Electron transfer from aromatic amines to excited coumarin dyes: Fluorescence quenching and picosecond transient absorption studies

Electron transfer (ET) from aniline, N-alkylanilines, N,N-dialkylanilines, and o-phenylenediamine to a number of excited (S-1) 4-CF3-coumarin dyes hav ing differently substituted 7-amino groups have been investigated in aceton itrile solutions using steady-state (SS) and time-resolved (TR) fluorescenc e quenching measurements. Direct evidence for the ET reactions in the prese nt systems have been obtained by characterizing the amine cation radicals u sing picosecond transient absorption measurements in the visible region. Th e experimentally determined bimolecular quenching constants (k(q)) are seen to correlate nicely with the free energy changes (Delta G degrees) for the ET reactions, within the framework of the Marcus ET theory. The total reor ganization energy (lambda) estimated from such correlation indicates that t he solvent reorganization (lambda(s)) plays the major role in governing the ET dynamics in the present systems. A comparison of the present results un der diffusive conditions with those reported earlier under nondiffusive con ditions indicates that the ET dynamics in the latter cases are much faster than in the former. The results are discussed considering a structural diff erence between the encounter complexes formed under diffusive and nondiffus ive conditions.