Solid-state thermochromism studied by variable-temperature diffuse reflectance spectroscopy. A new perspective on the chromism of salicylideneanilines

This paper reports a successful application of variable-temperature diffuse reflectance spectroscopy for the study of the solid-state thermo- and photochromism of salicylideneanilines, which belong to a class of the most well-known chromic organic compounds. This study disproves fundamental aspects that have been widely accepted for as long as 40 years and provides a new perspective on the chromism. The enol form of salicylideneaniline, which has been wrongly regarded as pale yellow, is in fact colorless, and the pale yellow is due to a trace of the cis-keto form, which is in equilibrium with the enol form. The phototransformation from the cis-keto to the trans-keto form was observed for the first time. Although it has been accepted that a given modification of salicylideneanilines is either photochromic or thermochromic but not both, this study reveals that such an exclusive property on the chromism does not exist and that salicylideneanilines are generally thermochromic in the solid state, whether they are photochromic or not. This study demonstrates that the color of materials is controlled by a trace of colored species and that the variable-temperature diffuse reflectance spectroscopy is quite an effective tool for its detection and therefore for the study of the chromism in the solid state.