Design, synthesis and characterization of triphenylamine-based conjugated porous polymers as fluorescent receptors for nitroaromatic derivatives

Abstract

Two series of hyperbranched conjugated polymers were synthesized via straightforward oxidative coupling polymerization, starting from star-shaped triphenylamine compounds with different internal π-spacers, each varying through the electron-withdrawing properties and rigidity. Excellent total reaction yields were achieved, exceeding 80 % under emulsion reaction conditions, particularly when employing compounds with high electron-withdrawing character (-CN). The polymers containing p-phenylene-vinylene and mixed p-phenylene (vinylene)-ethynylene structures were obtained predominantly as insoluble powders, mainly due to their fully aromatic backbone structure or highly cross-linked formed networks, and their chemical structures were validated by FT-IR analysis. Optical studies revealed that polymers synthesized under classical reaction conditions exhibited enhanced π-conjugation compared to those obtained via emulsion oxidative conditions, which was clearly reflected in a red-shift of the absorption and emission maxima. Conversely, the use of surfactants along with planar and rigid triple bonds facilitated the formation of more porous polymer networks, with the highest specific surface areas of 492 m2g-1, compared to analogous networks based on vinylene hyperbranched polymers (99 m2g-1) and (cyano)vinylene hyperbranched polymers (44 m2g-1). The fluorescence emission of these polymers as solid dispersions in chloroform, although of low intensity for the CN-containing polymers, enables them to detect the nitro-aromatic derivatives via fluorescence quenching. The dependence of both detection limit and emission quenching efficiency on the polymers backbone structure and employed synthesis method was obvious. Notably, the Stern-Volmer quenching constant (KSV) was highest for TNP, reaching 17.9 × 103 M-1 in the case of p-phenylene-vinylene polymer synthesized under emulsion reaction conditions, indicating promising avenues for the development of solid-state fluorescence sensors.