The First Observation of Hidden Hysteresis in an Iron(III) Spin‐Crossover Complex

Abstract Molecular magnetic switches are expected to form the functional components of future nanodevices. Herein we combine detailed (photo‐) crystallography and magnetic studies to reveal the unusual switching properties of an iron(III) complex, between low (LS) and high (HS) spin states. On cooling, it exhibits a partial thermal conversion associated with a reconstructive phase transition from a [HS‐HS] to a [LS‐HS] phase with a hysteresis of 25 K. Photoexcitation at low temperature allows access to a [LS‐LS] phase, never observed at thermal equilibrium. As well as reporting the first iron(III) spin crossover complex to exhibit reverse‐LIESST (light‐induced excited spin state trapping), we also reveal a hidden hysteresis of 30 K between the hidden [LS‐LS] and [HS‐LS] phases. Moreover, we demonstrate that FeIII spin‐crossover (SCO) complexes can be just as effective as FeII systems, and with the advantage of being air‐stable, they are ideally suited for use in molecular electronics.