Dyes That Bear Thiazolylazo Groups as Chromogenic Chemosensors for Metal Cations

Abstract

A family of dyes (L 1-L 6) that contain a thiazolylazo group as signalling subunit and several macrocyclic cavities with different ring sizes and type and number of heteroatoms as binding sites has been synthesized and characterized. Solutions of L 1-L 6 in acetonitrile show broad and structureless absorption bands in the 554-577 nm range with typicalmolar absorption coefficients that range from 20000 to 32000 M -1 cm -1. A detailed protonation study was carried out with solutions of L 1, L 2 and L 5 in acetonitrile. Addition of one equivalent of protons to L 1 and L 2 resulted in the development of a new band at 425 and 370 nm, respectively, which was ascribed to protonation in the aniline nitrogen. In contrast, protonation of L 5 resulted in a bathochromic shift of 25 nm of the absorption band that was conceivable with protonation of one of the nitrogen atoms of the azo moiety. These results were in agreement with 1H NMR spectroscopic data. Theoretical studies on the model ligand L 1 and on different possible protonation species were also performed by using density functional theory (DFT) quantum mechanical calculations. Colour modulations in solutions of L 1-L 6 in acetonitrile in the presence of the metal cations Fe 3+, Ni 2+, Zn 2+, Cd 2+, Pb 2+ and Hg 2+ have been studied. A selective chromogenic response of L 4 in the presence of Pb 2+ and L 5 in the presence of Hg 2+ was observed. To get a better insight into the chromophoric nature in the presence of metal cations, the interaction of Hg 2+ with the model compound L 1 in two different coordination modes was studied theoretically by using density functional theory (DFT) quantum mechanical calculations.