An Iron-Based MRI Probe with Tunable Spin State for Bioorthogonally Catalyzed Activation Imaging

species under photophysical conditions. This transition led to over a 100-fold enhancement in relaxivity with a high turnover number (>200), generating robust MRI contrast. The ruthenium complex was conjugated to bovine serum albumin and predelivered to the tumor site. In vivo bioorthogonal catalysis was subsequently demonstrated in a subcutaneous tumor model. At the tumor site, the catalyst specifically triggers a "turn-on" in relaxivity of the MRI probe, producing markedly enhanced MRI contrast via bioorthogonally catalyzed activation. By combining the specificity of bioorthogonal chemistry with the MRI probe with a tunable spin state, this work establishes the potential of the spin-modulated probe for targeted imaging with low background signals and high imaging contrast.