Page 179 - Photoreactive Organic Thin Films

I JJg MIKHAILV. KOZLOVSKY, LEV M. BLINOV, AND WOLFGANG HAASE

irradiation with nonpolarized UV light increases drastically the birefringence
induced by polarized visible light: An ind=±0.045 for the illuminated film
versus An ind=±0.001 for that reached without UV treatment. This gives rise to
the possibility of amplifying the sensitivity of the material to visible light by
previous exposure to UV light. To some extent, the phenomenon is similar to
the "nonvolatile holographic storage effect" observed recently in a doubly
83
doped inorganic ferroelectric. Our data show, however, that the UV sensibi-
78 80 84
lization is much more pronounced for the copolyrners of the KW series " '
77 81
than for the SK and KM series. ' That can be explained by the stronger
electron-accepting substituents at the tail of the azobenzene chromophores
for the latter two series, -CN for SK copolyrners and -OCF 3 for KM copoly-
rners as compared with -OCH 3 for KW copolyrners, which lead to reduced
stability and shorter lifetimes for the os-isomers, as reported by Ikeda
5
etal. *- 52
There is one more possibility for substantially increasing the stability of
photoinduced birefringence in chiral LC copolyrners. If the light-induced
reorientation of photochromic azo groups occurs not in a well-formed
(mature) mesophase, but during phase formation, it is less restricted by main--
chain conformation, so that higher An values can be achieved. Moreover,
corresponding structural reorganization should involve the polymer main
chain itself. As a result, its conformation "memorizes" the preferred orienta-
tion of chromophores and the thereby-induced cooperative reorientation of
colorless mesogenic moieties in glass, and the copolymer film keeps the
macroscopic orientation for much longer times. 85
Table 5.3 compares An values recorded in a 22-um film of KW19 after
20 min of irradiation with polarized white light from a polarizing microscope
for isothermal recording at 26°C and recording during sample cooling from
66 to 26°C. It is clearly seen that the thermally assisted recording is twice as
efficient and much more stable. To illustrate the stability of such an "imprinted"
birefringence (i.e., photoinduced during film cooling), Figure 5.14 presents a
2.5-year-old birefringent spot, induced in the copolymer film during micro-
scopic observations.
The kinetically determined Iso-TGB A* phase transition in copolyrners
of the KM series (shown in Figure 5.7) allows also for isothermal nonequilib-
rium photorecording in the copolyrners. Figure 5.15 shows the evolution of
photoinduced birefringence in a film just cooled from 100°C, where the

TABLE 5.3 Comparison of the Isothermal!/ Recorded and "Imprinted"
Birefringence in Dye Copolymer Film (KW 19,22-um Film)

Birefringence, An Isothermal recording Thermally assisted recording
(20 min at 26°C) (20 min as cooledfrom 66°C to 26°C)

Immediately after 0.0008 0.0020
the recording
18 h later 0.0003 0.0019