Nature materials: the team and collaborators of Professor Li Feng of Jilin University made a breakthrough in the field of stable organic light-emitting free radical materials

Recently, nature materials magazine published the team of Professor Li Feng, National Key Laboratory of supramolecular structure and materials, Jilin University, the team of Professor Jean Luc br é DAS, Georgia Institute of technology and the team of Professor Richard h friend, Cavendish Laboratory, Cambridge University High stability and luminescence efficiency in donor acceptor neutral radicals not following the Aufbau principle, nature materials, 2019, DOI: 10.1038/s41563-019-0433-1) Organic light-emitting diode (OLED) has great market value and application prospect in the field of display and lighting due to its advantages of high contrast, ultra-thin and bendable, and has been partially applied in mobile phones, televisions and other devices Due to the fact that 75% of the triplet excitons in OLED do not emit because of the transition forbidden, how to use triplet excitons to achieve 100% internal quantum efficiency (IQE) has become a research hotspot and difficulty in the field of OLED in the past 30 years In general, scientists mainly use phosphorescence and thermal activation delayed fluorescence (TADF) to achieve 100% Iqe by using the singlet and triplet excitons produced in the process of electroluminescence Professor Li Feng engaged in In the past 20 years of OLED research, a new principle of OLED luminescence based on two-wire exciton is proposed after realizing the essence of three-wire exciton transition forbidden Using free radical luminescent materials, only two-wire excitons are formed in OLED luminescence region, and the two-wire excitons have no self rotation forbidden problem in the transition process The Iqe of the device is 100% theoretically, thus avoiding the three-wire excitons for a long time Utilization of Based on this, Professor Li Feng's research team first applied organic light-emitting radical ttm-1cz to OLED light-emitting layer in 2015 to produce red light-emitting device, and confirmed that the light-emitting source of the device was from two-wire exciton (angel Chem., int ed., 2015, 54, 7091-7095); in the subsequent research work, the research team continuously improved the free radical light-emitting materials and device structure, making the device efficiency continuously improved, and also confirmed that OLED based on organic light-emitting radicals can achieve 100% of the two-wire exciton generation ratio (ACS appl Mater Interfaces, 2016, 8, 35472-35478; Chem Mater., 2017, 29, 6733-6739; J Phys Chem Lett., 2018, 9, 6644-6648); the research team also prepared stable room temperature light-emitting diphenylmethyl radicals and expanded the material system of light-emitting radicals (angelw Chem., int ed., 2018, 57, 2869-2873); in 2018, the research team synthesized two efficient free radicals ttm-3ncz and ttm-3pcz, with the light-emitting efficiency of the doped films reaching 90% and 61%, respectively The maximum external quantum efficiency (EQE) of OLED is 27% and 17%, respectively (nature, 2018, 563, 536-540) In order to further improve the stability of light-emitting radicals, Professor Li Feng's research team took TTM or PTM radicals as acceptors, pdcz, PPTA and 3ncz carbazole derivatives as donors, and obtained a series of self-supporting radicals such as ptm-3ncz, PTM pdcz and ttm-ppta This kind of donor acceptor molecular structure design makes this kind of free radicals have the electronic structure of anti Aufbau, which greatly improves the stability of the light-emitting free radicals and maintains high light-emitting efficiency Compared with TTM radicals, the stability of receptor radicals was improved by 2-4 orders of magnitude In 1923, Niels Bohr first proposed the construction principle of the molecular structure (source: Chittagong News Center) for the receptor free radicals Its basic meaning is that when electrons fill the atomic (or molecular) orbit, they fill the orbit in pairs according to the energy from low to high The anti structure principle refers to that in a molecule (or atom), when the low-energy orbit is not filled, the high-energy orbit has been filled with electrons in pairs A series of experiments carried out by the research team, including ultraviolet electron spectroscopy (UPS), cyclic voltammetry (CV), absorption and emission spectra, confirmed that these molecules have special electronic structures of anti structure principle In order to further confirm and discuss the special electronic structure, the team worked with Professor Jean Luc br é Das of Georgia Institute of technology to carry out a series of DFT and multi configuration self consistent field methods（ The results of the quantum chemical calculation of mcscf) not only confirm that the electronic structure of the acceptor free radicals does not follow the principle of structure, but also analyze the formation conditions of the electronic structure of the principle of anti structure in molecules from the perspective of orbital hybridization a) The electronic structure of molecular ground state following the principle of structure; b) the electronic structure of molecular ground state not following the principle of structure (source: Jida News Center), supported by Peiying program of Jilin University, through further cooperation with Professor Richard h friend of Cambridge University, Using ptm-3ncz as the luminescent body, the EQE of NIR OLED based on the solution method is 5.3%, which reaches the highest efficiency of pure organic NIR OLED This research achievement is a major breakthrough in the field of light-emitting radicals and OLED, and shows the application prospect of light-emitting radicals in the field of organic optoelectronics The research work has been supported by the National Natural Science Foundation of China (91833304, 51673080, 11804156), the key research and development plan of the Ministry of science and technology and the "973" plan (2016yfb0401001, 2015cb655003), the Peiying project of Jilin University and the construction project of high-level scientific and technological innovation team.