Role of Topology in Spin Alignment of Organic Materials
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ROLE OF TOPOLOGY IN SPIN ALIGNMENT OF ORGANIC MATERIALS KOICHI ITOH, TAKEJI TAKUI, YOSHIO TEKI AND TAKAMASA KINOSHITA Osaka City University, Faculty of Science, Department of Chemistry, Sumiyoshi-ku, Osaka 558, Japan
Sugimoto,
ABSTRACT
It is shown that the spin alignment in organic molecules as determined by single crystal ESR is highly dependent on the topological nature of their molecular pi electron networks. The three topological isomers of a high-spin molecule, biphenyl-n,n'-bis(phenylmethylene), abbreviated as BP-3,3'-BPM, BP3,4'-BPM and BP-4,4'-BPM are taken as model compounds. BP-3,3'-BPM has a unique spin alignment for which the simple molecular orbital and valence bond methods predict different ground-state spin multiplicities. The above remarkable feature of spin alignment in organic high-spin molecule is interpreted in terms of their spin density distributions which have been determined by a single crystal ENDOR technique and compared with theoretical values calculated on the basis of a generalized Hubbard model as well as the Heisenberg model. This approach is extended to magnetic polymers in order to characterize their structure of the spin sites.
INTRODUCTION
In recent years, much interest has been focused on ferromagnetism of organic materials, and a few polymers have been reported to exhibit ferromagnetic behaviors [1-3]. However, neither the mechanisms interpreting such behaviors nor their spin structures have been clarified. It is needed for characterization and molecular design of such organomagnetic materials, at this stage, to understand the mechanism of spin alignment characteristic of organic molecules from basic studies and to establish a method for characterizing their spin structures. The aim of this paper is to elucidate an important role of of topology in spin alignment in organic materials and to demonstrate the utility of an electron-nuclear double resonance (ENDOR) technique to the determination of spin alignment not only in model high-spin molecules but also in magnetic polymers. Since the detection of m-phenylenebis(phenylmethylene), the first highspin molecule, in 1967 [4], we have synthesized and characterized a series of organic high-spin molecules as models for organic ferromagnets [5]. Very recently, an oligomer of a one-dimensional ferromagnetic polymer has been obtained with the spin multiplicity as high as undecet (S = 5) [6]. The molecular design for most of these compounds has been based on the utilization of pi spins in topologically-degenerate non-bonding MO's (NBMO) as well as of sigma or "n" spins in the localized non-bonding orbitals on the divalent carbon atoms of carbenes. In the present work, we have studied the spin alignment of three topological isomers of a high-spin molecule, biphenyl-n,n'-bis(phenylmethylene) (n = 3, 4) which is abbreviated as BP-n,n'-BPM, and found a dramatic change in the ground-state spin depending on the topology of their pi electron networks. This remarkable feature of spin alignment is interpreted in terms of their spin density dis
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