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 dimethylaniline (DMA) and diethylaniline (DEA) to rhodamine 6G (R6G) are elucidated using complementary information from transient grating (TG) and three-pulse photon echo peak shift (3PEPS) measurements. The data are contrasted with those obtained from TG and 3PEPS studies in the "unreactive" solvents ethanol and dimethyl sulfoxide. New methods are employed to model these data using nonlinear response functions expressed in terms of both solvation dynamics and reaction kinetics. A three-level model, including a component in the response function to account for excited-state absorption, is used to model the 3PEPS and TG data. It is also demonstrated that 3PEPS retrieves information concerning the reaction coordinate as well as solvation information. We conclude that for R6G/DMA, rapid photoinduced ET occurs on a time scale of τ_a ∼ 85 fs and for the R6G/DEA system τ_a ∼ 160 fs. An excited-state absorption contribution to the signals that we associate with back-electron transfer was observed with time constants rb = 4.0 ps for R6G/DMA (15% contribution) and τ_b = 6.9 ps for R6G/DEA (20% contribution). Subsequently, the cooling and relaxation (i.e. ground-state recovery) occurs on a time scale of τ_c = 19 ps (R6G/DMA) and τ_c = 50 ps (R6G/DEA). We attribute the τ_c to solvent-limited reequilibration on the ground-state free energy curve.