Chemistry Reference
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SCHEME 7.2
Cartoon representations of (a) unrestricted interfacial orientation of dendrimer
10
at low pH, and (b) electrostatically restricted orientation of dendrimer
10
at high pH, on
positively charged gold electrodes.
the ferrocenyl residue is clearly observed in square wave voltammetric (SWV)
experiments. This SWV peak is progressively broadened and eventually disappears
as the pH increases. At pH
7 the voltammetric peak is no longer observed in the
surveyed potential range, but it fully develops again if the solution pH is decreased
again. This pH dependence was shown to be completely reversible in several cycles,
suggesting that decomposition of the ferrocene residue is not behind the loss of
voltammetric response at high pH values. After additional experimentation we
concluded that the best rationalization of the data relies on the pH-dependent
electrostatic orientation of the dendrimer at the electrode solution interface [18]
(Scheme 7.2). At low pH, the dendrimer has no charge and can adopt a range of
orientations at the interface. In many of these, the ferrocenyl group faces the
electrode, which gives rise to relatively fast electron transfer. At high pH, the
dendrimer is negatively charged and electrostatic attraction between the positively
charged electrode surface and the negatively charged carboxylates on the dendrimer
surface orients the dendrimer in such away that the ferrocenyl group is kept far away
from the electrode surface. As a result, the electron-transfer rate is much slower in
this situation. This rationalization of the data is further supported by the fact that the
third-generation dendrimer, compound
>
12
, shows similar behavior, while the first-
generation dendrimer,
10
, is too small to show these pronounced orientation
effects [18].
The demonstration that open core ferrocenyl-containing dendrimers show a
pronounced pH controlled, interfacial orientation dependence on their rates of
heterogeneous electron transfer was a strong validation of our dendrimer design, as
it shows that these macromolecules exhibit properties similar to those of redox
proteins. We also studied the electrochemical behavior of hydrophobic (
19-21
) and
hydrophilic (
) cobaltocenium-containing dendrimers and the results were
similar to those obtained with the ferrocenyl systems [19]. The viologen-containing
22-24
