Thermally activated delayed fluorescence material and preparation method

Abstract

The application discloses a kind of thermally activated delayed fluorescence materials and preparation method, belong to the technical field of organic luminescent material.The application selects triazine as acceptor and carries out proper modification, selects acridine as donor, constructs a series of donor-acceptor type thermally activated delayed fluorescence materials, with delayed fluorescence molecular properties.The material has good luminescent performance and thermal stability in thin film state, with good device performance, and low device efficiency roll-off.The preparation process is simple and highly reproducible, and there is a possibility of large-scale industrial production.

Description

Technical Field

[0001] This invention relates to the field of organic synthesis technology, and specifically to a method for preparing a thermally activated delayed fluorescence material. Background Technology

[0002] Organic light-emitting materials are simple to synthesize and environmentally friendly, and have broad application prospects in fields such as electroluminescence, sensors, bioimaging and anti-counterfeiting.

[0003] First-generation organic light-emitting diodes (OLEDs) typically use organic fluorescent dye molecules as the emitting layer. Most fluorescent molecules have opposite spins in their ground and excited states, so electronic transitions between them are spin-allowed. Theoretically, only 25% of singlet excitons in this type of OLED are used for emission, resulting in purer colors. However, OLEDs using fluorescent emitters cannot utilize triplet excitons, and their external quantum yield (EQE) is capped at only 5%.

[0004] To utilize the remaining 75% of triplet excitons, researchers have developed second-generation phosphorescent OLEDs and third-generation TADF (thermally activated delayed fluorescent) OLEDs. Phosphorescent OLEDs introduce heavy metal atoms into organic dye molecules. The spin-orbit coupling effect of these heavy metal atoms promotes electron spin flipping, allowing triplet excitons to return to the ground state and emit phosphorescence via radiative transitions. Besides directly utilizing the generated triplet excitons (T1), phosphorescent materials can also convert singlet excitons into triplet excitons through intersystem crossing processes. Therefore, the internal quantum efficiency (IQE) of phosphorescent OLED devices can theoretically reach 100%. Currently, red and green phosphorescent materials are used in commercial OLED devices. However, devices made from blue, especially deep blue, materials have short lifespans, limiting their large-scale commercial application. Compared to traditional fluorescent OLEDs, phosphorescent OLEDs have lower driving voltages and higher external quantum efficiency, but their disadvantages are also obvious. They require the use of expensive and rare precious metals, which greatly increases the cost of the device. They also pose potential environmental pollution problems, which greatly limits the development of phosphorescent OLEDs.

[0005] The latest generation of TADF OLEDs is based on purely organic small molecules. TADF molecules require minimizing the bandgap difference ΔE between the lowest excited singlet state S1 and the lowest excited triplet state T1. STThis process induces excitons to undergo a reverse intersystem crossing from the T1 state to the S1 state and then back to the S0 state to emit light. This characteristic allows TADF emitters to simultaneously utilize both singlet and triplet excitons for emission, achieving 100% exciton utilization. Furthermore, these materials are mostly pure organic small molecules, avoiding the use of precious metals and significantly reducing device fabrication costs. The ease of modification of small molecules also opens up more possibilities for TADF OLEDs.

[0006] Thermally activated delayed fluorescence (TADF) technology is a leading technology for third-generation OLEDs. TADF emitters are designed and synthesized using low-cost organic donor and acceptor derivatives. Compared to phosphorescent dopants based on metal complexes, TADF emitters are potential candidates for next-generation display technologies. To date, TADF materials have utilized donor units such as carbazole, aniline, and acridine, while acceptor units have included benzonitrs, triazines, and sulfones, as well as combinations of these units. Among these, triazine units are considered excellent structural units for constructing donor-acceptor TADF materials due to their good planarity, strong electron-withdrawing ability, and three modification sites.

