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

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. 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. [Display omitted] • 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%. [ABSTRACT FROM AUTHOR]