Electron transfer from diphenyl and triphenyl amines to excited coumarin dyes

Electron transfer (ET) from diphenylamine (DPA) and triphenylamine (TPA) to a series of excited coumarin dyes having differently substituted 7 amino g roups have been investigated in acetonitrile solution. The bimolecular quen ching constants (k(q)) obtained from steady-state (SS) and time-resolved (T R) fluorescence measurements are the same within the experimental error. Di rect evidence for the ET reactions have been obtained using picosecond tran sient absorption measurements by characterising the DPA cation radical. Exp erimentally determined k(q) values are seen to correlate well with free ene rgy changes (Delta G(0)) for the ET reactions within the framework of Marcu s ET theory. Under the present experimental conditions, where diffusion of the reactants is a prerequisite for the reaction to take place, the solvent reorganisation appears to play the main role in governing the ET dynamics. The contribution from intramolecular reorganisation appears to be negligib le, though under non-diffusive conditions it is reported that the ET for th e similar systems can occur much faster than the solvation dynamics through the participation of the intramolecular modes. Differences in the ET dynam ics for diffusive and non-diffusive conditions have been discussed in the l ight of the possible structural differences of the encounter complexes form ed under the two experimental conditions. (C) 2000 Elsevier Science S.A. Al l rights reserved.