[0007] However, TADF devices still face the problem of efficiency roll-off, meaning that the luminous efficiency of the device decreases significantly at high brightness. The main reason for this efficiency roll-off is that long-lived triplet excitons release energy at high concentrations through non-radiative transitions such as triplet-triplet annihilation (TTA) and triplet-polaron quenching. Microsecond-level delayed fluorescence lifetimes have been achieved through molecular structure modification, effectively utilizing triplet exciton luminescence to reduce efficiency roll-off. Short-lived TADF molecules can rapidly convert triplet excitons into singlet excitons, emitting light through radiative transitions, significantly reducing the accumulation of long-lived excitons and thus suppressing the efficiency roll-off of OLED devices at high brightness. Summary of the Invention

[0008] This invention discloses a method for preparing thermally activated delayed fluorescence (TADF) materials. Through specific molecular structure design, this invention modulates the molecular structure based on traditional triazine-based TADF materials by introducing a trifluoromethyl group. This enhances the acceptor's electron-withdrawing ability while simultaneously increasing the electron cloud separation distance, thereby reducing the overlap between the HOMO and LUMO, decreasing ΔEST, and exhibiting a faster delayed fluorescence lifetime at room temperature. This indicates a faster reverse intersystem crossing rate, which is beneficial for reducing the accumulation of long-lived triplet excitons, thus effectively suppressing the efficiency roll-off of TADF devices caused by triplet annihilation.

[0009] First, this invention provides a TADF material having the following general structural formula:

[0010]

[0011] R is selected from methyl, cyano, and hydrogen.

[0012] Furthermore, the molecular structure of the TADF material is as follows:

[0013]

[0014]

[0015]

[0016] As a preferred experimental scheme, in the general formula of the TADF material of the present invention: R is preferably derived from cyano.

[0017] As the optimal technical solution, the molecular structure of the TADF material described in this invention is as follows:

[0018]

[0019] The method for preparing TADF material molecules according to the present invention includes the following steps:

[0020]

[0021] S1: Synthesis of intermediate compound II:

[0022] 4-(trifluoromethyl)benzoamide hydrochloride and 2-fluoro-5-(hydroxymethyl)benzonitrile were reacted at a molar ratio of 1:1.2~1.5. After the reaction was completed, the reaction solution was extracted and dried, and then purified by silica gel column chromatography (petroleum ether: ethyl acetate = 10:1) to obtain intermediate compound II.

[0023] S2: Synthesis of TADF material I:

[0024] Intermediate compound II reacted with 9,10-dihydro-9,9-dimethylacridine at a molar ratio of 1:1.2~1.5. After the reaction was completed, the reaction solution was extracted and dried, and then purified by silica gel column chromatography (petroleum ether: dichloromethane = 5:1) to obtain TADF molecule I.

[0025] In the preparation methods described above in this invention, the solvents are preferably dehydrated solvents.

[0026] The purification methods described above in this invention all employ conventional methods without particular limitations. Dichloromethane and methanol are preferred as eluents for column separation, recrystallization, or a combination of both.

[0027] The raw materials used in the preparation methods described above in this invention can all be prepared by hand or by methods known in the art;

[0028] The structures of the compounds synthesized by the above preparation methods of the present invention were confirmed by high-resolution mass spectrometry and proton nuclear magnetic resonance spectroscopy.

[0029] The beneficial effects of this invention are as follows: The biggest drawback of traditional TADF materials is their long exciton lifetime (microseconds), which makes excitons more prone to triplet-triplet annihilation at high current densities. The TADF material in this application exhibits ultrashort nanosecond-level delayed fluorescence compared to traditional TADF materials, reducing triplet exciton stacking annihilation, improving exciton utilization, and effectively suppressing efficiency roll-off. The optimal TADF material in this invention achieves an efficiency roll-off of 7.87%, which is within the high range for traditional TADF materials. Attached Figure Description

[0030] Figure 1 This is the absorption spectrum of Example 1 in solvent state.

[0031] Figure 2 This is the absorption spectrum of Example 2 in solvent state.

[0032] Figure 3 This is the absorption spectrum of Example 3 in solvent state.

[0033] Figure 4 This is the fluorescence emission spectrum of Example 1 in solvent state.

[0034] Figure 5 This is the fluorescence emission spectrum of Example 2 in solvent state.

[0035] Figure 6 This is the fluorescence emission spectrum of Example 3 in solvent state.

[0036] Figure 7 This is the fluorescence lifetime curve of Example 1 in the thin film state at room temperature.

[0037] Figure 8 This is the fluorescence lifetime curve of Example 2 in the thin film state at room temperature.

[0038] Figure 9 This is the fluorescence lifetime curve of Example 3 in the thin film state at room temperature.

[0039] Figure 10 This is the thermogravimetric analysis curve of Example 1.

[0040] Figure 11 This is the thermogravimetric analysis curve of Example 2.

