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

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 (2.7 kJ mol−1) and by quantum chemistry calculations (4.7 kJ mol−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 540 nm 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 (623 cm−1) is observed when the luminescence of complex 1 is detected at 77 K in rigid matrix instead of at ambient temperature in liquid phase. This blue shift is considerably larger (1923 cm−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.