Parent, Unsubstituted Corrphycene: Synthesis and Properties

We report the synthesis of a porphyrin isomer that, until now, has only been obtained in the form of multiply substituted derivatives. Structural, spectral, and electrochemical characteristics of unsubstituted corrphycene (Cp) are described along with the data for its precursor, 11,18-di-tert-butylcorrphycene. Similar to porphine, Cp turns out to be a soft chromophore, characterized by very similar energy splittings between the pairs of frontier HOMO and LUMO orbitals. Magnetic circular dichroism spectra indicate that the splitting in the HOMO orbitals is larger than in the LUMO, opposite to the DFT predictions. Corrphycene exists in the form of a doubly degenerate trans tautomeric species. The two intramolecular H-bonds are not equivalent, which strongly suggests a stepwise mechanism of trans-trans conversion. Being a nearly perfect soft chromophore, Cp emerges as an attractive candidate for sensing changes in its environment.