Electronic spectra and symmetry of metalloporphyrins in low-temperature rare gas and nitrogen matrices

The electronic absorption and luminescence spectra of zinc, magnesium and free base porphine have been measured in low temperature nitrogen, argon, krypton and xenon matrices. The absorption spectra of the three compounds show similar multiplet structures, characteristic of each matrix. Analogies are discussed between the splitting of spectral lines in free base porphine, due to phototautomerization, and the crystal field induced lifting of the degeneracy of the Jahn-Teller unstable S1 state in metalloporphyrins. The similarity between the absorption spectral pattern observed in free base porphine and metalloporphyrins indicates that the symmetry distortion in the latter occurs along the N-N axis. The magnitude of the crystal field splitting is correlated with the nature of the matrix.