Harnessing chemical exchange: 19F magnetic resonance OFF/ON zinc sensing with a Tm(iii) complex.

A fluorinated, thulium(iii) complex (Tm-PFZ-1) serves as an off-on 19F magnetic resonance probe for Zn(ii). Rapid exchange among different conformations combined with paramagnetic relaxation and chemical shift effects of Tm(iii) effectively eliminate the 19F NMR/MRI signal in Tm-PFZ-1. Chelation of Zn(ii) induces increased structural rigidity and reduces exchange rate, affording a robust 19F NMR/MRI signal. Tm-PFZ-1 represents a first-in-class paramagnetic 19F MR agent that exploits a novel sensing mechanism for Zn(ii) and is the first 19F MR-based scaffold to provide an "off-on" response to Zn(ii) in aqueous solution.