Chemistry Reference
In-Depth Information
Fig. 20 Chemical structure of the biscalix[5]arene host 49 [
157
], the “jaws” receptor 50 [
158
],
and X-ray crystal structure of the 50·C
60
complex
Fig. 21 Structure of the switchable receptor 51 and scheme showing its switching between the
“off” and “on” states and its binding to C
60
“on” state, the binding constant was estimated to be log
K
a
¼
3.7 in toluene/
CH
2
Cl
2
(50:1) at room temperature.
A distinctive advantage of the simplicity of the tweezers-like design is that it can
easily be adapted to construct more elaborate supramolecular assemblies at rela-
tively low synthetic cost. For instance, in our group, based on the exTTF tweezers
46, we have built both linear [
162
] and hyperbranched [
163
] supramolecular
polymers [
164
], covalent dendrimers capable of associating several units of C
60
[
165
], and have extended the design to associate with and solubilize carbon
nanotubes in aqueous solution.
On the negative side, tweezers are not well preorganized unless very rigid spacers
are used to link the two recognizing units. This results in relatively modest associa-
tion constants, in the case of fullerenes typically in the order of 10
3
-10
4
M
1
.To
obtain complexes of higher stability, a tried and tested strategy is to move from
tweezers to macrocycles. Macrocycles are much better preorganized, and have
better expressed binding sites, increasing the binding constants significantly.
In return, they are more challenging synthetic targets, and the cavity of the host
needs to be fine-tuned to match the size of the guest, or the preorganization will
