Quenching of indole luminescence by copper ions: A distance dependence study

From the decrease in fluorescence and phosphorescence quantum yield in rigid glass matrices it was established that both excited singlet and triplet states of idole compounds are quenched by Cu2+ ions and that the interaction extends over distances that exceed 14 Å. Analyzing time-resolved phosphorescence data in terms of electron-transfer theory, we find that (1) the distance dependence of the rate is well described by an exponential law with an attenuation length of 0.55 Å; (2) electron transfer from the excited triplet state is at least 6 orders of magnitude slower than the maximum theoretical value or the value estimated for the excited singlet state. The quenching ability of copper is maintained also in the reduced state. A comparison of singlet and triplet bimolecular quenching rate constants shows that in fluid solution Cu+, relative to Cu2+, is an equally good if not a better quencher of indole emission. © 1991 American Chemical Society.