CONCERTED BOND FORMATION WITH ELECTRON TRANSFER TO RUTHENIUM POLYPYRIDYL EXCITED STATES
Ward, William Maurice
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Electronic excited states possess an empty electron orbital at a lower energy than the ground state compound and therefore may participate in redox reactions that are energetically unfavorable for the ground state compound. These photo-initiated one-electron redox reactions often result in unstable compounds that undergo subsequent bond breakage or bond formation through a stepwise mechanism. However, some reactions that are energetically favorable overall are hindered due to a high energy product of the initial electron transfer step. In these cases, a concerted mechanism, where bond breakage or formation is coincident with electron transfer, may provide an alternative low-energy pathway for the reaction to follow. In the case of bond formation, ion-pairing in a low-dielectric solvent enables the simultaneous encounter between the two reactants and the photo-oxidant. Several ruthenium and osmium polypyridyl compounds were studied as excited-state photo-oxidants for halide ions. Reactions fell into one of three zones depending on the reduction potentials of both the halide and the excited-state compound. Combinations of transition metal compounds and halides in Zone 1 were known to proceed through the stepwise mechanism and suspected to also proceed through the concerted mechanism in a low-dielectric solvent. Combinations in Zone 2 were found to react only through the Concerted mechanism. Time-resolved spectroscopy indicated the simultaneous disappearance of reactants and appearance of products. In Zone 3 both mechanisms were energetically unfavorable and the photophysical effects of ion-pairing were studied that were otherwise masked by redox chemistry.