Donor–acceptor-structured 1,4-diazatriphenylene derivatives exhibiting thermally activated delayed fluorescence: design and synthesis, photophysical properties and OLED characteristics

Nuclear magnetic resonance (NMR) spectra were measured on an Avance III 500 spectrometer (Bruker) and JNM-ECP400 spectrometer (JEOL). NMR chemical shifts are reported in parts per million (ppm) using tetramethysilane as an internal standard (TMS = 0.0 ppm). Elemental analyses were carried out with a Yanaco MT-5 CHN corder. UV–vis absorption and photoluminescence (PL) spectra of organic films were measured with a UV-2550 spectrometer (Shimadzu) and FluoroMax-4 spectrofluorometer (Horiba Scientific), respectively. PL quantum yield was measured by an absolute PL quantum yield measurement system (Hamamatsu Photonics C11347-10). Luminescence intensity and lifetime of solutions and organic films were measured with a Quantaurus-Tau system (Hamamatsu Photonics) and streak camera (Hamamatsu Photonics C4334). Organic films were excited by an N₂ gas laser (λ = 337 nm, pulse width = 500 ps, repetition rate = 20 Hz) under a vacuum of < 0.4 Pa. Samples were cooled to 4 K with a cryostat (Iwatani Industrial Gases). All quantum chemical calculations were performed using the Gaussian 09 programme package [16]. Geometry optimization was carried out using the M06-2X functional and cc-pVDZ basis set [17, 18]. Low-lying excited singlet and triplet states were computed using the optimized structures with time-dependent density functional theory (TD-DFT) [19, 20] at the M06-2X/cc-pVDZ level.

All reagents and solvents were purchased from Sigma-Aldrich, Tokyo Chemical Industry (TCI) or Wako Pure Chemical Industries, and used as received unless otherwise noted. All reactions were performed under an N₂ atmosphere in dry solvents.

To measure EL in OLED devices, clean 110 nm-thick indium tin oxide (ITO)-coated glass substrates with a 2 mm stripe pattern were used. The substrates were degreased with distilled water, a neutral detergent, acetone, isopropyl alcohol, and cleaned in a UV–ozone chamber (Nippon Laser and Electrics Laboratory, NLUV253) before being loaded into an evaporation system. Organic layers were thermally evaporated on substrates under a vacuum of < 7 × 10⁻⁴ Pa at an evaporation rate of < 0.3 nm s⁻¹. For all of the devices, a cathode aluminium (Al) layer was deposited through a 1 mm diameter opening in a shadow mask. The devices were exposed once to N₂ gas after the formation of the organic layers because a metal mask was included to define the cathode area. The current density–voltage–luminance characteristics were evaluated using a Keithley 2400 source metre and an absolute external quantum efficiency measurement system (Hamamatsu, C9920-12).

The synthetic route to obtain 9,9-dimethylacridane (7) is outlined in scheme 1. A mixture of methyl anthranilate (29.1 g, 0.19 mol), iodobenzene (17 mL, 0.15 mol), K₂CO₃ (26.7 g, 0.19 mol), and copper powder (1.6 g, 30 mmol) was stirred for 4 h at 180 °C. After cooling to room temperature, the reaction mixture was diluted with EtOAc (100 mL), and filtered through a Celite^® pad. The filtrate was concentrated by evaporation. The crude product was purified by column chromatography (silica gel, eluent: hexane/EtOAc = 5:1 v/v) to afford diphenylamine 5 as a colourless oil (yield = 25 g, 72%). ¹H NMR (500 MHz, CDCl₃): δ 9.47 (brs, 1H), 7.95 (dd, J = 8.0, 1.5 Hz, 1H), 7.44–7.10 (m, 6H) 7.07 (ddd, J = 8.0, 7.5, 1.5 Hz, 1H), 6.71 (td, J = 7.5, 1.5 Hz, 1H), 3.88 (s, 3H).

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MeLi (1.5 M in Et₂O, 160 mL) was added dropwise to a solution of 5 (13.6 g, 60 mmol) in Et₂O (67 mL) at −80 °C. The mixture was then stirred for 3 h at room temperature. The reaction was quenched by adding water (30 mL). The product was extracted with EtOAc three times, and the combined organic layers were washed with brine and dried over anhydrous MgSO₄. After filtration, the filtrate was concentrated by evaporation to afford carbinol 6 as a pale yellow oil (yield = 13.2 g, 97%). The product was used in the next step without further purification. ¹H NMR (500 MHz, CDCl₃): δ 7.78 (brs, 1H), 7.36 (dd, J = 8.0, 1.5 Hz, 1H), 7.28–7.17 (m, 3H), 7.15 (ddd, J = 8.0, 7.5, 1.5 Hz, 1H), 7.05 (dd, J = 7.5, 1.0 Hz, 2H), 6.92–6.78 (m, 2H), 2.48 (brs, 1H), 2.01 (s, 6H).

