying the Ir-D modes of 2 in the MLCT excited state. DFT calculations were also used to assign the structuring of the Ir-D stretch in the

FTIR data as mixed bpy and Ir-D vibrations.

0.20 0.15

0.10 Chloroform blank

Transmittance 0.05 Acetonitrile blank Dichloromethane blank* 0.00 2200 2150 2100 2050 2000 1950 1900 -1 Wavenumbers (cm )

Chloroform 0.12 Acetonitrile Dichloromethane* 0.08

Absorbance 0.04

0.00 2200 2150 2100 2050 2000 1950 1900 -1 Wavenumbers (cm )

For the purposes of performing TRIR studies, the selection of a suitable solvent was necessary using liquid FTIR. Figure 3.3 depicts the Ir-H stretch of 1 in three different solvents that showed the most promise for TRIR studies. Chloroform would in theory be the best solvent choice as the double-top feature is resolved; however, the sample proved to be sparingly soluble in this solvent which is detrimental to obtaining high enough concentrations for TRIR. Even a small change in absorbance at 2125 cm-1 between acetonitrile and chloroform blanks leads to a loss in resolution of the symmetric and antisymmetric stretches of 1. Dichloromethane was also selected because of its low absorption in the selected window (note that in Figure 3.3, the spacer width is almost doubled for the sample in dichloromethane to demonstrate the large effect spacer width has in signal intensity). However, the double-top feature is still not resolved in this solvent and the complex proved to be unstable after continued light irradiation. Acetonitrile was therefore chosen as the solvent for continued photophysical studies.

3.4.3. Ultrafast Transient Absorption (TA) Spectroscopy

The MLCT excited states of both complexes 1 and 2 were measured on sub-nanosecond timescales using transient absorption difference spectra and single wavelength kinetics following excitation at 480 nm. Four positive features were evident at 370, 430, 500, and the tail end of a broad feature starting below 900 nm, as can be seen in Figure 3.4 and Figure B8. Absorption features at 370 and 500 nm are common markers used to confirm the formation of bpy•- which results from transient MLCT one-electron oxidation of the metal center and one-electron reduction of bpy.35–37 The NIR difference spectra also agree with established spectra of other bpy-containing MLCT complexes where a broad absorption feature appears between 600-1000 nm.37,38 The entire absorption profile of the excited state follows the identical decay kinetics as a

function of monitoring wavelength, suggesting all the transient features observed originate from the same lowest energy MLCT excited state.

The MLCT excited state in 1 is short lived, as presented in Figure B9, with a single exponential lifetime of 25 ps for all transient features and no long-lived excited states were persistent in supra-nanosecond transient absorption spectroscopy experiments. TA data for 2 revealed similar spectral features and excited state lifetime, suggesting no obvious deuterium isotope effect on MLCT excited state decay. The short lifetime and resulting photostability of these complexes verified their utility as non-reactive species ideal for photophysical investigations of Ir(III) metal hydrides. Photostability was confirmed by UV-Vis absorption spectroscopy performed before and after all TA experiments confirming no changes to their ground state absorption spectra, highlighting the robustness of these molecules for photophysical investigations.

2.0 -2 ps -2 ps 0 ps 20 0 ps 5.5 ps 10.5 ps 5 ps 15.5 ps 10 ps 21.5 ps 15 ps 1.5 31.5 ps 15 20 ps 65.5 ps 105 ps 30 ps 400 ps

55 ps -3

-3 10 75 ps 215 ps 1.0 Ax10 355 ps Ax10 ∆ ∆ 5 0.5

0.0

350 400 450 500 550 600 650 900 1000 1100 1200 Wavelength (nm) Wavelength (nm)

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 performed under identical conditions.

+ 3.4.4. Comparison to cis-[Irbpy2Cl2]

The effect of hydride substitution of the analogous dichloride complex was briefly

studied. The absorption of cis-[Ir(bpy)2Cl2]PF6 contains d- π* transitions in the visible with π- π*

bands dominating the UV region (Figure B10). There is a marked bathochromic shift in the

MLCT band upon replacement of the chloride ligand with H-. This is in result of the increase in

15 -2 ps 0 ps 5.5 ps 11.5 ps 2.5 23.5 ps 33.5 ps 10 2.0

105 ps

500 ps -3 -3 1 ns 1.5 2 ns Ax10

Ax10 3 ns ∆ 1.0 ∆ 5 6.2 ns

0.5

0.0 0 0 50 100 150 Time (ps) 350 400 450 500 550 600 650 Wavelength (nm)

+ 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.

electron density on the metal center, therefore lowering the energy of the MLCT transition. The

well-studied room temperature emission of the dichloride (Figure B12) has been assigned as

originating from an equilibrated (π–π*)-(d–π*) excited state. 20,21,39–45 77 K steady state emission

of the dichloride reveal structured bands which have been assigned as originating from a ligand

delocalized excited state.20,21,39–45 In contrast, the proteo and deutero complexes 1 and 2 are non-

emissive at room temperature and exhibit little structuring in their emission profiles at 77 K with

a long-lifetime component of 8.2 μs (Figure B13). This is once again rationalized as being a

result of the increase in ligand field splitting after hydride substitution. However, the small

amount of structuring in the low temperature emission profiles suggests some LC character present in the emissive excited state despite the overall broad shape characteristic of MLCT transitions.

The dichloride was also investigated using ultrafast and nanosecond TA spectroscopy.

Although the short-lived features seen in Figure 3.5 are similar to those of 1 and 2, the lifetime is approximately half that of 1 and 2, 12 ps. Strikingly, there is a long-lived excited state not seen in the TA difference spectra in 1 or 2. This long-lived feature, also assigned as belonging to bpy•- is visible using nanosecond TA as displayed in Figure B10.35–37 These stark differences between the dichloride and 1 and 2 suggest replacing the chlorides with strongly σ-donating hydrides separates the very small energy gap believed to exist between the π- π* and d- π* states in the dichloride.20,21,39–45

3.4.5. Ultrafast Transient IR (TRIR) Spectroscopy of 1 and 2

The high photostability of this molecule allows for high excitation powers (5 μJ/pulse) to be used without use of a flow cell to reconcile the low extinction coefficients characteristic of the

Ir-H vibrations. Transient bleaching signals were observed for both the proteo and deutero compounds at 2128 and 1523 cm-1, respectively (Figure 3.6 and Figure B14). These observed bleaches were broad, structureless, and do not resolve the symmetric and anti-symmetric H-M-H stretches. However, this is unsurprising given the loss of resolution in the chosen acetonitrile solvent as seen in Figure 3.3. No other vibrational modes were observed (or calculated to be present) across the 2000-2200 cm-1 energy window allowing for clear fingerprint observation of the stretching modes of 1 in the MLCT excited state. This proteo molecule exhibited two distinct transient absorption features, one that is better resolved at higher energy and another that is less

resolved and broad at lower energy. The higher energy stretch is similar in intensity and shape to the bleaching feature and was expected as a result of MLCT oxidation of the Ir(III) center to

Ir(IV), which should transiently increase the strength of the Ir-H bonds in the excited state. The window could not be moved to higher energy to observe more of this feature as the solvent

A S

OD OD ∆

∆

-2 ps 0 ps 4.3 ps 12.3 ps 25.5 ps A 380 ps S

2200 2150 2100 2050 2000 1950 2200 2100 2000 -1 -1 Wavenumber (cm ) WavenumberWavenumber (cm -1))

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 signifies and antisymmetric stretch and S labels a symmetric stretch).

absorbs appreciably past this point. The significantly broader lower energy feature was unexpected and as the presented spectra are the result of background subtraction, is not simply a result of experimental artifacts. One possible explanation for the appearance of this second feature is the larger energy separation between symmetric and anti-symmetric stretching modes of H-Ir-H modes in the triplet excited state, as revealed through the electronic structure calculations described below. The extreme broadness of this feature is also unusual, and when

the window is shifted to lower energy as in Figure B15, it appears to end just below 1900 cm-1.

To test for possible solvent interactions, this experiment was repeated in DCM, though 1 proved photochemically unstable in this solvent. No features were observed when 2 was examined at this window.

The Ir-D stretches appear at much lower energy due to the increase in reduced mass.

Unfortunately, these stretching modes are moved to the center of the spectroscopic window occupied by multiple bpy vibrations, which can be seen in the FTIR spectra presented in Figure

-1 3.2. All TRIR studies performed in the 1400-1650 cm window had to be measured in CD3CN which doesn’t absorb as strongly as CH3CN in this region of the spectrum. The TRIR difference spectra revealed multiple transient features at high and low energy when compared to the bleach at 1523 cm-1, making it difficult to identify any pure Ir-D stretching mode shift resulting from

MLCT excited state formation. When observing the relevant shifts for 1 in this lower energy window, the same positive features were still present confirming they do not belong to any Ir-D related stretch (Figure B14). The locations of bpy vibrations present for 1 and 2 agree with

+ 46–48 previous transient infrared experiments performed on [Ru(bpy)3] . Lifetimes of the features within the 1430-1650 cm-1 window were measured to fall between 25 and 30 ps, suggesting all transient features originate from the identical excited state, confirming there was no observed isotope effect on the MLCT excited state lifetimes (Figure B16). Although infrared spectroelectrochemistry would be a useful technique here to further evaluate these molecules, enabling confirmation of all TRIR features, 1 unfortunately irreversibly oxidizes at the Ir(III) center on electrochemical time scales,18,19 precluding any possible bulk electrolysis characterization.

3.4.6. DFT Calculated IR Difference Spectra

Calculated IR difference spectra were generated for comparison to the experimental

TRIR data. All electronic structure and spectral simulation calculations were performed using the

Gaussian09 software package. Geometries of both Ir(III) complexes were optimized at the

B3LYP-D3/6-31G*/LANL2DZ level of theory. The polarizable continuum model was used to

+ simulate the acetonitrile solvent environment used in all TRIR experiments. [Ir(dmbpy)2Cl2]

(dmbpy = 4,4′-dimethyl-2,2′-bipyridine) was used as the benchmark for geometry optimizations as there are no available crystal structures for 1 or 2.49 Calculated TRIR spectra were produced by subtracting the optimized ground state’s structure’s normalized IR data from that of the lowest triplet excited state; both are shown in Figure B18.

Upon inspection, the experimentally determined TRIR frequencies for 1 as presented in

Figure 3.6 (left) and Figure B15 agree well with calculated difference spectra shown in Figure

3.6 (right) and Figure B19. According to the simulated difference spectrum, the symmetric and anti-symmetric stretches are essentially split into two different energy regions in the MLCT state of 1. This most likely explains the appearance of the two energetically well-separated transient features in the experimental TRIR data occurring at both low and high energy with respect to the vibrations in the ground state of this molecule. Based on the electronic structure calculations, the higher energy feature (2156 cm-1) is therefore assigned to the symmetric stretch while the lower energy feature (2086 cm-1) is attributed to the anti-symmetric stretch in the MLCT excited state.

The calculated IR difference spectrum of 2 also agrees with experimental TRIR results in that there is good overlap between the observed vibrations in the 1430-1650 cm-1 window of 1 and 2, except for the ground state bleach occurring near 1500 cm-1 that is only present in 2.

3.5. Conclusions

+ + The photophysical characterization of cis-[Ir(bpy)2H2] and cis-[Ir(bpy)2D2] has been presented in order to better understand the fundamental spectroscopy of the MLCT excited state of Ir(III) diimine hydrides. These compounds proved to be particularly robust, without producing permanent photoproducts, rendering them ideal for fundamental photophysical studies of the Ir-

H bond vibrational dynamics. Short-lived (τ = 25 ps) MLCT excited states were observed in both compounds using ultrafast transient UV-vis and mid-IR absorption spectroscopy. TRIR results revealed two Ir-H stretching modes surrounding the ground state bleach of the dihydride at 2130 cm-1. These features have been assigned to symmetric and anti-symmetric stretching modes of the H-Ir-H fragment in the MLCT excited state using computational methods. Similar observations could not be made for the deutero compound, where the Ir-D stretching modes in the excited state were masked by bpy-localized vibrations. The transient absorption and IR spectroscopic features presented here provide insight into the photophysical behavior of Ir(III) diimine hydrides and can be utilized in future investigations involving photocatalytic hydrides and their highly reactive intermediates, particularly in relation to the production of solar fuels.

3.6. Acknowledgments

This work was supported by the National Science Foundation (CHE-1465068). The solid-state

Raman spectra were acquired at the Analytical Instrumentation Facility (AIF) at North Carolina

State University, which was supported by the State of North Carolina and the National Science

Foundation (Award Number ECCS-1542015). The AIF is a member of the North Carolina

Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology

Coordinated Infrastructure (NNCI).

3.7. References

(1) Perutz, R. N.; Procacci, B. Photochemistry of Transition Metal Hydrides. Chem. Rev. 2016,

116, 8506–8544.

(2) Pitman, C. L.; Brereton, K. R.; Miller, A. J. M. Aqueous Hydricity of Late Metal Catalysts as

a Continuum Tuned by Ligands and the Medium. J. Am. Chem. Soc. 2016, 138, 2252–2260.

(3) Brereton, K. R.; Bellows, S. M.; Fallah, H.; Lopez, A. A.; Adams, R. M.; Miller, A. J. M.;

Jones, W. D.; Cundari, T. R. Aqueous Hydricity from Calculations of Reduction Potential and

Acidity in Water. J. Phys. Chem. B 2016, 120, 12911–12919.

