Abstract
Synthesis of the new scorpiand ligand L composed of a [9]aneN3 macrocyclic ring bearing a CH2CH2NHCH2-anthracene tail is reported. L forms both cation (Zn2+) and anion (phosphate, benzoate) complexes. In addition, the zinc complexes of L bind these anions. The equilibrium constants for ligand protonation and complex formation were determined in 0.1 M NaCl aqueous solution at 298.1 ± 0.1 K by means of potentiometric (pH-metric) titrations. pH Controlled coordination/detachment of the ligand tail to Zn2+ switch on and off the fluorescence emission from the anthracene fluorophore. Accordingly, L is able to sense Zn2+ in the pH range 6–10 down to nM concentrations of the metal ion. L can efficiently sense Zn2+ even in the presence of large excess of coordinating anions, such as cyanide, sulphide, phosphate and benzoate, despite their ability to bind the metal ion.
Highlights
Scorpiand type aza-macrocyclic ligands, that is aza-macrocyclic ligands with coordinating tails [1], and their metal complexes have attracted considerable interest thanks to the numerous uses they have been tested for, including but not limited to various biomedical applications [2,3] (MRI contrast agents [4,5], radioisotopes complexation and radiolabelling [6,7,8,9], radiotherapy [10,11,12,13,14], chelation therapy [15,16,17], antiproliferative treatments [18,19,20,21], enzyme mimicking [22,23,24,25,26,27,28,29]), catalysis [30,31,32,33,34,35,36] and chemosensing [37,38,39,40,41,42,43]. Receptors that signal their interaction with substrate species through a modification of their fluorescence emission properties have aroused wide interest, in particular for their efficiency, in terms of sensitivity and response times, as well as for their relatively easy preparation and for their ability to detect a variety of chemical species [37,38,39,40,41,42,43,44,45,46,47,48,49,50]
[53] and commonly an efficient to polyamine reported in Table through chemosensors the chelation enhancement of fluorescence (CHEF) effect [51]
A new scorpiand-like ligand (L) based on [9]aneN3 and bearing a coordinating tail terminated with an anthracene fluorophore has been prepared by a simple synthetic procedure and studied as fluorescent chemosensor for Zn2+
Summary
Scorpiand type aza-macrocyclic ligands, that is aza-macrocyclic ligands with coordinating tails [1], and their metal complexes have attracted considerable interest thanks to the numerous uses they have been tested for, including but not limited to various biomedical applications [2,3] (MRI contrast agents [4,5], radioisotopes complexation and radiolabelling [6,7,8,9], radiotherapy [10,11,12,13,14], chelation therapy [15,16,17], antiproliferative treatments [18,19,20,21], enzyme mimicking [22,23,24,25,26,27,28,29]), catalysis [30,31,32,33,34,35,36] and chemosensing [37,38,39,40,41,42,43].As regards the latter, receptors that signal their interaction with substrate species through a modification of their fluorescence emission properties (fluorescent chemosensors) have aroused wide interest, in particular for their efficiency, in terms of sensitivity and response times, as well as for their relatively easy preparation and for their ability to detect a variety of chemical species [37,38,39,40,41,42,43,44,45,46,47,48,49,50].Fluorescent chemosensors based on polyamines have proved very efficient for signalling both metal cations and anions in solution, even in water, where most chemosensors that function in less polar solvents fail to give adequate responses due to low solubility or weak binding to the target species.To this purpose, either chelation enhancement of fluorescence (CHEF) or chelation enhancement of quenching (CHEQ) experienced by the fluorophore upon interaction with the target species can be used.Molecules 2020, 25, 1355; doi:10.3390/molecules25061355 www.mdpi.com/journal/molecules behaviour in sensing Zn(II) ions in water. L is a scorpiand type molecule constituted by a macrocyclic [9]aneN3 (1,4,7-triazacyclononane) binding unit connected to an anthracene fluorophore through a linker containing an additional nitrogen donor. [9]aneN3 has been widely used for the construction of chemosensors thanks to its peculiar coordination properties and the availability of consolidated synthetic procedures for its symmetric and non-symmetric functionalization [52]
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