Design and synthesis of efficient blue thermally activated delayed fluorescence molecules bearing triarylborane and 10,10-dimethyl-5,10-dihydrophenazasiline moieties
Graphical abstract
Introduction
Starting from the pioneering work of Tang and VanSlyke,1 organic light-emitting diodes (OLEDs) have been widely investigated and applied to solid-state lightings and flat panel displays.2, 3 In general, holes and electrons injected from both electrodes recombine to form 25% of singlet excitons and 75% of triplet excitons in an emitting layer of OLEDs, resulting in electroluminescence (EL). Since normal fluorescence materials can only harvest the 25% of singlet excitons as EL, 75% of the energy is wasted as a nonradiative relaxation. To convert the all excitons into EL, the heavy metal-containing phosphorescence materials are well studied as an emitter for OLEDs owing to efficient intersystem crossing (ISC) from the lowest singlet excited state (S1) to the lowest triplet excited state (T1) and a fast radiative relaxation from T1 induced by high spin–orbit coupling.2, 4 Since notable achievement by Adachi and co-workers,5 thermally activated delayed fluorescence (TADF) materials are also getting much attention as highly efficient emitters for OLEDs exhibiting high external quantum efficiency (ηext).6 In the TADF process, triplet excitons are converted into singlet excitons via reverse intersystem crossing (RISC) with the aid of thermal activation. The small energy gap between S1 and T1 (ΔEST) is required for the efficient RISC process and is achieved by reducing spatial overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).3(b), 7 This matter is realized in molecules by introducing electron-donating moieties (HOMO) and electron-accepting moieties (LUMO) with a proper connection between them.
Triarylborane units with a vacant 2p orbital on the boron atom are widely utilized as a π-electron accepting unit for various optical applications.8 With the characteristics of the triarylborane units in mind, we recently found that TADF materials bearing triarylborane units as an acceptor and diaryl amino groups as a donor exhibited light blue and green emission with high photoluminescence quantum yields (PLQYs) and ηext.9 Adachi group, Kaji group, and Hatakeyama group also reported excellent boron-based TADF materials exhibiting very high ηext over 20%.10 To utilize these valuable properties for the highly efficient OLEDs, molecular design for the emission colors is an important issue as well as the overall device design to bring out the theoretical limit of efficiency. Generally, changing the energy gap between HOMO and LUMO similarly changes the energy gap between S1 and ground state (S0), which also changes the emission color of the luminescent molecule. It is common approach that the energy gap is varied by introduction of substituents into the appropriate position of the luminescent molecule. On the other hand, we focus on changing the cross-linking atom of a donor unit of TADF molecule. The diaryl amine derivatives such as 9,9-dimethylacridane and phenoxazine are widely used as an donor unit for TADF molecules,6 and therefore changing the cross-linking atom would vary the energy level of HOMO and the emission color. Herein, we describe novel boron-based TADF molecules 1a and 1b bearing a silicon atom-linking diarylamine moiety, 10,10-dimethyl-5,10-dihydrophenazasiline, as a donor unit, wherein the acceptor units are 10H-phenoxaboryl or dimesitylboryl groups (Scheme 1). These molecules showed much bluer emission color than the 9,9-dimethylacridane derivatives 2a and 2b reported by our group9 previously. Compounds 1a and 1b also possessed TADF characteristics with very good PLQYs. As for TADF molecules bearing a silicon atom-linking diaryl amine moiety, there is only one example by Kim group, in which they reported TADF molecule consisting of triazine moiety and 10,10-diphenyl-5,10-dihydrophenazasiline moiety.11
Section snippets
Results and discussion
To design boron-based TADF molecules consisting of 10,10-dimethylphenazasiline as a donor unit, the ground state (S0) geometry of compounds 1a and 1b were calculated using density functional theory (DFT) at the M06/6-31G(d) level with Gaussian 09 according to our previous reports.9, 12 As shown in Figure 1, their HOMO and LUMO were distributed on the 10,10-dimethyl-5,10-dihydrophenazasiline moieties and the triarylborane moieties, respectively. The dihedral angles between the phenylene and the
Conclusion
In conclusion, we have succeeded in synthesis of novel boron-based TADF molecules bearing 10,10-dimethyl-5,10-dihydrophenazasiline moiety as a donor unit and demonstrated that the TADF molecules showed deep blue and pure blue emissions with high PLQYs of 81%. Further investigations to apply the TADF materials to OLEDs as an emitter and optimize the device structures are now underway.
Acknowledgments
We thank Prof. T. Hattori (Tohoku University) for courteous permission to use their instruments and technical guidance.
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