5 REDOX STAGES OF TETRAOXAPORPHYRIN - A UV/VISIBLE/NEAR-IR, ESR, AND MO-THEORETICAL STUDY

Starting from the dication of tetraoxaporphyrin and the corresponding neutral compound, i.e., tetraoxaisophlorin, three further redox stages were generated: the radical cation, the radical anion, and the dianion. Electronic absorption spectra were registered for all five redox stages in the UV/visible/near-IR region (300-900 nm), together with the emission bands for the three fluorescent diamagnetic species (dication, neutral, dianion). The patterns of these spectra suggest differences in electronic structure and symmetry, particularly between the ions and the neutral molecule. Calculations by the PPP method performed on the three closed-shell systems and starting with D4h symmetry indicate that this symmetry is retained in the two diions; a degenerate pair, e(g), of frontier MO's is vacant in the dication and fully occupied in the dianion. By contrast, in the neutral tetraoxaisophlorin, the symmetry is lowered to D2h due to bond-length alternation along the 20-membered pi-perimeter; the pair e(g) splits into a HOMO b2g and a LUMO b3g. The observed electronic transitions in the diions and the neutral compound are satisfactorily reproduced by the PPP calculations with the participation of two further occupied and two unoccupied MO's, in addition to e(g) or b2g and b3g. The MO schemes of the radical ions have been assumed to be similar to those of the corresponding diions with a single and triple occupancy of e(g) or nearly degenerate e(g)-like orbitals in the cation and anion, respectively. This assumption accounts qualitatively for the observed electronic transitions in the radical ions, and it is consistent with the hyperfine pattern of their ESR spectra which exhibit an effective D4h symmetry. The width of the hyperfine lines and their behavior on saturation strongly suggest that both radical ions are subject to a dynamic Jahn-Teller effect. The pi-charge distribution in the dication is adequately described by one unit of positive charge being shared by the four oxygen atoms. This charge is only slightly reduced on the taking up of additional electrons which thus are accommodated in the 20 carbon pi-centers of the perimeter.