Triazolyl benzothiadiazole fluorescent chemosensors: A systematic investigation of 1,4- or 1,5-disubstituted mono- and bis-triazole derivatives

Cu(I)-catalyzed azide–alkyne cycloaddition represents one of the most efficient approaches for chemical transformation to create new fluorescent chemosensors. To elucidate the roles of 1,2,3-triazole moiety in fluorescent chemosensors, 1,4- or 1,5-disubstituted triazolyl benzothiadiazoles have been synthesized using azide–alkyne cycloaddition reaction between benzothiadiazole mono- or bis-alkynes and azido amino acid derivatives. Photophysical studies demonstrate that incorporation of one or two 1,2,3-triazolyl moieties into benzothiadiazole can extend the conjugated system and subsequently change the spectral properties, which are dependent on the position of substituents on triazole ring, the number of triazole unit but almost independent on the amino acid moiety. 1,4-Disubstituted triazolyl derivatives showed binding ability to Ni2+, Cu2+, Hg2+ and Co2+ in MeCN, thanks to a dual binding unit formed with benzothiadiazole moiety. In contrast, 1,5-disubstituted counterpart showed selective response to Hg2+ but no affinity to Ni2+, Cu2+, or Co2+, with the triazole ring probably as the sole binding site.