Thermomagnetic Bistability, Colossal Negative Thermal Expansion, and THz‐Heat‐Induced Switching in Cu‐Nitroxide Complex

Bistability in molecule‐based magnets is crucial for their applications in functional devices, and thermally induced excited spin state trapping (TIESST) represents a promising route to maintain two distinct spin states for a significant period of time. Heterospin compounds based on Cu(II) and nitroxide radicals demonstrate highly tunable non‐classical spin transitions, nevertheless they did not reveal TIESST effects up to date. Here, we report the first Cu(II)‐nitroxide complex that exhibits TIESST‐driven bistability up to 116 K. The TIESST temperature is close to temperatures of the spin transition, which is accompanied by a hysteresis of 30 K ( T _1/2↑ = 144 K, T _1/2↓ = 114 K). The trapped excited state features a very slow relaxation that exceeds 9 h at 95 K. Fast switching is also feasible for this complex: almost complete conversion from the ground to excited state occurs in a single THz pulse within 50 ms. The slow relaxation and preservation of the crystal quality enabled us to perform an XRD study and demonstrate that the trapped and stable phases coexist during the transition. It was demonstrated, that relaxation of excited state proceeds via an autocatalytic mechanism, previously unreported for spin‐crossover systems, which introduces a new paradigm in spin relaxation dynamics.