Page 107 - Photoreactive Organic Thin Films

ZOUHEIR SEKKAT

DR1/PMMA (2.5 wt.9
- Linear Fit " PUR-3
D PUR-4
Linear Fit

1.0 2.0 3.0
5
10" *1/F'

-12
-14
0.0 0.2 0.3
5
1CT *1/F
FIGURE 3.13 The reciprocal of the order parameter versus the reciprocal of the irradiation
intensity for PUR-1 and PUR-2. The insets show this dependence for PUR-3 and PUR-4, and
DRI/PMMA,as adapted from reference 20.

predicted by Equation 3.23 and that is referred to as the photo-orientation
constant, or the constant of photo-orientation by photoisomerization. This
value, -2/13, sets the maximum orientation that can be achieved by
photoisomerization and rationalizes, at least for high irradiation intensity, the
concept of a uniform stationary-state molecular order for both the A and B
isomers. Together with Azo-PURs, amorphous azo-polymers should exhibit a
photostationary-state order parameter at infinite irradiation flux near the
photo-orientation constant. A good example from the literature is provided
by DR1 molecules introduced as guests into films of PMMA (Figure 3.3). In
this figure, both the anisotropy and the isotropic absorbance were measured
as functions of the irradiating intensity, and the order parameter adapted
from those measurements (shown as an inset to Figure 3.13) also exhibits an
infinite flux value of S near -2/13. Even though the analysis wavelength,
514 nm, was different from the irradiation wavelength, 488 nm, these
wavelengths are close enough within the same absorption band of the DR1
chromohopre.
To evaluate photoisomerization and photo-orientation parameters, e A
and e B should be known. S A was calculated from the absorption spectrum of
the polymer solution before irradiation, assuming the same extinction
coefficient in the film and in solution; s% was determined by the Fisher's
method, modified by Rau, which holds not only for isotropic but also for
anisotropic samples when the isotropic absorbance is considered (vide infra).
For this determination, the isotropic absorbance change was recorded versus
the irradiating light intensity, and the sample absorbance change was
extracted for an irradiation flux extrapolated to infinity for three different
combinations of irradiation and analysis wavelengths: 488-488, 532-488,
and 532-532 nm, irradiation and analysis, respectively. These experiments