Synthesis, Structural Characterization, and Photophysical, Spectroelectrochemical, and Anion-Sensing Studies of Heteroleptic Ruthenium(II) Complexes Derived from 4′-Polyaromatic-Substituted Terpyridine Derivatives and 2,6-Bis(benzimidazol-2-yl)pyridine

Heteroleptic bis-tridentate
ruthenium­(II) complexes of composition
[(H2pbbzim)­Ru­(tpy-Ar)]­(ClO4)2, where
H2pbbzim = 2,6-bis­(benzimidazol-2-yl)­pyridine and tpy-Ar
= 4′-substituted terpyridine ligands with Ar = phenyl (2), 2-naphthyl (3), 9-anthryl (4), and 1-pyrenyl (5) groups, have been synthesized and
characterized by using standard analytical and spectroscopic techniques.
The X-ray crystal structures of the complexes [(H2pbbzim)­Ru­(tpy-Naph)]­(ClO4)2 (3), [(pbbzim)­Ru­(tpy-Naph)]·(CH3)2CO·H2O (3a), and
[(H2pbbzim)­Ru­(tpy-Py)]­(ClO4)2 (5) have been determined. The absorption, steady-state, and
time-resolved luminescence spectral properties of the complexes were
thoroughly investigated in dichloromethane. The compounds display
strong luminescence at room temperature with lifetimes (τ2) in the range of 5.5–62 ns, depending upon the nature
of the polycyclic aromatic moiety as well as the solvents. The complexes
are found to undergo one reversible oxidation in the positive potential
window (0 to +1.5 V) and four successive quasi-reversible reductions
in the negative potential window (0 to −2.4 V). The anion-sensing
properties of the receptors were thoroughly investigated in acetonitrile/dichloromethane
(1/9 v/v) solutions (2 × 10–5 M) using absorption,
steady-state, and time-resolved emission spectroscopic studies. 1H NMR titration experiments, on the other hand, were carried
out in either CD3CN or DMSO-d6. The anion-sensing studies revealed that the receptors act as sensors
for F–, CN–, AcO–, and SO42‑ and to some extent for HSO4– and H2PO4–. It is evident that, in the presence of excess anions, deprotonation
of the imidazole N–H fragments of the receptors occurs, which
is signaled by the change of color from yellow-orange to violet visible
with the naked eye. From the absorption and emission titration studies
the binding/equilibrium constants of the receptors with the anions
have also been determined. Anion-induced lifetime quenching and/or
enhancement make the receptors suitable lifetime-based sensors for
selective anions. Cyclic voltammetric (CV) measurements of the compounds
carried out in acetonitrile have provided evidence in favor of anion-dependent
electrochemical responses with F– and AcO– ions. Spectroelectrochemical studies have also been carried out
for both the protonated and deprotonated forms of the complexes in
the range of 300–1200 nm. With successive oxidation of the
Ru­(II) center, replacement of MLCT bands by LMCT bands occurs gradually
with observation of sharp isosbestic points in all cases.