Salicylidene class of molecules exhibit photochromism. It is due to tautomerism (intramolecular proton transfer). Article Photochromism and Thermochromism of Schiff Bases in the Soli...

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The most industrially important photochromic molecules are naphthopyrans and spiroindolinonaphthoxazines. They absorb near UV light and ring-open to produce photomerocyanine species which are intensely coloured. The reverse reaction back to their colourless forms is generally thermally driven. These types of molecules thus exhibit what is primarily known as 'T-type photochromism'. The vast majority (>95% in value and volume) of commercial photochromic colorants are employed in the production of ophthalmic lenses. Much smaller volumes are used in security printing and novelty items like toys. The scientific literature features many reports concerning diarylethenes and other types of photochromic colorant that exhibit P-type photochromism, i.e. they are switched between colourless and coloured states by means of UV light and visible light - there is no thermal reversion. The authors of these reports are targeting applications such as optoelectronic logic circuitry, data storage, and other hi-tech functions - however such uses are many years away and may never happen. Please see attached publication and presentation for more details. The Achilles heel of photochromic compounds is their fastness to light: if you can find photochromic systems that are stable to exposure of a decade or two of daylight then you will have made a huge advance in the field!