The Excited State Properties of Dirhodium (II,II) Complexes: Application for Solar Energy Conversion Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Congcong Xue Graduate Program in Chemistry The Ohio State University 2019 Dissertation Committee Dr. Claudia Turro, Advisor Dr. Terry Gustafson Dr. Yiying Wu 1 Copyrighted by Congcong Xue 2019 2 Abstract Transition metal complexes have been widely explored for solar energy conversion. The absorption of high energy ultraviolet and blue photons typically results in more energetic excited states, which renders them better reductants and/or oxidants as compared to their ground states. The excited states of dirhodium(II,II) complexes were recently discovered, which may serve as dyes in dye-sensitized solar cell applications owing to their strong, panchromatic absorption throughout the visible and near-infrared spectral regions. These "black dyes" are important to maximize the absorption of the photons from sunlight that reach the earth. T In the present work, the energy of the triplet excited states, E00 , and the redox potentials of a series of dirhodium complexes with the type cis-[Rh2(μ- DTolF)2(L)2][BF4]2, where L = np (1; 1,8-naphthyridine), npCOOH (2; 1,8- naphthyridine-2-carboxylic acid), phen (3; 1,10-phenanthroline), dpq (4; dipyrido[3,2- f:2’,3’-h]quinoxaline), dppz (5; dipyrido[3,2-a:2’,3’-c]phenazine), and dppn (6; benzo[i]dipyrido[3,2-a:2’,3’-h]quinoxaline); DTolF = N,N’-di-p-tolylformamidinate), were measured. Complexes 1 – 6 are not emissive, such that their triplet state energies were determined from energy transfer quenching experiments with a series of organic 3 T sensitizers with known * excited state energies, resulting in E00 values estimated to be ~1.1 eV for 1 and 2, ~1.5 eV for 3, and ~1.2 eV for 4 – 6 (vs Ag/AgCl). ii The reductive excited states of the dirhodium complexes make them potential dyes for n-type dye-sensitized solar cell (DSSC) applications. For this purpose, two new Rh2(II,II) dyes with methyl ester anchoring groups, [Rh2(DTolF)2(menp)2][BF4]2 (7; menp = 4-carbomethoxy-1,8-naphthyridine) and [Rh2(DTolF)2(dmeb)2][BF4]2 (8; dmeb = 4,4ʹ-carbomethoxy-2,2ʹ-bipyridine) were synthesized and their photophysical properties were characterized and compared. Femtosecond transient absorption (fsTA) and time- resolved infrared (fsTRIR) spectroscopies reveal that the initially populated 1ML-LCT excited states of 7 and 8 decay to the corresponding 3ML-LCT excited states with time constants of 4 ps and 2.5 ps, respectively. The 3ML-LCT excited states of 7 and 8 repopulate the ground state with lifetimes of 460 ps and 56 ps, respectively. The shorter 1ML-LCT and 3ML-LCT lifetimes of 8 as compared to those of 7 are attributed to the longer Rh-Rh bond in the former, which provides a fast deactivation pathway through a metal-centered (MC) state that involves population of the Rh2(*) molecular orbital. Photoinitiated electron injection into the semiconductor TiO2 upon low energy light irradiation was achieved through the excitation of Rh2(II,II) dyes associate to TiO2 nanoparticles through the methyl-ester substituent (7@TiO2 and 8@TiO2) and ultrafast electron injection was observed by fsTRIR with low energy excitation, 600 nm for 7 and 520 nm for 8. 2+ Complex 11, Rh2(DPhF)2(bncn)2 (form= diphenylformamidinate, bncn = benzo[c]cinnoline), with a shorter bridging bncn ligand was synthesized and characterized. The crystal structure indicates that 11 has a shorter Rh-Rh bond length compared to 1 – 8, which leads to a longer triplet excited state lifetime, T ~ 19 ns. Bulk iii electrolysis of 11 with trifluoromethanesulfonic acid shows H2 production with 98% Faradaic efficiency. The highly oxidative and nanosecond long triplet excited state of 11, together with its catalytic active bimetallic core, result in photocatalytic activity. Irradiation at 670 nm in acidic solutions with a sacrificial electron donor, 1-benzyl-1,4- dihydronicotinamide (BNAH), results in the catalytic production of hydrogen exceeding 170 turnovers (TON) in 24 hours with an initial rate of 28 TONs per hour. The catalysis proceeds through two stepwise excited state redox events, a feature previously unknown in homogeneous photocatalysis, which permits the storage of two redox equivalents on the dirhodium catalyst using low energy light with high efficiency. Lastly, the important intermediate, the one-electron reduced complex [11]1−, exhibits a triplet lifetime T~ 0.5 ns and Ered* ~ +0.66 eV, which makes the second electron transfer event favorable with BNAH as an electron donor to support the proposed mechanism. iv Dedication For my parents. Thanks for all your love and supports. v Acknowledgments First and foremost, I’d like to thank my