Accessing Triplet State via Enhanced Intersystem Crossing in Covalent Organic Frameworks for Natural Sunlight‐Driven Photocatalysis

Small changes in molecular structure can modify the energy landscape by altering the electronic functions of the covalent organic frameworks (COFs) toward photocatalytic reactions. Herein, porphyrin-based visible-light photosensitizers embedded in crystalline and porous COFs are showcased, offering substantial utilization of natural sunlight during photocatalysis. A detailed investigation of photophysical, photoelectrochemical, and photocatalysis reaction kinetics, along with controlled experiments, suggests that π-conjugation in COFs plays an indispensable role in highly selective and efficient photocatalysis by in situ generating singlet oxygen (¹O₂) from triplet molecular oxygen via an energy or electron-transfer mechanism. The pharmaceutically important sulfoxide precursors with various functional group tolerances were synthesized via selective oxidation under mild and environmentally friendly synthesis conditions. The metal-free COF catalyst was recycled at least five times without deteriorating the photocatalytic activity. The density functional theory calculation further reveals that efficient access to the low-energy triplet state, via enhanced intersystem crossing efficiency, relies on the molecular design of sustainable COF catalysts that influence ¹O₂ generation kinetics, high selectivity, and conversion. Sunlight-driven photocatalysis under mild conditions without requiring toxic reagents or nonrecyclable additives is an emerging strategy to access value-added chemicals in a "greener" and sustainable fashion, considering the energy efficiency and environmental safety.