Magnetic circular dichroism of octaethylcorrphycene and its doubly protonated and deprotonated forms

Electronic absorption and magnetic circular dichroism (MCD) spectra of 2,3,6,7,11,12,17,18-octaethylcorrphycene, a constitutional isomer of porphyrin., were recorded for the neutral, doubly protonated, and doubly deprotonated forms. The spectra closely resemble those of the parent porphyrin and are quite different from those of other porphyrin isomers such as porphycene and hemiporphycene. Theoretical considerations based on Michl's perimeter model allow one to understand the spectral properties observed within the series of porphyrin isomers composed of porphyrin, porphycene, hemiporphycene, and corrphycene. Differences in spectral features within members of the group are caused by varying orbital splitting patterns. In perimeter model terminology, corrphycene is a soft chromophore,with approximately equal energy splittings between the pairs of highest occupied and lowest unoccupied molecular orbitals (DeltaHOMO approximate to DeltaLUMO). As a consequence, a strong spectral sensitivity to structural peturbations is predicted for corrphycene. This behavior contrasts with what is expected for the porphyrin isomers porphycene and hemiporphycene, which are negative-hard chromophores (DeltaHOMO < DeltaLUMO). The experimentally observed excited-state characteristics are reproduced satisfactorily using time-dependent DFT (B3LYP/6-31G**) calculations. Finally, it is shown that the four-orbital model of Gouterman, which is widely used to interpret the spectral properties of porphyrin and its derivatives, is also applicable to corrphycene.