Achieving selectivity for copper over zinc with luminescent terbium probes bearing phenanthridine antennas.

A family of terbium probes was synthesized and evaluated for the luminescence detection of copper and zinc in water at neutral pH. Each probe incorporates a terbium ion chelated by a macrocyclic polyaminocarboxylate and conjugated to either one, two, or three phenanthridine antennas via a diamine linker. All three probes, Tb-1Phen, Tb-2Phen, and Tb-3Phen, exhibit similar responses toward copper and zinc. In each case, the terbium-centered time-gated phosphorescence decreases upon binding either CuI or CuII but not upon addition of ZnII. The phosphorescence of Tb-2Phen is also not significantly affected by other metal ions including MgII, CaII, MnII, FeII, NiII, CdII, and HgII. Tb-1Phen, on the other hand, responds weakly to MnII, FeII and NiII. The lack of affinity of each probe for ZnII was further confirmed by competition experiments with CuI and CuII. Notably, whereas the terbium-centered emission of each probe is quenched upon copper coordination, the phenanthridine-centered luminescence emission is not. As such, each probe functions as a ratiometric probe for the selective detection of copper over zinc. Theoretical calculations further demonstrate that the turn off response of the probe is due to an increase in the distance separating the lanthanide ion from its phenanthridine antennas upon coordination of copper, which in turn decreases the efficiency of terbium sensitization by the phenanthridines.