Metal-Ligand Cooperativity for Photovoltaic and Electrocatalytic Applications

This dissertation serves to investigate molecular mechanisms for solar-to-electricity conversion, i.e. photovoltaics, and for the catalytic storage of electrical energy in the form of chemical bonds via electrocatalytic CO2 reduction. A general theme across all chapters is to utilize unique ligand platforms to provide an added degree of control over complex photochemical and electrochemical properties. Chapters 1 serve as an introductory chapter to summarize the relevant literature for both themes (photovoltaics & electrocatalysis) of this dissertation. Chapters 2, 3 & 4 investigate non-innocent ligand (NIL) effects on the electronic properties of ruthenium based photosensitizers. Chapter 2 presents the fundamental electronic and photophysical properties for a series of [Ru(bpy)2(R-CAQN)]+ complexes where CAQN– is the 8-carboxy amidoquinolate ligand scaffold. This work has been published in the ACS journal Inorganic Chemistry (Inorg. Chem. 2016, 55, 2460-2472; DOI: 10.1021/acs.inorgchem.5b02834) thank to the contributions from our collaborators including David J. Szalda from Baruch College and Ralph T. Weber from Bruker BioSpin Corporation. Chapter 3 introduces a series of [Ru(bpy)2(XQN)]+ where XQN– is bidentate quinolate ligand having a heteroanion ‘X’ in the 8-position. The series reflects the effect of introducing the heteroanion in the coordination sphere on electronic and spectroscopic features of ruthenium polypyridyl complexes. Chapter 4 investigate the optical and redox properties of ruthenium photosensitizers in dye sensitized solar cells (DSSC) with four different non-innocent ligand platforms: NCS–, ppy–, OQN–, and CAQN– where NCS– is thiocyanate, and ppy– is 2-phenylpyridine. The results from the frontier orbital mixing contribute to the tunability of the redox properties and visible electronic transitions which could improve the power conversion efficiency in DSSC device. This work has been published in the Chemistry - A European Journal (Chem. Eur. J., 2017, 23, 7497–7507; DOI: 10.1002/chem.201605991). While it is feasible that our Ru-NIL photosensitizers may be also utilized for photocatalytic CO2 these studies are beyond the scope of this thesis and may possibly be pursued by future graduate students in our lab. Alternatively, my studies of CO2 reduction incorporate the NIL directly into the inner-coordination sphere of rhenium and manganese based catalysts which are probed using electrochemical methods. Using an electrocatalytic approach allows for a comprehensive evaluation and characterization of such catalysts as discussed in chapter 1. Specifically, chapter 5 discusses the electrochemical and photophysical properties of the fac-Re(AQN)(CO)3(CH3CN) complex where AQN– = N-phenyl-8-amidoquinolate. The electrocatalytic activity for CO2 to CO conversion exhibited by fac-Re(AQN)(CO)3(CH3CN) upon electrolysis is presented with GC analysis confirming the presence of CO in the products. In chapter 6 the more abundant first-row manganese metal center is utilized for a pair of bulky 6,6’-substituted 2,2’-bipyridyl (bpy) ligand complexes. Electrocatalytic properties are discussed for the {fac-MnI([(MeO)2Ph]2bpy)(CO)3(CH3CN)}(OTf) and [fac-MnI(mes2bpy)(CO)3(CH3CN)](OTf) where [(MeO)2Ph]2bpy is 6,6′-bis(2,6-dimethoxyphenyl)-2,2′-bipyridine and mes2bpy is 6,6\u27-dimesityl-2,2\u27-bipyridine. Second coordination sphere Brønsted acid-base effects are utilized at the bpy 6,6’ positions for the former catalyst to drive the catalytic cycle through a low overpotential pathway and optimize catalyst turnover frequencies for CO2 to CO. This work has been published in the Journal of the American Chemical Society (J. Am. Chem. Soc., 2017, 139, 2604–2618; DOI: 10.1021/jacs.6b08776) thank to the contributions from our collaborators including David C. Grills and Mehmed Z. Ertem from Brookhaven National Laboratory