Colorimetric sensing of metal ions by bis(spiropyran) podands: Towards naked-eye detection of alkaline earth metal ions

In order to develop a selective and sensitive colorimetric sensor for metal ions, a series of oligoether-linked bis(spiropyran) podands were prepared and their ability to colorimetrically sense alkaline earth metal ions was investigated. UV–vis absorption and 1H NMR spectroscopic studies showed that the podands isomerized from the colorless spiropyran form to the colored merocyanine form upon complexation with alkaline earth metal ions, whereas spectral changes were not visible in the presence of alkali metal ions. Podands of spiropyran subunits linked by a spacer of a 3-oxapentan-1,5-dioxy group, i.e.X-BSP-1 (X = MeO, tBu, iPr, H, Cl, Br), exhibited high selectivity to Ca2+. Introduction of an electron-donating group to the 5-position of each indoline ring of the podand gave rise to an increase in affinity to alkaline earth metal ions, enhancing the sensitivity. The absorption maximum of the colored metal–X-BSP-1 complex varied over a range of ca. 30 nm, from reddish purple to bluish purple. Clear discrimination of the Ca2+ complex from that of Mg2+ was possible using the naked eye. These results indicate the potential application of the X-BSP-1 podands as a colorimetric sensor for Ca2+.