Fast thermal cis–trans isomerization of heterocyclic azo dyes in PMMA polymers

A series of heterocyclic azo dyes were dispersed in poly(methyl methacrylate) (PMMA) matrix and the photochromic properties of the coloured films studied by UV–Vis spectroscopy. Visible irradiation of the coloured films lead to a fast decrease in the colour intensity due to the trans–cis photoisomerization reaction. When the light source was removed the spontaneous thermal cis–trans reverse isomerization occurred bringing the absorbance back to initial value with variable speed. Contrary to common azobenzenes that after photoisomerization with UV or visible light exhibit very slow thermal cis–trans back re-isomerization, that can last for hours or days, azo dyes bearing pyrrole, thiophene, thiazole or thiadiazole heterocycles display very fast switching between the two isomers (few seconds) even when dispersed in PMMA polymers.

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► PMMA thin films doped with heterocyclic azo dyes showed photochromic properties.
► E–Z isomerization of heterocyclic azo dyes bearing thiazole, benzothiazole, thiadiazole, thienylpyrrole and bithiophene.
► The switching speed of heterocyclic azo dyes is faster than common azo benzenes derivatives.