The photocatalytic aerobic oxidation of organic substrates using hydrazone-based 2D-COF-1 as a heterogeneous catalyst is governed by a well-defined radical-mediated mechanism involving reactive oxygen species (ROS) generation and sequential electron transfer processes. To elucidate the underlying pathway, a series of mechanistic investigations were conducted, including radical quenching experiments, in situ electron spin resonance (ESR) spectroscopy, and quantum efficiency measurements. These studies collectively confirm that the catalytic cycle proceeds through both energy transfer (ET) and single electron transfer (SET) pathways, with molecular oxygen activation playing a central role.
Radical scavenging experiments revealed that the addition of stoichiometric amounts of TEMPO or BHT—common radical inhibitors—led to a dramatic decrease in product yields across all tested systems, indicating the involvement of radical intermediates in the transformation. Specifically, when TEMPO was introduced, the reaction was almost completely suppressed, suggesting that the formation of carbon-centered radicals is essential for the oxidation process. Similarly, the use of DABCO and benzoquinone (BQ), which are known to quench singlet oxygen and superoxide radicals respectively, resulted in significant yield reductions, further supporting the participation of both ¹O₂ and O₂⁻ in the reaction network.
In situ ESR spectroscopy provided direct evidence for the presence of these key ROS. Upon irradiation of 2D-COF-1 in MeCN under air, characteristic signals corresponding to DMPO–¹O₂ adducts and TEMP–O₂⁻ adducts were observed, confirming the generation of singlet oxygen and superoxide radical anions via photoexcited 2D-COF-1. The intensity of these signals increased over time, correlating with reaction progress and demonstrating the sustained production of ROS during illumination. This data strongly supports a dual mechanism: one where the excited state of 2D-COF-1 transfers energy to ground-state O₂ to form ¹O₂ (Type II pathway), and another where it donates an electron to O₂ to generate O₂⁻ (Type I pathway).
Additional experiments with CuCl₂ and FeSO₄, which are known to interfere with SET processes, led to reduced yields, indicating that electron transfer from the substrate to the photocatalyst is crucial. The suppression of the reaction under these conditions suggests that the formation of a radical cation intermediate is a key step. Moreover, the absence of autocatalytic behavior and the relatively low but consistent apparent quantum efficiencies (A.Q.E. values between 11.3% and 19.6%) rule out a radical chain propagation mechanism, reinforcing the idea of a controlled photocatalytic cycle initiated by discrete photoactivation events.1404-90-6 medchemexpress
Based on these findings, a plausible reaction mechanism is proposed.CD62L Antibody Formula Upon visible-light excitation, 2D-COF-1 enters its excited state, which either transfers energy to O₂ to produce ¹O₂ or donates an electron to form O₂⁻.PMID:35246690 The substrate then undergoes single electron transfer (SET) with the oxidized photocatalyst (2D-COF-1⁺), forming a radical cation intermediate. This intermediate is subsequently attacked by ¹O₂ or O₂⁻, leading to the formation of the final oxidation product through a series of rapid steps. In some cases, direct reaction with ¹O₂ may also occur without prior SET, depending on the substrate’s redox potential and reactivity.
This comprehensive mechanistic analysis not only validates the role of 2D-COF-1 as an efficient oxygen activator but also highlights the importance of balancing redox properties, light absorption, and charge separation in designing high-performance COF-based photocatalysts. The ability to selectively generate and utilize multiple ROS types within a single catalytic system offers new opportunities for developing sustainable oxidative transformations with enhanced selectivity and efficiency.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
