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Redox-responsive molecular helices with highly condensed π-clouds

Abstract

Helices have long attracted the attention of chemists, both for their inherent chiral structure and their potential for applications such as the separation of chiral compounds or the construction of molecular machines. As a result of steric forces, polymeric o-phenylenes adopt a tight helical conformation in which the densely packed phenylene units create a highly condensed π-cloud. Here, we show an oligomeric o-phenylene that undergoes a redox-responsive dynamic motion. In solution, the helices undergo a rapid inversion. During crystallization, however, a chiral symmetry-breaking phenomenon is observed in which each crystal contains only one enantiomeric form. Crystals of both handedness are obtained, but in a non-racemic mixture. Furthermore, in solution, the dynamic motion of the helical oligomer is dramatically suppressed by one-electron oxidation. X-ray crystallography of both the neutral and oxidized forms indicated that a hole, generated upon oxidation, is shared by the repeating o-phenylene units. This enables conformational locking of the helix, and represents a long-lasting chiroptical memory.

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Figure 1: Synthesis and schematic structures of OPnH (n = 8, 16, 24, 32, 40 and 48) and OP8NO2.
Figure 2: Schematic representation of the helical inversion dynamics of OP8NO2 in solution and in the solid state.
Figure 3: Chiroptical features and helical inversion dynamics of the neutral and oxidized forms of OP8NO2.
Figure 4: Chiral symmetry-breaking in the crystallization of OP8NO2.
Figure 5: Crystal structures of OP8NO2 and OP8NO2•+[SbF6] adopting a heavily twisted 31-helical structure.
Figure 6: Density functional theory (DFT) calculation of a model compound of OP8NO2 (OP8NO2m) at the B3LYP/6-31G(d) level.

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Acknowledgements

This work was supported by KAKENHI (21350108). The authors thank S. Ohkoshi and K. Nakabayashi (University of Tokyo) for the measurement of the ESR spectrum.

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Authors

Contributions

T.F. and T.A. designed the work. E.O., T.F. and T.A. wrote the paper. E.O., H.S., S.A. and A.K. performed the experiments. Single-crystal X-ray diffraction studies were carried out through the collaboration of D.H., M.Y. and K.H., A.M., H.U. and K.Y. were responsible for DFT calculations.

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Correspondence to Takanori Fukushima or Takuzo Aida.

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The authors declare no competing financial interests.

Supplementary information

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Supplementary information (PDF 1736 kb)

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Crystallographic data for compound OP8Br (CIF 38 kb)

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Crystallographic data for compound OP8NO2 (neutral) (CIF 41 kb)

Supplementary information

Crystallographic data for radical cation OP8NO2•+, counter anion SbF6- (CIF 44 kb)

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Ohta, E., Sato, H., Ando, S. et al. Redox-responsive molecular helices with highly condensed π-clouds. Nature Chem 3, 68–73 (2011). https://doi.org/10.1038/nchem.900

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