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 having differently substituted 7-amino groups have been investigated in acetonitrile solutions using steady-state (SS) and time-resolved (TR) fluorescence quenching measurements. Direct evidence for the ET reactions in the present systems have been obtained by characterizing the amine cation radicals using picosecond transient absorption measurements in the visible region. The 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 reorganization energy (lambda) estimated from such correlation indicates that the solvent reorganization (lambda(s)) plays the major role in governing the ET dynamics in the present systems. A comparison of the present results under diffusive conditions with those reported earlier under nondiffusive conditions indicates that the ET dynamics in the latter cases are much faster than in the former. The results are discussed considering a structural difference between the encounter complexes formed under diffusive and nondiffusive conditions.