NC–(CF2)4–CNSSN˙ containing 1,2,3,5-dithiadiazolyl radical dimer exhibiting triplet excited states at low temperature and thermal hysteresis on melting–solidification: structural, spectroscopic, and magnetic characterization

A high yield, one-pot synthesis of the 1,2,3,5-dithiadiazolyl radical NC–(CF2)4–CNSSN˙ by reduction of the corresponding 1,3,2,4-dithiadiazolium salt is reported. In the solid state, the title compound is dimerized in trans-cofacial fashion with intra-dimeric Sδ+⋯Nδ− interactions of ca. 3.2 A, and the dimeric units are linked by electrostatic –CNδ−⋯Sδ+ interactions forming an infinite chain. Magnetic susceptibility measurements performed on the solid state sample indicate a magnetic moment of 1.8 μB per dimer (1.3 μB per monomer) at 300 K and a good fit to the Bleaney–Bowers model in the temperature range 2–300 K with 2J = −1500 ± 50 cm−1, g = 2.02(5), ρ = 0.90(3)%, and TIP = 1.25(4) × 10−3 emu mol−1. The [NC–(CF2)4–CNSSN˙]2 dimer is the second example of a 1,2,3,5-dithiadiazolyl radical dimer with an experimentally detected triplet excited state as probed by solid-state EPR [2J = −1730 ± 100 cm−1, |D| = 0.0278(5) cm−1, |E| = 0.0047(5) cm−1]. The value of the singlet–triplet gap has enabled us to estimate the “in situ” dimerization energy of the radical dimer as ca. −10 kJ mol−1. The diradical character of the dimer was calculated [CASSCF(6,6)/6-31G*] as 35%. The title radical shows magnetic bistability in the temperature range of 305–335 K as probed by the solid-state EPR presumably arising from the presence of a metastable paramagnetic supercooled phase. Bistability is accompanied by thermochromic behavior with a color change from dark green (dimeric solid) to dark brown (paramagnetic liquid).