Tetrahydroindazolone substituted 2-aminobenzamides as fluorescent probes: switching metal ion selectivity from zinc to cadmium by interchanging the amino and carbamoyl groups on the fluorophore

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 F-Zn(2+)-F-Cd(2+) = 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 F-Cd(2+)-F-Zn(2+) = 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 lambda(em) 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 lambda(em).