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.