Sugar-dependent spectral responses of azobenzene glycopyranoside monolayers

Novel. amphiphilic glycopyranosides containing an azobenzene chromophore an d an amide-benzene auxochrome are synthesized. A sugar headgroup is attache d to the auxochrome unit that links directly to azobenzene. The UV-vis spec tral response of the monolayer to a change of the molecular area is found t o depend on the chemical structure of the sugar unit, alpha -D-glucopyranos e or alpha -D-galactopyranose. When spread on the water surface at large mo lecular areas, both compounds show;pi-pi* long-wavelength peaks at around 4 60 nm. Upon compression of the films, the peaks for the glucopyranoside exh ibits a red shift, whereas that for the galactopyranoside gives a blue shif t. 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 p ressure isotherm and a morphological study suggest that a similar aggregati on is responsible for the blue shift due to the galactopyranoside. FTIR spe ctroscopy indicates that the amide group of the glucopyranoside contains mo stly the -C=N- resonance form. This resonance form increases the planarity of the azobenzene amide-benzene units, resulting in the observed red shift. This study offers a basic chemical structure for systematic controls of dy e aggregates by sugar.