M. Vitasovic et al., Calculations on the origin of hyperporphyrin spectra in sequentially protonated meso-(dimethylaminophenyl) porphyrins, J PORPHYR P, 5(3), 2001, pp. 191-197
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.