Probing solvation and reaction coordinates of ultrafast photoinduced electron-transfer reactions using nonlinear spectroscopies: Rhodamine 6G in electron-donating solvents

The reaction kinetics as well as the solvation dynamics of the photoinduced electron-transfer (ET) reaction from the electron-donating solvents dimeth ylaniline (DMA) and diethylaniline (DEA) to rhodamine 6G (R6G) are elucidat ed using complementary information from transient grating (TG) and three-pu lse photon echo peak shift (3PEPS) measurements. The data are contrasted wi th those obtained from TG and 3PEPS studies in the "unreactive" solvents et hanol and dimethyl sulfoxide. New methods are employed to model these data using nonlinear response functions expressed in terms of both solvation dyn amics and reaction kinetics. A three-level model, including a component in the response function to account for excited-state absorption, is used to m odel the 3PEPS and TG data. It is also demonstrated that 3PEPS retrieves in formation concerning the reaction coordinate as well as solvation informati on. We conclude that for R6G/DMA, rapid photoinduced ET occurs on a time sc ale of tau(a) similar to 85 fs and for the R6G/DEA system tau(a) similar to 160 fs, An excited-state absorption contribution to the signals that we as sociate with back-electron transfer was observed with time constants tau(b) = 4.0 ps for R6G/DMA (15% contribution) and tau(b) = 6.9 ps for R6G/DEA (2 0% contribution). Subsequently, the cooling and relaxation (i.e. ground-sta te recovery) occurs on a time scale of tau(c) = 19 ps (R6G/DMA) and tau(c) = 50 ps (R6G/DEA). We attribute the tau(c) to solvent-limited reequilibrati on on the ground-state free energy curve.