Abstract: The effects of the rigidity of molecular recognition sites in fluorine-based conjugated polymers P1 and P2 in solution on metal ion sensing have been investigated. The structures of polymer P1 and P2 have twisted 2,2′-bipyridine and planar 1,10-phenanthroline units, respectively, which alternate with one fluorene monomer unit. It is found that absorption and emission bands of 1,10-phenanthroline-based polymer P2 exposed to metal ions can be red -shifted up to 30 nm, and emission intensity can be quenched up to 100％, depending on metal ions present, which is very similar to the behavoir of 2,2′-bipyridine-based analog P1. And P2 shows much higher sensitivity to metal ion than that of P1. The origins of ionochromic effects of 2,2′-bipyridine-based conjugated polymer due to the metal ion chelation have been attributed to both conformational changes and electron density variations on the polymer chains caused by introducing positively charged metal ions. Based on the fact that conformational changes are not required in the ion responsive process of phen ion-recognition unit, we demonstrate that the electron density variations play more important roles in metal ion induced red shifts in absorption and fluorescence quenching in photoluminescence. The higher sensitivity of P2 films to metal ions compared with P1 suggest the use of rigid units as molecular recognition sites in the fluorescent ions-sensory conjugated polymer for achieving higher sensing sensitivity, which is also significant for studying the nature of ionchromatic effect of conjugated polymer. The study present herein has, to a certain degree, elucidated the nature of metal ion and polymer interactions and demonstrated a new approach to improve the metal ions sensing properties of conjugated polymer.

Key words: Conjugated polymers, 2,2′-bipyridine, 1,10-phenanthroline, 　Fluorescence quenching, Metal ions