Chemistry Reference
In-Depth Information
TABLE 7.3 Half-Wave Potentials
a
(E
1/2
1
and E
1/2
2
in V versus Ag/AgCl) for
dendrimers 37-41 Measured at 23
C in Organic Solutions Also Containing 0.1M
Tetrabutylammonium Hexafluorophosphate as Supporting Electrolyte
Solvent
37
38
39
40
41
E
1/2
1
DCM
0.240
0.240
0.277
0.351
0.292
E
1/2
2
DCM
0.755
0.764
0.722
0.821
0.784
E
1/2
1
THF
0.276
0.271
0.285
0.320
0.300
E
1/2
2
THF
0.676
0.676
0.685
0.706
0.708
E
1/2
1
ACN
0.325
0.322
0.345
0.369
0.358
E
1/2
2
ACN
0.754
0.756
0.770
0.776
0.772
a
Estimated error margin for all potential values is
0.005V.
hybrid dendrimer. We elected to designate these dendrimers as “hybrid” to emphasize
the fact that their structures combine two entirely different types of dendritic
components. Using first- to third-generation dendrons, we could prepare up to nine
unsymmetric dendrimers in this series, but we decided to prepare only the five
macromolecules shown in Figure 7.7, not only for reasons of economy but also
because we were able to answer a good number of questions related to the interplay
between the two types of dendrons with this selected set of dendrimers [34].
As anticipated the cathodic voltammetric behavior of dendrimers
is
characterized by the stepwise reduction of the viologen nucleus, which takes place
as two consecutive one-electron reduction processes. The half-wave potentials
measured in dichloromethane (DCM), tetrahydrofuran (THF), and acetonitrile (ACN)
are given in Table 7.3.
The potential values are clearly affected by dendron growth. The first reduction
half-wave potentials (E
1/2
1
) spread over a range of 110mVinDCMsolution, while the
covered range is narrower in THF and ACN [34]. The reason behind this is probably
related to the low polarity of DCM, which gives rise to the most pronounced polarity
contrast with the inner phase of the dendrons, particularly with the amide-rich and
polar Newkome dendrons. Therefore, the careful analysis of the potential values
recorded for these dendrimers in DCM solution is of substantial interest. First, we
should note that the E
1/2
1
value is almost invariant in going from
37-41
37
to
38
, where the
only change is the size of the Fr
echet dendron (first to second generation). Conversely,
D
E
1/2
1
of
from
37mV, which is obviously due to the
growth of the Newkome dendron from first to second generation, while the Fr
38
to
39
, there is a measurable
echet
dendron is identical (second generation). Clearly the reduction potential is more
sensitive to Newkome dendron growth than to Fr
echet dendron growth. A more
pronounced size differential in the Newkome dendron, from dendrimer
37
to
40
leads
D
E
1/2
1
to a more acute potential shift of
111mV. It is instructive to compare this
value with that observed in going from
52mV). In the latter case, the
potential shift results from replacing the first generation Newkome dendron by its
third-generation analogue while keeping the second generation Frechet dendron
unchanged. However, the magnitude of the potential shift is decreased (compared to
the
38
to
41
(
D
E
1/2
1
value
37
40
to
). This means that the growth of the Frechet dendron
