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Temperature dependence of the rate of deactivation of triplet phenanthrene by metal ions. Evidence for energy and electron transfer

The deactivation of triplet phenanthrene by the metal ions Cu2+, Pr3+, Nd3+, Eu3+, Tb3+, Ho3+, Yb3+ has been studied over a range of temperatures in methanol + water (9/1 by volume). The temperature dependence may be subdivided into three regions. (a) For T T > 140 K the activation energies are generally small and the mechanism is ascribed to exchange-mediated energy transfer. The rate constants correlate qualitatively with Franck–Condon factors calculated from spectral overlap, although the large number of potential final states obviates a quantitative correlation. (c) For T > 200 K, the activation energy increases for some of the quenchers. This is ascribed to the onset of a rate determining step involving ligand displacement for Pr3+, Nd3+ and Ho3+, whilst an electron transfer mechanism is suggested in the case of Cu2+ and Eu3+. The rate constant increased when chloride ions were added to solutions containing Cu2+ and the effects are attributed to the formation of complexes of the type CuCl2–nn. The use of Cu(ClO4)2 facilitated measurement of the quenching rate constants for solvated copper (II) ions.