Dyes That Bear Thiazolylazo Groups as Chromogenic Chemosensors for Metal Cations

Abstract

AbstractA family of dyes (L1–L6) 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 L1–L6 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 L1, L2 and L5 in acetonitrile. Addition of one equivalent of protons to L1 and L2 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 L5 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 L1 and on different possible protonation species were also performed by using density functional theory (DFT) quantum mechanical calculations. Colour modulations in solutions of L1–L6 in acetonitrile in the presence of the metal cations Fe3+, Ni2+, Zn2+, Cd2+, Pb2+ and Hg2+ have been studied. A selective chromogenic response of L4 in the presence of Pb2+ and L5 in the presence of Hg2+ was observed. To get a better insight into the chromophoric nature in the presence of metal cations, the interaction of Hg2+ with the model compound L1 in two different coordination modes was studied theoretically by using density functional theory (DFT) quantum mechanical calculations.