Langmuir-Blodgett films of photochromic polyglutamates 3. Spectroscopic studies on LB films of photochromic polyglutamates with alkylspacers of different length

Abstract

Abstract Poly (5-(2-(4-(4-hexylphenylazo) phenoxy) ethyl)- l -glutamate) (P2) and poly(5-(6-(4-(4-hexylphenylazo) phenoxy) hexyl)- l -glutamate (P6) are “hairy rod” polymers (i.e stiff polymer backbone with flexible side chains) bearing photochromic azobenzene moieties in the side chain coupled to the main chain via spacers of different length. Laagmuir-Blodgett films of these polymers were investigated using X-ray diffraction, FTIR and UVVIS spectroscopy. The structure of freshly prepared LB films is very similar for both polymers and is built up by deformed “hairy rods” (i.e with a non-symmetrical distribution of side chains around the polymer backbone) arranged in layers with main chains preferentially oriented in the dipping direction. The side chains are preferentially oriented normal to the surface of the substrate, additionally the projection of the long axis of the azobenzene moiety into the film plane lies preferentially parallel to the main chains. On isomerization of the azobenzene moieties by irradation, the molecular arrangement is changed dramatically. The layered structure is lost, due to rearrangement of the deformed “hairy rods” to a more symmetrical distribution of the side chains around the main chain, whereas the preferred orientation of the main chains is retained. In addition, for P2 projection of the azobenzene long axis changes from being parallel to the main chain to a more perpendicular orientation. Due to group interaction, as in the combined main chain and side chain liquid crystals, the preferred orientation of the chromophores parallel to the main chain is preserved in the case of P6.