Clevertex - Grado Zero Espace Srl

esume their original chemical structure and the colour will disappear-making the process fully
reversible [141].
activation
A(λ 1 ) B(λ 2 )
deactivation
There are different kinds of chemical processes which are responsible for the transition from form A to
form B:
• a variation in the oxidation condition of a metal;
• trans-cis isomerisation;
• a homolytic or heterolytic breaking of the chemical bond
These reactions obviously have to be reversible in order to be catalogued as a photochrome reaction.
If not, an ordinary photochrome reaction takes place.
Photo-chromic dyes
A very important natural photo-chromic dye with reversible colour changeability is rhodopsin, which is
present in the retina of the eye. Rhodopsin is activated by light and induces nerve stimuli, which are
transmitted to the brain where they provide the perception of colours. Due to extraordinary stability to
photo degradation, rhodopsin is a potential photoswitchable biomaterial which is suitable for various
applications.
Photochromic dyes are less used than thermochromic dyes due to poor stability.
• Colour pigments that change in response to temperature and light conditions are utilized for
camouflage purposes [142].
• Photochromic dyes:
used for optical memories and switches
filters
First fabrics with photochromic dyes appeared in the 1980ties. The Japanese producer Kanebo Ltd.
used microencapsulated spiropyrans, which were printed on textile materials. Under the influence of
UV rays with wavelength 350-400nm, the dyes reversibly changed from light blue to dark blue. As
spiropyrans are unstable, they were replaced by more stable spirooxazines in the end of the 1980ties.
At this time Toray Industries Inc. developed the process of coating fabrics with photochromic dyes.
The fabric, known under the tradename Sway UV®, was able to turn blue or violet under the influence
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