Perylene diimide covalent organic frameworks super-reductant for visible light-driven reduction of aryl halides

Abstract

In recent years, there has been a growing interest in the utilization of molecular photocatalysts in their radical ionic forms, especially as visible-light super-reductants. These catalysts exhibit remarkable capabilities in facilitating otherwise inert high-potential organic reactions, such as the reduction of aryl halides to aryl radicals. However, the development of heterogeneous super-reductants has lagged behind due to the deactivation effect caused by molecular aggregation. This study presents a novel approach to address this limitation by heterogenizing perylene diimides (PDIs) super-reductants with a consecutive photo-induced electron transfer mechanism into two-dimensional donor-acceptor (D-A) covalent organic frameworks (COFs). Both COFs, possessing D-A electronic structures and photothermal effects, demonstrated superior visible-light photocatalytic performance compared to their homogeneous counterparts. They achieved up to 99% conversion in the dehalogenation of aryl halides, primarily through a hydrogen atom trapping aryl radical mechanism. Additionally, we conducted a comparative investigation of the excited states of radical anionic D-A-type COFs and DPPDI using femtosecond transient absorption spectroscopy. Notably, the lifetimes of COFs were significantly prolonged, measuring 210 and 260 ps, respectively, compared to the 150 ps lifetime of (DPPDI•−). This study offers valuable insights into the design of efficient free radical ion-type photocatalysts, with potential applications in various chemical transformations.