Magnetic Bistability in Crystalline Organic Radicals: The Interplay of H‑bonding, Pancake Bonding, and Electrostatics in 4‑(2′-Benzimidazolyl)-1,2,3,5-dithiadiazolyl

The neutral radical 4-(2′-benzimidazolyl)-1,2,3,5-dithiadiazolyl
(HbimDTDA) exhibits a first order phase transition around 270 K without
symmetry breaking, preserving its orthorhombic Pbca space group between 340 and 100 K. Associated with this reversible
single-crystal-to-single-crystal phase transition, thermal hysteresis
of the magnetic susceptibility is observed. The low temperature (LT)
phase is diamagnetic owing to pancake bonding between the π-radicals.
In the paramagnetic high temperature (HT) phase, the pancake bonds
are broken, and new electrostatic contacts are apparent. As a result
of the dense 3D network of supramolecular contacts, which includes
H-bonds, the HbimDTDA system provides the first example of magnetic
bistability for a DTDA radical.