(4) Suenobu, T.; Guldi, D. M.; Ogo, S.; Fukuzumi, S. Excited-State Deprotonation and H/D

Exchange of an Iridium Hydride Complex. Angew. Chemie - Int. Ed. 2003, 42, 5492–5495.

(5) Barrett, S. M.; Pitman, C. L.; Walden, A. G.; Miller, A. J. M. Photoswitchable Hydride Transfer

from Iridium to 1-Methylnicotinamide Rationalized by Thermochemical Cycles. J. Am.

Chem. Soc. 2014, 136, 14718–14721.

(6) Barrett, S. M.; Slattery, S. A.; Miller, A. J. M. Photochemical Formic Acid Dehydrogenation

by Iridium Complexes: Understanding Mechanism and Overcoming Deactivation. ACS Catal.

2015, 5, 6320–6327.

(7) Chambers, M. B.; Kurtz, D. A.; Pitman, C. L.; Brennaman, M. K.; Miller, A. J. M. Efficient

Photochemical Dihydrogen Generation Initiated by a Bimetallic Self-Quenching Mechanism.

J. Am. Chem. Soc. 2016, 138, 13509–13512.

(8) Kaesz, H. D.; Saillant, R. B. Hydride Complexes of the Transition Metals. Chem. Rev. 1972,

72, 231–281.

(9) Colombo, M.; W. George, M.; N. Moore, J.; I. Pattison, D.; N. Perutz, R.; G. Virrels, I.; Ye,

T.-Q. Ultrafast Reductive Elimination of Hydrogen from a Metal Carbonyl Dihydride

Complex; a Study by Time-Resolved IR and Visible Spectroscopy. J. Chem. Soc., Dalt.

Trans. 1997, No. 17, 2857–2860.

(10) Lian, T.; Bromberg, S. E.; Asplund, M. C.; Yang, H.; Harris, C. B. Femtosecond Infrared

Studies of the Dissociation and Dynamics of Transition Metal Carbonyls in Solution. J. Phys.

Chem. 1996, 100, 11994–12001.

(11) George, M. W.; Turner, J. J. Excited States of Transition Metal Complexes Studied by Time-

Resolved Infrared Spectroscopy. Coord. Chem. Rev. 1998, 177, 201–217.

(12) Doorn, S. K.; Dyer, R. B.; Stoutland, P. O.; Woodruff, W. H. Ultrafast Electron Transfer and

Coupled Vibrational Dynamics in Cyanide Bridged Mixed-Valence Transition-Metal Dimers.

J. Am. Chem. Soc. 1993, 115, 6398–6405.

(13) Frederix, P. W. J. M.; Adamczyk, K.; Wright, J. A.; Tuttle, T.; Ulijn, R. V; Pickett, C. J.;

Hunt, N. T. Investigation of the Ultrafast Dynamics Occurring during Unsensitized

Photocatalytic H2 Evolution by an [FeFe]-Hydrogenase Subsite Analogue. Organometallics

2014, 33, 5888–5896.

(14) Kuimova, M. K.; Alsindi, W. Z.; Blake, A. J.; Davies, E. S.; Lampus, D. J.; Matousek, P.;

McMaster, J.; Parker, A. W.; Towrie, M.; Sun, X.-Z.; et al. Probing the Solvent Dependent

Photophysics of Fac-[Re(CO)3(dppz-X2)Cl] (Dppz-X2 = 11,12-X2-dipyrido[3,2-a:2′,3′-

C]phenazine); X = CH3, H, F, Cl, CF3). Inorg. Chem. 2008, 47, 9857–9869.

(15) Kuimova, M. K.; Grills, D. C.; Matousek, P.; Parker, A. W.; Sun, X.-Z.; Towrie, M.; George,

M. W. Picosecond Time-Resolved Infrared Investigation into the Nature of the Lowest

Excited State of Fac-[Re(Cl)(CO)3(CO2Et-Dppz)] (CO2Et-Dppz =

dipyrido[3,2a:2′,3′c]phenazine-11-Carboxylic Ethyl Ester). Vib. Spectrosc. 2004, 35, 219–

223.

(16) Kuimova, M. K.; Alsindi, W. Z.; Dyer, J.; Grills, D. C.; Jina, O. S.; Matousek, P.; Parker, A.

W.; Portius, P.; Zhong Sun, X.; Towrie, M.; et al. Using Picosecond and Nanosecond Time-

Resolved Infrared Spectroscopy for the Investigation of Excited States and Reaction

Intermediates of Inorganic Systems. Dalt. Trans. 2003, No. 21, 3996–4006.

(17) Kuimova, M. K.; Sun, X. Z.; Matousek, P.; Grills, D. C.; Parker, A. W.; Towrie, M.; George,

M. W. Probing Intraligand and Charge Transfer Excited States of Fac-[Re(R)(CO)3(CO2Et-

+ Dppz)] (R = Py, 4-Me2N-Py; CO2Et-Dppz = dipyrido[3,2a:2’,3’c]phenazine-11-Carboxylic

Ethyl Ester) Using Time-Resolved Infrared Spectroscopy. Photochem. Photobiol. Sci. 2007,

6, 1158–1163.

(18) Sullivan, B. P.; Meyer, T. J. Synthesis of Hydrido-Iridium(III) Complexes from a

Trifluoromethanesulphonato Intermediate. J. Chem. Soc. Chem. Commun. 1984, No. 7, 403–

405.

(19) Bolinger, C. M.; Story, N.; Sullivan, B. P.; Meyer, T. J. Electrocatalytic Reduction of Carbon

Dioxide by 2,2’-bipyridine Complexes of Rhodium and Iridium. Inorg. Chem. 1988, 27,

4582–4587.

(20) Soman, S. Novel Iridium (III) Complexes for Photocatalytic H2 Generation from H2O Using

Sunlight. PhD Dissertation, Dublin City University, 2012.

(21) Soman, S.; Younis, H. M.; Browne, W. R.; Vos, J. G.; Pryce, M. T. Synthesis and Isotope

Effects on the Excited State Properties of N^N Bound [Ir(polypyridyl)2Cl2]PF6 Complexes.

Eur. J. Inorg. Chem. 2017, 2017, 5598–5603.

(22) Watts, R. J.; Harrington, J. S.; Van Houten, J. A Stable Monodentate 2,2’-bipyridine Complex

of Iridium(III): A Model for Reactive Intermediates in Ligand Displacement Reactions of

Tris-2,2’-bipyridine Metal Complexes. J. Am. Chem. Soc. 1977, 99, 2179–2187.

(23) Garakyaraghi, S.; Danilov, E. O.; McCusker, C. E.; Castellano, F. N. Transient Absorption

Dynamics of Sterically Congested Cu(I) MLCT Excited States. J. Phys. Chem. A 2015, 119,

3181–3193.

(24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J.

R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; et al. Gaussian 09, Revision

D.01; Gaussian, Inc.: Wallingford, CT, 2016.

(25) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio

Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements

H-Pu. J. Chem. Phys. 2010, 132, 154104.

(26) Becke, A. D. Density‐functional Thermochemistry. III. The Role of Exact Exchange. J. Chem.

Phys. 1993, 98, 5648–5652.

(27) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations.

Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270–283.

(28) Hariharan, P. C.; Pople, J. A. The Influence of Polarization Functions on Molecular Orbital

Hydrogenation Energies. Theor. Chim. Acta 1973, 28, 213–222.

(29) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self—Consistent Molecular Orbital Methods. XII.

Further Extensions of Gaussian—Type Basis Sets for Use in Molecular Orbital Studies of

Organic Molecules. J. Chem. Phys. 1972, 56, 2257–2261.

(30) Becke, A. D. A New Mixing of Hartree–Fock and Local Density‐functional Theories. J.

Chem. Phys. 1993, 98, 1372–1377.

(31) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations.

Potentials for K to Au Including the Outermost Core Orbitals. J. Chem. Phys. 1985, 82, 299–

310.

(32) Wadt, W. R.; Hay, P. J. Ab Initio Effective Core Potentials for Molecular Calculations.

Potentials for Main Group Elements Na to Bi. J. Chem. Phys. 1985, 82, 284–298.

(33) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. New Developments in the Polarizable

Continuum Model for Quantum Mechanical and Classical Calculations on Molecules in

Solution. J. Chem. Phys. 2002, 117, 43–54.

(34) Soman, S.; Manton, J. C.; Inglis, J. L.; Halpin, Y.; Twamley, B.; Otten, E.; Browne, W. R.;

De Cola, L.; Vos, J. G.; Pryce, M. T. New Synthetic Pathways to the Preparation of Near-

Blue Emitting Heteroleptic Ir(III)N6 Coordinated Compounds with Microsecond Lifetimes.

Chem. Commun. 2014, 50, 6461–6463.

(35) Finlayson, M. F.; Ford, P. C.; Watts, R. J. Excited States and Transients Formed in Laser

Flash Photolysis of Iridium(III) Complexes of 2,2’-Bipyridine. J. Phys. Chem. 1986, 90,

3916–3922.

(36) Buntinx, G.; Naskrecki, R.; Poizat, O. Subpicosecond Transient Absorption Analysis of the

Photophysics of 2,2‘-Bipyridine and 4,4‘-Bipyridine in Solution. J. Phys. Chem. 1996, 100,

19380–19388.

(37) Ichimura, K.; Kobayashi, T.; King, K. A.; Watts, R. J. Excited-State Absorption Spectroscopy

of Ortho-Metalated Iridium(III) Complexes. J. Phys. Chem. 1987, 91, 6104–6106.

(38) Creutz, C. Bipyridine Radical Ions. Comments Inorg. Chem. 1982, 1, 293–311.

(39) Divisia, B.; Ford, P. C.; Watts, R. J. Determination of Rate Constants for Photosolvation

Reactions: Solvent Dependence of Chloride Substitution Rates for Excited States of Cis-

dichlorobis(2,2’-bipyridine)iridium(III). J. Am. Chem. Soc. 1980, 102, 7264–7268.

(40) Watts, R. J.; Efrima, S.; Metiu, H. A Novel Deuterium Effect on Dual Charge-Transfer and

Ligand-Field Emission of the Cis-dichlorobis(2,2’-bipyridine)iridium(III) Ion. J. Am. Chem.

Soc. 1979, 101, 2742–2743.

(41) Ohashi, Y.; Nakamura, J. Solvent Effect on Excited States of [IrCI2(bpy)2]Cl, [IrCl2(phen)2]Cl

and Related Complexes. Chem. Phys. Lett. 1984, 109, 301–305.

(42) Watts, R. J. Luminescence Tuning of 2,2’-Bipyridine and 1,10-Phenanthroline Complexes of

Iridium(III) in Fluid Solution. Inorg. Chem. 1981, 20, 2302–2306.

(43) Wallace, L.; Heath, G. A.; Krausz, E.; Moran, G. Pathological Luminescence Characteristics

+ of cis-[Ir(bpy)2Cl2] . Inorg. Chem. 1991, 30, 347–349.

(44) Watts, R. J.; Crosby, G. A.; Sansregret, J. L. Excited States of Transition Metal Complexes.

Spectroscopic Measurement of dπ-ππ* Interactions in Iridium(III) Complexes. Inorg. Chem.

1972, 11, 1474–1483.

(45) Ohashi, Y.; Kobayashi, T. A Study on Electronic Excited States of Iridium(III) Complexes

Containing Bipyridine and Phenanthroline Ligands. Solvent Effect on Triplet-Triplet

Absorption Spectra. Bull. Chem. Soc. Jpn. 1979, 52, 2214–2217.

(46) Butler, J. M.; George, M. W.; Schoonover, J. R.; Dattelbaum, D. M.; Meyer, T. J. Application

of Transient Infrared and near Infrared Spectroscopy to Transition Metal Complex Excited

States and Intermediates. Coord. Chem. Rev. 2007, 251, 492–514.

(47) Omberg, K. M.; Schoonover, J. R.; Treadway, J. A.; Leasure, R. M.; Dyer, R. B.; Meyer, T.

2+* J. Mid-Infrared Spectrum of [Ru(bpy)3] . J. Am. Chem. Soc. 1997, 119, 7013–7018.

(48) Sun, Q.; Dereka, B.; Vauthey, E.; Lawson Daku, L. M.; Hauser, A. Ultrafast Transient IR

Spectroscopy and DFT Calculations of Ruthenium(II) Polypyridyl Complexes. Chem. Sci.

2017, 8, 223–230.

(49) Yoshikawa, N.; Sakamoto, J.; Kanehisa, N.; Kai, Y.; Matsumura-Inoue, T. Cis -

Dichlorobis(4,4′-Dimethyl-2,2′-bipyridine)iridium(III) Hexafluorophosphate. Acta

Crystallogr. Sect. E Struct. Reports Online 2003, 59, m155–m156.

