Radical Activity of Binary Melamine-Based Hydrogen-Bonded Self-Assemblies

Melamine cyanurate and melamine barbiturate self-assembled materials were prepared and studied by the electron paramagnetic resonance (EPR) spectroscopy, optical and scanning electron microscopy, X-ray powder diffraction (XRD). Both optical and electron microscopy show the formation of microscale crystalline particles possessing complex layered structures with a highly stable appearance. Both assemblies tend to form twinned crystals, which in the barbiturate case leads to multiple twinning in every particle. Optical microscopy shows high anisotropy and birefringence of both materials. XRD data represent a high crystallinity of melamine barbiturate and much lower for melamine cyanurate. Studied materials reveal the ability to incorporate radicals that correlate with their crystal structure quality. It is attributed to the structure-dependent stabilization of active radical superoxide species in structure voids or defects. In the example of melamine barbiturate, it is shown that the number of active paramagnetic centers increases at ca. 22% when a substance is irradiated with UV + Vis light. It reaches saturation in approximately 20 min, whereas only ca. 14% decrease was observed in a week.