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.

The thermochromism of salicylideneanilines in the solid state has been so far explained only in terms of the temperature-induced change of light absorption, which is caused by the shift of the tautomeric equilibrium between the enol and cis-keto forms. In this study, we measured variable temperature diffuse reflectance and fluorescence spectra of the crystalline powders of salicylideneanilines. We also determined their fluorescence quantum yields. The chromaticity coordinates at each temperature were calculated from these data and compared to the perceived colors of the powders. We found that the temperature-induced shift of the tautomeric equilibrium is not the main cause of the thermochromic color change of the thermochromic crystals and that the thermochromism can be explained only by taking account of the temperature-induced change of fluorescence.

Many salicylideneanilines have been known to exhibit thermochromism in the solid state. In contrast, in the solution, the thermochromism has rarely been observed. This paper reveals that salicylideneanilines are generally thermochromic in the solution of hydrocarbon solvents and that the proton tautomerization, which is the origin of the thermochromism, is controlled by the aggregation of molecules. This is evidenced by the electronic absorption spectra of salicylideneaniline ( 1), N-(5-chloro-2-hydroxybenzylidene)aniline (2), N-(2-hydroxy-4-methoxybenzylidene)aniline (3), N-(2-hydroxy-5-nitrobenzylidene)aniline (4), N-(2-hydroxy-5-nitrobenzylidene)-2,6-dimethylaniline (5), and N-(3,5-di-tert-butyl-2-hydroxybenzylidene)aniline (6) in isopentane or the mixture of isopentane and methylcyclohexane (volume ratio ) 3:1) at different temperatures between 297 and 77 K. They exhibited a remarkable spectral change with lowering temperature, except for 6. At 297 K the OH form is exclusively populated. When the temperature is lowered, the absorption bands of the OH form decrease in intensity and new bands that are assigned to the NH form emerge. At 77 K, the OH form disappears and the NH form is exclusively populated. The spectra of 6 do not change with lowering of the temperature.

X-Ray crystallographic and electronic absorption spectroscopic studies on N-(2-hydroxy-3-nitrobenzylidene)aniline revealed that this compound exists as a mixture of the OH and NH forms with nearly the equal populations in crystals.

Tautomeric equilibrium of salicylideneanilines is in favor of the enol form in most organic solvents at room temperature. We have previously reported that the equilibrium in saturated hydrocarbon solvents is reversed at low temperature, being totally in favor of the cis-keto form, which is unfavorable at room temperature. Although this stabilization of the cis-keto form has been explained in terms of aggregation of molecules at low temperature, the structure of the aggregate has remained uncharacterized. In this study, we measured UV-vis absorption spectra of salicylideneanilines including those which have tert-butyl groups at various positions of the benzene rings. The cis-keto form did not appear at low temperature, when salicylideneanilines have a tert-butyl group at a specific position or is restrained to a planar conformation. From these results we propose here that the temperature-induced reversal of the tautomeric equilibrium proceeds through the formation of intermolecular hydrogen-bonded cyclic dimers and the subsequent formation of the higher-order aggregates.