A Fluorescent Chemosensor for Zn2+ Based on a C3-Symmetrical and Pre-Organized 2,2′,2″-Nitrilotribenzoic Acid Material

A C3-symmetrical 4,4″,4⁗-nitrilotris(2′-methyl-[1,1′-biphenyl]-3-carboxylic acid) (4) derived from nitrilotriacetic acid (NTA) was found to selectively bind Zinc(ii) ions both in DMSO or MeOH. A synergistic effect of the anionic counter ion SO42− on the sensing behaviour of 4 to metal ions was clearly observed in DMSO. Interestingly, 4 showed a rapid hypochromatic shift in emission ascribed to the deprotonation and the concomitant formation of a 4–metal complex upon the addition of Zn2+ ions, instead of the bathochromic shift and emission enhancement attributed to the SO42−-involved hydrogen-bonding interaction for Ni2+, Li+, Mg2+, and Na+ ions at ratios below 1:1 in DMSO. The observed sensing process of sulfate salts associated with the SO42−-involved hydrogen-bonding interaction, deprotonation, and the concomitant complexation can also be clearly monitored by titration methods utilising UV-vis, fluorescence, and NMR spectroscopy in solution. In comparison with 4, compound 1 showed an obvious difference in the binding interaction with zinc sulfate in MeOH, probably owing to the decreased acidity. Anion-induced hydrogen-bonding interactions and deprotonation of the COOH protons in the excited state also endowed 4 versatile spectroscopic properties. The addition of F− and SO42− anions resulted in a remarkable enhancement probably related with a rigidifying effect. 2,2′,2″-Nitrilotribenzoic acid can be utilised as a potential scaffold to build a series of conjugated fluorescent sensors by its chelation effect owing to the rigid cavity pre-organised by the triphenylamine moiety and the carboxylic groups and the conjugation extension in the 4,4′,4″ positions.