Photochromism and thermochromism derived from proton tautomerism

4 Photochromism and thermochromism derived from proton tautomerism 

This section describes the characteristics of the photo- and thermochromism derived from proton tautomerization in crystals, using the photo- and thermochromic behaviour of 2-alkylisophthalaldehydes, salicylideneanilines, and dinitrobenzylpyridines as typical examples. 

Colourless crystals of 2.5-dimethylisophthalaldehyde [29] change to red when placed in sunlight. The red colour fades when the sample is kept in the dark for more than five minutes. The back reaction is accelerated by warming the sample in the dark. This whole process occurs repeatedly at least for several cycles. Similar photochromic behaviour is observed in 2-methyl-5-isopropylisophthalaldehydes [30] (Raju and Krishna Rao, 1987). It is claimed 

Salicylideneaniline [32] and its derivatives are known to exhibit photochromic behaviour. The colour of the crystal of salicylideneaniline changes from pale yellow to red, and the red colour changes back to the original one in a thermal process. On the other hand, other derivatives of salicylideneaniline show only thermochromic behaviour. It is usually the case that a photochromic crystal never exhibits thermochromism, and vice versa. 

Incidentally the photochromic behaviour of anils cannot be observed in fluid media. This is because, even though isomerization around the C-N bond occurs photochemically, the reverse isomerization occurs readily in a thermal process through the inversion of the nitrogen atom. Thus, fading of colours takes place rapidly in fluid media. 

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