TETRAOXAPORPHYCENE - ESR/ENDOR, UV/VISIBLE/NEAR-IR, AND MO-THEORETICAL STUDY OF ITS 5 REDOX STAGES

The radical cation (3.+), radical anion (3.-), and dianion (3(2-)) of tetraoxaporphycene were generated from the neutral compound (3) and the dication (3(2+)). The two paramagnetic species, 3.+ and 3.-, give rise to highly resolved ESR spectra, which have also-been investigated by ENDOR and TRIPLE-resonance spectroscopy. The ion pairs formed by the radical anion 3.- with alkali-metal counterions become less tight when the size of the cation increases from Li+ through Na+ and K+ to Cs+. Proton, Li-7, Na-23, K-39, and Cs-133 hyperfine data are consistent with the structure of the ion pairs, in which the counterion is situated on the 2-fold axis above or below the molecular plane. The absorption spectra of all five redox stages in the UV/visible/near-IR region (300-950 nm) and the emission bands of the three diamagnetic species are similar to those of the isomeric and isoelectronic tetraoxaporphyrins, 4(2+)-4(2-), which have recently been investigated. The absorption bands were assigned to electronic transitions with the use of PPP (Pariser-Parr-Pople) calculations. Bond-length alternation along the 20-membered pi-perimeter is predicted by theory for the neutral compound 3, in accord with the NMR spectra and the X-ray structural data. In the dication 3(2+), the four oxygen atoms should bear one positive pi-charge, which is largely preserved in the consecutive reduction stages, as additional electrons are almost exclusively accommodated by the perimeter.