Photophysical Properties of Ruthenium Bipyridine (4-Carboxylic acid-4‘-methyl-2,2‘-bipyridine) Complexes and Their Acid−Base Chemistry^†

Photophysical properties such as absorption and emission spectra, lifetimes, and redox potentials of eight ruthenium complexes, Ru(LL)₂(MebpyCOOH)²⁺, where LL represents bpy, phen, Me₂bpy, Me₄bpy, (MeO)₂bpy, (EtO)₂bpy, Cl₂bpy, and NO₂phen, have been measured. The acid dissociation constants of ground and excited states have been determined. The ground-state pK_a values were obtained from the pH dependence of the complex absorbance changes. The excited-state pK_a* values were extracted from the emission titration curve and corrected for the excited-state lifetime of both protonated and deprotonated species. The largest ΔpK_a, pK_a* − pK_a, found for Ru(Me₂bpy)₂(MebpyCOOH)²⁺ and Ru(Me₄bpy)₂(MebpyCOOH)²⁺ of 1.7 indicate that MebpyCOOH gains most of the MLCT excited-state electron. The big negative ΔpK_a found for Ru(Cl₂bpy)₂(MebpyCOOH)²⁺, −4.2, clearly indicates the metal-to-ligand charge transfer to the Cl₂bpy ligands.