Urea-based supramolecular gelators: molecular structure-gelation relationship and sensing of organophosphorus compounds

Abstract

The aim of this thesis was to study the phenomenon of supramolecular gelation of urea-based gelators and their application as sensors for neutral organophosphorus species such as nerve agents. The work was therefore naturally divided into two major parts. The first part consisted in an investigation of the structure-gelation relationship of a series of urea gelators, in an effort to overcome the serendipitous approach that is widely applied to their discovery. Among the components of the common gelator scaffold that were optimised to deliver the best gelation performance, particular attention was given to the role of the head substituent on its benzene ring. Crystal structure prediction calculations together with liquid- and solid-state NMR were used to understand the molecular reasons behind the observed macroscopic properties of supramolecular gels formed either by nitro- or methoxy-substituted gelators. Remarkably, this approach demonstrated that, rather than electronic effects, it was the nitro substituent’s ability to interfere with the urea hydrogen bond network to cause the differences observed in the gel formation experiments, when compared to the methoxy-analogue. The second part focused on the possible application of bis/tris(urea)-based supramolecular gels as organophosphorus warfare agents’ sensors. After the development of a fast and easily interpretable in-house test, it was possible to observe the effectiveness of different candidates in responding to the presence of either the nerve agent Soman or its simulant dimethyl methyl phosphonate. It was observed that in the presence of the guest molecules gelation could be delayed or even suppressed, suggesting the formation of hydrogen bonds between guest and host that were interfering with the self-assemble of the gelator molecules. Conversely it was also found that, if present in lower amount, dimethyl methyl phosphonate could instead induce a detectable thermo-mechanical reinforcement of the gel network, as confirmed by rheology and calorimetry results, which was ascribed to solvophobic effects.

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ORCID for Philip Gale: orcid.org/0000-0001-9751-4910

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