Chemistry Reference
In-Depth Information
TABLE 7.2 Equilibrium Association Constants (M
1
) for Complex
Formation between the Host BPP34C10 and Viologen-Containing
Open Core Dendrimers at 25
C
K (in acetone)
a
K (in acetonitrile)
a
Guest
Methyl viologen
560
200
13
370
93
14
210
85
NB
b
15
92
31
480
100
32
630
100
33
670
85
34
420
70
35
560
100
36
740
75
a
Error margin no larger than
12%.
b
No binding detected.
13-15
group to the focal point of the Newkome dendrons in
. No such tether exists in
the Frechet dendrimers
. Despite the closer proximity of the viologen group to
the dendrons in this case and in marked contrast to the behavior observed with
13-15
,
the K values measured with the two series of Fr
31-36
echet dendrimers do not show any
decreasing trend as the dendron grows. If anything, dendron growth results in small
gains in complex stability with these two series of macromolecules. No significant
differences in the binding affinities were observed between the two series of Fr
echet
dendrimers, which differ in the structure of the substituents connected to the viologen
residue on the opposite side to the dendron.
The structure of the complex between BPP34C10 and methyl viologen is well
known [32,33]. Briefly, the crown's oxygen atoms interact with the two positive
charges on the viologen while the two
-donor hydroquinol units on the crown
develop charge-transfer interactions with the
p
-acceptor bipyridinium residue.
Overall the crown encircles the viologen groups and the stability of the resulting
complex is rather modest, which facilitates the interference on the complexation
process by steric hindrance from the growing dendritic mass. Clearly, this is the case
with the Newkome dendrons, but Fr
p
echet dendrons fail to bring about similar effects.
The more rigid character of the polyaryl ether (Fr
echet) dendrons, built around the
1,3,5-substitution pattern on the phenyl ring, may lead to wedge-like dendron shapes,
which afford less effective encapsulation of the viologen residues as compared to the
more flexible Newkome dendrons. This interpretation is also supported by the very
small dendron size effects on the half-wave potentials for viologen reduction that were
recorded with dendrimer series
[25].
We conclude that the investigation of these host-guest binding interactions leads
to useful insights into the space filling and encapsulating properties of these two
very different, albeit popular, types of dendron that are often used in dendrimer
design.
31-33
and
34-36
