Design and synthesis of bipyridyl-containing conjugated polymers: Effects of polymer rigidity on metal ion sensing

Three conjugated polymers comprised of 9,9-dioctylfluorene and 2,2'-bipyridine, which are alternatively linked by the C-C single bond (Pl), vinylene bond (P2), or ethynylene bond (P3), have been synthesized via the Suzuki reaction, the Wittig-Horner reaction, and the Heck reaction, respectively. The optical, electrochemical, and other physical properties of the polymers are dependent on the linkers. The polymer linked by the C-C single bond exhibits a much larger Stokes shift compared with the other two polymers, indicative of higher extended and rigid backbone conformations in the polymers linked by the vinylene and ethynylene bonds. All the three polymers are sensitive to the existence of a variety of transition metal ions due to the chelation between the 2,2'-bipyridyl moieties and the metal ions. For the metal ions which have moderate and weak coordination ability with the 2,2'-bipyridyl moieties, an obvious difference in response sensitivity is observed among the three polymers: P1 has the highest sensitivity, which is followed by P2, and P3 always exhibits the lowest sensitivity. The different sensing sensitivity is attributed to the different backbone rigidity of the three polymers, which is caused by the three different linkers. The results suggest the use of C-C single bond linker in the molecular design toward the 2,2'-bipyridyl-based conjugated polymer chemosensors for achieving higher sensing sensitivity.