Role of High-Spin Molecules as Models for Organic Ferro- and Ferri-Magnets, and Pi-Toporegulated Magnetic Polymers
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ROLE OF HIGH-SPIN MOLECULES AS MODELS FOR ORGANIC FERRO- AND FERRI-MAGNETS, AND PI-TOPOREGULATED MAGNETIC POLYMERS TAKEJI TAKUI, MAKOTO ENDOH, MASAYUKI OKAMOTO, KAZUNOBU SATOH, TOYOHIRO SHICHIRI, YOSHIO TEKI, TAKAMASA KINOSHITA AND KOICHI ITOH Osaka City University, Faculty of Science, Department of Chemistry, Sugimoto, Sumiyoshi-ku, Osaka 558, JAPAN ABSTRACT A chemical modification exploiting functional groups such as ether and methylene as bridges between high-spin assemblies has been carried out as a usable method to increase dimensionality of spin structure. Exchange interaction via an ether or a methylene bridge between two high-spin assemblies has been studied by single-crystal ESR spectroscopy. Whether it is ferromagnetic or antiferromagnetic depends upon the substituted position of the bridge, demonstrating the important role of the topological nature in spin alignment. It turns out that superexchange interaction or hyperconjugation mechanism can dominate spin alignment between high-spin assemblies. Using the criteria obtained, model compounds for units of organic ferrimagnets have been synthesized. They possess antiferromagnetically-exchange coupled heterospins and the salient features of their spin structures are characterized Xn terms of the spin density distribution as determined by single-crystal rH-ENDOR (Electron-Nuclear-DOuble Resonance) spectroscopy. INTRODUCTION The quest for organic magnetism is the focus of current topics in many fields of both pure and applied science. Organic high-spin molecules have accepted increasing interest as models for synthetic organo-magnetic materials such as organic superpara-,ferro-, and ferri-magnets, and the related experimental[1-31] and theoretical work[32-53] have been done to serve their molecular design for the last two decades. Our continuing study of the highspin molecules can date back to the detection of the first high-spin hydrocarbon, m-phenylenebis(phenylmethylene) in 1967 by one of the authors [1,2] and to the proposal of pi-toporegulated ferromagnetic polymers[36-38]. Our strategy for obtaining organic magnetism contains molecular design for high-spin assemblies exploiting topologically-degenerate pi non-bonding MOs[36-38,7,8,29]. An alternative approach to organic magnetism has been to utilize intermolecular charge-transfer interaction between organic electron donors and acceptors which are properly stacked in crystals. This approach has been based on the McConnell's idea[34,35] and several modifications have been proposed [9,10,13,18,28,52]. For the last two years several papers have appeared reporting synthesis of magneto-active organic materials which exhibited ferromagnetic behavior[57-61]. Most of high-spin molecules reported so far have only quasi onedimensional spin structure. Thus it is required to extend them to polymers or macromolecules with two- or three-dimensional spin structure of magnetic ordering for obtaining synthetic organo-magnetic material, since purely onedimensional systems can not have macroscopic magnetic properties such a
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