A versatile carbazole donor design strategy for blue emission switching from normal fluorescence to thermally activated delayed fluorescence. (October 2021)

• Record Type: Journal Article
• Authors: Niu, Rui; Li, Jiuyan; Liu, Di; Dong, Ruizhi; Wei, Wenkui; Tian, Houru; Shi, Chunlong
• Is Part Of: Dyes and pigments. Volume 194(2021)
• Publication Date: 2021-10
• Subjects: Blue thermally activated delayed fluorescence (TADF) -- Methyl-substituted carbazole -- Steric hindrance -- Organic light-emitting diodes (OLEDs); Dyes and dyeing -- Periodicals; Pigments -- Periodicals; 667.2
• DOI: 10.1016/j.dyepig.2021.109581
• Languages: English
• ISSNs: 0143-7208

Abstract

Abstract: The weak electron donating ability of carbazole is most suitable for constructing the donor-acceptor type blue emitters, but its low spatial requirements caused by the five-membered ring bridged structure is not favorable for spatial separation of frontier molecular orbitals. So many carbazole based compounds do not exhibit thermally activated delayed fluorescence (TADF). Herein, a more sterically demanding group was introduced at the 1-site of carbazole to form 1-methylcarbazole (1-MeCz), in order to construct a versatile donor for blue TADF materials. 1-MeCz was used as donor in combination with triazine and pyrimidine as acceptors to design novel compounds 1-MeCz-TRZ and 1-MeCz-Pm. It was observed that the presence of the methyl group at the 1-site of carbazole enhanced the twisted angles and reduced frontier molecular orbital overlapping, successfully switching the blue emission from the normal fluorescence of the methyl-free reference compounds (Cz-TRZ and Cz-Pm) to TADF of 1-MeCz-TRZ and 1-MeCz-Pm. The organic light-emitting diodes of 1-MeCz-TRZ and 1-MeCz-Pm exhibited blue emission at 450 and 458 nm with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.11) and (0.17, 0.18), and external quantum efficiencies of 13.07% and 7.53%. This study provides a versatile, simple and practical design strategy with 1-MeCz to construct pure blue TADF emitters in combination with various acceptors. Graphical abstract: Increasing the torsion angle of molecule by introducing methyl at 1-site of carbazole is proved to be an effective donor design strategy for blue emitters to change mechanism from normal fluorescence to thermally activated delayed fluorescence. Image 1 Highlights: 1-Methylcarbazole (1-MeCz) was designed and prepared as universal donor to construct pure blue TADF emitters. The steric hindrance effect of the methyl group is capable of facilitating donor twisting and HOMO/LUMO separation and lead to TADF. Two novel blue TADF-OLEDs of these emitters realized pure blue emission with CIE (0.15, 0.11) and external quantum efficiencies of 13.07%.