Thermalization process after the relaxation of electronically excited states: Intramolecular proton transfer systems studied by the transient grating method

Photophysical and thermalization processes after photoexcitation of 2-(2'-hydroxy-5'-methylphenyl)benzotriazole (HMPB) in cyclohexane and 2-hydroxybenzophenone (HBP) in various solvents were investigated by the transient grating (TG) method. From the time profiles of the population grating (PG) signals, two distinct kinetics were observed for HMPB and HBP. For HMPB, the faster (600 fs) kinetics is attributed to the back proton transfer reaction in the ground state, and the faster process of HBP (400 fs) is attributed to the vibrational cooling in the S-1 state. The slower one (similar to 30 ps) of both compounds is assigned to the vibrational cooling in the S-0 state. In ethanol (hydrogen bonding solvent), the PG signal originated from the T-1 state of HBP is apparent. However, the thermal energy from the T-1 state is negligibly small and the triplet quantum yield was found to be less than 0.05. The photoexcited HBP relaxes to the ground state by the internal conversion dominantly even in ethanol. The thermalization rates of these molecules were measured from a point of view of the translational energy of solvents by the acoustic peak delay method of the TG signal. The results show that in the early step of the thermalization, there is a very fast cooling process (less than a few ps) which is due to the energy transfer from the photoexcited solute to (several) effectively coupled solvent molecule(s), and then the heated solvent molecule becomes cool by the thermal diffusion to the bulk solvents. The thermalization processes depend on both of the solute and solvent properties. The time development of the temperature calculated based on this thermalization model explains the experimental observations. (C) 1999 American Institute of Physics. [S0021-9606(99)00723-0].