Fluorescence quenching of benzo(a)pyrene by oxygen and carbon tetrabromide in n-hexane under high pressure: estimation of the diffusion coefficient of quencher

Abstract

The contribution of diffusion to the fluorescence quenching by oxygen and carbon tetrabromide (CBr 4) of benzo(a)pyrene (BZPY) in n-hexane at pressures up to 400 MPa was investigated. For both BZPY/O 2 and BZPY/CBr 4, the fluorescence quenching rate constant, k q, decreased significantly with increasing pressure. The apparent activation volume for k q, Δ V q ≠, was 10.1±0.1 and 13.2±1.0 cm 3 /mol for BZPY/O 2 and BZPY/CBr 4, respectively. It was also found that the plot of ln k q against ln η, where η is the solvent viscosity, showed significant downward curvature. From these results, it was concluded that the quenching competes with diffusion for both of the systems at the lower-pressure region. The contribution of diffusion to the quenching was analyzed on the basis of a kinetic model with solvent cage in which the quenching occurs, and the observed k q was separated into the contributions of the bimolecular rate constant in the solvent cage, k bim, and the rate constant for diffusion, k diff. By using the values of k diff thus separated, together with those of the diffusion coefficient of BZPY that was measured by other workers, the diffusion coefficients for O 2 and CBr 4 were estimated.