One-pot synthesis of conjugated vinylene-extended viologen ionic radical polyacetylenes for visible light-promoted photocatalytic CO2 cycloaddition

Abstract

In this work, we reported a facile one-pot strategy to construct conjugated vinylene-extended viologen-based ionic radical polyacetylenes (denoted as VIRPs) for visible light-promoted photocatalytic carbon dioxide (CO₂) cycloaddition under ambient conditions. As expected, the ionic polymer VIRP-1 was directly prepared in situ by the quaternization-induced spontaneous polymerization (QISP) reaction between 1,2-bis(4-pyridyl)ethylene (BPE) and propargyl bromide (PGB) in CH₃CN at 100 °C for 24 h without any catalysts and initiators. To our delight, the radical character of the ionic radical polymer VIRP-2 could be significantly enhanced by using K₂CO₃ as an alkaline reductant in the QISP reaction process, similar to the synthesis of VIRP-1, which was confirmed by electron paramagnetic resonance (EPR) spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found that the use of K₂CO₃ not only achieved the in situ one-electron reduction of vinylene-extended viologen cations to stable viologen cationic radicals, but also could remove a large proportion of Br⁻ anions and afford non-halogen anions CO₃²⁻ within VIRP-2 by anion exchange during the synthetic process. Both ionic radical polyacetylenes VIRP-1 and VIRP-2 exhibit strong and broad visible light-harvesting ability; however, VIRP-2 with much stronger radical intensity possesses better photoinduced charge separation and migration ability. Therefore, the ionic radical polymer VIRP-2 was regarded as a highly efficient and green metal-free heterogeneous photocatalyst in the visible-light-promoted catalytic CO₂ conversion with various epoxides into cyclic carbonates at room temperature and atmospheric pressure without using any cocatalysts and solvents. This work affords a new direction to construct task-specific ionic radical polymers towards photocatalytic CO₂ cycloaddition under ambient conditions.