Thermochromism of salicylideneanilines in solution: Aggregation-controlledproton tautomerization

Many salicylideneanilines have been known to exhibit thermochromism in the solid state. In contrast, in the solution, the thermochromism has rarely be en observed. This paper reveals that salicylideneanilines are generally the rmochromic in the solution of hydrocarbon solvents and that the proton taut omerization, which is the origin of the thermochromism, is controlled by th e aggregation of molecules. This is evidenced by the electronic absorption spectra of salicylideneaniline (1), N-(5-chloro-2-hydroxybenzylidene)anilin e (2), N-(2-hydroxy 4-methoxybenzylidene)aniline (3), N-(2-hydroxy-5-nitrob enzylidene)aniline (4), N-(2-hydroxy-5-nitrobenzylidene)-2,6- dimethylanili ne (5), and N-(3,5-di-tert-butyl-2-hydroxybenzylidene)aniline (6) in isopen tane or the mixture of isopentane and methylcyclohexane (volume ratio = 3:1 ) at different temperatures between 297 and 77 K. They exhibited a remarkab le spectral change with lowering temperature, except for 6. At 297 K the OH form is exclusively populated. When the temperature is lowered, the absorp tion bands of the OH form decrease in intensity and new bands that are assi gned 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.