Organic free radicals, also known as "free radicals" in chemical terms, refer to atoms or groups with unpaired electrons formed by homolytic covalent bonds of compound molecules under external conditions such as light and heat. The organic free radicals in the magnetic coupling between unpaired electrons are one of their most typical properties, making them widely used in high-tech fields such as high storage density materials, aerospace materials, biocompatible materials, stealth materials, etc.
Figure 1. Schematic diagram of NITR free radicals
The magnetic behavior of organic radicals mainly depends on the type of close contact between adjacent molecules, that is, the crystal packing structure. Therefore, controlling the stacking of single crystals plays an important role in the successful design of molecular magnetic materials. The following three hydrogen bonds are taken as examples to illustrate the relationship between free radical structure and magnetism.
- Hydrogen-bonded phenolic hydroxyl radicals: A lot of researches have been done on the magnet structure of stable radicals with phenolic hydroxyl functional groups. The hydrogen bond itself does not provide the main electron exchange, its main function is to make the free radical spin vector enter the crystal lattice to produce intermolecular exchange. Some researchers have studied a series of nitroxide radicals of phenolic hydroxyl groups, including ortho, meta, and para derivatives and catechol derivatives. All derivatives exhibit extended hydrogen bond interaction chains or bands, Including the interaction of NO···HO and NO···CH3.
- The formation of free radical dimers through hydrogen bond heterospins: Pairing free radical units with complementary single-crystal structures is an important way to design diverse free radical magnetic materials. In this way, hetero-spin dimers (or larger aggregates) can be obtained from different spin units. Some researchers have studied the 2'-deoxyuridine functionalized with phenyl aminoxyl radical, and this system forms multiple hydrogen bond chains. Ferromagnetic exchange is caused by the intermolecular contact between the nitroxide functional group and the phenyl group in the adjacent molecule. This contact plays an important role in dispersing the electron density of the nitroxide functional group.
- Free radicals self-assembled through the hydrogen bond of benzimidazole: Benzimidazole (BIm) crystallizes under the action of a nearly linear NH···N donor-acceptor. From the direction of forming the one-dimensional chain axis, benzene rings alternately appear in a zigzag pattern. When there are substituents on the 2-position carbon and the benzene ring, this zigzag structure can provide a certain space for the substituents. Some researchers have synthesized CIBABI, Me2BABI, and BABI. These molecules are all formed based on the self-assembly of hydrogen bonds provided by BIm, and they all have tert-butyl nitroxide radicals attached to the 2-carbon. As long as the acyloxy radicals attached to BIm are not twisted, the single-electron spin will be well dispersed on the BIm ring.
Organic diradicals can be divided into localized diradicals and delocalized diradicals. Delocalized diradicals are further divided into Kekulele diradicals and non-Kekulele diradicals. In addition, non-aromatic diradical molecules can also be classified as delocalized diradicals. The research on magnetic molecular materials based on organic free radicals has attracted much attention. Among them, the most widely studied organic free radicals are indolinic aminooxy (IA), phenoxyl (PO), dithiadiazolyl (DTDA), verdazel (VER), and Nitroxide radicals including t-butyl-nitroxide (NO), nitroxyl nitroxide (NN), imino nitroxide (IN), 2,2,6,6-Tetramethylpiperidinoxy (TEMPO) and so on.
Figure 2. The chemical structure of TEMPO
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- De Novo. (2019) "Synthesis of Highly Functionalized Benzimidazolones and Benzoxazolones through an Electrochemical Dehydrogenative Cyclization Cascade." Angew. Chem. Int. Ed. 58, 9017-9021.