Synthesis and optical properties of conjugated polymers bearing a 1,8-difunctionalized carbazole unit

Abstract

Three conjugated polymers P1–P3 bearing the 1,8-difunctionalized/9-unsubstituted carbazole unit were synthesized by the Suzuki coupling polymerization technique using 9,9-dihexylfluorene-2,7-diboronic acid as the co-monomer. The functional groups at the 1,8-positions were imine (P1), 1,3-oxazoline (P2) and pyridine (P3). The conjugated polymers bearing 1,8-dipyridyl-9-methylcarbazole (P4) and 1,8-diphenylcarbazole (P0) were also prepared. Although the imine C=N bond in 3,6-dibromo-1,8-bis((phenylimino)methyl)carbazole was partially hydrolyzed under the polymerization condition, the carbazole monomer that had the sterically demanding imine group successfully gave the defect-free conjugated polymer P1. From the 1H-NMR (nuclear magnetic resonance) and infrared spectra, the intramolecular hydrogen bonding between the carbazole NH proton and the C=N moiety included in the functional group was confirmed. Compared with the conjugated polymer P0, the conjugated polymers P1 and P3 had the absorption maxima at the longer wavelength region due to the intramolecular hydrogen bonding. The redshift of the ultraviolet-visible spectrum of the conjugated polymer P4 can be attributed to the donor(carbazole)–acceptor(pyridine) interaction. The conjugated polymers P1 and P3 showed similar photoluminescence spectra, and the fluorescence quantum yields were quite low. Although the conjugated polymer complex P3–Cu with the coordinated copper(II) ion was non-fluorescent, the conjugated polymer complex P3–Zn with the zinc(II) ion produced the longer wavelength emission than the undoped polymer P3. Poly(carbazole-3,6-diyl-alt-9,9-dihexylfluorene-2,7-diyl)s bearing the functional groups such as imine, 1,3-oxazoline and pyridine at the 1,8-positions of the carbazole unit were synthesized by the Suzuki coupling polymerization to examine the structure–properties relationship. The formation of an intramolecular hydrogen bonding between the carbazole NH proton and the functional group influenced the UV–Vis and PL spectra, and generally decreased the fluorescence quantum yield. The chelate coordination of metal ions also had an impact on the optical properties.