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T. A. Khattab and M. S. Abdelrahman
process only necessitates visible light absorption to revert back to its pristine state
[48, 49].
Those organic photochromic colorants generally possess problems upon application in a variety of commercial products, such as the steric hindrance effect arising
upon encapsulation in a film matrix leading to inhibiting their optical characteristics.
Conversely, inorganic photochromic colorants, such as strontium aluminate, exhibit
a better photochromic performance because they do not possess such steric hindrance effect since their photophysical change is not accompanied with molecular
structure change [50, 51]. The long time exposure of organic photochromic colorants to ultraviolet radiation can stimulate their degradation and consequently the
gradual decrease of their photochromic response leading to low photostability which
limits their usage in commercial products for an outdoor environment. Strontium
aluminum oxide doped with lanthanides (SrAl 2 O 4 :Eu
2+ /Dy
3+ ) or strontium aluminate is an inorganic colorant that has been known as long-lasting phosphorescent
material. It has been extensively applied as a photochromic colorant for a variety of
textile and non-textile applications. It is characterized by highly photostability under
ultraviolet irradiation, excellent fatigue resistance and fast reversibility [52–55].
3.1 Photochromism in Textiles
Some early azo disperse dyestuffs, such as azobenzene derivatives, demonstrated a
visual color change upon exposure to strong sunlight particularly when applied to cellulose acetate clothing. The process was reversible in the dark. This was attributed to
molecular switch from trans-azo isomer to the less stable cis-azo isomer upon ultraviolet irradiation [56]. Even though recent publications indicate rising interest for
smart textile products, there have been relatively few reports on photochromic textiles due to technical difficulties associated with the application process and product
performance. There are some reports on studying the exhaustion dyeing of spirooxazines onto synthetic fabrics introducing photochromic clothing able to change from
colorless or weakly colored to blue. However, this process was generally characterized by low dyestuff exhaustion. Polyester fibers dyed with phenoxyanthraquinone
dyestuffs displayed color variation from yellow to orange after ultraviolet irradiation
[57]. Spirooxazine and naphthopyran derivatives were used in the dyeing process
of polyester fibers as a commercial photochromic disperse colorants via exhaustion
dyeing. Blue spirooxazine offered the most effective results affording a fabric that
demonstrated obvious color change upon exposure to ultraviolet light. There have
been a few reported research work on photochromic dyestuffs designed for textile purposes by dyeing, such as spirooxazines bearing water solubilizing sulfonate
functional groups to introduce acid dyestuffs toward the production of photochromic
polyamide fibers [58]. In addition, a spirooxazine bearing a dichlorotriazine moiety
has been described as a fiber-reactive dyestuff appropriate for polyamide-based textiles [59]. Although, this molecular modification of photochromic dyes is arguable,
T. A. Khattab and M. S. Abdelrahman
process only necessitates visible light absorption to revert back to its pristine state
[48, 49].
Those organic photochromic colorants generally possess problems upon application in a variety of commercial products, such as the steric hindrance effect arising
upon encapsulation in a film matrix leading to inhibiting their optical characteristics.
Conversely, inorganic photochromic colorants, such as strontium aluminate, exhibit
a better photochromic performance because they do not possess such steric hindrance effect since their photophysical change is not accompanied with molecular
structure change [50, 51]. The long time exposure of organic photochromic colorants to ultraviolet radiation can stimulate their degradation and consequently the
gradual decrease of their photochromic response leading to low photostability which
limits their usage in commercial products for an outdoor environment. Strontium
aluminum oxide doped with lanthanides (SrAl 2 O 4 :Eu
2+ /Dy
3+ ) or strontium aluminate is an inorganic colorant that has been known as long-lasting phosphorescent
material. It has been extensively applied as a photochromic colorant for a variety of
textile and non-textile applications. It is characterized by highly photostability under
ultraviolet irradiation, excellent fatigue resistance and fast reversibility [52–55].
3.1 Photochromism in Textiles
Some early azo disperse dyestuffs, such as azobenzene derivatives, demonstrated a
visual color change upon exposure to strong sunlight particularly when applied to cellulose acetate clothing. The process was reversible in the dark. This was attributed to
molecular switch from trans-azo isomer to the less stable cis-azo isomer upon ultraviolet irradiation [56]. Even though recent publications indicate rising interest for
smart textile products, there have been relatively few reports on photochromic textiles due to technical difficulties associated with the application process and product
performance. There are some reports on studying the exhaustion dyeing of spirooxazines onto synthetic fabrics introducing photochromic clothing able to change from
colorless or weakly colored to blue. However, this process was generally characterized by low dyestuff exhaustion. Polyester fibers dyed with phenoxyanthraquinone
dyestuffs displayed color variation from yellow to orange after ultraviolet irradiation
[57]. Spirooxazine and naphthopyran derivatives were used in the dyeing process
of polyester fibers as a commercial photochromic disperse colorants via exhaustion
dyeing. Blue spirooxazine offered the most effective results affording a fabric that
demonstrated obvious color change upon exposure to ultraviolet light. There have
been a few reported research work on photochromic dyestuffs designed for textile purposes by dyeing, such as spirooxazines bearing water solubilizing sulfonate
functional groups to introduce acid dyestuffs toward the production of photochromic
polyamide fibers [58]. In addition, a spirooxazine bearing a dichlorotriazine moiety
has been described as a fiber-reactive dyestuff appropriate for polyamide-based textiles [59]. Although, this molecular modification of photochromic dyes is arguable,