Electron transfer from diphenyl and triphenyl amines to excited coumarin dyes

Abstract

Electron transfer (ET) from diphenylamine (DPA) and triphenylamine (TPA) to a series of excited coumarin dyes having differently substituted 7 amino groups have been investigated in acetonitrile solution. The bimolecular quenching constants ( k q) obtained from steady-state (SS) and time-resolved (TR) fluorescence measurements are the same within the experimental error. Direct evidence for the ET reactions have been obtained using picosecond transient absorption measurements by characterising the DPA cation radical. Experimentally determined k q values are seen to correlate well with free energy changes (Δ G 0) for the ET reactions within the framework of Marcus 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 negligible, though under non-diffusive conditions it is reported that the ET for the similar systems can occur much faster than the solvation dynamics through the participation of the intramolecular modes. Differences in the ET dynamics for diffusive and non-diffusive conditions have been discussed in the light of the possible structural differences of the encounter complexes formed under the two experimental conditions.