MAGNETIC MOLECULES

摘要

Molecular magnetism has had a rapid increase in the last twenty- twenty five years. The original goal, which marks the transition from magnetochemistry, is that of using the techniques of molecular chemistry in order to design and synthesise new classes of magnetic materials, which are based on molecular lattices, rather than the continuous lattices of classical magnets. In a sense this development parallels that of molecular conductors, with the additional difficulty of the intrinsic instability of organic materials with unpaired electrons. In fact at the beginning the major difficulty was that of showing that indeed purely organic magnetism is indeed possible. In order to understand this point one has to recall that all the classical magnets are based on the presence of unpaired d or/electrons, while no genuine magnet based on s or p electrons was known up to the beginning of the nineties. Things started to change when the pioneering work of Itoh and Iwamura showed that indeed strong ferromagnetic coupling can be realised in polycarbenes, of which an example is sketched below. The two unpaired electrons on each centre are in orthogonal orbitals, therefore they are in a S= 1 state. The disposition in meta around the aromatic rings results in quasi degenerat molecular orbitals, with strong fereromagnetic coupling. Later Kinoshita reported convincing evidence for the ferromagnetic ordered state of the organic nitronyl nitroxide radical sketched below. The critical temperature is very low, 0.6 K, but the theoretical doubt was rapidly dissolved: organic magnetism is possible. Many other organic radicals of the class of the nitroxides were investigated. The highest critical temperature reported so far for this class of materials is 1.4 K.