Ground and excited states of Mg porphin studied by the SAC/SAC-CI method

The SAC (symmetry adapted cluster)/SAC-CI method is applied to the ground and excited states of magnesium porphin (MgP). The π interaction between the Mg atom and the prophin ring is small and, therefore, the essential difference between MgP and free base porphin (FBP) lies in symmetry; the former is D4h and the latter D2h. The degenerate excited states in MgP split into two in FBP. The SAC-CI results for the excitation energy and the oscillator strength compare reasonably well with the experimental spectra for Mg etioporphyrin (MgEtio) and Mg tetraphenylporphin (MgTPP) and the natures of the excited states are clarified. Goutermanʹs four-orbital model holds well for the Q band, but the excitations from the 2a2u MO below the four orbitals mix in the B band, as was found previously for FBP. The natures of the N bands are different between MgP and FBP.