The control and prediction of the self-assembly process in multicomponent supramolecular gels are challenging because the structure and properties rely mostly on the geometry and spatial arrangement of the building blocks. The understanding of noncovalent interactions between the individual gelators at the molecular level will enable us to tune the gelation properties of multicomponent gels. We have studied the self-assembly process of multicomponent gel based on enantiomers and herin we report the first crystallographic evidence of specific co-assembly in mixed enantiomeric gel, which is supported by scanning electron microscopy and atomic force microscopy images. The mode of interactions between the individual gelators from the molecular to macroscopic level, which are responsible for co-assembled fibers, was identified by single-crystal X-ray diffraction. We have proved that specific co-assembly leads to enhanced mechanical and thermal stability in the mixed gel compared to the meso and individual enantiomeric gels.
Mixing supramolecular gels based on enantiomers leads to re-arrangement of gel fibers at the molecular level, which results in more favorable packing and tunable properties. Bis(urea) compounds tagged with a phenylalanine methyl ester in racemic and enantiopure forms were synthesized. Both enantiopure and racemate compounds formed gels in a wide range of solvents and the racemate (1-rac) formed a stronger gel network compared with the enantiomers. The gel (1R+1S) obtained by mixing equimolar amount of enantiomers (1R and 1S) showed enhanced mechanical and thermal stability compared to enantiomers and racemate gels. The preservation of chirality in these compounds was analyzed by circular dichroism and optical rotation measurements. Analysis of the scanning electron microscopy (SEM) and atomic force microscopy (AFM) images revealed that the network in the mixed gel is a combination of enantiomers and racemate fibers, which was further supported by solid-state NMR. The analysis of the packing in xerogels by solid-state NMR spectra and the existence of twisted-tape morphology in SEM and AFM images confirmed the presence of both self-sorted and co-assembled fibers in mixed gel. The enhanced thermal and mechanical strength may be attributed to the enhanced intermolecular forces between the racemate and the enantiomer and the combination of both self-sorted and co-assembled enantiomers in the mixed gel.
ARTICLE
This journal isWe report the selective gelation properties of copper(II) complexes of N-(4pyridyl)nicotinamide (4PNA). The morphology of the xerogels was examined by scanning electron microscopy (SEM). Correlation of X-ray powder diffraction (XRPD) pattern of the xerogels and single crystal structure of copper(II) acetate complex suggests that the single crystal X-ray data represents a good structural model for the gel fibers themselves and that gelation arises from the presence of a 1D hydrogen bonded chain between gelator amide groups and coordinated anions, while the presence of strongly bound water in non-gelator systems results in the formation of more extensively hydrogen bonded crystalline networks. The selective gelation of all the copper(II) salts compared to the other metal salts may be attributed to Jahn-Teller distorted nature of copper(II), which weakens water binding in all copper(II) salts.
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