Screening metal-free photocatalysts from isomorphic covalent organic frameworks for the C-3 functionalization of indoles

The visible-light-driven organic transformation using two-dimensional covalent organic frameworks (2D-COFs) as metal-free heterogeneous photocatalysts is a green and sustainable approach, and it has gained a surge of interest by virtue of the photosensitizer's high crystallinity, abundant porosity, outstanding stability, excellent light-harvesting ability and tunable structure. However, the guiding principle for designing, constructing and selecting COF-based photocatalysts has not been put forward so far. Herein, we contribute a fascinating strategy to guide the acquisition of excellent framework photocatalysts, which is to screen them from a series of isomorphic COFs. As a proof of concept, three new isomorphic pyrene-based 2D-COFs (COF-JLU23, COF-JLU24 and COF-JLU25) with variable linkers were successfully synthesized. In addition to having similar crystallinity and porosity with the same pore size and shape, their absorption, emission, bandgap, energy level, transient photocurrent response and photocatalytic activity could be easily adjusted via configuring different linkers in frameworks. Indeed, COF-JLU24 with electron donor-acceptor characteristics exhibited the best photocatalytic activity among the three isomorphic COFs for C-3 functionalization reactions of indoles, even better than that of the metal-free photocatalyst g-C3N4. More importantly, the screened COF-JLU24 as a metal-free photocatalyst still displayed extensive substrate adaptability and excellent recyclability. We anticipate that this strategy will become a robust rule of thumb for fast access to COF-based photocatalysts. In addition, we still highlight that the present study broadens the applied frontier of COF-based photocatalysts.