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.