Covalent Organic Frameworks Enabling Site Isolation of Viologen‐Derived Electron‐Transfer Mediators for Stable Photocatalytic Hydrogen Evolution

Abstract

Electron transfer is the rate‐limiting step in photocatalytic water splitting. Viologen and its derivatives are able to act as electron‐transfer mediators (ETMs) to facilitate the rapid electron transfer from photosensitizers to active sites. Nevertheless, the electron‐transfer ability often suffers from the formation of a stable dipole structure through the coupling between cationic‐radical‐containing viologen‐derived ETMs, by which the electron‐transfer process becomes restricted. Herein, cyclic diquats, a kind of viologen‐derived ETM, are integrated into a 2,2′‐bipyridine‐based covalent organic framework (COF) through a post‐quaternization reaction. The content and distribution of embedded diquat‐ETMs are elaborately controlled, leading to the favorable site‐isolated arrangement. The resulting materials integrate the photosensitizing units and ETMs into one system, exhibiting the enhanced hydrogen evolution rate (34600 μmol h−1 g−1) and sustained performances when compared to a single‐module COF and a COF/ETM mixture. The integration strategy applied in a 2D COF platform promotes the consecutive electron transfer in photochemical processes through the multi‐component cooperation.

Ionic Covalent Organic Frameworks for Energy Devices

“…Typically, iCOFs are constructed from charged monomers or by postfunctionalization of reserved active sites on COF backbones with charged groups. [ 173,319 ] Recently, Li et al. introduced the concept of intentionally synthesizing defective COFs, and exploiting the defective sites as locations for functionalization.…”

Photocharging of Materials

“…Construction of microporous COFs that possess photoredox active moieties and interconected by low-molecular weight linkers might provide a class of materials with high δ max . [132], [133], [134] Overall, microporous structure, high ionic conductivity and highly positive VB potential are three the most important parameters to reach high δ max , R PC andR DC .…”

Promises and Challenges in Photocatalysis

“…Today, photocatalytic technology has been mainly applied in the treatment of the industrial wastewater, including papermaking, printing, dyeing and electroplating industry, etc. As for other photocatalysis research fields, such as solar water splitting (Mi et al, 2021), photocatalytic CO 2 reduction (He et al, 2019), photocatalytic CH 4 activation (Ma et al, 2021), nitrogen fixation , and photocatalytic fine chemicals synthesis (Leng et al, 2020;Tan et al, 2021), they often stuck at the proof-of-concept level.…”