Fe(II)-based spin-crossover complexes hold tremendous promise as multifunctional switches in molecular devices [1]. However, real-world technological applications are only feasible if the excited spin-states are kinetically stable – a feature that has only been achieved at cryogenic temperatures, most prominently in the light-induced excited spin-state trapping (LIESST) effect. Both the analysis and manipulation of the associated intersystem crossing dynamics are usually based on a single dominant reaction coordinate that preserves the symmetry of the complex: a symmetric stretching mode of the Fe-ligand bonds. We now go beyond this picture and achieve significant kinetic stabilization of a chiral Fe(II)-complex in solution through the control of its symmetry-breaking torsional distortion in the photo-excited quintet state.

The chiral tris-chelate complex [Fe(4-4’-Me₂bpy)₃]²⁺ is configurationally labile in solution, with the racemization known to proceed via a trigonal twisting mode. Already 40 years ago, Purcell suggested that this structural distortion must involve a spin-crossover [2]. Here, we reverse this perspective: through supramolecular complex formation with enantiopure counterions [3], we obtain configurationally stable Fe(II) complexes with preferential Λ or Δ configuration and thus effectively block the racemizing trigonal twist. We first demonstrate kinetic stabilization by measuring a four-fold increase in lifetime of the photo-excited quintet state compared to the labile complex. We then employ time-resolved circular dichroism (CD) spectroscopy [4] to extract the CD spectrum of the excited quintet state and track its evolution with sub-picosecond time resolution. This lets us identify a symmetry-breaking torsional distortion as the main reaction coordinate in its relaxation, thus providing direct evidence for the central importance of this mode in the intersystem crossing of Fe(II) complexes. Most importantly, however, we demonstrate that the control of their chirality provides a powerful new strategy for manipulating their spin-crossover dynamics.

[1] A. Bousseksou et al., Chem. Soc. Rev. 2011, 40, 3313
[2] K.F. Purcell, J. Am. Chem. Soc. 1979, 101, 5147
[3] J. Lacour et al., Angew. Chem. Int. Ed. 1998, 37, 2379
[4] M. Oppermann et al., Optica 2019, 6, 56