High Pressure Effect on the Exothermic Energy Transfer from the Lowest Triplet States of Benzophenone and Triphenylene to Naphthalene in Viscous Solvents

Abstract

Exothermic energy transfer from the lowest triplet states of benzophenone (³BZP*) and triphenylene (³TPH*) to naphthalene (N) as a function of pressure in 2-butanol (BuOH) and 2, 2, 4, 4, 6, 8, 8-heptamethylnonane (HMN) with high viscosity at 0.1 MPa and 25 °C was investigated. The quenching rate constant, k_q, was significantly decreased with increasing pressure. The activation volumes for k_q at 0.1 MPa fell in 22.3 and 32.2 cm³/mol for ³BZP*/N, and 18.1 and 30.5 cm³/mol for ³TPH*/N in BuOH and HMN, respectively. The fluorescence quenching of TPH (¹TPH*) by BZP was also investigated in BuOH and the activation volume was found to be 22.1 cm³/mol. The activation volume for the solvent viscosity was calculated to be 23.7 and 42.1 cm³/mol in BuOH and HMN, respectively. The result that the activation volume for k_q is nearly equal to that for solvent viscosity, being independent of solvent polarity, implies that the quenching is diffusion-controlled even at 0.1 MPa. The pressure dependence of k_q was discussed from the solvent viscosity dependence induced by pressure, and the difference between the activation volumes for k_q and solvent viscosity was interpreted by the degree of the contribution of diffusion to the quenching. By the analysis, the observed k_q was separated into the rate constants for diffusion, k_diff, and the bimolecular rate constant for the quenching in the solvent cage, k_bim.