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.