CdII/ZnII discrimination using 2,5-diphenyl[1,3,4]oxadiazole based fluorescent chemosensors

Abstract

The coordination properties and photochemical response of two fluorescent ligands 2,5-bis{2-[bis(2-pyridylmethyl)aminomethyl]phenyl}[1,3,4]oxadiazole (L1) and 2,5-bis{2-[(1,4,7-triazacyclononane-1-yl)methyl]phenyl}[1,3,4]oxadiazole (L2) towards Zn^II and Cd^II are reported. The ligands contain the 2,5-diphenyl[1,3,4]oxadiazole (PPD) fluorescent probe linked to two coordinating units, namely dipicolylamine (DPA) for L1 and 1,4,7-triazacyclononane (TACN) for L2, through a methylene spacer. Both ligands form mononuclear and dinuclear complexes with these metal ions in aqueous or hydro-alcoholic media. With regard to L1, only the mononuclear Cd^II complex is highly fluorescent at 368 nm, while the mononuclear Zn^II and dinuclear Cd^II and Zn^II complexes are not emissive. Ligand L2 forms an excimer emitting at 474 nm only in the presence of one equivalent of Cd^II, and potentiometric, spectrophotometric, spectrofluorimetric, and NMR studies and DFT calculations were performed to explain this peculiarity. The discrimination between Cd^II and Zn^II is mainly due to the different coordination environments of the metal ions in the mononuclear complexes; namely Cd^II is coordinated by both the side arms of each ligand, while Zn^II is bound only to one coordinating unit.