Page 217 - Photoreactive Organic Thin Films

196 HENNING MENZEL

In polyurethane 33, the azobenzene moieties are separated from each
other along the polymer backbone by isophorone units and have the free
volume necessary for isomerization and molecular reorientation. This poly-
mer's azobenzene moieties can be photoisomerized readily in mixed LBK.
films, as demonstrated by measuring the optically induced birefringence that
originates from the photoreorientation of the chromophore upon polarized
69
irradiation (see Section 6.5.2).

33

A system similar to the partially substituted poly(vinylalcohol) was
2S
proposed by Penner et al. They prepared copolymers of a mesogenic
azobenzene monomer and hydroxyethylacrylate (HEA) 34 with different
compositions.

Copolymers 34 with a 1:5 or 1:10 composition transferred readily as
multilayers. UV-vis spectroscopy revealed through a strong blue shift of the
n-n*-band compared to the solution that the azobenzene moieties are highly
aggregated in the LBK films at room temperature. The extent of the blue shift
suggests a crystalline order in the LBK film. The situation changed when the
LBK film was heated into the liquid crystalline phase (around 50 to 60°C for
the LBK film). The phase transition was accompanied by shift in the peak
position from 335 nm (H-aggregates) to 355 nm (dimers/non aggregated). 28
That is, by heating into the liquid crystalline phase, the packing of the chro-
mophores was significantly loosened; there was still sufficient interaction to
stabilize the LBK film, however. 28
Illumination of LBK films of copolymers 34 (1:2.4) in the liquid
crystalline state at 63°C, with polarized light at 457 nm, results in a photo-
induced optical anisotropy. That is, the chromophores can be reoriented,
70
which indicates that efficient photoisomerization is possible. Photo-
reorientation is also possible at room temperature after the original structure
of the LBK film has been broken up by irradiation with UV light (that is,
after a photostationary state with a high content of «s-isomer has been
established). 71
The copolymers 35 and the corresponding homopolymer (n = 0) are
72 75
liquid crystalline, too, and have been deposited as LBK films. " In the