Page 220 - Photoreactive Organic Thin Films

6, PHOTO1SOMERIZATION IN IANGMUIR-BLODGETT-KUHN STRUCTURES

FIG. 6.16 Schematic representation of the deformation of photoehromic "hairy rod"-like pofy(t-
gtutamate)s 38 at the air/water interface through interaction with the water surface and the applied
surface pressure, and the resulting bilayered structure of LBK films.

poly(L-glutamate)s 38 having an alkyl tail, whereas poly(L-glutamate)s with
an alkoxy tail group 39 did not have the required flexibility of the side
chains. Most likely, the ether bridge enables the tail to form a higher ordered
state that lacks the flexibility of a slightly lower ordered liquid crystalline
33 39
state. ' However, polymers 39 can be deposited if the os-form is spread,
because the crooked ds-isomer cannot form highly ordered assemblies. 90
The structure of mono- and multilayers of polymers 38 was investigated
29 91
in detail. ' The polymers form domains when spread at the air/water
interface. From the peak position of the azobenzene 7t-7C*-band, it can be
concluded that the chromophores are strongly aggregated in the domains
91
even if no surface pressure is applied. Furthermore, it was found that the
"hairy rods," which should have a symmetrical distribution of the side chains
around the main-chain helix, are deformed by contact with the water surface
and the applied surface pressure (see Figure 6.16).
This deformation can be followed by UV-vis spectroscopy directly at
the air/water interface. Upon compression of the monolayer, the absorbance
changes not very much around the point where the first increase in surface
pressure is recorded (see Figure 6.17). This is caused by the disappearance of
the voids between the domains. Above the kink in the isotherm, however,
the n~n* band (~323 nm) decreases while the band at 248 nm increases,
This increase is due to the increase in chromophore concentration upon
compression of the monolayer. The decrease of the n-n* band is caused by a
preferential orientation of the chromophores perpendicular to the surface.
Chromophores oriented perpendicularly are not detected because the transi-
tion dipole of the n-n* band is parallel to the probing light. The band at
248 nm, however, is not sensitive to the orientation of the chromophore,
because the transition dipole moment has a component perpendicular to the
89
long axis of the molecule (see Figure 6.17).
The structure with the deformed "hairy rods" is preserved when multi-
layers are deposited on solid supports, as evidenced by X ray reflectometry
29
revealing a marked bilayered structure (see Figure 6.16). Photoisomeriza-
tion of the aggregated azobenzene chromophores in LBK films is possible and
leads to significant structural changes. The bilayered structure is completely