Intramolecular binding site competition as a means of tuning the response of a colourimetric anion sensor

Two new viologen-based anion hosts, 2 and 3, are reported, incorporating two tripodal binding domains. Binding of carboxylate anions, particularly acetate, malonate and succinate by pyridinium derivative 2 results in an intense purple colouration. DFT calculations on representative system 2·malonate reveal that this absorption arises from charge-transfer from the anion to the bipyridinium unit. Anion binding by 3 does not initially result in a colour change because anions are bound at the periphery of the receptor; however, addition of more that two equivalents of acetate or more than one equivalent of dicarboxylate turn the colourimetric response on.