Hysteretic Room‐Temperature Magnetic Bistability of the Crystalline 4,7‐Difluoro‐1,3,2‐Benzodithiazolyl Radical

Abstract The title radical R⋅, synthesized by reduction of the corresponding cation R + , is thermally stable up to ~380 K in the crystalline state under anaerobic conditions. With SQUID magnetometry, single‐crystal and powder XRD, solid‐state EPR and TG‐DSC, reversible spin‐Peierls transition between diamagnetic and paramagnetic states featuring ~10 K hysteretic loop is observed for R⋅ in the temperature range ~310–325 K; ΔH=~2.03 kJ mol −1 and ΔS=~6.23 J mol −1 K −1 . The transition is accompanied by mechanical movement of the crystals, i. e ., by thermosalient behavior. The low‐temperature diamagnetic P‐1 polymorph of R⋅ consists of R⋅ 2 π‐dimers arranged in (…R⋅ 2 …) n π‐stacks; whereas the high‐temperature paramagnetic P2 1 /c polymorph, of uniform (…R⋅…) n π‐stacks. With the XRD geometries, CASSCF and broken‐symmetry DFT jointly suggest strong antiferromagnetic (AF) interactions within R⋅ 2 and weak between R⋅ 2 for the (…R⋅ 2 …) n stacks; and moderate AF interactions between R⋅ for the (…R⋅…) n stacks. The fully hydrocarbon archetype of R⋅ does not reveal the aforementioned properties. Thus, the fluorinated 1,3,2‐benzodithiazolyls pave a new pathway in the design and synthesis of metal‐less magnetically‐bistable materials.