Chemistry Reference
In-Depth Information
SCHEME 7.3
Binding of dendrimer
36
by BPP34C10 crown ether host.
in aqueous media. The binding interactions of the viologen open core dendrimers
16-18
with the cucurbit[8]uril host have allowed us to report interesting redox-based
methods to control the self-assembly of these dendrimers [27-29]. This body of work
has been the subject of other reviews [30,31], so we will not devote a lot of space to
these issues in this chapter. However, the well-known host-guest interactions
between dibenzocrown ether hosts and viologen guests in solvents of intermediate
polarity, such as acetonitrile, appeared as an interesting way to compare the
properties of viologen open core dendrimers based on Newkome (
16-18
) and
Frechet (
) dendrons. Therefore, we decided to determine the
corresponding equilibrium association constants (K) between these dendronized
viologens and bis-p-phenylene-24-crown-8 (BPP34C10, see structure in Scheme 7.3)
in acetone and acetonitrile solutions. The K values (Table 7.2) were determined by
recording the absorbance of the charge transfer band of the complex in titrations in
which the concentration of crown host was varied while maintaining the concen-
tration of dendrimer fixed. The measured absorbance values were fitted to a 1:1
binding isotherms using standard methods [25].
Several trends are clearly evident in the data. First, all complexes show higher K
values in acetone than in acetonitrile solution. This finding reflects the ion-dipole
nature of the main intermolecular forces holding the complexes together. The more
polar solvent, acetonitrile, solvates more effectively the positively charged nitrogen
atoms on the viologen and the oxygen atoms on the crown, decreasing the complex
stability. The stability of the BPP34C10 complexes with the open core Newkome
dendrimers (
31-33
and
34-36
) decreases with dendrimer growth. At the limit of this behavior, no
complex formation was observed between the third-generation dendrimer
13-15
and the
crown host in acetonitrile solution [25]. Afive-methylene tether connects the viologen
15
