Ru(II) and Os(II) Complexes Based on Terpyridyl-Imidazole Ligand Rigidly Linked to Pyrene: Synthesis, Structure, Photophysics, Electrochemistry, and Anion-Sensing Studies

We report in this work a new family of bis-tridentate ruthenium(II) and osmium(II) complexes bearing a terpyridyl ligand rigidly link to pyrenyl-benzimidazole moiety (tpy-HImzPy = 10-(4-[2,2':6',2 ''-terpyridine]terpyridin-4'-yl-phenyl)-9H-9,11-diaza-cyclopenta[e]pyrene) along with other tridentate ligands such as 4'-(2-naphthyl)-2,2':6',2 ''-terpyridine (tpy-NaPh) and 2,6-bis(benzimidazole-2-yl)pyridine (H(2)pbbzim). All the complexes are thoroughly characterized by their elemental analysis, ESI mass spectrometry, and H-1 NMR spectroscopy. The molecular structures of two complexes [Ru(tpy-HImzPy)(2)](ClO4)(2) (3) and [(pbbzim)Ru-(tpy-HlmzPy)] (2a) in the solid state were determined by X-ray crystallography. The absorption, steady-state, and time-resolved luminescence and electrochemical properties of all the four compounds have been studied. On excitation at their MLCT bands, all four compounds exhibit moderately strong room-temperature luminescence with lifetimes ranging between 3.8 and 161.1 ns in aerated condition, whereas in the deaerated (N-2 purged) condition, the lifetimes vary between 8.2 and 199.1 ns, depending upon the nature of the solvents. The presence of imidazole N-H protons in all the complexes motivates us to study anion sensing properties of the complexes in solution through different channels. Spectrophotometeric, fluorometric, H-1 NMR spectroscopic, and cyclic voltammetric studies of the complexes in presence of anions reveal that the complexes sense principally F-, CN-, and to a lesser extent for AcO-. Multichannel anion sensing studies also indicate that anion-induced deprotonation of the imidazole N-H protons occur in all four compounds. The equilibrium constant of this deprotonation steps have been estimated from UV-vis absorption and emission titration data. Anion-induced modulation of lifetimes makes all the four complexes suitable for lifetime-based sensors for selective anions.