Reversible change in color of substances with variation of the temperature is known as thermochromism and has attracted much interest from chemist for a long time. Salicylideneanilines belong to a class of the most popular compounds that show thermochromism in the solid state. We recently succeeded for the first time in the observation of the crystal structure change associated with the thermochromism and in the determination of the structure of the colored form. Furthermore, we discovered that salicylideneanilines are generally thermochromic in the fluid solution of a saturated hydrocarbon solvent, although it has been believed that salicylideneanilines are thermochromic only in the solid state. Our study revealed that the association of the molecules plays an essential role in the thermochromism.
