TIME-RESOLVED FLUORESCENCE STUDIES OF THE INTERMOLECULAR EXCIMER FORMATION OF PYRENE IN SOLUTION UNDER HIGH-PRESSURE

All the rate constants associated with the pyrene excimer formation in six solvents were determined as a function of pressure up to 300 MPa. The radiative rate constants for the monomer increased slightly with increasing pressure, but those for the other radiative and nonradiative processes associated with the monomer and the excimer were almost independent of pressure. The activation volumes fell in the range from -3 to 0 cm3/mol. The rate constant for the excimer formation, k3, decreased monotonically with increasing pressure, as a result of the increase in the solvent viscosity. The activation volumes for k3 were correlated linearly with that for the solvent viscosity, passing through the origin. And also, a linear plot of ln k3 against ln eta, where eta-is the solvent viscosity induced by pressure, was observed in solvents examined. From these results, it was concluded that the excimer formation process is fully diffusion controlled. On the other hand, the rate constant for the dissociation of the excimer, k4, showed minimum values in nonaromatic hydrocarbons with increasing pressure. Furthermore, temperature dependence of k4 in methylcyclohexane revealed that the activation energy changed gradually from ca. 33 kJ/mol at 0.1 MPa to -37 kJ/mol at 300 MPa. These results on the step of k4 in nonaromatic hydrocarbons were interpreted by assuming the processes associated with the solvent viscosity as well as the bond-breaking processes when the excimer dissociates into two species in the solvent cage.