Carbazole-Benzothiadiazole Functionalized Metallacycle for Enhanced Charge Separation and Efficient Photocatalytic Cross-Dehydrogenative Coupling^†

A novel square-shaped metallacycle M, functionalized with carbazole and benzothiadiazole, was synthesized through coordination-driven self-assembly. The discrete metallacyclic architecture endows M with superior optical properties owing to its rigid metallacyclic skeleton and donor-acceptor electronic structure. The femtosecond transient absorption (fs-TA) spectroscopic measurements demonstrated that the macrocyclic skeleton significantly promotes the intramolecular charge transfer efficiency and the rapid formation of triplet states. Furthermore, leveraging M as a photocatalyst enabled to drive the cross-dehydrogenative coupling (CDC) reactions with >90% efficiency, which was facilitated by its persistent charge separation states and long-lived triplet states. This work highlights the critical role of metallacycle engineering in optimizing photophysical dynamics and advancing applications in smart optoelectronics and sustainable photocatalysis.