William Edwards

Solvent–gelator interactions—using empirical solvent parameters to better understand the self-assembly of gel-phase materials

By studying a family of L-lysine bis-urea gelators with variable peripheral groups in different solvents, a more detailed understanding of the way in which gelator fibres interact with the surrounding solvent environment is obtained. In all cases, these gelators establish the same hydrogen bonding molecular recognition pathways—however, this process is mediated by the nature of the solvent. In terms of Kamlet–Taft parameters, the α parameter of the solvent (hydrogen bond donor ability) has primary importance in controlling whether the gelator can establish a hydrogen bond network; the β parameter (hydrogen bond acceptor ability) plays a secondary role in tuning the thermal stability of the gel, and the π* parameter (polarisability) controls the solvation of the peripheral groups on the gelator by the solvent, and hence tunes the gel stability and the ability of the gelator to establish fibre–fibre interactions, as evidenced by scanning electron microscopy imaging. Considering solvent parameters allows us to gain a unique insight into hierarchical assembly processes at different length scales, i.e., molecular scale gelator–gelator interactions, and nanoscale fibre–fibre and fibre–solvent interactions. These processes are at the heart of developing effective models for the dynamic assembly of gel-phase soft materials.

Self-organisation effects in dynamic nanoscale gels self-assembled from simple mixtures of commercially available molecular-scale components

In this paper we report a new, simple gelation system based on an acyl hydrazone building block. These gels can be formed by simple mixing of aldehyde and hydrazide followed by a heat/cool cycle, and constitute a simple type of multi-component gel, in which a new covalent bond is formed. Importantly, we demonstrate two different outcomes when this gelation system is challenged with mixtures of different aldehydes – in some cases, only one of the aldehydes will react with the hydrazide to preferentially form the effective gelator and assemble into organised gels, while in other cases, both aldehydes will react with the hydrazide to yield co-assembled systems. Detailed insight into these assembly processes is obtained through NMR and differential scanning calorimetry. Understanding self-organising systems is a vital step towards the self-assembly of multi-functional advanced materials from complex systems, and also indicates how self-assembly processes, when coupled with chemical reactivity, allow order to spontaneously rise from chaotic mixtures.

Using EPR spectroscopy as a unique probe of molecular-scale reorganization and solvation in self-assembled gel-phase materials

We describe the synthesis of spin-labeled bis-ureas which coassemble with bis-urea gelators and report on self-assembly as detected using electron paramagnetic resonance spectroscopy (EPR). Specifically, EPR detects the gel-sol transition and allows us to quantify how much spin-label is immobilized within the gel fibers and how much is present in mobile solvent pools-as controlled by temperature, gelator structure, and thermal history. EPR is also able to report on the initial self-assembly processes below the gelation threshold which are not macroscopically visible and appears to be more sensitive than NMR to intermediate-sized nongelating oligomeric species. By studying dilute solutions of gelator molecules and using either single or double spin-labels, EPR allows quantification of the initial steps of the hierarchical self-assembly process in terms of cooperativity and association constant. Finally, EPR enables us to estimate the degree of gel-fiber solvation by probing the distances between spin-labels. Comparison of experimental data against the predicted distances assuming the nanofibers are only composed of gelator molecules indicates a significant difference, which can be assigned to the presence of a quantifiable number of explicit solvent molecules. In summary, EPR provides unique data and yields powerful insight into how molecular-scale mobility and solvation impact on assembly of supramolecular gels.

Chiral Assembly Preferences and Directing Effects in Supramolecular Two-Component Organogels

The impact of chirality on the self-assembly of supramolecular gels is of considerable importance, as molecular-scale programming can be translated into nanostructuring and ultimately affect macroscopic performance. This paper explores the effect of chirality on the assembly of two-component gels comprised of a second-generation dendritic lysine peptide acid, containing three chiral centres, and an amine. This combination forms an acid–amine complex that assembles into nanofibres through peptide-peptide hydrogen bonds, leading to organogels. With achiral amines, a racemic mixture of l,l,l and d,d,d dendritic peptide acids surprisingly forms the best gels—more commonly, mixing enantiomers suppresses gelation. Thermodynamic studies demonstrate that depending on the amine, the greater stability of heterochiral gels can either be entropically or enthalpically driven. With amines possessing “R” chirality, the l,l,l peptide acid consistently forms more effective gels than its d,d,d analogue. Furthermore, in mixed gels, l,l,l sometimes imposes its assembly preference onto d,d,d. In summary, this paper demonstrates a rare example in which heterochiral gels are preferred, and also explores directing effects when each component in a two-component gel is chiral.