Quenching of tris (2,2′-bipyridine) chromium (III) and tris(1,10-phenanthroline)chromium (III) excited states by phenols: Temperature and pH effect

Abstract

The quenching rate constant k q of the excited states *Cr(bpy) 3 3+ and *Cr(phen) 3 3+ by several substituted phenols was investigated as a function of temperature using the time resolved luminescence quenching technique. The activation parameters showed a pH dependence. The most important contribution to the total activation entropy in basic media was found to be the electrostatic factor. The data obtained from log k q vs. Δ G 0 plot were analyzed on the basis of the current theories for electron transfer processes. The best fitting analysis in the Δ G # vs. Δ G 0 plot yielded reorganization energies of 59 and 55 kcal/mol for the bipyridine and phenanthroline chromium complexes, respectively. The high values of reorganization energies found ( λ) were interpreted as a hydrogen bond term contribution ( λ H) to the outer-sphere reorganization energy, λ o′, where λ o′ = λ o+ λ H. The effect of the ionic strength and [Cl −] on the quenching rate constant for the reaction of *Cr(phen) 3 3+ with 2,6-dimethylphenol was investigated. It was found that neither the ionic pair formation nor the [Cl −] concentration seem to affect the quenching mechanism.