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

Abstract

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 < 140 K all the quenchers display diffusion-controlled rate constants. (b) For 200 > 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 CuCl^2–n_n. The use of Cu(ClO₄)₂ facilitated measurement of the quenching rate constants for solvated copper (II) ions.