Preparation and characterisation of the C60 charge transfer complex C60−.[1,1′,3,3′-tetramethyl-Δ2,2′-bi(imidazolidine)]+. Original Research Article

Ferromagnetism in the C60 charge transfer salt C60−.[1,1′,3,3′-tetramethyl-Δ2,2′-bi(imidazolidine)]+. (C60[TMBI]) with Curie temperatures Tc above 140 K and coercive forces Hc of about 1000 Oe were reported by Wang and Zhu (J. Phys. Chem. Solids 55 (1994) 437). This dramatic increase of Tc and Hc by a small variation of the counterion compared to the first C60 ferromagnet C60−.[tetrakis(dimethylamino)ethylene]+. (Allemand et al., Science 253 (1991) 301) triggered us to elucidate the charge transfer from TMBI to C60 by absorption measurements in the VIS/NIR. We found that the reaction kinetics is much more complex than in C60[TDAE]. Solid state investigations of C60[TMBI] with EPR and microwave conductivity are in clear disagreement with the results published by Wang and Zhu. We found instead strong evidence that C60[TMBI] is rather a diamagnetic C60 CT complex with a lot of structural defects than a high temperature molecular ferromagnet.