Calculations on the origin of hyperporphyrin spectra in sequentially protonated meso-(dimethylaminophenyl) porphyrins

The spectra of porphyrins meso-substituted by 4-dimethylaminophenyl groups and protonated at the central pyrrole nitrogens show strong new bands in th e visible (450-600 nm) and far-red (700-780 nm). On complete protonation th e observed spectra are essentially normal except for enhanced intensity in the farthest red band at similar to 650 nm. The new bands observed with par tial protonation have been qualitatively interpreted as hyperporphyrin spec tra due to transitions from 'the filled pi orbitals on the amino nitrogen a toms... to the porphyrin LUMOs' [Ojadi EGA, Linschitz M, Gouterman M, Waite r RI, Lindsey JS, Wagner RW,I. We now report spectra calculated using the C AChe computer program package. The best structures are determined by energy minimization using a semi-empiral AM1 algorithm; a semi-empirical INDO/CI calculation is then used to obtained the excited state transition energies and intensities. The calculated spectra, although generally blue-shifted co mpared to experiment, reproduce the hyperporphyrin spectra in those cases w here they are observed, in these cases, in contrast to species with normal porphyrin spectra, the HOMO or HOMO-1 is nob an orbital on the porphyrin ri ng but is rather a pi -orbital on the unprotonated amino nitrogen that exte nds over the associated phenyl ring. The calculations suggest that the hype rporphyrin transitions are of Ph(pi) --> Por(pi*) origin, where Ph(pi) is a n orbital on the phenylamine substitutent and Por(pi*) is a LUMO on the por phyrin. It is further suggested that in the fully protonated species the in tensity enhancement of the farthest red band in an otherwise normal porphyr in spectrum may relate to phenyl character in the HOMO or HOMO-1. Copyright (C) 2001 John Wiley & Sons, Ltd.