Organic Magnets
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Organic Magnets Jaume Veciana and Hiizu Iwamura Introduction The notion of organic molecular materials showing metallic properties, such as electric conductivity or ferromagnetism, started several decades ago as a mere dream of some members of the chemical community. The goal was to create an assembly of organic molecules or macromolecules containing only light elements (C, H, N, O, S, etc.) and yet possessing the electron/hole mobility or spin alignment that is inherent in typical metals or their oxides and different from the isolated molecular materials. Organic molecular conductors initially were developed during the 1960s, but the first examples of organic molecular magnets took several more decades to be discovered, owing to the more subtle and complex structural and electronic aspects of these materials. The flurry of activity in this field can be traced to the widely held belief that even the most sophisticated properties can be rationally designed by a systematic modification of organic molecular structures. This motivation was further fueled by increased synthetic capabilities, especially for obtaining large organic molecules with suitable structures and topologies, and also by the spectacular progress of supramolecular chemistry for materials development witnessed in recent years. Also noteworthy is the pioneering work performed in the 1960s by several physical organic chemists who unraveled different ways of aligning spins within open-shell molecules (i.e., triplet diradicals, carbenes, etc.), working against nature’s tendency to align them in an antiparallel manner. Magnetic interactions between unpaired electrons, located on the singly occupied molecular orbitals (SOMOs) of di- and polyradicals, or between the adjacent open-shell molecules in crystals, are a crucial issue in this evolving field. Thus, depending upon the symmetry, degeneracy, and topological characteristics of SOMOs and also on the mode of arrangement of the molecules in a crystal, the resulting interaction can align the neighboring spins parallel or antiparallel (see the introductory article by Miller and Epstein in this issue of MRS Bulletin). Two aspects are to be considered in the development of strong magnetic proper-
MRS BULLETIN/NOVEMBER 2000
ties in molecular materials. They are the spin-containing building blocks (i.e., free radicals and radical ions) and their coupling routes (mechanisms). In this article, we will briefly summarize the essence of both of these issues, which are important for the design of this class of materials and the understanding of their physical properties. We will also review some of the most important research results achieved worldwide in this rapidly developing area of research.
Magnetic Interactions and Magnetic Ordering Magnetic exchange interactions alone determine the magnetic behavior of organic compounds made of light elements at temperatures well above 0.1 K. Other kinds of magnetic interactions (such as hyperfine, dipolar, or spin–orbit interactions) can be considered negligible above this
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