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

In this work, highly emissive and soluble pi-conjugated poly(benzodithieno-imidazole-alt-carbazole) pi-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 pi-conjugated polymers have been characterized by multinuclear NMR spectra as well as by tetradetector GPC studies showing molecular weight (M-n) of 16.5 and 14.3 kDa with the polydispersity indices of 1.36 and 1.31 respectively. The emissive pi-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 pi-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 10(5) 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.