Page 201 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

186 Principles and Methods

The type II photosensitization pathway involves the transfer of excited
spin-state energy from the sensitizer to another molecule. Type II sen-
sitization does not depend upon the presence of a donor molecule, but
only requires a long-lived triplet excited state. In the presence of oxygen,
this triplet state is quenched by ground-state oxygen, which is trans-
formed into singlet oxygen. As noted previously, singlet oxygen is a reac-
tive oxygen species that can participate in reactions in solution that are
spin-forbidden in the case of ground-state molecular oxygen. Singlet
oxygen formation via type II photosensitization and quenching has been
reviewed extensively by Wilkinson et al.
A typical photosensitizing molecule in the ground state is represented
by S 0 with S 1 and T 1 representing the lowest energy singlet and triplet
states, respectively. Figure 5.22 is a graphic representation of their
main photosensitized pathways.
Light within the absorbance range of the photosensitizer is absorbed
and promotes electrons into the excited singlet state (Eq. 80).

k w
S 1 hv h S 1 (80)
0
The intensity and wavelength of the light will govern the rate of S for-
1
mation (k ). The singlet state then decays via at least three different
W
pathways: fluorescence, internal conversion, or intersystem crossing

80 81 82 83 86 87 105
88 89

103
91 92 97 98 94 95 96

104

Figure 5.22 General photosensitization kinetic scheme.