Hysteretic spin crossover between a bisdithiazolyl radical and its hypervalent σ-dimer

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• The bisdithiazolyl radical 1a exists in two crystalline forms: α and β.
• The α-phase features π-stacked radicals, while the β-phase consists of π-stacked dimers linked by S···S-S···S σ-bonds.

Purpose of the Study:

• To investigate the structural, magnetic, and conductive properties of the dimorphic bisdithiazolyl radical 1a.
• To elucidate the nature of the phase transitions in the β-phase induced by temperature and pressure.

Main Methods:

• Variable-temperature magnetic susceptibility measurements.
• Powder X-ray diffraction (XRD) under varying temperature and pressure.
• Variable-temperature and -pressure conductivity measurements.

Main Results:

• The α-phase behaves as a Curie-Weiss paramagnet, exhibiting 1D Heisenberg chain antiferromagnetic coupling at low temperatures.
• The β-phase is diamagnetic up to 380 K, transitioning to a paramagnetic state via dimer-to-radical conversion.
• Phase transitions in β-1a are triggered by temperature (380 K) and pressure (0.65–0.98 GPa), involving the cleavage of the S···S-S···S σ-bond.
• Both phases exhibit insulating behavior, with conductivity significantly enhanced under pressure, particularly for the β-phase during its transition.

Conclusions:

• The dimorphism of bisdithiazolyl radical 1a leads to distinct magnetic and electronic properties.
• The β-phase exhibits a unique reversible dimer-to-radical phase transition driven by thermal and mechanical stimuli.
• The study highlights the crucial role of intermolecular bonding in dictating the solid-state behavior of radical systems.