Exploring the impact of self-assembly pathways on the structures and properties of small-molecule gels

Abstract

Gels based on low-molecular-weight gelators (LMWGs) often consist of highly extended sheets or fibrils, which fuse or intertwine to produce a sample-spanning network. The aim of this investigation was to gain insight into such hierarchical processes by characterising a variety of urea-based LMWGs, and modelling the impact of key structural features on the outcome of self-assembly.

Self-assembly of bis(urea) LMWGs often produces chains of urea-urea hydrogen bonds known as α tape motifs. One series of bis(urea)s with a sterically hindered spacer vary greatly in their gelation abilities, and form single crystals comprising a diverse range of α tape networks. Molecular dynamics simulations suggest that gels arise when crystal growth is outcompeted by the spontaneous scrolling of isolated lamellar assemblies. Thus, gelation is mainly associated with species that self-assemble into asymmetric lamellae, which undergo scrolling due to the differing forces exerted by the upper and lower faces.

Even small changes to a self-assembly pathway can dramatically alter the resulting material. Photoisomerisation of bis(urea)s with terminal salicylidene-aniline (anil) moieties requires that the surface imine groups of crystalline lamellae can freely rotate, and is inhibited when gelation or co-crystallisation results in a less optimal molecular arrangement. Likewise, the anion affinities and aggregate microstructures of five isomeric linear tris(urea)s depend strongly on their spacer configurations. Although these molecules are not effective LMWGs, more extended oligo(urea)s can deliver higher gelation capacities. Remarkably, one achiral pentakis(urea) self-assembles into amyloid-like braided helices that may be chirally enriched by a template material. In addition, the tris(urea) analogue of this compound can form interfacial “lilypad” metallogels under vapour-diffusion conditions, demonstrating a novel mode of self-assembly with potential applications in metal sequestration. Building on these results could lead to supramolecular material with more complex microstructures and stimuli-responsive functionalities, and aid our understanding of the hierarchical self-assembly processes observed in biological systems.