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

Abstract

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.

Temperature dependence of the nearly diffusion-controlled fluorescence quenching by oxygen of 9,10-dimethylanthracence in liquid solution

“…In our previous works,11,16 we measured the apparent activation volume, from the pressure dependence of *V q E at 25 ¡C. The values of are summarized in Table 1, k q *V q E together with those of the activation volume, deter- mined from the pressure dependence of the solvent viscosity.11, 16 The signiÐcant di †erence between and *V q E *V i E was attributed to the contribution of di †usion to the quenching.11 In agreement with the conclusion from the high pressure study, the present results from the temperature variable study suggest that the quenching is not fully but nearly di †usion-controlled.…”

“…3 and 4 show the plots of against cg/T obtained by c/k q changing the temperature at 0.1 MPa, together with the results obtained by changing the pressure at 25 ¡C. 16 It can be seen that the plots bend slightly downward, especially in MCH, and also that they are approximately on a smoothed line in n-hexane and MCH, respectively. In the previous work,9 it was found that the plot using eqn.…”

“…In the previous work,16 we examined the Ñuorescence quenching of DMEA by in n-O 2 alkanes (C4 to C7) including n-hexane, and found that aex is in good agreement with aSW in solvents with to but not in C 4 C 6 , n-heptane (aex \ 690 and aSW \ 1137). 16. Fig.…”

“…Also in Table 2, they are compared with the values of aex and obtained from the high pressure study in nk bim 0 hexane and in MCH at 25 ¡C. 16 It can be seen that the plots bend slightly downward, especially in MCH, and also that they are approximately on a smoothed line in n-hexane and MCH, respectively. The discrepancy in nheptane and MCH may be due to soluteÈsolvent interactions as well as to the shape of the solvent molecule.…”

Radiationless deactivation of singlet oxygen (1Δg) sensitized by 9-acetylanthracene in liquid and supercritical ethane: local density augmentation in the vicinity of the singlet oxygen and sensitizer molecules

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The lifetime of singlet oxygen ( 1 D g ) sensitized by apparently non-fluorescent 9-acetylanthracene (ACA) was determined at pressures up to about 35 MPa in supercritical ethane (SCF ethane;35, 45 and 55 C) as well as in liquid ethane (25 C) from the phosphorescence decay measurements. For all the systems examined, the decay rate constant (t D À1 ) increased significantly at the lower pressure region and monotonically as pressure increased further.

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