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

Photophysical and thermalization processes after photoexcitation of 2-(2'-h ydroxy-5'-methylphenyl)benzotriazole (HMPB) in cyclohexane and 2-hydroxyben zophenone (HBP) in various solvents were investigated by the transient grat ing (TG) method. From the time profiles of the population grating (PG) sign als, two distinct kinetics were observed for HMPB and HBP. For HMPB, the fa ster (600 fs) kinetics is attributed to the back proton transfer reaction i n 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 stat e is negligibly small and the triplet quantum yield was found to be less th an 0.05. The photoexcited HBP relaxes to the ground state by the internal c onversion dominantly even in ethanol. The thermalization rates of these mol ecules were measured from a point of view of the translational energy of so lvents 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), a nd then the heated solvent molecule becomes cool by the thermal diffusion t o the bulk solvents. The thermalization processes depend on both of the sol ute and solvent properties. The time development of the temperature calcula ted based on this thermalization model explains the experimental observatio ns. (C) 1999 American Institute of Physics. [S0021-9606(99)00723-0].