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

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 for
the intramolecular dimerization and the solid-state behavior. Furthermore,
intramolecular diradical dimers have greatly enhanced temperature-responsive
behavior compared to their intermolecular counterparts. Crystalline
and amorphous powders of the same radicals feature similar spin switching
behavior, but the crystalline materials have slower bond-rupture kinetics
at higher temperatures, suggesting that solid-state packing effects
are an important kinetic consideration. An interesting feature of
these systems is that, upon cooling down to room temperature after
heating, some radicals remain trapped in the solids, indicating magnetic
bistability, while others partially or fully return to the diamagnetic
dimers. This work provides insights into how chemical structure affects
spin crossover in the solid state for this new class of air-stable
radicals, the knowledge of importance for the construction of dynamically
responsive solid-state materials, and organic spin crossover polymers.