Precise Modulation of Conjugative Structure and Porosity in Covalent Organic Frameworks for Transition Metal Free Visible-Light Catalysis of Trifluoromethylation

Abstract

The precise modulation of conjugative structure and porosity parameters in photoactive covalent organic frameworks (COFs) remains challenging yet attractive in the photocatalytic field. This work presents three porous donor−acceptor COFs constructed from phenothiazines and triazines. Their photocatalytic performance and mechanism of trifluoromethylation for oxindoles and heteroaromatic compounds are studied. High trifluoromethylation conversions up to 95% are achieved under visible light (430 nm) at room temperature in the absence of any transition metal. Of interest is the observation that the photocatalytic efficiencies are remarkably improved by the modulation of π-conjugation length and position of the imine spacer between the donor−acceptor units. The reason is explained by virtue of the different light harvesting, separation capability of photogenerated electrons and holes, charge transport, and pore size in the three COFs, as reflected in their variations of fluorescence lifetimes, current intensities, distances, and impedances of charge transfer. The revealed results are helpful for an in-depth understanding of the effects of conjugative donor−acceptor structure and pore size on the photocatalytic activity of COFs. Moreover, the created catalysts show promising application in transition metal-free trifluoromethylation of heterocyclic pharmaceuticals.