CHAPTER 4: Ultrafast Dynamics of Re(I) Diimine-Containing Di- and Tricarbonyls

4.1. Background and Scope

Re(I) diimine-containing tricarbonyls have been studied for several decades, with significant emphasis being placed on the photophysical implications of their low-lying and tunable metal-to-ligand charge transfer (MLCT) excited states.1–7 Their facile synthetic flexibility enables chromophoric and ancillary ligand modifications to introduce IR functional group tags that permit close monitoring of the excited state processes through time-resolved infrared (TRIR) techniques.2,8–11 Importantly, the presence of strongly IR-absorbing CO groups to both the metal center (as C≡O) and the chromophoric ligand (as C=O) facilitate straightforward detection of electron density movement in the molecule subsequent to photoexcitation. However, a large drawback to the presence of three metal-carbonyls in the case

- of Re(N^N)(CO)3R chromophores (R = X or a neutral monodentate ligand) is the fast

ES ES

GS GS Tricarbonyl Dicarbonyl

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.

nonradiative decay processes promoted by their high frequency vibrations strongly coupled to solvent modes, which markedly reduces the lifetimes and photoluminescence quantum yields of these particular MLCT excited states.5,12–14

Therefore, reducing the number of M-C≡O bonds can strategically decrease nonradiative decay rates as describe below while simplifying vibrational spectroscopy assignments, leading to the recent research on Re(I) dicarbonyl diimine containing species.15–17 Figure 4.1 qualitatively depicts the differences in vibrational overlap between ground and excited states in Re(I) dicarbonyl and tricarbonyl complexes. A higher degree of destructive interference between the ν

= 0 level of the excited state potential surface and the vibrational levels (ν = 0, 1, 2, 3…) of the ground state potential energy surface results in smaller vibrational wave function overlap leading to lower rates of non-radiative decay (knr) in the dicarbonyl species. Higher energy C≡O stretching modes in the tricarbonyl structures signify more distributed pi-backbonding when compared to Re(I) dicarbonyls, wherein the metal contributes electron density to two C≡O π* orbitals instead of three. Dividing the pi-backbonding between two C≡O π* orbitals in these latter molecules results in significantly lower C≡O vibrational frequencies that do not couple as effectively to the solvent-based modes. The net result of this lessening vibrational overlap is a reduction in nonradiative decay leading to the enhancement of valuable excited state photophysical properties including photoluminescence quantum yield and excited state lifetime.

The addition of highly conjugated diimine ligands containing C=O bonds introduces ligand-centered (LC) excited states that can be readily monitored using TRIR spectroscopy.2,18,19

Remote from the metal d-orbitals, these ligand-localized C=O bonds exhibit IR absorptions significantly lower in energy than the M-C≡Os, leading to two distinct windows in which to observe changes in electron density in different portions of the molecule upon photoexcitation.

CO + CO + CO + CO CO CO N Re CO N Re CO N Re CO

N N NCCH3 N N PPh3 N N DMAP N N N O O O O O O N N N C8H17 C8H17 C8H17 O O O 1A 1B 1C

Scheme 4.1 Structures of the Re(I) tricarbonyl molecules investigated here.

The research presented in this Chapter focuses on the ultrafast dynamics of a class of

Re(I) tricarbonyl complexes in which the ancillary ligand was altered and a class of Re(I) dicarbonyl classes in which the substitution pattern on the diimine ligands used was

+ systematically varied. The tricarbonyls investigated 1A-1C, [Re(CO)3(NIBI-phen)L] , where L is

CH3CN, PPh3, or DMAP, are shown in Scheme 4.1. Scheme 4.2 depicts the Re(I) dicarbonyl

Scheme 4.2 Structures of the Re(I) dicarbonyls examined in this chapter. Labels indicate distinguishing diimine ligands.

complexes 1-9 that were investigated. In particular, ultrafast TRIR will be utilized to identify the nature of the lowest excited states in both classes of molecules while gleaning insight into the photophysical steps leading to the formation of these states. Electronic structure calculations were also performed to assist in assignments of the time-resolved infrared spectra reported here.

4.2. Experimental

All Re(I) tricarbonyl complexes were synthesized by a previous Castellano group member Dr. Rosalynd Joyce and the synthesis and structural characterization of these molecules are provided in her Dissertation.20 The Re(I) dicarbonyl complexes were synthesized by a current

Castellano group member, Ms. Hala Atallah, and the synthesis and structural characterization of these molecules are provided here in Appendix C. All solvents used for spectroscopic measurements were spectroscopic grade and used as received from commercial sources.

4.2.1. Ultrafast UV-VIS Transient Absorption (TA) Spectroscopy

Sub-picosecond transient absorption (TA) measurements were performed using an amplified Ti:sapphire laser system described previously.21 Briefly, a portion of the output from a

1 kHz Ti:sapphire Coherent Libra regenerative amplifier (4 mJ, 100 fs (fwhm) at 800 nm) was split into the pump and probe beams. The pump beam was directed into an optical parametric amplifier (Coherent OPerA Solo) to generate the 470 nm (tricarbonyls) or 500 nm (dicarbonyls) pump pulse used in these experiments, while the probe beam was delayed in a 6.6 ns optical delay stage. The probe beam was focused into a CaF2 crystal to generate white light continuum

(WLC) spanning 340−750 nm or into the NIR with a proprietary crystal. The two beams were focused and spatially and temporally overlapped into a spot on the sample, with the relative polarizations of the pump and probe beams set at the magic angle. All solvents were fresh and

spectrophotometric grade, and the ground-state absorption spectra were taken before and after each experiment using an Agilent 8453 UV−visible spectrophotometer to ensure there was no sample decomposition. Samples were prepared in 2 mm path length quartz cuvettes. The transient spectra and kinetics were obtained using a commercially available transient absorption spectrometer (Helios, Ultrafast Systems), averaging at least three scans and using 2 s of averaging at every given delay. Transient kinetics were evaluated using Igor Pro 7.

4.2.2. Ultrafast Mid-IR Transient Absorption (TRIR) Spectroscopy

Sub-picosecond mid-infrared time-resolved measurements were performed using an in- house constructed pump-probe transient infrared absorption spectrometer. The output from a 1 kHz Ti:Sapphire Coherent Libra regenerative amplifier (4 mJ, 100 fs (fwhm) at 800 nm) was split into the pump and probe beams. The pump beam was directed into an optical parametric amplifier (Coherent OPerA Solo, UV-VIS) to generate tunable excitation (480 nm), delayed in a

3.3 ns optical delay stage, and then focused into the sample. An optical chopper synchronized with the laser output at 500 Hz was placed in the pump beam for delta OD calculation, and a wavelength-appropriate half-wave plate rotated polarization to ensure the excitation at the magic angle.

The probe beam was directed into another optical parametric amplifier (Coherent OPerA

Solo, DFG) to generate the mid IR probe beam. After entering an N2-purged sample compartment, the probe beam was split 50/50 into probe and reference beams which were both focused into the sample sealed in a demountable flow cell with round 25 mm Diam. x 1 mm

BaF2 windows and a variable spacer but only the probe was overlapped with the pump. Both probe and reference were re-focused onto the entrance slit of a Horiba Scientific iHR320 imaging spectrometer. The signal was collected using a 64x2 dual array MCT liquid N2–cooled

detector (FPAS integrator and electronics from Infrared Systems Development Corporation). The experiment was controlled by in-house coded LabVIEW software. Typically, 1600 laser pulses were averaged to generate sufficient kinetic transients. The sample solutions were prepared to maintain FTIR absorbance values of ca. 0.6 at the metal carbonyl stretch. The ground state IR absorption spectra were taken before and after each experiment using a Bruker Vertex 80V FTIR spectrophotometer operating with OPUS v.7.2 software to ensure there was no sample decomposition observable in the mid-IR fingerprint regions.

4.2.3. Electronic Structure Calculations

The electronic structure calculations utilized in this study were performed using the

Gaussian 0922 software package and the computation resources of the North Carolina State

University High Performance Computing Center. Geometry optimizations were performed at the

PBE0-D3/Def2-SVP/SDD23–26 level of theory for the Re(I) tricarbonyl series and the B3LYP-

D3/6-31G*/LANL2DZ level of theory for Re(I) dicarbonyl sereies.27–34 The polarizable continuum model (PCM) was used to simulate the solvent environments for all calculations.35

Frequency calculations were performed on all optimized structures and no imaginary frequencies were obtained in any instance.

4.3. Ultrafast TRIR Spectroscopy of Re(I) Tricarbonyls Containing NIBI-phen and an

Ancillary Ligand

Static FT-IR spectra of the Re(I) tricarbonyl complexes are presented in Figure C22 and reveal several IR absorptions ranging from 1600 to 2100 cm-1 and the carbonyl absorption features measured are listed in Table 4.1. The strong modes at higher energy are assigned to the stretching modes of the three metal-bound carbon monoxide ligands whereas the carbonyl features at lower

energy belong to the chromophore fastened to the NIBI moiety appended to the phenanthroline ligand.

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.

-1 -1 Complex Solvent νRe-CO (cm ) νNIBI-CO (cm )

CH3CN 2040, 1936, 1906 (sh) 1727, 1707, 1668 1A THF 2037, 1932, 1900 (sh) 1726, 1708, 1670

CH3CN 2041, 1956, 1925 1727, 1707, 1668 1B THF 2037, 1951, 1923 1728, 1708, 1670

CH3CN 2031, 1923, 1899 (sh) 1727, 1707, 1668 1C THF 2028, 1920, 1894 (sh) 1726, 1708, 1670

Important to note is the range of frequency values for the νRe-CO modes, which shift to higher (as in the case of PPh3) or lower (as in DMAP) energies depending on the metal-bound ancillary ligand. As anticipated, the assigned νNIBI-CO modes are invariant to ancillary ligand identity as the chromophoric ligand they are bonded to remains fixed in complexes 1A to 1C.

Interestingly, the FT-IR spectra of both CH3CN and DMAP-containing complexes measured in

THF or CH3CN produce only 2 clearly observable metal-carbonyl stretching modes. However, a weak shoulder is apparent in these spectra that is assigned as the third C≡O stretching mode.

This assignment is consistent with the DFT-calculated IR spectra for these molecules, in which the two lower energy Re-CO modes are overlapped significantly (Figure C23).

Ultrafast TRIR experiments were performed on the three Re(I) tricarbonyl complexes

1A-1C in both acetonitrile and tetrahydrofuran to map the movement of electron density following excitation at 470 nm. The resulting IR difference spectra recorded for [Re(NIBI- phen)(CO)3CH3CN]PF6 in THF and CH3CN are presented in Figure 4.2. Across the full spectral window evaluated (1850 – 2150 cm-1), there was a fast kinetic component detected followed by a long-lived transient that survives through the end of the delay line at 3.3 ns. To aid in visualizing the spectral shapes of these distinct kinetic components, the difference spectra in acetonitrile were separated into short and long time delays in Figure C24. The dynamics were mapped using single wavelength kinetics analyzed using Igor Pro 7. A summary of select frequencies and traces are included in Table C1 and Figure C25. The metal carbonyl window at higher energy

8 0 ps

6 25 ps

75 ps 4

550 ps

mOD 2

∆ 3.7 ns

-2 freq recalc Solvent = Acetonitrile -4 2150 2100 2050 2000 1950 1900 1850 -1 Wavenumber (cm ) 5 0 ps

4 25 ps 3 100 ps

610 ps 2 3.7 ns mOD ∆ 1 0

-1 freq recalc Solvent = Tetrahydrofuran

2150 2100 2050 2000 1950 1900 1850 -1 Wavenumber (cm )

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)

captures one visible bleach at 1930 cm-1 and two excited state maxima at 1950 and 2030 cm-1.

There are very minor changes in the difference spectra within this spectral window across the entire delay line, but major spectral changes were observed in the lower energy window (1580 –

1740 cm-1) when comparing short and long delay times. Research performed by Dr. Rosalynd

Joyce and Dr. James Yarnell suggest that upon excitation of [Re(NIBI-phen)(CO)3CH3CN]PF6 into its low energy band, the S1 state was initially populated, which is mainly intraligand charge transfer (ILCT) in character with some degree of MLCT character. Intersystem crossing then occurs to populate the lowest energy triplet state, which was shown to reside almost completely on the chromophoric NIBI-phen ligand according to spin density calculations, before relaxing back to its initial ground state. According to these results, the majority of electron density lies on the NIBI-phen ligand throughout the relaxation process, agreeing with the lack of significant energy shifts in the metal carbonyl stretching frequencies after initial pulsed laser excitation.

However, the small amount of MLCT character in these excited states does lead to a general shift of these carbon monoxide absorptions to higher energy as M-CO backbonding is reduced once electron density is removed from the metal center, corresponding to the transient strengthening of the C≡O bonds. The low energy window centered on the NIBI carbonyl stretching modes depicts a general shift to lower energy of the excited state features compared to the visible ground state bleaching features. The fast decay of the broad absorbance at short times is followed by more structured absorbance features that last through the end of the delay line.

