Synthesis, Characterization, Photophysical, and Anion-Binding Studies of Luminescent Heteroleptic Bis-Tridentate Ruthenium(II) Complexes Based on 2,6-Bis(Benzimidazole-2-yl)Pyridine and 4'-Substituted 2,2′:6′,2" Terpyridine Derivatives

A series of heteroleptic tridentate ruthenium(II) complexes of composition [(H(2)pbbzim)Ru(tpy-X)(PF6)(2) (1-7), where H(2)pbbzim = 2,6-bis(benzimidazole-2-yl)pyridine and tpy-X = 4'-substituted tepyridine ligands with X = H, p-methyl phenyl (PhCH3), p-bromomethylphenyl (PhCH2Br), p-dibromomethylphenyl (PhCHBr2), p-cyanomethylphenyl (PhCH2CN), p-triphenylphosphonium methylphenyl bromide (PhCH2PPh3Br), and 4'-phenylformyl (PhCHO) groups, has been synthesized and characterized by using standard analytical and spectroscopic techniques. These compounds were designed to increase the excited-state lifetime of ruthenium(II) bisterpyridine-type complexes. The X-ray crystal structure of a representative compound 2, which crystallized with monoclinic space group P2(1)/c, has been determined. The absorption spectra, redox behavior, and luminescence properties of the ruthenium(II) complexes have been thoroughly investigated. All of the complexes display moderately strong luminescence at room temperature with lifetimes in the range of 10-58 ns. Correlations have been obtained for the Hammett sigma(p) parameter with their MLCT emission energies, lifetimes, redox potentials, proton NMR chemical shifts, etc. The anion binding properties of all the complexes as well as the parent ligand H(2)pbbzim have been studied in acetonitrile using absorption, emission, and H-1 NMR spectral studies, and it has been observed that the metalloreceptors act as sensors for F-, AcO-, and to some extent H2PO4-. At a relatively lower concentration of anions, a 1:1 H-bonded adduct is formed; however, in the presence of an excess of anions, stepwise deprotonation of the two benzimidazole N-H fragments occurs, an event which is signaled by the development of vivid colors visible with the naked eye. The receptor-anion binding constants have been evaluated. Cyclic voltammetric (CV) measurements carried out in acetonitrile-dimethylformamide (9:1) provided evidence in favor of anion (F-, AcO-) concentration dependent electrochemical responses, enabling 1-7 to act as suitable electrochemical sensors for F- and AcO- ions.