Photophysical and transition-metal ion signaling behavior of a three-component system comprising a cryptand moiety as the receptor

The synthesis, photophysical behavior, and transition-metal ion signaling ability of a three-component system (I) comprising a 4-aminophthalimide moiety as the fluorophore, a cryptand moiety as the metal ion receptor, and a dimethylene group as the spacer have been described. A lower fluorescence efficiency and a shorter fluorescence lifetime of the system compared to the corresponding properties of the bare fluorophore (4-aminophthalimide) are attributed to photoinduced intramolecular electron transfer between the fluorophore and the receptor moiety of the molecule. In the presence of transition-metal ions, I exhibits significant changes in its absorption and fluorescence properties, which have been used to estimate the strength of the binding interaction between the system and the metal ions. Even though I exhibits fluorescence enhancement in the presence of the transition-metal ions, which are notorious for their fluorescence quenching abilities, the magnitude of the enhancement is found to be significantly lower than that exhibited by a structurally similar svstem (II) containing a much simpler an-Lino group as the metal ion binding site. This observation has been interpreted by taking into consideration the efficiency of the photoinduced intramolecular electron-transfer process in the two systems, and the implication of this result in the design of fluorosensors for transition-metal ions has been highlighted.