Specific Ca 2+ Fluorescent Sensor: Signaling by Conformationally Induced PET Suppression in a Bichromophoric Acridinedione

A series of acridinedione‐based bichromophoric podand systems 1a – c were synthesized and characterized. Among these, bichromophore 1c shows specific binding of Ca 2+ in the presence of other biologically important metal ions like Na + , K + , Mg 2+ , and Zn 2+ . The selective complexation was proved by steady‐state emission, time‐resolved emission, and 1 H NMR titration. Signaling of the binding event was achieved by Ca 2+ ‐induced folding of the bichromophore, resulting in PET suppression in the acridinedione chromophore. Involvement of a PET process in the optical signaling was confirmed by comparing bichromophores 1a – c with non‐PET compound 2 and monochromophore model compound 3 . Non‐PET compound 2 failed to give optical response upon Ca 2+ binding as a result of the absence of a PET process in the Ca 2+ ‐bound complex. Monochromophore 3 shows a similar optical response, which is the same as that in 1c . Titration of the metal‐ion‐bound complex of 1c with EDTA released the metal ion from the complex, thereby regaining the original photophysical properties of the bichromophore.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)