A New Benzoxazole-Based Fluorescent Macrocyclic Chemosensor for Optical Detection of Zn2+ and Cd2+

Abstract

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 (L) are reported. L is a cyclophane macrocycle containing the 1,3-bis(benzo[d]oxazol-2-yl)phenyl (BBzB) fluorophore and an aliphatic tetra-amine chain to form the macrocyclic skeleton. Methods: Spectrophotometric and spectrofluorimetric measurements, 1H NMR analysis, and DFT calculations were performed. Results: L 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 Zn2+ and Cd2+ in an aqueous medium (acetonitrile–water, 4:1 v/v) at neutral pH through a CHEF effect upon metal ion coordination. Paramagnetic metal ions (Cu2+) and heavy atoms (Pb2+, Hg2+) resulted in a quenching of fluorescence or very low emission. Conclusions: The new cyclophane macrocycle L was revealed to be a selective PET-regulated chemosensor for Zn2+ and Cd2+ 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.

A novel 2,6-bis(benzoxazolyl)phenol macrocyclic chemosensor with enhanced fluorophore properties by photoinduced intramolecular proton transfer

“…8A, left), as observed for an analogous ligand lacking the OH group recently reported by our research group. 13 The addition of Cd(II) and Pb(II) ions caused an increase in the absorbance of the deprotonation band only up to one equivalent, suggesting the formation of mononuclear complexes (Fig. 8B and C left).…”

“…1 H and 13 C NMR spectra were recorded on a Bruker Avance 400 instrument, operating at 400.13 and 100.61 MHz, respectively, and equipped with a variable temperature controller. The temperature of the NMR probe was calibrated using 1,2-ethanediol as a calibration sample.…”

“…Coupling constants ( J values) are given in hertz (Hz). 1 H-1 H and 1 H- 13 C correlation experiments were performed to assign the signals. pH titrations were performed at 298.1 K, in a mixed acetonitrile-d 3 /D 2 O 60 : 40 v/v solution to prevent solubility issues.…”

“…8A, left), as observed for an analogous ligand lacking the OH group recently reported by our research group. 13…”

Design, Synthesis, and Anti-Fungal Evaluation of Heterocyclic Benzoxazole Derivatives

“…There are also other pharmaceutical applications such as antibiotic [ 12 , 13 ], anticancer, [ 14 , 15 , 16 ] anti-inflammatory applications [ 17 ]. Due to their structural characteristics, they can be used in material industry such as fluorescent chemosensor for special ions [ 18 ] and mesogenic applications [ 19 ].…”