Similar spectral features and dynamics were observed in the ultrafast TRIR difference spectra of [Re(NIBI-phen)(CO)3PPh3]PF6 (Figure 4.3). When separating these spectra into

“short” and “long” time delay times as performed in Figure C26 it is straightforward to conclude that no new features are formed in the higher energy window across the entire spance of the

10 10 freq recalc freq recalc 3.7 ns 2.8 ns 8 8 2.1 ns 1.08 ns 893 ps 608 ps 500 ps 6 6 310 ps 230 ps 172 ps 107 ps 4 104 ps 4 62 ps 50 ps

mOD 40 ps ∆ 20 ps 2 mOD 2 14 ps ∆ 10 ps 8.6 ps 7.2 ps 6.0 ps 0 0 4.9 ps 3.8 ps 2.9 ps 2.1 ps -2 -2 1.3 ps 0.6 ps 0 ps

-4 -4 2150 2100 2050 2000 1950 1900 1850 1750 1700 1650 1600 -1 -1 Wavenumber (cm ) Wavenumber (cm )

6 3.1 ns 6 2.8 ns 2.1 ns 1.1 ns 900 ps 600 ps 4 500 ps 310 ps 4 230 ps 170 ps 100 ps 60 ps 50 ps 40 ps 2 21 ps mOD

∆ 14 ps mOD 2 10.5 ps ∆ 9 ps 7 ps 6 ps 4.5 ps 0 3.5 ps 2.5 psp 0 1.5 ps 1 ps Time 0 -2 freq recalc -2 1750 1700 1650 1600 2150 2100 2050 2000 1950 1900 1850 -1 -1 Wavenumber (cm ) Wavenumber (cm )

Figure 4.3 Ultrafast TRIR difference spectra of [Re(NIBI-phen)(CO)3PPh3]PF6 after excitation at 470 nm in acetonitrile (top) and tetrahydrofuran (bottom). (spacer width = 390 μm). delay line while the ligand carbonyl window once again initiates as a broad absorbance that evolves into the structured features measured throughout the remainder of the delay line. The stark similarities to the TRIR difference spectra presented in Figure 4.2 are consistent with the previously proposed photophysical pathway that is identical to that of [Re(NIBI- phen)(CO)3PPh3]PF6; the highly ILCT S1 excited state, with some MLCT character, is initially populated prior to intersystem crossing to the T1 state which is also localized on the NIBI-phen chromophoric ligand as suggested by spin density calculations.20 Nanosecond step-scan TRIR would be a valuable tool in further probing the nature of the triplet excited state as previous

TRIR experiments on 3LC states typically expose a decrease in C≡O stretching frequency, which can be visualized on longer timescales.2,36 However, electronic structure calculations reported in the next section qualitatively match the long-lived absorption seen in Figures 4.5 and 4.6, suggesting the T1 state may not exhibit this shift to lower energy.

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)

However, [Re(NIBI-phen)(CO)3DMAP]PF6 was proposed to undergo a different excited state pathway with respect to the other two molecules, which leads to changes in the ultrafast

TRIR difference spectra in acetonitrile as presented in Figure 4.4; this particular complex proved to be photochemically unstable in THF mandating the exclusive use of CH3CN. Although the difference spectra across the lower energy window is very similar to those of complexes 1A and

1B, there are major differences in the higher energy window centered in the metal carbonyl region, as shown in Figure C28. At early delay times, the absorbances in this window are much broader before the two sharper long-lived positive features become visible, ultimately matching the absorption pattern measured in [Re(NIBI-phen)(CO)3CH3CN]PF6. The previously proposed excited state pathway for this molecule suggests that upon photoexcitation, the S2 state is initially

populated, which was determined to be predominately MLCT in nature. This assignment is consistent with the large changes seen in the metal-bonded C≡O features at early delay times before internal conversion to S1 and subsequent intersystem crossing to the ligand centered

NIBI-phen triplet state occurs.

In an effort to help with the assignments of the TRIR long lived spectroscopic features,

DFT calculations were performed to first optimize the structures of the complex at the S0 and T1 geometries. This was followed by simulating their associated TRIR spectra at long times by subtracting the calculated IR spectra at the S0 geometry from that at the T1 geometry (Figure

C30).

OD ∆

L = CH3CN 2200 2150 2100 2050 2000 1950 1850 1800 1750 1700

∆

L = PPh3 2200 2150 2100 2050 2000 1950 1850 1800 1750 1700

∆

L = DMAP 2200 2150 2100 2050 2000 1950 1850 1800 1750 1700 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.

Comparing the simulated spectra in Figure 4.5 with the general shape of the longer-lived spectral features determined experimentally, there are clear similarities, further suggesting the identity of these features as belonging to the lowest T1 excited state. DFT geometry optimization of 1A in the doublet state were also performed to simulate the IR spectra of the short time component in the lower energy window and the resulting difference spectrum presented in

Figure C31. The spectrum is also similar to the experimental results wherein a strong bleaching feature is visible at higher energy and a weaker one is nearly overlapped with the lower energy positive features, which also appear overlapped in the experimentally observed spectra.

4.3.1. Conclusions and Future Directions

Three Re(I) tricarbonyl complexes containing a highly conjugated chromophoric ligand,

NIBI-phen, and ancillary ligands were examined using ultrafast TRIR spectroscopy. The complicated dynamics were compared to a previously proposed excited state relaxation scheme.20 In summary, chromophores 1A and 1B exhibit similar excited state dynamics in which small shifts to higher energy in metal C≡O modes suggests some MLCT character in the S1 state despite the largely ILCT character that was assigned via TD-DFT calculations. A concurrent decrease in the energy of the NIBI C=O vibrations was also observed, consistent with previously reported ultrafast dynamics of Re(I) tricarbonyl molecules.2 Complex 1c exhibited similar absorbance features and excited state dynamics in the lower-energy C=O window, although a larger hypsochromic shift was observed in the high energy C≡O window at early times, suggesting proportionally higher MLCT character in this excited state configuration.

Nanosecond step-scan TRIR would be invaluable for properly assigning the long-lived triplet

excited state and and for comparisons to the electronic structure calculations and ultrafast results presented here.

4.4. Ultrafast Dynamics of Re(I) Diimine-containing Dicarbonyls

The Re(I) dicarbonyl complexes presented in Scheme 1.2 were examined via ultrafast time-resolved techniques and their more symmetric (approximate C2) structure and straightforward diimine ligand substitutions lead to easier-to-interpret IR-absorptions and dynamics. Without the complications of highly conjugated chromophoric ligands, as in the previous section covering Re(I) NIBI-phen tricarbonyls, the excited states are likely to be solely

MLCT in nature. MLCT excited states are easily identified via TRIR spectroscopy as an increase in C≡O stretching frequency is anticipated since backbonding will necessarily be reduced after oxidation of the metal center. The UFTA difference spectra of these dicarbonyls (Figure 4.6 and

4.7) overlap perfectly with the nsTA difference spectra presented in Figure C32, in which two major positive features surround a strong ground state bleach near 500 nm when measured in dichloromethane. We assign these positive features as belonging to the 3MLCT excited state. The lifetimes of the triplet MLCT excited states are well modeled using single-exponential kinetics as determined by visual inspection of the residuals where time-resolved photoluminescence and nsTA studies yielded lifetimes ranging from 130 to 510 ns for the phen-based ligand grouping

(1-5) and 20-60 ns for the bpy-based complexes (6-9). These lifetime differences are largely a result of the differences in ligand rigidity between the phen and bpy moieties (Figure C33 and

C34 and Table C2). Previous research on bpy and phen-containing osmium(II) complexes confirmed the effect of adding the chemical link between pyridyl ligands in knr. Although their energy gaps were comparable, the nonradiative decay in the bpy complex was about four times

as much as the analogous phen complex. This was explained via Franck-Condon analysis of their photoluminescence emission spectra, which revealed a smaller equilibrium displacement between ground and excited state in phen. This confirmed a marked increase in rigidity in the phenanthroline ligand compared to that of bpy.37,38

In the ultrafast time regime, a short-lived relaxation process precludes the long-lived decay of the 3MLCT excited state, with time constants ranging from 60 to 150 ps for complexes

1-5 and 10 to 20 ps for complexes 6-9.

-3 8 x10 -2 ps 1 2 -3 6 x10 0 ps 10 ps 6 50 ps 4 108 ps 506 ps 4 1.1 ns 2 2.5 ns 5.5 ns 2 A A ∆ ∆ 0 0 -2 ps 0 ps -2 -2 10.5 ps 109 ps 1.1 ns -4 -4 5.5 ns

-6 400 500 600 700 800 400 500 600 700 800 Wavelength (nm) Wavelength (nm) -2 ps 3 4 0 ps -2 ps -3 4 x10 10 ps -3 0 ps 55 ps 4 x10 500 ps 108 ps 3.1 ns 505 ps 5.5 ns 2 1.1 ns 2.5 ns 2 5.5 ns A 0 A ∆

∆ 0 -2

-2 -4 -4 400 500 600 700 800 400 500 600 700 800 Wavelength (nm) Wavelength (nm)

-3 15 x10 5 -2 ps 0 ps 10 ps 10 55 ps 108 ps 505 ps 1.1 ns A 5 5.5 ns ∆

-5 400 500 600 700 800 Wavelength (nm) 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

-3 6 7 8 x10 -3 -2 ps 6 x10 -2 ps 0 ps 0 ps 10 ps 10 ps 6 55 ps 108 ps 108 ps 4 1.1 ns 505 ps 2.5 ns 1.1 ns 5.5 ns 4 2.5 ns 5.5 ns A A 2 ∆ ∆

2 0 0 -2 -2

400 500 600 700 800 400 500 600 700 800 Wavelength (nm) Wavelength (nm) -3 4 x10 8 9 -3 6 x10 -2 ps -2 ps 0 ps 3 0 ps 10 ps 50 ps 108 ps 1.1 ns 4 500 ps 3.1 ns 1.1 ns 2 5.5 ns 2.5 ns 5.5 ns

A A 2 ∆ ∆ 1

0 0

-2 -1

400 500 600 700 800 400 500 600 700 800 Wavelength (nm) Wavelength (nm) 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.

Similar excited state dynamics were mirrored in the ultrafast TR-IR difference spectra of

1-9 measured in dichloromethane following excitation at 500 nm. The static FT-IR data reveal two metal-bound C≡O stretching modes, belonging to the symmetric and antisymmetric modes

(Table C3). These stretching modes range in energy from 1820 to 1900 cm-1, typical of the two

+ 16 previously known cis-[Re(N^N)2(CO)2] complexes. Upon selective MLCT excitation at 500 nm, the metal center is transiently oxidized, leading to a reduction in pi-backbonding and concomitant increase of the C≡O bond order. This translates as an increase in energy of these vibrational modes in the TR-IR difference spectra, as shown in Figure 4.8. The excited state features are long-lived, with little decay occurring over the 4 ns delay line of these experiments,

101

1 2

3 4 5

6 7

8 9

Figure 4.8 Ultrafast transient infrared difference spectra following 500 nm excitation of the [Re(N^N) 2(CO)2]PF6 complexes 1-6 in dichloromethane, where N^N is phen-based (top) or bpy- based (bottom).

102

consistent with the exclusive formation of 3MLCT excited states in all instances. To garner additional evidence for this MLCT assignment, DFT calculations were performed in a similar manner as described in previous sections of this dissertation in which the S0 and T1 state geometries were optimized and their calculated IR spectra subtracted to simulate the experimental TRIR difference spectra. The general spectral profiles of the DFT-calculated difference spectra agree very well with the experimental data in which both the symmetric (at higher energy) and antisymmetric (lower energy) stretching modes clearly shift to higher energy upon formation of the 3MLCT excited state

4.4.1. Conclusions and Future Directions

The examined dicarbonyl complexes 1-9 are highly symmetric and their excited state pathways are not complicated by the addition of the highly conjugated NIBI group on the diimine ligand as seen in the previous section. The TRIR spectra were similar across the entire series in which the C≡O symmetric and antisymmetric stretching modes were shifted to higher energy upon MLCT excitation after transient oxidation of the Re(I) center to Re(II), consistent with the DFT results. The triplet excited state survives beyond the end of the 4 ns delay line into tens to hundreds of nanoseconds, which is mirrored in the nsTA difference spectral kinetics.

Future work will involve further functionalization of the diimine ligands and possible addition of low-lying LC-based excited states through replacement of the phen and bpy ligands with highly conjugated chromophoric-bearing ligands, such as the NIBI-phen moiety discussed earlier in this

Chapter. It should be noted that the Castellano group maintains a substantial inventory of such pi-conjugated diimine ligands and these are readily available for exploitation in the Re(I) dicarbonyl structural motif.

103

4.5. References

(1) Kirgan, R. A.; Sullivan, B. P.; Rillema, D. P. Photochemistry and Photophysics of

Coordination Compounds: Rhenium BT - Photochemistry and Photophysics of

Coordination Compounds II; Balzani, V., Campagna, S., Eds.; Springer Berlin

Heidelberg: Berlin, Heidelberg, 2007; pp 45–100.

(2) Yarnell, J. E.; Deaton, J. C.; McCusker, C. E.; Castellano, F. N. Bidirectional “Ping-Pong”

Energy Transfer and 3000-Fold Lifetime Enhancement in a Re(I) Charge Transfer

Complex. Inorg. Chem. 2011, 50 (16), 7820–7830.

(3) Wrighton, M.; Morse, D. L. Nature of the Lowest Excited State in Tricarbonylchloro-

1,10-Phenanthrolinerhenium(I) and Related Complexes. J. Am. Chem. Soc. 1974, 96 (4),

998–1003.

(4) Van Wallendael, S.; Shaver, R. J.; Rillema, D. P.; Yoblinski, B. J.; Stathis, M.; Guarr, T.

F. Ground-State and Excited-State Properties of Monometallic and Bimetallic Complexes

Based on Rhenium(I) Tricarbonyl Chloride: Effect of an Insulating vs a Conducting

Bridge. Inorg. Chem. 1990, 29 (9), 1761–1767.

