Photo-, thermo- and electrochromic behaviour

Photochromic LB materials offer much potential for applications in optical data storage. The molecular processes involved include proton exchange, tautomerism, charge transfer and cis-trans isomerization. 

Spiropyrans (Fig. 3.9a) and fulgides (Fig. 3.9b) provide examples of tautomeric-based photochroism. In the case of (44), UV-induced 

While the stability of LB spiropyrans needs to be improved for longterm device use (thermal fatigue being the main problem), these materials have much potential as components in multifrequency memory and optical logic devices. This can be achieved by using a multilayer structure 

The materials (42)-(47) illustrate the photochromic behaviour exhibited by relatively simple molecules. However, photochroism is also known for polymeric materials and often involves cis-trans isomerization of an azobenzene unit incorporated into the polymer. For example, photochromic behaviour has been demonstrated when the azobenzene core is incorporated as either a main-chain polymer (48) [76] or as a side-chain in a phospholipid polymer (49) [77]. 

Electrochromic effects have been observed for LB films of rare-earth-substituted phthalocyanines (2), with X = Yb, Pt [82]. In general, the compressed monolayers are only found to be sufficiently stable when dimers of the type YbPc2 are used. However, electrochromic switching is only observed when alternative Y-type layers of the dimer are deposited with fatty acids. It has been established that a self-reorganization mechanism of the bisphthalocyanines operates, and that the alkyl-chain length of the acid can significantly affect the switching mechanism. For example, if the chain contains fewer than 20 carbon atoms, the electrochromic process is not impeded if 20 or 21 carbon atoms are involved, the 

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