PROTONATION OF THE EXCITED-STATES OF RUTHENIUM(II) COMPLEXES CONTAINING 2,2'-BIPYRIDINE, 2,2'-BIPYRAZINE, AND 2,2'-BIPYRIMIDINE LIGANDS IN AQUEOUS-SOLUTION

The apparent pK(a)* values and the rate constants for the quenching by H+ of the excited states of nine homo- and heteroleptic complexes of Ru(II) that contain bpm (2,2'-bipyrimidine), bpz (2,2-bipyrazine), and bpy (2,2'-bipyridine) as ligands were determined in aqueous solution; the excited state can be described as a one-electron reduced ligand that is coordinated to a Ru(III) center with the electron localized uniquely on the most easily reduced ligand (bpz > bpm > bpy). As is the case with the one-electron reduced bpm and bpz complexes, which exhibit pK(a) values in the range from 6.3 to 9.2, the excited states can be protonated, resulting in spectral shifts and marked lowering of the life time and emission intensity. Where luminescence from the monoprotonated excited state is observable, the lifetimes and pK(a)* values can be directly determined; in the other cases, estimates of the deprotonation rate constants can be made. The relationship pK(a) - pK(a)* = 4.8 is obtained from the data. Correlations with the values of E(red)-degrees show that protonation occurs on the uniquely reduced ligand and that the bpm and bpz ligands can be distinguished. Values of pK(a)* for Ru(bpz)32+ were determined as a function of temperature; activation energies for the protonation and deprotonation reactions, as well as thermodynamic parameters, were obtained from the data.