Synthesis, Structural Characterization, and Photophysical, Spectroelectrochemical, and Anion-Sensing Studies of Heteroleptic Ruthenium(II) Complexes Derived from 4′-Polyaromatic-Substituted Terpyridine Derivatives and 2,6-Bis(benzimidazol-2-yl)pyridine

Abstract

Heteroleptic bis-tridentate ruthenium(II) complexes of composition [(H2pbbzim)Ru(tpy-Ar)](ClO4)2, where H2pbbzim = 2,6-bis(benzimidazol-2-yl)pyridine and tpy-Ar = 4'-substituted terpyridine ligands with Ar = phenyl (2), 2-naphthyl (3), 9-anthryl (4), and 1-pyrenyl (5) groups, have been synthesized and characterized by using standard analytical and spectroscopic techniques. The X-ray crystal structures of the complexes [(H2pbbzim)Ru(tpy-Naph)](ClO4)2 (3), [(pbbzim)Ru(tpy-Naph)]·(CH3)2CO·H2O (3a), and [(H2pbbzim)Ru(tpy-Py)](ClO4)2 (5) have been determined. The absorption, steady-state, and time-resolved luminescence spectral properties of the complexes were thoroughly investigated in dichloromethane. The compounds display strong luminescence at room temperature with lifetimes (τ2) in the range of 5.5-62 ns, depending upon the nature of the polycyclic aromatic moiety as well as the solvents. The complexes are found to undergo one reversible oxidation in the positive potential window (0 to +1.5 V) and four successive quasi-reversible reductions in the negative potential window (0 to -2.4 V). The anion-sensing properties of the receptors were thoroughly investigated in acetonitrile/dichloromethane (1/9 v/v) solutions (2 × 10(-5) M) using absorption, steady-state, and time-resolved emission spectroscopic studies. (1)H NMR titration experiments, on the other hand, were carried out in either CD3CN or DMSO-d6. The anion-sensing studies revealed that the receptors act as sensors for F(-), CN(-), AcO(-), and SO4(2-) and to some extent for HSO4(-) and H2PO4(-). It is evident that, in the presence of excess anions, deprotonation of the imidazole N-H fragments of the receptors occurs, which is signaled by the change of color from yellow-orange to violet visible with the naked eye. From the absorption and emission titration studies the binding/equilibrium constants of the receptors with the anions have also been determined. Anion-induced lifetime quenching and/or enhancement make the receptors suitable lifetime-based sensors for selective anions. Cyclic voltammetric (CV) measurements of the compounds carried out in acetonitrile have provided evidence in favor of anion-dependent electrochemical responses with F(-) and AcO(-) ions. Spectroelectrochemical studies have also been carried out for both the protonated and deprotonated forms of the complexes in the range of 300-1200 nm. With successive oxidation of the Ru(II) center, replacement of MLCT bands by LMCT bands occurs gradually with observation of sharp isosbestic points in all cases.