under vacuum prior to the insertion of neutral guests [50]. Neutral guests that have
been inserted are reported in Table 2.
Some molecules remain typically adsorbed at the outer surface of the ZL crystals
independent of the synthesis procedure. It is therefore often necessary to remove
them in order to obtain the desired product. Different procedures have been used
successfully, depending on the guest. Anions cannot enter the channels of ZL unless
they form ion pairs. This means that molecules at the outer surface can be destroyed
by using an oxidizing agent such has hypochlorite [26] (see, e.g., Fig. 1) or by
applying dithionite as a reducing agent [51]. The molecules inside of the channels
are not accessible to anionic species unless drastic conditions are applied. A common
method used to remove objects from the outer surface is washing the samples with a
solvent that is not or only slowly capable of entering the channels and passing the
guest. Examples are 1-butanol [50, 52] and DCM [16], but also washing solutions
containing an appropriate detergent can be used to remove the molecules on the
outer surface [14, 53].
A molecule that can enter a channel can also exit, if no measures are taken to
avoid this process, depending on the environmental conditions. An illustrative
example is the observation that the speed to substitute resorufin (ResH, Table 2)
located inside of the channels by solvent molecules depends on their size. The
following pattern was observed at r.t.: water >> methanol > ethanol > 1propanol ! 1-butanol. The reason is that water molecules can pass the ResH inside
of the narrow channels, while 1-propanol and especially 1-butanol cannot [52]. The
displacement of para-terphenyl (pTP, Table 2) by water was observed to be reversible. Exposure of pTP-ZL composites to laboratory air at rt. of about 20% relative
humidity causes the pTP to leave the channels and to adsorb at the outer surface of
the ZL crystals. Interestingly, the pTP slips back into the channels upon drying the
samples at about 200
C. This process can be repeated many times [54]. It is
interesting to compare this observation with the behavior of fluorenone. It was
observed that this molecule is not substituted by water despite of the fact that it is
smaller than pTP or similar molecules which are displaced [55]. The reason for this
behavior was not understood at the time of its discovery. The puzzle was solved by
performing extensive first-principles investigations on the fluorenone inside ZL,
both at dry conditions and in the presence of water. It turned out that the interaction
of the fluorenone carbonyl group with the ZL extra framework potassium cations is
responsible for the dye stabilization in the ZL nanochannels. The interaction with
potassium extra framework cations stabilizes the carbonylic dye fluorenone inside
the nanochannels, leading to a water-resistant dye-ZL composite. A selection of the
resulting structures calculated for dry conditions is shown in Fig. 4. The fluorenone
is strongly bound to one K
+ via the carbonyl oxygen in the minimum energy
structure seen in the bottom panels. Contact to two K
+ seen in the upper left panel
is only 2.1 kcal/mol less stable. On the contrary, the upper right panel represents a
configuration in which the carbonyl group of the dye does not interact with the
potassium cation. Such a configuration is much less stable than the minimum
structure. The comparison demonstrates that configurations in which the carbonyl
group is not in contact with K
+ are strongly energetically disfavored with respect to
Guests in Nanochannels of Zeolite L
11
been inserted are reported in Table 2.
Some molecules remain typically adsorbed at the outer surface of the ZL crystals
independent of the synthesis procedure. It is therefore often necessary to remove
them in order to obtain the desired product. Different procedures have been used
successfully, depending on the guest. Anions cannot enter the channels of ZL unless
they form ion pairs. This means that molecules at the outer surface can be destroyed
by using an oxidizing agent such has hypochlorite [26] (see, e.g., Fig. 1) or by
applying dithionite as a reducing agent [51]. The molecules inside of the channels
are not accessible to anionic species unless drastic conditions are applied. A common
method used to remove objects from the outer surface is washing the samples with a
solvent that is not or only slowly capable of entering the channels and passing the
guest. Examples are 1-butanol [50, 52] and DCM [16], but also washing solutions
containing an appropriate detergent can be used to remove the molecules on the
outer surface [14, 53].
A molecule that can enter a channel can also exit, if no measures are taken to
avoid this process, depending on the environmental conditions. An illustrative
example is the observation that the speed to substitute resorufin (ResH, Table 2)
located inside of the channels by solvent molecules depends on their size. The
following pattern was observed at r.t.: water >> methanol > ethanol > 1propanol ! 1-butanol. The reason is that water molecules can pass the ResH inside
of the narrow channels, while 1-propanol and especially 1-butanol cannot [52]. The
displacement of para-terphenyl (pTP, Table 2) by water was observed to be reversible. Exposure of pTP-ZL composites to laboratory air at rt. of about 20% relative
humidity causes the pTP to leave the channels and to adsorb at the outer surface of
the ZL crystals. Interestingly, the pTP slips back into the channels upon drying the
samples at about 200
C. This process can be repeated many times [54]. It is
interesting to compare this observation with the behavior of fluorenone. It was
observed that this molecule is not substituted by water despite of the fact that it is
smaller than pTP or similar molecules which are displaced [55]. The reason for this
behavior was not understood at the time of its discovery. The puzzle was solved by
performing extensive first-principles investigations on the fluorenone inside ZL,
both at dry conditions and in the presence of water. It turned out that the interaction
of the fluorenone carbonyl group with the ZL extra framework potassium cations is
responsible for the dye stabilization in the ZL nanochannels. The interaction with
potassium extra framework cations stabilizes the carbonylic dye fluorenone inside
the nanochannels, leading to a water-resistant dye-ZL composite. A selection of the
resulting structures calculated for dry conditions is shown in Fig. 4. The fluorenone
is strongly bound to one K
+ via the carbonyl oxygen in the minimum energy
structure seen in the bottom panels. Contact to two K
+ seen in the upper left panel
is only 2.1 kcal/mol less stable. On the contrary, the upper right panel represents a
configuration in which the carbonyl group of the dye does not interact with the
potassium cation. Such a configuration is much less stable than the minimum
structure. The comparison demonstrates that configurations in which the carbonyl
group is not in contact with K
+ are strongly energetically disfavored with respect to
Guests in Nanochannels of Zeolite L
11