[0041] Figure 12 This is the thermogravimetric analysis curve of Example 3.

[0042] Figure 13 This is the brightness efficiency curve of Example 1.

[0043] Figure 14 This is the brightness efficiency curve of Example 2.

[0044] Figure 15 This is the brightness efficiency curve of Example 3. Detailed Implementation

[0045] The present invention will be further described below with reference to embodiments, but it should be understood that the scope of protection of the present invention is not limited to the embodiments. Unless otherwise specified, the experimental methods used are conventional methods, and the materials and reagents used can be purchased commercially.

[0046] The raw material specifications, process equipment and testing methods involved in the embodiments are all common knowledge in the field. Any adaptive adjustments made by those skilled in the art based on actual production conditions should be regarded as not departing from the protection scope of the core process of the present invention.

[0047] Example 1: Preparation of compound ACD-TRZ-CN:

[0048]

[0049] S1: Synthesis of intermediate compound II-1:

[0050]

[0051] In a 100 mL round-bottom flask, 1 g (5.315 mmol) of 4-(trifluoromethyl)benzoamide hydrochloride and 0.8033 g (5.315 mmol) of 2-fluoro-5-(hydroxymethyl)benzonitrile and 20 mL of toluene were added, followed by 0.1448 g (0.7972 mmol) of Cu(OOCCH3)2·H2O. The mixture was then heated under reflux for 24 hours. After the reaction was complete, the mixture was extracted three times with ethyl acetate and saturated sodium chloride aqueous solution. The organic phase was collected and dried over anhydrous sodium sulfate. The dried organic phase was concentrated and purified by silica gel column chromatography using a 10:1 (v / v) mixture of petroleum ether and ethyl acetate as the eluent. The petroleum ether and ethyl acetate mixture was then recrystallized to give a white flocculent solid intermediate II-1 in approximately 46.5% yield.

[0052] S2: Synthesis of TADF material ACD-TRZ-CN:

[0053]

[0054] 0.2 g (0.4098 mmol) of intermediate 5-(4,6-bis(4-(trifluoromethyl)phenyl)-1,3,5-triazin-2-yl)-2-fluorobenzonitrile, 0.0822 g (0.4918 mmol) of carbazole, 0.2 g of cesium carbonate, and 20 mL of N,N-dimethylformamide were added to a 50 mL round-bottom flask and refluxed at 120 °C for 12 h under nitrogen protection. The mixture was extracted three times with ethyl acetate and saturated sodium chloride aqueous solution, and the organic phase was collected and dried over anhydrous sodium sulfate. The dried mixture was concentrated and purified by column chromatography using a 5:1 (v / v) mixture of petroleum ether and dichloromethane as the eluent. The mixture was then recrystallized to give a yellow solid in approximately 45% yield. 1H NMR (400 MHz, Methylene Chloride-d2) δ 9.40 (s, 1H),9.34 - 9.24 (m, 1H), 8.97 (d, J = 8.1 Hz, 4H), 7.87 (dd, J = 39.2, 8.3 Hz,5H), 7.57 (d, J = 6.4 Hz, 2H), 7.08 - 6.91 (m, 4H), 6.25 (d, J = 5.6 Hz, 2H), 1.75 (s, 6H).

[0055] Example 2 Preparation of compound ACD-TRZ-Me:

[0056]

[0057] S1: Synthesis of intermediate compound II-2:

[0058] Same as step S1 in Example 1, except that 2-fluoro-5-(hydroxymethyl)benzonitrile is replaced with (4-bromo-3-methylphenyl)methanol to obtain intermediate compound II-2.

[0059] S2: Synthesis of TADF material ACD-TRZ-Me:

[0060]

[0061] 0.2 g (0.3724 mmol) of intermediate 2-(4-bromo-3-methylphenyl)-4,6-bis(4-(trifluoromethyl)phenyl)-1,3,5-triazine, 0.06219 g (0.3724 mmol) of 9,10-dihydro-9,9-dimethylacridine, 0.0478 g (0.4974 mmol) of sodium tert-butoxide, 0.0074 g (0.0332 mmol) of palladium acetate, 0.0134 g (0.0663 mmol) of tritert-butylphosphine, and 20 mL of toluene were added to a 50 mL round-bottom flask and refluxed at 120 °C for 12 h under nitrogen protection. The mixture was extracted three times with ethyl acetate and saturated sodium chloride aqueous solution, and the organic phase was collected and dried over anhydrous sodium sulfate. The dried mixture was concentrated and purified by column chromatography using a 5:1 (v / v) mixture of petroleum ether and dichloromethane as the eluent. The mixture was then recrystallized to give 135.9 mg of a yellow solid, with a yield of 53.3%. ¹H NMR (400 MHz, MethyleneChloride-d²) δ 9.02 - 8.80 (m, 5H), 8.04 - 7.84 (m, 5H), 7.51 (s, 3H), 6.96 (s, 4H), 6.21 (d, J = 7.8 Hz, 2H), 2.26 (s, 3H), 1.70 (s, 6H).

