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The data show clearly that the pore size has a significant impact
on the enantioselectivity. The enantioselectivity does not follow
such a simple trend as the adsorption capacity, which increases
with increasing pore size. The highest enantiomeric excess was
found for a pore size of 0.8 nm, which was found to be the medium
case. It can be assumed that the differences of the loadings are
caused by the different alignments of the chiral guest molecules
adsorbed in the pores, where the stereogenic centers have a
different impact on the enantiomer selectivity. As visualization,
Fig. 4 shows a sketch of (S)-limonene in the isoreticular homochiral
MOFs. This can be interpreted in the following way: If the pore size
is ‘‘too’’ small (a), the guest molecules are ‘‘forced’’ to adsorb in the
pores in such a position, where the impact of the stereogenic center
in the framework is small. If the pore size is ‘‘too’’ large (c), the
molecules can adsorb all over the large pore and the impact of the
stereogenic center is small, too. If the pore size is well adjusted,
roughly as large as the guest molecule (b), the stereogenic center
has the highest impact on the guest molecule, resulting in the
highest enantiomer separation.
In conclusion, the enantioselectivity of isoreticular chiral
MOFs with identical stereogenic centers and different pore
sizes was investigated. The enantioselective uptake of the chiral
probe molecules, (R)- and (S)-limonene, by thin MOF films of type
Cu2(Dcam)2(dabco), Cu2(Dcam)2(BiPy) and Cu2(Dcam)2(BiPyB) was
measured by using a QCM. It was found that the adsorption capacity
increases with increasing pore size. A more complex situation
was found for the enantiomer selectivity, where the highest
enantiomeric excess is in SURMOFs with medium pore size,
while the enantiomeric excess for very small and large pores is
significantly smaller. This study demonstrates that not only the
stereogenic center, but also the pore size have to be adjusted for
gaining highest enantioselectivities in chiral nanoporous materials
and thereupon enabling a significantly more efficient enantiomer
separation.
Fig. 3 (a) (S)- and (R)-limonene uptakes by the series of isoreticular
homochiral SURMOFs, Cu2(Dcam)2(L) with L = dabco, BiPy or BiPyB,
relative to the SURMOF mass. (b) The (theoretical) enantiomeric excess
in the isoreticular chiral MOF depends strongly on the pore size.
there is no interaction between the different enantiomers, Fig. 3b.
This means it corresponds to the enantiomeric excess at very low
concentrations. It was found that the (theoretical) enantiomeric
excess of (S)-limonene versus (R)-limonene changes significantly
for the different MOF structures; namely approximately 8% for
Cu2(Dcam)2(BiPyB), 17% for Cu2(Dcam)2(dabco) and 35% for
Cu2(Dcam)2(BiPy).
The reliability of the data is checked by carrying out the
experiments with SURMOFs of type Cu2(Lcam)2(BiPy), the
enantiomeric mirror image of Cu2(Dcam)2(BiPy). The QCM data
(Fig. SI5, ESI†) show that the (theoretical) enantiomeric excess
of (R)-limonene versus (S)-limonene is 34%, which is in perfect
agreement with the data determined for Cu2(Dcam)2(BiPy).
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