Kinetic Study of the External Heavy-Atom Quenching of Fluorescence in Liquid Solution under High Pressure

Abstract

The rate constants, kq, for the fluorescence quenching of pyrene by six heavy-atom quenchers, carbon tetrabromide (CBr4), pentabromoethane (PENTBE), 1,2-tetrabromoethane (TETRBE), 1,1-dibromo-2-bromoethane (TRIBE), 1,2-dibromoethane (DIBE), and bromoethane (BE), were measured in methylcyclohexane (MCH) at pressures of up to 650 MPa and 25 °C. The plots of ln kq against pressure show a monotonical decrease for CBr4 and PENTBE, a maximum for TETRBE and TRIBE, and a monotonical increase for DIBE and BE. The activation volumes were estimated as 18.9, 12.0, −10.2, −8.0, −7.3, and −7.1 cm3/mol for CBr4, PENTBE, TETRBE, TRIBE, DIBE, and BE, respectively. For CBr4 and PENTBE, the plots of 1/kq against η, where η is solvent viscosity, were approximately linear with positive intercepts, whereas for TETRBE, TRIBE, DIBE, and BE, they deviated significantly from linearity. These results are interpreted to occur by a quenching mechanism via an encounter complex formed between the singlet state of pyrene and heavy-atom quencher molecules, which is followed by exciplex formation in the solvent cage. It was shown that the ratio kdiff/k-diff, of the rate constant for the formation (kdiff) to that for the dissociation (k-diff), of the encounter complex involved in the kinetic model increases with increasing pressure, and also that the pressure dependence of kdiff/k-diff is described satisfactorily by the radial distribution function at the closest approach distance between the solute molecules. From these facts, together with the finding that kdiff is approximately inversely proportional to the solvent viscosity, the pressure dependence of kq observed in this work was attributed to the competing rate processes of the bimolecular quenching constant, kbim, with kdiff.