Application of Michl’s 4N+2 perimeter model: Effect of N- and neo-confusion, and reduced pyrrole and direct pyrrole-pyrrole bonds on the optical spectra of free base porphyrinoids

Michl’s 4N+2 perimeter model is used to analyze trends in the optical spectra and electronic structures of a series of free base porphyrin model ligands to identify key structure-property relationships associated with the N- and neo-confusion, and the introduction of reduced pyrrole and direct pyrrole-pyrrole bonds to the porphyrin ligand. Following Michl’s predictions, the calculated Q band intensities at the CAM-B3LYP/6-31G(d) are found to correlate with [Formula: see text]HOMO 2 – [Formula: see text]LUMO[Formula: see text], where [Formula: see text]HOMO and [Formula: see text]LUMO refer to the energy gaps between the MOs arising from the frontier occupied and unoccupied MOs with M L = ± 4, ± 5 angular nodal patterns, but only when dyes with the same parent hydrocarbon perimeter is involved. Corrole analogues such as N-confused corroles and C-fused norroles are predicted to have the most intense Q bands, which is inconsistent with what has been observed experimentally. Combining N- and neo-confusion with the reduction of peripheral pyrrole bonds in the context of chlorins and bacteriochlorins does not appear to result in more favorable properties for biomedical singlet oxygen photosensitizer applications.