From Murex Purpura to Sensory Photochromic Textiles

Abstract

Term photochromism in physics refers to a reversible change in colors of compounds inuced by light (usually UV). Photochromic phenomenon within the realm of textiles goes way back to the era of Alexander the great (around 350 b.c.). The first dye exhibiting photochromic phenomenon was obtained from sea snails Murex brandaris and Murex Purpura. Once applied to the textile the dye would be in colourless state, exhibiting its true colour (purple) only after being exhibited to sunlight. The reaction, however, was irreversible. List of materials exhibiting this phenomenon ranges from inorganic, minerals and organic, only the latter being appropriate for use on textile materials in the aim of obtaining photochromic sensors. Organic molecules (dyes) of photochromic properties are divided into five main classes which can to some extent satisfy the ideal requirements for any class of organic photochromic material ; spiropyrans (spiroindolinobenzopyrans), spironaphtoxazines, naphopyrans, fulgides and diarylethenes. The ideal requirements on this molecules include rapid colour development, controllable return into colorless state of the matter, the colour palette should be wide and the rest state should have as little colour as possible, preferably colourless. Photochromic molecules are characterized by a non-planar molecular structure in which the flow of -electrons is prevented resulting in a colorless form. Source of UV radiation induces breakage of the C(spiro-oxazin)-O bond which enables formation of planar ring opened molecular structure with an enlarged system of double conjugated bonds. Chemical constitution of these molecules has so far limited their use on textile fibers from the aspect of fastness to light, care, washing and durability in general. Today, photochromic dyes are synthesized and applied to textile material using different technological processes, which includes inserting into the fiber matrix by a common exhaustion dyeing technique, blending with a polymer, then extruding or wet spinning into photochromic fibres or mixing with a resin and surface coating onto a fiber or textile surface (printing technique). Commercially available photochromic dyes, act as disperse dyes, solvable in most non-polar solvents. Application range is wide from indicators of intensity of UV light on children's clothing, indicators of authenticity of brand design and origin and as elements of camouflage applied in the segment of military clothing and technical textiles (uniforms, tents, nets etc.) This paper gives an insight into the state-of the-art photochromic textiles, reviewing major achievements in the area of obtaining photochromic dyes and finally sensory textiles. It will cover synthesis basis of photochromic dyes and discuss the importance of radicals on the molecular level onto the spectrophotometric properties of dye molecule. Solvent effects marked as key element in understanding the photochromic behaviour and dependencies among elements of eventual photochromic system are observed from an aspect of the influence of solvent polarity (dielectric constant) and caused max shifts in spectral curves.