triplet, which is formed with nearly unity quantum yield, but the
radiative lifetime of triplet of triplet PdBPheo is still sufficiently
long, ca. 3ms, for Tookad to produce singlet oxygen withFD1 in
organic solvents andFD0.5 in 3%TX-100/D 2 O( 79 ). However,
Stakel does not produce detectable amounts of singlet oxygen.
The predominance of electron transfer in Stakel at the expense of
energy transfer, as opposed to Tookad, is in line with their oxida-
tion potentials. The oxidation potential of Tookad is 0.34 V higher
than that of MgBChl ( 24 ) but that of Stakel is only 0.24 V higher
which facilitates electron transfer ( 80 ).
Another spectroscopic technique that should be able to quan-
tifyFOis transient absorption of the photosensitizer radical cat-
ion. This was attempted in combination with a peculiar approach
to lower the value of the oxidation potential of the photosensi-
tizer, and hence to increase the energy of the HOMO and make
HOMO–LUMO gap smaller than the triplet state energy. As
shown inTable I, this can be achieved with Zn rather than Pd
metalloporphyrins derivatives. In fact, the zinc complex of the
bacteriopurpurinimide derivative identified in Fig. 8, ZnBPur,
hasET¼1.4 eV,E1/2ox¼0.45V, andE1/2red¼–0.81V versus SCE
( 81 ), and the following sequence of reactions becomes exothermic
in polar solvents
(^3) ZnBPurþZnBPur!ZnBPurþþZnBPur
ZnBPurþO 2 !ZnBPurþO 2
The radical anion and the radical cation of ZnBPur were
identified by transient absorption and EPR. The EPR spectra
were also consistent with O 2 forming a complex with the photo-
sensitizer in glassy benzonitrile. ZnBPur yieldedFD¼0.58, much
smaller thanFD¼0.94 of PdBPur but higher thanFD¼0.33 of
H 2 BPur (81,82). This is consistent with the expected heavy-atom
effect and unrelated to a compensation betweenFDandFO, as
expected from the fact that O 2 is not generated in a competi-
tive reaction mechanism.
Chemical methodscan alsobeemployed to assess the competition
between the formation of^1 O 2 and O 2 . A specific chemical probe
may react either with^1 O 2 or O 2 and yield a fluorescent product.
For example, nonfluorescent 3^0 -(p-aminophenyl)fluorescein reacts
with hydroxyl radicals, formed subsequently to the formation of
O 2 , to give fluorescein as oxidation product( 83 ), whereas Singlet
Oxygen Sensor GreenÒ(Molecular Probes) specifically reacts with
(^1) O
2 to give another fluorescent product (^84 ). Admittedly, this again
is not adirect comparison between^1 O 2 and O 2 ,andtheconversion
216 LUIS G. ARNAUT