MeSO₃H (35.1 g, 0.37 mol) was added to a mixture of crude 6 (14.0 g, 62 mmol) and polyphosphoric acid (83.1 g) in toluene (390 mL). The mixture was stirred for 2 h at 60 °C, and then the reaction was quenched by adding water (100 mL). The product was extracted with EtOAc three times, and the combined organic layers were washed with saturated aqueous NaHCO₃ and brine, and dried over anhydrous MgSO₄. After filtration and evaporation, the product was purified by column chromatography (silica gel, eluent: hexane/EtOAc = 5:1 v/v), washed with hexane, and dried under vacuum to give 9,9-dimethylacridane 7 as colourless crystals (yield = 10.0 g, 78%). ¹H NMR (500 MHz, CDCl₃): δ 7.38 (dd, J = 8.0, 1.3 Hz, 2H), 7.10 (ddd, J = 8.0, 7.5, 1.3 Hz, 2H), 6.91 (dd, J = 7.5, 7.5 Hz, 2H), 6.69 (d, J = 7.5 Hz, 2H), 6.13 (brs, 1H), 1.58 (s, 6H).

The synthetic route to obtain 3-(diphenylamino)carbazole (9) is outlined in scheme 2. 4-(N,N-dimethylamino)pyridine (DMAP, 0.80 g, 8.2 mmol) was added to a solution of 3-bromocarbazole (20.3 g, 82 mmol) and Boc₂O (24.8 g, 114 mmol) in THF (300 mL). The mixture was stirred for 4 h at room temperature. The reaction was concentrated by evaporation. The residue was purified by column chromatography (silica gel, eluent: toluene/hexane = 1:1 v/v) to afford carbamate 8 as a white solid (yield = 28.7 g, quantitative yield). ¹H NMR (500 MHz, CDCl₃): δ 8.40 (brd, J = 7.5 Hz, 1H), 8.20 (brd, J = 7.5 Hz, 1H), 7.90 (d, J = 2.0 Hz, 1H), 7.51 (dq, J = 7.5, 0.5 Hz, 1H), 7.40 (dd, J = 8.5, 2.0 Hz, 1H), 7.33 (ddd, J = 7.5, 7.5, 1.0 Hz, 1H), 7.14 (ddd, J = 8.5, 7.5, 1.0 Hz, 1H), 1.39 (s, 9H).

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Pd(OAc)₂ (97 mg, 440 μmol) was added to a mixture of 8 (3.0 g, 8.7 mmol), diphenylamine (2.2 g, 13 mmol), t-BuONa (1.0 g, 10 mmol), and P(t-Bu)₃ (0.18 g, 0.87 mmol) in toluene (80 mL). The mixture was heated under reflux for 1 h. After cooling to room temperature, the reaction mixture was filtered through a Celite^® pad. The filtrate was washed successively with water, saturated aqueous NH₄Cl, and brine, and dried over anhydrous MgSO₄. After filtration and evaporation, the crude product was used in the next step without further purification. A mixture of the obtained crude product, 1,4-dioxane (50 mL), and hydrochloric acid (36%, 5.0 mL) was stirred for 3 h at 100 °C. After cooling to room temperature, the resulting mixture was neutralized with saturated aqueous NaHCO₃. The product was extracted with toluene three times, and the combined organic layers were washed successively with saturated aqueous NaHCO₃ and brine, and dried over anhydrous MgSO₄. After filtration and evaporation, the crude product was purified by column chromatography (silica gel, eluent: toluene/hexane = 1:1 v/v) to afford 9 as a white solid (yield = 2.1 g, 71%). ¹H NMR (500 MHz, toluene-d₈): δ 7.94 (d, J = 1.5 Hz, 1H), 7.55 (dd, J = 7.5, 0.5 Hz, 1H), 7.36–7.18 (m, 3H), 7.17–6.98 (m, 9H), 6.94 (dd, J = 8.5, 0.5 Hz, 1H), 6.69 (tt, J = 7.5, 1.0 Hz, 2H), 6.49 (brs, 1H).