[0062] Example 3 Preparation of compound ACD-TRZ:

[0063]

[0064] S1: Synthesis of intermediate compound II-3:

[0065] Same as step S1 in Example 1, except that 2-fluoro-5-(hydroxymethyl)benzonitrile is replaced with 4-bromobenzyl alcohol to obtain intermediate compound II-3.

[0066] S2: Synthesis of TADF material ACD-TRZ:

[0067]

[0068] The same ACD-TRZ-Me synthesis method was used, except that intermediate II-2 was replaced with intermediate II-3, yielding a yellow solid with a yield of approximately 51.24%. ¹H NMR (400 MHz, Methylene Chloride-d²) δ 9.02 - 8.80 (m, 5H), 8.04 - 7.84 (m, 5H), 7.51 (s, 3H), 6.96 (s, 4H), 6.21 (d, J = 7.8 Hz, 2H), 2.26 (s, 3H), 1.70 (s, 6H).

[0069] Example 4:

[0070] TADF material molecules were dissolved in DCM solution to prepare a 3 mM stock solution. 0.3 mL of the stock solution was pipetted onto a quartz glass slide using a 1 mL pipette, and then heated at 70 °C for 15 min to remove the solvent. Once the sample was completely dry, it was ready for subsequent thin film testing. To prepare a liquid sample, 3 mL of the corresponding test solvent was pipetted into a cuvette, and then 10 μL of the stock solution was pipetted into the cuvette and mixed thoroughly.

[0071] Steady-state spectral testing:

[0072] UV-Vis absorption spectroscopy: The UV-Vis absorption spectra of the solution samples in the range of 200-800 nm were measured using a UV-Vis spectrophotometer. The absorption spectra show that molecules ACD-TRZ, ACD-TRZ-Me, and ACD-TRZ-CN all exhibit two solvent absorption peaks in different polar solvents. The strong and narrow absorption peak at 270 nm for all three molecules corresponds to the absorption of the π-π* transition in the benzene ring. The weak and broad absorption peaks at 390 nm, 400 nm, and 420 nm for the three molecules correspond to the absorption of the intramolecular charge-transfer (ICT) state.

[0073] Steady-state fluorescence spectroscopy: The maximum absorption wavelength in the UV-Vis absorption spectrum was selected as the excitation wavelength. Steady-state / transient fluorescence spectrometry was used for testing, with a xenon lamp as the light source. The fluorescence emission spectra of materials ACD-TRZ, ACD-TRZ-Me, and ACD-TRZ-CN are shown in the figure. The emission of molecules ACD-TRZ, ACD-TRZ-Me, and ACD-TRZ-CN in five solvents of different polarities all exhibited a significant solvation effect; the emission redshifted with increasing solvent polarity, a characteristic of typical ICT effect emission. In highly polar solvents, the fluorescence emission peak of molecule ACD-TRZ-CN was quenched, with energy consumed through non-radiative transitions.

[0074] Example 5:

