Page 142 - Photoreactive Organic Thin Films

4. PHOTOISOMERIZATION AND PHOTO-ORIENTATION OF AZO DYE IN FILMS OF POLYMER ( 2 |

both increase by nearly the same amount (see the UV columns). This shows
that the polarizability of the azobenzene molecules breaks down because of
the change in their electronic and structural properties induced by the
photoisomerization from a planar to a bend structure. This also can be seen
at the value of n (the mean refractive index), which is not conserved after the
UV irradiation (see the UV columns in Figure 4.9).
The thickness of the LBK film does not change under UV and blue light
irradiation. This was confirmed by the X-ray experiments and has also been
a
shown, by means of X-ray reflectometry, for P 2,6 °d P<$,6- When the sample is
exposed to blue light, n z increases, and n x and n y decrease to nearly their
initial values before UV irradiation. This means that the packing of the end
chains cannot be broken by the isomerization. The film structure is stable and
the chromophores retain full memory of their initial orientation in the dark
state before UV irradiation. This order can be explained by the crystallinity
of the tails; their packing is the ordering force that stabilizes the film
structure. The isomerization in this film can be repeated without fatigue
under successive UV and blue light irradiations (Figure 4.9), and it always
achieves the same refractive indices. The stability of the structure was also
confirmed by X-ray reflectometry experiments. As a result of the dramatic
photoinduced change in the molecular orientational order, the three-
dimensional optical order parameter of the sample changes forth and back by
about one order of magnitude under UV and blue light irradiation.
For comparison, we also prepared LBK structures with the P 2>6 and P 6>6
polymers. Table 4.1 summarizes the thickness and the refractive indices in the
three principal directions of P 2)6 and P 6>6, together with the data for P^io- In
this case, the dipping direction is represented by the x-axis. All three materials
show an in-plane (n x - n y) anisotropy directed in the dipping direction, but
the main contribution is found perpendicular to the surface, due to the
monolayer structure at the air/water interface, which is conserved by the
transfer. The nearly complete loss of anisotropy in the cis isomer (labeled

TABLE 4.1 Thickness (d) and the 633-nm Indices of Refraction (n x, n y, n z) in the
p
P
Three Principal Directions of Different Azo-Polyglutannates' (P 2,io» 2,6» *,i) LBK
Structures 2
n x n y n, d, A
PZ,IO new 1.552 1.533 1.678 23.7
UV 1.574 1.566 1.586 23.8
B 1.545 1.537 1.690 23.8
new 1.572 1.538 1.689 24.2
P 2, 6
UV 1.598 1.589 1.603 24,3
B 1.583 1.592 1.636 24.3
P 6j6 new 1.589 1.547 1.637 24.4
UV 1.581 1.573 1.568 24.8
B 1.611 1.562 1.606 24.8
a
Here, x refers to the dipping direction. New refers to freshly prepared samples, and UV and B
refer to samples irradiated with UV and blue light, respectively.