Page 209 - Photoreactive Organic Thin Films

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LBK films of 7, measured by electrochemical methods, indicated activation
energies similar to those measured for solutions. This was interpreted to be
the result of the sterically demanding os-isomer having a rather unfavorable
42
state. Furthermore, the kinetics may be used as an indication that, in the
LBK film, the back isomerization proceeds via the inversion mechanism {see
Chapter 1). The thermal back reaction of salkylidene anilines 3 in LBK films
was also found to deviate significantly from a first-order kinetics. 9
These examples and investigations on azobenzene moieties in polymers 43
show that the photochromic behavior is mainly controlled by the free volume
distribution around the chromophore. To obtain LBK films in which azoben-
zene moieties can undergo photoisomerization, therefore, the free volume
around the azobenzene chromophore must be controlled precisely to allow
for the molecular rearrangement inherent in the reversible trans to cis photo-
isomerization. This is possible by (1) mixing with other amphipmles, (2)
adjusting the architecture of the amphiphile, or (3) attaching the chro-
mophore to a polymer either by coulomb interaction or covalently.

6.4.1 Mixing with Other Amphiphiles
The molecular aggregation of the azobenzene arnphiphile 7 can be reduced
significantly by mixing it with phospholipids having saturated or unsaturated
alkyl chains. The trans to cis photoisomerization in a mixed monolayer is
accelerated. However, the contribution of the saturated and unsaturated
44
lipids in the mixed monolayers was found to be different. Like low molecu-
lar weight amphiphiles, surface active polymers can also be used as a matrix
for azobenzene moieties. For example, helical copolyglutamates, which form
very good monolayers, have been used as a matrix for oleophilic azodyes in
Langmuir-Blodgett films. The dye is "dissolved" in the side chain region of
45
the polymer. Isomerization of the azobenzene dyes was not investigated
specifically, but the resulting LBK films were successfully tested in optical
data storage experiments. Irradiation resulted in slightly different optical
properties, which could be detected by surface plasmon spectroscopy. The
46
origin of the changes to the optical properties was not investigated in detail.
A problem often associated with the approach of mixing a photoactive
amphiphile with a nonphotoactive arnphiphile is that not necessarily molecu-
47 48
lar mixing occurs, but domains of the azobenzene amphiphile are formed. '
To overcome this problem, the molecular mixing has been enforced by
tailored interactions between the different amphiphiles. For example, low
molecular weight azobenzene dyes can be dissolved in polyglutamates having
long alkyl chains and some azobenzene groups in the side chain. The molecu-
lar mixing is supported by the interaction of the polymer-bound azobenzene
with the admixed chromophore. Because the azobenzene is molecularily
dissolved in the side chain region, it can be readily isomerized by irradiation.
Irradiation causes demixing, however, because the as-azobenzene units do
not have the interactions that are present in the all-trans film and are expelled
49
from the side chain region.
A mixture that is stable in the ess-form was achieved by mixing an ionic
azobenzene amphiphile having a long spacer between the head group and the