Thermochromism of salicylideneanilines in solution: Aggregation-controlled proton tautomerization

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.