Supramolecular Gels from Bis-amides of L-Phenylalanine: Synthesis, Structure and Material Applications

A series of small organic molecules having a bis-amide backbone containing hydrogen-bond functionalities were rationally designed, synthesized and characterized to examine their ability to act as potential low-molecular-weight gelators (LMWGs). All the bis-amides were decorated with identical 3-pyridyl amide of L-phenylalanine moieties along with variously substituted terminal benzoyl groups. Gelation studies revealed that only 4-methylphenyl substituted bis-amide (PME) was capable of gelling both aqueous (DMSO/water) and methyl salicylate (MS) (an important solvent for topical formulation for medical applications) solvents; whereas 4-chlorophenyl and 4-bromophenyl substituted bis-amides (PCL, PBR, respectively) acted as organogelator for various organic solvents. On the contrary, 4-nitrophenyl as well as 3,5-dinitrophenyl substituted bis-amides (PNI, DNI, respectively) were unable to gel any solvents studied herein. The corresponding aqueous gel namely PME-HG and three methyl salicylate gels PME-MS, PCL-MS and PBR-MS were characterized by dynamic and table top rheology followed by electron microscopy. Single crystal X-ray diffraction (SXRD) data revealed crucial insights into the supramolecular assembly of all the gelator and nongelator bis-amides. Both PME-HG and PME-MS were rheoreversible - an important property in material applications. Interestingly, PME-MS displayed remarkable material properties such as shape-sustaining, loadbearing and self-healing. Selected MS and aqueous gels loaded with nano-molar iodine were found to possess anti-bacterial property as revealed by zone inhibition assay.