Enhanced Mechanical and Thermal Strength in Mixed-Enantiomers-Based Supramolecular Gel

Abstract

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.

Urea Derivatives as Functional Molecules: Supramolecular Capsules, Supramolecular Polymers, Supramolecular Gels, Artificial Hosts, and Catalysts

“…Domodaran et al. investigated the gelation abilities of bis‐urea compounds with two chiral centers [109] . The trend in this study is different from that of Yamanakas’ results, and all homochiral, heterochiral, and racemic bis‐ureas show gelation ability.…”

Using chirality to influence supramolecular gelation

“…Marchesan et al showed that changing the chirality in the N-terminus of a tripeptide could result in a non-gelator becoming a gelator, with for example VFF becoming a gelator when the d -isomer of the terminal valine was used 13,14. In rare examples, a mixture of two enantiomers has been shown to lead to gels with improved properties 9,15,16…”

Chirality, Gelation Ability and Crystal Structure: Together or Apart? Alkyl Phenyl Ethers of Glycerol as Simple LMWGs

“…While many of them form gels (not always identical in characteristics) regardless of the enantiomeric composition, relatively simple compounds of this class exhibited their specific properties only in the enantiopure form and did not exhibit in the racemic form, and vice versa. Examples of such behavior are given in reviews [22,23] and this effect has also been noted in recent publications [24][25][26][27]. Subsequently, this minor mystery of chiral gels was obscured by other, significant in meaning and spectacular in illustrations, problems of supramolecular chirality [28,29] and chiral recognition [30], which are clearly manifested in nanostructured media.…”