Squaramides for colorimetric and fluorescent anion sensing

Abstract

A series of fluorophore-containing squaramides were synthesised to investigate the fluorescent and colorimetric response to anions of squaramides containing large aromatic substituents. Squaramides in which the fluorophores were conjugated directed to the squaramide motif were found to have highly acidic NH protons, giving rise to complicated fluorescence behaviour and colour changes upon addition of anions. In contrast, squaramides incorporating a methylene spacer to electronically insulate the fluorophore from the squaramide gave a relatively simple fluorescence response upon addition of anions, which was attributed to a decrease in excimer emission relative to monomer emission intensity, likely due to a disaggregation mechanism. The ratio of excimer : monomer emission was used to provide a ratiometric fluorescent response to anions, with the most selective molecules demonstrating a preference for binding to sulfate over other anions.