Two in one: A Brønsted acid grafted photoactive covalent organic framework as metal-free dual photocatalyst for aerobic oxidative C-C cleavage

Covalent organic frameworks (COFs) are highly promising photocatalysts with tunable photophysical properties. So far, they have served both as a photosensitizer and as solid support. Introducing a second catalytic site in the photoactive COF scaffold to form a dual photocatalyst is the logical next step towards green photocatalytic chemistry by expanding the boundaries of visible-light-driven photoredox catalysis. Herein, we present a dual quinoline COF-based photocatalyst in which Brønsted acid sites are incorporated in a photoactive COF to form a metal-free photocatalyst for aerobic oxidative C-C bond cleavage of cycloalkanones. The Brønsted acid catalytic sites activate the substrates, while the highly conjugated COF uses visible light to create singlet oxygen for the actual C-C bond cleavage. The resulting COF-367-SO3H exhibits high catalytic activity, broad substrate adaptability, as well as outstanding recyclability and stability due to the large surface area, permanent porosity, long-range conjugation as well as an orderly arrangement of catalytic sites. This is the first report on a dual Brønsted acid / photoactive COF scaffold and is another step forward in the development of metal-free photocatalysts for organic transformations.