DEPENDENCE OF THE HEAVY-ATOM INDUCED QUENCHING OF THE FLUORESCENCE OF N-METHYLACRIDONE UPON THE SOLVENT

The quenching constants K and the rate constants kq 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 τfw of the first excited singlet state of N-methylacridone in the solvents, whereas the values of kq are not affected by the solvent, being appreciably smaller than the rate constants of diffusion, kdiff. The increase of kq with decreasing ionisation potential of the quencher IQV is attributed to the CT interactions between the fluorophore 1A{A figure is presented} and the heavy-atom quencher Q involving a CT encounter complex 1(A{A figure is presented} ...Q), with 1A{A figure is presented} acting as an acceptor on account of the correlation between kq and IQV. Assuming that the formation of the encounter complex is diffusion controlled (k1≈kdiff), 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 kisc. The resultant small association constant k1/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 kq with respect to kdiff. © 1979.