Anion sensing studies of luminescent bis-tridentate ruthenium(II) and osmium(II) complexes based on terpyridyl-imidazole ligand through different channels

Abstract

The anion sensing properties of homo- and heteroleptic bis-tridentate ruthenium(II) and osmium(II) complexes of compositions [(tpy-PhCH3)Ru(tpy-HImzPh3)](ClO4)2 (1), [(H2pbbzim)Ru(tpy-HImzPh3)] (ClO4)2 (2) and [M(tpy-HImzPh3)2](ClO4)2 [M=RuII (3) and OsII (4)], where tpy–PhCH3=p–methylphenyl terpyridine, H2pbbzim=2,6–bis(benzimidazole–2–yl)pyridine and tpy-HImzPh3=4′-[4-(4,5-Diphenyl-1H-imidazol-2-yl)-phenyl]-[2,2′:6′,2″]terpyridine have been studied in solution using absorption, steady state and time-resolved emission, and 1H NMR spectral measurements. It has been observed that the complexes 1, 3 and 4 act as sensors for F− and to a lesser extent for CN−, whereas 2 acts as sensor for F−, AcO−, CN−, SO42−, BF4− and to some extent for H2PO4−. Cyclic voltammetric (CV) measurements provided evidence in favor of anion concentration dependent electrochemical responses, enabling 1–4 to act as suitable electrochemical sensors for anions. It is evident from sensing studies that in the presence of excess of anions stepwise deprotonation of the imidazole N–H fragments occur in all the cases, an event which is signaled by the development of vivid colors visible with the naked eye. Time-resolved photoluminescence decays were also measured for the complexes to test their viability as a lifetime-based sensor for anions. The receptor–anion binding/equilibrium constants have also been evaluated for the metalloreceptors.