Three fluorescent probes CdABA′, CdABA and ZnABA′, which are structural isomers of ZnABA, have been designed with N,N-bis(2-pyridylmethyl) ethylenediamine (BPEA) as chelator and 2-aminobenzamide as fluorophore. These probes can be divided into two groups: CdABA, CdABA′ for Cd2+ and ZnABA, ZnABA′ for Zn2+. Although there is little difference in their chemical structures, the two groups of probes exhibit totally different fluorescence properties for preference of Zn2+ or Cd2+. In the group of Zn2+ probes, ZnABA/ZnABA′ distinguish Zn2+ from Cd2+ with FZn2+–FCd2+ = 1.87–2.00. Upon interchanging the BPEA and carbamoyl groups on the aromatic ring of the fluorophore, the structures of ZnABA/ZnABA′ are converted into CdABA/CdABA′. Interestingly, the metal ions selectivity of CdABA/CdABA′ was switched to discriminate Cd2+ from Zn2+ with FCd2+–FZn2+ = 2.27–2.36, indicating that a small structural modification could lead to a remarkable change of the metal ion selectivity. 1H NMR titration and ESI mass experiments demonstrated that these fluorescent probers exhibited different coordination modes for Zn2+ and Cd2+. With CdABA′ as an example, generally, upon addition of Cd2+, the fluorescence response possesses PET pathway to display no obvious shift of maximum λem in the absence or presence of Cd2+. However, an ICT pathway could be employed after adding Zn2+ into the CdABA′ solution, resulting in a distinct red-shift of maximal λem.
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Organic & Biomolecular Chemistry
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