Visible-Light-Active CuO/Carbon Dots/g-C₃N₄ Heterojunction Triggered Click Reaction: Scope and Mechanistic Insights

Azide-alkyne cycloaddition has emerged as a key reaction in modern synthetic chemistry, enabling the rapid and efficient synthesis of a wide range of molecules with several uses across multiple disciplines. Recently, photoredox catalysis triggered by visible light has evolved as a novel activation method that can potentially act as a green alternative to the existing thermal catalytic systems. Herein, for the first time, we investigated the photocatalytic activity of ternary CuO/carbon dots/graphitic carbon nitride (CCC) heterojunctions for azide-alkyne cycloaddition (AAC) under exposure to blue LED. The minimal loading of CuO on carbon dots/g-C3N4 (CDCN) serves as an ideal heterogeneous photocatalyst for disubstituted triazole synthesis. The designed catalytic system is analyzed using various characterization techniques. The structural properties, functional groups, optical properties, surface morphology, interfacial properties, and elemental composition of the prepared CCC heterojunctions were determined using XRD, FTIR, UV-vis DRS, UV-vis spectroscopy, BET, FESEM, HRTEM, EIS, and XPS. HRTEM analysis reveals that the CDs are monodispersed and spherical, with an average diameter of 7.47 nm. The average particle size and d-spacing value of CuO in CCC are determined to be 25.72 and 0.26 nm, respectively. The existence of monodispersed carbon dots (CDs) (HRTEM) in the catalyst serves as an electron bridge, thus facilitating the dual charge transfer Z-scheme pathway. Scavenging investigations identify the generation of holes and electrons during the photocatalytic processes. In addition, blue LED-mediated photocatalytic degradation studies were conducted to assess the inhibition of reactive oxygen species to support the proposed photocatalytic mechanism. The designed catalyst shows excellent product yields of up to 92%, high turnover number (TON, 66.9) and turnover frequency (TOF, 6.7 h-1), and excellent reusability for up to six cycles. The hot filtration test reveals the retainability of heterogeneous nature and nonleaching properties of the prepared nanocomposite. In summary, our trapping investigations validate the broad aspects of the proposed plausible mechanism. Additionally, it provides deep mechanistic insights that could be beneficial for comprehending and improving visible-light blue LED-induced heterogeneous copper-catalyzed Click reaction.