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.