Magnetogenic probes that respond to chemical stimuli in an off–on mode

Probing chemical events can be achieved by several forms of readout. Very popular among designers of responsive probes is an optical/fluorescent output, and their operation in a truly binary off–on mode is greatly desired in view of the associated maximal signal-to-background ratio and detection simplicity. Promotion of another form of physical readout, a change in the magnetization of the sample, detectable for example by Magnetic Resonance Imaging, has been hampered by the inability to propose a molecular probe that is initially silent (off). This focus article discusses a recently proposed solution that consists in the design of suitable multi-dentate ligands whose diamagnetic ferrous complexes (off) turn into paramagnetic ones (on) in aqueous solution upon modification by an external chemical stimulus.