Page 202 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Reactive Oxygen Species Generation on Nanoparticulate Material 187

(Eqs. 81 83). Intersystem crossing is a spontaneous nonradiative
transition from the singlet to the triplet electronic state.

k F
S 1 h S 0 1 hv F (81)
k ic
S h S 0 (82)
1
k isc
S 1 h T 1 (83)
The rate of singlet decay is taken as the sum of these rates of singlet
, (Eq. 84) and the quantum yield, , is defined by the frac-
decay, k SD T
tion of the singlet state that decays to the triplet state (Eq. 85).
1
k SD 5 k 1 k 1 k isc 5 (84)
F
ic
sS d
1
k isc
5 (85)
T
k SD
The singlet form of a photosensitizer can also be quenched by oxygen to
form the triplet state of the sensitizer, generating either singlet oxygen
(Eq. 86) or ground-state oxygen (Eq. 87). It can also be quenched to the
ground state (Eq. 88) or the sensitizer may be altered through a reac-
tion to form a new product (Eq. 89).
k O 2
S 1 ∗
1
2
1
S 1 O h T 1 O 2 (86)
O 2
k isc
3
S 1 1 O 2 h T 1 1 O 2 (87)
k O 2
Sd 3
S 1 O 2 h S 0 1 O 2 (88)
1
O 2
k Sr
S 1 O h reaction product (89)
2
1
k O 2 5 k O 2 1 k O 2 1 k O 2 1 k O 2 (90)
SQ S isc Sd Sr
The triplet state in turn decays by phosphorescence (Eq. 91) or inter-
system crossing (Eq. 92). Phosphorescence releases light energy as elec-
trons fall to a lower energy level, and internal conversion involves the
same drop in energy without the associated energy release. The rate of
triplet decay is the sum of these processes (Eq. 93).
k Tp
T h S 1 hv p (91)
1
0
k Td
T h S 0 (92)
1
1
5 k 1 k 5 (93)
k TD Tp Td
sT d
1