“…There is still a general consensus that the deviation from the planarity, the dihedral angle between the aromatic subunits, can be up to 25 • [25,26] for aniline derivatives, while o-hydroxy imines with a spacer between the imine N-atom and the aromatic rings of the corresponding amine are excluded from that empirical rule [27,28]. The accepted mechanism for thermochromism is a temperature-dependent keto-enol tautomeric change between an uncolored enol form and a yellow cis-keto form, while the existence of the trans-keto form changes the color to red [27,28]. In many cases, keto-amine forms were not found in the crystal structure o-hydroxy Schiff bases; in some cases, the molecules are in keto tautomeric form, and the samples are not of the expected color; and in some cases, keto-enol tautomeric equilibrium was found, and its position is affected by a temperature change without a color change; the postulates of Schiff base thermochromism are still taken as definite facts.…”