Page 176 - Photoreactive Organic Thin Films

5. CHIRAL POLYMERS WITH PHOTOAFFECTED PHASE BEHAVIOR FOR OPTICAL DATA STORAGE 155

8 Darkness;
r
\ Yellow light

0,0
300 400 500 300 400
A,, nm , nm

FIG. 5.9 (A) Changes in the UV absorption spectra of copolymer films under UV irradiation. (B)
Dark relaxation and yellow light illumination afterwards (I) virgin film; (2) UV illumination for 5 s; (3)
the same for 30 and 60 s; (4-7) dark relaxation for 3 min, 6 min, 9 min, and 24 h, respectively; (8) illumi-
nation with yellow light, 30 s.

as compared with other photochromic materials based on azobenzene deriva-
tives, including low molar mass chiral dyes and achiral LC polymers (see
Table 5.2). This opens up the possibility of preparing thin polymer films
(both cast and spin-coated) in their ds-form, from UV-irradiated solutions.
After evaporation of the solvent, the films keep the acquired ds-form for
77
hours and can be further used for photorecording purposes.
As mentioned in Section 5.1, the reorientation of azobenzene photo-
chromic moieties under illumination with polarized light and the birefringence
that is induced from it are well known and covered in many publications.

TABLE 5.2 Lifetime of cis-Forms for Various Azobenzene Derivatives

Azo-dye compound Temperature, °C Relaxation time, T Reference
Chiral low molar mass dye

25 ~ 3 min

Achiral LC polymer

0= 50 ~ 2. 5 min 52
O— (CH 2) 6— O— < X
Chiral LC copolymer
SK8 20 2.5 h 76,77
KW40 20 10 h 76, 78, 80