The dependence of the heavy atom induced quenching of the fluorescence of n-methylacridone upon the solvent

Abstract

Abstract The quenching constants K and the rate constants k q of the heavy-atom-induced fluorescence quenching of N-methylacridone were obtained from Stern-Volmer plots for nine different solvents. Iodomethane, iodoethane, iodobutane and iodobenzene were used as quenchers. It was found that the solvent dependence of K is due to the varying fluorescence decay times τ f w of the first excited singlet state of N-methylacridone in the solvents, whereas the values of k q are not affected by the solvent, being appreciably smaller than the rate constants of diffusion, k diff . The increase of k q with decreasing ionisation potential of the quencher I Q V is attributed to the CT interactions between the fluorophore 1 A and the heavy-atom quencher Q involving a CT encounter complex 1 (A …Q), with 1 A acting as an acceptor on account of the correlation between k q and I Q V . Assuming that the formation of the encounter complex is diffusion controlled (k 1 ≈k diff ), the kinetics of the quenching process can be interpreted thus that the back-reaction k −1 proceeds at a much faster rate than the heavy-atom-induced spin inversion k isc . The resultant small association constant k 1 /k −1 of the CT encounter complex accounts for the weak CT interaction between N-methylacridone and heavy-atom quencher and the comparatively low values of k q with respect to k diff .