Solvent and Pressure Effects on Quenching by Oxygen of 9,10-Dimethylanthracene Fluorescence in Liquid n-Alkanes:  A Study on Solvent Cage Effects

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2006-09, Vol.110 (36), p.10601-10606
• Main Authors: Okamoto, Masami, Tanaka, Fujio
• Format: Article
• Language: English
• Subjects: Alkanes - chemistry; Anthracenes - chemistry; Ethane - chemistry; Fluorescence; Oxygen - chemistry; Pressure; Propane - chemistry; Viscosity
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp062517o

The fluorescence quenching by oxygen of 9,10-dimethylanthracene (DMEA) in liquid ethane and propane at pressures up to 60 MPa and 25 °C was investigated. The apparent activation volumes for the quenching rate constant, k q, , were 5.0 ± 3.4 and 7.4 ± 1.0 cm3/mol, whereas those for the solvent viscosity, η, , were 190 ± 22 and 42 ± 1 cm3/mol in ethane and propane at 6.0 MPa, respectively. These results were discussed together with those in n-alkanes (C4−C7) and methylcyclohexane (MCH) that were previously reported, and it was found that increases monotonically but decreases rapidly with increasing the number of carbon atoms in n-alkanes. The plot of ln k q against ln η showed a leveling-off with decreasing η. These observations were analyzed satisfactorily by the pressure dependence of the solvent viscosity on k q coupled with that of the radial distribution function, g(σ), at contact with a hard sphere assumption. The apparent bimolecular rate constant, k bim,0, for the quenching in the solvent cage was evaluated by extrapolating to g(σ)η = 0 in the plot of g(σ)/k q against g(σ)η, and it was found that k bim,0 decreased with increasing the radius of the solvent molecule. From the solvent size dependence of k bim,0, the solvent cage effect was discussed phenomenologically.