N-Bromosuccinimide (NBS, 27 g, 0.15 mol) was added slowly to a solution of phenanthrenequinone (15 g, 72 mmol) in H₂SO₄ (95%, 150 mL) at 0 °C. The mixture was stirred for 2 h at room temperature. The reaction mixture was poured into water with crushed ice (ca. 400 mL). The resulting precipitate was collected by filtration. The product was washed with hot EtOAc (300 mL). After filtration, the product was dried under vacuum to provide 1 as an orange solid (yield = 22.4 g, 85%). ¹H NMR (500 MHz, CDCl₃): δ 8.25 (d, J = 8.5 Hz, 2H), 8.08 (d, J = 2.5 Hz, 2H), 7.96 (dd, J = 8.5, 2.5 Hz, 2H).

Ethylenediamine (20 mL, 0.30 mol) was added dropwise to a suspension of 1 (10 g, 27 mmol) in EtOH (150 mL) at 0 °C. The mixture was heated under reflux for 1 h, and then AcOH (250 mL) was added. The mixture was heated under reflux for further 2 h without N₂ flow. After cooling to room temperature, the reaction mixture was poured into water (ca. 100 mL), and the resulting precipitate was collected by filtration. The product was washed successively with hot MeOH (200 mL) and acetone (200 mL). After filtration, the solid product was dried under vacuum to afford 2 as a pale-yellow solid (yield = 5.9 g, 56%). ¹H NMR (500 MHz, DMSO-d₆): δ 9.23 (d, J = 2.0 Hz, 2H), 9.13 (s, 2H), 8.83 (d, J = 8.5 Hz, 2H), 8.06 (dd, J = 8.5, 2.0 Hz, 2H).

ATP–PXZ. Pd(OAc)₂ (60 mg, 0.26 mmol) was added to a suspension of 2 (2.0 g, 5.2 mmol), phenoxazine (3.0 g, 16 mmol), P(t-Bu)₃ (0.11 g, 0.52 mmol), and K₂CO₃ (1.5 g, 11 mmol) in toluene (150 mL) at room temperature. The mixture was heated under reflux for 15 h. After cooling, the reaction was quenched by adding MeOH (150 mL). The resulting precipitate was collected by filtration. The product was washed successively with hot MeOH (20 mL) and acetone (20 mL). After filtration, the solid product was dried under vacuum to afford ATP–PXZ as a pale-yellow solid (yield = 2.7 g, 88%). ATP–PXZ was further purified by temperature-gradient sublimation under vacuum. ¹H NMR (400 MHz, DMSO-d₆): δ 9.17 (d, J = 8.6 Hz, 2H), 9.15 (d, J = 2.2 Hz, 2H), 9.03 (s, 2H), 7.92 (dd, J = 8.6, 2.2 Hz, 2H), 6.80 (dd, J = 7.7, 1.5 Hz, 4H), 6.73 (ddd, J = 7.7, 7.7, 1.5 Hz, 4H), 6.66 (ddd, J = 8.1, 7.7, 1.5 Hz, 4H), 6.10 (dd, J = 8.1, 1.5 Hz, 4H). Elemental analysis (%) calculated for C₄₀H₂₄N₄O₂: C, 81.07; H, 4.08; N, 9.45; found: C, 81.02; H, 3.98; N, 9.46.

ATP–ACR and ATP–CZ were prepared in a similar manner. Spectroscopic and analytical data for these compounds are described below.

ATP–ACR. Pale-yellow powder (yield = 37%). ¹H NMR (400 MHz, DMSO-d₆): δ 9.19 (d, J = 8.6 Hz, 2H), 9.12 (d, J = 2.4 Hz, 2H), 9.00 (s, 2H), 7.89 (dd, J = 8.6, 2.4 Hz, 2H), 7.60–7.48 (m, 4H), 7.03–6.89 (m, 8H), 6.42–6.32 (m, 4H), 1.73 (s, 12H). Elemental analysis (%) calculated for C₄₆H₃₆N₄: C, 85.68; H, 5.63; N, 8.69; found: C, 85.69; H, 5.62; N, 8.69.

