Accessing Triplet State via Enhanced Intersystem Crossing in Covalent Organic Frameworks for Natural Sunlight-Driven...

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.