Page 167 - Photoreactive Organic Thin Films

MIKHAIL V. KOZLOVSKY, LEV M. BLINOV, AND WOLFGANG HAASE

X < 400 nm

X > 400 nm, /, A
U-form (or trans-isomsr) Z-form (or cw-isomer)

In this process, the trans-to-cis transformation is induced by UV light illumi-
nation, whereas the backward conversion at ambient temperature occurs
spontaneously but can be accelerated by increasing the temperature or by
illumination with yellow light.
The E/Z-isomerization process is characterized by angular-dependent
excitation and leads, therefore, to the photoselection of a preferred azobezene
dye orientation. In other words, the dichroic dye units choose an orientation
where the electronic transition moment is perpendicular to the light electric
vector. It promotes, in turn, the cooperative reorientation of neighboring
moieties, which include other fragments of the macromolecule, such as the
main chain or photochemically inactive comonomer units, and low molar
mass additives. Thus, a macroscopic orientation of the sample arises, and it
remains long after the illumination is stopped and all the dye moieties return
to the thermodynamically equilibratory E-state.
To date, various possibilities for incorporating an azobenzene dye into a
polymer system have been suggested and tested. First, solid dispersions of
4 8
low molar mass azo dyes in amorphous polymers were studied. " Then, the
amorphous copolymers containing chemically bound azobenzene fragments
16
9
were thoroughly investigated. "" Many attempts at combining the photo-
chromism of azobenzene systems with liquid crystallinity have also been per-
formed, probably because of the prominent role of liquid crystals in modern
display technology. In the simplest way, azobenzene dye can be dissolved in
17 19
an LC polymer matrix. " In addition, with polymer-stabilized liquid
crystals (PSLCs), one can introduce orientation into a low molar mass liquid
20 21
crystal through azobenzene-containing polymer networks, ' while other
authors, in an opposite manner, use LC polymer networks and low molar
22 23
mass azo dyes. ' Optical and electro-optical switching in polymer-dispersed
liquid crystals (PDLCs) with azobenzene molecules has been reported as
24 25
well. ' It is worth noting as well of that Wu et al, have performed systematic
studies on contactless photoalignment of liquid crystals by thin surface layers
26
of azodye polymers. "" 28
The most intensively developed materials, however, are the azobenzene-
containing side chain LC polymers, which show a unique combination of
liquid crystallinity and photochromic behavior in a single macromolecule,
whereas copolymerization of comonomers with different functionality allows
fine tuning of phase behavior, photo-optical properties, and other parameters
to the requirements of a particular application. Starting from the first publi-
29 31
cations of Eich et #/., ~ work in that area has been done by several research
32 42
groups. ' As compared with amorphous polymers, the liquid crystalline
systems possess an initial preorientation, which should be overcome during
the photo-orientation process. Generally, the higher the degree of mesogenic
group ordering, the lower the values of the photoinduced birefringence,