Photocatalytic Performance Enhancement Modulated by Skeleton π-Conjugation Extension of Imide-Linked Covalent Organic Frameworks

Conjugated covalent organic frameworks (COFs) with excellent semiconductor properties and photocatalytic activity can serve as fascinating photocatalysts. The rational design and systematic exploration of the influence of π-conjugation extension of COFs are appealing and quite significant for developing high-performance COF photocatalysts. In this work, three imide-linked COFs (TAPT-PyrD, TAPT-NapD, and TAPT-PerD) and comparative TAPT-PhtA COF of an imine linkage with analogous topological structures and gradually increased π-conjugation extension (a phenyl core for PyrD and PhtA, naphthalene for NapD, and perylene for PerD, with the same 1,3,5-tris(4-aminophenyl)triazine (TAPT) triamine monomer) have been synthesized and employed as photocatalysts to systematically explore the relationship between photocatalytic activity and the degree of π-conjugation extension. The introduction of electron-withdrawing imide linkages into COFs facilitates charge separation and transfer after photoexcitation. The TAPT-NapD COF with moderate π-conjugation extension reveals significantly enhanced photocatalytic performance compared to that of TAPT-PyrD COF. However, the TAPT-PerD COF with the largest π-conjugation structure manifests drastically decreasing photocatalytic efficiency because recombination becomes dominant at excessive degrees of conjugation, though it was remarkedly improved with addition of a hole sacrificial agent. Density functional theory (DFT) calculations demonstrate that the increased degree of π-conjugation extension indeed boosts the light-harvesting activity and photogenerated exciton dissociation, while the significantly deteriorated photocatalytic conversion of the TAPT-PerD COF with excessive π-conjugation extension, without considering its imperfection with a lower crystallinity, results probably from the self-quenching of local excitons and the hindered electron and hole transfer. Our findings may provide a certain reference value for the rational design of COF photocatalysts via regulation of the degree of π-conjugation extension.