3.2.2 Guests That Can Pass Each Other
Small molecules such as H 2 , O 2 , N 2 , Cl 2 , H 2 O, CH 3 O, toluene, and others can often
pass bulkier guest inside of the channels and can thus modify properties of the
composites in many ways. This is, e.g., important for using ZL in catalysis [81]. An
interesting example that explains the importance of controlling the co-guests is the
behavior of ResH (Table 2). This molecule is strongly fluorescent in solution in its
anionic form Res
À
. The fluorescence was observed to be completely quenched when
the molecule was inserted into the channels of ZL using a procedure that results in a
fully hydrated composite [52]. The properties of Res-ZL composites change dramatically, however, if prepared under different conditions so that completely
dehydrated Res-ZL results, which is observed to be strongly luminescent
[82]. Scheme (b) in Fig. 5 illustrates such situations. Different properties result if a
small co-guest must first be positioned in the well of the channel in a way that larger
guest inserted afterward can slip past them. Once both guests, the smaller and the
bulkier one, are inside, the smaller guest remains trapped in his trench, as exemplified in Scheme (c) of Fig. 5. An example of such a situation is IMZ
+ (Table 1) which
has frequently been used in order to substitute some of the 3.6 per u.c. charge
compensation K
+ located in the channels [16, 83].
4 Functionalization: Stopcocks, Coat, Polymer Brushes
Hydroxy groups are distributed all over the external surface of ZL crystals. This
provides an opportunity for functionalization by means of covalent bonds. The base
and coat hydroxy groups, furthermore, show distinctly different reactivity. This can
be understood by comparing them as illustrated in Fig. 6. The figure explains that the
channel entrances are exclusively located at the base. The red lines point to the base
of the SEM image of a ZL crystal and connect it with the drawing on the left showing
the hexagonally arranged channel entrances and the sketch which shows how the
channel entrances can be functionalized selectively with stopcocks. The scheme
illustrates a situation where the channels are filled with two different guests, arranged
in a sandwich structure. The yellow line connects the coat of a ZL crystal, seen in the
SEM image, with the coat in the scheme which is functionalized by binding the
yellow objects to the OH groups located at the surface [14, 15, 20].
4.1 Stopcocks
The stopcock principle was invented for selective modification of the ZL channel
entrances [84, 85]. A simple scheme of the stopcock principle is illustrated in Fig. 7a.
The head of these molecules is too large to enter the channels. Since stopcock
Guests in Nanochannels of Zeolite L
15
Small molecules such as H 2 , O 2 , N 2 , Cl 2 , H 2 O, CH 3 O, toluene, and others can often
pass bulkier guest inside of the channels and can thus modify properties of the
composites in many ways. This is, e.g., important for using ZL in catalysis [81]. An
interesting example that explains the importance of controlling the co-guests is the
behavior of ResH (Table 2). This molecule is strongly fluorescent in solution in its
anionic form Res
À
. The fluorescence was observed to be completely quenched when
the molecule was inserted into the channels of ZL using a procedure that results in a
fully hydrated composite [52]. The properties of Res-ZL composites change dramatically, however, if prepared under different conditions so that completely
dehydrated Res-ZL results, which is observed to be strongly luminescent
[82]. Scheme (b) in Fig. 5 illustrates such situations. Different properties result if a
small co-guest must first be positioned in the well of the channel in a way that larger
guest inserted afterward can slip past them. Once both guests, the smaller and the
bulkier one, are inside, the smaller guest remains trapped in his trench, as exemplified in Scheme (c) of Fig. 5. An example of such a situation is IMZ
+ (Table 1) which
has frequently been used in order to substitute some of the 3.6 per u.c. charge
compensation K
+ located in the channels [16, 83].
4 Functionalization: Stopcocks, Coat, Polymer Brushes
Hydroxy groups are distributed all over the external surface of ZL crystals. This
provides an opportunity for functionalization by means of covalent bonds. The base
and coat hydroxy groups, furthermore, show distinctly different reactivity. This can
be understood by comparing them as illustrated in Fig. 6. The figure explains that the
channel entrances are exclusively located at the base. The red lines point to the base
of the SEM image of a ZL crystal and connect it with the drawing on the left showing
the hexagonally arranged channel entrances and the sketch which shows how the
channel entrances can be functionalized selectively with stopcocks. The scheme
illustrates a situation where the channels are filled with two different guests, arranged
in a sandwich structure. The yellow line connects the coat of a ZL crystal, seen in the
SEM image, with the coat in the scheme which is functionalized by binding the
yellow objects to the OH groups located at the surface [14, 15, 20].
4.1 Stopcocks
The stopcock principle was invented for selective modification of the ZL channel
entrances [84, 85]. A simple scheme of the stopcock principle is illustrated in Fig. 7a.
The head of these molecules is too large to enter the channels. Since stopcock
Guests in Nanochannels of Zeolite L
15