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Reactive Oxygen Species Generation on Nanoparticulate Material 189

energy. Increasing the lifetime of the triplet state and minimizing the
effect of the nonproductive pathways will result in higher quantum yields
for ROS production; these pathways are controlled by the concentration
and proximity of the photosensitizer in the solution.
Type I reactions can occur in parallel with type II reactions, when the
photosensitizers are in the presence of electron donors. Type I reactions are
initiated by the reduction of the triplet state by an electron donor (Eq. 103).
The donor in this case has a reduction potential lower than either the
ground state (S ) or the excited state (T or S ) of the sensitizer (Eqs. 2 to 5).
1
0
1
Once the reduction occurs, the sensitizer takes the form of a radical anion
.2
(S 0 ) that has the possibility of reducing oxygen to superoxide (Eq. 104) and
subsequently returning to the ground state. Reduction of the singlet state
is more thermodynamically favorable but is kinetically limited because of
the short lifetime of the singlet excited state (Eq. 105). As a result, singlet-
state reactions with the electron donor are not likely to occur.
D
k T1 2. .1
T 1 1 D h S 0 1 D (103)
O2
k S ? 2
.2
So
S 0 1 O 2 h O .2 1 S 0 (104)
2
D .2 .1
k S1
S 1 D h S 0 1 D (105)
1
Thus, the proportion of oxygen reacting with the triplet-state sensitizer
must be modified to express the reactions between the triplet state and
the electron donor (Eq. 106).
O 2 ⎡ ⎤
k TQ ⎣ O 2 ⎦
P T O 2 TD ( T1 ⎣ ⎦)
D
TQ ⎣
AN ⎣ ⎦
SQ ⎣ ⎦

k k O2 O ⎡ 2 ⎦ ⎤ k S 0 S ⎡ ⎤ k T 1 T ⎡ ⎤ k D ⎡ ⎤
1
0
(106)
In order to determine the quantum yield for superoxide formation, the
following assumptions are made: anion radical sensitizers react with
II
oxygen to form superoxide (i.e., f sup 1); anion radicals are unlikely to
be formed (Eq. 105) due to the short lifetime of S 1 ; and the donor reacts
only with the triplet-state molecule, T 1 , of Eq. 103.
Given these assumptions, the proportion of the triplet-state mole-
cules that react with a donor is given by Eq. 107 with the corresponding
quantum yield given by Eq. 108.
D D ⎡ ⎤
k
D T1 ⎣ ⎦
P
T TD ( O S T 1 T1 ⎣ ⎦)
D
S ⎡ ⎤
0
2
k k TQ ⎣ O ⎡ 2 ⎦ ⎤ k SQ ⎣ ⎦ k AN ⎣ ⎦ k D ⎡ ⎤
T ⎡ ⎤
N
1
0
(107)