Anomalous Hammett's plot in the quenching of Ru(bpy)32+ phosphorescence by p-substituted phenols

Tris(2,2'-bipyridine) Ru(II) (Ru(bpy)(3)(2+)) phosphorescence is readily quenched by p-substituted phenols in a bimolecular process whose rate, in aqueous solution, is determined by the pH of the solution and the electron-affinity of the substituent. A Hammett's type plot based on data obtained at pH 10.4 in terms of sigma parameters conforms to a parabola with a minimum rate for phenol and two branches, one comprising compounds with negative sigma values (electron donors) and other with positive sigma values (electron acceptors groups). This anomalous behaviour takes place at a constant pH and it can be explained in terms of two quenching mechanisms: a simple electron transfer from the phenolate to the Ru(II) complex when pKa << pH, and a PCET (proton coupled electron transfer) mechanism for compounds where the protonated phenol predominates (pKa > pH). (C) 2016 Elsevier B.V. All rights reserved.