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j 40 ZOUHEIR SEKKAT AND WOLFGANG KNOLL

studied in this chapter. In particular, we adjusted an exponential decay to the
first 10 seconds of the recovery at all 6 pressures, and we found a thermal
1
isomerization rate for PMMA-DR1 in the range 0.17 - 0.23 s^ , with no
particular dependence on pressure. This result rules out pressure-induced
static effects and reinforces the friction effects discussed; it also shows that if
trans has enough sweep volume to isomerize to cis, cis will also have enough
sweep volume to isomerize back, a feature supported by the more compact
and globular, i.e., twisted, conformation of the cis- versus the trans-DRl.

4.6 CONCLUSION

Photoisomerization and photo-orientation of azobenzenes and their
derivatives in films of polymer are influenced by the environment of the
chromophore. In highly organized supramolecular structures constructed by
LBK multilayers, the azobenzene molecules can be trapped in a well-defined
manner; in amorphous spin-cast films, molecular movement is strongly
influenced by the structure of the unit building blocks of the polymer. Besides
the thermodynamical parameters, temperature and pressure, the factors that
strongly influence photoisomerization and photo-orientation in films of polymer
are the free volume, the nature of the connection of the chromophore to the
polymer main chain, the rigidity of the backbone, the molecular weight, and
the glass transition temperature. This chapter summarizes a number of
examples and proofs of environmental and polymeric structural effects on
photoisomerization and photo-orientation. In LBK azo-polyglutamates, the
lengths of the spacer and the tail, i.e., the alkyl chain, attached to the
chromophore clearly influence the movement of the azobenzene side chains,
and studies in loosely packed, molecularly thin SAMs reveal a similar
isomerization and reorientation process in the bulk and at the molecular
level. In clear contrast to spin-cast films, photo-(re)orientation does not appear
in LBK structures due to strong intermolecular interactions. In high-Tg, nonlinear
optical polyimides, the isomerization is slowed down when the chromophore
is firmly embedded into a rigid backbone, and photoisomerization is capable
of inducing molecular orientation as much as 325°C below Tg of a polyimide
containing no flexible connector or tether. In azo-polyurethanes, the azo
dye photo-orientation efficiency is influenced by the polymer structure as well
as by the isomerization rate and the photochemical quantum yields, and
pressure studies in films of PMMA-DR1 show that the chrornophore's local
free volume plays a major role in photoisomerization and photo-orientation.

This work is based on collaborations with many friends and colleagues, and it is our pleasure to
thank them for their support and for many helpful discussions. In particular, J. Wood participated
in most of the work, Y. Geerts and K. Mullen provided the azo-silane compound, M. Biichel and
S. Paul contributed to the azo-polyglutamate LBK experiments, and B. Weichart and H. Menzel
from Hannover Universitat provided the azo-polyglutamate polymers. The work on azo-
polyimides was done at the University of California, Davis and IBM Almaden (San Jose) with