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Recently, numerous efforts have been devoted to sky blue or greenish blue organic light-emitting diodes (OLEDs), nevertheless, the deep blue and pure blue OLEDs still remain a formidable challenge. Herein, a series of deep blue and pure blue donor–acceptor–donor (D–A–D) type emitters, namely Cz-DMTD, PhCz-DMTD and TMCz-DMTD, were constructed. For PhCz-DMTD which exhibited no delayed fluorescence component, the extended delocalization of donors and the enhancement of oscillator strength were both observed through theoretical calculation. For methyl-engineered TMCz-DMTD, the degenerate charge transfer singlet excited (1CT) and triplet excited (3CT) states and localized triplet excited (3LE) state endowed a thermally activated delayed fluorescence (TADF) character. Moreover, PhCz-DMTD and TMCz-DMTD based OLEDs displayed emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.164, 0.077) and (0.157, 0.107) and with maximum external quantum efficiencies (EQEmax) of 4.6% and 8.7%, respectively. It is the first time that the electroluminescence wavelength of TMCz based OLEDs reached below 450 nm with a CIEy below 0.11. Molecular engineering of the carbazole moiety is expected to provide a valuable strategy for molecular design of more deep blue emitters. [ABSTRACT FROM AUTHOR]

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]