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heterogeneous electron-transfer processes involving these macromolecules. Some of
our reasons have been described before in this chapter, especially those related to
biological mimicry. We were also interested on the idea of detecting dendron back
folding and its effect on the microenvironment of the redox active residue through
electrochemical potential measurements. Finally, a powerful motivator for the
preparation of these dendrimers was the investigation of their heterogeneous rates
of electron transfer.
Our first series of electroactive dendrimers was prepared by reaction of chloro-
carbonylferrocene with the amine Newkome dendrons, giving rise to the hydrophobic
series of macromolecules
[16]. These hydrophobic dendrimers were investigated
in dichloromethane solutions (about 1 mM) also containing 0.1M tetrabutylammo-
nium hexafluorophosphate as the supporting electrolyte. Our data showed a very clear
effect of dendrimer generation on the half-wave potential (E
1/2
) for one-electron
oxidation of the ferrocenyl residue. As the dendron grows, the E
1/2
value shifts to less
positive values, suggesting that the generation of positive charge is favored by the
dendritic mass [16]. This finding was rationalized by considering that the dendrimer
inner phase contains amide groups and its polarity is, thus, higher than that of the bulk
dichloromethane solution. As the dendron grows, it folds back and starts to affect the
microenvironment around the ferrocenyl residue, increasing its relative polarity and
leading to a greater thermodynamic ease for conversion from ferrocene (neutral) to
ferrocenium (
7-9
1 charge).
Voltammetric experiments were also performed to measure the standard rate
constants for electron transfer (k
รพ
o
) between the ferrocenyl residues in the dendrimer
and the electrode surface [13,16]. Our data show a clear tendency to lower k
o
values
with dendrimer growth. This trend has been commonly observed in redox core
dendrimers, in which dendrimer growth generally attenuates the electrochemical
rate constant [17]. Our hydrophobic ferrocenyl dendrimers (
) are no exception.
It is widely accepted that the decreased rate of heterogeneous electron transfer is due
to the longer average distance of maximum approach between the redox center and
the electrode surface, which is anticipated as the molecular weight of the macro-
molecule increases.
We also investigated dendrimers
7-9
in aqueous solution. Here the pH of the
solution becomes extremely important as the dendrimers go from bearing no charge
at acidic pH values to bearing a relatively large negative charge upon ionization, that
is, when the solution pH is neutral or basic. We were very interested on the
possibility of using this anionic charge development to orient the dendrimers on
the electrode surface. Experiments with bare gold electrodes showed that the
voltammetric parameters are rather insensitive to the solution pH. In order to
emphasize the electrostatic effects, we decidedtoexperimentwithgoldworking
electrodes derivatized with a monolayer of cystamine. These electrodes maintain a
positively charged surface throughout the pH range of our experiments (roughly
from pH 3 to 8), as the terminal amine group of cystamine remains fully protonated.
Under these conditions, we observed that the voltammetric behavior of the second-
generation dendrimer, compound
10-12
, is strongly pH dependent [18]. At low pH,
when the dendrimer is fully protonated and uncharged, the one-electron oxidation of
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