Chemistry Reference
In-Depth Information
the observed rate constant of electron transfer (
k
obs,et
) is defi ned as:
(
)
k
=
v
N
κ
exp
−∆
G
‡
RT
(8.13)
obs,et
where
v
N
is the nuclear frequency factor and k is the electron transfer coeffi cient.
The free energy of activation,
∆
G
‡
, is expressed as:
2
G
0
λ
∆
!
#
∆
G
‡
=
1
+
(8.14)
4
λ
where l is the reorganizational energy and D G
0
is the change in standard free energy
of redox reactants and products. Experimentally
k
et
can be determined by Stern-
Volmer kinetic analysis as described below.
Type I Photooxidation Reactions in PDT
Reductive quenching of an excited photosensitizer by DNA constitutes a Type I
photooxidation reaction, as defi ned by Foote.
46
For example, consider the mecha-
nism of decay of the
3
MLCT state of [(phen)Ru(HAT)
2
]
2+
in the presence of DNA,
phen = 1,10 - phenanthroline, HAT = 1,4,5,8,9,12 - hexaazatriphenylene.
I
[
(
)
(
)
]
+→
2
+
a
[
(
)
(
)
]
2
+
phen Ru HAT
hv
*
1
phen Ru HAT
(8.15)
2
2
k
[
(
)
(
)
]
→
2
+
[
(
)
(
)
]
2
+
1
*
phen Ru HAT
isc
*
3
phen Ru HAT
(8.16)
2
2
k
[
(
)
(
)
]
→
2
+
[
(
)
(
)
]
2
+
3
*
phen Ru HAT
nr
phen Ru HAT
(8.17)
2
2
k
[
(
)
(
)
]
→
2
+
p
[
(
)
(
)
]
+
2
+
3
*
phen Ru HAT
phen Ru HAT
h
′
(8.18)
2
2
k
[
(
)
(
)
]
+
2
+
q
[
(
)
(
)
]
2
+
*
phen Ru HAT
DNA
→
phen Ru HAT
+
DNA
(8.19)
2
2
k
[
(
)
(
)
]
+
2
+
et
[
(
)
(
)
]
+
*
phen Ru HAT
DNA
→
phen Ru HAT
+
DNA
+
(8.20)
2
2
Kinetically the quantum yield of the electron transfer reaction (Equation 8.20)
is expressed as:
k
kk k k
[ ]
++ +
DNA
et
ΦΦ
et
=
(8.21)
isc
(
)
[
]
DNA
q
p
nr
et
where F
et
is the quantum yield of electron transfer, F
isc
is the effi ciency of popula-
tion of the
3
MLCT state,
k
p
and
k
nr
are the respective unimolecular phosphorescence
and nonradiative decay rate constants,
k
et
is the rate constant of electron transfer,
k
q
is rate constant of bimolecular deactivation and [DNA] is the effective concentration
of the DNA quencher in molarity of base pairs. Effi ciency and rates of excited state
reactions can be determined by probing the quantum yield of phosphorescence F
p
:
o
Φ
Φ
p
(
)
[
]
=+
1
τ
kk
+
DNA
(8.22)
o
et
q
p
