The Role of Metal Ions and Counterions in the Switching Behavior of a Carboxylic Acid Functionalized Spiropyran
Abstract
The ability of a carboxylic acid functionalized spiropyran, SP, to act as a reversible and selective receptor for metal ions has been systematically examined. A series of spectroscopic measurements, combined with single-crystal X-ray diffraction have been employed in order to determine metal-ion sensitivity and selectivity, binding constants, solution compositions, binding kinetics as well as solid-state structure of a metal-complex of this ligand. This molecule acts as a chelating ligand and binds to metal ions using a triad of functionalities and will release the metal ion by application of visible light. Altering the complex from ZnCl2 and Zn(ClO4)2, leads to only a small change in the complex absorption maxima from 494 to 492 nm, but produced a drastic increase in the sensitivity for the zinc(ii) ion. Fluorescence spectroscopy was employed to establish that the detection limit for Zn(ii) is approximately 3 × 10-7 M for Zn(ClO 4)2. The counterion also influences binding constants and the equilibrium constants for Cu(ClO4)2 and CuCl 2 are K = 3.37 × 104 M-1 to K = 2.04 × 103 M-1, respectively. Additionally, in the presence of perchlorate, the spiropyran-Cu complex formation is fast enough to allow for real-time naked eye detection of this metal ion in less than three minutes; the formation of other metal complexes takes between 50 minutes to 2 hours. Finally, the single-crystal structure determination of the SP-zinc chloride complex shows that the Zn(ii) ion displays a distorted square-pyramidal arrangement, and neighbouring metal ions are bound together into a 1-D coordination polymer with a hydrophilic core, surrounded by hydrophobic ‘cladding’. © 2010 The Royal Society of Chemistry.
Silvia Giordani
Full Professor Chair of Nanomaterials
My research interests are in the design, synthesis, and characterization of hybrid smart nanomaterials for biomedical, energy and environmental applications