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transfer in which the excitation energy can
be transferred directly into high energy
vibrational modes that are quickly trans-
ferred to the surroundings (e.g. the OH
effect) (Foote and Denny, 1971); energy
transfer to another excited state that would
then be funnelled to surrounding energy
vibrational modes (e.g. the carotenoids)
(Schmidt, 2004), and reversible electron
transfer reactions (Schweitzer and Schmidt,
2003) (Fig. 6.12). These processes explain
why
1
O
2
has short lifetimes in water and
protic solvents (methanol and ethanol;
Table 6.1) and also why the azide anion is
an excellent quencher of singlet oxygen
(Wilkinson
et al
., 1995). The OH group is
present in a large number of singlet oxygen
suppressors and has one of the highest
vibrational energy levels; its overtone tran-
sition is near to the lower energy level of
1
O
2
(96 kJ/mol) (Krasnovskii, 1976), facilitating
the electronic-vibrational coupling. Foote
and Ogilby showed in the 1970s that
1
O
2
has
longer lifetimes in deuterated solvents and
performs photo-oxidation reactions, unrav-
elling the possible role of
1
O
2
(Foote and
Denny, 1971; Ogilby and Foote, 1982).
The other two physical processes
through which
1
O
2
decays without forming
products are the reversible electron and
energy transfer transition states (≠) formed
with the azide ion (Catalan
et al
., 2004) and
b-carotene (Wilkinson
et al
., 1995; Schmidt,
2004), respectively (Fig. 6.12). Both mecha-
nisms lead
1
O
2
to decay to the ground state
(
3
O
2
). The constants involved in these pro-
cesses are diffusion controlled (
~
10
10
mol.l
−1
.s
−1
).
In others words, there are effective and fast
ways to suppress
1
O
2
. It is important to
emphasize that under physical quenching
the excited state energy of singlet oxygen is
dissipated in the surroundings as heat. Both
oxygen and the quencher agents return to
their original state.
Chemical quenching includes all the
reactions described in the last section, plus
the simple electron transfer reactions that
convert singlet oxygen into anion radical
superoxide. Molecules that present ade-
quate E
0
values and that stabilize well posi-
tive charges are good candidates to suppress
singlet oxygen by this specific mechanism
(Oliveira
et al
., 2011), which depends on
solvent stabilization of the involved inter-
mediates (Machado
et al
., 1995; Schweitzer
and Schmidt, 2003). In general, the interac-
tion of singlet oxygen with molecules
favours more than one mechanism simulta-
neously. For example, carotenoids are the
most efficient known singlet oxygen sup-
pressors reacting mostly by physical mech-
anisms, although chemical quenching is
also observed. Proteins, enzymes and DNA
also quench singlet oxygen by chemical
and physical mechanisms (Lu
et al
., 2000;
Schmidt, 2004).
Nature looks for strategies to protect
itself from these oxidation reactions. For
protection against sunlight, humans have
melanin to avoid light from reaching the
[
1
O
2
...
HO-H]
3
O
2
+ HO-H
(a)
[
1
O
2
...
N
3
-
]
3
O
2
+ N
3
-
(b)
H
3
C
H
3
C
[
1
O
2
...
]
CH
3
CH
3
CH
3
H
3
C
CH
3
H
3
CCH
3
CH
3
(c)
3
O
2
+
H
3
C
H
3
C
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
Fig. 6.12.
Main deactivation routes of singlet oxygen: (a) electronic-vibrational coupling, (b) reversible
electron transfer and (c) energy transfer transition states.
Table 6.1.
Lifetimes of singlet oxygen in various solvents (
1
D
g
).
Solvent
H
2
O
MeOH
C
6
H
6
CS
2
CCl
4
C
6
F
6
D
2
O
Air, 1 atm.
t
(
m
s)
3
7
24
200
700
3,900
70
~76,000
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