An "off-on" fluorescent and colorimetric probe bearing fluorescein moiety for Mg(2+) and Ca(2+) via a controlled supramolecular approach.

In the present work, a new kind of fluorescein-based chemosensor L was designed and synthesized to selectively recognize Ca(2+) or Mg(2+) over other competing ions. The chemosensor showed "off-on" fluorescent and color changes upon the addition of Ca(2+) and Mg(2+). The dynamic binding events with the formation of 1:1(L/M) and 1:2(L/M) complexes were examined. The cation-driven conformation changes of L were understood and proposed rationally by the UV-vis, FL, and (1)HNMR titrations. By this allosteric effect, Ca(2+) and Mg(2+) could be selectively recognized with the 1:2 stoichiometry by fluorescent changes, which were different from other known reports on chemosensors that the host-cation complexion was exploited for controlling chromophore interaction as the model of signaling.