Radical Activity of Binary Melamine-Based Hydrogen-Bonded Self-Assemblies
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Applied Magnetic Resonance
ORIGINAL PAPER
Radical Activity of Binary Melamine‑Based Hydrogen‑Bonded Self‑Assemblies Vladimir V. Shilovskikh1 · Alexandra A. Timralieva1 · Elena V. Belogub2 · Elizaveta A. Konstantinova3 · Alexander I. Kokorin4,5 · Ekaterina V. Skorb1 Received: 12 July 2020 / Revised: 5 August 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract 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.
* Vladimir V. Shilovskikh [email protected] 1
Infochemistry Scientific Center, ITMO University, Lomonosova str. 9, Saint Petersburg 191002, Russia
2
South Urals Federal Research Center of Mineralogy and Geoecology, Urals Branch of Russian Academy of Science, Miass 456317, Russia
3
Department of Physics, M. V. Lomonosov Moscow State University, Moscow, Russia
4
N.N. Semenov Federal Research Center for Chemical Physics, Russian Academy of Sciences, Moscow, Russia
5
Plekhanov Russian University of Economics, Moscow, Russia
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1 Introduction The pronounced radical activity of substances that do not exhibit homolytic bond break at mild stress and do not produce radicals themselves is an intriguing question. Such behavior may result from several causes, such as intensive irradiation of biological samples [1], or paramagnetic probe injection [2]. Such injection of a sensitive probe into a structure is a powerful method of the local structure investigation. For example, semiconductor silicon surface bombardment with protons [3] reveals not only a number, but also a local surrounding of defects and their type. Not only that, crystals are known to stabilize radical states and significantly change their properties [4]. Electron paramagnetic resonance (EPR) studies of glasses treated with ion irradiation make it possible to identify ion surroundings and, therefore, to describe prevailing loca
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