(5) Caspar, J. V; Meyer, T. J. Application of the Energy Gap Law to Nonradiative, Excited-

State Decay. J. Phys. Chem. 1983, 87 (6), 952–957.

(6) Wallace, L.; Rillema, D. P. Photophysical Properties of Rhenium(I) Tricarbonyl

Complexes Containing Alkyl- and Aryl-Substituted Phenanthrolines as Ligands. Inorg.

Chem. 1993, 32 (18), 3836–3843.

(7) Striplin, D. R.; Crosby, G. A. Nature of the Emitting 3MLCT Manifold of Rhenium(I)

104

(Diimine) (CO) 3Cl Complexes. Chem. Phys. Lett. 1994, 221 (5), 426–430.

(8) Blanco-Rodríguez, A. M.; Towrie, M.; Sýkora, J.; Záliš, S.; Vlček, A. Photoinduced

Intramolecular Tryptophan Oxidation and Excited-State Behavior of [Re(L-AA)(CO)3(α-

Diimine)]+ (L = Pyridine or Imidazole, AA = Tryptophan, Tyrosine, Phenylalanine).

Inorg. Chem. 2011, 50 (13), 6122–6134.

(9) Adams, B. S.; Shillito, G. E.; van der Salm, H.; Horvath, R.; Larsen, C. B.; Sun, X.-Z.;

Lucas, N. T.; George, M. W.; Gordon, K. C. Alteration of Intraligand Donor–Acceptor

Interactions Through Torsional Connectivity in Substituted Re-Dppz Complexes. Inorg.

Chem. 2017, 56 (21), 12967–12977.

(10) Vlček, A.; Kvapilová, H.; Towrie, M.; Záliš, S. Electron-Transfer Acceleration

Investigated by Time Resolved Infrared Spectroscopy. Acc. Chem. Res. 2015, 48 (3), 868–

876.

(11) Kuimova, M. K.; Grills, D. C.; Matousek, P.; Parker, A. W.; Sun, X.-Z.; Towrie, M.;

George, M. W. Picosecond Time-Resolved Infrared Investigation into the Nature of the

Lowest Excited State of Fac-[Re(Cl)(CO)3(CO2Et-Dppz)] (CO2Et-Dppz =

Dipyrido[3,2a:2′,3′c]Phenazine-11-Carboxylic Ethyl Ester). Vib. Spectrosc. 2004, 35 (1),

219–223.

(12) Baiano, J. A.; Murphy, W. R. Nonradiative Decay in Rhenium(I) Monometallic

Complexes of 2,3-Di-2-Pyridylpyrazine. Inorg. Chem. 1991, 30 (24), 4594–4598.

(13) Baiano, J. A.; Kessler, R. J.; Lumpkin, R. S.; Munley, M. J.; Murphy, W. R. Nonradiative

105

Decay in Rhenium(I) Monometallic Complexes of 2,3-Di(2-Pyridyl)Pyrazine and 2,3-

Di(2-Pyridyl)Quinoxaline. J. Phys. Chem. 1995, 99 (50), 17680–17690.

(14) Worl, L. A.; Duesing, R.; Chen, P.; Ciana, L. Della; Meyer, T. J. Photophysical Properties

of Polypyridyl Carbonyl Complexes of Rhenium(I). J. Chem. Soc. Dalt. Trans. 1991, No.

S, 849–858.

(15) Black, D. R.; Hightower, S. E. Preparation and Characterization of Rhenium(I)

Dicarbonyl Complexes Based on the Meridionally-Coordinated Terpyridine Ligand.

Inorg. Chem. Commun. 2012, 24, 16–19.

(16) Smithback, J. L.; Helms, J. B.; Schutte, E.; Woessner, S. M.; Sullivan, B. P. Preparative

Routes to Luminescent Mixed-Ligand Rhenium(I) Dicarbonyl Complexes. Inorg. Chem.

2006, 45 (5), 2163–2174.

(17) Koike, K.; Tanabe, J.; Toyama, S.; Tsubaki, H.; Sakamoto, K.; Westwell, J. R.; Johnson,

F. P. A.; Hori, H.; Saitoh, H.; Ishitani, O. New Synthetic Routes to

Biscarbonylbipyridinerhenium(I) Complexes Cis,Trans-[Re(X2bpy)(CO)2(PR3)(Y)]N+

(X2bpy = 4,4‘-X2-2,2‘-Bipyridine) via Photochemical Ligand Substitution Reactions, and

Their Photophysical and Electrochemical Properties. Inorg. Chem. 2000, 39 (13), 2777–

2783.

(18) Butler, J. M.; George, M. W.; Schoonover, J. R.; Dattelbaum, D. M.; Meyer, T. J.

Application of Transient Infrared and near Infrared Spectroscopy to Transition Metal

Complex Excited States and Intermediates. Coord. Chem. Rev. 2007, 251 (3), 492–514.

(19) Scattergood, P. A.; Delor, M.; Sazanovich, I. V; Bouganov, O. V; Tikhomirov, S. A.;

106

Stasheuski, A. S.; Parker, A. W.; Greetham, G. M.; Towrie, M.; Davies, E. S.; et al.

Electron Transfer Dynamics and Excited State Branching in a Charge-Transfer

Platinum(Ii) Donor–Bridge-Acceptor Assembly. Dalt. Trans. 2014, 43 (47), 17677–

17693.

(20) Joyce, R. Excited-State Manipulations in Mo(II) Dimers and Re(I) MLCT Complexes,

North Carolina State University, 2018.

(21) Garakyaraghi, S.; Danilov, E. O.; McCusker, C. E.; Castellano, F. N. Transient

Absorption Dynamics of Sterically Congested Cu(I) MLCT Excited States. J. Phys. Chem.

A 2015, 119 (13), 3181–3193.

(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J.

R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; et al. Gaussian 09, Revision

D.01. 2016.

(23) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods without Adjustable

Parameters: The PBE0 Model. J. Chem. Phys. 1999, 110 (13), 6158–6170.

(24) Weigend, F. Accurate Coulomb-Fitting Basis Sets for H to Rn. Phys. Chem. Chem. Phys.

2006, 8 (9), 1057–1065.

(25) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and

Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys.

Chem. Chem. Phys. 2005, 7 (18), 3297–3305.

(26) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Energy-Adjustedab Initio

107

Pseudopotentials for the Second and Third Row Transition Elements. Theor. Chim. Acta

1990, 77, 123–141.

(27) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio

Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94

Elements H-Pu. J. Chem. Phys. 2010, 132 (15), 154104.

(28) Becke, A. D. Density‐functional Thermochemistry. III. The Role of Exact Exchange. J.

Chem. Phys. 1993, 98 (7), 5648–5652.

(29) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations.

Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82 (1), 270–283.

(30) Hariharan, P. C.; Pople, J. A. The Influence of Polarization Functions on Molecular

Orbital Hydrogenation Energies. Theor. Chim. Acta 1973, 28 (3), 213–222.

(31) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self—Consistent Molecular Orbital Methods.

XII. Further Extensions of Gaussian—Type Basis Sets for Use in Molecular Orbital

Studies of Organic Molecules. J. Chem. Phys. 1972, 56 (5), 2257–2261.

(32) Wadt, W. R.; Hay, P. J. Ab Initio Effective Core Potentials for Molecular Calculations.

Potentials for Main Group Elements Na to Bi. J. Chem. Phys. 1985, 82 (1), 284–298.

(33) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations.

Potentials for K to Au Including the Outermost Core Orbitals. J. Chem. Phys. 1985, 82

(1), 299–310.

(34) Becke, A. D. A New Mixing of Hartree–Fock and Local Density‐functional Theories. J.

108

Chem. Phys. 1993, 98 (2), 1372–1377.

(35) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. New Developments in the Polarizable

Continuum Model for Quantum Mechanical and Classical Calculations on Molecules in

Solution. J. Chem. Phys. 2002, 117 (1), 43–54.

(36) Schoonover, J. R.; Strouse, G. F.; Dyer, R. B.; Bates, W. D.; Chen, P.; Meyer, T. J.

Application of Time-Resolved, Step-Scan Fourier Transform Infrared Spectroscopy to

Excited-State Electronic Structure in Polypyridyl Complexes of Rhenium(I). Inorg. Chem.

1996, 35 (2), 273–274.

(37) Arias, M.; Concepción, J.; Crivelli, I.; Delgadillo, A.; Díaz, R.; Francois, A.; Gajardo, F.;

López, R.; Leiva, A. M.; Loeb, B. Influence of Ligand Structure and Molecular Geometry

on the Properties of D6 Polypyridinic Transition Metal Complexes. Chem. Phys. 2006,

326 (1), 54–70.

(38) Treadway, J. A.; Loeb, B.; Lopez, R.; Anderson, P. A.; Keene, F. R.; Meyer, T. J. Effect

of Delocalization and Rigidity in the Acceptor Ligand on MLCT Excited-State Decay.

Inorg. Chem. 1996, 35 (8), 2242–2246.

(39) Langdon-Jones, E. E.; Williams, C. F.; Hayes, A. J.; Lloyd, D.; Coles, S. J.; Horton, P. N.;

Groves, L. M.; Pope, S. J. A. Luminescent 1,8-Naphthalimide-Derived ReI Complexes:

Syntheses, Spectroscopy, X-Ray Structure and Preliminary Bioimaging in Fission Yeast

Cells. Eur. J. Inorg. Chem. 2017, 2017 (44), 5279–5287.

(40) Kurtz, D. A.; Dhakal, B.; Hulme, R. J.; Nichol, G. S.; Felton, G. A. N. Correlations

109

between Photophysical and Electrochemical Properties for a Series of New Mn Carbonyl

Complexes Containing Substituted Phenanthroline Ligands. Inorganica Chim. Acta 2015,

427, 22–26.

110

APPENDICES

111

Appendix A

bpy phen 20 dtbb dcb )

-1 15 cm -1 M (

3 10 x10 ε 5

0 300 400 500 600 Wavelength (nm) Figure A1. Absorption spectrum of [Ir(Cp*)(N^N)Cl]Cl measured in methanol.

1 Figure A2. 400 MHz H NMR spectrum for [Ir(Cp*)(bpy)H]PF6 in acetone-d6.

112

1 Figure A3. 400 MHz H NMR spectrum for [Ir(Cp*)(phen)H]PF6 in acetone-d6.

1 Figure A4. 400 MHz H NMR spectrum for [Ir(Cp*)(dtbb)H]PF6 in acetone-d6.

113

Figure A5. IR spectrum of [Ir(Cp*)(bpy)H]PF6.

Figure A6. IR spectrum of [Ir(Cp*)(phen)H]PF6.

Figure A7. IR spectrum of [Ir(Cp*)(dtbb)H]PF6.

114

Figure A8. HRESIMS of [Ir(Cp*)(bpy)H]PF6 in methanol.

Figure A9. HRESIMS of [Ir(Cp*)(dtbb)H]PF6 in methanol.

115

Figure A10. 1H NMR spectrum of [RhCp*(bpy)Cl]Cl in CDCl3.

1.0

bpy phen 0.8 dtbb

0.6

0.4 Normalized Absorbance

0.2

0.0 300 400 500 600 700 Wavelength (nm) Figure A11. Absorbance of [Ir(Cp*)(N^N)H]PF6 in methanol.

116

0 -3 x10 -2

-3 10 x10

0 200 400 600 800 1000 1200 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.

80 4

2 -3 Time 0 0

10 ns -2 x10 20 ns -4 60 30 ns 50 ns 70 ns 100 ns 30 -3

Ax10

-3 20 ∆ Ax10

20 ∆ 10

0 0

350 400 450 500 550 600 650 -200 0 200 400 600 800 Wavelength (nm) Time (ns) Figure A13. nsTA of [Ir(Cp*)(dtbb)H]PF6 in acetonitrile after 430 nm excitation (left). Kinetic trace and singlet exponential fit at 500 nm; τ = 52.0 ± 0.4 ns (right).

117

-3

10 Ax10 ∆

-10 400 500 600 700 800 Wavelength (nm) Figure A14. nsTA spectrum of [Ir(Cp*)(phen)H]PF6 at time 0 in acetonitrile after excitation at 430 nm. Complex determined to be unstable under these conditions.

430 nm 702 nm 420 nm 694 nm

a b

420 nm 687 nm

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.

1.8 9 Time 0 1.6 Time 0 8 150 µs 150 us 7 1.4 600 µs 600 us 2 ms 1.2 1 ms 6 4 ms

1.0 -2 -1 4 0.8 Ax10 Ax10 ∆ ∆ 3 0.6 2 0.4 1 0.2 0 0.0 400 450 500 550 600 650 700 750 800 400 500 600 700 Wavelength (nm) Wavelength (nm)

Figure A17. nsTA difference spectra of [Ir(Cp*)(dtbb)H]PF (left) and 6 [Ir(Cp*)(phen)H]PF (right) in deaerated methanol (λ = 430 nm). 6 ex

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

Table A1. Summary of lifetimes measured via nsTA spectroscopy in methanol and acetonitrile. *indicates complex not stable in acetonitrile

Methanol Acetonitrile Hydride l (nm) t1 (ms) t2 (ms) l (nm) t1 (ns)

[Ir(Cp*)(bpy)H]PF6 480 3.5 ± 0.2 14.8 ± 0.3 500 82.7 ± 0.7

[Ir(Cp*)(dtbb)H]PF6 475 0.9 ± 0.05 2.9 ± 0.02 500 52.0 ± 0.4

[Ir(Cp*)(phen)H]PF6 600 4.1 ± 0.1 12.9 ± 0.4 --* --*

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)

λ (nm) t1 t2 t3* 350 11.5 ± 0.4 ps (rise) 1.0 ± 0.1 ns (decay) >6.3 ns (decay) 440 27.5 ± 2.5 ps (rise) 747.4 ± 17.5 ns (rise) >6.3 ns (decay) 500 11.7 ± 0.8 ps (rise) 271.1 ± 21.4 ns (decay) > 6.3 ns (rise) 580 1.14 ± 0.01 ns -- --

121

Figure A19. Kinetic traces of [Ir(Cp*)(bpy)H]PF6 at select wavelengths after 400 nm excitation in deaerated methanol.

