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

Abstract

The radical cation (3˙⁺), the radical anion (3˙^–) and the, diamon (3^2–) of 2,3,9,10,16,17,23,24-octaethyltetraoxa[26]porphyrin were generated from the neutral compound (3) and the dication (3²⁺). The electronic spectra of these five redox stages resemble those in the tetraoxaporphyrin series, 1²⁺, 1˙⁺, 1,1˙^– and 1^2–, but are shifted to lower energies by 0.7–1.0 eV. Thus, the Soret 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 4nπ-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 π-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 3²⁺ 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.