Amorphous conjugated polymer networks

More amorphous, more disordered, more flexible, ...

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Our group has published ‘Amorphous 2D materials containing a conjugated-polymer network’ in Communications Chemistry 2019, 2, 97 .1 This paper reported on amorphous two-dimensional (2D) materials based on conjugated-polymer network synthesized by random copolymerization of the two monomers, such as benzoquinone (BQ) and pyrrole (Py) (Figure 1). The random stacking of the amorphous polymer networks forms the low-crystalline layered structures leading to the exfoliation into the nanosheets. The resultant BQ-Py nanosheets showed the high catalytic performance for hydrogen evolution reaction. 

Figure 1.   Schematic illustrations of the BQ-Py amorphous covalent organic networks and their exfoliation into the nanosheets.

    As described in the Behind the Paper, this material was a serendipity for me. No collaborators aimed to synthesize such materials. This new material was fortunately found by a reference experiment of the other project. Nevertheless, the results have been fundamental and suggestive for us to design new concepts and materials.

    In recent years, metal-organic and covalent-organic frameworks (MOFs and COFs) have attracted much interest as a new family of self-organized materials containing regularly arranged functional molecules and pores. My collaborators might be interested in such beautiful, defined, and well-ordered architectures. Nevertheless, unfortunately, our materials had the ‘disordered’ networks and their ‘random’ stacking. They might feel why our materials are not beautiful, organized, and … at the bottom of their mind. After the paper, I have realized that the disordered and random natures are the structure advantages of our material. We have focused on the ‘amorphous covalent organic networks’ distinguished from the crystalline architectures, such as main- and side-chain polymers, MOFs and COFs (Figure 2).2

Figure 2.   Schematic illustrations of the assembled architectures containing functional molecules (orange spheres). (a) Side-chain polymers. (b) Main-chain polymers. (c) Framework structures, such as MOFs and COFs. (d) Our proposed amorphous covalent organic networks. Ref (2) published by The Royal Society of Chemistry.

   If the multiple reactions simultaneously proceed at different rates and in different growth directions, the random copolymerized networks can be obtained. The functional molecules can be dispersed in the network structures without formation of the crystalline aggregates (Figure 3). Moreover, the low-crystalline assembly of the network polymers facilitates the exfoliation into the nanostructures, such as particles, flakes, and sheets. Therefore, the functional molecules as the units can be efficiently used compared with the rigid crystalline assemblies. The amorphous polymer networks are regarded as a new assembly state of functional molecules (Figure 3). After the paper, we designed three types of the similar amorphous covalent organic networks containing BQ moieties. The resultant nanoflakes based on the amorphous covalent organic network exhibited the high-specific capacity originating from the redox reactions of the BQ moieties. The specific capacity is one of the highest performances in the previous works about aqueous supercapacitor. These new concepts were introduced in our recent paper.2 

 

Figure 3. Molecular design of amorphous covalent organic networks containing redox-active BQ moieties, such as BQ–DAT (a), BQ–DVB (b), and BQ–HQ (c) polymers. Ref (2) published by The Royal Society of Chemistry.

   After the paper, two collaborators have supported us to further advances. Dr. Kosuke Sato at Sagami Chemical Research Center, an alumnus of my research group, has continued to support this research project. Dr. Chi-How Peng, a specialist of polymer synthesis, and his coworkers in National Tsing Hua University have helped us to characterize the molecular structures. I appreciate their kind supports and contributions. In my group, the paper in Communications Chemistry is highly motivating for further development of amorphous covalent organic networks containing desired functional molecules. Now, we enjoy design and syntheses of  more amorphous and disordered states for development of advanced functional materials.

References

(1) S. Yano, K. Sato, J. Suzuki, H. Imai, *Y. Oaki, “Amorphous 2D materials containing a conjugated polymer network”, Communications Chemistry 2019, 2, 97

(2) J. Suzuki, A. Ishizone, K. Sato, H. Imai, Y.-J. Tseng, C.-H. Peng, *Y. Oaki, “Amorphous flexible covalent organic networks containing redox-active moieties: A noncrystalline approach to assembly of functional molecules”, Chemical Science 2020, 11, 7003 (Selected as an Inside Back Cover).

Yuya Oaki

Associate Professor, Keio University

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