ATP–CZ. Off-white powder (yield = 74%). ¹H NMR (400 MHz, DMSO-d₆): δ 9.37 (d, J = 2.0 Hz, 2H), 9.20 (d, J = 8.1 Hz, 2H), 9.06 (s, 2H), 8.28 (d, J = 7.8 Hz, 4H), 8.17 (dd, J = 8.1, 2.0 Hz, 2H), 7.57 (d, J = 8.3 Hz, 4H), 7.48 (dd, J = 8.3, 6.8 Hz, 4H), 7.53 (dd, J = 7.8, 6.8 Hz 4H). Elemental analysis (%) calculated for C₄₀H₂₄N₄: C, 85.69; H, 4.31; N, 9.99; found: C, 85.64; H, 4.24; N, 9.98.

Bromine (13.9 g, 87 mmol) was added to a suspension of phenanthrenequinone (50 g, 0.24 mol) and benzoylperoxide (2.5 g, 6.5 mmol) in nitrobenzene (250 mL) at room temperature. The mixture was stirred and heated to 120 °C. After the formation of gaseous HBr started, further bromine (69.2 g, 0.43 mol) was added dropwise to the mixture. The mixture was stirred for 2 h at 120 °C. After cooling to room temperature, EtOH (250 mL) was added to the reaction mixture. The resulting precipitate was collected by filtration and washed with EtOH. The product was dried under vacuum to afford 3 as an orange solid (yield = 75.5 g, 85%). ¹H NMR (500 MHz, CDCl₃): δ 8.12 (d, J = 2.0 Hz, 2H), 8.07 (d, J = 8.5 Hz, 2H), 7.67 (dd, J = 8.5, 2.0 Hz, 2H).

Ethylenediamine (40 mL, 0.60 mol) was added dropwise to a suspension of 3 (20 g, 55 mmol) in EtOH (300 mL) at 0 °C. The mixture was heated under reflux for 3 h, and then AcOH (250 mL) was added. The mixture was heated under reflux for a further 3 h without N₂ flow. After cooling to room temperature, the reaction mixture was added to MeOH (ca. 500 mL), and the resulting precipitate was collected by filtration. The product was washed successively with hot MeOH (200 mL) and acetone (200 mL). After filtration, the solid product was dried under vacuum to afford 4 as a pale-yellow solid (yield = 11.9 g, 56%). ¹H NMR (500 MHz, DMSO-d₆): δ 9.22 (d, J = 2.0 Hz, 2H), 9.11 (s, 2H), 9.06 (d, J = 8.5 Hz, 2H), 8.03 (dd, J = 8.5, 2.0 Hz, 2H).

m-ATP–PXZ. Pd₂(dba)₃ · CHCl₃ (70 mg, 68 μmol) was added to a suspension of 4 (1.0 g, 2.6 mmol), phenoxazine (1.5 g, 8.1 mmol), cataCXium^® PIntB (0.09 g, 0.26 mmol), and t-BuONa (0.57 g, 6.0 mmol) in toluene (60 mL) at room temperature. The mixture was heated under reflux for 12 h. After cooling, the reaction mixture was added to MeOH (150 mL). The resulting precipitate was collected by filtration. The product was washed successively with MeOH (20 mL) and acetone (20 mL). The solid product was dried under vacuum to afford m-ATP–PXZ as a pale-yellow solid (yield = 1.4 g, 91%). This compound was further purified by temperature-gradient sublimation under vacuum. ¹H NMR (500 MHz, DMSO-d₆): δ 9.42 (d, J = 8.5 Hz, 2H), 9.17 (s, 2H), 9.02 (d, J = 2.0 Hz, 2H), 7.86 (dd, J = 8.5, 2.0 Hz, 2H), 7.12 (dd, J = 7.7, 1.5 Hz, 4H), 6.93 (ddd, J = 8.0, 7.7, 1.5 Hz, 4H), 6.88 (ddd, J = 8.0, 8.0, 1.5 Hz, 4H), 6.33 (dd, J = 8.0, 1.5 Hz, 4H). Elemental analysis (%) calculated for C₄₀H₂₄N₄O₂: C, 81.07; H, 4.08; N, 9.45; found: C, 81.16; H, 4.00; N, 9.43.

m-ATP–ACR, m-ATP–CZ, and m-ATP–CDP were prepared in a similar manner. Spectroscopic and analytical data for these compounds are described below.

