A series of acridinedione-based bichromophoric podand systems 1a–c were synthesized and characterized. Among these, bichromophore 1c shows specific binding of Ca2+ in the presence of other biologically important metal ions like Na+, K+, Mg2+, and Zn2+. The selective complexation was proved by steady-state emission, time-resolved emission, and 1H NMR titration. Signaling of the binding event was achieved by Ca2+-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 Ca2+ binding as a result of the absence of a PET process in the Ca2+-bound complex. Monochromophore 3 shows a similar optical response, which is the same as that in 1c. Titration of the metalion- bound complex of 1c with EDTA released the metal ion from the complex, thereby regaining the original photophysical properties of the bichromophore.
Eur. J. Org. Chem. 2009, 5941–5947