228. Chiral control of spin-crossover dynamics in Fe(II) complexes
     Malte Oppermann, Francesco Zinna, Jérôme Lacour, Majed Chergui
     Nat. Chem. 2022, 14, 739-745

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.

Preprint in: ChemRxiv 2021, 10.26434/chemrxiv-2021-v0pjv

Highlighted in News and Views:  Johansson, J.O. Plot twist in the iron spin saga. Nat. Chem. 2022, 14, 722-723. 10.1038/s41557-022-00986-1

DOI : 10.1038/s41557-022-00933-0 

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