2,4-Diphenyl-6-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-1,3,5-triazine: Uses, Synthesis and Application

Physicochemical property

2, 4-diphenyl -6-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan 2-yl)phenyl]-1,3,5-triazine is white powder or crystal for phenyl. It has a melting point of 204 °C. Its boiling point is predicted to be 641.3±57.0 °C and its density is predicted to be 1.21±0.1 g/cm³.

Synthetic routes

Fig. 2 The synthetic step 1 of 2,4-Diphenyl-6-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-1,3,5-triazine.

Add Pd (PPh₃)₄ (0.345 g, 0.3 mmol) quickly to a mixture of 2-chloro-4, 6-diphenyl-1, 3, 5-triazine (5.35 g, 20 mmol), 3-bromophenylboronic acid (4.01 g, 20 mmol) and Na₂CO₃ aqueous solution (2 mol L .-.1, 20 mL) in toluene (100 mL) and ethanol (20 mL) under nitrogen. Stir the reaction at 90 °C for 12 h and cool to room temperature. Remove the volatile under reduced pressure and extract the residue with CH₂Cl₂/H₂O. Separate the combined organic layer and dry over anhydrous MgSO4. Filter and concentrate in vacuo. Purify the crude product by column chromatography using petroleum ether as eluent. Scale gram ¹H NMR (500 MHz, CDCl₃) δ 8.89 (t,J= 1.75 Hz, 1H), 8.80.-.8.74 (m, 4H), 8.73.-.8.68 (m, 1H), 7.76.-.7.71 (m, 1H), 7.66.-.7.55 (m, 6H), 7.45 (t, J= 7.8 Hz, 1H) [1].

Fig. 3 The synthetic step 2 of 2,4-Diphenyl-6-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-1,3,5-triazine.

Add Pd (PPh₃)₂Cl₂ (0.11 g, 0.15 mmol) to a mixture of 2- (3-bromophenyl) -4, 6-diphenyl-1, 3, 5-triazine (3.88 g, 10 mmol), bis (pinacolato) diboron (3.04 g, 12 mmol) and anhydrous KOAc (2.94 g, 30 mmol) in THF (80 mL) under nitrogen. Stir the reaction at 80°C for 12 h and cool to room temperature. Remove the volatile under reduced pressure and extract the residue with CH₂Cl₂/H₂O. Separate the combined organic layer and dry over anhydrous MgSO₄. Filter and concentrate in vacuo. Purify by column chromatography using petroleum ether/CH₂Cl₂ (v/v = 1/1) as eluent. ¹H NMR  (500 MHz, CDCl₃) δ 9.14 (s, 1H), 8.88.-.8.86 (m, 1H), 8.83.-.8.75 (m, 4H), 8.05.-.8.07 (m, 1H), 7.74.-.7.46 (m, 7H), 1.41 (s, 12H) [1].

Application

An electron-transport triarylphosphine oxide-triazine

An organic electron-transport compound for phosphorescent OLEDs is reported, which possesses the advantages of low molecular weight, enhanced glass transition temperature and electron mobility upon doping with 8-hydroxyquinolatolithium (Liq) as well as facile synthesis and purification. There is 2, 4-Diphenyl -6-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan 2-yl)phenyl]-1,3,5-triazine in synthesis. The analytically pure NaAN-m-TRZ (m/z=611.73) is obtained through coupling the 2,4-Diphenyl-6-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-1,3,5-triazine linker with 10-(naphth-2-yl)-anthracen-9-yl moiety. The residual bromo intermediate could be easily removed by column chromatography and/or recrystallization from CH₂Cl₂, hence eliminating a high-risk factor for OLED stability. Thermal analyses show that it exhibits a Tg of 157℃ and decomposition temperature of 353℃ at 1% weight loss. NaAN-m-TRZ has a HOMO level of -5.76 eV determined by the ultraviolet photoelectron spectroscopy measurement and an estimated LUMO level of -2.84 eV. Doping NaAN- m-TRZ with 50% (mass fraction) Liq yields impressive electron mobility of 6.23×10-5~7.19×10-4 cm2·V-1·s-1@E=(2~5)×105 V·cm-1 using space-charge-limited current model, which contributes to suppressing triplet-polaron annihilation in the phosphorescent OLEDs. Consequently, based on the single NaAN-m-TRZ:Liq electron-transport layer, the top-emission green phosphorescent OLED involving Ir(ppy)2(m-mbppy) produces extraordinary durability with projected lifetime t97 of 2 567 h@1 000 cd·m-2 as well as a luminous efficiency of 72.2 cd·A-1 and power efficiency of 81 lm·W-1@1 000 cd·m-2 [2].

