A crystal-engineering approach to organic ferrimagnets is reported. Coulombic energy between an anionic biradical with 5 = 1 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 Δ/kB ≈ 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 (ΔE/kn = -5 K), exemplifying that meto-(3,5)-linkage of unpaired electrons in π-aromatic rings does not necessarily give a triplet ground state for heteroatomic-substituted π 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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