Recently, the research team of Wang Qing, research fellow with the School of Chemistry and Chemical Engineering(SCCE) of IMU who joined IMU through talent attraction program “Steed Program”, has made important progress in the research on the charge transfer of radicals. The team’s research result was published in Angew. Chem. Int. Ed., a top international chemical journal. The research result is named “Symmetry-Broken Intermolecular Charge Separation of Cationic Radicals” (URL of the article:https://onlinelibrary.wiley.com/doi/10.1002/anie.202301348). The first authors are Liu Guanghua, a PhD candidate of SCCE and Gao Lei, a master candidate of the school. The corresponding authors are Wang Qing, research fellow with SCCE, Prof. Chi Chunyan with National University of Singapore and Prof. Dai Gaole with Hanzhou Normal University. IMU is the institution which the first authors are with.
Cationic radicals play a significant role in the organic optoelectronic research, such as its application in spin and solar thermal devices. Due to spin-spin interactions, cationic radicals often undergo dimerization and generate high-symmetry compounds of charge delocalization. And this kind of reactions are termed as “ symmetry-driven” processes. The “symmetry-broken” actions(i.e. two identical cationic radicals do not cause dimerization but lead to disproportion reaction to generate asymmetric products) have never been reported although they are very significant in research: (1) It will replace the commonly utilized donor-acceptor strategy to build optoelectronic materials to accelerate the development of single-molecule device; (2) It will bring forth a new method to prepare mixed valence complexes; (3) It will make possible the development of the organic- compound-based ferroelectric materials.
On account of this, the research team of Wang Qing prepared a quadrupolar compound Pyr-BA with two electron-rich pyrrole-type nitrogen atoms and the other two electron-withdrawing pentafluorophenyl groups. The way to dope nitrogen atoms is hopeful to enhance the interaction between molecules and reduce the energy barriers for molecular disproportion to confer the unique properties on the cationic radical(Figure 1).
Figure1 Molecular design strategy
The single-crystal structure shows that although the structure of the cationic radical has a net form of (2Pyr-BA)2+2SbF6-, it is, actually, an asymmetric mixed-valence complex of Pyr-BA0.6+···Pyr-BA1.4+. And Pyr-BA0.6+（B）has a planar structure and Pyr-BA1.4+（A）bowl-shaped structure(Figure 2a). The phenomenon is totally different from the previously reported symmetric dimerized cases. The authors assumed that it was caused by 0.4 electron transfer between 2 identical cationic radicals. And variable-temperature (VT) single-crystal analysis (Figure 2b), VT absorption spectra(Figure 2c) and VT EPR experiments, all confirmed that aggregation and lower temperature would help to facilitate this SB-CS process.
Figure 2 VT physical characterization
This research confirmed for the first time with single-crystal structure that ionic radicals have symmetry-broken charge separation. This work will not only bring about new opportunities for material application but also inject fresh blood into the basic theories of charge transfer.
The research is supported by the National Natural Science Foundation of China (Grant No. 22001135, 22261041), and the Education Department of Inner Mongolia Autonomous Region (Grant No. NJYT22098).
A brief introduction to Wang Qing: Wang Qing is a PhD candidate supervisor and a research fellow with SCCE of IMU who joined IMU through the university’s talent attraction program-“Steed Program”. In July, 2014, Wang graduated from Xi’an Jiaotong University as BS. In August, 2018, she graduated from the National University of Singapore as a PhD. Then Wang started her postdoctoral research at Nagoya University. In December, 2019, Wang joined IMU. She focuses on the research of organic functional materials with large conjugated system.
URL of the research team: https://m.x-mol.com/groups/wang_qing_0114