An Fe complex for 19 F magnetic resonance-based reversible redox sensing and multicolor imaging.

We report a first-in-class responsive, pentafluorosulfanyl (-SF 5 )-tagged 19 F MRI agent capable of reversibly detecting reducing environments via an Fe II/III redox couple. In the Fe III form, the agent displays no 19 F MR signal due to paramagnetic relaxation enhancement-induced signal broadening; however, upon rapid reduction to Fe II with one equivalent of cysteine, the agent displays a robust 19 F signal. Successive oxidation and reduction studies validate the reversibility of the agent. The -SF 5 tag in this agent enables 'multicolor imaging' in conjunction with sensors containing alternative fluorinated tags and this was demonstrated via simultaneous monitoring of the 19 F MR signal of this -SF 5 agent and a hypoxia-responsive agent containing a -CF 3 group.