m-ATP–ACR. Pale-yellow powder (yield = 79%). ¹H NMR (500 MHz, DMSO-d₆): δ 9.51 (d, J = 8.5 Hz, 2H), 9.22 (s, 2H), 9.05 (d, J = 2.0 Hz, 2H), 7.79 (dd, J = 8.5, 2.0 Hz, 2H), 7.50 (dd, J = 8.0, 1.5 Hz, 4H), 6.91 (ddd, J = 8.0, 7.0, 1.5 Hz, 4H), 6.87 (ddd, J = 8.0, 7.0, 1.0 Hz, 4H), 6.15 (dd, J = 8.0, 1.0 Hz, 4H), 1.65 (s, 12H). Elemental analysis (%) calculated for C₄₆H₃₆N₄: C, 85.68; H, 5.63; N, 8.69; found: C, 85.60; H, 5.62; N, 8.65.

m-ATP–CZ. Off-white powder (yield = 94%). ¹H NMR (500 MHz, DMSO-d₆): δ 9.48 (d, J = 8.5 Hz, 2H), 9.33 (d, J = 2.0 Hz, 2H), 9.21 (s, 2H), 8.27 (d, J = 7.5 Hz, 4H), 8.12 (dd, J = 8.5, 2.0 Hz, 2H), 7.59 (d, J = 8.5 Hz, 4H), 7.45 (ddd, J = 8.0, 7.5, 1.0 Hz, 4H), 7.31 (ddd, J = 8.0, 7.5, 1.0 Hz 4H). Elemental analysis (%) calculated for C₄₀H₂₄N₄: C, 85.69; H, 4.31; N, 9.99; found: C, 85.77; H, 4.26; N, 9.98.

m-ATP–CDP. Pale-yellow powder (yield = 78%). ¹H NMR (500 MHz, DMSO-d₆): δ 9.46 (d, J = 8.8 Hz, 2H), 9.35 (d, J = 2.0 Hz, 2H), 9.19 (s, 2H), 8.20 (d, J = 8.0 Hz, 2H), 8.12 (dd, J = 8.8, 2.0 Hz, 2H), 8.05 (d, J = 2.0 Hz, 2H), 7.59 (d, J = 8.5 Hz, 2H), 7.56 (d, J = 8.0 Hz, 2H), 7.41 (ddd, J = 8.5, 8.0, 1.3 Hz, 2H), 7.33–7.15 (m, 12H), 7.08–6.85 (m, 12H). Elemental analysis (%) calculated for C₆₄H₄₂N₆: C, 85.88; H, 4.73; N, 9.39; found: C, 85.75; H, 4.66; N, 9.39.

To realize small ΔE_ST in TADF materials, it is critical to separate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) to give small exchange energy. Steric hindrance that introduces twisting between donor and acceptor units can successfully achieve this purpose [21–23]. As shown in figure 2, we designed and synthesized a series of donor–acceptor–donor (D–A–D)-type molecules composed of an ATP acceptor core and peripheral aromatic donor moieties such as phenoxazine (PXZ), 9,9-dimethylacridane (ACR), carbazole (CZ) and 3-(diphenylamino)carbazole (CDP). The donor units were attached at 6, 11- and 7,10-positions on the central ATP core to systematically investigate the physicochemical properties of these regioisomers.

The calculated HOMO and LUMO distributions of the PXZ-functionalized molecules are shown in figure 3. The M06-2X/cc-pVDZ method was used for the calculations because by taking solvent effects into account within the polarizable continuum model, it well reproduced the emission wavelengths of our compounds in organic solvent media. The HOMOs of ATP–PXZ and m-ATP–PXZ are predominantly distributed on the peripheral PXZ units, whereas the LUMOs of both materials are localized over the ATP core. This offers clear spatial separation of the HOMO and LUMO because of the large dihedral angles (ca. 80–90°) between the ATP core and PXZ peripheries. The calculated ΔE_ST, HOMO and LUMO energy levels, and oscillator strength for the S₁ ← S₀ excitation of these molecules are summarized in table 1. Although the theoretical calculation seems to overestimate ΔE_ST values, it offers qualitative understanding of the relationship between ΔE_ST and HOMO–LUMO overlap. ΔE_ST is calculated to be 0.18 eV for ATP–PXZ and 0.17 eV for m-ATP–PXZ. The ΔE_ST values of ATP–PXZ and m-ATP–PXZ are somewhat smaller than that of 4CzIPN (ΔE_ST = 0.3 eV using the same functional), which is a representative TADF material we reported previously [9], so they should be sufficiently small to induce TADF characteristics. Likewise, for ACR-functionalized ATP–ACR and m-ATP–ACR, small ΔE_ST values of approximately 0.28 and 0.25 eV, respectively, were obtained, suggesting high potential as TADF emitters. In contrast, the CZ substituents do not provide enough twist (i.e., orthogonal configuration) between the donor and acceptor subunits, resulting in a rather large ΔE_ST of 0.60 eV for both the 6,11-substituted ATP–CZ and 7,10-substituted m-ATP–CZ.

