Nanocolumnar
Si substrates (porous silicon (PSi)) have been functionalized
with a quinoxaline-bridged (EtQxBox) cavitand in which the quinoxaline
moieties are bonded to each other through four ethylendioxy bridges
at the upper rim of the cavity. The receptor, which is known to selectively
complex aromatic volatile organic compounds (VOCs) even in the presence
of aliphatic compounds, has been covalently anchored to PSi. The larger
surface area of PSi, compared to that of flat substrates, allowed
one to study the recognition process of the surface-grafted receptors
through different techniques: Fourier-transform infrared spectroscopy,
thermal desorption, and X-ray photoelectron spectroscopy. The experiments
proved that surface-grafted cavitands retain the recognition capability
toward aromatic VOCs. In addition, the affinities of EtQxBox for various
aromatic compounds (i.e., benzene, toluene, nitrobenzene, and
p
-nitrotoluene) have been studied combining density functional
theory computations and thermal desorption experiments. Computational
data based on the crystal structures of the complexes indicate that
this cavitand possesses a higher affinity toward aromatic nitro-compounds
compared to benzene and toluene, making this receptor of particular
interest for the detection of explosive taggants. The results of computational
studies have been validated also for the surface-grafted receptor
through competitive recognition experiments. These experiments showed
that EtQxBox-functionalized PSi can recognize nitrobenzene in the
presence of a significant excess of aromatic vapors such as benzene
(1:300) or toluene (1:100).
The complexation of L-lactic acid by two isomeric diphosphonate cavitands was studied, both in solution and in the solid state. 1 H and 31 P NMR experiments and X-ray diffraction analysis on single crystals evidenced the formation of a supramolecular host-guest adduct with both synthetic receptors, driven by a combination of H-bonding and C-H⋯π interactions.
A new inherently chiral cavitand is described, in which the desymmetrization of the rigid concave cavity is achieved by introducing three different bridging groups at the upper rim, namely two inward P=O groups, one inward P=S moiety and one methylene bridge in the AABC mode. The racemic mixture of cR and cS enantiomers is resolved by semi-preparative chiral HPLC and the enantiopurity confirmed through circular dichroism. The properties of the enantiopure cR-cav in the enantioselective recognition of racemic 2-butanol is studied by co-crystallization experiments. The exclusive formation of the complex R-BuOH@cR-cav demonstrates that cR-cav is able to discriminate between the two enantiomers of the alcohol in the solid state. The enantiospecific recognition exhibited by the cR-cav towards racemic 2-butanol is particularly relevant because of the low degree of chirality of the alcohol (1.9 calculated by CCM algorithm)
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