Squaramide-Based Self-Associating Amphiphiles for Anion Recognition

The synthesis and characterisation of two novel self-assembled amphiphiles (SSAs) SQS-1 and SQS-2 are reported. Both compounds, based on the squaramide motif, were fully soluble in a range of solvents and were shown to undergo self-assembly through a range of physical techniques. Self-assembly was shown to favour the formation of crystalline domains on the nanoscale but also fibrillar film formation, as suggested by SEM analysis. Moreover, both SQS-1 and SQS-2 were capable of anion recognition in DMSO solution as demonstrated using ¹ H NMR and UV/Vis absorption spectroscopy, but displayed lower binding affinities for various anions when compared against other squaramide based receptors. In more competitive solvent mixtures SQS-1 gave rise to a colourimetric response in the presence of HPO₄^2- that was clearly visible to the naked eye. We anticipate that the observed response is due to the basic nature of the HPO₄^2- anion when compared against other biologically relevant anions.