Ultrafast vibrational and thermal relaxation of dye molecules in solutions

Intra- and intermolecular relaxations of dye molecules are studied after the excitation to the high-lying excited states by a femtosecond laser pulse, using femtosecond time-resolved stimulated emission pumping fluorescence depletion spectroscopy (FS TR SEP FD). The biexponential decays indicate a rapid intramolecular vibrational redistribution (IVR) depopulation followed by a slower process, which was contributed by the energy transfer to the solvents and the solvation of the excited solutes. The time constants of IVR in both oxazine 750 and rhodamine 700 are at the 290-360 fs range, which are insensitive to the characters of solvents. The solvation of the excited solutes and the cooling of the hot solute molecules by collisional energy transfer to the surrounding takes place in the several picoseconds that strongly depend on the properties of solvents. The difference of Lewis basicity and states density of solvents is a possible reason to explain this solvent dependence. The more basic the solvent is, which means the more interaction between the solute and the neighboring solvent shell, the more rapid the intermolecular vibrational excess energy transfer from the solute to the surroundings and the solvation of the solutes are. The higher the states density of the solvent is, the more favorable the energy transfer between the solute and solvent molecules is.