Page 131 - Photoreactive Organic Thin Films

J I Q ZOUHEIR SEKKAT AND WOLFGANG KNOLL

evidence clearly shows that molecular movement depends on the structure of
the unit building blocks of the polymer.
Organized azo-molecular assemblies allow for the study of photoisomer-
ization and photo-orientation in sterically and orientationally well-defined
media, such as LBK multilayers, the molecular-interactions-based order of
30
which can be altered by photoisomerization of the azo units. In this chapter,
we also discuss the photoisomerization-induced changes in the structural and
optical properties of highly organized organic films containing azobenzenes.
Photoisomerization and photo-orientation of azobenzenes is compared in
amorphous spin-cast films, in LBK supramolecular assemblies, and in self-
assembled monolayers.
This chapter is organized as follows. Section 4.2 addresses the study of
photoisomerization and photoinduced orientation of azobenzene molecules at
the molecular level in SAMs of azo-silane molecules. Section 4.3 discusses
photoinduced effects in supramolecular assemblies, i.e., LBK multilayer structures
containing azobenzene molecules, and compares the photoinduced movement
of azobenzenes in these structures to that observed in spin-cast films. Section
4,4 focuses on the isomerization and sub-Tg photoinduced orientation in a
series of very high Tg (up to 350°C) nonlinear optical polyirnide and thermoplastic
donor-embedded polyurethane polymers containing azo dye, especially focusing
on polymer structure-Tg-photoinduced molecular movement relationships.
Section 4.5 describes pressure effects on photoisomerization and photo-orientation
in films of a PMMA polymer containing azo dye. Finally, we make some
concluding remarks in Section 4.6.

4.2 PHOTOISOMERIZATION OF AZOBENZENES IN MOLECULARLYTHIN SELF-
ASSEMBLED MONOLAYERS: PHOTO-ORIENTATION AND PHOTO-MODULATION OF
THE OPTICAL THICKNESS

In this section, we discuss the photo-orientation of azobenzenes in molecularly
thin SAMs by means of UV-vis spectroscopy and surface plasmons (SPs).
The structural formula of 4-(6-carboxy-(3-amidopropyl)triethoxysilane)-4'~
pentylazobenzene, referred to as azo-silane, which leads to a self-assembled
monolayer, is shown in Figure 4.1 (top). Azo-silane SAMs (see schematic in
Figure 4.1, bottom) for the UV-vis spectroscopy and surface plasmons experiments
were prepared as reported elswhere. 39

4.2.1 Photoisomerization of Azo-SAMs
UV (360 nm) and blue (450 nm) light irradiations of the ultrathin azo-silane
SAMs clearly induce the forth, i.e., trans—»cis, and back, i.e., cis—Hrans,
photoisomerization of azobenzene molecules (see Figure 4.2A). The real-time
dependence of the absorbance of the sample during the thermal cis—>trans
back reaction is not a monoexponential decay (see Figure 4.2B). This decay
shows a complex multiexponential relaxation behavior that could be fit
neither by a monoexponential decay nor by a biexponential relaxation.
Nevertheless, a monoexponential decay could be fit to the data acquired over