Fluorescence quenching of aromatic molecules by SCN− in acetonitrile: effect of molecular size on electron transfer fluorescence quenching

Rate constants of fluorescence quenching by SCN− and efficiencies of enhanced intersystem crossing and free radical generation in fluorescence quenching were measured for several aromatic fluorescencers in acetonitrile. The results indicate that the quenching mechanism depends on δG and the switchover of quenching mechanism from exciplex formation to long-distance electron transfer occurs at δG ≈ −1.1 eV. The large shift of the switchover δG from −0.4 eV for aromatic quenchers to −1.1 eV for SCN− can be accounted for the difference in molecular size between aromatic quencher and SCN−.