Mechano-responsive calix[4]arene-based molecular gels: agitation induced gelation and hardening

Two novel cholesteryl derivatives of calix[4]arene with L- or D-phenylalanine residues in the linkers (1 and 2, respectively) were designed and synthesized. The gelation behaviors of the compounds in 30 organic solvents were tested. It was demonstrated that 1 gels n-butanol and n-pentanol at room temperature, but 2 gels isopropanol only under the treatment of heating-cooling cycling or energy supplementing via sonication, vortex or agitation at room temperature. AFM and SEM measurements revealed that the structures of the gel networks are greatly affected by the spatial configurations of the linkers contained in the calix[4] arene derivatives. Furthermore, mechanical treatment not only promotes the gelation of the system of 2-isopropanol, but also enhances the strength of the gel. Specifically, 18 min of agitation at room temperature makes the storage modulus and the yield stress of the gel (3.5%, w/v) exceed 1 x 10(6) Pa and 6 x 10(3) Pa, respectively, which are second only to the mechanically strongest low molecular mass gelator- based molecular gel reported until now. XRD analysis revealed the hexagonal packing structure of 1 in its n-pentanol gel.