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) (Product No. D2295) was the first organic ferromagnet in a pure organic radical.⁴⁾ Nogami et al. reported that 2,2,6,6-tetramethylpiperidine 1-oxyl (= TEMPO) (Product No. T1560) and its analogues became ferromagnets or antiferromagnets.^5,6) Furthermore, 1,1-diphenyl-2-picrylhydrazyl (= DPPH) (Product No. D4313) is an antiferromagnet at extremely low temperature.