Page 156 - Photoreactive Organic Thin Films

4, PHOTOiSOMERIZATION AND PHOTO-ORIENTATION OF AZO DYE IN FILMS OF POLYMER | J 5

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activation and reaction volumes in solution. For photo and thermal
isomerization reactions, kinetics of both ground- and excited-state isomerization
are used to assess solvent and high-viscosity solution effects on isomerization
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rates and isomerization movement. " Regarding the effects of pressure on
photoisomerization and photo-orientation in polymeric thin films, we show
that both photoisomerization and photo-orientation of disperse red one (DR1),
an azo dye, flexibly tethered to a poly(methyl-methacrylate) (PMMA) polymer,
referred to in the text as PMMA-DRl, are hindered by the application of
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hydrostatic pressure.
We used ATR prism coupling as well as a Kerr gate optical setup to probe
the influence of pressure on photoisomerization and photo-orientation in
PMMA-DRl. The polymer was obtained from IBM Almaden; its chemical
structure is shown in Figure 4.22. The dye is tethered to the PMMA copolymer
with 10% DR1 per monomer unit. The molecular weight (M m) and glass
transition temperature (Tg) of this polymer are M n = 163000 g/mol and Tg =
123°C, respectively. PMMA-DRl films (thickness ~ 1 pm) were prepared by
spin-coating from a diethyleneglycol dimethyl ether solution either directly on
top of a sapphire slide for the Kerr gate experiments, or on top of a 50 nm~
thick gold layer that was then evaporated directly on top of a sapphire slide
for the ATR prism coupling experiments. The films were dried at 90°C for 12 h
and heated at 130°C for several minutes in a vacuum to remove traces of the
remaining solvent. After heating, the films were allowed to cool slowly to
room temperature. Details about the characteristics of the pressure chamber
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can be found elsewhere. The temperature inside the chamber was controlled
by a thermostat, and all the experiments discussed in this paper were
performed at 25°C. Water was used as a pressure medium. For the Kerr gate
optical experiments, the pressure cell containing the film sample was placed
between crossed polarizers oriented at 45 and -45 degrees with respect to the
vertical, and a probe beam propagated in succession through the polarizer,

FIG. 4.22 Chemical structure of PMMA-DRI.