advisor, Professor Claudia Turro for her guidance and enlightenment. I’m so grateful that she always helps me see the positive part when I encounter any difficulties and encourages me when I doubt myself. I’d like to thank her for shaping me into a more confident version of myself. Claudia gave me a lot of opportunities to collaborate with amazing people and work on interesting projects. She also allowed me to travel around the country to share my research results, talk to fantastic people and get inspired. I genuinely appreciate all my incredible experience in the past five years as a Turro group member. I also want to acknowledge the amazing group. Dr. Tyler Whittemore, you’re like a mentor and role model to me. Working with you is such an incredible experience, you’re so brilliant yet hardworking and your positivity keeps influencing me. You taught me all the laser techniques, answered so many of my questions and guided me through a lot of tough situations. Dr. Hannah Sayre and Dr. Suzanne Witt, you’re like big sisters to me in the group and taught me everything about catalysis and electrochemistry. Hannah influences me not only as a scientist but also as a human being who cares about the world around us, environment, women’s rights and so much more fun topics you brought up with. Suzanne, thank you for laying such a good foundation on the energy side of the group and the talk with you about future career. Dr. William Kender, you’re so smart and vi just like a walking Wikipedia and thank you for all the enlightenment when I lost the direction. Dr. Travis and Jessica White, you’re the first mentors I had in group. I also appreciate the opportunity to collaborate with Travis when he’s a professor at OU. Lauren, you’ve always been super helpful, thoughtful and sweet. Jie, thanks for taking your own time and synthesizing all the complexes for me. To my current and past coworkers, Regina, TJ, Malik, Katy, Camila, Hemanthi, Austin, Sean, Jessica, Matt, Massiel, Shaoyang and Allen. It’s been such a pleasure to work with all of you guys. Thanks for my dearest friends Juan, Fen, Celia and Jiahui, without you girls grad school wouldn’t be so enjoyable. To my Mon and Dad, thanks for encouraging me to go aboard and pursue my PhD. I cannot go through these alone without your support and video calls every weekend. vii Vita June 2014 .......................................................B.S. Chemistry, Beijing University of Chemical Technology, Beijing, China 2014 to present ..............................................Graduate Research and Teaching Associate, Department of Chemistry, The Ohio State University Publications C. Xue, H. J. Sayre, C. Turro, Chem. Comm., 2019, DOI: 10.1039/c9cc04677a S. Saeedi, C. Xue, B. McCullough, S. Roe, B. Neyhouse, T. White, ACS Appl. Energy Mater., 2019 T. N. Rohrabaugh Jr., K. A. Collins, C. Xue, J. K. White, J. J. Kodanko, C. Turro, Dalton Trans. 2018, 47, 11851 T. J. Whittemore, A. Millet, H. J. Sayre, C. Xue, B. S. Dolinar, E. G. White, K. R. Dunbar, C. Turro, J. Am. Chem. Soc., 2018, 140, 5161 T. J. Whittemore, H. J. Sayre, C. Xue, T. A. White, J. C. Gallucci, C. Turro, J. Am. Chem. Soc., 2017, 139, 14724 S. Chen,† W. Zhou,† Y. Cao, C. Xue, and C. Lu, J. Phys. Chem. C, 2014, 118, 2851 viii Fields of Study Major Field: Chemistry ix Table of Contents Abstract ............................................................................................................................... ii Dedication ........................................................................................................................... v Acknowledgments.............................................................................................................. vi Vita ................................................................................................................................... viii Table of Contents ................................................................................................................ x List of Tables ................................................................................................................... xiii List of Figures .................................................................................................................. xiv Chapter 1. Introduction and Background ....................................................................... 1 1.1 Solar Energy Need .................................................................................................... 1 1.2 Dye Sensitized Solar Cell ......................................................................................... 4 1.2.1 Operational Principle ........................................................................................