Femtosecond Studies of Charge Separation in the Fluorescent State of DCM

Abstract

Solvation dynamics has long been recognized to be of great importance for the understanding of the rate of intramolecular electron transfer (ET) reactions1,2. Especially when the ET reaction takes place in the so-called strong adiabatic coupling limit, solvation dynamics may predominantly determine the ET rate constant. Under such conditions one has κ ≫ 1, where the adiabaticity parameter3, κ, is given by κ = π V2 〈τ〉 /ħλm, in which V is the electronic coupling between the locally excited (LE) and charge transfer (CT) states involved in the electron transfer process, 〈τ〉 is the solvent relaxation time and λm is the solvent reorganization energy. Recently, for a number of molecules solvent controlled ET in the photo-excited state has been considered in the strong adiabatic coupling regime4. In this report, we present results of a femtosecond pump-probe study of the charge separation reaction in the photo-excited state of a large laser dye molecule, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4-H-pyran (DCM). We have performed a subpicosecond study of the solvatochromism of the DCM molecule. A major finding is that following the pulsed excitation of the molecule, the integrated intensity of the spontaneous fluorescence exhibits a transient decay to approximately 50% of its initial value in a few picoseconds. The results are consistent with strongly mixed LE and the CT excited states for DCM (i.e., the adiabatic coupling limit applies) so that the solvation dynamics constitutes the rate determining step in the ET process.