Page 154 - Photoreactive Organic Thin Films

4. PHOTOISOMERIZATiON AND PHOTO-ORIENTATION OF AZO DYE IN FILMS OF POLYMER 133

polymer backbone, i.e., the presence of the aromatic ring into the backbone
of PUR-1, clearly influences the photo-orientation dynamics and efficiency
for PUR-1 versus PUR-2. This influence of the polymer backbone was also
observed for PUR-3 versus PUR-4, confirming the influence of polymer
structural effects on photo-orientation.
The levels of photo-induced anisotropy in all four Azo-PURs are also
correlated to the polymer structure. Figure 4.20 shows real-time dichroism
values of the photoinduced anisotropy with 488 nm irradiation and analysis
in all four Azo-PURs obtained at near the steady state of irradiation versus
the irradiation intensity. The observed polymer structural effects of Figure
4.20 are confirmed by other data (not shown) independently obtained after
photoisomerization by linearly polarized 488 nm irradiation of all four
polymers and measurement of Abs// and Abs± by a UV-vis spectrophotometer
at the maximum wavelength, i.e., 470 nm for PUR-1 and PUR-2, and 440 nm
for PUR-3 and PUR-4. Figure 4.20 clearly shows a higher photostationary-
state anisotropy for PUR-1 than for PUR-2, PUR-3, and PUR-4. Even though
the observed anisotropy depends on the cis and trans balance in concentration,
on the isomers' extinction coefficients, and on the isomers' orientation, polymers
with the same chromophore but different backbones exhibit different levels
of induced anisotropy under the same irradiation conditions. The series of
data taken by both real-time dichroism and UV-vis steady-state values demonstrate
that the photo-orientation efficiency in the azo-polyurethanes studied decreases
according to the series PUR-1 >PUR-2>PUR-3>PUR-4. The fast cis-»trans
thermal isomerization and the addition of the aromatic ring into the polymer
backbone facilitate the movement of the chromophore.
Next, we discuss whether the chromophores' spontaneous, i.e., thermally
activated, relaxation is primarily influenced by the movement of the polymer
backbone (a-relaxation), by that of the chromophore (^-relaxation}, or by

0.00
-•-PUR-1
-0.02
•-0- PUR-2
-*- PUR-3
-0.04 -O- PUR-4
-0.06

-0.08
-0.10
-0.12
-0.14
-0.16
0 20 40 60 80
Irradiation Intensity (mW/cm ) 2
FIG. 4.20 Photostationary-state anisotropy in Azo-PURs versus irradiation intensity. Each data point
was taken after 5 minutes of irradiation. The data are those of real-time dichroism with 488 nm
irradiation and analysis.