Photoinduced intramolecular energy transfer and anion sensing studies of isomeric RuIIOsII complexes derived from an asymmetric phenanthroline–terpyridine bridge

Abstract

Two heterobimetallic Ru(II)–Os(II) complexes of compositions [(bpy)₂M^II(phen-Hbzim-tpy)M′^II(tpy-PhCH₃)]⁴⁺, where M^II = Ru and M′^II = Os (4) and M^II = Os and M′^II = Ru (5), phen-Hbzim-tpy = 2-(4-(2,6-di(pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5][1,10]phenanthroline, bpy = 2,2′-bipyridine, and tpy-PhCH₃ = 4′-(4-methylphenyl)-2,2′:6′,2′′-terpyridine have been synthesized and characterized by elemental analyses, ESI mass spectrometry, and ¹H NMR and UV-vis absorption spectroscopy. The absorption spectra, redox behavior, and luminescence properties of the complexes have been thoroughly investigated and compared with that of monometallic model compounds [(bpy)₂M^II(phen-Hbzim-tpy)]²⁺ [M^II = Ru (1) and M^II = Os (2)] and [(phen-Hbzim-tpy)Ru^II(tpy-PhCH₃)]²⁺ (3). The complexes display very intense, ligand-centered absorption bands in the UV and moderately intense MLCT bands in the visible regions. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations, whereas the monometallic complexes display one-electron oxidation in the positive potential window. Steady state and time-resolved luminescence data at room temperature show that an efficient intramolecular electronic energy transfer takes place from the Ru-center to the Os-based component in both the heterometallic dyads in all the solvents. The complexes under investigation contain an imidazole NH proton which became appreciably acidic due to metal coordination and can be utilized for recognition of selective anions in solution either via hydrogen bonding interaction or by proton transfer. Accordingly, the anion binding properties of the two heterobimetallic complexes as well as parent bridging ligand, phen-Hbzim-tpy, have been studied in solutions using absorption, steady state and time-resolved luminescence spectral measurements. The metalloreceptors act as sensors for F⁻, CN⁻ and AcO⁻ ions. It is evident from sensing studies that in the presence of excess of selective anions, deprotonation of the imidazole N–H proton occurs in all cases.