Poly(3,9-carbazole)s: A Chemically Stable Extended Form of Polyaniline for Nitro-Aromatic Sensor Applications

Self-polycondensation of 3-iodocarbazole derivatives in the presence of CuI catalysts produces the corresponding 3,9-carbazole-based polymers. Unsubstituted poly(3,9-carbazole) P2 has a limited solubility, whereas poly(3,9-carbazole) P1 with a tert-butyl group at the 6-position of the carbazole is soluble in common organic solvents. These polymers are regarded as an extended form of polyaniline. However, contrary to polyaniline, they are thermally and chemically stable owing to the highly twisted structure between adjacent repeat units. In addition, P1 is highly stable when applying potentials of up to 1.50 V. Due to the high stability toward oxidation, the cast films of these polymers display well-defined visible fluorescence in their neutral states. When the films are exposed to nitro-aromatic vapors (modeled by nitro-explosives), energy transfer induced fluorescent quenching occurs. Upon exposure of the quenched films to air, the fluorescence is restored. Overall, the substituted P1 film exhibits better sensing abilities than the unsubstituted P2 film due to the bulky tert-butyl groups. Poly(3,9-carbazole) derivatives are synthesized by Cu-catalyzed self-polycondensation of 3-iodocarbazole monomers. Despite the same backbone structure as polyaniline, they are thermally and chemically robust in the neutral states and display a noticeable stability toward oxidation. In addition, the presence of tert-butyl substituents enhanced the solubility in common organic solvents and nitro-aromatic sensing properties.