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, Me 2bpy, 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 pKa values were obtained from the pH dependence of the complex absorbance changes. The excited-state pKa* 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, pKa* - pKa, found for Ru(Me 2bpy)2(MebpyCOOH)2+ and Ru(Me 4bpy)2(MebpyCOOH)2+ of 1.7 indicate that MebpyCOOH gains most of the MLCT excited-state electron. The big negative ΔpKa found for Ru(Cl2bpy)2-(MebpyCOOH) 2+, -4.2, clearly indicates the metal-to-ligand charge transfer to the Cl2bpy ligands.
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