[0075] Transient photoluminescence decay spectroscopy: A steady-state / transient fluorescence spectrometer was used. For nanosecond-level lifetime testing, single-photon counting (TCSPC) mode was selected. An EPL 375 or VPL 375 laser was chosen as the light source based on the maximum absorption wavelength in the UV-Vis absorption spectrum. Transient photoluminescence spectra of the samples in different solvents were tested. For microsecond-level lifetime testing, multichannel scanning (MCS) mode was used, with a VPL 375 laser as the light source. To verify whether the material possesses delayed fluorescence properties, the transient fluorescence spectra of molecules ACD-TRZ, ACD-TRZ-Me, and ACD-TRZ-CN in thin film state were measured. Their fluorescence decay curves are shown in the figure. All three molecules exhibit double exponential decay at room temperature and demonstrate a long lifetime in the microsecond range, proving that the molecules possess TADF properties. The shorter lifetime (τ1) is the instantaneous fluorescence process of a singlet exciton emitting light through radiative transition, while the longer lifetime (τ2) is the delayed fluorescence process of a triplet exciton upconverting to a singlet state via reverse intersystem crossing and then emitting light through radiative transition. For the molecule ACD-TRZ, τ1 = 14.21 ns and τ2 = 1.31 μs; for ACD-TRZ-CN-Me, τ1 = 14.03 ns and τ2 = 920 ns; and for ACD-TRZ-CN, τ1 = 14.07 ns and τ2 = 660 ns. The extremely short delayed fluorescence lifetime helps mitigate exciton annihilation in TADF-OLEDs and suppresses efficiency roll-off at high brightness. This extremely short delayed fluorescence is mainly attributed to the lowest singlet and triplet excited states characteristic of the near-degenerate CT features of the molecules.

[0076] Example 6:

[0077] Thermogravimetric analysis (TGA) was used for testing under argon protection. The program was set to increase the temperature from 50 °C to 100 °C at a rate of 20 K / min, and then hold at 100 °C for 100 min to remove any crystal water that might be present in the sample. The temperature was then increased from 100 °C to 500 °C at a rate of 10 K / min, and then decreased at a rate of 10 K / min. The temperature at which the sample lost 5% of its weight was taken as the thermal decomposition temperature (Tg). The thermal stability of the three molecules was studied by TGA under an argon atmosphere. As shown in the figure, the thermal decomposition temperatures of the three molecules at a 5% weight loss were 396.83 °C, 379.83 °C, and 338.83 °C, respectively. All three molecules exhibit good thermal stability and are suitable for fabricating OLED devices via vacuum evaporation.

[0078] Example 7:

[0079] Fabrication of TADF-OLED Devices and Electroluminescence Testing: All three molecules possess good photophysical properties and thermal stability, which is beneficial for fabricating OLED devices via vacuum evaporation. Using the three molecules as guest dopant layers in the OLED device and mCP as the host dopant layer (doping ratio 30%), the following OLED structure was fabricated: (ITO) / HAT-CN (5nm) / TAPC (20nm) / TCTA (5nm) / 30wt% emitters: mCP (20nm) / TmPyPB (40nm) / LiF (1nm) / Al (200nm). ITO, Al, HATCN, LiF, TAPC, and TmPyPB serve as the anode, cathode, hole injection layer, electron injection layer, hole transport layer, and electron transport layer, respectively. TCTA, due to its high triplet energy level, acts as an exciton blocking layer, restricting exciton recombination within the EML to generate photons and emit light. mCP serves as the host material for the doped device. The external quantum efficiencies of the molecules ACD-TRZ, ACD-TRZ-Me, and ACD-TRZ-CN are 10.06%, 12.13%, and 7.18%, respectively; the efficiency roll-offs are 13.7%, 11.16%, and 7.87%, respectively. Low efficiency roll-off is one of the hallmarks of high-performance devices.

[0080] The above embodiments are only used to illustrate the present invention. Any equivalent transformations and improvements made on the basis of the technical solutions of the present invention should not be excluded from the protection scope of the present invention.

Claims

1. A type of TADF material, characterized in that, The TADF material has the following general structural formula: ； R is selected from methyl, cyano, and hydrogen.

2. The TADF material according to claim 1, characterized in that, The structure of the TADF material is as follows: 。 3. The method for preparing TADF material according to claim 2, characterized in that, Includes the following steps: ； S1: Synthesis of intermediate compound II: 4-(trifluoromethyl)benzoamide hydrochloride and 2-fluoro-5-(hydroxymethyl)benzonitrile were reacted at a molar ratio of 1:1.2~1.

5. After the reaction was completed, the reaction solution was extracted and dried, and then purified by silica gel column chromatography to obtain intermediate compound II. S2: Synthesis of TADF material I: Intermediate compound II reacted with 9,10-dihydro-9,9-dimethylacridine at a molar ratio of 1:1.2~1.

5. After the reaction was completed, the reaction solution was extracted and dried, and then purified by silica gel column chromatography to obtain TADF molecule I.

4. The application of the TADF material according to claim 1, characterized in that: The TADF material is used in organic light-emitting diodes.

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