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Fig. 19 Chemical structure of receptors 46-48 and molecular models of their complexes calcu-
lated at the BH&H/6-31 G** level
the synthesis of gram quantities of [8]- and [10]cycloparaphenylenes, and the
crystal structure of the [10]CPP·C
60
complex [
150
].
We have reported extensively on the ability of the curved electron donor exTTF
to serve as a recognition motif for fullerenes. The geometric and electronic com-
plementarity between the concave aromatic face of exTTF and the convex surface
of the C
60
was exploited to build very simple tweezers-like receptors (46, in Fig.
19
)
which associate with C
60
with respectable binding constants in the order of log
K
a
¼
3-4, in several solvents at room temperature [
151
,
152
]. Thanks to their
synthetic accessibility we could access a collection of hosts in which we
could investigate the specific contribution of concave-convex interactions to
the molecular recognition of C
60
(46-48 in Fig.
19
)[
153
]. All three receptors
bear the same number of aromatic rings and are approximately equal in size, so
the contribution of
and dispersion interactions can be considered equiva-
lent. Hosts 46 and 47 both feature concave recognition motifs, and differ only in
their electronic character: while the exTTF moiety in 46 is an electron donor,
the TCAQ units in 47 are electron acceptors. Meanwhile, the anthraquinone
fragments in 48 have a similar electronic character to 47, but are completely
flat. Unsurprisingly, 46 shows the highest association constant, due to the
combination of electronic and geometric complementarity towards C
60
. Remark-
ably, there is a noticeable difference between the binding affinities of 47 and 48
that can be attributed to concave-convex interactions. As we noted in our
original communication, whether concave-convex interactions should be treated
as a new kind of intermolecular force or just a particular case of preorganization
is not straightforward. In any case, it is worth considering them for the few
curved guests of interest, mainly fullerenes and carbon nanotubes [
154
].
We have also investigated the binding abilities of
ˀ
-extended derivatives of
tetrathiafulvalene with a truxene core (truxTTF), which feature up to three dithiole
units connected covalently to the conjugated aromatic core. To accommodate the
ˀˀ
