Recently, Professor Zhang Qichun and his team from City University of Hong Kong, based on the B-N bond, have successfully constructed two types of polymer single crystals using different functionalized ligands.

The introduction of different functional groups has resulted in significant differences in the physical and chemical properties of the two polymers. The crystal structure analysis of them provides a clear and intuitive explanation for these differences.

Zhang Qichun stated that this work proves that the functionalization of B-N polymers is indeed feasible. He believes that by further regulating the basic structure, more diversified and specialized properties can be endowed to the polymers.

In addition, the spatial compactness of the polymer chain has very high requirements for semiconductors, conductors, and superconductors of the polymer. Polymer single crystals are very promising in achieving polymer superconductivity, and even achieving high-temperature superconductivity.

In the research, after introducing tetrathiafulvalene as a functional group, they obtained polymer crystals. After iodine doping, the corresponding devices not only showed excellent low-voltage operating characteristics but also showed obvious photosensitivity.This allows the polymer crystals in this study to have certain application potential in low-power artificial vision devices.

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In addition, due to the easily oxidized characteristics of tetrathiafulvalene, the polymer crystals in this study may respond to some oxidative gases, and thus are expected to be used for gas sensing.

At the same time, while functionalizing the polymer, the team also obtained accurate structural information of the polymer.

Where does the functional study of B-N polymers face difficulties?According to the introduction, due to the advantages of light weight, high strength, and high flexibility, organic polymers have been widely used in fields such as automobiles, medical care, and health care.

Multifunctional organic polymers have a high potential for industrial applications. To regulate their performance, it is necessary to understand the structure of the polymers in depth.

Understanding the stacking mode and interaction of organic polymer molecules inside is of great guiding significance for regulating the physical and chemical properties of the polymers.

However, for most organic polymers, due to the influence of chain entanglement and uneven molecular weight distribution, they usually appear in the form of amorphous or crystalline powder, making it difficult to directly study their internal molecular structure.

Although the ideal molecular model can be indirectly obtained by combining diffraction analysis and simulation calculations, the molecular model usually differs from the actual structure.Thus, people turned to the study of organic polymer single crystals. The reason is: for the structure resolution of single crystals, it can provide the most intuitive and accurate structural information.

For example, studying the interactions and arrangement structures between polymer molecules can help us understand the relationship between crystallization dynamics and the performance of polymer materials.

Research has found that reversible covalent bonds can promote self-correction of defects and promote the growth of polymer single crystals. As a widely known dynamic bond, the B-N bond has a strong covalent nature.

Although some B-N bond-based polymer materials have been developed, previous research has been more focused on analyzing their structures. There has been little previous research on the functionalization of B-N polymers.Do not be limited to existing methods.

It is reported that for many years, the research group has been engaged in the study of organic framework materials (COF, Covalent Organic Frameworks).

They have noticed that the vast majority of COF single crystals reported to date are based on reversible covalent bonds, such as those based on boron-oxygen bonds or imine bonds.

The formation of reversible bonds and structural self-healing play a key role in achieving long-range, orderly growth, which can transform amorphous network structures into regular COF structures, and it is even very likely to produce polymer single crystals.

For understanding the relationship between polymer "material-properties," polymer single crystals can provide profound guidance. Taking the reversible bonds commonly used in the preparation of COF single crystals as an example, it can be imagined that other reversible bonds can also be used to develop polymer single crystals.Based on this assumption, the B-N bond caught the attention of the team. The B-N bond also possesses dynamic reversible covalent properties, and has previously been used to prepare B-N polymer crystals, which appears to be feasible at first glance.

Therefore, the research group decided to use the B-N bond to study polymer crystals. After a period of literature research, they found that current research on B-N polymers is more focused on structural studies, with very little research on the functionalization of materials.

As a result, they decided to introduce some functional groups to see if they could achieve the functionalization of polymers. During this process, they focused on tetrathiafulvalene and porphyrin, both of which are electron-rich functional groups.

The reason is that both of them have electron-donating characteristics and have different oxidation potentials, thus providing a good contrast.

Based on this, the research group used tetrakis(4-pyridyl)tetrathiafulvalene and tetrakis(4-pyridyl)porphyrin as ligands.Initially, they referred to the conventional preparation methods in the existing literature. In the system with porphyrin as the functional center, the preparation of crystals was easier, and the process of screening conditions was also relatively smooth.

However, when preparing crystals with tetrakis(4-pyridyl)tetrathiafulvalene, despite trying all the methods that had been reported, the products obtained were always powders, leading them to once think that it was impossible to succeed based on this system.

Zhang Qichun said, "We once doubted that there was a problem with the experimental scheme itself, and we were also considering whether we needed to redesign the experimental ideas. In the end, we decided not to be limited to existing methods and directly screen a large number of reagents and preparation methods."

So, they first chose dioxane as the solvent, but after trying methods such as evaporation, diffusion, and recrystallization, they found that evaporation at a certain temperature was more likely to form crystals. And only by using dioxane as the solvent could crystals be obtained.

"Under step-by-step optimization, we finally found the appropriate crystallization conditions. Although this research direction is very difficult and the experimental conditions are also very demanding, as long as the right conditions are found, there is always hope," said Zhang Qichun.Ultimately, they finally obtained two types of polymer crystals and conducted performance tests on them. Due to the previously reported thiophene-based COF, which can adsorb iodine and exhibit certain photosensitivity.

Therefore, the research team also focused on the optoelectronic properties of the two polymer crystals. They found that after iodine vapor treatment, both polymer crystals exhibited different optoelectronic properties.

Especially in the system of tetra(4-pyridyl)thiophene, the material after iodine treatment showed excellent low-power performance and significant photosensitivity.

This indicates that they not only achieved the functionalization of polymer single crystals but also inspired them to further study multifunctional polymers.

Recently, the related paper was published in Advanced Functional Materials[1] with the title "Constructing Covalent Organic Polymers through Dative B─N Bonds: Synthesis, Single Crystal Structures, and Physical Properties".

[1] Please note that the reference number [1] is a placeholder and should be replaced with the actual reference number or citation as it appears in the source material.Xin Wang is the first author, and Zhang Qichun serves as the corresponding author.

Currently, the research team has only studied the impact of the introduction of two types of electron-donating groups on the properties of the polymer.

Next, they plan to introduce some electron-withdrawing functional groups into the polymer to form a contrast with the current work, and to see what specific changes will occur in the physical and chemical properties of the polymer.

In addition to studying the one-dimensional chain structure, the team is also researching the combination of different ligands to see if it can change the way the basic units are connected, hoping to obtain a two-dimensional or three-dimensional framework structure.