Thermal Hysteresis in Neutral and Ionic Main Group Radicals
Aaron Mailman
a Nanoscience Center, Department of Chemistry, Ä¢¹½Ö±²¥, Finland.
aaron.m.mailman@jyu.fi, aaronmailman@gmail.com
There has been a growing interest in the use of organic radicals as building blocks for functional materials, due to their technologically relevant properties, such as, ferromagnetism, electrical conductivity, and molecular bistability [1][2]. The open-shell nature of radicals enables unique electronic and magnetic properites unattainable in closed-shell molecules. We have demonstrated that small, stable heterocyclic thiazyl (and their heavier selenium congener) radicals can be designed with tuneable electronic properities and predictable solid-state assembly, offering potential applications in batteries, sensor, and information storage.
Recently, we showed that binary organic radical-ion salts containing the σ-dimer of TCNQ radical anions exhibit thermally triggered dynamic covalent bonding, unlocking potential for next generation electronics, sensors, and hybrid materials [3]. Reverisble thermal hysteresis in organic radicals remains relatively rare compared to the well-studied spin-crossover (SCO) phenomenon in metal complexes. While the SCO involves a spin transition between a high-spin (HS) and low-spin (LS) state [4], organic radicals undergo an interconversion between a radical (S = ½)and its π- or σ-dimer (S = 0) due to external stimuli. These reversible solid-state to solid-state (or liquid state) transitions can be structurally characterized by single crystal and/or powder X-ray diffraction. Furthermore, the distinct electron-exchange interactions between phases generates hysteresis loops in the magnetic susceptibility measurements.
Recent advances in steric and electronic stabilization have enabled the design of robust thiazyl radicals with molecular bistability, governed by structural and solid-state interactions. Controlling the crystalline forms of these materials through solvent engineering, process optimization, and anion selection continues to present substantial challenges despite recent breakthroughs. We highlight main-group radical materials exhibiting thermal hysteresis under thermal, pressure and light stimuli, underscoring their promise for adaptive technologies.
Figure 1. Temperature dependence of the χT product between 1.85 − 380 K at 1 T with different heating and cooling rates for the phase transition between a TCNQ σ-dimer dianion and two TCNQ radical-anions in an organic radical-ion salt (Sulfur = yellow, Nitrogen = blue, Carbon = grey; Intermolecular interactions = light-blue dashed lines).
References:
[1] Hicks, R. Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds; Wiley,
2011.
[2] Sato, O. Dynamic Molecular Crystals with Switchable Physical Properties. Nat. Chem. 2016, 8 (7),
644.
[3] Taponen, A.; Ayadi, A.; Lahtinen, M. K.; Bonhommeau, S.; Oyarzabal, Rouzières, M.; Mathonière, M.;
Tounonen, H. M.; Clérac, R. J. Am. Chem. Soc. 2021, 143 (39), 15912.
[4] Spin-Crossover Materials; Halcrow, M. A., Ed.; John Wiley & Sons Ltd.: Chichester, U. K., 2013.