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Scheme 3 outlines the synthetic approach used to prepare the designed ATP-based molecules. The dibromo intermediates 1 and 3 were prepared by regioselective bromination of 9,10-phananthrenequinone according to the literature [24–26]. The ionic mechanism led to the 2,7-dibromo derivative 1, whilst the radical mechanism provided the 3,6-dibromo derivative 3. The subsequent cyclization of 1 and 3 with ethylenediamine and aromatization afforded dibromo-1,4-diazatriphenylenes 2 and 4. The target materials were synthesized via palladium-catalyzed aminations of 2 and 4 with two equivalents of the corresponding arylamine (i.e., aromatic donor units) in moderate to high yields (see experimental details). Final materials were purified by repetitive temperature-gradient sublimation before characterization and OLED fabrication.

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The photophysical properties of the ATP-based compounds were examined by UV–vis absorption and PL spectroscopies; the results are summarized in table 2. Figure 4 shows UV–vis absorption and PL spectra of the PXZ-functionalized derivatives, ATP–PXZ and m-ATP–PXZ, in toluene. There are two clear absorption bands in the UV–vis spectra: one is an intense absorption at shorter wavelength (< 350 nm) corresponding to the π–π* transition of the ATP core, while the other is a weaker absorption corresponding to an intramolecular charge transfer (CT) transition in the range of ca. 350–450 nm related to the D–A–D structure. Both ATP–PXZ and m-ATP–PXZ exhibit structureless green PL emission with the maximum (λ_PL) at 546 nm in toluene solution. A slightly smaller Stokes shift is observed for m-ATP–PXZ than ATP–PXZ, which implies that a smaller degree of structural change will take place in the 7,10-substituted derivative upon photoexcitation. Similar intramolecular CT transition characteristics in the S₁ states are observed for other ACR-, CZ-, and CDP-functionalized compounds as a result of their D–A–D structures.

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Next, we studied the delayed fluorescence characteristics of the ATP-based materials by means of transient PL decay measurements. As reported in previous studies [5–9, 21–23], TADF materials in N₂-saturated solution typically show two distinct components of prompt decay originating from fluorescence, and delayed decay based on TADF. In this series of materials, ATP–PXZ, m-ATP–PXZ, ATP–ACR, m-ATP–ACR and m-ATP–CDP ⁶ are found to show noticeable TADF emission components in N₂-saturated solution. Figure 5 presents the transient PL decay curves for ATP–PXZ in toluene before and after N₂ bubbling. The PL profile exclusively exhibits rapid decay before N₂ bubbling with an emission lifetime of τ = 31 ± 2 ns. In contrast, after N₂ bubbling, ATP–PXZ exhibits both a nano-second-scale prompt component (τ_p = 73 ± 3 ns) and micro-second-scale delayed component (τ_d = 1.9 ± 0.1 μs) at room temperature (300 K). The latter long-tail emission should originate from TADF. In addition, a PL quantum yield (Φ_PL) of 24 ± 1% was obtained after N₂ bubbling, which is six times higher than that measured before bubbling (Φ_PL = 4 ± 1%), because of suppression of excited state energy transfer to triplet oxygen molecules. Meanwhile, CZ-functionalized derivatives ATP–CZ and m-ATP–CZ did not show obvious TADF characteristics in solution regardless of N₂ bubbling, which is ascribed to their relatively large ΔE_ST (tables 1 and 2).

