Chemistry Reference
In-Depth Information
dendrimers deserve a separate section as we investigated them in more detail, which
eventually led us in some new research directions.
7.2.3 Viologen-Containing Open Core Dendrimers
Viologens is the common name given to the salts of 4,4
0
-bipyridinium dications.
These compounds undergo two consecutive one-electron reduction processes [20].
The first reduction yields a radical cation, which is typically deep blue, but becomes
violet upon dimerization. The radical cation can be further reduced at more negative
potentials to form a neutral, quinonoid species. These reductions are fully reversible,
from a chemical and electrochemical standpoint.
Our general interest in viologen derivatives led Carlos Peinador, a visiting
professor from Universidad de A Coru
~
na, Spain, to prepare a series of viologen
core dendrimers (compounds
echet dendrons [21] to surround the
viologen nucleus [22]. The structures of these macromolecules are shown in
Figure 7.4. Investigation of their voltammetric behavior in acetonitrile solution
produced surprising results, as their electrochemical behavior was found to be fully
reversible (fast in the voltammetric time scale) as the size increases from first to third
generation. This was an unusual result, since electroactive core dendrimers had been
found to suffer a pronounced attenuation of electrochemical kinetics with dendrimer
growth [17]. Even considering that these dendrimers are not very large, their fast
electrochemistry was surprising in light of the existing body of literature data.
However, our results were confirmed by the simultaneous publication by Balzani
and coworkers of similar data on the same structures [23].
Intrigued by the voltammetric findings with compounds
28-30
), using Fr
, we decided to
prepare viologen dendrimers using our open core design with Newkome dendrons.
We encountered problems with compound stability until we settled on a five-
methylene spacer between the viologen nucleus and the amide connection to the
focal point of the dendrimer (see structures
28-30
in Figure 7.3). This
spacer was not required in the case of ferrocene or cobaltocenium redox residues, and
we initially worried that it might diminish the effects of dendron growth on the
microenvironment of the viologen nucleus. However, our voltammetric data quickly
dispelled this notion, because the half-wave potentials showed a clear dependence on
dendron size [24]. In fact, the voltammetric behavior of compounds
13-15
and
16-18
, as their
hexafluorophosphate salts, could be investigated in various solvents with a substantial
range of polarities, which was not possible with the ferrocenyl dendrimers due to
solubility limitations.
Figure 7.5 shows voltammetric data obtained from these solvent studies [24,25]. In
dichloromethane solution (Figure 7.5a), we observed that the first reduction potential
shifts to more negative values as the dendrimer grows. This clearly suggests that
dendritic growth differentially favors the dicationic versus the cationic form of the
viologen nucleus. This is the same trend recorded with the ferrocenyl dendrimers
13-15
7-9
in dichloromethane solution and suggests that, in both cases, the inner phase of the
dendrimer is more polar than the bulk dichloromethane solution. Thus, when the
dendrimer component grows the oxidation state with the highest positive charge is
