Chemistry Reference
In-Depth Information
electron transfer processes at the molecular level. In these systems the rate of
charge transfer (CT) is a combination of a strongly distance-dependent tunneling
mechanism (or superexchange) and a weakly distance-dependent incoherent trans-
port (or hopping).
The attenuation factor,
ʲ
, is the parameter usually employed to describe the
quality of a system as molecular wire, so that the lower the
ʲ
value the longer the
ʲ
distance that the charge can be moved efficiently. The
parameter is characteristic
for the decay of CT rate constant,
k
ET
, as a function of distance,
R
DA
:
k
0
e
ʲ
R
DA
k
ET
¼
values can vary from 1.0-1.4
˚
1
for proteins to 0.001-0.06
˚
1
for
highly conjugated bridges [
241
]. Other parameters exert an impact on the rate of
charge transfer,
Typical
ʲ
ʔ
G
),
in particular
the underlying driving forces
(
the
), and the electronic couplings (
V
) between
the donor and acceptor moieties. Therefore
ʻ
corresponding reorganization energies (
value depends not only on the bridge
but rather on the wire system as a whole, that is, the DBA system, whether the D and
A termini are molecular units or metallic contacts.
The unique electronic properties of fullerenes have prompted the study of the
molecular wire behavior of different molecular bridges connected to fullerene C
60
as the electron acceptor and different electron donor fragments [
242
,
243
]. The
study of these systems with various bridge lengths allows the measurement of
distance-dependent charge separation (CS) and charge recombination (CR) rates
and, therefore, the determination of the corresponding
ʲ
values.
It is well documented that molecular wires that exhibit
para
-conjugation facili-
tate charges to be transferred over larger distances and hence show lower
ʲ
values
related to non-conjugated bridges. Very fast electron transfer for both CS and CR
has been reported for compound 59, with two acetylene bonds connecting the C
60
unit with the
ʲ
10
10
s
1
)[
244
].
In contrast, for compounds 60, in which the polyacetylene bridge is connected
through a phenyl ring in the meso position of the porphyrin, CS and CR are in the
regions of 7.5
10
11
s
1
,
k
CR
¼
ʲ
-position of ZnP (
k
CS
>
1
2.5
10
9
s
1
10
6
s
1
, with a
value of
0.06
˚
1
in PhCN [
245
]. This difference in the rate of electron transfer is due, at
least in part, to more favorable orbital interactions between ZnP and C
60
in 59 and
suggests that the phenyl ring retards the electron transfer processes (Fig.
32
).
When comparing these systems with a ZnP-
p
-phenylenebutadiynilenes-C
60
series (61), the
k
CS
and
k
CR
values decrease drastically as the distances increase
from 22 to 40
˚
2.4
and 1.6
0.2
ʲ
10
8
s
1
and
k
CR
range
from 2.1
10
6
s
1
to 4.6
10
5
s
1
) giving a quite large attenuation factor of
0.25
˚
1
in PhCN [
246
]. This
ʲ
value suggests that the phenyl ring inserted in the
polyalkyne bridge acts as a resistor for electron transfer, giving rise to a less
effective superexchange mechanism.
For
10
10
s
1
to 1.1
(
k
CS
range from 1.0
-substituted ZnP-oligophenyleneethynylene-C
60
systems 62
(ZnP-oPPE-C
60
), a strong increase in the lifetime of the charge separated state
the
ʲ
