Labile Electronic Ground State of the π-Cation Radical of Zinc(II) Tetraphenylporphyrin: a Variable-temperature ESR Investigation

The UV-vis and variable-temperature X-band EPR spectral studies (298–77 K) on π-cation of zinc(II) tetraphenylporphyrin ( ZnTPP ) have revealed for the first time a labile electronic ground state. The electronic structure changes from 2 A 2u to 2 A 1u as the temperature is lowered from 298 to 150 K. Axial ligands and σ-donor solvents show a significant effect on the spectral data. The lability in the ground state electronic configuration is attributed to the flexibility of the porphyrin moiety in changing from a ‘ruffled’ to a lower symmetry.