We report a first-in-class responsive, pentafluorosulfanyl (-SF5)-tagged 19F MRI agent capable of reversibly detecting reducing environments via an FeII/III redox couple. In the FeIII form, the agent displays no 19F MR signal due to paramagnetic relaxation enhancement-induced signal broadening; however, upon rapid reduction to FeII with one equivalent of cysteine, the agent displays a robust 19F signal. Successive oxidation and reduction studies validate the reversibility of the agent. The -SF5 tag in this agent enables 'multicolor imaging' in conjunction with sensors containing alternative fluorinated tags and this was demonstrated via simultaneous monitoring of the 19F MR signal of this -SF5 agent and a hypoxia-responsive agent containing a -CF3 group.
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