Organic Radicals [Magnetic Materials]

　Ferromagnetic and ferrimagnetic properties of metal compounds are based on contributions of unpaired d- or f-electrons of transition or lanthanoid metals. On the other hand, magnetic properties of metal-free organic compounds are due to unpaired π-electrons. Normally, π-electrons are favorable to electrical conduction in material science, because they are movable by the π-conjugation effect, but they have received less attention for magnetic materials. However, diverse chemical modifications of an organic molecule have produced organic radical-based ferromagnets.^1,2) It is known that the charge transfer complex obtained from fullerene with tetrakis(dimethylamino)ethylene (TDAE) becomes an organic magnet.³⁾ In addition to using a π-conjugated organic radical, localized free radical species also provide organic magnets. 4-Nitrophenylnitronyl nitroxide (NPNN) was the first organic ferromagnet in a pure organic radical.⁴⁾ Nogami et al. reported that 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and its analogues became ferromagnets or antiferromagnets.^5,6) Furthermore, 1,1-diphenyl-2-picrylhydrazyl (DPPH) is an antiferromagnet at extremely low temperature.