Molecular orientation of azobenzene amphiphiles in surface monolayers and Langmuir-Blodgett multilayers

Monolayer characteristics at the air/water interface and multilayer formation by the Langmuir-Blodgett (LB) technique were examined for a series of bilayer-forming azobenzene amphiphiles. The amphiphiles with the ammonium head group formed well-spread monolayers, whereas those with the ethylenediamine and oligo (ethylene oxide) head groups gave crystalline dispersions on water. The ammonium monolayers became either expanded or condensed in the π-A isotherm by complexation with anionic polymers in the subphase. In reflectance spectroscopy, blue shifts (λ_max, 300-340 nm) typical of the parallel azobenzene stacking were observed for condensed monolayers, whereas expanded monolayers gave λ_max at 360-370 nm which suggested the presence of non-interacting azobenzene units or of tilted azobenzene stacking. LB films were readily obtained from polyion-complexed monolayers. The chromophore orientation in the LB film after each deposition was inferred from difference spectra. The parallel orientation in condensed monolayers was maintained after deposition. The tilted chromophore arrangements also remained unchanged, and some expanded monolayers gave parallel (condensed) stacking upon deposition.