Quenching by Oxygen of the Lowest Singlet and Triplet States of Pyrene and the Efficiency of the Formation of Singlet Oxygen in Liquid Solution under High Pressure

Quenching by oxygen of the lowest singlet (S1) and triplet (T1) states of pyrene at pressures up to 400 MPa in liquid solution was investigated. The rate constant of the S1 state, kqS, decreased significantly with increasing pressure, while that of the T1 state, kqT, was nearly independent of pressure at the lower pressure region (<100 MPa) and decreased monotonically with further increase in pressure. The activation volume for kqS, ΔVqS⧧, at 0.1 MPa fell in the range of 12−16 cm3/mol, depending on the solvents examined, whereas that for kqT, ΔVqT⧧, was nearly zero. It was found that both the activation volumes, ΔVqS⧧ and ΔVqT⧧, are significantly smaller than those determined from the pressure dependence of the solvent viscosity, η, ΔVη⧧ (22−25 cm3/mol). For the quenching of the S1 state, the large difference between ΔVqS⧧ and ΔVη⧧ was interpreted in terms of the competition of the quenching with diffusion, and kqS was separated into the contributions of the rate constants for diffusion, kdiff, and for th...