Synthesis, Structural Characterization, Photophysical, Electrochemical, and Anion-Sensing Studies of Luminescent Homo- and Heteroleptic Ruthenium(II) and Osmium(II) Complexes Based on Terpyridyl-imidazole Ligand

A series of hetero- and homoleptic tridentate ruthenium(II) and osmium(II) complexes of compositions [(tpy-PhCH3)Ru(tpy-HImzphen)](ClO4)(2) (1), [(H(2)pbbzim)Ru(tpy-HImzphen)](ClO4)(2) (2), and [M(tpy-HImzphen)(2)](-) (ClO4)(2) [M = Ru-II (3) and Os-II (4)] where tpy-PhCH3 = 4'-(4-methylphenyl)-2,2':6',2 ''-terpyridine, H(2)pbbzim = 2,6-bis-(benzimidazole-2-yl)pyridine and tpy-HImzphen = 2-(4-[2,2':6',2 '']terpyridine-4'-yl-phenyl)-1H-phenanthro[9,10-d]-imidazole, have been synthesized and characterized by using standard analytical and spectroscopic techniques. X-ray crystal structures of three complexes 2, 3, and 4 have been determined. The absorption spectra, redox behavior, and luminescence properties of the complexes have been thoroughly investigated. All of the complexes display moderately strong luminescence at room temperature with lifetimes in the range of 10-55 ns. The effect of solvents on the absorption and emission spectral behavior of the complexes has also been studied in detail. The anion sensing properties of all the complexes have been studied in solution using absorption, emission, and H-1 NMR spectral studies and by cyclic voltammetric (CV) measurements. It has been observed that the complexes 1, 3, and 4 act as sensors of F- only, whereas 2 acts as sensor for F-, AcO-, and to some extent for H2PO4-. It is evident that in the presence of excess of anions deprotonation of the imidazole N-H fragment(s) occurs in all cases, an event which is signaled by the development of vivid colors visible with the naked eye. The receptor-anion binding/equilibrium constants have been evaluated.