Abstract
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.
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Data availability
The TA, TAA and TRCD datasets analysed and discussed in this publication are available from the Zenodo repository at https://doi.org/10.5281/zenodo.6255933.
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Acknowledgements
We thank J. Spekowius and J. Helbing (Zürich University) for adapting and sharing the B-Matrix referencing methodology and S. Grass (Geneva University) for the preparation of the enantiopure ammonium TT salts. We also thank L. Müller and B. Bauer (École Polytechnique Fédérale de Lausanne (EPFL)) for assistance in the laboratory and X. Kong and C. Heinis (EPFL) for providing access to a steady-state circular dichroism spectrometer. Finally, we thank L. M. Lawson Daku (Geneva University), G. Pescitelli and F. Santoro (Pisa University) for helpful discussions. This work was supported by the Swiss National Science Foundation (SNSF) through the National Center of Competence in Research (NCCR) Molecular Ultrafast Science and Technology (MUST). M.O. was supported by a fellowship within the postdoc programme of the German Academic Exchange Service (DAAD).
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J.L. and M.O. conceived the original idea. M.O. coordinated and carried out all aspects of the research (experiments, data analysis and interpretation), discussing them regularly with M.C.; F.Z. and J.L. contributed to sample preparation and manipulation and to discussions about the stereochemistry of the complexes. M.O. wrote the manuscript with contributions from all authors.
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Extended data
Extended Data Fig. 1 Steady-state characterization of the diastereomeric ion pair [Fe(dm-bpy)3][Λ-TT]2.
Detailed comparison of the spectroscopic properties of the diastereomeric ion pair in its Λ configuration when dissolved in CHCl3 and CH2Cl2: Comparison of molar extinction as a function of wavelength (a), comparison of CD spectra normalized to their maximum amplitude (b), spectral decomposition of the normalized absorption spectrum in CHCl3 (c), and spectral decomposition of the normalized CD spectrum in CHCl3 (d). The inset in panel (a) displays the structure of the Λ configuration of the TT anion.
Extended Data Fig. 2 Results from the global analysis of the TA experiments.
Decay-associated spectra (DAS, left panels) and species-associated spectra (SAS, right panels) obtained from a global analysis of the TA data of [Fe(dm-bpy)3][Λ-TT]2 in CHCl3 (a,b), in CH2Cl2 (c,d), and [Fe(dm-bpy)3][PF6]2 in CH2Cl2 (e,f) after photoexcitation of the MLCT band at 530 nm. For the plots of the DAS, a TA spectrum at 5 ps is included to show the positions of the GSB and ESA bands.
Extended Data Fig. 3 Results from the global analysis of the TAA experiments.
Decay-associated spectra (DAS, left panels) and species-associated spectra (SAS, right panels) obtained from a global analysis of the TAA data of [Fe(dm-bpy)3][Λ-TT]2 in CHCl3 (a,b), in CH2Cl2 (c,d), and [Fe(dm-bpy)3][PF6]2 in CH2Cl2 (e,f) after photoexcitation of the MLCT band at 530 nm. For both the DAS and the SAS, a TAA spectrum at 5 ps is included to show the positions of the main TAA bands and zero-crossings.
Extended Data Fig. 4 Spectrally resolved data from the TRCD experiments.
TRCD spectra as a function of pump-probe delay for Λ-Fecont(a), Λ-Fecont(b), Λ-Fesep(c), and Λ-Fesep(d). For the Λ configurations a total of 26 pump-probe delays are displayed, whereas 9 pump-probe delays are displayed for the Λ configurations. For all samples, the maximum absorbance near 295 nm was approximately 0.7 OD in a 0.5 mm pathlength flow cell. The samples were photoexcited at 530 nm with a peak fluence of approximately 3.5 mJ cm−2.
Extended Data Fig. 5 Model calculations of the HS-state TAA as a function of the ligands’ conformational ensemble.
Calculation of expected anisotropy values from the long-axis ligand centred (LC) transition dipoles of a tris-chelate complex photoexcited via one of its MLCT transitions (here: \(\stackrel{\rightharpoonup}{M}_{1}\) on ligand 1). a) Anisotropy calculated for individual LC transition dipoles as a function of their out-of-ligand-plane rotation angle. The inset displays the employed coordinate system and the labelling of the transition dipoles. b) Anisotropy calculated for individual LC transition dipoles as a function of their in-ligand-plane rotation angle around the origin of the coordinate system. c) Anisotropy obtained from a conformational ensemble over the in- and out-of-ligand-plane rotation angles as a function of the standard deviation of the associated two-dimensional Gaussian distribution. The average anisotropy value (solid black line) corresponds to the value obtained in an experimental measurement. d) One-dimensional Gaussian distributions with selected standard deviations, illustrating the conformational ensembles associated with the anisotropy values measured in the presented experiments.
Extended Data Fig. 6 Decomposition of representative TA and TRCD spectra into ground state bleach and excited state absorption contributions.
Spectral decomposition of the simultaneously acquired TA (a) and TRCD (b) spectrum of Λ-Fecont at 5 ps into a GSB and a HS-state contribution consisting of Gaussian bands. Note that in (a) the GSB and ESA bands are scaled by a factor of 0.5 for a better comparison with the TA spectrum.
Extended Data Fig. 7 Assessment of the sensitivity of the TRCD experiments and the suppression of polarization artefacts.
The left-most panels display the TRCD spectrum at -0.1 ps and 10 ps for racemic [Fe(bpy)3]Cl2 in H2O, where any non-zero signal is attributed to polarization artifacts. These measurements were performed prior to each of the reported TRCD experiments: a) for Λ-Fecont, b) for Λ-Fesep, and c) for Λ-Fecont and -Fesep, which were measured back-to-back. The right panels display the TRCD spectra at -0.1 ps for each of the four samples, including the associated standard error as a shaded area.
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Supplementary Figs. 1–27, Tables 1–5 and Discussions on the steady-state sample characterization, the TA experiments, the TAA experiments and the TRCD experiments.
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Oppermann, M., Zinna, F., Lacour, J. et al. Chiral control of spin-crossover dynamics in Fe(II) complexes. Nat. Chem. 14, 739–745 (2022). https://doi.org/10.1038/s41557-022-00933-0
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DOI: https://doi.org/10.1038/s41557-022-00933-0
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