A New Benzoxazole-Based Fluorescent Macrocyclic Chemosensor for Optical Detection of Zn<sup>2+</sup> and Cd<sup>2+</sup>
2022
Daniele Paderni | Luca Giorgi | Maria Voccia | Mauro Formica | Lucia Caporaso | Eleonora Macedi | Vieri Fusi
Background: Benzoxazole-containing ligands find many applications both in medicinal chemistry, catalysis and fluorescence chemosensing. Benzoxazole-containing macrocycles could be therefore a good strategy to achieve stable and selective fluorescent complexes with suitable metal ions. In this work, the synthesis, binding, and photochemical properties of a new fluorescent ligand (<b>L</b>) are reported. <b>L</b> is a cyclophane macrocycle containing the 1,3-bis(benzo[<i>d</i>]oxazol-2-yl)phenyl (BBzB) fluorophore and an aliphatic tetra-amine chain to form the macrocyclic skeleton. Methods: Spectrophotometric and spectrofluorimetric measurements, <sup>1</sup>H NMR analysis, and DFT calculations were performed. Results: <b>L</b> behaves as a PET-mediated chemosensor, being emissive at 390 nm at acidic pH and non-emissive at basic pH. The chemosensor is able to detect Zn<sup>2+</sup> and Cd<sup>2+</sup> in an aqueous medium (acetonitrile–water, 4:1 <i>v/v</i>) at neutral pH through a CHEF effect upon metal ion coordination. Paramagnetic metal ions (Cu<sup>2+</sup>) and heavy atoms (Pb<sup>2+</sup>, Hg<sup>2+</sup>) resulted in a quenching of fluorescence or very low emission. Conclusions: The new cyclophane macrocycle <b>L</b> was revealed to be a selective PET-regulated chemosensor for Zn<sup>2+</sup> and Cd<sup>2+</sup> in an aqueous medium, being able to bind up to two and one metal cations, respectively. The molecule showed a shifted emission towards the visible region compared to similar systems, suggesting a co-planar conformation of the aromatic fragment upon metal coordination. All these data are supported by both experimental measurements and theoretical calculations.
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