Sol–gel phase transition induced by fiber–vesicle structural changes in sugar-based bolaamphiphiles

Abstract

The research objects of this paper were to design bolaamphiphilic gelators utilizing a sugar family as a source of solvophilic groups and an azobenzene segment as a solvophobic group and to monitor the aggregation mode utilizing the spectroscopic properties of the azobenzene chromophore. The results indicated that the bolaamphiphiles act, although only for specific DMSO–water mixtures, as gelators and form a unique supramolecular helical structure in the gel phase. The UV-Vis and CD spectra showed that the azobenzene segments adopt H-type face-to-face orientation and the dipole moments are arranged in the right-handed (R)-helicity. Since the fibrils as observed by electron microscope possess the right-handed helical structure, one may consider that the microscopic azobenzene–azobenzene orientation is reflected by the macroscopic supramolecular structure. When boronic acid-appended poly(L-lysine) was added, the gel phase was changed into the sol phase in the macroscopic level and the fibrous aggregate was changed into the vesicular aggregate in the microscopic level. These changes, which are usually induced by a temperature change, are due to the specific boronic acid–sugar interaction occurring at the constant temperature. Interestingly, when D-fructose which shows high affinity with the boronic acid group was added, the sol phase and the vesicular aggregate were changed back to the gel phase and the fibrous aggregate, respectively. This means that the phase and morphological changes in the sugar-integrated bolaamphiphiles can be controlled reversibly.