Spectroscopic investigation of photo-induced proton-coupled electron transfer and Dexter energy transfer in model systems

Abstract

Spectroscopic investigations of systems designed to advance the mechanistic interrogation of photo-induced proton coupled electron transfer (PCET) and proton-coupled (through-bond) energy transfer (PCEnT) are presented. PCET is ubiquitous in Nature, where it is at the heart of bioenergy conversion and catalysis (Chapter I). Systems of relevance to mechanistic studies of PCET and PCEnT are the central tenet of this work. In uni-directional PCET, electron transfer (ET) occurs from an electron donor (De) to an electron acceptor (Ae) through a hydrogen bonded proton interface. The proton interface plays a vital role in mediating ET. Thus, the exact ionization configuration of the interface must be uncovered to fuIIy realize the influence of the interface. SpecificaIIy, does the interface exists in the non-ionized (i.e. amidine-carboxylic acid) or ionized (i.e. amidinium-carboxylate) form. Strategies to spectraIIy monitor the interface ionization state by extending electronic communication from a porphyrinic chromophore to its pendant amidinium functionality are pursued through examination of an alkynylamidinium Ni(II) porphyrin (Chapter II) and an amidinium appended Zn(II) purpurin (Chapter III). With the ionization state of the interface resolved, mechanistic studies of photo-induced PCET between an identical De and Ae pair juxtaposed by a non-ionized (amidine-carboxylic acid) and an ionized (amidinium-sulfonate) interface are undertaken to reveal that PCET occurring through an ionized interface is more strongly coupled to the surrounding solvent environment (Chapter IV). Work on this system is extended to a second solvent of similar dielectric constant to establish that molecular variation of the solvent environment impacts PCET, likely through its interaction with the proton interface (Chapter V). Two water-soluble amidinium-appended ferrocene moieties are presented as building blocks for aqueous bi-directional PCET in which PT occurs to the bulk and ET occurs along a covalently bound coordinate (Chapter VI). ET and through-bond EnT are described by the semiclassical nonradiative decay formalism, meaning both processes should be sensitive to an intervening proton network. For the first time, PCEnT is established using ferrocenyl-amidine moieties bound through an amidinecarboxylic acid interface to Ru(II) polypyridyl complexes (Chapter VII).