Chiral control of spin-crossover dynamics in Fe(II) complexes

Iron-based spin-crossover (SCO) complexes hold tremendous promise as multifunctional switches in molecular devices. However, real-world technological applications require the excited high-spin (HS) state to be kinetically stable – a feature that has only been achieved at cryogenic temperatures in the light-induced excited spin-state trapping effect. Here we demonstrate HS state trapping by controlling the chiral configuration of FeII(4,4’-dimethyl-2,2’-bipyridine)3 in solution, associated for stereocontrol with enantiopure ∆- or Λ-TRISPHAT anions. We characterize the HS state relaxation using a newly developed ultrafast circular dichroism technique in combination with transient absorption and anisotropy measurements. We find that the decay of the HS state is accompanied by ultrafast changes of its optical activity, reflecting the coupling to a symmetry-breaking torsional twisting mode, contrary to the commonly assumed picture. Furthermore, we show that the diastereoselective ion-pairing with the enantiopure anions suppresses the vibrational population of the identified twisting mode, thereby achieving a four-fold extension of the HS lifetime. Transferred to the solid state, this novel strategy may thus significantly improve the kinetic stability of iron(II)-based magnetic switches at room temperature.