In principle, EPR of the spin adduct formed under laser irradia-
tion by a specific spin trap of O 2 should give a good estimate of
FO. In practice, this measurement gives the amount of spin adduct
formed and stable in the time window of the experiment, which is
only indirectly related toFO. Charge recombination with the pho-
tosensitizer radical cation, competitive reactions of O 2 , and
decomposition of the spin adduct contribute to diminish the
amount of O 2 measured with this procedure. Lower limits for
FOwere determined by this method for two photosensitizers in
clinical trials, Tookad and Stakel, shown inFig. 8. The values
obtained wereFO¼0.001 and 0.003 for Tookad ( 79 ) and Stakel
( 80 ), respectively, but are likely to largely underestimate of the
actual amount of O 2 produced. Interestingly, these palladium
complexes of bacteriopheophorbide derivatives make use of the
heavy-atom effect of Pd to enhance intersystems crossing to the
FIG. 8. Molecular structures of photosensitizers employed in
photodynamic therapy.
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 215