A study of the complex relationship between self-assembly, activity and material properties of low molecular weight gelators

The self-assembly of small organic molecules can lead to interesting materials such as non-Newtonian fluids and gels. These have potential applications ranging from cell culturing to organic electronics. In this work, a selection of molecules that self-assemble and form low molecular weight gels are studied. In a high throughput photocatalysis experiment, five amino acid functionalised perylene bisimides under a range of variables were tested for the photocatalytic hydrogen evolution reaction from water. The results showed that the formation of 1D nanofibres with a specific molecular packing and surface charge produced optimum rates of H2. Two gelator molecules that formed 1D nanofibres but possessed a different molecular packing were inactive, highlighting the importance and sensitivity of the self-assembly process for any potential application. As well as manipulating the self-assembled structures that form, the ordering of those nanostructures can be controlled through the formation of so called ‘gel noodles’. The influence that nanostructure alignment would have on photocatalysis was of interest but PBIs did not readily form gel noodles. Using a model system of a functionalised dipeptide (1ThNapFF), it was determined that pre-gel solutions containing 1D nanofibres that result in viscous shear-thinning fluids were required to form gel noodles. Highly aligned gel noodles with a uniform size distribution could be prepared by using a spinning technique to subject the pre-gel solution to a stretching and narrowing of diameter (extensional deformation). In the literature, an isoleucine-functionalised PBI (PBI-I) was reported to form highly viscous pre-gel solutions. These solutions were tested. Aligned gel noodles were prepared without the need for the spinning technique. The subtle change in molecular structure resulted in vastly different shear viscosities, facilitating the formation of the gel noodles. Un-aligned gel noodles could be prepared through delayed gelation. However, no definitive conclusion as to whether or not alignment improved hydrogen evolution could not be drawn. A third, more aligned sample, was of interest for the photocatalysis but attempts using the spinning technique on the PBI-I solutions were unsuccessful. The PBI-I pre-gel solution did not stretch during spinning. Instead resulted in numerous broken-up small lengths of unaligned gel noodle. It was hypothesised that this was due to the PBI-I solution possessing a low extensional viscosity relative to 1ThNapFF. A literature technique (called ‘dripping-onto-substrate’) to measure extensional relaxation time was setup, validated and used to show that the 1ThNapFF solutions behave as elastic fluids with long extensional relaxation times. whereas the PBI-I solutions did not. While both solutions self-assembled to form wormlike micelles with a similar shear viscosity, they still possessed fundamental differences in fluid properties, highlighting the complexity of studying and manipulating these systems. It is evident that there is no ‘catch all’ approach to analysing them, and a range of techniques are required