Pyrene and imidazole functionalized luminescent bimetallic Ru(ii) terpyridine complexes as efficient optical chemosensors for cyanide in aqueous, organic and solid media

Abstract

We report in this work the anion recognition and sensing aspect of a new family of bimetallic Ru(II) complexes derived from a symmetrical bridging 5,11-bis(4-([2,2′:6′,2′′-terpyridine]-4′-yl)phenyl)-4,12-dihydropyreno[4,5-d:9,10-d‘]diimidazole (tpy-H₂PhImzPy-tpy) terpyridine ligand in solution as well as in the solid sate through different channels such as absorption, steady state and time-resolved emission, and ¹H NMR spectroscopic techniques. Interestingly, the complexes exhibit luminescence in the red region with moderately long lifetimes compared with the related terpyridine complexes of Ru(II). In DMSO, complexes 1 and 2 act as sensors for F⁻ and to a lesser extent for AcO⁻, CN⁻ and H₂PO₄⁻, whereas 3 acts as a sensor for F⁻, AcO⁻, CN⁻ and to some extent for H₂PO₄⁻. In contrast to DMSO, all the complexes exhibit very high selectivity towards cyanide ions in the presence of an excess of other anions in aqueous medium. The complexes display visual detection of cyanide with the detection limit lying in the range of 1.01 × 10⁻⁷ to 9.79 × 10⁻⁸ M. Equilibrium constants for the interaction of the complexes with the anions were evaluated from absorption and emission titration profiles and were found to lie in six orders of magnitude. It is observed that the excited-state lifetimes of the complexes were modulated to a significant extent by the selected anions in all the three media proving the utility of such complexes to act as lifetime-based sensors for anions. The fact that all the complexes can selectively sense cyanide in the presence of other anions with their detection limits lying in the range of 10⁻⁷ M–10⁻⁸ M in aqueous solution is particularly important for their practical applicability. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) studies were performed to understand the nature of the ground and excited states of the complexes with detailed assignments of the orbitals involved in absorption transitions. In particular, the red-shifts of the absorption and emission bands in the presence of selective anions have been well reproduced by computations.