Gelation and self-assembly of glutamate bolaamphiphiles with hybrid linkers: effect of the aromatic ring and alkyl spacers

Novel bolaamphiphilic gelators on the basis of L-glutamic acid, which use rigid aromatic substituents together with different lengths of alkyl chains as spacers, were synthesized. Depending on the structure, some of the molecules can gel a broad variety of solvents, from hexane to water/methanol mixture. The corresponding gels were characterized with UV-vis, CD, FT-IR spectroscopy, XRD, atomic force microscopy (AFM) and transmission electron microscopy (TEM). Nanofiber structures were obtained in nearly all the organogels. The results revealed that the introduction of the aromatic ring as well as the variation of alkyl spacer length greatly changes the gelation property and the corresponding gel structures. In pure organic solvents, the hydrogen bonding from amide groups play an important role and the even-odd alkyl chain can affect the gelating and self-assembly. In water/methanol, bolaamphiphiles with odd number methylene units can also form gels with strong pi-pi stacking working as the driving force. A possible model was proposed to explain the different self-assembly properties of the bolaamphiphiles.