Pressure effect on the lifetime of singlet oxygen in solutions

Lifetimes of singlet oxygen ({sup 1}O{sub 2}; {sup 1}{Delta}{sub g}) were measured in four solvents as a function of pressure in the range of 0.1-400 MPa at 25{degree}C. It was found that the lifetimes decrease monotonically with increasing pressure in solvents examined. The activation volumes for the quenching by solvent fall in the range of {minus}4 to {minus}9 cm{sup 3}/mol depending on solvent. The magnitude of which was well interpreted by taking into account the pressure dependence on the collision frequency estimated by the hard sphere model. From the analysis, it was concluded that the influence of pressure on the lifetime is attributed not only to the increase of collision frequency due to the increased solvent concentration by pressure but also to the extra increase of collision frequency characteristic to a given solute-solvent pair depending on the packing fraction and the size of molecules.