Realizing efficient single organic molecular white light emitting diodes from contractual isometrization of quinazoline based emitters
Corresponding author: Zhang Yong, Harbin University of technology; Wang Ying, Institute of physical and chemical technology, Chinese Academy of Sciences
By: Bowen Li, Zhiyi Li, Fengyun Guo, Jingsheng song, Xi Jiang, Ying Wang, Shiyong Gao, Jinzhong Wang, Xinchang Pang, Liancheng Zhao, and Yong Zhang
Because of its excellent spectral stability and simple device preparation process, the single molecule organic white light materials have been widely concerned in the field of basic science and practical lighting applications in recent years. Among them, due to the significant differences in electronic configuration and / or structure, the organic molecules with two configurations usually exhibit different photophysical properties at the same time, such as the realization of different wavelengths of emission. However, the research in this field is still in its infancy, and the understanding of molecular design and its structure-activity relationship is very limited, especially the relationship between photophysical properties and configuration changes. Recently, Professor Zhang Yong of Harbin University of technology and Wang Ying, researcher of Institute of physical and chemical technology of Chinese Academy of Sciences, took phenylpyrimidine (or quinazoline, PQ) as the receptor unit and phenothiazine (PTZ) as the donor unit, and obtained two d-π-a-type molecules with double configuration through central benzene ring connection, namely 2pq-ptz and 4pq-ptz (Figure 1a). 2pq-ptz molecule has unique equal proportion double structure It has almost ideal flexibility to realize white light emission. It is applied in LED / OLED and shows excellent white light device performance.
Figure 1. (a) molecular structure of 2pq-ptz and 4pq-ptz, (B-E) crystal structure of 2pq-ptz
Through single crystal X-ray diffraction analysis, there are two configurations in the crystal structure of 2pq-ptz. One is the dihedral angle between the central benzene ring and PTZ unit is almost 0, but there is a large bend angle of 80.5o, which is called "quasi ax" for short, while the other is 101.8o, which is called "quasi EQ" for short Type, and the two configurations form a precise accumulation of layers, coexisting in a ratio of 1:1 (Fig. 1b-e). Theoretical calculation shows that the ground state potential energy of two configurations of 2pq-ptz single crystal is equal. 2pq-ptz shows dual wavelength emission in toluene solution (Fig. 2a) and doped film (Fig. 2b). White light emission can be realized by adjusting solution concentration or doping concentration. In addition, through the study of energy levels and transient spectra of "ax" and "EQ" configurations, it is found that "ax" configuration belongs to the traditional fluorescence, in the blue light emission region; while "EQ" configuration has the characteristics of thermal activation delayed fluorescence (TADF), in the orange light region; the complementary emission spectra of both provide the possibility to achieve efficient white OLED.
Fig. 2. (a) absorption and emission spectra in toluene solution, (b) emission spectra of 2pq-ptz doped in MCP, (c) luminescent process and energy level diagram of "ax" and "EQ" of 2pq-ptz two configurations, (d) transient spectra of 2pq-ptz in doped film
By using a simple drop coating technology, 2pq-ptz and PMMA films with different doping concentration are uniformly coated on the surface of the blue LED chips on the market, and white LED devices with CIE of (0.32, 0.34) and cri of 89.3 can be obtained. In addition, when it is applied to OLED, the external quantum efficiency of white OLED based on 2pq-ptz is 10.1%, which is one of the highest efficiency reported so far. At the same time, in order to make full use of the TADF characteristics of the "EQ" configuration, an efficient orange TADF OLED based on 2pq-ptz is obtained by increasing the doping concentration, and its external quantum efficiency can reach 25.0% (Figure 3).
Figure 3. (a) CIE color coordinate of white OLED, (b) device performance of white OLED, (c) el spectrum of yellow OLED, (d) device performance of yellow OLED
The results were published in the recent ACS Applied Materials & interfaces journal. The paper also got the help of Professor Song Jinsheng of Henan University and Professor Pang Xinchang of Zhengzhou University.
Realizing efficient single organic molecular white light emitting diodes from conventional isometrization of quinazoline based emitters Bowen Li, Zhiyi Li, Fengyun Guo, Jingsheng song, Xi Jiang, Ying Wang *, Shiyong Gao, Jinzhong Wang, Xinchang Pang, Liancheng Zhao, Yong Zhang * ACS Appl. Mater. Interfaces, 2020, 12, 14233-14243, DOI: 10.1021/acsami.9b20162Publication Date: February 27, 2020Copyright ? 2020 American Chemical Society
(from ACS Publications)
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