The contribution of diffusion to the fluorescence quenching by oxygen of 9,10-dimethylanthracene (DMEA) in liquid CO2 and supercritical CO2 (SCF CO2) at pressures up to 60 MPa was investigated. For comparison, the fluorescence quenching by CBr4 of DMEA was also investigated. The apparent activation volume of the quenching rate constant, k q, was 8 ± 1 and 10 ± 3 cm3/mol for DMEA/O2, and 42 ± 7 and 400 ± 90 cm3/mol for DMEA/CBr4 in liquid CO2 (25 °C, 10 MPa) and SCF CO2 (35 °C, 8.5 MPa), respectively. For DMEA/O2, the plots of ln k q against ln η, where η is the solvent viscosity, showed a leveling-off with decreasing the solvent viscosity, whereas for DMEA/CBr4 they were almost linear in both liquid and SCF CO2. The results, together with those of the pressure and the pressure-induced solvent viscosity dependences of k q for DMEA/O2 in n-alkanes (C4 to C7) and for DMEA/CBr4 in n-hexane, revealed that the quenching competes with diffusion. The contribution of diffusion to the quenching was analyzed on the basis of a kinetic model with solvent cage in which the quenching occurs. The bimolecular rate constant for the quenching in the solvent cage, k bim, was 6.0 × 1010 and 12 × 1010 M-1 s-1 in liquid CO2 (25 °C, 10 MPa) for DMEA/O2 and DMEA/CBr4, respectively, and 5.7 × 1010 and 12 × 1010 M-1 s-1 in SCF CO2 (35 °C, 8.5 MPa) for DMEA/O2 and DMEA/CBr4, respectively. The pressure dependence of k bim and the contribution of diffusion to the quenching are discussed.

The mechanism for the quenching by oxygen of the lowest triplet state (T1) of 9-acetylanthracene(ACA) in five saturated hydrocarbon solvents at pressures up to 400 MPa was investigated. The quenching rate constants of the T1 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 cm3/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 T1 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 (S1) 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 cm3/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 S1 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 (1Δg) formed by the quenching of the T1 state of ACA, ΦΔ, were measured, and the values of ΦΔ 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.

The mechanism for the quenching by oxygen of the singlet and triplet states of several anthracene derivatives in methylcyclohexane (MCH) under pressures of up to 700 MPa was investigated. The value for the rate constant of fluorescence quenching, k s, at 0.1 MPa is found to vary from (3.2 f 1.1) X lo9 M-' s-* for 9,10-dicyanoanthracene (DCNA) to (2.88 f 0.27) X IOy0 M-l s-I for 9-methylanthracene (MEA), whereas values for the rate constant of the triplet-state quenching process, k at 0.1 MPa are similar for each of the anthracenes, being in the range (3.0-3.8) X IO9 M-' s-I. The values for the pressure increase. A linear relation between In k," and In 7 is found for anthracene (A) and MEA, with slopes of -0.57 f 0.04 and -0.64 f 0.02, respectively. However, plots of this relation show a distinct downward curvature for 9,lO-dichloroanthracene (DCLA) and DCNA. It is also found that In kqT does not vary linearly with In 7 for any of the derivatives examined. The activation volumes of k," for A and MEA are estimated to be in the range 12-14 cm3 mol-I. These values are about 2 times larger than those determined for k T, but are only half of the value reported for the activation volume of the viscosity of MCH. The ratio of k, ' to k: for MkA and DCLA at 0.1 MPa is reasonably close to the predicted value of 1/9 and increases with pressure, reaching a value of approximately 4 9 for DCLA at 700 MPa. These results suggest that, is not conserved, may come to play an important role in the quenching of the triplet state as the pressure is increased. Dynamic aspects of the fluorescence quenching are also discussed in terms of the transient decay feature characterized by the function predicted by the Smoluchowski model. k: and k, P' decrease with increasing pressure, mainly as a result of the increase in viscosity of the solvent that accompanies in addition to '(AOz)*, encounter complexes of the form 3(AOz)* or / (A02)*, for which the total spin angular momentum IntroductionMolecular oxygen is an efficient quencher of the electronically excited states of many organic molecules. In most cases, the quenching by oxygen is so eMicient that the reaction rate is believed to be diffusion-limited. However, quenching rate constants reported in the literature vary from compound to compound, for example, 3.3 X IO'O M-' s-' for p-methoxybiphenyl and 4.4 X lo9 M-l s-I for flu0ranthene.I Despite this wide range of magnitudes,

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, k q S, decreased significantly with increasing pressure, while that of the T1 state, k q T, was nearly independent of pressure at the lower pressure region (<100 MPa) and decreased monotonically with further increase in pressure. The activation volume for k q S, ΔV q S⧧, at 0.1 MPa fell in the range of 12−16 cm3/mol, depending on the solvents examined, whereas that for k q T, ΔV q T⧧, was nearly zero. It was found that both the activation volumes, ΔV q S⧧ and ΔV q T⧧, are significantly smaller than those determined from the pressure dependence of the solvent viscosity, η, ΔV η ⧧ (22−25 cm3/mol). For the quenching of the S1 state, the large difference between ΔV q S⧧ and ΔV η ⧧ was interpreted in terms of the competition of the quenching with diffusion, and k q S was separated into the contributions of the rate constants for diffusion, k diff, and for the bimolecular quenching in the solvent cage, k S,bim. For the quenching of the T1 state, it was found that k q T/k diff increases over 1/9 and approaches 4/9 with increasing pressure. The oxygen concentration dependence on the quantum yield for the formation of singlet oxygen, ΦΔ, in methylcylohexane (MCH) was measured at pressures up to 400 MPa in order to separate ΦΔ into the contributions of the S1 and T1 states. From the results, together with those of the pressure dependence of the quantum yield of T1 state, the branching ratio for the formation of singlet oxygen in the T1 state, f Δ T, was found to decrease with increasing pressure. The oxygen quenching of the T1 state from these results was discussed by using the mechanism that involves the encounter complex pairs with singlet, triplet, and quintet spin multiplicities.