occurs in femtoseconds. However, PdIITPP, which is also diamag-
netic and d^8 , does not show fluorescence but is strongly phospho-
rescent. This difference was assigned to the tightly bond d
orbitals of Pd that push the intermediate states closer in energy
to the ground state, disfavoring the radiationless path ( 7 ).
The electronic structure of FeIITPP is quite complex, with the
four 3d-like orbitals lying close together and above the porphyrin
a1uand a2uorbitals. The orbitals generate low-lying triplet states
and promote the deactivation of the excited states. Additionally,
FeIITPP has a CT band with a maximum at 714 nm. As a result,
no luminescence is observed from FeIITPP. However, FeIITPP
reacts with O 2 to form a dioxygen adduct that eventually leads
to [FeIIITPP]þwith axial ligands(38,43). High spin [FeIIITPP]þ
also exhibits CT absorption bands in the 650–750 nm region,
assigned to a1u,a2u!eg transitions, whose exact positions
depend on the electronegativity of the axial ligand ( 44 ). Weak
luminescence is observed from the tripmultiplet state of
[FeIIITPP]þwhen it is positioned below the CT state ( 38 ).
The strongly antibonding dx (^2) – y 2 of [MnIIITPP]þremains empty,
but the other four d orbitals are singly occupied. The [MnIIITPP]þ
absorption spectrum provides an impressive example of low-
energy CT absorption bands characteristic of hyper-type spectra.
The dp(dxzan dyz) orbitals of MnIIIraise in energy with respect to
those of FeIII and approach the energies of the porphyrin eg
orbitals ( 45 ). The energetic proximity of orbitals withpsymmetry
leads to substantial mixing of the orbitals and enhances the inten-
sity of the CT bands. The strong perturbation, the porphyrinp
orbitals, leads to the split Soret band with peaks at 370 and
477 nm. The Q band is observed at 596 nm, and the other bands
in the visible part of the spectrum are CT bands. Electronic excita-
tion rapidly populates a tripquintet state that repopulates the
quintet ground state in less than 30 ps; a competitive decay chan-
nel populates a tripseptet state that decays in 80 ps to the ground
state presumably via quintet CT state ( 46 ). The very weak lumi-
nescence of [MnIIITPP]þresembles that of [FeIIITPP]þ( 38 ).
Long-lived excited states can be found in free-base porphyrins
and closed-shell metalloporphyrins. Open-shell diamagnetic
metalloporphyrins may lead to long-lived^3 (p,p) states in high
yields provided that they lie below the CT states, as in PdIITPP
or ClInIIITPP. Thus, the most promising photosensitizers are
likely to come from these types of compounds. This does not
imply that paramagnetic metalloporphyrins cannot have inter-
esting photochemistry. For example, irradiation of ClMnIIITPP
at 416 nm was shown to induce the photoreduction of MnIII
to MnII porphyrin with the concomitant loss of the chlorine
202 LUIS G. ARNAUT