understanding of electronic states and electronic transitions in
porphyrins and metalloporphyrins, and of the lifetimes and
yields of their singlet and triplet electronic states. From this
spectroscopic and photochemical viewpoint, we address the
interaction between electronically excited porphyrins or
porphyrins derivatives and molecular oxygen because this
interaction can lead to ROS of general interest in
photomedicine. Finally, we present some recent results of dyes
of this class designed for PDT.
II. Molecular and Electronic StructureThe core of aporphyrinis a tetrapyrrole in which the four rings
of the pyrrole type are linked together by methine carbon atoms.
The most common reduced porphyrins are dihydroporphyrins,
and the parent compound of this series is called chlorin.
Tetrahydroporphyrins in which the saturated carbon atoms are
located at nonfused carbon atoms of two diagonally opposite pyr-
role rings are calledbacteriochlorins. An interesting common fea-
ture of these species is that they share the same basic
chromophore, because the porphyrin macrocycle has 22
p-electrons, but only 18p-electrons are considered to lie on the
main delocalization pathway. Figure 2 illustrates these
structures and the numbering scheme recommended by IUPAC.
The 2, 3, 7, 8, 12, 13, 17, and 18 positions are commonly referred
as “beta-positions” and the 5, 10, 15, and 20 positions are
referred as“meso-positions.”
A simple and yet meaningful model to interpret the electronic
structure of porphyrins was proposed by Guterman based on
the two highest occupied molecular orbitals (HOMOs) and
the two lowest unoccupied molecular orbitals (LUMOs) (7,8).
The macrocycles of porphin (H 2 P) and 5,10,15,20-
tetraphenylporphyrin (H 2 TPP) are distorted from the
planar (D4h) geometry owing to the presence of the two
hydrogens bonded to two opposing central nitrogens and assume
aD2hsymmetry. The two HOMOs in theD4hpoint group labeled
a1uand a2ucorrespond to auand b1u, respectively, in theD2h
point group, and the two LUMOs transform from egx and egy
inD4hto b2gand b3ginD2h. Thexmolecular axis is defined by
the central NH bonds, and the y axis bisects the central
nitrogen electron lone pairs, as shown in Fig. 2. In porphyrins,
the HOMO auand b1uorbitals lie close enough in energy that
they be viewed as being essentially degenerate, and the b2gand
b3gorbitals are even closer in energy. As a result, the allowed
190 LUIS G. ARNAUT