Squaraine rotaxane shuttle as a ratiometric deep-red optical chloride sensor

A new squaraine rotaxane molecular shuttle exhibits high chemical stability and acts as a deep-red, fluorescent and colorimetric sensor for Cl− anion with reversible, ratiometric response. The molecular design encapsulates a dihydroxyl substituted squaraine dye inside an anthracene-containing tetralactam macrocycle and a “clicked capping” reaction was used to convert an appropriate pseudorotaxane precursor into a permanently interlocked rotaxane in high yield. Reversible binding of Cl− to the rotaxane in solution, or on the surface of prototype dipsticks, causes lateral displacement of the surrounding macrocycle away from the central squaraine station and a substantial 30–40 nm shift in the squaraine absorption/fluorescence maxima that can be easily detected by the naked eye. The collective attributes of intense absorption/emission and ratiometric response at deep-red wavelengths is a significant advance in optical Cl− sensor performance by an organic molecule.