Luminescent bis-tridentate ruthenium(ii) and osmium(ii) complexes based on terpyridyl-imidazole ligand: synthesis, structural characterization, photophysical, electrochemical, and solvent dependence studies

Abstract

Homo- and heteroleptic bis-tridentate ruthenium(II) and osmium(II) complexes of compositions [(tpy-PhCH₃)Ru(tpy-HImzPh₃)](ClO₄)₂ (1), [(H₂pbbzim)Ru(tpy-HImzPh₃)] (ClO₄)₂ (2) and [M(tpy-HImzPh₃)₂](ClO₄)₂ [M = Ru^II (3) and Os^II (4)], where tpy-PhCH₃ = p-methylphenyl terpyridine, H₂pbbzim = 2,6-bis(benzimidazole-2-yl)pyridine and tpy-HImzPh₃ = 4′-[4-(4,5-diphenyl-1H-imidazol-2-yl)-phenyl]-[2,2′:6′,2′′]terpyridine, have been synthesized and characterized by using standard analytical and spectroscopic techniques. These compounds were designed to increase the room temperature excited-state lifetimes of bisterpyridine-type ruthenium(II) and osmium(II) complexes. The X-ray crystal structures of two homoleptic complexes 3 and 4 have been determined and show that both the compounds crystallized in orthorhombic form with space group Fddd. The photophysical and redox properties of the complexes have been thoroughly investigated. All the complexes display moderately strong luminescence at room temperature with lifetimes in the range of 6–35 ns. The complexes are found to undergo one reversible oxidation in the positive potential window (0 to +1.6 V) and one irreversible and two successive quasi-reversible reductions in the negative potential window (0 to −2.0 V). The influence of solvents on the photophysical properties of the complexes has also been investigated in detail.