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dithioles, the truxene moiety breaks down its planar structure and adopts an all-
cis
sphere-like geometry with the three dithiole rings protruding. The concave shape
adopted by the truxene core perfectly mirrors the convex surface of fullerenes,
indicating that van der Waals and concave-convex
interactions between them
should be maximized. Indeed, the association of trux-TTF and fullerenes in solution
was investigated by
1
H NMR titrations with C
60
and C
70
as guests affording binding
constants of log
K
a
¼
ˀˀ
3.1 and 3.9 for C
60
and C
70
in CDCl
3
/CS
2
, respectively
[
155
].
4.3 Tweezers and Macrocycles for the Molecular Recognition
of Fullerenes
As we have seen in some of the examples described above, tweezer-like hosts have
been a particularly popular design for the construction of receptors for fullerenes.
This is so because tweezers are usually synthetically accessible, since it requires
only connecting two recognizing units symmetrically via a spacer.
One of the earliest examples of molecular tweezers for C
60
was reported by the
group of Fukazawa in 1998 [
156
]. They connected two units of calix[5]arene
through a variety of rigid spacers, and obtained the best results for host 49, which
showed a binding constant of log
K
a
¼
4.9 in toluene at room temperature, a world
record in complex stability at the time.
The positive interaction between porphyrins and fullerenes has often been
exploited to construct this kind of receptor [
157
]. The first example of such
porphyrin tweezers for fullerenes was reported by Boyd, Reed and co-workers
over a decade ago [
158
]. They connected two porphyrin units appended with
pyridine ligands through coordination of palladium. The structure of the tweezers
50 is shown in Fig.
20
. The binding constant of this receptor towards C
60
was
estimated to be log
K
a
¼
3.7 in toluene-
d
8
at room temperature.
After this first example the same authors reported very similar receptors, in
which the coordination link between the porphyrin units was substituted with
covalent bonding through the amides of either isophthalic or terephthalic acid
[
159
]. This resulted in a decrease in the association constant. Later, in collaboration
with the group of Armaroli, they replaced the benzenedicarboxamide spacers with
several calixarenes, reaching binding constants as high as log
K
a
¼
5.4 and 6.4 for
C
60
and C
70
respectively, both in toluene at room temperature [
160
].
A more sophisticated example of molecular tweezers for C
60
(51 in Fig.
21
),
with a mechanism to turn “on” and “off” their ability to bind the fullerene guest,
was reported by Shinkai and co-workers [
161
]. As synthesized, each of the
appending pyridines coordinates to the porphyrin metal, keeping the two porphyrins
on opposite sides and preventing association of C
60
. When an external palladium
center is added, the conformation changes to bring both porphyrins to the same side
of the molecule, allowing for association with fullerene, as shown in Fig.
21
. In the
