Room-temperature magnetic bistability in organic radical crystals: Paramagnetic-diamagnetic phase transition in 1,3,5-trithia-2,4,6-triazapentalenyl

An organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), exhibits a first-order phase transition between a paramagnetic high-temperature (HT) phase and a diamagnetic low-temperature (LT) phase, with a surprisingly wide thermal hysteresis loop over the temperature range 230–305 K. The crystal structure of the HT phase consists of a one-dimensional (1D) regular π-stacking column with short $\mathrm{S}\cdot \cdot \cdot \mathrm{N}$ and $\mathrm{S}\cdot \cdot \cdot \mathrm{S}$ intercolumn contacts. The intra- and intercolumn exchange coupling constants are estimated to be ${J/k}_{\mathrm{B}}=-320\hspace{0.5em}\mathrm{K}$ and ${\mathrm{zJ}}^{\prime }{/k}_{\mathrm{B}}=-360\hspace{0.5em}\mathrm{K},$ indicating a 3D magnetic interaction. The LT phase has a diamagnetic property, caused by a strong alternation in the stacking column. The differential scanning calorimetry measurements reveal an exothermic and an endothermic transition upon cooling and heating, respectively, with the hysteresis in agreement with static magnetic measurements. The observed transition entropy is larger than the maximum estimation of the spin contribution, $R\ln 2,$ suggesting incorporation of the lattice system in this phase transition. The electron paramagnetic resonance measurements also demonstrate the paramagnetic-diamagnetic phase transition with the hysteresis. The reflection spectra suggest a change in dimensionality at the phase transition, which is consistent with the results of the intermolecular overlap-integral calculations.