Iron-based spin-crossover complexes hold tremendous promise as multifunctional switches in molecular devices. However, real-world technological applications require the excited high-spin state to be kinetically stable—a feature that has been achieved only at cryogenic temperatures. Here we demonstrate high-spin-state trapping by controlling the chiral configuration of the prototypical iron(II)tris(4,4′-dimethyl-2,2′-bipyridine) in solution, associated for stereocontrol with the enantiopure Δ- or Λ-enantiomer of tris(3,4,5,6-tetrachlorobenzene-1,2-diolato-κ²O¹,O²)phosphorus(V) (P(O₂C₆Cl₄)₃^– or TRISPHAT) anions. We characterize the high-spin-state relaxation using broadband ultrafast circular dichroism spectroscopy in the deep ultraviolet in combination with transient absorption and anisotropy measurements. We find that the high-spin-state decay 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. The diastereoselective ion pairing suppresses the vibrational population of the identified reaction coordinate, thereby achieving a fourfold increase of the high-spin-state lifetime. More generally, our results motivate the synthetic control of the torsional modes of iron(II) complexes as a complementary route to manipulate their spin-crossover dynamics.

铁基自旋交叉复合物作为分子器件中的多功能开关具有巨大的前景。然而，现实世界的技术应用需要激发的高自旋态在动力学上是稳定的——这一特性只有在低温下才能实现。在这里，我们通过控制溶液中原型铁 (II)tris(4,4'-二甲基-2,2'-联吡啶) 的手性构型来证明高自旋态捕获，与对映体纯 Δ- 或 Λ 的立体控制相关-三(3,4,5,6-四氯苯-1,2-diolato-κ ² O ¹ ,O ² )磷(V)的对映体 (P(O ₂ C ₆ Cl ₄ ) ₃ ^–或 TRISPHAT) 阴离子。我们使用深紫外中的宽带超快圆二色光谱结合瞬态吸收和各向异性测量来表征高自旋态弛豫。我们发现高自旋态衰减伴随着其光学活动的超快变化，反映了与对称破坏扭转模式的耦合，这与通常假设的情况相反。非对映选择性离子配对抑制了已识别反应配位的振动群体，从而实现了高自旋态寿命的四倍增加。更一般地说，我们的结果激发了对铁 (II) 配合物扭转模式的合成控制，作为控制其自旋交叉动力学的补充途径。