Optical chemosensors for metal ions in aqueous medium with polyfluorene derivatives: Sensitivity, selectivity and regeneration

Six polyfluorene derivatives, P1-P6, were synthesized and investigated as responsive materials for the optical sensing of metal ions in an aqueous medium. They were designed by combining carbazole with fluorene units within the backbone. Carbazole was N-functionalized with three coordinating groups, 2-pyridyl-benzimidazole (P1 and P4), 2-phenyl-benzimidazole (P2 and P5) and 4-phenyl-terpyridyl (P3 and P6), respectively. P1-P3 are random copolymers with fluorene:carbazole ratios of 9:1 for P1 and P2, and 9.7:0.3 for P3; P4-P6 are the corresponding alternating polymers. This design lead to polymers made of a conjugated backbone and pendant coordinating groups. The optical properties of the monomers were impacted in various ways by metal ions, and the formation of the [NiM3]2+ and [ZnM3]2+ and [ZnM32]2+ were evidenced with association constants of 105.22, 106.45 and 1014.0, respectively. The emission of the polymers was afterwards found to be influenced by theses metal ions with different sensitivity and selectivity. P1 was found to be more sensitive to the Ni2+ and Cu2+ ions with a better selectivity for Ni2+. Emission of the corresponding alternating polymer P4 was more efficiently quenched by these two ions with respect to P1, in addition of being sensitive to the Ca2+ and Al3+ ions. P3 showed sensitivity to the Ni2+, Cu2+, Al3+, Ca2+, and Zn2+ ions. The luminescence of P6 was much more pronounced with the Ni2+, Cu2+, Cd2+, Zn2+, Al3+, Fe2+, and Fe3+ ions with respect to P3. More remarkably, the presence of the Zn2+ or Cd2+ ions resulted in a new emission band, leading to the possibility to selectively sense these two ions. Relatively high Stern-Volmer constants (in the 106-105 range) were obtained, and sensitivities down to the ppb level were reached, especially for the Ni2+ ion. Influence of both the coordinating group and the polymer backbone on the polymers sensitivity and selectivity was emphasized. Finally, the recyclability of some representative optical sensors was shown both in solution and in the solid state. In particular, thin films were shown to be easily regenerated, which opens the way to the elaboration of reusable optical sensors.