122

a b -3 1.2 x10

-3 1.0 x10 1.0

0.8

A A 0.6 0.5 ∆ ∆ 0.4

0.2 0.0 0.0

400 450 500 550 600 650 400 450 500 550 600 Wavelength(nm) Wavelength(nm)

-3 2.0 x10 c d

-3 1.5 1.5 x10

1.0 1.0 A

A ∆ ∆ 0.5 0.5

0.0 0.0

400 450 500 550 600 650 400 450 500 550 600 650 Wavelength(nm) Wavelength(nm) Figure A20. UFTA spectra at time 0 of Ir(III) chlorides (a) bpy (b) phen (c) dtbb (d) dcb in methanol after 400 nm excitation.

123

a b

c d

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.

Table A3 Measured UFTA lifetimes of Ir(III) chlorides in methanol and water. Complex Solvent Growth t (ps) Decay t (ns)

Water 41.03 ± 6.15 0.788 ± 0.007 [IrCp*bpyCl]Cl Methanol 38.84 ± 5.64 0.792 ± 0.007 Water 50.71 ± 2.81 0.924 ± 0.033 [IrCp*phenCl]Cl Methanol 55.49 ± 7.22 0.887 ± 0.019 Water 10.31 ± 4.05 1.18 ± 0.016 [IrCp*dtbbCl]Cl Methanol 64.24 ± 20.04 1.02 ± 0.015 Water 12.18 ± 4.07 0.610 ± 0.008 [IrCp*dcbCl]Cl Methanol 14.24 ± 1.56 0.723 ± 0.004

124

4 50 min 45 min 40 min 35 min 30 min 20 min 3 15 min 10 min 9 min 8 min 7 min 6 min 5 min 4.5 min 4 min 2 3.5 min 3 min 2.5 min

Absorbance 2 min 1.5 min 45 sec 30 sec 10 sec 1 0 sec Before addition of formate

400 500 600 700 800 Wavelength (nm)

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.

Water Acetonitrile Methanol

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

Table A4 Summary of 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) Energy of Wavefunction Complex Solvent Transition Character Coefficient (nm / eV) Water 564 / 2.20 Homo → Lumo 0.700 (98.0%) [Rh(Cp*H)bpy]+ Acetonitrile 562 / 2.21 Homo → Lumo 0.670 (89.8%) Methanol 564 / 2.20 Homo → Lumo 0.700 (98.0%) Water 412 / 3.01 Homo → Lumo 0.693 (96.0%) [RhCp*bpyH]+ Acetonitrile 413 / 3.00 Homo → Lumo 0.670 (89.8%) Methanol 413 / 3.00 Homo → Lumo 0.672 (90.3%)

HOMO LUMO

Rh(III)-H

Rh(I) (Cp*H)

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

Absorbance

0 500 1000 1500 2000 2500 3000 -1 Wavenumber (cm ) Figure A25. Simulated IR spectrum of [RhCp*(6,6'-Me-bpy)H]+ in acetonitrile.

Absorbance

0 500 1000 1500 2000 2500 3000 3500 -1 Wavenumber (cm ) 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.

3 14 x10

12 10

8 6

4 EmissionIntensity 2

0 550 600 650 700 750 800 Wavelength (nm) Figure A28. Steady-state emission of [Ru(C6Me6)(bpy)H]PF6 in acetonitrile. Negative artifact at 550 nm is the result of solvent subtraction. 128

Table A5 Summary of UFTA single wavelength kinetic fits of [Ru(C6Me6)(bpy)H]PF6 in different solvent environments with 500 nm excitation.

Wavelength Solvent τ (ps) τ (ps) (nm) 1 2 360 2.5 ± 1.6 ps 22.8 ± 2.4 ps 390 1.0 ± 0.1 ps 16.8 ± 2.5 ps Acetonitrile 470 1.5 ± 0.7 ps 16.2 ± 1.7 ps 540 0.7 ± 0.3 ps 22.2 ± 1.8 ps 645 0.34 ± 0.1 ps 15.9 ± 4.4 ps 360 0.97 ± 0.03 ps 13.6 ± 0.5 ps 390 0.79 ± 0.03 ps 11.2 ± 0.8 ps Methanol 470 11.1 ± 0.2 ps -- 545 0.8 ± 0.1 ps 12.6 ± 0.2 ps 640 0.54 ± 0.09 ps 12.2 ± 1.0 ps 360 1.4 ± 0.1 ps 21.0 ± 2.3 ps 390 0.93 ± 0.05 ps 15.0 ± 1.6 ps Tetrahydrofuran 475 1.3 ± 0.4 ps 11.5 ± 0.5 ps 545 18.7 ± 0.8 ps -- 640 0.43 ± 0.04 ps 13.3 ± 4 ps

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

-3 -2 ps 4 x10 -1 ps -3 0 ps 3 x10 0 ps 0.5 ps 0.2 ps 1 ps 0.5 ps 3 1.4 ps 2 ps 2.6 ps 2 5.4 ps 5.5 ps 10 ps 10.1 ps 24 ps 27 ps 55 ps 2 54 ps 1 108 ps 108 ps 235 ps 235 ps A A ∆ ∆ 1 0

0 -1

-1 -2 a b -2

350 400 450 500 550 600 650 700 350 400 450 500 550 600 650 700 Wavelength (nm) Wavelength (nm)

A -2 ∆

-1 ps 0 ps 0.5 ps -4 1.5 ps 5.3 ps 10.6 ps 24.8 ps 109 ps 6.1 ps -3 c -6 x10 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure A30. UFTA difference spectra of [Ru(C6Me6)(bpy)H]Cl in (a) acetonitrile

(b) methanol (c) tetrahydrofuran after 500 nm excitation.

130

Table A6. Summary of UFTA single wavelength kinetic fits of [Ru(C6Me6)(bpy)H]Cl in different solvent environments with 500 nm excitation. (Preliminary data)

Wavelength Solvent τ1 (ps) τ2 (ps) (nm) 355 1.1 ± 0.4 ps 22.6 ± 1.0 ps 390 0.93 ± 0.06 ps 16.8 ± 1.4 ps Acetonitrile 470 1.8 ± 0.5 ps 13.7 ± 1.0 ps 550 0.6 ± 0.1 ps 17.4 ± 0.6 ps 635 0.8 ± 0.3 ps 20.2 ± 2.8 ps 360 1.0 ± 0.2 ps 13.4 ± 0.9 ps

390 0.77 ± 0.06 ps 10.2 ± 1.0 ps

Methanol 460 11.5 ± 0.5 ps --

550 1.1 ± 0.1 ps 11.7 ± 0.6 ps

640 0.6 ± 0.1 ps 8.3 ± 2.2 ps 390 11.1 ± 0.9 ps -- Tetrahydrofuran 460 1.5 ± 0.2 ps 12.5 ± 1.0 ps 540 0.95 ± 0.02 ps 10.2 ± 1.6 ps

100 100 50 100

-6 -6 0 0 -6 0 -50 x10 x10 -100 -100 x10 -100 -150

0.0 Coefficient values ± one standard deviation 2.0 y0 =-9.5346e-06 ± 1.78e-05 2.5 A1 =0.00031712 ± 5.75e-05 Coefficient values ± one standard deviation tau1 =1.1169e-12 ± 4.48e-13 y0 =0.0001162 ± 1.62e-05 -0.5 A1 =0.0010208 ± 7.64e-05 A2 =0.0020657 ± 4.13e-05 2.0 Coefficient values ± one standard deviation tau2 =2.2563e-11 ± 9.83e-13 tau1 =1.0414e-12 ± 1.63e-13

1.5 y0 =-0.00017572 ± 1.33e-05 Constant: A2 =0.0016808 ± 6.56e-05 -3 -3 -3 -1.0 A1 =-0.00096452 ± 8.79e-05 X0 =1.77e-13 tau2 =1.3446e-11 ± 8.78e-13 1.5 Constant: tau1 =1.4579e-12 ± 2.35e-13 X0 =1.74e-13 A2 =-0.001485 ± 8.76e-05 tau2 =1.2476e-11 ± 9.8e-13 1.0 Ax10 Ax10 Ax10 -1.5

∆ Constant: ∆ ∆ 1.0 X0 =2.83e-15 0.5 -2.0 a 0.5 b c -2.5 0.0 0.0

0 50 100 150 200 0 50 100 150 200 0 100 200 300 400 Time (ps) Time (ps) Time (ps)

Figure A31. Select kinetic traces, biexponential fits (black), and residuals (top) from UFTA spectra of [Ru(C6Me6)(bpy)H]Cl measured in (a) acetonitrile (360 nm) (b) methanol (360 nm) and (c) tetrahydrofuran (460 nm).

131

-3

Ax10 2 ∆

300 400 500 600 700 800 Wavelength (nm)

Figure A32. nsTA spectrum of [Ru(C Me )(bpy)H]Cl in tetrahydrofuran after 6 6 excitation at 500 nm.

+ Figure A33. HOMO and LUMO of [Ru(C6Me6)(bpy)H] at the B3LYP/ECP28MWB/6-31G** (PCM = acetonitrile) level of theory using Gaussian09.

132

S0 T1 Absorbance

2200 2100 2000 1900 -1 Wavenumber (cm )

Figure A34. Simulated IR spectra of [Ru(C6Me6)(bpy)H]PF6 ground state and lowest triplet state at the B3LYP/ECP28MWB/6-31G** (PCM = acetonitrile) level of theory.

133

Appendix B

Figure B1. 1H NMR spectrum of cis-[Ir(bpy) Cl ]PF in DMSO-d (400 MHz). 2 2 6 6

134

1 Figure B2. H NMR spectrum of [Ir(bpy)2(CF3SO3)]CF3SO3 in DMSO-d6 (400 MHz).

135

Figure B3. 1H NMR spectrum of cis-[Ir(bpy) H ]PF in DMSO-d . Inset shows the highly 2 2 6 6 shielded hydride resonance at -17.92 ppm. The spectrum of cis-[Ir(bpy) D ]PF was exactly the 2 2 6 same, except for the undetected hydride peak resulting from isotopic substitution. (400 MHz)

136

13 Figure B4. C NMR spectrum of cis-[Ir(bpy)2D2]PF6 in DMSO-d6. (100 MHz)

Figure B5. The 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] .

1.0

0.8

0.6

0.4

Normalized Absorbance 0.2

2500 2000 1500 1000 500 -1 Frequency (cm )

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

2.0 -2 ps -2 ps 0 ps 0 ps 5 ps 10 ps 1.5 5 ps 15 ps 10 ps 1.5 19.5 ps 15 ps 30 ps 65 ps 20 ps 100 ps 30 ps 400 ps

55 ps 1.0

-3 75 ps -3 1.0 215 ps

Ax10 355 ps Ax10 ∆ ∆ 0.5 0.5

0.0 0.0

350 400 450 500 550 600 650 900 1000 1100 1200 Wavelength (nm) Wavelength (nm) Figure B8. Ultrafast TA difference spectra of 2 in acetonitrile (λex = 480 nm). NIR difference spectra are also shown on right. Laser scatter at excitation wavelength removed for clarity.

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. Lifetimes of 1 and 2 were determined to be 24.4 ± 0.3 and 24.8 ± 0.5 ps, respectively.

139

30 0 ns 15 250 ns 1 μs 20 10

-3 -3 10

Ax10 Ax10 ∆ ∆ 5

350 400 450 500 550 600 0 1000 2000 Wavelength (nm) Time (ns) + Figure B10. Nanosecond TA of [Irbpy2Cl2] in acetonitrile (left) after excitation at 420 nm and biexponential fit of feature at 470 nm (right).

Lifetime found to be 14.9 ± 3.5 ns, 331 ± 3.2 ns.

1.2

1.0

0.8

0.6

0.4 Norm. Absorbance Norm. 0.2

0.0 200 300 400 500 600 700 800 Wavelength (nm)

Figure B11. UV-vis spectra of 1 and [Ir(bpy)2Cl2]PF6.

140

5 4 x10 Time 0 7 25 ns 6 x10 50 ns 3 150 ns 5 250 ns 350 ns 4 500 ns 2 750 ns 3 1 µs 2 µs 2 1

Emission Intensity K @ 293 1 Emission Intensity @ K 77

0 0

400 500 600 700 800 500 600 700 Wavelength (nm) Wavelength (nm)

Figure B12. Room temperature time-resolved photoluminescence in acetonitrile (left) and 77 K steady-state emission in 4:1 ethanol: methanol (right) spectra of [Ir(bpy)2Cl2]PF6.

