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modulates down the effects of Newkome dendron growth on the other side of the
viologen nucleus. Consistent with this conclusion, the
D
E
1/2
1
value measured with
the open core viologen dendrimers
140mV in DCM solution. In other
words, our data reveals that the first to third generation dendron-induced
13-16
was
D
E
1/2
1
value
is tempered by the steric bulk of the other viologen substituent. These findings
indicate that, in spite of its five-methylene tether to the viologen residue, Newkome
dendrons are effective at modifying the core's microenvironment. The sign of
the recorded
D
E
1/2
1
values in DCM solution is explained by the greater polarity
of the Newkome dendron inner phase, which differentially stabilizes the oxidation
states with the larger positive charges (
þ
2 versus
þ
1 in the first-reduction process
and
1 versus 0 in the second). The potential values recorded in THF and ACN
follow similar trends, although the magnitude of the potential shifts is generally
smaller [34].
We also assessed the standard rate constants for heterogeneous electron transfer
with these viologen core dendrimers. While the smaller dendrimers,
þ
37-39,
exhibit
essentially reversible voltammetric behavior, the two largest dendrimers
41
show measurably slower (quasireversible) voltammetric behavior [34]. These two
compounds are the largest molecular weight viologen core dendrimers ever studied
by our group and, thus, the loss of voltammetric reversibility is anything but
unexpected.
The voltammetric behavior of these hybrid dendrimers is interesting because it
allows us to compare the behavior of two different types of dendrons present in the
same macromolecule. The results are consistent with previous data obtained by our
group and suggest important differences in the rates of space filling and relative inner
polarities of Frechet and Newkome dendrons.
40
and
7.4 HYBRID FR
´
CHET-NEWKOME DENDRIMERS WITH A
TRIAZINE CORE
Our interest on the preparation of dendrimer systems containing both Fr
echet and
Newkome dendrons eventually led us to consider triazine-based dendrimers of the
kind developed by Steffensen and Simanek [35], taking advantage of the differential
reactivity of trichlorotriazine (cyanuric chloride). This group and others [36] have
demonstrated that it is possible to sequentially replace one, two, or three of the chloro
substituents by controlling the temperature of the reaction. In our synthetic proce-
dures, we started by reacting cyanuric chloride with a Fr
echet dendron (in its -OH
terminated form) at 0
C in THF. In a second step, we reacted the resulting Fr
echet
dendronized dichlorotriazinewith an amine Newkome dendron in acetone solution (at
room temperature). This leads to the nucleophilic substitution of the second chloro
substituent and the attachment of the Newkome dendron to the triazine nucleus.
Finally, the last chloro substituent is replaced by treatment with aminoferrocene in dry
refluxing THF, yielding a triazine-based dendrimer containing three different sub-
stituents: a Frechet dendron, a Newkome dendron, and aminoferrocene [37]. The
resulting dendrimers are redox active due to the presence of the ferrocenyl residue.
