Page 221 - Photoreactive Organic Thin Films

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n-n" band
323 nm

transition
dipole

<——> 248 nm
4/ transition

0.20 0.25 0.30 0.35
2
Area per Monomeric Unit [nm ]
FIG. 6.17 Absorbance at 243 nm and 323 nm for monolayers of polymer 38 (n = 2) at the
air/water interface as a function of the surface pressure and the corresponding surface pressure
area/isotherm.

lost, indicated by the loss of the Bragg reflexes in the X ray reflectometry 29
(details of the structural changes will be discussed later; see Section 6.5.1)
Furthermore, the optical properties, namely the refractive indices of the LBK
92
films, change, as can be established by surface plasmon spectroscopy. The
structural changes are accompanied by a significant deaggregation of the
azobenzene chromophores. After the structural changes have taken place,
the azobenzene moieties can be photoreoriented. 69

Poly(L-glutamate)s having both azobenzene and mesogenic, nonphoto-
chromic biphenyl side chains (40) have been prepared to investigate the effect
of aggregation on photoisomerization, and in particular on the possibility of
93
reorienting the chromophores by polarized irradiation. The polymers form
mono- and multilayers that have the same densely packed, bilayered structure
as the homopolymers, but the azobenzene moieties are spatially decoupled
and are not aggregated in the LBK films. Nevertheless, photoreorientation by
polarized irradiation is hindered, as is the case in LBK films of the homo-
93
polymer 38 (n = 2).
Azobenzene chromophores have been incorporated not only in side chain
polymers, but also in other polymer structures. There are several examples
94 97
of incorporation of azobenzene into the polymer backbone " or in the
98
backbone and in the side chain. In these works, the main interest was to
43 94 95
realize photoinduced changes of the polymer coil in solution. ' ' Some of
the main-chain azobenzene polymers were investigated in monolayers at the