Conjugated polymers have the advantages of easy modification of chemical structure, tunable energy band and spectrum, etc., and have great potential in the field of organic optoelectronics. At present, the synthesis of conjugated polymers is mainly prepared by cross-coupling methods such as Suzuki, Stille, and direct arylation polymerization. However, these polymerization reactions are generally carried out at high temperature, which not only consumes energy, but also generates difficult-to-separate homopolymeric structural defects in the polymer, which significantly degrades the material quality and device performance. Compared with the high temperature reaction, the room temperature reaction is beneficial to reduce the homopolymeric structural defects. In the synthesis of polymer semiconductors, currently only living polymerization or free radical polymerization can realize the preparation of homopolymeric compounds, and the problem of chemical selectivity is the main problem that restricts the preparation of alternating conjugated polymers by this method. Therefore, the development of a new method for cross-coupling at room temperature is of great research significance for reducing costs and improving material quality.
On the basis of in-depth research on carbon-sulfur bond activation (Nat. Chem., 2017, 9, 188. Inorg. Chem., 2018, 57, 9266.), in 2019, the team of Huang Hui from the University of Chinese Academy of Sciences first used aromatic sulfides as Alternative electrophiles for aryl halides in organic semiconductor synthesis (Angew. Chem. Int. Ed. 2019, 58, 5044). Aromatic sulfides, as electrophiles, have unique reaction characteristics, such as a wide range of functional group tolerance, so this strategy is suitable for the synthesis of various organic semiconductor materials; in addition, although the carbon-sulfur bond is easy to poison the catalyst, the synthesis is difficult, but effective The catalyst and additive strategy can still promote the efficient formation of carbon-carbon bonds (Liebeskind-Srogl Coupling); finally, the probability of self-coupling of carbon-sulfur bonds at room temperature is much lower than that of carbon-halogen bonds, so it is suitable for non-homopolymerization structures. Synthesis of defective polymers. Therefore, the development of carbon-sulfur bond activation has guiding significance for the design and preparation of conjugated polymers.
Recently, the Huang Hui/Shi Qinqin team and the Northwestern University Professor Tobin J. Marks and other research groups have developed a room-temperature Stille-type cross-coupling polymerization methodology using aryl disulfides as electrophiles. Compared with the traditional thermally activated Stille coupling reaction, this method has a faster polymerization rate and fewer self-coupling defects, and has a wide range of substrates, which is a new method for the preparation of polymer semiconductor materials.
Figure 1. A new method for preparing polymers with C-S bonds, and the difference from classical thermal activation for preparing semiconductor materials
In order to further understand the key factors controlling the reaction during the carbon-sulfur bond reaction, the authors conducted small molecule competition experiments and control experiments to show that the rate-determining step of the reaction mainly lies in the process of metal transfer, and verified that the electron-donating ability is stronger The reaction rate of the nucleophile is fast and the yield is high. Finally, the reaction mechanism of this method is proposed.
Figure 2. Mechanistic studies: (a) competition reaction; (b) controlled experimental study; (c) cross-coupling mechanism based on C-S cleavage.
In order to further illustrate the advantages of the new method compared with the traditional Stille coupling to prepare organic polymer semiconductors, the authors, together with researcher Lin Yuze from the Institute of Chemistry and associate professor Zhang Fengjiao from the University of Chinese Academy of Sciences, conducted research on the structural defect states of materials, the density of defect states in material thin films, and the performance of organic electronic devices. We cooperated and verified the advantages of this method for preparing high-quality polymer semiconductor materials.
Figure 3. Defect study: (a) Self-coupling side reaction study; (b) Molecular structures of P2-CS (room temperature C-S activation), P2-CI (traditional Stille) and PBBT (homopolymeric structural defect); (c) ) NMR spectra of P2-CS, P2-CI and PBBT (9.0 ~ 6.5 ppm); (d) defect density of states (tDOS) of P2-CS and P2-CI films; (e) OFET transfer curves of P2-CS films ; (f) OFET transfer curves of P2-CI films.
In conclusion, this work develops the first Stille cross-coupling polymerization utilizing carbon-sulfur bond activation. Under mild reaction conditions, a series of polymer semiconductor materials that are difficult to obtain by high-temperature Stille reaction were prepared, thus providing a new research idea for the synthesis of low-cost, high-performance organic/polymer semiconductor materials.
The related results were published on Nat. Commun. with the title of "Efficient Room Temperature Catalytic Synthesis of Alternating Conjugated Copolymers via C-S Bond Activation.". Li Zijie, a doctoral student at the University of Chinese Academy of Sciences, is the first author, and Associate Professor Shi Qinqin is the first author of this paper. The authors, Professor Huang Hui, Associate Professor Shi Qinqin and Professor Tobin J. Marks are the co-corresponding authors of this article. The above work was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences and the National University of Science and Technology.
Article link: https://www.nature.com/articles/s41467-021-27832-