and bacteriochlorin (H 2 TOHPB) follow the same trend as H 2 TPP,
H 2 TPC, and H 2 TPB in Table I, the photodecomposition quantum
yield in PBS:methanol seems to increase by a factor of 10 as the
oxidation potential is reduced in 0.1 V. The presence of two Cl
atoms in theortho-positions of the phenyl rings of H 2 TCl 2 PBEt
increase the photostability of this bacteriochlorin to that of a por-
phyrin. The nature of the substituents and metal ions also
influences Fpd. For example, Tookad has a higher oxidation
potential than bacteriochlorophylla, which is similar to that of
H 2 TPB, but the photodegradation quantum yields of H 2 TOHPP
and Tookad are similar. Stakel has an oxidation potential 0.1 V
less positive than that of Tookad ( 80 ) and was reported to be less
photostable than Tookad. Stakel does not form detectable
amounts of^1 O 2. Its facile photooxidation in PBS leads O 2 and
OHradicals, and the generation of these species is accompanied
by the consumption and degradation of a similar number of
Stakel molecules ( 80 ).
Table IV also shows that the quenching rate constants of trip-
let bacteriochlorins by molecular oxygen approach the value of
4/9kdiff, indicative of the formation of CT complexes. The
quenching of triplet porphyrins and chlorins is close to 1/9kdiff,
which is consistent with the high energy of excited state CT
complexes in these systems ( 65 ).
The data published over the past 8 years shows the relevance
of the CT channel in the quenching of triplet bacteriochlorins
by molecular oxygen. These sensitizers contrast with triplet
porphyrins that have higher oxidation potentials, lower
quenching rates but an upper limit of unity for the singlet oxygen
quantum yield. Whereas PDT with triplet porphyrins will be
dominated by type II mechanism, triplet bacteriochlorins may
also make use of type I mechanism. The comparison between
thein vitroPDT efficacy of porphyrin and bacteriochlorins may
inform on the relevance of type I and type II mechanisms. Addi-
tionally, it is of general interest to assess the progress achieved
by the photosensitizers presented inFig. 8 over the most popular
photosensitizers in clinical use: Photofrin and Foscan.
V. Photodynamic Therapy
Photofrin (a mixture of hematoporphyrin derivatives including
various dimers) and Foscan (H 2 TOHPC) are the most widely
used sensitizers in oncologic PDT and are indicated for a variety
of cancers(102,103). In vitroandin vivostudies reported for
these sensitizers and for the new generation of bacteriochlorin
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 221