Hydrogen Transfer-Driven Photocatalysis in a Hydrogen-Bonded Melamine-Barbiturate Assembly

Photogeneration of reactive oxygen species (ROS) by supramolecular assemblies based on hydrogen bonds is of considerable interest as an alternative to metal-containing photocatalysts. In this work, the mechanisms of ROS and radical generation in melamine barbiturate (MB) crystals are investigated. Using EPR spectroscopy, it was shown that UV irradiation (360 nm) induces the formation of singlet oxygen (1O2) with a steady-state concentration of (1.2 ± 0.3) × 10–14 M, while radical formation in the visible region did not exceed the experimental background within the detection limits. Quantum chemical modeling confirmed that 1O2 reacts with barbituric acid to form a hydroperoxyl radical (HOO·) and a carbon-centered radical. A new approach was developed to detect short-lived radicals generated inside the crystal lattice: a supramolecular MB assembly encapsulating the TEMPO-ol nitroxyl probe was synthesized. X-ray diffraction and EPR data confirmed the successful incorporation of the probe into the crystal structure without a change in its morphology. Kinetic experiments and calculations of thermodynamic parameters demonstrated that TEMPO-ol selectively captures mobile HOO· radicals in situ through exchange interaction, followed by thermodynamically favorable radical termination. Thus, this work not only reveals the mechanism of ROS photogeneration in MB assemblies but also proposes a new strategy for radical detection in heterogeneous environments with limited availability.