Review and Perspectives of β-Keto-enamine-Based Covalent Organic Framework for Photocatalytic Hydrogen Evolution

In recent years, covalent organic framework (COFs) have emerged as a new class of photocatalysts due to their structural diversity, band gaps tunability, and high porosity of their structures. Among these, β-keto-enamine COFs are an important type that benefited from their high structural stability, low-cost and convenient synthesis, intrinsic charge-separation ability, and excellent visible-light-absorption ability. The 1,3,5-triformylphloroglucinol (Tp)-based COFs are the most primary linkage in assembling β-keto-enamine COFs, also named Tp-COFs. Tp-COFs are a type of highly stable COFs, known earlier by researchers, that were considered as a milestone in the development of COF materials. The structural features of Tp-COFs provide an ideal platform for the extensive research on their photocatalytic activity. Until now, the highest photocatalytic hydrogen activity of COF materials is from Tp-COFs, and the number of reported Tp-COF-based hybrid photocatalysts is the largest in various COF-based composite photocatalysts. Thus, it is necessary to timely summarize the advances of Tp-COF photocatalysts in hydrogen evolution. Different from previous reviews summarizing a broad introduction of all types of COF-based photocatalysts, this review specializes in the advances of Tp-COFs for photocatalytic hydrogen evolution. The advantages of Tp-COFs as visible-light-driven photocatalysts are discussed here first; afterward, a brief timeline of the developments and typical examples of Tp-COF photocatalysts are provided with the aim of presenting the great achievements in this field. Finally, the challenges remaining and personal perspectives on further development of this type of material for photocatalysis are presented.