Effect of pH on the Photophysical and Redox Properties of a Ruthenium(II) Mixed Chelate Derived from Imidazole-4,5-dicarboxylic Acid and 2,2′-Bipyridine: An Experimental and Theoretical Investigation

Combined experimental and DFT–TD-DFT computational studies were utilized to investigate the structural and electronic properties of mixed-ligand monometallic ruthenium­(II) complexes of compositions [(bpy)₂Ru­(H₂Imdc)]⁺ (<b>1</b>^<b>+</b>), its N–H deprotonated form [(bpy)₂Ru­(HImdc)] (<b>1</b>), and COOH deprotonated form [(bpy)₂Ru­(Imdc)]⁻ (<b>1</b>^<b>–</b>), where H₃Imdc = imidazole-4,5-dicarboxylic acid and bpy = 2,2′-bipyridine. The optimized geometrical parameters for the complexes computed both in the gas phase and in solution are reported and compared with the previously reported X-ray data. The influence of pH on the absorption, emission, and redox properties of [(bpy)₂Ru­(H₂Imdc)]⁺ (<b>1</b>^<b>+</b>) has been thoroughly investigated. The absorption titration data were used to determine the ground state p<i>K</i> values, whereas the luminescence data were utilized for the determination of excited state acid dissociation constants. The proton-coupled redox activity of <b>1</b>⁺ has been studied over the pH range 2–12 in acetonitrile–water (3:2). From the <i>E</i>_1/2 versus pH profile, the equilibrium constants of the variously deprotonated complex species in Ru^II and Ru^III oxidation states have been determined. As compared to the protonated complex (<b>1</b>^<b>+</b>), which undergoes reversible oxidation at 0.96 V (vs Ag/AgCl) in acetonitrile, the redox potential of the fully deprotonated complex (<b>1</b>^<b>–</b>) is shifted to a much lower value, viz., 0.52 V. Density functional theory (DFT) and time-dependent DFT (TD-DFT) study provides insight into the nature of the ground and excited states with resulting detailed assignments of the orbitals involved in absorption and emission transitions. In particular, the red-shifts of the absorption and emission bands and the cathodic shift in the oxidation potential of <b>1</b>⁺ compared to <b>1</b> and <b>1</b>^<b>–</b> are also reproduced by our calculations