Organic covalent frame rotational isomers and their special gas adsorption behavior

On April 10, 2023, the team of Professor Dan Zhao of the National University of Singapore, the team of Professor Huang Zhehao of Stockholm University in Sweden, and the team of Professor Ryotaro Matsuda of Nagoya University in Japan published an article entitled “Covalent Organic Framework Atropisomers with Multiple Gas-Triggered Structural” in the journal Nature Materials Flexibilities”.

This achievement reports the rotational isomerization phenomenon in organic covalent frameworks, expands the scope of flexible porous crystal materials based on covalent bond connection, and further enriches the application of such materials in the fields of gas adsorption, separation and storage. The corresponding authors of the paper are Zhao Dan, Huang Zhehao, Matsuda Ryotaro; The tied first authors are Kang Chengjun and Zhang Zhaoqiang.

Soft/flexible porous crystals (SPCs) are a special class of porous materials with guest response or adaptability, which combine high crystallinity and structural variability. Unlike rigid porous materials, guest molecules can change the pore structure and interaction behavior of SPCs, giving SPCs significant advantages in gas adsorption, separation and storage. Mesh chemistry has led to the development of new porous crystalline materials in metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). At present, most of the reported SPCs are MOFs, mainly because the relatively weak coordination bonds in MOFs favor reversible bond breaking and bending, thereby giving MOFs crystallinity and flexibility. However, the stability of flexible MOFs linked to coordination bonds is poor, which limits their wide application. In contrast, covalent bonds confer greater stability on COFs, but the crystal structures they connect to are usually rigid. Therefore, it will be of great significance to develop a porous material with high stability, high crystallinity and structural flexibility that can have covalent connections at the same time.

Conformational isomerism is a type of stereoisomerism, and conformations can be transformed into each other by single-key rotation. As a special class of conformational isomers, rotational resistance isomers refer to conformational isomers with single-bond rotation obstruction. First reported in 1922, spin-resistant isomers are commonly found in molecules with large steric hindrance and play an important role in biological systems and drug discovery. However, in infinitely malleable covalent framework crystal structures, rotational isomers are still rarely reported.

Professor Dan Zhao’s team at the National University of Singapore found that by changing the growth conditions of three-dimensional (3D) COF, such as temperature, the same group of monomers can grow different crystal structures. One crystal has a pore size of 9.7 Å and the other crystal has a pore size of 3.7 Å, which is also the minimum pore size that can be achieved by COF materials. The team, together with the team of Professor Zhehao Huang of Stockholm University in Sweden, analyzed the single crystal structure through three-dimensional electron diffraction, and found that the two crystals have the same chemical and interspersed structure, but the connection structure between the adjacent three-dimensional repeating units has spatial conformational differences. And due to the spatial limitations of unit cells, this conformational difference cannot be transformed into each other by single bond rotation, resulting in the two crystals produced cannot be converted into each other. The authors name these crystals with the same chemical and interspersed structure, but different crystal structures and cannot be converted into each other, as the rotational isomers of COF crystals. This is also the first time that rotational isomerism has been found in a covalent crystal structure with infinite extension. Importantly, the two rotational isomers have very different gas adsorption behaviors, with one isomer (pore size 9.7 Å) being rigid and the other (pore size 3.7 Å) having special gas adsorption flexibility behavior. The team, together with the team of Professor Ryotaro Matsuda of Nagoya University in Japan, found that the newly synthesized COF rotational isomer had a variety of crystal structure conversion modes by using in situ gas adsorption-powder crystal X-ray diffraction characterization: when adsorbing ethylene, the COF rotational isomer showed discontinuous crystal structure transformation; When adsorbing carbon dioxide, it shows continuous crystal structure changes; However, when adsorbing acetylene, continuous and discontinuous crystal structure changes exist in the same system.

Figure 1: Synthesis of 3D COF spin-resisting isomers and single crystal structure analysis

Figure 2: Ethylene adsorption behavior of 3D COF rotaphrating isomers

Figure 3: CO2 adsorption behavior of a 3D COF spin-blocking isomer

Figure 4: Acetylene adsorption behavior of 3D COF rotary isomers

Figure 5: Theoretical calculation of the interaction of 3D COF spin-resisting isomers with guest molecules

This series of studies has expanded the rotational isomer from molecular structure to infinitely extended covalent crystal structure, and advanced COFs from the main rigid structure to the field of covalently linked SPCs, further promoting the application of SPCs in gas adsorption, separation and storage. (Source: Science Network)

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