'Bifunctional' sugar-integrated gelators for organic solvents and water - On the role of nitro-substituents in 1-O-methyl-4,6-O-(nitrobenzylidene)-monosaccharides for the improvement of gelation ability

The influence of modifications at the benzylidene protection group on the gelation ability of 1-O-methyl-4,6-O-benzylidene-monosaccharides was investigated. For α-D-glucose and α-D-mannose the ability for self-aggregation was found to be significantly extended by the introduction of a p-nitro-substituent. As a result, a new type of the so called 'bifunctional gelators', that are able to gelate to the same extent nonpolar and aromatic solvents as well as alcohols and water, was obtained. This effect was found to be strictly limited to nitro-substituents in the para-position. In contrast, ortho- and meta-derivatives show a drastic decrease in the gelation ability for all types of solvents. CD spectroscopy and experiments with solvent mixtures suggest that two aggregation modes are used for different solvents. In nonpolar liquids the establishment of hydrogen-bonds rules the self-aggregation process, whereas in protic solvents such as alcohols and water other aggregation modes such as hydrophobic interactions and dipole-dipole interactions are likely to be the main driving forces for gelation. To characterise the morphology of the gel network scanning electron microscopy (SEM) was carried out with the xerogels. Thus, this paper describes a new and facile method to design robust and versatile low molecular weight gelators.