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Thermochromism in an organic crystal based on the coexistence of σ- and π-dimers

Nature Materials volume 7, pages 48–51 (2008)Cite this article

Abstract

Transition-metal complexes and organic radical molecules can be used to make electric conductors and ferromagnets1,2,3, the optical properties of which can be controlled by changing temperature and are used as molecular switches and sensors4,5,6,7,8. Whereas a number of organic radicals in solution show temperature-dependent optical properties5,6,9,10,11,12, such behaviour in crystalline forms is more rare13,14,15. Here, we show a fully reversible continuous thermochromism with a unique mechanism in purely organic crystals of diazaphenalenyl radical. This behaviour is based on changes in the diazaphenalenyl dimers coexisting in the crystal. From the X-ray crystal structure analyses and temperature-dependent visible spectra, we conclude the presence of a thermal equilibrium between σ-bonded and π-bonded dimers, which are separated by 2.62(6) kcal mol−1. This conclusion is supported by room-temperature electron spin resonance spectra of the solid, which showed signals that are attributable to a thermally accessible triplet state of the π-dimer structure. This proves the coexistence of two dimers of different bonding natures in the crystal, causing it to demonstrate thermometer-like behaviour.

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Figure 1: Molecular formula and crystal structure of compound 2.
Figure 2: Images of the 0.15-mm-thick crystal of 2 in the range of temperature between 100 K and 300 K.
Figure 3: Two types of dimer structure of 2 disordering in the crystalline state.
Figure 4: Temperature dependence of the oscillator strength, solid-state ESR spectra and experimentally determined energy diagram of the crystal.

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Acknowledgements

This work was partly supported by PRESTO-JST, Grants-in-Aid for Scientific Research in Priority Areas ‘Application of Molecular Spins’ (Area No. 769) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, and 21COE program ‘Creation of Integrated EcoChemistry’ of Osaka University.

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Authors and Affiliations

  1. Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

    Yasushi Morita, Shuichi Suzuki & Kazuhiro Nakasuji

  2. PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan

    Yasushi Morita & Kozo Fukui

  3. Departments of Chemistry and Materials Science, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan

    Shigeaki Nakazawa, Daisuke Shiomi, Kazunobu Sato & Takeji Takui

  4. Department of Chemistry, Graduate School of Science, Kyushu University, Fukuoka, Fukuoka 812-8581, Japan

    Hiroshi Kitagawa

  5. Department of Advanced Materials Science, Graduate School of Frontier Sciences, University of Tokyo, Kashiwa, Chiba 277-8561, Japan

    Hideo Kishida & Hiroshi Okamoto

  6. Faculty of Engineering, Yokohama National University, Hodogaya-ku, Yokohama 240-0085, Japan

    Akira Naito

  7. Department of Chemistry and Materials Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan

    Akiko Sekine & Yuji Ohashi

  8. Rigaku Corporation, Akishima, Tokyo 196-8666, Japan

    Motoo Shiro & Katsunari Sasaki

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  1. Yasushi Morita
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Contributions

Y.M., S.S., K.F., T.T. and K.N. planned this project, and carried out the experimental and theoretical work with the other authors. A.N., S.N., D.S. and K.S. carried out the ESR and magnetic susceptibility measurements and analyses. H.K. (Kyushu Univ.), H.K. (Univ. of Tokyo) and H.O. measured the temperature-dependent electronic spectra. A.S., Y.O., M.S. and K.S. (Rigaku Corp.) carried out the X-ray crystal structure analysis.

Corresponding authors

Correspondence to Yasushi Morita or Takeji Takui.

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Supplementary Information

Supplementary information - figures S1-S12 and tables S1-S11 (PDF 1751 kb)

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Morita, Y., Suzuki, S., Fukui, K. et al. Thermochromism in an organic crystal based on the coexistence of σ- and π-dimers. Nature Mater 7, 48–51 (2008). https://doi.org/10.1038/nmat2067

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