4 8 x10 0 ns 0 ns 4 25 ns 25 ns 4 x10 100 ns 100 ns 200 ns 200 ns 300 ns 300 ns 500 ns 6 400 ns 1 μs 500 ns 3 μs 750 ns 3 6 μs 1 μs 10 μs 2 μs 20 μs 3 μs 40 μs 6 μs 80 μs 4 10 μs 20 μs 2 40 μs 80 μs

1 Emission77Intensity@ K

Emission77Intensity@ K 0 0 400 500 600 700 800 400 500 600 700 800 Wavelength (nm) Wavelength (nm)

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 in acetonitrile-d3 following 480 nm excitation.

1.0

0.5

0.0 mOD ∆ -0.5

-1.0

2150 2100 2050 2000 1950 1900 -1 Wavenumber (cm ) 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

1.0 1.0

0.8 0.5 0.6 mOD mOD 0.0 ∆ ∆ 0.4

0.2 -0.5 0.0

0 100 200 300 400 0 50 100 150 200 Wavelength (nm) Time (ps) Time (ps)

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.

6 6 4 4 2 2 mOD

mOD ∆ ∆ 0 0

-2 -2

-4 -4 0 100 200 300 400 0 100 200 300 400 Time (ps) Time (ps) 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. Lifetimes determined to be 28.9 ± 1.7 and 28.2 ± 2.

143

Absorbance

3000 2000 1000 -1 Wavenumber (cm )

Figure B18. Calculated IR spectra of the ground states of 1 (red) and 2 (blue) and their respective 3MLCT excited states (black).

OD ∆

2400 2200 2000 1800 1600 1400 -1 Wavenumber (cm )

Figure B19. Calculated IR difference spectra of 1 (red) and 2 (blue) using the simulated spectra from Figure B18.

144

Appendix C

+ Synthesis of Dicarbonyl Complexes, [Re(N^N)(CO)2]

Synthesis was completed by Ms. Hala Atallah

Complexes fac-Re(N-N)CO3Cl , fac-[Re(N-N)(CO)3(NCMe)]BF4 and fac-Re(CO)3(N-

20,39,40 N)(CF3SO3) were prepared as described in literature.

cis-[Re(CO2)(3,4,7,8-Me4phen)2](CF3SO3) (1). By a modified procedure described by

16 Smithback. fac-Re(3,4,7,8-Me4Phen)CO3(CF3SO3) (100 mg, 0.15 mmol) and 2.2 eq of 3,4,7,8- tetramethyl-1,10-phenanthroline (3,4,7,8-Me4phen) were placed in a test tube with a stir bar. The tube was capped with a septum and purged with N2 for 30 mins. The mixture was stirred and heated at 275oC for 15 mins. The mixture was cooled then MeOH was added, resulting in a red solid precipitate. The precipitate was filtered and purification was achieved by column chromatography on acidic alumina with CH2Cl2/CH3CN mixture. The product was reprecipitated through dissolving in methylene chloride and adding dropwise to stirring diethyl ether. Yield: 67

1 mg (48%). H NMR (400 MHz, Methylene Chloride-d2) δ 9.66 (s, 2H), 8.32 (d, J = 9.5 Hz, 2H),

8.21 (d, J = 9.4 Hz, 2H), 7.23 (s, 2H), 2.93 (s, 6H), 2.73 (s, 6H), 2.66 (s, 6H), 2.13 (s, 6H). 13C

NMR (101 MHz, Methylene Chloride-d2) δ 201.7 (x2), 156.3 (x2), 148.5 (x2), 146.7 (x2), 146.3

(x2), 146.0 (x2), 144.9 (x2), 135.6 (x2), 134.3 (x2), 130.1 (x2), 129.6 (x2), 124.5 (x2), 124.1

(x2), 18.1 (x2), 18.0 (x2), 15.3 (x2), 15.2 (x2). ESI-HRMS. Found: m/z 713.2069. Calcd for

185 C34H32N4O2 Re: m/z 713.2055.

145

cis-[Re(CO2)(4,7-Me2phen)2]PF6 (2). Synthesis was the same as for 1 except 100 mg (0.16 mmol) of fac-Re(4,7-Me2phen)CO3(CF3SO3) and 2.2 eq of 4,7-dimethyl-1,10-phenanthroline

(4,7-Me2phen) were used. Purification was done by dissolving the mixture in MeOH and the product was precipitated by the addition of aqueous solution of ammonium hexaflurorophosphate. The mixture was left in the fridge for an hour then filtered and washed with H2O. Column chromatography was performed on acidica alumina with CH2Cl2/CH3CN mixture. The product was then dissolved in methylene chloride and precipitated in stirring

1 diethyl ether. Yield: 65 mg (51%). H NMR (400 MHz, Acetonitrile-d3) δ 9.74 (d, J = 5.4 Hz, 2

Hz), 8.36 (d, J = 9.3 Hz, 2H), 8.24 (d, J = 9.3 Hz, 2H), 7.85 (d, J = 5.4 Hz, 2H), 7.41 (d, J = 5.3

Hz, 2H), 7.27 (d, J = 5.3 Hz, 2H), 3.02 (s, 6H), 2.73 (s, 6H). 13C NMR (101 MHz, Acetonitrile- d3) δ 202.4 (x2), 156.4 (x2), 150.1 (x2), 148.8 (x2), 148.4 (x2), 147.7 (x2), 147.3 (x2), 131.9

(x2), 131.0 (x2), 128.3 (x2), 126.8 (x2), 125.4 (x2), 124.9(x2), 19.1 (x2), 19.0 (x2). ESI-HRMS.

185 Found: m/z 657.1432. Calcd for C30H24N4O2 Re: m/z 657.1429.

cis-[Re(CO2)(5,6-Me2phen)2]PF6 (3). Synthesis and purification were the same as for 2 except that 100 mg (0.16 mmol) of fac-Re(5,6-Me2phen)CO3(CF3SO3) and 2.2 eq of 5,6-dimethyl-1,10-

1 phenanthroline (5,6-Me2phen) were used. Yield: 33 mg (24%). H NMR (400 MHz, Acetonitrile- d3) δ 9.87 (d, J = 5.2 Hz, 2H), 8.89 (d, J = 8.8 Hz, 2H), 8.61 (d, J = 8.3 Hz, 2H), 8.01 (dd, J = 8.4,

13 5.3 Hz, 2H), 7.45 (m, 2H), 2.84 (s, 6H), 2.70 (s, 6H). C NMR (400 MHz, Acetonitrile-d3) δ

201.5 (x2), 155.7 (x2), 148.1 (x2), 147.1 (x2), 146.7 (x2), 136.3 (x2), 134.8 (x2), 133.5 (x2),

133.0 (x2), 132.9 (x2), 132.0 (x2), 127.4 (x2), 126.0 (x2), 15.7 (x2), 15.5 (x2). ESI-HRMS.

185 Found: m/z 657.1439. Calcd for C30H24N4O2 Re: m/z 657.1429.

146

7 cis-[Re(CO2)(phen)2](CF3SO3) (4). Synthesis and purification were the same as for 1 except that 100 mg (0.17 mmol) of fac-Re(phen)CO3(CF3SO3) and 2.2 eq of phen were used. Yield: 51

1 mg (41%). H NMR (400 MHz, Acetonitrile-d3) δ 9.93 (dd, J = 5.3, 1.3 Hz, 2H), 8.77 (dd, J =

8.2, 1.3 Hz, 2H), 8.50 (dd, J = 8.2, 1.4 Hz, 2H), 8.23 (d, J = 8.9 Hz, 2H), 8.11 (d, J = 8.9 Hz, 2H),

8.03 (dd, J = 8.2, 5.3 Hz, 2H), 7.58 (dd, J = 5.2, 1.4 Hz, 2H), 7.46 (dd, J = 8.2, 5.1 Hz, 2H).

7 cis-[Re(CO2)(4,7-Ph2phen)2]PF6 (5). Synthesis and purification were the same as for 2 except

100 mg (0.14 mmol) of fac-[Re(4,7-Me2phen)(CO)3(MeCN)]BF4 and 2.2 eq of 4,7-diphenyl-

1 1,10-phenanthroline (4,7-Ph2phen) were used. Yield: 19 mg (13.2%). H NMR (400 MHz,

Acetonitrile-d3) δ 10.02 (d, J = 5.6 Hz, 2H), 8.19 (d, J = 9.5 Hz, 2H), 8.06 (d, J = 9.4 Hz, 2H),

8.00 (d, J = 5.5 Hz, 2H), 7.82 (d, J = 5.4 Hz, 2H), 7.79 – 7.75 (m, 4H), 7.74 – 7.64 (m, 6H), 7.56

(t, J = 3.3 Hz, 6H), 7.50 – 7.43 (m, 6H).

cis-[Re(CO2)(5,5’-Me2bpy)2]PF6 (6). Synthesis and purification were the same as for 2 except

100 mg (0.17 mmol) of fac-[Re(5,5’-Me2bpy)(CO)3(MeCN)](BF4) and 2.2 eq of 5,5’-dimethyl-

2,2’-bipyridine (5,5’-Me2bpy) were used and the reaction time was increased to 4h. Yield: 84 mg

1 (64.7%). H NMR (400 MHz, Acetonitrile-d3) δ 9.27 (s, 2H), 8.51 (dd, J = 11.7, 8.6 Hz, 4H), 7.92

(dd, J = 8.3, 1.3 Hz, 2H), 7.83 (dd, J = 8.4, 1.3 Hz, 2H), 7.06 (s, 2H), 2.55 (s, 6H), 2.17 (s, 6H).

13 C NMR (101 MHz, Methylene Chloride-d2) δ 200.9 (x2), 155.5 (x2), 154.0 (x2), 153.9 (x2),

147.3 (x2), 140.4 (x2), 138.6 (x4), 137.9 (x2), 124.5 (x2), 124.0 (x2), 18.7 (x2), 18.6 (x2). ESI-

185 HRMS. Found: m/z 609.1438. Calcd for C30H24N4O2 Re: m/z 609.1429.

147

cis-[Re(CO2)(4,4’-Dtbbpy)2]PF6 (7). Synthesis and purification were the same as for 2 except

100 mg (0.15 mmol) of fac-[Re(4,4’-Dtbbpy)(CO)3(MeCN)](BF4) and 2.2 eq of 4 4'-di-tert- butyl-2 2'-bipyridine (4,4’-Dtbbpy) were used and the reaction time was increased to 3h.The reprecipitation was done in hexanes instead of diethyl ether. Yield: 60 mg (43.3%). 1H NMR (400

MHz, Acetonitrile-d3) δ 9.29 (d, J = 6.0 Hz, 2H), 8.44 (s, 2H), 8.38 (s, 2H), 7.63 (d, J = 6.0 Hz,

13 2H), 7.35 (d, J = 6.0 Hz, 2H), 1.50 (s, 18H), 1.33 (s, 18H). C NMR (101 MHz, Acetonitrile-d3)

δ 202.1 (x2), 165.2 (x2), 163.4 (x2), 156.9 (x2), 156.6 (x2), 156.2 (x2), 148.2 (x2), 126.3 (x2),

125.2 (x2), 122.9 (x2), 118.3 (x2). ESI-HRMS. Found: m/z 777.3300. Calcd for

185 C38H48N4O2 Re: m/z 777.3307.

16 cis-[Re(CO2)(bpy)2]PF6 (8). Synthesis was the same as for 1 except 100 mg (0.18 mmol) of fac-[Re(bpy)(CO)3(MeCN)](BF4) and 2.2 eq of bpy were used and the reaction time was increased to 4h at 230oC. The mixture was cooled and MeOH was added and black precipitate appeared and was filtered out. Aqueous solution of ammonium hexafluorophosphate was added to the filtrate and red precipitate appeared. The mixture was left in the fridge overnight and the resulting precipitate was filtered out, dissolved in DCM and precipitated in diethyl ether (x2).

1 Yield: 20 mg (17%). H NMR (400 MHz, Acetonitrile-d3) δ 9.45 (d, J = 5.8 Hz, 2H), 8.46 (d, J =

8.3 Hz, 2H), 8.38 (d, J = 8.2 Hz, 2H), 8.14 (t, J = 8.0 Hz, 2H), 8.02 (t, J = 7.5 Hz, 2H), 7.65 –

7.58 (m, 2H), 7.41 (d, J = 5.6 Hz, 2H), 7.36 – 7.30 (m, 2H).

cis-[Re(CO2)(4,4’-Me2bpy)2]PF6 (9). Synthesis and purification were the same as for 2 except

100 mg (0.17 mmol) of fac-[Re(4,4’-Me2bpy)(CO)3(MeCN)](BF4) and 2.2 eq of 4,4’-dimethyl-

2,2’-bipyridine (4,4’-Me2bpy) were used and the reaction time was increased to 4h. Yield: 32 mg

148

1 (24.5%). H NMR (400 MHz, Acetonitrile-d3) δ 9.23 (d, J = 5.9 Hz, 2H), 8.31 (s, 2H), 8.24 (s,

2H), 7.44 (dd, J = 5.8, 1.1 Hz, 2H), 7.22 (d, J = 5.7 Hz, 2H), 7.15 (d, J = 5.8 Hz, 2H), 2.61 (s,

13 6H), 2.42 (s, 6H). C NMR (101 MHz, Acetonitrile-d3) δ 202.0 (x2), 156.5 (x2), 156.3 (x2),

156.0 (x2), 153.0 (x2), 151.1 (x2), 147.9 (x2), 129.8 (x2), 128.8 (x2), 126.2 (x2), 125.3 (x2),

185 21.3 (x4). ESI-HRMS. Found: m/z 609.1443. Calcd for C30H24N4O2 Re: m/z 609.1429.

