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The 4′-alkoxy-tetrafluorophenyl dithiadiazolyls, ROC6F4CNSSN [R = Me (1), Et (2), nPr (3), nBu(4)] all adopt cis-oid dimers in the solid state. The methoxy derivative 1 adopts a π-stacked AA’AA’ motif, whereas propoxy (3) and butoxy (4) derivatives exhibit an AA’BB’ stacking. The ethoxy derivative (2) is polymorphic. The α-phase (2α) adopts an AA’BB’ motif comparable with 3 and 4, whereas 2β and 2γ are reminiscent of 1 but combine a mixture of both monomers and dimers in the solid state. The structure of 2β exhibits Z’ = 6 with two dimers and two monomers in the asymmetric unit but undergoes a thermally induced phase transition upon cooling below −25 °C to form 2γ (Z’ = 14) with six dimers and two monomers in the asymmetric unit. The transition is associated with both rotation and translation of the dithiadiazolyl ring. Detailed differential scanning calorimetry and variable temperature powder X-ray diffraction studies coupled with SQUID magnetometry have been used to show that 2α converts irreversibly to 2β upon heating and that 2β and 2γ interconvert through a reversible phase transition with a small thermal hysteresis in its magnetic response., We would like to thank the Canada Research Chairs Program for financial support (J.M.R.), the University of Windsor for a scholarship (Y.B.) and C.F.I./O.R.F. support for infrastructure. We thank the Inorganic Chemistry Exchange (I.C.E.) Program for the opportunity for R.S. to visit U. Windsor. We also acknowledge support from the Ministerio de Economia y Competividad of Spain (Grant No. MAT2015-68200-C2-2-P). Additional support from Diputacion General de Aragon (DGA-M4) is also acknowledged. (J.C. and A.A.)

The syntheses and characterization of the aryl-substituted dithiadiazolyls, 2ʹ-Cl-5ʹ-\ud X-C6H3CNSSN• [1 (X = F), 2 (X = Cl), 3 (X = Br), 4 (X = I)] are described. In all four cases the\ud radicals adopt distorted stacks of π*-π* dimers with inter-stack S···X contacts. In 1 (monoclinic\ud P2/c) S···Cl contacts are manifested through a non-crystallographic 3-fold axis forming\ud supramolecular trimers whereas the inter-stack S…X contacts in 2 (triclinic P-1), 3 and 4 (which\ud form an isostructural pair, orthorhombic Pna21) form supramolecular chains. While all the\ud structures adopt π*-π* cis-oid dimer motifs, tuning the halogen modifies the intra-dimer S···S\ud distance. Variable temperature SQUID magnetometry and X-band CW-EPR studies revealed the\ud presence of a thermally accessible triplet state in all cases, with the singlet-triplet separation\ud appearing in the order 1 > 2 > 3 > 4, consistent with a reduction in the overlap integral with\ud increasing intra-dimer S···S separation. Variable temperature structural studies on both 2 and 4\ud reveal a structural evolution from a distorted π-stack motif towards a regular π-stacked array on\ud warming. This is particularly pronounced in 2 where the intermolecular S…S separations along\ud the stacking direction converge on a regular 3.6 Å spacing at ambient temperature.

The fluorescent 9′-anthracenyl-functionalized dithiadiazolyl radical (3) exhibits four structurally determined crystalline phases, all of which are monomeric in the solid state. Polymorph 3α (monoclinic P21/c, Z′ = 2) is isolated when the radical is condensed onto a cold substrate (enthalpically favored polymorph), whereas 3β (orthorhombic P212121, Z′ = 3) is collected on a warm substrate (entropically favored polymorph). The α and β polymorphs exhibit chemically distinct structures with 3α exhibiting face-to-face π–π interactions between anthracenyl groups, while 3β exhibits edge-to-face π–π interactions. 3α undergoes an irreversible conversion to 3β on warming to 120 °C (393 K). The β-phase undergoes a series of reversible solid-state transformations on cooling; below 300 K a phase transition occurs to form 3γ (monoclinic P21/c, Z′ = 1), and on further cooling below 165 K, a further transition is observed to 3δ (monoclinic P21/n, Z′ = 2). Both 3β → 3γ and 3γ → 3δ transitions are reversible (single-crystal X-ray diffraction), and the 3γ → 3δ process exhibits thermal hysteresis with a clear feature observed by heat capacity measurements. Heating 3β above 160 °C generates a fifth polymorph (3ε) which is distinct from 3α–3δ based on powder X-ray diffraction data. The magnetic behavior of both 3α and the 3β/3γ/3δ system reflect an S = 1/2 paramagnet with weak antiferromagnetic coupling. The reversible 3δ ↔ 3γ phase transition exhibits thermal hysteresis of 20 K. Below 50 K, the value of χmT for 3δ approaches 0 emu·K·mol–1 consistent with formation of a gapped state with an S = 0 ground-state configuration. In solution, both paramagnetic 3 and diamagnetic [3][GaCl4] exhibit similar absorption and emission profiles reflecting similar absorption and emission mechanisms for paramagnetic and diamagnetic forms. Both emit in the deep-blue region of the visible spectrum (λem ∼ 440 nm) upon excitation at 255 nm with quantum yields of 4% (3) and 30% ([3][GaCl4]) affording a switching ratio [ΦF(3+)/ΦF(3)] of 7.5 in quantum efficiency with oxidation state. Solid-state films of both 3 and [3][GaCl4] exhibit emission bands at a longer wavelength (490 nm) attributed to excimer emission., We would like to thank the Canada Research Chairs Program for financial support (J.M.R.) and the University of Windsor for a scholarship (Y.B.). E.G. and M.P. would like to thank NSERC. A.A. and J.C. acknowledge support from grant MAT2015-68200-C2−2-P from the Ministerio de Economía y Competividad of Spain and the European Regional Development Fund. Additional support from Diputacion General de Aragon (DGA-M4) is also acknowledged.