multiplicity, three are sublevels of the encounter pair with triplet
multiplicity, and the ninth one is the singlet encounter pair.
In the absence of perturbations, only the singlet encounter pair
eventually leads to singlet oxygen. According toFig. 7, the sin-
glet path leads to singlet oxygen with a unit quantum yield and
the triplet quenching rate constant is ( 70 – 72 )
kD¼1
9kdiff (14)In the presence of CT and for small energetic differences between
singlet and triplet states of CT intermediates, the singlet quan-
tum yield should drop to 0.25 and the quenching rate constant
should increase to( 73 )
kCT¼4
9kdiff (15)The quintet path does not correlate to any products, and the
quintet encounter pairs are purely dissociative.
Molecular oxygen is a good electron acceptor, with a reduction
potentialEAred¼–0.78V versus SCE in dimethylsulfoxide( 74 ),
but not a good electron donor. Thus, CT to molecular oxygen
increases as the oxidation potential of the donor decreases. The
donor is an electronically excited state, and its oxidation
potential is given by
EoxD¼EoxDE (16)where we haveEDox¼ 0.95–1.43¼–0.48V for H 2 TPP. However,
for H 2 TPB, the corresponding value isEDox¼0.40–1.30¼–0.90V.
(^3) M+O 2 ( (^3) Σg–)
1 kdiff (^3) M...O 2 ( (^3) Σ–g)
k–diff
(^1) kct
(^1) kΔ
(^3) kct
(^1) k–ct
3,1kisc 1,3k
isc
(^3) kic
(^1) kic
k–diff (^3) k–ct
k–diff
k–diff
(^111) M... (^1) O 2 ( (^1) Δg)
(^1) M... (^3) O 2 ( (^3) Σg–) M + O 2 (^3 Σg)
M +^1 O 2 (^1 Δg)
3
(^1) Mδ+...O 2 δ−
Mδ+...O 2 δ−
CT channel
nCT channel
(^33) M...O 2 ( (^3) Σg–) 3
(^53) M...O 2 ( (^3) Σg–)
/ 9
(^3) / 9 kdiff
(^5) / 9 kdiff
k–diff
FIG. 7. Wilkinson mechanism for triplet quenching by molecular
oxygen.
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 213