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

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journal contribution
posted on 2007-06-21, 00:00 authored by Chien-Hua Su, Hsuan-Ying Chen, Kelvin Yun-Da Tsai, I-Jy Chang
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 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 ΔpKa, pKa* − pKa, 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 ΔpKa found for Ru(Cl2bpy)2(MebpyCOOH)2+, −4.2, clearly indicates the metal-to-ligand charge transfer to the Cl2bpy ligands.