From Smart Materials to Chromic Textiles
261
O
N
N
R 1 R 2
R
X
Y
X
N
R 1 R 2
R
N
O
colourless spiropyran
coloured photomerocyanine
UV
Y
Fig. 3 Derivatives of spirooxazine chromic colorants
ultraviolet irradiation [46, 47]. Naphthopyrans, also known as chromenes, have been
widely studied as the most significant group of photochromic colorants. Similarly as
spiropyran and spirooxazine, the photochromic behavior of naphthopyrans involves
light-stimulated ring-opening to provide the more coplanar colored photomerocyanine as shown in Fig. 4. Naphthopyrans give photochromic colors in the spectrum
range between yellow, orange, red, violet and blue depending on the substituents
pattern. Neutral colors such as gray and brown which are important for ophthalmic
applications are also accessible [19]. Diarylethenes have been also studied in devices
using optical switch as photochromic colorants able to switch between colorless or
weakly colored ring-opened and colored ring-closed forms (Fig. 5). The reversible
O
R
R 1
R 2
O
R 1
R
R 2
UV
Fig. 4 Naphthopyran derivatives as chromic colorants
S
S
S
S
Visible light
F
F
F
F
F
F
F
F
F F
F F
Colorless - Open Form
Red - Closed Form
UV
Fig. 5 Diarylethene photochromic molecular switching dye