Azobenzene derivative monolayer film

Figure 23. (a) Schematic representation of an anionic surfactant azobenzene derivative monolayer film at the air-water interface. (i>) Schematic representation of the stable monolayer film formed from the polyion complex of anionic surfactant azobenzene derivatives with a cationic polymer. Note the difference in free volume around the reactant chromophores in the two monolayers. 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

If one makes use of the rather limited information available and given above one may infer that a tilt of between 20° and 30° is normal for straight chain azobenzene derivatives when deposited as LB films, even when a homeotropic phase exists. Such a structure can only be produced in a rather loosely packed film. At the moment it is an open question whether monolayers of these materials exist in the hexatic phase as is the case for fatty acids or whether the structure more nearly corresponds to the smectic-A phase. In the case of the birefringent phase described by Jones et al. [151] it was shown that, once this phase was established, further layers deposited by the LB technique go down in an epitaxial manner. 

In addition to crown-ethers, calix[4]resorcinarenes 19 have also been used as head groups to obtain azobenzene amphipbiles with sufficient photo-isomerization in LBK films. For this purpose, azobenzene moieties have been tethered to the lower rim of the crown conformer of the calixarene. O-octacarboxymethoxylated calix[4]resorcinarenes 19 (X = CH2-COOH) display efficient trans to cis photoisomerizability in densely packed mono-layers on a water surface, in LBK films, and in surface-adsorbed monolayers, whereas the noncarboxymetylated 19 (X = H) derivative gives films that are too densely packed. However, the aggregation is already suppressed efficiently compared to the azobenzene derivative without calixaren. ... 

To summarise, azobenzene derivatives can be used to form high quality monolayer films. The azo-linkage provides a masked functionality which is suited to a wide range of chemical and photochemical manipulations. In particular, it offers a means to fabricate patterns of adsorbed species in the plane of the monolayer, using photolithography. 

Besides azobenzene, spirobenzopyran, and salicylidene 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 the area at constant pressure upon irradiation. Diphenyldiacetylerie chromophores 6 (Figure 6.5) have shown photoinduced anisotropy upon polar-... 

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