Page 203 - Photoreactive Organic Thin Films

132 HENNING MENZEL

hv
COOH COOH

FIG. 6.3 Reversible photochroism of amphiphilic saiicylidene aniline 3 in LBK films.

6.2.3 Spiropyrans
The photoreaction of spiropyrans 4 comprises a bond cleavage resulting in a
zwitterionic species, the merocyanine, and subsequent cis to trans isomeriza-
tion (see Figure 6.4). There are several species in equilibrium, so the
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chemistry and photochemistry of the spiropyrans are rather complex. The
photoreaction of spiropyrans is sterically quite demanding and has been
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reported to be restricted in LBK films.
There are also reports describing unhindered photoreaction of spiropy-
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rans in LBK films. Nevertheless, the matrix formed by the LBK films influences
the reaction, and the optical properties of the material can be fine-tuned by
the choice of the film. For example, it is possible to selectively aggregate the
chromophores and to build up a layered multifrequency recording medium
u
for optical data storage as has been shown by Hibino et al. The authors
used amphiphiles with differing polarities and concentrations to generate
a medium of tailored polarity that favors the formation of aggregates.
Furthermore, the colored ionic form was stabilized by adsorption to a mono-
layer of a cationic amphiphile, or if it forms a monolayer, by adsorption of an
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polyelectrolyte.

6.2.4 Other Chromophores
Besides azobenzene, spirobenzopyran, and saiicylidene derivatives, other
photochromic dyes have been used in LBK films. For example, anthocyanine
dyes 5 (see Figure 6.5) have been used to obtain monolayers that change
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the area at constant pressure upon irradiation. Diphenyldiacetyiene chro-
mophores 6 (Figure 6.5) have shown photoinduced anisotropy upon polar-

NO,
FIG. 6.4 Photochemical reactions for spirobenzopyrans 4.