QUENCHING OF TRIPLET-STATES OF AROMATIC KETONES BY SULFUR-CONTAINING AMINO-ACIDS IN SOLUTION - EVIDENCE FOR ELECTRON-TRANSFER

The mechanism for quenching triplet states of benzophenones by sulfur- containing amino acids in water/acetonitrile solution was investigated by laser flash photolysis. The amino acids in the study were methioni ne, S-methylcysteine, and S-carboxymethylcysteine, and the eight aroma tic triplets were those of benzophenone and its derivatives possessing electron-withdrawing or electron-donating groups. The presence of rad ical ions in the transient spectra and correlations of the quenching r ate constants with the free energy change for electron transfer are st rong indications that the process involves an electron transfer. These correlations were displayed as Rehm-Weller plots (logarithm of quench ing rate vs free energy). Classical theoretical formulations of the Re hm-Weller correlations were used to estimate the intrinsic barriers an d the transmission coefficients for the electron-transfer processes. A pplying both ''quadratic'' Marcus and ''asymptotic'' Agmon-Levine free energy relationships led to the values of intrinsic barriers lower th an the solvent reorganization energy calculated within the framework o f the dielectric continuum model. These relationships also led to low electronic transmission coefficients. The low values of the intrinsic barriers for electron transfer were also obtained using the recently d eveloped Tachiya approach which allowed for variable electron-transfer distance. Possible explanations were given for the resulting multiple Rehm-Weller plots with different plateau values for each of the amino acids studied.