TASK 4 Supramolecular Chemistry (Calixarene – A Versatile Host) BY Sri Mulyanti
Calixarene – A Versatile Host
Calixarenes were developed later
than crown ethers and cyclodextrins but have still been extensively researched.
Macrocycles of calix[n]arenes are constructed by linking a number of phenol
residues via methylene moieties
Fig. 1. Calix[n]arene
Like crown ethers, the name
“calixarene” reflects the structures of these molecules, since a calix is a
chalice. Calixarenes with various cavity sizes have been designed, each of
which has conformation isomers, and their phenolic hydroxyl groups are often
modified. These structural characteristics allow us to create calixarene
derivatives with various structural modifications.
The conformational isomers of a calixarene with four phenol residues are shown in Fig. 2
Fig 2.
The isomers vary in terms of the orientations of their phenol
groups:
(a) has a cone structure with all
of the phenols pointing to the same direction;
(b) has a
partial cone structure with one phenol pointing in a different direction to the
others;
(c) has a 1,3-alternate structure
with neighboring phenols pointing in opposite directions.
These isomeric hosts have
different selectivities for metal ion inclusion in the upper cavity and the lower
cavity. Of course, changing the number of phenol residues alters the guest size
appropriate for effective inclusion.
Fig. 3 Binding of fullerene by calix[8]arene
The calix[8]arene depicted in Fig. 2.18 can bind fullerenes (see Chap. 3); the fullerene “soccer ball” is trapped in the calix. Fullerenes are usually prepared as mixture of C60, C70, C76, and so on, and separating them is not always easy. The calix[8]arene has a cavity with an inner diameter of ∼1nm, which is therefore suitable for C60, since it has a diameter of ∼0.7nm. When the calixarene is added to a toluene solution of a mixture of fullerenes, a 1:1 complex of the calixarene and C60 selectively precipitates. Isolation of the precipitates followed by dispersion of them in chloroform results in the precipitation of dissociated C60. Repeating these processes results in C60 with high purity. Since the phenolic hydroxyl groups can be modified in various ways, we can design an array of functionalized hosts.
Fig. 4 Calixcrown
Figure 4 shows the
structure of calixcrown, in which two hydroxyl groups in calix[4]arene are
bridged by an oligoethylene glycol chain. The flexibility of the crown part is
highly restricted in this structure, resulting in highly selective molecular
recognition. The size of this binding site is quite close to the size of a
sodium ion. The binding affinity of the calixcrown to a sodium ion is 100 000
times greater than that observed for a potassium ion.
Another
interesting example involves a calixarene that exhibits a color change upon the
binding of a chiral guest. When converting the chiral recog- nition phenomenon
into a change of color, the design of the host molecule attaching to the
chromophore is critical. The host molecule shown in Fig. 5
Fig. 5. Chiral recognition by a dye-carrying calixarene
possesses
two dye moieties and a chiral binaphthyl group. When a guest molecule (phenyl
glycinol) is added to the host (dissolved in ethanol), the solution color
changes depending on the chirality of the guest. The original color of the
guest-free host is red; addition of R-phenyl glycinol changes the color to
blue-purple. In contrast, the solution color remains red upon the ad- dition of
S-phenyl glycinol. When R-phenyl glycinol is bound to the host, the left-hand
indophenol (dye A) in the host is deprotonated and the right-hand indophenols (dye
B) interacts more with the hydrophobic environment of the binaphthyl group.
These changes cause the complex to change color.
Sumber: Katsuhiko Ariga · Toyoki Kunitake Supramolecular Chemistry – Fundamentals and Applications
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