Molecular Recognition and Component Selection in Supramolecular Gels

This thesis will cover aspects of molecular recognition and component selection in supramolecular gels. In the first section a bola-shape alkene appended organogelator is tested for its response to a number of cations. The gelator was found to undergo a gel-sol transition upon addition of Li+ and Ag+ and not when either Na+ or K+ was added. The response to Li+ was caused by the higher charge density of the ion compared to the other group I metals, allowing it to disrupt the intermolecular hydrogen bonding between the gelator molecules. The response to Ag+ was shown to be due to an interaction between this ion and the alkene groups of the gelator. This demonstrated that the silver(I)-alkene interaction can be utilised in the gel phase.
The second part of this thesis will investigate the formation of multi-component gels based on 1:1 complexes formed between a lysine based, carboxylic acid bearing dendron and a monoamine. The forces underpinning gelation as well as the effect of changing the solvent and changing the amine used are all investigated. This section then explores component selection, where a number of possible amines are added to a starting mixture but the dendron will form a gel with predominately one of the amines in preference to the others. The amines that are not incorporated into the network were easily viewed by NMR spectroscopy. A number of different amines were tested which all formed gels of different thermal stabilities with the dendron and also had different pKa values. Both of these factors were shown to be important in driving the preferential selection of a certain amine as they described the amines ability to form a complex with the dendron (pKa) and assemble as part of a gel network (thermal stability).
The thesis then describes the effect of using chiral amines, enantiomers of which form complexes with the chiral dendron which are diastereomeric. The effect of using different enantiomers of an amine was studied and found to dramatically alter the structure of the gelator network and the behaviour of the gel formed. These effects could be tuned by mixing different amounts of each enantiomer into the sample. As the gels formed with different enantiomers had different thermal stabilities they were able to be used in a number of experiments to test for component selection. These did indeed show this effect to be taking place, with the enantiomer which formed the most stable gel preferentially incorporated into the gelator network.
The final section of this thesis reverses the concept in the previous section and investigates the effect of changing the chirality of the lysine based dendron when mixing it with non-chiral amines, or a single enantiomer of a chiral amine. When mixed with non-chiral amines, as expected, changing the chirality of the dendron changed the structure of the gelator network and the properties of the gel formed and this could be tuned by mixing different amounts of the dendron enantiomers. Unexpectedly, this resulted in more thermally stable gels being formed with the racemic mixtures rather than their enantiopure equivalents, a very rare occurrence. When chiral amines were used the results were very similar to the previous section, with different enantiomers of the dendron changing the nano-scale structure of the network and the properties of the gels formed. Again this effect could be tuned by using different amounts of the dendron enantiomers to form the gel. Finally these systems were also tested to see if they could exhibit component selection but this study was inconclusive, partly down to the disruptive effect excess dendron had on the gel network formed.