Sugar-dependent spectral responses of azobenzene glycopyranoside monolayers

Novel amphiphilic glycopyranosides containing an azobenzene chromophore and an amide-benzene auxochrome are synthesized. A sugar headgroup is attached to the auxochrome unit that links directly to azobenzene. The UV-vis spectral response of the monolayer to a change of the molecular area is found to depend on the chemical structure of the sugar unit, α-D-glucopyranose or α-D-galactopyranose. When spread on the water surface at large molecular areas, both compounds show π-π* long-wavelength peaks at around 460 nm. Upon compression of the films, the peaks for the glucopyranoside exhibits a red shift, whereas that for the galactopyranoside gives a blue shift. A reference compound having a carboxylic acid in place of the sugar unit produces a blue shift because of the formation of H-like aggregates. The pressure isotherm and a morphological study suggest that a similar aggregation is responsible for the blue shift due to the galactopyranoside. FTIR spectroscopy indicates that the amide group of the glucopyranoside contains mostly the -C=N- resonance form. This resonance form increases the planarity of the azobenzeneamide-benzene units, resulting in the observed red shift. This study offers a basic chemical structure for systematic controls of dye aggregates by sugar.