Page 143 - Photoreactive Organic Thin Films

I 22 ZOUHEIR SEKKAT AND WOLFGANG KNOLL

"UV" in the table) is also seen for the three polymers. It can be seen that P 2j6
and P 2ji(b which have the same spacer-length, show similar behaviors when
the LBK structures are freshly prepared (labeled "New" in the table). In the
case of the New P 6)6 film, the anisotropy in the dipping direction is more
pronounced. After the first UV-blue irradiation cycle, the optical anisotropy
of the P 2j6 LBK structure is partially lost compared to the P 210 LBK structure.
There are two reasons for this: (1) the molecular tail-length is longer for P 2)10
than for P 2j6, leading to a higher Van der Waals interaction; and (2) there is
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additional electrostatic interaction of the oxygen in the alkoxy-chains of
P 2jlo compared to the alkyl-chains of P 2>6. These interactions in the side chains
of P 2>io conserve the high order and produce the stability. Table 4,1 also
shows that the refractive index of the New P 6>6 LBK structure in the z
direction is smaller than for the P 2)6 and P 2nio LBK structures, but the in-plane
anisotropy is larger. After the first UV-blue irradiation cycle, this in-plane
anisotropy is even more pronounced compared to P 2>6 and P 2jio, which
become nearly isotropic, whereas the anisotropy of P 6 6 observed between the
out-of-plane and in-plane directions becomes smaller. This can be explained
by the longer spacer in the case of P^; this would allow more mobility to the
side chains, which prefer orientation in the direction of the main chains.
For all the azo-polyglutamate LBK-polymer films, all the experiments
mentioned previously were repeated setting the UV and blue lights to be
linearly polarized, in the plane of the sample, successively parallel and
perpendicular to the dipping direction; the same results were obtained for
both UV and blue-light directions of polarization. In other words, no photo-
selection effect was detected in these LBK structures. This contrasts with the
results obtained with spin-cast films from the P 2jlo polymer (vide infra). This
may be due to the subtle nature of the reorientation of the azobenzenes
imposed by the LBK deposition technique, because efficient photoselection,
following photoisomerization-induced reorientation, can be achieved in spin-
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cast azo-polymer films. " To isomerize, azobenzene molecules must feel
highly organized supramolecular assemblies constructed by means of strong
molecular physico-chemical interactions. In contrast to spin-cast films, for
which the molecular units are initially randomly distributed, the degree of
freedom for the azo units is considerably reduced in such supramolecular
assemblies. This can be seen from the behavior of the azobenzene molecules
in the P 2>10 LBK structures, which have to recover their initially highly
oriented arrangement after a complete trans<-»cis photoisomerization cycle.
The azo units are constrained by the LBK structure to a highly oriented trans
configuration or to a bend cis configuration. Next, we discuss polymer
structural effects on photoisomerization and photo-orientation in spin-cast
polymer films, where the azo dyes are initially randomly distributed without
inter molecular interaction imposed on the molecular order.

4.4 POLYMER STRUCTURAL EFFECTS ON PHOTO-ORIENTATION

Photo-orientation results in anisotropy and depends on the environment of
the photoisomerizable chromophore. To increase the stability of photo-