High-Pressure Study on the Quenching Mechanism by Oxygen of the Lowest Triplet State in Solution

The mechanism for the quenching by oxygen of the lowest triplet state (T 1 ) of 9-acetylanthracene(ACA) in five saturated hydrocarbon solvents at pressures up to 400 MPa was investigated. The quenching rate constants of the T 1 state of ACA, k q T, decrease monotonically with increasing pressure, and the apparent activation volumes for k q T vary in the range 0.8−5.3 cm 3 /mol. It was also found that the plots of ln k q T against ln η, where η is solvent viscosity, show significant downward curvatures in all the solvents examined. From measurements by the time-resolved thermal lensing at 0.1 MPa, together with measurements of triplet−triplet absorption spectra as a function of pressure, the yields of the T 1 state of ACA were found to be approximately unity in the experimental conditions examined. The quenching rate constants, k q S, by oxygen of the lowest excited singlet state (S 1 ) of 9,10-dimethylanthracene (DMEA) whose van der Waals radius is nearly equal to that of ACA decrease strongly with increasing pressure, and the apparent activation volumes for k q S fall in the range of 9.4−14.9 cm 3 /mol. It was also found that the plots of ln k q S against −ln η are linear, with a slope of 0.59−0.71 depending on solvent. These results of k q S are consistent with our previous conclusion that the oxygen quenching of the S 1 state of DMEA is diffusion controlled. The ratio, k q T / k q S, is approximately 1/9 in methylcyclohexane but is less than 1/9 in n -butane, n -pentane, n -hexane, and n -heptane at 0.1 MPa and 25 °C, and the ratio was found to increase over 1/9 with increasing pressure in all the solvents examined. By the bleaching method of DPBF, coupled with time-resolved luminescence measurements, the yields of singlet oxygen ( 1 Δ g ) formed by the quenching of the T 1 state of ACA, Φ Δ, were measured, and the values of Φ Δ were found to be approximately unity. These results were explained by a kinetic model in which the intersystem crossings between encounter complexes with different spin multiplicities are taken into account. From the analysis based on this model, the pressure dependence of k q T / k q S is discussed.