Synthesis, Structural Characterization, Photophysical, Electrochemical, and Anion-Sensing Studies of Luminescent Homo- and Heteroleptic Ruthenium(II) and Osmium(II) Complexes Based on Terpyridyl-imidazole Ligand

A series of hetero- and homoleptic tridentate ruthenium­(II)
and
osmium­(II) complexes of compositions [(tpy-PhCH3)­Ru­(tpy-HImzphen)]­(ClO4)2 (1), [(H2pbbzim)­Ru­(tpy-HImzphen)]­(ClO4)2 (2), and [M­(tpy-HImzphen)2]­(ClO4)2 [M = RuII (3) and OsII (4)], where tpy-PhCH3 = 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine,
H2pbbzim = 2,6-bis­(benzimidazole-2-yl)­pyridine and tpy-HImzphen
= 2-(4-[2,2′:6′,2″]­terpyridine-4′-yl-phenyl)-1H-phenanthro­[9,10-d]­imidazole, have been
synthesized and characterized by using standard analytical and spectroscopic
techniques. X-ray crystal structures of three complexes 2, 3, and 4 have been determined. The absorption
spectra, redox behavior, and luminescence properties of the complexes
have been thoroughly investigated. All of the complexes display moderately
strong luminescence at room temperature with lifetimes in the range
of 10–55 ns. The effect of solvents on the absorption and emission
spectral behavior of the complexes has also been studied in detail.
The anion sensing properties of all the complexes have been studied
in solution using absorption, emission, and 1H NMR spectral
studies and by cyclic voltammetric (CV) measurements. It has been
observed that the complexes 1, 3, and 4 act as sensors for F–only, whereas 2 acts as sensor for F–, AcO–, and to some extent for H2PO4–. It is evident that in the presence of excess of anions deprotonation
of the imidazole N–H fragment­(s) occurs in all cases, an event
which is signaled by the development of vivid colors visible with
the naked eye. The receptor–anion binding/equilibrium constants
have been evaluated.