Page 144 - Photoreactive Organic Thin Films

4, PHOTOISOMERlZATtQN AND PHOTO-ORIENTATION OF AZO DYE IN FILMS OF POLYMER J 23

orientation, azo dyes have been attached to polymer main chains as pendant
side groups. In this section, we discuss the correlation of the polymer
architecture to sub-Tg, light-induced molecular movement in high-temperature
NLO azo-polyimides and in donor embedded azo-polyurethanes, and we
show that light can still create orientation of azo dyes up to 325°C below the
Tg of rigid azo-polyimides. The chromophores were embedded rigidly into
rigid polyimide backbones without any flexible connector or tether. This rigid
embedding of the chromophores into a rigid polyimide backbone decreases
appreciably the rate of the cis->trans thermal isomerization. It will be shown
that the isomerization process itself depends on the molecular structure of the
unit building blocks of the polymer. The effect of the cis—>trans thermal
isomerization rate on photo-orientation will be discussed for azo-polyurethaees
in which the azo-chromophore is substituted with groups of different electron-
withdrawing strengths, i.e., Cyano versus Nitro groups. It will also be shown
that the photo-orientation dynamics and efficiency are strongly influenced by
a seemingly small difference into the polyurethane backbone.

4.4,1 Photoisomerization and Photo-Orientation of High-Temperature Azo-Polyimides
We used spin-coated films of high-Tg, nonlinear optical (NLO) azo-polyimides,
i.e., PI-1, PI-2, PI-3a and PI-3b (see Figure 4.11), the unit building blocks of
which have distinct molecular structures. PI-1 and PI-2 are both donor-embedded
systems in which the NLO chromophore is incorporated rigidly into the
backbone of the polymer without any flexible connector or tether. PI-3a and
PI3b, on the other hand, are true side-chain systems in which the NLO azo
dye is attached to the main chain via a flexible tether. Compared with the
donor-embedded system, the flexible side chain system allows freer movement
of the azo-chromophore. Details of the polymer synthesis and characterization
and of the sample preparation can be found in references 59 and 60. The Tg
values of the polyimides were determined by differential scanning calorimetery
at a heating rate of 20°C per minute. The Tg values for PI-1, PI-2, PI-3a, and
PI~3b were 350, 252, 228, and 210°C, respectively. Note that introducing a
flexible unit into the polyimide backbone via the precursor dianhydride
lowers the Tg of the donor-embedded polymers (PI-1 and PI-2) by ~ 100°C
(350 versus 252°C).
Figure 4,12 shows 633-nm ATR modes in PI-1 before and after TE-
2
polarized 532-nm (30 mW/cm ) irradiation. The mode shifts after irradiation
showed that birefringence is achieved in PI-1 at room temperature. The ATR
accurate measurement of the refractive index components n x and n y (in-plane)
and n z (normal to the plane) yielded ^=^=1.649 and « z=1.628 before irradiation,
and w x=1.652, « y=1.617, and « z=1.635 35 minutes after irradiation, where
the y direction corresponds to the polarization direction of the irradiation
light. The same observation was made for PI-2. Irradiation of the samples at
room temperature, therefore, induces considerable birefringence in the samples.
2
For both PI-1 and PI-2, assuming that n - E (n, mean refractive index) for
optical frequencies, the mean dielectric constant, (E) decreases upon irradiation
(Aepn = - 0.023, A£ PI_ 2 = - 0.016). The dielectric constant is proportional to
the chromophore density and the decrease upon irradiation suggests a quasi-