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The PL properties of the ATP-based TADF emitters were also studied in doped thin films with an amorphous host matrix. We used 4,4'-bis(9-carbazolyl)-1,1'-biphenyl (CBP; T₁ = 2.7 eV), 1,5-bis(9-carbazolyl)benzene (mCP; T₁ = 2.9 eV) and 3,3'-bis(9-carbazolyl)-1,1'-biphenyl (mCBP; T₁ = 2.8 eV) as host materials on account of their wide energy gaps and high T₁ energy levels [27, 28]. Figure 6 depicts the PL spectra of 6 wt%-emitter:host codeposited thin films. In the doped films, sky-blue to green emissions were observed with λ_PL ranging from 483 to 529 nm. The λ_PL positions of the codeposited thin films are hypsochromically shifted by about 10–30 nm compared with those measured in toluene solution. Compared with their solution states, Φ_PL of all of these compounds increased considerably in the solid state (Φ_PL = 49–81%), most likely because the fixation of fragmental rotation and conformational changes can suppress the non-radiative decay to some degree. We also investigated the transient PL characteristics of the emitter:host codeposited thin films. As displayed in figure 7, a 6 wt%-ATP–PXZ:CBP film clearly exhibits both prompt and delayed PL components. Because the prompt fluorescence and delayed emission spectra are coincident, the long-tail emission should originate from TADF. The overall Φ_PL of the 6 wt%-ATP–PXZ:CBP film was 63 ± 1% at room temperature (table 2), in which the prompt (Φ_F) and TADF (Φ_TADF) efficiencies were estimated to be 14% and 49%, respectively.

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The performances of ATP-based TADF emitters were investigated by fabricating multilayer OLEDs with the device configurations of ITO (100 nm)/α-NPD (35 nm)/6 wt%-emitter:host (15 nm)/TPBi (65 nm)/LiF (0.8 nm)/Al (70 nm) for green emitters (ATP–PXZ and m-ATP–PXZ), and ITO (100 nm)/α-NPD (35 nm)/mCP (10 nm)/6 wt%-emitter:host (15 nm)/PPT (10 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (70 nm) for sky-blue emitters (ATP–ACR, m-ATP–ACR and m-ATP-CDT). We used 4,4'-bis[N-(1-naphthyl)-N-phenylamino]-1,1'-biphenyl (α-NPD) as a hole-transporting layer and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) as an electron-transporting layer. For sky-blue TADF-OLEDs, thin layers (10 nm) of mCP and 2, 8-bis(diphenylphosphoryl) dibenzo-[b,d]thiophene (PPT) [29] with high T₁ energies were inserted to suppress triplet exciton quenching of the sky-blue emitters with T₁ of ca. 2.8 eV at the interfaces in the devices.

Figure 8 depicts the external quantum efficiency (η_ext) versus current density (J) curves and current density–voltage–luminance (J–V–L) characteristics of the fabricated TADF-based OLEDs. Device performances are also summarized in table 3. The green-emitting OLEDs containing ATP–PXZ and m-ATP–PXZ display higher η_ext (11.7% and 12.6%, respectively) than those of the sky-blue-emitting OLEDs with ATP–ACR and m-ATP–ACR (7.5% and 8.7%, respectively), because of the higher Φ_PL of the PXZ-functionalized ATPs in doped thin films than those with ACR. The ATP–PXZ-based OLED shows a maximum current efficiency (η_c) of 37.9 cd A⁻¹ and power efficiency (η_p) of 24.8 lm W⁻¹. Moreover, as listed in table 3, their EL maxima (λ_EL) are similar to the corresponding λ_PL, confirming that EL emission originates from the same intramolecular CT excited (S₁) states of the D–A–D-structured ATP derivatives.

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In the TADF-based OLEDs under electrical excitation, the triplet excitons are directly generated by carrier recombination and successively converted to the S₁ state via efficient reverse ISC. Accordingly, the theoretical maximum of internal EL quantum efficiency (η_int) can be given by the following equation [6]:

$$\begin{eqnarray*}&&{{\eta }_{\operatorname{int}}}={{\eta }_{r,S}}\times {{\Phi }_{{\rm{F}}}}+{{\eta }_{r,S}}\times {{\Phi }_{{\rm{TADF}}}}+{{\eta }_{r,T}}\times {{\Phi }_{{\rm{TADF}}}}/{{\Phi }_{{\rm{ISC}}}},\end{eqnarray*}$$

where η_r,S denotes the singlet-exciton production rate (25%), η_r,T represents the triplet-exciton production rate (75%), and Φ_ISC (≈86%) is the efficiency of ISC. We estimate that η_int for the ATP–PXZ-based device is 58%, so the theoretical maximum η_ext is approximately 12% assuming a light out-coupling efficiency of 20%. The experimental η_ext of 11.7% obtained for the ATP–PXZ-based device is consistent with the theoretical value, suggesting that an appropriate charge carrier balance and effective triplet exciton confinement are attained in the emitting layer.