New dinuclear Ru(ii) complexes containing free chelating polypyridine sites within the bridging ligands: absorption spectra, luminescence properties, redox behavior and sensing properties

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Abstract
A series of dinuclear Ru(II) polypyridine complexes have been prepared and their absorption spectra and luminescence properties (both at room temperature in acetonitrile fluid solution and at 77 K in butyronitrile rigid matrix) have been investigated. The species studied are [(bpy)2Ru(L1)Ru(bpy)2]4+ (1; bpy = 2,2′-bipyridine), [(tpy)Ru(Ln)Ru(tpy)]4+ (2, Ln = L2; 3, Ln = L3; 4, Ln = L4; tpy = 2,2′:6′,2″-terpyridine; for L1–L4 bridging ligands, see Fig. 1). All the compounds exhibit intense absorption bands in the UV region, assigned to spin-allowed ligand-centered (LC) transitions, and moderately intense spin-allowed metal-to-ligand charge-transfer (MLCT) absorption bands in the visible. The compounds also exhibit relatively intense emissions, originating from triplet MLCT levels, both at 77 K and at room temperature. All the new compounds contain a free chelating bipyridine site within their bridging ligand structure, and this confers to the new species interesting properties as far as the effect of perturbation (e.g., addition of acid or zinc salts) on the absorption and luminescence properties is concerned. Indeed, the luminescence intensity of each species is strongly affected by the presence of protons or cations. In particular, upon acid or zinc salts addition the luminescence intensity of 1 decreases, while the luminescence intensity of 2–4 increases. This different behaviour is related to the different dominating pathways for MLCT excited-state decay in Ru(II) chromophores containing tridentate or bidentate polypyridine ligands. The redox behavior of 1 and 2 has also been investigated in acetonitrile solution in the absence and presence of zinc salts. It has been found that the electronic interaction between the peripheral chromophores is enhanced by zinc coordination.

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