Page 223 - Photoreactive Organic Thin Films

202 HENNING MENZEL

reported an example of a structural change that takes place in an LBK film of
101
a double-chain azobenzene amphiphile complexed with polyviologen. With
repeated isomerization cycles, the tilt angle of the azobenzene moieties
increases significantly. Upon storage in the dark, however, the original chro
mophore orientation—almost perpendicular to the substrate—was restored.
Another interesting example of photoinduced structural changes involves
molecules made up of two flat, ring-shaped cyclic octapeptides connected by
an azobenzene moiety (see Figure 6.18). The peptide rings were designed in
such a way that they form hydrogen bonds on only one side, resulting in
dimerization only. In solution, fraws-isomers have been shown to form inter-
molecular assemblies (Figure 6,18), whereas ds-isomers form discrete dimers

FIG. 6* 18 Photoactive peptide made up of two fiat, ring-shaped cyclic octapeptides connected by
azobenzene, and their cylindrical aggregates in the trans state (intermolecular aggregate) and in the as
state (intramolecular aggregate). (Reproduced from reference 102 with permission from Wiley-VCH).