isoelectronicwithFeII(TPP)andcanalsobindaxialligands.FeII-por-
phyrin systems have been much investigated because of the
biological relevance of its ability to bind CO and O 2 axially. Weakly
binding ligands are high spin, either in five- or in six-coordinate
complexes. Increasing the binding constant of the ligands leads to a
change from high to low spin as the electrons are displaced from the
dz 2 orbital( 25 ). Unlike FeII-porphyrin, the four-coordinate species
FeIII(TPP)is unknown. Five-coordinate species such as FeIII(TPP)
Cl are high spin, and its electrochemical reduction to FeII(TPP)
occurs at 0.31 V versus saturated calomel electrode (SCE) in
CH 3 ClCH 3 Cl ( 17 ). Under similar conditions, the reduction potential
of pentacoordinated high-spin MnIII(TPP)Cl is0.29 V ( 26 ).
Two other metals may play important roles in
metalloporphyrins designed for PDT: palladium and indium.
The first oxidation and reduction potentials of PdIITPP are
þ1.02 and1.00 V versus SCE( 27 ), which are consistent with
ligand-centered redox processes and with the assignment of the
Q band to a porphyrin transition. The singlet state energy of
PdIITPP is somewhat higher than that of H 2 TPP. The nature of
the axial ligand of InIII(TPP)þinfluences its redox properties.
When that ligand in Cl, the first oxidation and reduction
potentials areþ1.16 and1.09 versus SCE in CH 2 Cl 2 , and the
maximum of the Q band is observed at 597 nm in CHCl 3 ( 28 ).
The one-electron oxidation or reduction of InIII(TPP)Cl is also
p-ring-centered reactions.
Breaking the degeneracy of the HOMOs increases the intensity
of the Q band. In bacteriochlorins, this is accompanied by a
decrease in the HOMO–LUMO gap, and an intense Q band is
observed in the near infrared. Following the labeling of the
orbitals shown in Fig. 3, the lowest energy transition in a
bacteriochlorin represents mainly the configuration a1u!egx
(HOMO!LUMO) with a small contribution from a2u!egy
(HOMO 1 !LUMOþ1) and is labeled Qy (Fig. 2). In
tetraphenylbacteriochlorins, this very intense band is observed
at 740–750 nm. The Qx band has a smaller intensity and is
observed at 519–529 nm. It represents mainly a2u!egxwith a
smaller amount of a1u!egy. The By absorption near
350 – 360 nm and the Bxat 370–380 nm are the split Soret band.
The Byband is made up mainly of the a2u!egyconfiguration
while the Bxband is mainly a1u!egy( 29 ).
The intense near infrared absorption exhibited by bacteriochlo-
rophyllais a most interesting property for a sensitizer, but bac-
teriochlorophyll a is known to be extremely sensitive to
oxidation. For many years, the lability of the bacteriochlorins
available directed the research efforts on new photosensitizers
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 197