A New Route towards Redox Bistability through the Inspection of Manganese–Porphyrin Complexes

The redox and spin versatilities of manganese–porphyrin complexes [MnIIP] are examined to construct a redox-switchable device. The electronic structure of [MnIIIP]+ was analyzed by using wavefunction-based calculations (complete active spaces plus single excitations on top of the active spaces, that is, CAS+singles). A non-negligible σ-type electron-transfer configuration is present in the [MnIIIP]+S=2 ground state. By contrast, the [MnIIP.]+ valence tautomer is a purely π-type intramolecular charge transfer, thus reflecting an S=3 spin state as a result of the strong ferromagnetic interaction (J=30 meV) between the S=5/2 MnII ion and the S=1/2 porphyrin radical cation P.+. The change of the redox-sensitive site in the valence tautomer leads to a ‘triangular scheme’ that involves a critical step in which a simultaneous electron transfer and spin change are expected to induce bistability. From the wavefunction inspection, a meso-substituted porphyrin candidate was designed to support this scenario. The complete active-space second-order perturbation theory (CASPT2) adiabatic energy difference between the S=2 and the S=3 spin states was reduced down to 0.15 eV, thereby giving rise to a metastable S=3 state characterized by a 0.10 Å extension of the porphyrin cavity radius. These results not only confirm the rather versatile nature of these inorganic systems but also demonstrate that redox and spin changes are intermingled in this class of compounds and can be used for applied devices.