Letter abstract
Nature Materials 7, 48 - 51 (2008)
Published online: 2 December 2007 | doi:10.1038/nmat2067
Subject Category: Optical, photonic and optoelectronic materials
Thermochromism in an organic crystal based on the coexistence of 
- and 
-dimers
Yasushi Morita1,2, Shuichi Suzuki1, Kozo Fukui2, Shigeaki Nakazawa3, Hiroshi Kitagawa4, Hideo Kishida5, Hiroshi Okamoto5, Akira Naito6, Akiko Sekine7, Yuji Ohashi7, Motoo Shiro8, Katsunari Sasaki8, Daisuke Shiomi3, Kazunobu Sato3, Takeji Takui3 & Kazuhiro Nakasuji1
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 
-bonded and 
-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 -dimer structure. This proves the coexistence of two dimers of different bonding natures in the crystal, causing it to demonstrate thermometer-like behaviour.
- Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
- PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan
- Departments of Chemistry and Materials Science, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan
- Department of Chemistry, Graduate School of Science, Kyushu University, Fukuoka, Fukuoka 812-8581, Japan
- Department of Advanced Materials Science, Graduate School of Frontier Sciences, University of Tokyo, Kashiwa, Chiba 277-8561, Japan
- Faculty of Engineering, Yokohama National University, Hodogaya-ku, Yokohama 240-0085, Japan
- Department of Chemistry and Materials Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan
- Rigaku Corporation, Akishima, Tokyo 196-8666, Japan
Correspondence to: Yasushi Morita1,2 e-mail: morita@chem.sci.osaka-u.ac.jp
Correspondence to: Takeji Takui3 e-mail: takui@sci.osaka-cu.ac.jp
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