Page 215 - Photoreactive Organic Thin Films

194 HENNING MENZEL

A B C
FIG. 6.14 Schematic representation of the pH-dependent change in the monolayer structure of
azobenzene amphiphile 22 spread on (A) a pure water subphase, (B) a subphase containing 28 at
pH = 6.0, and (C) at pH = 8.0 (adapted from reference 61 with permission from Elsevter Science).

TABLE 6.1 Dependence of the Area per Amphiphile 22 (n = 5) on the
Polymeric Counterfoil 61

Counterion Area per molecule 22 CPK model

2
Ba * 0.25 0.32 _
poly(vinylammonium) chloride 25 0.37 0.54 0.35-0.41
poly(allylamine) 26 0.39 0.50 0.36-0.43
poly(N-methyl vinylpyridium) iodide 27 0.41 0.61 0.41-0.63
poly(N-benzylvinyl,N,N,N-trimethyl 0.42 0.59 0.56-0.70
ammonium) chloride 28
poly(N,N-diallyl-N,N-dimethyl 0.45 0.65 0.64-0.76
ammonium) chloride 29

improved photoisomerization. Although photoisomerization is completely
2+
suppressed in the case of the densely packed LBK films with Ba counterions,
it is almost unrestricted in the case of the poly(N-benzylvinyl,N,N,N-
61
trimethyl ammonium) chloride 28 counterion. Similar observations have
been made for the complexes of a two-azobenzene-chain amphiphile 30 with
polycations, and an almost linear relation was found between the area per
62
molecule and the fraction of ds-isomer in the irradiated LBK-film (see
Figure 6.15).
Recent investigations of a polyelectrolyte complex of 31 and poly(diallyl-
dimethylammonium) chloride 29 have shown that although photoisomeriza-
tion is unrestricted in the LBK films, some mechanical stress is built up in the
films upon trans to cis isomerization. This was established by AFM, which
shows reversible morphological changes. On irradiation, a number of protru-
sions with a height of ca. 5 nm appeared on the surface. These structures