The freeenergyof full,outer-sphere, photoinducedelectron transfer
is given by
DGeET¼ EoxDEredA(^) e
e^20
erDA
(17a)
where the last term in negligible in polar solvents, and emphasis
is given to the measurement of redox potentials in solvents of
similar dielectric constante. This shows that outer-sphere elec-
tron transfer from H 2 TPP to O 2 is endothermic, but for H 2 TPB,
it may become exothermic in polar solvents and lead to the
superoxide anion, O 2 .
The generation of superoxide ion is facilitated by
photosensitizers with lowEDox. This outer-sphere electron trans-
fer should become controlled by diffusion whenDGET<–0.27 eV
(–6.2 kcal/mol) ( 75 ), that is, when EDox<–1.05V (e.g.,
ET¼1.30 eVandEDox<0.25V)versusSCEinpolarsolvents.Inter-
estingly, for such exothermic electron transfers, the electron trans-
fer rate becomes larger than the statistically limited energy
transfer rate, and superoxide ion becomes the predominant prod-
uctattheexpenseofsingletoxygen.Thisdoesnotnecessarilymean
that large amounts of superoxide ion will be formed at
DGET¼0.27 eV because back-electron transfer from O 2 to the
radical cation of the photosensitizer is exothermic by
DGeET¼ EoxDEredA
(^) e
e^20
erDA
(17b)
andDGET¼–1.03 eV is very close to the driving force of the
fastest charge recombinations, at the beginning of Marcus
inverted region( 76 – 78 ). Additionally, photosensitizers with low
oxidation potentials are more susceptible toward oxidative deg-
radation and may react with O 2 . The free radical route to cellu-
lar damage in PDT is called type I mechanism.
The most direct way to assess the competition between type I
and type II mechanisms in PDT is to measure the quantum
yields of singlet oxygen (FD) and of superoxide ion (FO) indepen-
dently of each other. The value ofFD is conveniently obtained
from the laser energy dependence of the singlet oxygen phospho-
rescence at 1270 nm because singlet oxygen has microsecond
lifetimes in many solvents. This dependence may become nonlin-
ear when a wide range of laser energies are employed and inform
on the presence of CT mechanisms( 65 ). However, only the value
ofFDcan be quantitatively obtained by this method.
214 LUIS G. ARNAUT