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cavities were occupied, and (c) the aromatic guests with electron-
withdrawing substituents diminished the affinity, whereas aromatic
guests with electron-donating substituents enhanced the affinity.
A similar study from the same group also used the thin film
of molecular squares
for VOC sensing [99]. The films
exhibited preference for aromatic guests (benzene) over aliphatic
substrates (cyclohexane) and for good electron donors over poor
ones. Size selectivity of the films was demonstrated with the uptake
of cyclic ethers of various sizes, such as dioxane (size
3a
and
3b
4.5 Å) and the
large 18-crown-6 cyclic ether (size
7.5 Å). This study showed that
the amount of 18-crown-6 uptake was 1
2 orders of magnitude less
than the amount of dioxane uptake, depending on the film thickness.
The quantitative assessments of VOC uptake were achieved via QCM
for thin-film arrays. The VOC uptake process is reversible and can be
regenerated by placing the loaded films under vacuum at ambient
temperature for 10 min.
10.9
Molecular Recognition From Three-
Dimensional Metallacyclic Cages
Current understanding of molecular recognition in biological systems
is based on the “induced-fit” mechanism, in which the substrate
induces the organization of the recognition site of the receptor,
rather than by a lock and key model [101,102]. Most examples of
induced-fit models use flexible artificial hosts, which show restricted
conformation only if they organize a specific guest [103,104]. One of
the few examples of induced-fit models in which the guest molecule
induces the organization of the host itself is provided by Fujita and
coworkers' metallacyclic supramolecular cage
56
[101]. They found
that the host (cage
) binds 4-methoxyphenylacetate as a guest in
1:1 stoichiometry in aqueous solution. Analysis of the
56
1
H NMR data
not only confirmed the presence of the guest molecule within the
cavity but also confirmed the
symmetry necessary for a trigonal
prism. Interestingly, in the absence of the specific guest, the self-
assembly resulted in the formation of a considerable amount of
oligomeric product; this disappeared on the addition of the guest
leading to the formation of the cage-like complex.
D
3h
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