Development of ternary squaraine-tryptamine-nitroaromatic charge-transfer complexes for potential use as colourimetric stains in gel electrophoresis. Part 1

A water-soluble dual pendant squaraine with tryptaminium cations has been prepared and mixed with dinitroaromatics (3,5-dinitrobenzoic acid, 3,5-dinitrosalicylic acid, and 2,4-dinitrophenol) in solution to create ternary complexes containing indole-nitroaromatic charge-transfer pairs. Examination of the X-ray crystal structures of just the three types of charge-transfer complexes, without the squaraine moiety, employed yielded six structures in which all bar one demonstrated charge-transfer pair or stacking associations between the indole and nitroaromatic rings. These charge-transfer associations were evident in the UV–visible absorption spectra of each complex, including those incorporating the squaraine. Examination of the fluorescence performance upon illumination with 300 nm light of each complex, particularly in the presence of an equimolar amount of protein, showed that although there was a ten-fold increase in the fluorescence of the squaraine moiety (above that without protein) the levels across the series were no more than that observed for the base bistryptaminium squaraine.