Ground and excited state dynamics of new dinuclear ruthenium complexes: NMR, UV–Vis, IR, electrochemical, photophysical characterization, and theoretical study of Ru(bpy)2(μ-dpp)Ru(CN–X)4n+ complexes

Abstract

Two novel binuclear complexes of ruthenium(II) have been synthesized and characterized by various spectroscopic (NMR, IR, UV–Vis absorption and emission) and electrochemical methods. Extensive and detailed NMR studies of [(bpy)2Ru(μ-dpp)Ru(CN)4] 1 and [(bpy)2Ru(μ-dpp)Ru(CNCH3)4]4+2 complexes have revealed a dynamic equilibrium between the stereo isomers of the ground-state complexes. Small energy difference between the two isomers of 1 has been estimated using NMR data (2.7kJmol−1) and by quantum chemistry calculations (4.7kJmol−1). The relatively broad lowest energy absorption band appearing in the visible range is assigned as overlapping metal-to-ligand charge-transfer (MLCT) dπ(Ru)A→π∗(μ-dpp) (MLCT1A) and dπ(Ru)B→π∗(μ-dpp) (MLCT1B) transitions. The MLCT bands of complex 1 are solvatochromic (e.g. the lowest energy MLCT1 band shifts from 506 to 540nm upon changing the solvent from water to MeOH), while the same band of the complex 2 is not sensitive to the solvent nature. The luminescence properties of the excited 1 and 2 complexes are significantly different: (i) a rather small blue shift of the emission band (623cm−1) is observed when the luminescence of complex 1 is detected at 77K in rigid matrix instead of at ambient temperature in liquid phase. This blue shift is considerably larger (1923cm−1) in the case of complex 2. (ii) Time resolved luminescence studies have revealed that the 3MLCT1A and 3MLCT1B excited states of complex 1 decay independently. On the other hand a dynamic equilibration occurs between two triplet excited states of complex 2. A slight interaction between the metal centers of complex 1 has been assumed by considering the electrochemical data, while a stronger coupling of the ruthenium atoms in complex 2 has been concluded by comparison of the oxidation potentials of the complex 2 with that of other binuclear ruthenium complexes of the metal centers possessing different chemical environment.