Structural characterization and spectroelectrochemical, anion sensing and solvent dependence photophysical studies of a bimetallic Ru(ii) complex derived from 1,3-di(1H-imidazo[4,5-f][1,10]phenanthroline-2-yl)benzene

Abstract

The X-ray crystal structure of a mixed-ligand bimetallic ruthenium(II) complex of composition [(bipy)₂Ru(H₂Impib)Ru(bipy)₂](ClO₄)₄ (1), where H₂Impib = 1,3-di(1H-imidazo[4,5-f][1,10]phenanthroline-2-yl)benzene and bipy = 2,2′-bipyridine, has been determined and showed that the compound crystallized in monoclinic form with the space group P2(1)/c. The absorption, steady state and time-resolved luminescence spectral properties of the complex were thoroughly investigated in different solvents. The compound displays strong luminescence at room temperature with lifetimes in the range of 140–470 ns, depending upon the nature of the solvent. Solvent-induced lifetime tuning makes the complex a suitable solvatochromic probe. The complex is found to undergo one simultaneous two-electron reversible oxidation in the positive potential window (0 to +1.6 V) and four quasi-reversible reductions in the negative potential window (0 to −2.2 V). Spectroelectrochemical studies have also been carried out for the bimetallic compound in the range of 300–1600 nm. With stepwise oxidation of the Ru(II) centers replacement of MLCT bands by LMCT bands occur with the development of a broad band at λ_max = 1260 nm, which is ascribed to inter-valence charge-transfer (IVCT) transition for the mixed-valence Ru^IIRu^III species. The anion sensing properties of the receptor were thoroughly investigated in acetonitrile solution using absorption, steady state and time-resolved emission spectroscopic studies. The anion sensing studies revealed that the receptor acts as sensor for F⁻, AcO⁻ and H₂PO₄⁻. It is evident that in the presence of excess F⁻ and AcO⁻ ions, deprotonation of the imidazole N–H fragments of the receptor occurs, an event which is signaled by the change of color from yellow to orange visible to the naked eye. From the absorption and emission titration studies the binding/equilibrium constants of the receptor with the anions have also been determined. Anion-induced lifetime quenching by F⁻ and AcO⁻ and enhancement by H₂PO₄⁻ makes the receptor a suitable lifetime-based sensor for selective anions. Cyclic voltammetry (CV) measurements of the compound carried out in acetonitrile have provided evidence in favor of anion-dependent electrochemical responses with F⁻ and AcO⁻ ions.