Isomeric Control of Structure and Properties in Tolyl-phenyl-1,2,3,5-dithiadiazolyl Radicals

An isomeric series of 1,2,3,5-dithiadiazolyl (DTDA) radicals, 4′-(p-tolyl)C6H4CNSSN (1), 3′-(p-tolyl)C6H4CNSSN (2), and 2′-(p-tolyl)C6H4CNSSN (3), were prepared and crystallized by vacuum sublimation. Radical 1 exhibits monotropic polymorphism, crystallizing as three polymorphs (1α, 1β, and 1γ) depending on the sublimation conditions. Each polymorph forms pancake-bonded dimers: 1α adopts a cis-oid dimer conformation, whereas both 1β and 1γ crystallize as trans-cofacial dimers, but with 1β and 1γ adopting different packing motifs. A combination of DSC and VT-PXRD revealed successive transformations 1α → 1β and 1β → 1γ upon heating, reflecting 1γ as the most entropically stable phase. Crystallization of 1 from the molten state is cooling-rate-dependent, affording 1β as the thermodynamic phase on slow cooling. The crystal structure of 2α comprises a mixture of both twisted and trans-cofacial dimer conformations, while in 3α, molecules adopts a trans-antarafacial dimer conformation to accommodate the steric hindrance of the ortho-tolyl group. SQUID magnetometry measurements reveal that 1α, 2α, and 3α are all diamagnetic at low temperature but exhibit an increase in sample paramagnetism on warming. Thermal studies reveal that 2α and 3α melt to form metastable glassy phases. VT EPR studies indicate that glassy 2 largely dimerizes upon cooling, whereas 3 largely retains its paramagnetism in the glassy phase.