Linear free-enthalpy relation for radical yield in fluorescence quenching by electron transfer

Abstract

The rate constant (kq) and net electron-transfer yield (Φr) in the quenching of an excited oxonine singlet by 24 aromatic amines and methoxy compounds were determined in methanol from stationary fluorescence measurements and flash spectroscopy. The systems investigated correspond to a variation of ΔGet°, the standard free reaction enthalpy of the electron transfer from the quencher to excited singlet oxonine, between -0.12 and -1.56 eV. Whereas the kq values observed are all close to the diffusion controlled limit, Φr, increases when ΔGet° becomes more negative, whereby the following linear correlation is observed: In Φr = -5.7 - (3.2 eV-1)ΔGet°. The implications of this relationship on the mechanism of free-radical formation and electronic deactivation are discussed. Mechanisms where either a solvent-shared radical pair or a relaxed exciplex are assumed as exclusive intermediates of the quenching reaction are shown to be incompatible with the experimental result. It is suggested that in the quenching reaction radical pairs and exciplexes are formed via a nonrelaxed charge-transfer state, the branching ratio between radical pair and exciplex formation depending on ΔGet°.