We have seen that the strong electronic absorption in the
phototherapeutic window observed with bacteriochlorins can
be reconciled with photostability increasing the oxidation poten-
tial (or the planarity) of the bacteriochlorins with electron-
withdrawing substituents. In particular, F or Cl atoms in the
ortho-positions of the phenyl rings of TPPs have electron-
withdrawing effects that stabilize the HOMO and increaseFT
to values close to unity. Thus, reduced porphyrins with halogen
atoms in these positions may combine the desired electronic
absorption with stability and long-lived triplet states. A word of
caution must be said about the use of electron-withdrawing sub-
stituents to stabilize reduced porphyrins. Substituents that are
particularly effective for this task, such as the nitro group, may
lead to low-lying CT states that effectively deactivate the triplet
state ( 69 ). Coordination with Pd or In also increasesFTand, with
proper care in the choice of the axial ligand of In complexes, may
not compromiseFD. However, the oxidation potential of PdIITPP
is only 0.07 V higher than that of H 2 TPP and can only make a
small contribution to the stability of hydroporphyrins. The oxida-
tion potential of ClInIIITPP is 0.21 V higher than that of TPP,
and indium hydroporphyrins should be more stable than their
palladium analogues. However, care must be exercised in the
choice of the axial ligand to allow for long-lived triplet states.
IV. Photoinduced Reactions with Molecular OxygenThe long-lived triplet states of free-base porphyrins have
energies between 33 and 36 kcal/mol (1.43–1.56 eV), and their
bacteriochlorin analogues have energies between 25 and 30 kcal/
mol (1.08–1.30 eV)( 65 ). This is lower than the energy of the sec-
ond excited state of molecular oxygen (^1
P
gO 2 ,E
P¼37.5 kcal/mol) but higher than the energy of singlet oxygen (^1 Dg O 2 ,
ED¼22.5 kcal/mol). Thus, we can expect that direct energy trans-
fer from triplet porphyrin or bacteriochlorin to molecular oxygen
leading to ground state sensitizer and singlet oxygen will be an
important decay mechanism in porphyrin-based photosensitizers.
In PDT, this is known as type II mechanism.
The mechanism of interaction between the triplet state of a
porphyrin and molecular oxygen involves the formation of an
encounter pair that may have singlet, triplet, or quintet spin
multiplicities. Spin statistics determine the probability of
forming each one of these pairs. When two triplets interact, they
give nine encounter pair spin states with equal probability, five
of which are sublevels of the encounter pair with quintet
212 LUIS G. ARNAUT