Study of Crystal Structure and Magnetic Properties of a 1,3,2‐Dithiazolyl Radical Crystal, BBDTA·Cl

We investigate the crystal structure and magnetic properties of an organic radical cation salt, BBDTA·Cl, based on the monocation of benzo[1,2-d:4,5-d′]bis[1,3,2]dithiazolyl (BBDTA) with one unpaired electron (spin quantum number S = 1/2) and a chloride anion. This material exhibits paramagnetic behavior with strong antiferromagnetic interactions between the radical species at around room temperature and a paramagnetic-to-diamagnetic phase transition with no thermal hysteresis at around 150 K. X-ray diffraction analysis indicates that the BBDTA+ cations form two types of regular π-stacking columns and short interatomic contacts between the columns at 200 K. At 90 K, lattice distortion is observed that accompanies the dimerization of the BBDTA+ cations in the π-stacking columns. This indicates that the origin of the magnetic change can be related to the structural change. The lattice parameters decreased monotonically and continuously with decrease in temperature around the transition temperature, whereas the differential scanning calorimetry (DSC) curve did not indicate any peak that corresponded to lateral heat. This essentially suggests second-order characteristics of the phase transition. Molecular-orbital calculations reveal that the magnetic interactions between the neighboring BBDTA+ cations in the π-stacking column are much stronger than the magnetic interactions between the columns, thereby suggesting that this material has one-dimensional magnetic characteristics in the higher-temperature region. This can be related to the spin-Peierls-like transition at around 150 K, in contrast to first-order characteristics of paramagnetic-to-diamagnetic phase transitions in some organic radicals with structural features similar to that of BBDTA·Cl. Differences in the phase-transition character between them have also been discussed.