Poly(benzodithieno-imidazole-alt-carbazole) based π-conjugated copolymers: Highly selective and sensitive turn-off fluorescent probes for Hg2+

Abstract

In this work, highly emissive and soluble π-conjugated poly(benzodithieno-imidazole-alt-carbazole) π-conjugated copolymers (P1 and P2) have been synthesized through Pd(0)-catalyzed Suzuki coupling polymerization protocol from the corresponding monomers. For solubilizing in common organic solvents, heptyl (in P1) or 2-(2-(2-methoxyethoxy)ethoxy)ethyl (in P2) substituents have been introduced at benzodithieno-imidazole and 9-position of carbazole unit. The well-defined and soluble π-conjugated polymers have been characterized by multinuclear NMR spectra as well as by tetradetector GPC studies showing molecular weight (Mn) of 16.5 and 14.3 kDa with the polydispersity indices of 1.36 and 1.31 respectively. The emissive π-conjugated polymers show remarkable selectivity and sensitivity towards Hg2+ ion as turn-off fluorescent probes as a result of molecular wire effect, without interference by other competing metal ions. The detailed photophysical and sensing studies have been demonstrated to understand the insight of polymer-metal ion interaction which is responsible for selective fluorescence quenching of the π-conjugated polymers. DFT studies have been performed to gain the insight of the electronic properties of the sensor by determining the frontier molecular orbitals and the HOMO-LUMO energy levels for a model probe (M1). The polymeric probes (P1 and P2) are reversible in nature, and can also act as secondary sensors for S2-. The limit of detection of the polymer probes towards Hg2+ is found to be in the range of 40–50 ppb with high association constant in the order of 105 M−1 suggesting its high applicability as chemosensor for Hg2+. The polymers have also been explored as thin film polymeric sensor towards Hg2+ to demonstrate its practical utility.