Understanding the Electronic Structures of First-Row Transition Metal Complexes for Solar Energy Conversion and Catalysis

Citation

Stroscio, Gautam Dutta (2021) Understanding the Electronic Structures of First-Row Transition Metal Complexes for Solar Energy Conversion and Catalysis. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4627-9w77. https://resolver.caltech.edu/CaltechTHESIS:02062021-054659966

Abstract

Chapter 1 discusses the major findings and themes of the studies presented in this thesis. Chapter 2 presents a DFT-based methodology for quantifying entatic states. Here it is applied to Cu-based photosensitizers used for solar electricity generation, solar fuels synthesis, organic light emitting diodes (OLEDs), and photoredox catalysis. The methodology can be used to decouple the steric and electronic contributions to excited state dynamics and, in turn, can be used to guide the design of future photosensitizers. The computed entatic energies in some of the photosensitizers were the largest quantified to date: ~20 kcal mol^-1 relative to the conformationally flexible [Cu(phen)₂]⁺. Of course, considering typical chemical barriers and driving forces, these values are significant.

Chapter 3 is an investigation of the ground and excited spin state energetics of a free carbene and several of its iron porphyrin carbene (IPC) analogs. Here it is shown that for the IPC models, multireference ab initio wave function methods give results most consistent with experiment. Specifically, the predicted, mixed singlet ground state is mostly dominated by the closed-shell singlet (Fe(II)←{:C(X)Y}⁰) configuration, with a small contribution from an Fe(III)–{C(X)Y}^–• open-shell singlet configuration (hole in d(xz)). This description differs from that obtained by using DFT. Also, using the multireference ab initio wave methods, elongation of the IPC Fe–C(carbene) bond increases the weighting of this particular open-shell configuration within the ground state singlet.

Chapter 4 also deals with a system where DFT and multireference ab initio results diverge: the light-induced Ni(II)–C homolytic bond dissociation in Ni 2,2'-bipyridine photoredox catalysts. DFT calculations give a barrier of ~30 kcal mol^-1 while multireference ab initio calculations giving a barrier of ~70 kcal mol^-1. Thus, within the latter description, a previously proposed mechanism of thermally assisted dissociation from the lowest energy triplet ligand field excited state is unfavorable. Instead, the mechanism given by the multireference description is initial population of a singlet Ni(II)-to-bpy metal-to-ligand charge transfer (¹MLCT) excited state followed by intersystem crossing and aryl-to-Ni(III) charge transfer. From accessible repulsive triplet excited states, homolytic bond dissociation can occur.

Thesis Files