| Host-guest science is a branch of supramolecular science. It describes the formation of supramolecular complexes via noncovalent interactions between host and guest moieties. The process that host and guest connected together is assembly. Building blocks in assembly are molecules, which determine that assembly is more complex, but more convenient than synthesis to form relatively huge architectures. Pressure is one of the most important thermodynamic parameters. It is able to decrease the distances between building blocks, tuning the weak noncovalent host-guest interactions therein, so as to change the assembly and generate some new structures and properties. This article combines the behaviors of host-guest interactions(Raman) and the whole structural evolution(in situ angle synchrotron x-ray diffraction, ADXRD) as a function of pressure, inferring the assembly changes between host and guest, as well as offering some new strategies for high pressure host-guest assembly study.Porous coordination polymer(PCP) is porous material with highly ordered and organized networks, consisting of inorganic metal ions/clusters and organic ligands, which are connected by the coordinate bonds and noncovalent interactions. PCP contains great value of promising applications in molecular recognition, storage, separation, ions exchange, optics, electricity, magnetism, and chemical catalysis/sensors. High pressure investigation of structure and host-guest assembly in PCP, not only studies the pressure-induced tuning of host-guest assembly, but also explores how the host-guest assembly affects the whole structure. In this study, we choose several typical PCP materials as models to explore their structural behaviors and host-guest assemblies at high pressure. The guest molecules here include both the “outer” gust molecules(such as small organic molecules and gas molecules), and the “inner” guest molecules, which have been already connected to the host framework within the PCP structures.Firstly, high pressure studies of Hofmann clathrate Ni(NH3)2Ni(CN)4·2C6H6(Ni-Bz) reveal that pressure is able to separate the host framework and “inner” guest molecules. That is, the pressure-induced disassembly. High pressure Raman experiments show that between 1.0 and 4.0 GPa, the guest benzene molecules in Ni-Bz gradually leave the host framework, accompanied by the distortion of benzene and enhancement of hydrogen bonding networks. As soon as all the guests have left the host, without the supporting of guest molecules, the layered structure of host is destroyed, and accompanied with the phase transition. Furthermore, when no “inner” guest molecules are applied in the host framework, the empty space between the layered structure, Ni(CN)4Ni(NH3)2·0.5H2O(Ni-Ni), is compressed rapidly and NH3 groups at the window of the pores are distorted. These two points jointly lead to the steric hindrance within the framework. Therefore, the “outer” guest molecules cannot insert into the pores of the host. It implies that in this Hofmann-type system, contrary to the pressure-induced disassembly, pressure cannot create the new assembly between the hosts and “outer” guest molecules. In this situation, “outer” guest molecules only function as the pressure transmitting medium(PTM) to tune the pressure condition. The combined high pressure studies of Ni-Bz and Ni-Ni shows that pressure is an effective method to induce the disassembly between host and guest. And steric hindrance plays very important roles in its opposite process, pressure-assisted assembly.Secondly, by choosing proper host and guest, we ensure that pressure is indeed an effective method to create the new assembly relationship between host PCP framework and “outer” guest molecules, the so called pressure-assisted assembly. High pressure studies of typical PCP material, Prussian blue(PB), show that PB undergoes two phase transitions with increasing pressure, and the “outer” guest molecules are squeezed into the host framework during the compression. At ~3.0 GPa, the first phase transition of PB is observed, accompanied by the distortion of the three-dimensional framework and the symmetry change from Pm-3m to P21212. And the “outer” guest molecules cannot influence this transition. With increasing pressure, PB undergoes the second phase transition that the framework is further distorted slightly. The symmetry of the structure is not changed through this transition process, and the whole structure becomes amorphous gradually with further compression. Significantly, with the existence of “outer” alcohol guest molecules, the critical pressure of the second phase transition increases from ~7.9 GPa to ~12.7 GPa. We infer it is the insertion of the “outer” guest molecules that prevent the structure from collapse, so as to delay the beginning of the second phase transition. Pressure changes the assembly relationship successfully, forming the new assembly between host framework of PB and the “outer” guest of small alcohol molecules. Such new host-guest systems are more stable and less compressible than the host system only.In addition, for some host-guest systems, their assembly relationships cannot be changed by pressure. However, external force is still able to tune the connected methods between host and guest. The mixed phases material, [NH3-(CH2)4-NH3]Cu Cl4(2C4Cu Cl4), undergoes two phase transitions at high pressure. Below 0.7 GPa, the conformation changes of the organic chains and rearrangement of hydrogen bonding networks lead to the first phase transition. The guest organic chains act as spring-layers to protect the host inorganic parts. And in higher pressure range, the destruction of the layered structure, as well as the distortion of the hydrogen bonding networks and the coordinated inorganic parts, together result into the second phase transition and the piezochromism of the material. Furthermore, in the system of {[Cu(CO3)2](CH6N3)2}n(GCC), the external force strengthens the hydrogen bonding between host and guest. Then the guest molecules, which locate at the window of the pores in the host framework, are distorted gradually and inserted into the empty space inside the pores, leading to the expansion of the whole structure. Meanwhile, without the supporting of guest molecules at the window, the structure becomes disordered simultaneously. In these systems, the host and guest have been connected within the structures. It is proven that pressure can control the connection method between the two building blocks, so as to influence the behaviors of the whole host-guest system.Finally, when the structure of PCP is stable enough, both the assembly relationship and assembly method cannot be changed at high pressure, PCP can still offer some unique structural behaviors. System of [Cu(4,4’-bpy)2(H2O)2]·Si F6(Cu(bpy)·Si F) shows the surprising negative linear compressibility(NLC) at high pressure, involving with the wine-rack mechanism, quasi-square grids in 2D networks transfer to rhombic ones with the expansion of the diagonals along c axis. And the NLC transfers to the nearly zero linear compressibility(ZLC) during further compression. The joint analysis of experiments and first-principle calculations demonstrates that the assembly way between guest and host is not changed, and guest molecules only function to support the host framework at high pressure. Furthermore, expansion degree and the critical pressure of transition are significantly modulated by the pressure condition. We infer that the supporting function of guest molecules prevents the amorphism of the structure at relatively high pressure. And this point is of pivotal importance for PCP to generate some unique structural behaviors during compression. |
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