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

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.