UV-VIS-near-IR, MO and EPR studies of five redox stages of an octaethyltetraoxa[26]porphyrin

The radical cation (3(.+)), the radical anion (3(.-)) and the dianion (3(2)-) of 2, 3, 9, 10, 16, 17, 23, 24-octaethyltetraoxa [26] porphyrin were generated from the neutral compound (3) and the dication (3(2+)). The electronic spectra of these five redox stages resemble those in the tetraoxaporphyrin series, 1(2+), 1(.+), 1, 1(.-) and 1(2-), but are shifted to lower energies by 0.7-1.0 eV. Thus, the Sorer and Q bands of the four charged species, 3(2+), 3(.+), 3(.-) and 3(2-), lie at 450-600 and 700-1300 nm, respectively, The spectrum of the neutral 3 is characteristic of 4n pi-systems. The absorption bands were assigned to electronic transitions with the use of PPP calculations. The diions 3(2+) and 3(2-) are predicted to preserve the symmetry D-4h of the x-system with cyclic delocalization along the 28-membered perimeter, whereas in the neutral 3 the symmetry is lowered to D-2h with localization of the double bonds. The four oxygen atoms in 3(2+) should bear one positive charge which is only slightly decreased on consecutive reductions, so that the additional negative charges are taken up almost exclusively by the perimeter. Proton-coupling constants were determined for the radical cation 3(.+) and the radical anion 3(.-) by EPR spectroscopy. Within the limits of experimental resolution, they are the same for both radical ions and consistent with an effective D-4h symmetry, The large inherent width of the EPR lines-and their reluctance to saturation-point to a degenerate or nearly degenerate ground state of 3(.+) and 3(.-) undergoing a dynamic Jahn-Teller effect.