Porous Organic Polymers (POPs) Based on Acridone as Heterogeneous Photocatalysts for Oxidative Dimerization and Cyclocondensation Reactions

Abstract

Porous organic polymers (POPs) are emergent tailor‐made materials, which can be accessed by covalent polymerization of diligently designed molecular building blocks in a bottom‐up fashion. Acridone, a well‐known chromophore that undergoes intersystem crossing (ISC) with near‐unity quantum yield akin to benzophenone, was structurally elaborated into a building block and subjected to Friedel‐Crafts polyalkylation to afford a series of POPs, namely, Ac‐MePOP, Ac‐OMePOP, and Ac‐CBPOP. These POPs exhibit remarkable porosity and significant absorption in the visible region. Of the three polymers, Ac‐CBPOP with the highest BET surface area of ca. 1027 m2/g is shown to serve as an excellent heterogeneous photocatalyst for visible light‐mediated oxidative transformations, that is, oxidative coupling of benzylamines to imines and cyclocondensation of o‐phenylenediamines with aldehydes to benzimidazoles under oxygen atmosphere. It is further shown that Ac‐CBPOP can be reused for several reaction cycles without any loss of catalytic activity with its stability intact. Mechanistic studies show that 1O2 serves as the key reactive oxygen species, formed by the energy transfer from Ac‐CBPOP to 3O2, in the photocatalytic oxidation reactions. The results demonstrate the development of stable photocatalytic POP materials with tailor‐made properties by a bottom‐up covalent polymerization of programmed molecular building blocks.