Room-temperature Magnets Based on 1,3,5-Triazine Linked Porous Organic Radical Frameworks

Abstract

Obtaining room-temperature (RT) molecule-based magnets is a long-sought-after goal in the materials community. However, so far, most of the reported magnets based on charge-transfer salts, pure organic radicals, and coordination polymers have shown low magnetic ordering temperatures. Herein, we propose an alternative approach for magnets by using covalently linked organic radical frameworks, in which neighboring radicals are ferromagnetically coupled. Stable hexacyanotrimethylenecyclopropanide radical anions ([CN6CP]M, M = K+ [1a], n-Bu4N+ [1b]) were found to undergo either thermal polymerization in the solid state at a relatively low temperature (300°C) without the need for ZnCl2 (for 1a) or trifluoromethanesulfonic-acid-mediated polymerization at 60°C (for 1b) to give 1,3,5-triazine-linked porous organic radical framework 2 or 3, respectively. The resulting material 2 exhibited spontaneous magnetization at RT with typical hysteresis of ferromagnets, and the ordering temperature was estimated to be 465 K, whereas the magnetic behavior of 3 is more like superparamagnetism. A long-sought-after dream in the materials community is to obtain room-temperature organic and molecule-based magnets, which would show considerable advantages over traditional metal and metal-oxide-based magnets. They can have potential applications in high-density data storage, quantum computing, spin sensors, spintronics, and so on. However, so far, most efforts of using pure organic radicals, charge-transfer salts, and coordination polymers have resulted in limited success, that is, the critical magnetic ordering temperature is usually lower than the boiling point of liquid nitrogen (77 K). Herein, we report a new approach by using covalently linked porous organic radical frameworks, in which the neighboring radicals are ferromagnetically coupled with each other. The newly synthesized 1,3,5-trizaine-linked porous organic radical frameworks exhibited spontaneous magnetization at room temperature with an estimated ordering temperature above 450 K. Wu and colleagues describe the synthesis of 1,3,5-triazine-linked porous organic radical frameworks by thermal or triflic acid assisted polymerization from the cyano-containing stable radical monomers. The radicals in the polymers are ferromagnetically coupled with each other through the newly formed 1,3,5-triazine connector. As a result, these materials exhibited spontaneous magnetization or superparamagnetism at room temperature.