Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers.
Transition-metal complexes and organic radical molecules can be used to make electric conductors and ferromagnets, the optical properties of which can be controlled by changing temperature and are used as molecular switches and sensors. Whereas a number of organic radicals in solution show temperature-dependent optical properties, such behaviour in crystalline forms is more rare. 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 sigma-bonded and pi-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 pi-dimer structure. This proves the coexistence of two dimers of different bonding natures in the crystal, causing it to demonstrate thermometer-like behaviour.
Morita
Yasushi
Y
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan. morita@chem.sci.osaka-u.ac.jp
Suzuki
Shuichi
S
Fukui
Kozo
K
Nakazawa
Shigeaki
S
Kitagawa
Hiroshi
H
Kishida
Hideo
H
Okamoto
Hiroshi
H
Naito
Akira
A
Sekine
Akiko
A
Ohashi
Yuji
Y
Shiro
Motoo
M
Sasaki
Katsunari
K
Shiomi
Daisuke
D
Sato
Kazunobu
K
Takui
Takeji
T
Nakasuji
Kazuhiro
K