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 (H2pbbzim).
All the complexes are thoroughly characterized by their elemental
analysis, ESI mass spectrometry, and 1H NMR spectroscopy.
The molecular structures of two complexes [Ru­(tpy-HImzPy)2]­(ClO4)2 (3) and [(pbbzim)­Ru­(tpy-HImzPy)]
(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 (N2 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, 1H 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.