| Abstract
| - Many salicylideneanilines have been known to exhibit thermochromism in the solid state. In contrast, in thesolution, the thermochromism has rarely been observed. This paper reveals that salicylideneanilines are generallythermochromic in the solution of hydrocarbon solvents and that the proton tautomerization, which is theorigin of the thermochromism, is controlled by the aggregation of molecules. This is evidenced by the electronicabsorption 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 orthe mixture of isopentane and methylcyclohexane (volume ratio = 3:1) at different temperatures between297 and 77 K. They exhibited a remarkable spectral change with lowering temperature, except for 6. At 297K the OH form is exclusively populated. When the temperature is lowered, the absorption bands of the OHform decrease in intensity and new bands that are assigned to the NH form emerge. At 77 K, the OH formdisappears and the NH form is exclusively populated. The spectra of 6 do not change with lowering of thetemperature.
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