Organic light-emitting diodes (OLEDs) have become an ideal candidate for flat planal display and solid-state lighting applications. To lower the fabrication cost, purely organic thermally activated delayed fluorescence (TADF) materials featuring full exciton utilization was proposed to take the place of novel-metal-containing phosphorescence materials. However, due to the spin-forbidden nature of triplet state, purely organic materials generally have long excited state lifetime (generally a few microseconds or longer). In OLED devices, the long exciton lifetime can cause bimolecular reactions such as singlet-triplet annihilation and triplet-triplet annihilation, dampening the efficiency at high brightness and operating lifetime. Therefore, shortening the excited state lifetime has become an urgent topic. As the conformation of TADF emitters have significant effect on the photo-physical properties, the conformation-property relationship in practical amorphous film state of OLED is needed to further develop OLEDs with high efficiency and low efficiency roll-off.
In recent years, our group has investigated the conformation-property relationships of purely organic emitters and developed multiple efficient TADF materials. For example, we achieved reversible switching between conventional fluorescence and thermally activated delayed fluorescence using a dual conformational phenothiazine donor, which exhibits changeable single component white-light emission.1 Further, we developed an adamantane-substituted acridine donor with dual fluorescence emissions, and efficient OLED with an external quantum efficiency (EQE) of 29% can be achieved.2 In addition, many efficient TADF materials with donor-acceptor structures were also reported by our group, especially a series of spiro donors.3-6 When comparing the excited state lifetime and efficiency roll-off characters, an interesting question arises: the TADF materials have similar small singlet-triplet energy gap according to fluorescence and phosphorescence spectra and theoretical calculations, but varied excited state lifetime. Combing the pioneer researches, we speculated that the hidden conformation distribution in amorphous film state might influent the excited state lifetime, which is usually neglected by researchers.
To investigate the role of conformation distribution in host-guest system on excited state lifetime, based on our previous investigations, we selected three different donors (1,3,6,8-tetramethyl-9H-carbazole, 9,9-dimethyl-9,10-dihydroacridine and 10H-spiro[acridine-9,2'-adamantane]) to be combined with a benzene-1,3,5-triyltris(phenylmethanone) acceptor, named TBP-3MCz, TBP-DMAc and TBP-3aDMAc respectively (Figure 1a). The selected molecules have varied donor plane rigidity, which is controlled by the bridge moiety of acridine or fused-ring in carbazole, corresponding to dual conformations of TBP-3aDMAc and twisted intramolecular charge transfer of TBP-3MCz and TBP-DMAc. In the doped film state, these molecules exhibit increasing excited state lifetime in the sequence of TBP-3MCz, TBP-DMAc and TBP-3aDMAc, which is closely related to the conformation heterogeneity originated from the rigidity of the donors (Figure 1b). Further, the conformation distributions of the host-guest systems were simulated by molecular dynamics and quantum chemistry calculations (Figure 1c). The rigid donor shows a confined conformation distribution whereas the flexible donor has a broad conformation distribution in the film state, resulting in a large disorder of singlet-triplet energy gap (ΔEST) and excited state lifetime. Some conformer distributions with large ΔEST would prolong the excited state lifetime in the film state, as shown in the scheme in Figure 1d. Guided by these results, a molecular design of adopting the rigid donor with steric hindrance for confined conformation distribution was proposed, and three TADF emitters were developed. The solution-processed green and blue OLEDs and sensitized multiple resonance TADF OLEDs based on these molecules showed high EQE greater than 20% along with excellent efficiency roll-off suppression, demonstrating the viability of the confined conformation distribution strategy for efficient OLEDs (Figure 1e).
We believe our wok extends the scope of TADF materials to conformation disorder in practical amorphous film state and provides a clear conformation-property relationship of TADF materials. Regulating conformation distributions in amorphous film state by donor rigidity can be an effective strategy for TADF OLEDs with short excited state lifetime. More information of this study can be found in our paper entitled "Confining donor conformation distributions for efficient thermally activated delayed fluorescence with fast spin-flipping" in Nature Communications. (https://doi.org/10.1038/s41467-023-38197-y)
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2. Li, W. et al. Adamantane-substituted acridine donor for blue dual fluorescence and efficient organic light-emitting diodes. Angew. Chem. Int. Ed. 58, 582-586, doi:10.1002/anie.201811703 (2019).
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