Organic electron-transport materials are an essential component to boost performances and stability of organic light-emitting diodes. We present a robust organic electron-transport compound 3-(6-(3-(4,6-bis(4-biphenylyl)-l,3,5-triazin-2-yl)phenyl)pyridin-2-yl)phenyldiphenylphosphine oxide by facilely coupling the triphenylphosphine oxide moiety to the 2,4-Diphenyl-6-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-1,3,5-triazine unit via a 2,6-pyridinylene linker. It is well soluble in weakly polar solvents and possesses a high Tg of 123 °C with an exceptional Td≈470 °C at 1% weight loss and deep HOMO/LUMO levels of ca. −6.45/-3.06 eV. The phos-phorescent spectrum measured in solid state at 77 K reveals a notable triplet energy of 2.88 eV. n-Doping with 8-hydroxyquinolatolithium (Liq) produces the electron mobility value of 4.66×10−5-3.21×10−4 cm V−1s−1 @(2–5)×10−5 V cm−1. Moreover, the contrasting solubility of the bromo reaction intermediate and the new compound in alcoholic solvents facilitates separation. The characterizations of bottom- and top-emission green phosphorescent OLEDs involving this single Liq-doped electron-transport layer reveal long stability. In particular, the latter provides outstanding performances with 77.4 cd A−1 (corresponding to an EQE of 18.7%) and 86.8 lm W−1@ca. 1000 cd m−2, based on the green emitter bis(2-phenylpyridine)(2-(4-methyl-3-phenylphenyl)pyridine)iridium(III). Moreover, driven by a constant current for ca. 640 h, the initial luminance of 1000 cd m−2 appears almost no decay [3].

There has been an increasing demand for high-performance and cost-effective organic electron-transport materials for organic light-emitting diodes (OLEDs). In this contribution, we present a simple compound 3-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1,10-phenanthroline（there is 2, 4-diphenyl -6-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan 2-yl)phenyl]-1,3,5-triazine in synthesis）through the facile Pd-catalyzed coupling of a triphenyltriazine boronic ester with 3-bromo-1,10-phenanthroline. It shows a high Tg of 112 °C. The ultraviolet photoelectron spectroscopy measurements reveal a deep HOMO level of −6.5 eV. The LUMO level is derived as −3.0 eV, based on the optical bandgap. The low-temperature solid-state phosphorescent spectrum gives a triplet energy of ∼2.36 eV. 2,4-Diphenyl-6-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-1,3,5-triazine (Liq, 1:1) leads to considerably improved electron mobility of 5.2 × 10−6–5.8 × 10−5 cm2 V−1 s−1 at E = (2–5) × 105 V cm−1, in contrast with the triarylphosphine oxide-phenantroline molecular conjugate we reported previously. It has been shown that through optimizing the device structure and hence suppressing polaron-exciton annihilation, introducing this single Liq-doped electron-transport layer could offer high-efficiency and stable phosphorescent OLEDs [1].

Highly efficient bipolar host material

Two novel indole-based, bipolar, host materials were designed and synthesized by introducing the triazine unit to the 5-position of indole moiety with a meta-linking strategy. The two host materials exhibited excellence bipolar transport abilities and the meta-linking analog presented a high Tg value (>120 °C). Furthermore, their electrochemical and photo-physical properties were also fully characterized and all the results exhibited that the meta-linking strategy made a significant effect on the physical properties of the two hosts. Moreover, the PHOLED devices using 9-(4-(5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl) phenyl)-9H-carbazole and 9-(4-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)phenyl)-9H-carbazole（these two compounds are synthesized from 2, 4-diphenyl -6-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan 2-yl)phenyl]-1,3,5-triazine) as hosts were also fabricated to evaluate the practical utilities of host materials. The devices achieved the maximum external quantum efficiencies of 17.53% for para-linking compound and 14.53% for meta-linking analog, respectively. In particular, para-linking compound based device revealed the maximum external quantum efficiency was twice that of the device using 2,4-Diphenyl-6-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-1,3,5-triazine as the host [4].

References

[1] Jin G, Liu J Z, Zou J H, et al. Appending triphenyltriazine to 1, 10-phenanthroline: a robust electron-transport material for stable organic light-emitting diodes[J]. Science Bulletin, 2018, 63(7): 446-451.

[2] Ling-ling C, Lin-ye W, Shu X, et al. Triazine-based electron-transport material for stable phosphorescent organic light-emitting diodes[J]. Chinese Journal of Liquid Crystal & Displays, 2021, 36(1).

[3] Chen L L, Peng L, Wang L Y, et al. Molecular engineering of an electron-transport triarylphosphine oxide-triazine conjugate toward high-performance phosphorescent organic light-emitting diodes with remarkable stability[J]. Science China Chemistry, 2020, 63(7): 904-910.

[4] Chen Y, Xie J, Wang Z, et al. Highly efficient bipolar host material based-on indole and triazine moiety for red phosphorescent light-emitting diodes[J]. Dyes and Pigments, 2016, 124: 188-195.

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