Rational design of a phenothiazine-based donor-acceptor covalent organic framework for enhanced photocatalytic oxidative coupling of amines and cyclization of thioamides

Abstract

Owing to the high structural designability and functional tunability, covalent organic frameworks (COFs) have emerged as a promising platform for designing excellent photocatalyst candidates. However, their photocatalytic performances are still hindered by insufficient separation and transfer of photo-generated charge carriers. The ordered alignment of the electron donor (D) and acceptor (A) in two-dimensional COFs can promote charge carrier separation upon photoexcitation and provide a favorable pathway for exciton transport, and thus is advantageous for photocatalysis. In this work, we report a new D-A COF that was constructed from the electron-rich phenothiazine (PTZ) and electron-deficient triazine (TTA) subunits, giving rise to a segregated bicontinuous D-A heterostructure within the framework. In comparison to the triphenylamine (TPA) based analogue COF with smaller D-A contrast, this newly designed PTZ-TTA-COF exhibited lower exciton binding energy and enhanced charge separation/transfer. As a consequence, it revealed remarkably enhanced photocatalytic ability in oxidative amine coupling and cyclization of thioamide to 1,2,4-thiadiazole reactions under visible light and in air. This study will provide a rational guidance for developing high-performance polymeric photocatalysts based on D-A structural COFs through the molecular-level design strategy.