The quenching constants K and the rate constants k of the heavy-atom-induced fluorescence quenching of B-methyltcridone 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??: 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 1: is attributed to the CT interactions between the fluorophore 'Ax and the heavy-atom quencher Q involving a CT encounter complex '(A*...Q), with 'A* acting as an acceptor on account of the correlation between kq and I:.
Assuming that the formation of the encounter complex is diffusion controlled (k,=kdiff 1, the kinetics of the quenching process can be interpreted thus that the baok-reaction k_, proceeds at a much faster rate than the heavy-atom-induced spin inversion kisc. The resultant small association oonstant k,/k_, 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 with respect to kdiff. q