Mn(III) porphyrins have great potential as Gd-free MRI contrast agents because both the cation and the ligand have interesting properties. The redox properties of the Mn(III)-ion can be exploited for the preparation of reactive oxygen species for therapy. Moreover, the porphyrin ligand allows these complexes to have a high affinity for tumor tissues. The inherent properties of the porphyrin ligands make these systems attractive for photothermal, photodynamic, and sonodynamic therapies. Therefore, these systems are attractive for the development of theranostics for MRI-guided therapy. For the magnetic field strengths at which most clinical MRI machines operate at present (0.5–1.5 T), the longitudinal relativity of low-molecular-weight complexes is even higher than that of the classical Gd-based contrast agents. This review gives an overview of the developments in the field of Mn(III) porphyrin contrast agents during the last 30 years.

The NMR relaxivities of the decatungstolanthanoate core-shell nanoparticles, prepared by encapsulating [Ln(W₅O₁₈)₂]⁹⁻ polyoxometalates (LnPOM) within amorphous silica shells (K₉[Ln(W₅O₁₈)₂]@SiO₂), were studied along the Ln series. The relaxivity of GdPOM is slightly higher than for Gd-DTPA due to second-sphere relaxation effects, but the values for the other paramagnetic LnPOMs are much smaller due to the short T_1e values of their Ln³⁺-ions. The NPs have core-shell spherical structures, with LnPOM-containing cores with 9.5–28 nm diameters, and 4.0–11.0 nm thick amorphous silica shells. In water suspensions, the NPs have negative zeta potentials (−32.5 to −40.0 mV) and time-dependent hydrodynamic diameters (31–195 nm) reflecting the formation of aggregates. The relaxivities of GdPOM@SiO₂ NPs suspensions (r₁=10.97 (mM Gd)⁻¹ s⁻¹, r₂=12.02 (mM Gd)⁻¹ s⁻¹, 0.47 T, 25 °C) are considerably larger than for the GdPOM solutions, indicating that their silica shell is significantly porous to water. This increase is limited by the agglomeration of the complexes in the NPs core, limiting their access to water to those at the core surface. Replacing half of the Gd³⁺ ions by Eu³⁺ decreases the NPs r₁ and r₂ relaxivities at 0.47 T to 20 % and 35 % of their initial values, which are still considerable, but does not affect the efficient luminescence properties of the Eu³⁺ centers. This indicates that the mixed NPs have potential as dual modality MRI/optical imaging contrast agents.