Cation-induced switching on–off luminescence in an imidazole 4,5-dicarboxylate-bridged RuIIOsII bipyridine complex: A combined experimental and DFT/TD-DFT investigation

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A heterobimetallic complex of the composition [(bpy)2Ru(Imdc)Os(bpy)2](ClO4), where Imdc3−=imidazole-4,5-dicarboxylate(3−) ion and bpy=2,2′-bipyridine, was synthesized and characterized using standard analytical and spectroscopic techniques, including X-ray crystallography. Steady state and time resolved luminescence data at room temperature showed that an efficient intramolecular electronic energy transfer takes place from the Ru center to the Os based component in the dyad. The cation binding properties of the complex were thoroughly investigated in solution using UV–Vis absorption, steady state and time-resolved emission spectroscopic techniques. It is interesting to note that the luminescence intensity, quantum yield as well as the lifetime of the complex were enhanced substantially in the presence of selective cations, showing cation-induced molecular switching behavior. The effect of different cations on the rate of photoinduced intramolecular energy transfer in the dyad was also explored. Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies were utilized to investigate the structural and electronic properties of the dyad. The optimized geometrical parameters for the dyad, computed both in the gas phase and in solution, are reported and compared with the single crystal X-ray data. Computational (TD-DFT) studies also provide insight into the nature of the ground and excited states, with detailed assignments of the orbitals involved in the absorption and emission transitions. In particular, the blue-shifts of the absorption and emission bands in the presence of selective cations are reproduced by our calculations.