DOES BACK ELECTRON-TRANSFER WITHIN GEMINATE REDOX PAIRS FORMED IN BIMOLECULAR QUENCHING FOLLOW A MARCUS BELL-SHAPED ENERGY-GAP DEPENDENCE

The temperature dependence of the cage escape yield (eta(ce)) of redox products from the quenching of Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine ) by aromatic amines in deaerated 1:1 (nu/nu) CH3CN/H2O solutions has been examined. Values of lambda, the solvent reorganization energy upo n electron transfer within the geminate redox pairs, have been extract ed from plots of log((eta(ce)(-1)-1)T-1/2) vs. 1/T; the more stericall y hindered two-ring amines exhibit a higher value of lambda (1.1 +/- 0 .08 eV) than do most of the one-ring amines (0.82 +/- 0.04 eV). A Marc us plot of log(eta(ce)(-1)-1) vs. Delta G(bt)(0) shows a correlation w ithin only the inverted region for systems with the same lambda; the e arlier identification of the results for the same photosensitizer and quenchers as a bell-shaped energy-gap curve is due to the coincidental overlap of two independent segments within the inverted region.