Boosting intramolecular charge transfer and photocatalytic hydrogen evolution activity of carbon nitride through molecular integration of benzene rings

Abstract

Regulating both the in-plane structure and interlayer forces through a molecular design strategy is an effective strategy to reinforce the spatial charge separation in nonmetallic organic polymer semiconductors but remains a difficult task. Herein, we report the substitution of triazine rings with π electron-rich benzene rings to establish an intramolecular donor–acceptor (D–A)-based g-C3N4 (CN) polymers featuring interlayer interaction. Experimental investigations and theoretical calculations have demonstrated that the D–A structure enables fixed-point electron transfer within the plane from the donor to the acceptor segments. Additionally, the interlayer driving force arising from the internal potential difference in benzene-doped CN (BDCN) can facilitate the transfer of electrons from the B-CN layer (benzene ring-doped CN layer) to the CN sublayer. Ultrafast spectroscopy has further quantitatively confirmed that the introduction of benzene rings can greatly improve in-plane and interlayer charge separation/transfer and in turn boost the photocatalytic efficiency. Moreover, extending the π-conjugated system in BDCN can also enhance the light absorption ability. Thus, the optimized 5BDCN (252.92 μmol) exhibits a 7.3-fold increase in the photocatalytic H2 evolution compared to pristine CN (34.48 μmol). In this study, a comprehensive understanding of the structure-performance relationship serves as a fundamental guideline for the rational design and synthesis of CN with an enhanced photocatalytic activity.