Structural investigations on LB films of “hairy rod” polymers with mesogenic groups in the side chains

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Poly(5-(2-(4-(4-hexylphenylazo)phenoxy)ethyl)- L-glutamate), poly(5-(6-(4-(4-hexylphenylazo)phenoxy)hexyl)- L- and poly(5-(4-phenylazo)benzyl- L -glutamate-co-5-(4-hexadecyloxy)benzyl- L glutamate) were synthesized by polymerization and copolymerization of the corresponding N-carboxyanhydrides. LB films of these “hairy rod” polymers bearing mesogenic groups within the side chains were investigated by means of X-ray diffraction and spectroscopic methods (Fourier transform infrared, UV-vis). The LB films are composed of deformed hairy rods ( i.e. with a non-symmetrical distribution of the side chains around themain chain helix) arranged in bilayers. When the azobenzene moiety is isomerized photochemically, the deformation relaxes towards a more symmetrical distribution of the side chains around the helix, and, depending on the length of the spacer, there is a reorientation of the azobenzene groups with respect to the helices. Furthermore, it was found that there is an aggregation of the chromophores causing a blue shift and a broadening of the main absorption band. Photoisomerization and concomitant structural changes cause a decrease in the amount of aggregated chromophores, the extent of which depends on the length of the spacer used to tether the chromophore. The structural changes are accompanied by changes in the optical properties, which can be detected by surface plasmon microscopy.