A series of indolo[3,2-b]indole (IDID) derivatives are designed as a novel structural platform for thermally activated delayed fluorescence (TADF) emitters. Intramolecular charge transfer (ICT)-type molecules consisting of IDID donor (D) and various acceptor (A) moieties are synthesized and characterized in the protocol of the systematical structure-property correlation. IDID derivatives exhibit high efficiency, prompt fluorescence as well as TADF with emission ranges tuned by the chemical structure of the acceptor units. Interestingly, almost all of the IDID derivatives show an identical energy level of the lowest triplet excited state (T) attributed to the locally excited triplet state of the IDID backbone (LE), while that of their lowest singlet excited state (S) is largely tuned by varying the acceptor units. Thus, we demonstrate the underlying mechanism in terms of the molecular engineering. Among the compounds, Tria-phIDID and BP-phIDID generate efficient delayed fluorescence based on the small energy gap between the lowest singlet and triplet excited states (ΔE) and mediation of the LE state. Organic light-emitting diodes with these Tria-phIDID and BP-phIDID as a dopant in the emitting layer show highly efficient electroluminescence with maximum external quantum efficiencies of 20.8% and 13.9%, respectively.
A series of twisted triaryl-s-triazine derivatives are used as acceptor fragments in design of deep-blue TADF emitters for OLED. Comprehensive photophysical investigations indicate high impact of structural relaxation on the TADF color and efficiency.
structures. [1] The promising mechanism is that TADF molecules convert the triplet states to singlet states via thermally activated reverse intersystem crossing (rISC). [2] Since the first report on the application of TADF molecule in organic light-emitting diodes (OLEDs) by Adachi group, [3] huge progress for TADF materials has been made over the past decades. The previous reports demonstrated that an effective strategy for TADF molecules is mainly composed by a twist donoracceptor (D-A) framework which could achieve a small energy gap (ΔE ST ) by the spatial separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). [4] To date, high external quantum efficiencies (EQEs) beyond 20% have been gained with the emission wavelength from deep blue to red. [5] However, most these high-efficiency devices were achieved by vacuum deposition procedure, which is unsuitable for the large area and inexpensive 3D printing owing to its complicated process and high cost. Therefore, it is necessary to develop the high-efficiency solution processable OLEDs based on TADF emitters.According to the design strategy of TADF molecules, proper donor and acceptor units should be carefully selected because Four crucifix-shaped molecules, named TPA-BPSB, DMAc-BPSB, MTPA-BPSB and MDMAc-BPSB, bearing the same acceptor fragment of bis(phenylsulfonyl)benzene (BPSB) and different donor segments (TPA and MTPA are the diphenylamine derivatives while DMAc and MDMAc are the 9,9-dimethylacridine derivatives) are synthesized and characterized by NMR, mass spectra, and single crystal X-ray crystallography. The molecular structure-property relationship of these crucifix-shaped molecules is systematically explored. All compounds display thermally activated delayed fluorescence (TADF) in the region of 500-550 nm. In addition, charming mechanochromic luminescence properties are observed for all these TADF molecules under external stimuli, such as grinding and exposure to CH 2 Cl 2 vapor. Four TADF molecules are used as the emitters in organic light-emitting diodes (OLEDs) fabricated via solution process. MTPA-BPSB-based device presents a best performance with a highest external quantum efficiency of ≈21%, which is among the highest efficiencies for reported BPSB-based solution-processable OLEDs so far. This research has an important significance in designing high-efficiency multifunctional TADF molecules.
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