Femtosecond solvation dynamics determining the band shape of stimulated emission from a polar styryl dye

Abstract

Spectra of transient absorption and stimulated emission are recorded for the styryl dye DASPI, after excitation at 470 nm, with experimental resolution of 100 fs. The evolution of the S1→S0 transition energy distribution is obtained for the solvents methanol and acetonitrile at several temperatures. It is described by the dependence of the mean (first moment), width, and asymmetry (second and third central moments) of the distribution on time. The observed time‐dependence of the mean transition energy is simulated by appropriate models for the solvation dynamics. In both methanol and acetonitrile an ultrafast component is observed. Width and asymmetry change most rapidly and characteristically during this initial part of solvation. In the evolution of the higher moments, different relaxation contributions apparently are better distinguished than in the evolution of the first moment. For methanol at 50 °C, an oscillatory evolution is observed mainly in the higher moments which may indicate underdamped coherent solvent motion.