Effect of Structure on the Spin Switching and MagneticBistability of Solid-State Aryl Dicyanomethyl Monoradicals and Diradicals

摘要

Stable organic radicals with switchable spin states have applications in medicine, biology, and material science. An emerging class of such spin-switchable radicals is based on dicyanomethyl radicals, which are typically thermally and air-stable species that form weakly bonded (closed-shell singlet) dimers at a lower temperature that rupture into electron paramagnetic resonance-active diradicals at a higher temperature. However, thus far, the study of these dicyanomethyl radicals has focused on their solution-phase behavior. An understanding of how chemical structure affects the solid-state spin switching behavior for these radicals is unknown. Here, we examine the solid-state spin crossover behavior of 6 monoradicals and 10 tethered diradicals and demonstrate that these species also undergo spin switching in the solid state. We find that the susceptibility for solid-state spin switching for the intermolecular dimers is weakly correlated to the solution-phase Gibbs free energies of dimerization, but no apparent correlations are seen between the solution-state free energies forthe intramolecular dimerization and the solid-state behavior. Furthermore,intramolecular diradical dimers have greatly enhanced temperature-responsivebehavior compared to their intermolecular counterparts. Crystallineand amorphous powders of the same radicals feature similar spin switchingbehavior, but the crystalline materials have slower bond-rupture kineticsat higher temperatures, suggesting that solid-state packing effectsare an important kinetic consideration. An interesting feature ofthese systems is that, upon cooling down to room temperature afterheating, some radicals remain trapped in the solids, indicating magneticbistability, while others partially or fully return to the diamagneticdimers. This work provides insights into how chemical structure affectsspin crossover in the solid state for this new class of air-stableradicals, the knowledge of importance for the construction of dynamicallyresponsive solid-state materials, and organic spin crossover polymers.