Colourimetric carboxylate anion sensors derived from viologen-based receptors

A series of tri- and tetrapodal viologen-based anion receptors showing a colourimetric response to carboxylates, such as acetate, have been synthesised. Alteration of the anion binding sites allows for binding site competition within a receptor. This results in a delayed colourimetric response for urea derivatives compared with pyridinium systems because the anions are initially bound to the periphery of the receptor, away from the viologen unit. DFT calculations and experimental measurements allow the colour change to be assigned to an anion-receptor charge-transfer process, facilitated by the exceptionally low reduction potential of the cationic host compounds. Evidence for electron transfer to give the viologen radical cation is also seen in some cases.