Diffusion-controlled electron transfer reactions: Subpicosecond fluorescence measurements of coumarin 1 quenched by aniline and N,N-dimethylaniline

Abstract

The fluorescence quenching of a 7-aminocoumarin dye [coumarin 1 (C1)] by amine electron donors (aniline or N,N-dimethylaniline) in methanol was examined by picosecond time-resolved and steady-state fluorescence measurements. The quencher concentration dependence of the data was analyzed using the classic Smoluchowski model and the Collins and Kimball model of diffusion-controlled reactions. In addition, the Wilemski and Fixman model, which includes a distance-dependent sink term, was used to analyze the data. We have conclusively shown that the Smoluchowski model does not describe either the C1-aniline or the C1-dimethylaniline fluorescence quenching data. It was found that the Collins and Kimball model accurately described the C1-aniline data, but was inappropriate for the C1-dimethylaniline results. The addition of a simple position-dependent sink term to the Collins and Kimball model enabled both the C1-aniline and the C1-dimethylaniline time-resolved data to be accurately described. Analysis with a model incorporating a nonadiabatic electron transfer sink function revealed that both reactions have a strong distance dependence and that only the C1-aniline reaction can be classified as solely nonadiabatic electron transfer. Based on these analyses, we conclude that the C1-dimethylaniline reaction encompasses both the adiabatic and nonadiabatic limits of electron transfer. We also analyzed the temperature dependence of the reaction rate using Marcus nonadiabatic electron transfer theory to estimate the activation energy, the solvent reorganization energy, and the electronic coupling matrix element of the intrinsic electron transfer reaction. The average bimolecular reaction rate found was 8.77×109 M−1 s−1 for the C1-aniline reaction and 1.52×1010 M−1 s−1 for the C1-dimethylaniline reaction.