molecular oxygen. Thus, virtually all triplets will interact with
molecular oxygen. The values ofFDcan be reliably measured
by singlet oxygen phosphorescence. The energy of the CT state
can be estimated from Eqs. (17a and 17b). The major uncertainty
in deriving the value ofF○from Eq. (19) and an independent
measurement ofFDis the estimate of the fraction of CT states
that separate into free ions and of the CT states that recombine.
A thorough investigation of the rates and energies of all the pro-
cesses involved in the interaction between halogenated and
sulfonated bacteriochlorins and molecular oxygen leads to the
scheme presented in Fig. 9. Typically, these photosensitizers
haveFD¼0.70.1 andF□¼0.30.1 ( 65 ), and lower values of
FDare closely associated with higher values ofF□.
The generation of O 2 by chlorinated and sulfonated bacteri-
ochlorins was also investigated by EPR. It was shown that O 2
is the primary photoproduct of electron transfer in aqueous solu-
tion, but it protonates and/or disproportionates to yield molecu-
lar oxygen and hydrogen peroxide ( 65 ). The formation of the
hydroxyl radical subsequent to the formation of O 2 and H 2 O 2
was demonstrated by the EPR spectra of spin adducts formed
with specific spin traps, and by the inhibition of their formation
in the presence of superoxide dismutase or catalase. The
hydroxyl radical cannot be formed neitherviathe Haber–Weiss
reaction because it is known to be inefficient in water norvia
the Fenton reaction because a chelating resin was employed to
remove metal ions from the solution. Alternatively, the
FIG. 9. Rates (in s^1 ) and energies in the interaction between
molecular oxygen and halogenated and sulfonated bacteriochlorins ( 65 ).
218 LUIS G. ARNAUT