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A crystal-engineering approach to organic ferrimagnetics is reported. Coulombic energy between cationic biradical with S=l and anionic radical with S=1/2 is a promising driving force of co-crystallizing the hetero-molecular assemblage in a controllable manner. As a cationic component of "organic salt femmagnetics", two kinds of nitronyl nitroxide biradicals, 2,6-and 3,5-substituted pyridine derivatives, were examined. It was predicted from semi-empirical molecular orbital calculations that both the 2,6-and 3,5-derivatives have the triplet ground states both in the neutral and cationic states. The molecular ground state of the 2,6-biradical in the neutral state was found to be triplet from magnetic susceptibility measurements, while the susceptibility and ESR measurements suggested a singlet ground state for the 3.5-biradical.

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A crystal-engineering approach to organic ferrimagnets is reported. Coulombic energy between an anionic biradical with S = I and a cationic monoradical with S = 1/2 can be utilized as a driving force of cocrystallization of open-shell molecules with different spin quantum numbers, leading to organic salt ferrimagnets. In this study, 3,5-substituted phenol and benzoic acid derivatives of nitronyl nitroxide biradicals were synthesized as an ionizable S = 1 component of organic salt ferrimagnets. The molecular ground states of the biradicals in the neutral state were examined by continuous wave electron spin resonance (ESR) spectroscopy and static paramagnetic susceptibility measurements in the solid state. The molecular ground state of the phenol derivative was found to be triplet (S = 1) with the singlet-triplet energy gap of AE/k5 25 K, indicating that the biradical can be a building block of organic salt ferrimagnetics. The benzoic acid derivative was found to have a singlet (S = 0) ground state (4E/k5 ? -5 K), exemplifying that meta-(3,5)-linkage of unpaired electrons in it-aromatic rings does not necessarily give a triplet ground state for heteroatomic-substituted it conjugation. The molecular ground states of the biradicals determined in the ESR experiments were confirmed by the susceptibility in the solid state.

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