Elsevier

Inorganica Chimica Acta

Volume 357, Issue 14, 15 November 2004, Pages 4078-4084
Inorganica Chimica Acta

Synthesis, photophysical characterisation and metal ion binding properties of new ligands containing anthracene chromophores

https://doi.org/10.1016/j.ica.2004.05.001Get rights and content

Abstract

Two new fluorescent chemosensors for heavy metal ions have been synthesised and their photophysical properties have been investigated. They present a pyridyl-thioether-based binding site and the anthracene moiety as a chromophore. In the experimental conditions used, no evidence is found for the formation of complexes with Pb2+, Zn2+, Cd2+, and Ag+ ions. On the contrary, in acetonitrile solutions both ligands strongly bind Cu2+ and Hg2+ cations according to a 1:1 and a 1:2 (metal:ligand) stoichiometry. In these complexes, the intense luminescence typical of anthracene derivatives is almost completely quenched and this phenomenon can be mainly attributed to an intraligand electron transfer process from the anthracene chromophore to the complexed pyridine. These results are of interest for the development of new chemosensors for the design of efficient electronic tongues for the detection of transition metal ions.

Ligands 1 and 2, presenting a pyridyl-thioether-based binding site and the anthracene moiety as a chromophore, show a high affinity for Hg2+ and Cu2+, whose complexation induces an almost complete quenching of the anthracene fluorescence. These features make these compounds interesting chemosensors for these metal ions, in particular in multisensory devices as the so-called electronic tongues. This could lead to the simultaneous detection of both these analytes in solution, of particular interest for practical applications.

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Introduction

The preparation and characterisation of new chemosensors represents the first step for the development and fabrication of efficient devices for the real time determination of chemical species [1], [2], [3], [4], [5], [6], [7], [8]. Of the various kinds of chemosensors, the luminescent ones present many advantages, since luminescence measurements are usually very sensitive, low cost, easily performed, and versatile [9]. Such devices find wide applications in many disciplines such as medical diagnostics, clinical and medical sciences, biochemistry, analytical chemistry and environmental science [6], [7], [8]. For this reason, many efforts are devoted to the design of more and more efficient chemosensors and, as a consequence, a huge number of papers have been recently published.

For the detection of the target analyte, two different processes are needed: molecular recognition and signal transduction, i.e., the mechanism by which the complexation of the sensor with the analyte causes a change in the properties of the sensor itself. Chemosensors present, therefore, components able to perform all these functions. In this manuscript, we report the synthesis, photophysical characterisation and metal ion binding properties of ligands 1 and 2. These ligands possess an anthracene chromophore as signalling unit, and a chelating unit composed of two sulfur atoms and a pyridine, designed to coordinate transition metal ions.

Section snippets

Materials and general procedures

Tetrahydrofuran (THF) was distilled from Na benzophenone just before use. N,N-Dimethylformamide (DMF) was purified by distillation from CaH2 and stored over 4 Å molecular sieves in a dark bottle.

The solvent used for photophysical measurements was acetonitrile from Merck (UVASOL). All other reagents were purchased from Fluka/Aldrich and used as such. All the metal ions were added as perchlorate salts.

2,6-Bis(phenylthiomethyl)-3-hydroxypyridine (3) [10] and 2,6-bis(chloromethyl)pyridine (4) [11]

Synthesis of the ligands

The fluorescent chemosensors we have developed to achieve this work incorporate a pyridyl-thioether-based binding site and the anthracene moiety as a fluorophore. The synthesis of sensor 1, Scheme 1, was obtained from the reaction of 2,6-bis(phenylthiolmethyl)-3-hydroxypyridine (3), prepared according to the literature procedures [10], with 9-chloromethylanthracene in the presence of K2CO3 using DMF as a solvent. The anthracene-based sensors 2 was successfully assembled by coupling

Conclusions

The anthracene containing ligands 1 and 2 have a strong affinity for both Hg2+ and Cu2+ ions. For both ligands, protonation and complexation with Hg2+ and Cu2+ ions leads to dramatic changes in the photophysical properties of the anthracene chromophore, and in particular to a strong quenching of their intense luminescence. The quenching can be mainly attributed to an intraligand electron transfer process from the anthracene chromophore to the complexed pyridine. In the case of copper, a

Acknowledgements

The authors thank MIUR, (FISR, project SAIA), and the University of Bologna (funds for Selected Topics) for funding.

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