Photomechanical Effects in Monolayers

Photomechanical effects have been also observed in monolayers obtained from poly(L-glutamic acid) modified with carbocyanine1831 and spiropyran dyes.184 In the latter case, irradiation at 254 nm produced changes in the molecular conformation, which in turn caused photomodulation of the surface pressure and surface area of the films. From all these examples, it appears that photoresponsive monolayers are quite fascinating systems, which may eventually come to be regarded as a machine to transform light into mechanical energy . 81  

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Menzel, H. Langmuir-Blodgett films of photochromic polyglutamates. 7. The photomechanical effect in monolayers of polyglutamates with azobenzene moieties in the side chains. Macromol. Chem. Phys., 195, 3747 (1994). 

Seki, T., and Tamaki, T. Photomechanical effect in monolayers of azobenzene side ch.iin polymers. Chem. Lett., 1739 (1993). 

Photomechanical effects in monolayers have also been observed in other cases, for example with so-called hairy-rod type poly(glutamatejs (see Chart 5.13) [58]. 

Menzel1811 more recently described the properties and behavior of monolayers prepared from azobenzene-containing poly(L-glutamate)s possessing the structures XIX and XX (n= 2) shown in Scheme 11. These monolayers showed photomechanical effects opposite to those described above for azo-modified poly(L-lysine)s. In fact, they expanded when exposed to UV light (trans—>tis isomerization), and shrank when exposed to visible light (ds— trans isomerization). The expansion was found to be smaller than expected from comparison of the monolayer isotherms obtained from irradiated and nonirradiated solutions. This was attributed to the trans-cis photoconversion of the azo units, which occurs with lower yields in monolayers... 

Another class of polymers equipped with azobenzene moieties comprises a-helical polypeptides, in particular pQly(L-glutamate)s and poIy(L-lysine)s. In solution, these azobenzene-modified polypeptides can undergo photoinduced helix-coil transitions. Polypeptides partially (30 to 50%) substituted with azobenzene moieties are surface active and form stable monolayers. Because of the partial substitution, there is sufficient free volume, and the azobenzene moieties can be isomerized in the monolayer. The photoisomerization changes the area per molecule, and the monolayer shows a photomechanical effect. LBK films of a photosensitive poly(L-lysine) with 31 mol... 

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