Imidazole Acceptor for Both Vacuum-Processable and Solution-Processable Efficient Blue Thermally Activated Delayed Fluorescence

The members of the imidazole family have been widely used for electron transporting, host, conventional fluorescent, and phosphorescent materials. Although the imidazole core also has great potential as an acceptor segment of deep-blue thermally activated delayed fluorescence (TADF) owing to its high triplet energy, the emission color of imidazole-based TADF organic light-emitting diodes (OLEDs) has so far been limited to blue to green. In this work, four acridan-imidazole systems are theoretically designed aiming for deep- or pure-blue emitters. All four emitters exhibit deep-blue to blue emission owing to the high energy levels of the lowest excited singlet states, exhibiting y coordinates of Commission Internationale de l’Eclairage coordinates between 0.06 and 0.26. The molecule composed of a trifluoromethyl-substituted benzimidazole acceptor in combination with a tetramethyl-9,10-dihydroacridine donor (named MAc-FBI) achieves a high maximum external quantum efficiency (EQEMAX) of 13.7% in its application to vacuum-processed OLEDs. The emitter has high solubility even in ecofriendly nonhalogenated solvents, which motivates us to fabricate solution-processed MAc-FBI-based OLEDs, resulting in an even higher EQEMAX of 16.1%.


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. Molecular structures, and HOMO and LUMO distributions of Ac-BI, MAc-BI, Ac-FBI, and MAc-FBI. Torsion angles between donor and acceptor segments are also indicated.

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Synthetic procedure 1 H and 13 C NMR spectra were recorded with a JEOL ECA600 or a Bruker Avance III 600US Plus NMR spectrometer. Chemical shifts were reported in δ ppm, using tetramethylsilane as an internal standard.
To a stirred solution of 4-bromo-1-iodo-2-methylbenzene (3.15 g, 10.6 mmol) in 35 mL superdehydrated THF, iPrMgCl‧LiCl solution in THF (1.3 M, 12.0 mL, 15.6 mmol) was added dropwise at 0 °C under argon atmosphere. After stirring the mixture at 0 °C for 1 h, dry dimethylformamide (3.11 mL, 44.8 mmol) was slowly added to the solution. The solution was stirred at room temperature overnight. The reaction was quenched by adding 1 N HCl solution and the organic layer was extracted with dichloromethane and washed with brine. The combined organic layer was dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography on silica-gel using AcOEt / n-hexane = 1 / 10 as an eluent. After removal of solvent, the product was obtained as colorless liquid with a yield of 99%. 1  To a solution of 4-bromo-1-iodo-2-(trifluoromethyl)benzene (3.01 g, 8.58 mmol) in 15 mL superdehydrated THF, n-butyllithium solution in hexane (1.6 M, 2.63 mL, 10.7 mmol) was added dropwise under argon atmosphere and stirred at −90 °C for 2 h. After the addition of dry dimethylformamide (6.68 mL, 34.2 mmol), the solution was stirred at −90 °C for another 1 h, warmed to room temperature, and stirred overnight. The reaction was quenched by adding saturated aqueous NH4Cl, and then diluted with water. The organic layer was extracted with AcOEt and dried over Na2SO4 and evaporated under reduced pressure. The crude product was purified by column chromatography on silica-gel using AcOEt / n-hexane = 1 / 10 as eluent. After removal of solvent, the product was obtained as a light 4-bromo-2-(trifluoromethyl)benzaldehyde (1.72 g, 6.81 mmol) and 2-aminodiphenylamine (838 mg, 4.55 mmol) was dissolved into 50 mL DMF and the solution was stirred at 140 °C overnight. The solution was diluted with 100 mL water and extracted with mixed organic solvent (AcOEt / n-hexane).
The combined organic layer was dried over Na2SO4 and evaporated under reduced pressure. The crude product was purified by column chromatography on silica-gel using AcOEt / n-hexane = 1 / 3 as eluent.

General procedure for preparation of the solution, films, and devices
For the solution measurements, 10 −5 M toluene solution was prepared using analysis-grade toluene (FUJIFILM Wako Pure Chemical Corporation, Japan). The residual oxygen was removed by Argon gas bubbling through the solution for > 15 min. Amorphous films used for the photophysical measurements were prepared by spin-coating 10 mg mL −1 of chloroform solution onto the quartz substrates, which were cleaned by 30 min UV-O3 exposure. For the fabrication of OLEDs, ITO patterned glass substrates were cleaned by 10 mins of sonication in detergent and H2O (twice).
Subsequently, the substrates were cleaned by hot steam of 2-propanol for 5 mins and were then exposed to 30 mins of UV-O3. Solution-processed OLEDs were fabricated by the following procedure and all solution-processed films were fabricated at the air condition. PEDOT:PSS (CLEVIOUS TM P VP CH 8000, Heraeus) diluted in ultrapure H2O at the ratio of 1:1 wt% was spin-coated onto the ITO substrate at 4000 rpm and baked at 150 °C for 10 mins. After cooling the substrates, 10 mg mL −1 of PVK (Mw ~ 1,100,000) solution dissolved in 1,2-dichlorobenzene was spin-coated at 2000 rpm and baked at 120 °C for 10 mins. After cooling the substrates, 10 mg mL −1 of emitter solution dissolved in toluene was spin-coated at 2200 rpm and baked at 100 °C for 10 mins. Following organic layers and Al cathode were fabricated by vacuum deposition. All depositions were conducted using a deposition apparatus (SE-4260, ALS Technology, Japan) at ~10 −4 Pa. OLEDs were encapsulated in the glove box with glass caps containing getter films to remove oxygen and moisture.

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General procedure for device characterization The performance of OLEDs was evaluated using an integrating sphere (C9920-12, Hamamatsu Photonics, Japan) with a Keithley 2400 Source Meter. The device performances were measured in the forward direction in 200-mV steps. S17 ΦPLs were determined to be average value ± error (95% confidence interval). a Measured after Ar bubbling. level considering toluene solvent polarity effects using PCM in Gaussian of c S0 geometry and d S1 geometry. S18 Figure S6. (a) PL spectra at room temperature, (b) fluorescence (Fluo.) and phosphorescence (Phos.) spectra of Ac-BI, MAc-BI, Ac-FBI, and MAc-FBI doped in DPEPO film at 77 K. Fluorescence and phosphorescence spectra were integrated in the time range of 10-50 µs and 300-600 ms, respectively, after the light pulse excitation. S19