Scheme C1. Generalized Synthetic Scheme for Rhenium(I) dicarbonyl complexes (1-9).

The intermediate complexes fac-Re(N-N)CO3Cl were synthesized using a procedure reported in the literature.40 This involves refluxing rhenium(I) pentacarbonyl chloride in a 1:1 eq. with the diimine ligand in toluene for two hours. The isolated product was then used to synthesized either the fac-[Re(N-N)(CO)3(NCMe)]BF4 or fac-Re(CO)3(N-N)(CF3SO3). The synthesis of the fac-

[Re(N-N)(CO)3(NCMe)]BF4 and fac-Re(CO)3(N-N)(CF3SO3) were prepared as described in

20,39 literature. This involves refluxing the fac-Re(N-N)CO3Cl with AgOTF or AgBF4 in ethanol or acetonitrile respectively to get the desired product. The final products were synthesized

149

utilizing a “melt” of the tricarbonyl-diimine with excess diimine ligand to produce the rhenium(I) diimine dicarbonyl complexes with two polypyridine ligands using a procedure similar to what is reported in the literature.16 Scheme C1 shows the generalized procedure for the

+ 1 13 synthesis of cis-[Re(N-N)2(CO)2] . The final prodcuts were characterized by H NMR, C

NMR, and HRMS.

1 Figure C1. H NMR of cis-[Re(CO2)(3,4,7,8-Me4phen)2](CF3SO3) (1) in DCM-d2 (100 MHz).

150

13 Figure C2. C NMR of cis-[Re(CO2)(3,4,7,8-Me4phen)2](CF3SO3) (1) in DCM-d2 (100 MHz).

1 Figure C3. H NMR of cis-[Re(CO2)(4,7-Me2phen)2]PF6 (2) in ACN-d3 (100 MHz).

151

13 Figure C4. CNMR of cis-[Re(CO2)(4,7-Me2phen)2]PF6 (2) in ACN-d3 (100 MHz).

152

1 Figure C5. H NMR of cis-[Re(CO2)(5,6-Me2phen)2]PF6 (3) in ACN-d3 (100 MHz).

153

13 Figure C6. C NMR of cis-[Re(CO2)(5,6-Me2phen)2]PF6 (3) in ACN-d3 (100 MHz).

154

1 Figure C7. H NMR of cis-[Re(CO2)(phen)2](CF3SO3) (4) in ACN-d3 (400 MHz).

1 Figure C8. H NMR of cis-[Re(CO2)(4,7-Ph2phen)2]PF6 (5) in ACN-d3 (400 MHz).

155

1 Figure C9. H NMR of cis-[Re(CO2)(5,5’-Me2bpy)2]PF6 (6) in DCM-d2 (100 MHz).

156

13 Figure C10. C NMR of cis-[Re(CO2)(5,5’-Me2bpy)2]PF6 (6) in DCM-d2 (100 MHz).

1 Figure C11. H NMR of cis-[Re(CO2)(4,4’-Dtbbpy)2]PF6 (7) in ACN-d3 (400 MHz).

157

13 Figure C12. C NMR of cis-[Re(CO2)(4,4’-Dtbbpy)2]PF6 (7) in ACN-d3 (100 MHz).

158

1 Figure C13. H NMR of cis-[Re(CO2)(bpy)2]PF6 (8) in ACN-d3 (400 MHz).

1 Figure C14. H NMR of cis-[Re(CO2)(4,4’-Me2bpy)2]PF6 (9) in ACN-d3 (100 MHz).

159

13 Figure C15. C NMR of cis-[Re(CO2)(4,4’-Me2bpy)2]PF6 (9) in ACN-d3 (100 MHz).

160

+ Figure C16. HRMS of cis-[Re(CO2)(3,4,7,8-Me4phen)2] (1).

161

+ Figure C17. HRMS of cis-[Re(CO2)(4,7-Me2phen)2] (2).

162

+ 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

L= CH3CN L= PPh3 L= DMAP

Solvent = CH3CN Solvent = CH3CN Solvent = CH3CN

L= CH3CN L= PPh3 L = DMAP

Solvent = THF Solvent = THF Solvent = THF

Figure C22. Ground state FTIR spectra of [Re(NIBI-phen)(CO)3L]PF6 in acetonitrile and tetrahydrofuran.

L= CH3CN L= PPh3 L= DMAP

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.

Table C1. Summary of lifetimes obtained at select vibrational frequencies in acetonitrile and tetrahydrofuran.

-1 Complex Solvent ν (cm ) τ1 (ps) τ2 (ps) 1650 4.58 ± 0.19 273.30 ± 70.10 CH CN 3 2030 3.01 ± 0.17 24.85 ± 3.11 [Re(NIBI-phen)(CO) CH CN]PF 3 3 6 1650 10.32 ± 2.20 91.34 ± 11.20 THF 2030 8.91 ± 1.92 89.04 ± 8.09 1650 4.79 ± 0.12 131.74 ± 29.40 CH CN 3 2030 4.54 ± 0.19 148.96 ± 41.70 [Re(NIBI-phen)(CO) PPh ]PF 3 3 6 1650 30.70 ± 2.62 377.84 ± 36.80 THF 2030 20.86 ± 1.08 309.71 ± 37.10 1660 5.05 ± 0.94 842.16 ± 175.13 [Re(NIBI-phen)(CO) DMAP]PF CH CN 3 6 3 2010 10.55 ± 0.68 87.28 ± 14.50

168

200 200 -6 100 -6 0 0 -200 x10 -100 x10 -200 -400

6 8 Curve Fit Results Curve Fit Results Coefﬁcient values ± one standard deviation Coefﬁcient values ± one standard deviation y0 =0.001651 ± 4.66e-05 5 y0 =0.00072997 ± 3.35e-05 A =0.0070107 ± 0.000126 A1 =0.0040372 ± 0.000167 tau =4.7012 ± 0.178 tau1 =2.8915 ± 0.188 6 Constant: A2 =0.0016567 ± 0.000164 X0 =8.944 4 tau2 =21.563 ± 3.01 Constant:

X0 =2.356 mOD mOD

3 ∆ ∆ L = CH CN L = CH3CN 4 3 2 Solvent = CH3CN Solvent = CH3CN -1 -1 ν = 2030 cm 2 ν = 1651 cm 1

0 0 50 100 150 0 20 40 60 80 Time (ps) Time (ps)

400 400

-6 200 -6 200 0 0 x10 -200 x10 -200 3.5 Coefﬁcient values ± one standard deviation Coefﬁcient values ± one standard deviation 6 y0 =0.00093439 ± 4.08e-05 3.0 y0 =0.0007338 ± 1.88e-05 A1 =0.0022265 ± 0.000268 A1 =0.00091901 ± 0.000106 tau1 =10.327 ± 2.2 tau1 =8.9096 ± 1.92 A2 =0.0029958 ± 0.00027 2.5 A2 =0.0017357 ± 0.000107 5 tau2 =91.342 ± 11.2 tau2 =89.04 ± 8.09 Constant: Constant: X0 =6.638 2.0 X0 =3.996 4 L = CH CN L = CH CN

3 mOD 3 mOD 1.5 ∆ 3 ∆ Solvent = THF Solvent = THF 1.0 -1 -1 ν = 2030 cm 2 ν = 1650 cm 0.5 1 0.0

0 1000 2000 3000 0 1000 2000 3000 Time (ps) Time (ps)

Figure C25. Kinetic fits of [Re(NIBI-phen)(CO)3CH3CN]PF6 at select vibrational frequencies in ultrafast TRIR experiments.

169

Short

Long

Figure C26. Ultrafast TRIR 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).

0.5

0.0 -3 200 -6 -0.5 0 -1.0 x10 x10 -1.5 -200 -2.0 5 8 Curve Fit Results 4 Coefﬁcient values ± one standard deviation y0 =-5.1217e-05 ± 2.8e-05 Curve Fit Results Coefﬁcient values ± one standard deviation A1 =0.0048154 ± 8.39e-05 tau1 =4.5393 ± 0.187 6 y0 =0.00063915 ± 3.64e-05 3 A1 =0.0077824 ± 8.1e-05 A2 =0.00052587 ± 6.06e-05 tau1 =5.0036 ± 0.113 tau2 =148.96 ± 41.7 A2 =0.0005661 ± 4.45e-05 Constant: tau2 =428.73 ± 99.3 mOD X0 =2.625 mOD 4 Constant: ∆ 2 ∆ X0 =6.004 L = PPh3 L = PPh3 Solvent = CH CN 1 Solvent = CH3CN 2 3 ν = 2030 cm-1 ν = 1650 cm-1 0 0 0 1000 2000 3000 0 1000 2000 3000 Time (ps) Time (ps) 400

200 -6 200 -6 0 0 x10 x10 -200 -200

6 Curve Fit Results Coefﬁcient values ± one standard deviation Coefﬁcient values ± one standard deviation y0 =0.00092921 ± 4.43e-05 4 y0 =0.00030035 ± 2.33e-05 A1 =0.003086 ± 0.000134 A1 =0.0025806 ± 6.46e-05 tau1 =30.703 ± 2.62 tau1 =20.859 ± 1.08 A2 =0.0025303 ± 0.000129 A2 =0.0010596 ± 6.21e-05 4 tau2 =377.84 ± 36.8 3 tau2 =309.71 ± 37.1 Constant: Constant: L = PPh3 X0 =8.101 L = PPh3 X0 =3.245 Solvent = THF mOD Solvent = THF mOD ∆ ∆ 2 ν = 2030 cm-1 2 ν = 1650 cm-1 1 0 0 0 1000 2000 3000 0 1000 2000 3000 Time (ps) Time (ps) Figure C27. Kinetic fits of [Re(NIBI-phen)(CO)3PPh3]PF6 at select vibrational frequencies in ultrafast TRIR experiments.

170

Short

Long

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).

2.0 400

200 -6 1.0 -3 0

0.0 -200 x10 -1.0 x10 -400

Coefﬁcient values ± one standard deviation y0 =0.0012634 ± 9.94e-05 A1 =0.0017659 ± 0.000161 5 4 tau1 =5.0458 ± 0.943 A2 =0.0013536 ± 9.76e-05 Curve Fit Results tau2 =842.16 ± 175 Coefﬁcient values ± one standard deviation Constant: 4 X0 =8.944 y0 =0.00029118 ± 1.37e-05 3 A1 =0.0017107 ± 6.77e-05 L = DMAP tau1 =10.549 ± 0.676 3 A2 =0.0005892 ± 6.76e-05 Solvent = CH3CN tau2 =87.287 ± 14.5 mOD mOD

∆ -1

∆ Constant: 2 ν = 1660 cm 2 X0 =3.603 L = DMAP Solvent = CH3CN 1 -1 1 ν = 2010 cm 0 0 1000 2000 3000 0 400 800 Time (ps) Time (ps) Figure C29. Kinetic fits of [Re(NIBI-phen)(CO)3DMAP]PF6 at select vibrational frequencies in ultrafast TRIR experiments.

171

L = CH3CN L = PPh3 L = DMAP

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.

OD ∆

1850 1800 1750 1700 1650 -1 Wavenumber (cm ) Figure C31. Simulated TRIR difference spectrum of + [Re(NIBI-phen)CH3CN] at early times. 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

1 2 3

4 5

Figure C33. UFTA single wavelength kinetics of Re(I) dicarbonyls 1-5 measured at 360 nm.

174

6 7

8 9

Figure C34. UFTA single wavelength kinetics of Re(I) dicarbonyls 6-9 at select wavelengths.

175

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)

Complex Wavelength (nm) τ1 (ps) τ2 (ns)* 1 360 62.95 ± 11.7 512 2 360 76.02 ± 9.45 212 3 360 80.04 ± 11.60 128 4 360 56.30 ± 9.98 131 5 360 148.41 ± 18.10 182 6 390 22.81 ± 1.93 60 7 370 13.42 ± 2.81 26 8 370 15.95 ± 2.68 25 9 380 9.24 ± 2.05 23

Table C3 Summary of solid-state FTIR-ATR metal carbonyl stretching frequencies in complexes 1-9.

-1 Complexes νCO (cm ) 1 1891, 1824 2 1899, 1828 3 1907, 1828 4 1905, 1828 5 1899, 1828 6 1893, 1828 7 1899, 1826 8 1907, 1834 9 1899, 1826

176

S S S 1 2 A 3 A A OD OD ∆ ∆

A A S A S S 2150 2100 2050 2000 1950 1900 1850 1800 2200 2100 2000 1900 1800 -1 -1 Wavenumber (cm ) Wavenumber (cm )

S 4 S 5 A A OD OD ∆ ∆

A A S S

2200 2100 2000 1900 1800 2200 2100 2000 1900 1800 -1 -1 Wavenumber (cm ) Wavenumber (cm )

S 7 A 6 A S OD ∆

A A S S 2200 2100 2000 1900 1800 -1 Wavenumber (cm )

A 8 S 9 S A mOD ∆

A A S S

2200 2100 2000 1900 1800 -1 Wavenumber (cm ) 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 solvent=DCM) level of theory.

177