Thermally responsive magnetic bistability through solid–liquid phase transitions in alkylthio-functionalized dithiadiazolyl radicals

Abstract

A series of p-alkylthio-substituted phenyl-1,2,3,5-dithiadiazolyl radicals, p-R-PhDTDA (R = SMe (2), SEt (3), SⁿPr (4), SⁱPr (5), S^tBu (6)), and the prototypical (R = H (1)) phenyl derivative exhibit a thermally driven solid–liquid transition accompanied by a spin-state change from diamagnetic (S = 0) π-dimers to a pair of paramagnetic (S = ½) radicals. X-ray crystallography reveals that most derivatives form phase-pure cis-cofacial dimers, except 3 which crystallizes as three distinct polymorphs: 3α displays a sandwich-type herringbone structure stabilized by intermolecular S⋯S chalcogen bonding (isostructural with 2) while 3β and 3γ feature slipped π-stacked herringbone arrangements (ABAB and AA′BB′, respectively). In these latter forms, the alkyl groups act as steric buffers between adjacent stacks, a structural motif retained in derivatives 4–6. Differential scanning calorimetry (DSC), hot-stage microscopy, and vibrating sample magnetometry (VSM) collectively demonstrate a hysteretic phase transition in 1–6 with magnetic bistability arising from supercooling of the paramagnetic liquid phase. These results establish a new design paradigm for stimuli-responsive materials in which molecular packing, thermal behavior, and spin states are intrinsically linked through phase transitions.