Triplet EPR Dynamics of Randomly Oriented Dications and Dianions of Tetraoxaporphyrinoids

This paper deals with the photoexcited triplet state of several tetraoxaporphyrinoids studied by time-resolved electron paramagnetic resonance (EPR). The porphyrinoids studied are tetraoxa[18]porphyrin ( 1a ⁺⁺), octaethyl-tetraoxa[18]porphyrin ( 1b ⁺⁺), tetra-n-butyl tetraoxa[18]porphyrin ( 1c ⁺⁺), octaethyl-tetraoxa[26]porphyrin ( 2 ⁺⁺), and tetraoxa[18]porphycene ( 3 ⁺⁺), all of them as per-chlorate salts. To qualify as stable aromatic 18π- (or 26π-) electron systems, they must exist as dications. Analysis of the triplet EPR spectra is characterized by unique features not found in their parent porphyrins and porphycenes. While in the case of the axially-symmetric tetraphenylporphyrin, the triplet EPR spectra indicate deviation from axial symmetry, the tetraoxaporphyrin 1a ⁺⁺ exhibits, as expected, a vanishing E-term of the zero-field splitting (ZFS) tensor. A conspicuous difference is found between the triplet EPR spectra of the dications and the dianions of all tetraoxa compounds. The results are discussed in terms of the electronic structures of the porphyrinoids.