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

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 pi-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.