A donor–π–σ–acceptor strategy in porous polymers for suppression of carrier recombination: boosting photoredox in a Csp2–N radical cross-coupling reaction

Abstract

The σ-linkage of the D–σ–A heterojunction can effectively modulate the excitonic effect and carrier transport or recombination, thereby regulating the optical or photoredox performance. Here, two imidazolium ion-based porous polymers, namely ETTA-σ-Im and ETTA-Im, are constructed via a Buchwald–Hartwig coupling reaction of tetraaniline ethylene (ETTA) and imidazolium ions (σ-Im/Im). The strong pull–push interaction between ETTA and imidazolium ions effectively reduces exciton binding energy and promotes the generation and transfer of free charge carriers. More notably, the incorporation of a σ linker between the D–π–A heterojunction efficiently suppresses photogenerated carrier recombination of polymers. Compared with ETTA-Im, ETTA-σ-Im shows slower OCP decay kinetics and exhibits remarkable performance in photooxidative cross-dehydrogenation-coupling (CDC) amidation between aldehydes and amines, while simultaneously coupling with photoreductive H₂O₂ production. This work provides a green strategy for CDC amidation without metal catalysts or additional bases. Theoretical and experimental investigations reveal that (1) O₂ can be absorbed at the imidazolium ion sites of polymers and the photogenerated electrons are concentrated on the adsorbed O₂ and imidazoliums in the excited state; (2) direct Csp²–N amidation proceeds via a radical cross-coupling mechanism and the photosynthesis of H₂O₂ proceeds through a 2e⁻ ORR pathway involving an O₂˙⁻ intermediate on the surface of ETTA-σ-Im. Moreover, ETTA-σ-Im exhibits a notable H₂O₂ yield of 1345 μmol g⁻¹ h⁻¹ and a remarkable apparent quantum efficiency (AQE) of 6.1% at 420 nm in a two-phase water/benzyl alcohol system, nearly 1.5 times that of ETTA-Im. These results demonstrate that the D–π–σ–A strategy offers novel insights for enhancing the photoredox performance of polymers through molecular engineering.