240
H. Ohtsu et al.
This is consistent with the fact that the largest fraction of the gaseous sulfur is in its
dimer state [65]. Therefore, S 3 encapsulation in the saddle [(ZnI 2 ) 3 (TPT) 2 ] network
likely occurred via a ship-in-a-bottle cluster assembly. First, S 2 (disulfur) molecules
enter the pores of the network and then are converted to S 3 (trisulfur) due to its higher
stability. Since the encapsulation is conducted under the equilibrium conditions, it
is difficult to determine the exact mechanism of disulfur molecule conversion. To
observe these processes, the guest molecules should be kinetically trapped before
the equilibrium state is reached. To achieve this, iodide sites could act as suitable
interactive sites to capture small sulfur species and prevent their aggregation. To
explore this possibility, we employed of kinetically assembled helical CuI network
with interactive pores, as discussed in Sect. 12.1.7.
12.1.9 X-Ray Snapshots of the S 2 Conversion Inside
an Interactive Pore
As mentioned in Sect. 12.1.4, pores lined up with interactive iodide sites could
serve as efficient traps to stabilize and visualize small sulfur species. Furthermore,
in Sect. 12.1.4, we also described sulfur encapsulation into the saddle ZnI network
performed under equilibrium conditions. Elemental sulfur and the network powder
were kept at 533 K under vacuum for 6 h to encapsulate sulfur gas. Because of
the equilibrium conditions, it was difficult to determine the conversion mechanism
before the sulfur species reached their equilibrium state inside the network pores. To
overcome this limitation, kinetic trapping of small metastable sulfur allotropes was
used to observe their conversion in the pore.
The CuI helical network was used for the trapping experiments due to the presence
of interactive iodide sites where the sulfur transformation could occur. Unlike earlier
encapsulation experiments, the sulfur trapping into the helical structure was carried
out under kinetic conditions. Elemental sulfur and the desolvated network powder
were placed at well-separated locations in a zig-zag glass tube, followed by sealing
it under vacuum (~10
–6 Torr). The sulfur side of the tube was heated to generate the
sulfur vapor, while the network side was kept at the room temperature creating a sharp
temperature gradient. This arrangement enabled the gaseous sulfur molecules to be
trapped kinetically by the porous material. After the reaction was complete, singlecrystal X-ray diffraction was performed on the resultant sample sequentially at 250,
300, 350, and again 250 K. The analysis of these diffraction patterns revealed the
structure of transient small sulfur allotropes. The initial structure at 250 K showed
the presence of two types of physisorbed guests in the network channels, S 2 and
bent-S 3 species. Upon increasing the temperature to 300 K, the crystal structure
analysis revealed the formation of cyclo-S 3 chemisorbed to the bridging iodide sites
in addition to bent-S 3 and cyclo-S 3 , both physisorbed in the channels. A theoretical
investigation showed that the stronger interacting cyclo-S 3 species should be in its
dicationic state, cyclo-S 3
2+ . Further heating to 350 K resulted in the opening of the
H. Ohtsu et al.
This is consistent with the fact that the largest fraction of the gaseous sulfur is in its
dimer state [65]. Therefore, S 3 encapsulation in the saddle [(ZnI 2 ) 3 (TPT) 2 ] network
likely occurred via a ship-in-a-bottle cluster assembly. First, S 2 (disulfur) molecules
enter the pores of the network and then are converted to S 3 (trisulfur) due to its higher
stability. Since the encapsulation is conducted under the equilibrium conditions, it
is difficult to determine the exact mechanism of disulfur molecule conversion. To
observe these processes, the guest molecules should be kinetically trapped before
the equilibrium state is reached. To achieve this, iodide sites could act as suitable
interactive sites to capture small sulfur species and prevent their aggregation. To
explore this possibility, we employed of kinetically assembled helical CuI network
with interactive pores, as discussed in Sect. 12.1.7.
12.1.9 X-Ray Snapshots of the S 2 Conversion Inside
an Interactive Pore
As mentioned in Sect. 12.1.4, pores lined up with interactive iodide sites could
serve as efficient traps to stabilize and visualize small sulfur species. Furthermore,
in Sect. 12.1.4, we also described sulfur encapsulation into the saddle ZnI network
performed under equilibrium conditions. Elemental sulfur and the network powder
were kept at 533 K under vacuum for 6 h to encapsulate sulfur gas. Because of
the equilibrium conditions, it was difficult to determine the conversion mechanism
before the sulfur species reached their equilibrium state inside the network pores. To
overcome this limitation, kinetic trapping of small metastable sulfur allotropes was
used to observe their conversion in the pore.
The CuI helical network was used for the trapping experiments due to the presence
of interactive iodide sites where the sulfur transformation could occur. Unlike earlier
encapsulation experiments, the sulfur trapping into the helical structure was carried
out under kinetic conditions. Elemental sulfur and the desolvated network powder
were placed at well-separated locations in a zig-zag glass tube, followed by sealing
it under vacuum (~10
–6 Torr). The sulfur side of the tube was heated to generate the
sulfur vapor, while the network side was kept at the room temperature creating a sharp
temperature gradient. This arrangement enabled the gaseous sulfur molecules to be
trapped kinetically by the porous material. After the reaction was complete, singlecrystal X-ray diffraction was performed on the resultant sample sequentially at 250,
300, 350, and again 250 K. The analysis of these diffraction patterns revealed the
structure of transient small sulfur allotropes. The initial structure at 250 K showed
the presence of two types of physisorbed guests in the network channels, S 2 and
bent-S 3 species. Upon increasing the temperature to 300 K, the crystal structure
analysis revealed the formation of cyclo-S 3 chemisorbed to the bridging iodide sites
in addition to bent-S 3 and cyclo-S 3 , both physisorbed in the channels. A theoretical
investigation showed that the stronger interacting cyclo-S 3 species should be in its
dicationic state, cyclo-S 3
2+ . Further heating to 350 K resulted in the opening of the