Full relaxation dynamics recovery from ultrafast fluorescence experiments by means of the stochastic model: Does the solvent response dynamics depend on the fluorophore nature?

Abstract

• The model of non-stationary charge-transfer dye fluorescence spectrum is presented. • The solvation dynamics depend on fluorophore. • The contribution of intramolecular vibrations in FLUPS is separated. • The solvent relaxation function of hexane is determined. An approach to analysing time-resolved fluorescence spectra has been developed for dyes with non-parabolic free energy curves of the excited and ground states. It incorporates the description of the solvent relaxation and redistribution/relaxation of intramolecular high-frequency vibrations proceeding in parallel with the pumping. The approach has been applied to simulate spectra of covalently linked perylene-dimethylaniline (PD) in a series of solvents of different polarity. The developed approach combined with the achieved temporal resolution of the modern fluorescence upconversion spectroscopy is shown to provide a possibility to disentangle the relaxation of high-frequency vibrations and the dynamics of the solvent. In this compound the reorganization of both intramolecular vibrations and the solvent strongly affects the absorption and fluorescence spectra so that their influence on spectral dynamics is highly entangled. The comparison of fluorescence spectral dynamics of two dyes, coumarin 153 and PD, shows that the solvation dynamics depend on the fluorophore. The fitting of simulated fluorescence spectral dynamics of PD to the experimental data enables us to determine the solvent relaxation function of hexane.