Hydrogen-Transfer and Charge-Transfer in Photochemical Reactions. Progress Report, October 1, 1976--September 31, 1977

摘要

Quantitative studies have been made of effects of sulfur compounds on photoreduction of benzophenone by amines. Aromatic mercaptans and disulfides are converted to photostationary equilibrium concentrations of the two forms, and retard photoreduction very efficiently, to a small extent by light absorption and quenching of triplet (approximately 10%), to a large extent (approximately 60%) by the repeated hydrogen transfer reactions, and the remainder by quenching of the charge transfer complex. Aliphatic disulfides are reduced, mercaptans are not oxidized, the two states are not equilibrated, and photoreduction by amines is accelerated by aliphatic mercaptans. The acceleration is attributed to catalysis of proton transfer in the charge-transfer complex. Ratios of rate constants for reduction of amine-derived radicals by mercaptans to oxidation by ketone are obtained. Effects of light absorption, triplet quenching and hydrogen transfer are calculated in retardation by mercaptans of photoreduction by alcohols. In reduction by 2-propanol and acetophenone, more hydrogen transfer is observed than would be calculated, indicating that thiyl radicals, in addition to ketone triplet, abstract hydrogen. In reduction by benzhydrol, retardation is due entirely to light absorption and quenching. Benzophenone ketyl radical is too highly stabilized for hydrogen abstraction to compete with radical demerization. Rate constants for abstraction of hydrogen from aromatic mercaptans by the ketyl radicals are estimated. In photoreduction of fluorenone by substituted dimethylanilines, low quantum yield due to an electron donating substituent is increased by decrease in polarity of solvent, and low quantum yield due to electron withdrawing substituents is increased by increase in polarity of solvent. The results are attributed to effects of substituents and solvent polarity on extent of charge transfer in the charge transfer complex. (ERA citation 03:015593)