Dual-Functional Photoredox Catalytic Thiocyanation and Hydroxylation Using a Donor-Acceptor COF

Abstract

Heterogeneous photoredox catalysis, a powerful approach for activating small molecules, is experiencing a notable resurgence due to the availability and eco-friendly benefits of natural sunlight. Most of the photoredox organic transformations are either electron or hole mediated pathways to drive the reaction with the assistance of sacrificial agents. Recently, the simultaneous use of photogenerated electrons and holes for reductive and oxidative reactions has emerged as an intriguing approach in organic photoredox reactions. In this study, we report the design and synthesis of a donor-acceptor imine based Py-Tz COF, composed of pyrene and tetrazine based building units and explored oxidative thiocyanation and reductive hydroxylation reactions. The efficient photogenerated electron-hole separation, driven by pyrene's strong electron-donating and tetrazine's electron-accepting properties, along with a low exciton binding energy (61.4 meV), facilitated both reactions efficiently as realized in the transformation of substrates having different functional groups. Py-Tz COF was further employed as a photocatalyst for CO₂ reduction, coupled with a thiocyanation reaction within a single redox cycle. By leveraging the distinct oxidation and reduction energy levels of the Py-Tz COF, oxidative thiocyanation and reductive hydroxylation were achieved in a one-pot transformation. The mechanism of each reaction was evaluated by identifying the reaction intermediates through in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), supported by the different photophysical and electron paramagnetic resonance (EPR) studies. Post-catalysis characterizations confirmed that the COF retained its crystallinity and photostability after the reactions. This report highlights the unique application of a metal-free, heterogeneous photocatalyst for two different photoredox reactions conducted in a single pot.