Do Nonplanar Porphyrins Have Red-Shifted Electronic Spectra? A DFT/SCI Study and Reinvestigation of a Recent Proposal

It is commonly believed that nonplanar distortions of the porphyrin skeleton bring about significant red shifts in the electronic absorption spectra. However, based on absorption spectroscopy and semiempirical AM1 studies of meso -tetrakis(perfluoroalkyl)porphyrins, DiMagno and co-workers have challenged this notion. Here we present density functional theory based configuration interaction singles calculations which uphold the traditional view. Both ruffling and saddling deformations do bring about significant red shifts in the electronic spectra. Nonplanarity-induced destabilization of the porphyrin HOMOs appears to be the principal cause of these red shifts.