Low refractive index capping layer material, method of making and use thereof

Abstract

The application provides a low-refractive-index cover layer material, which is a carbazolyl-oxazole derivative and benzene as a mother nucleus, and a plurality of steric groups and electron-withdrawing groups are introduced to regulate the packing density and electron density of the molecule to adjust the polarizability of the molecule, so as to obtain a low-refractive-index cover layer material with a chemical structure shown in formula 1. By matching the low-refractive-index cover layer material with a high-refractive-index material, the light interference effect can be utilized to further improve the light extraction efficiency of an organic electroluminescent device. The refractive index of the compounds in examples 1-155 is 1.411-1.519 in each color gamut, which is lower than the refractive index of the comparative examples Ref1-Ref11, and the extinction coefficient k value is almost 0 in each color gamut, which does not affect the light emission of the light-emitting layer. The device performance of the device examples 1-155 prepared by combining the compounds prepared in examples 1-155 with the high-refractive-index CPL is better than the device performance of the device prepared by only containing the high-refractive-index CPL.

Description

Technical Field

[0001] This invention relates to the field of organic light-emitting materials technology, and in particular to a low refractive index capping layer material, its preparation method, and its application. Background Technology

[0002] Organic light-emitting diodes (OLEDs) are optoelectronic devices based on the electroluminescent properties of organic materials, typically consisting of an anode, an organic functional layer, and a cathode. The light efficiency of OLEDs is generally categorized into internal quantum efficiency and external quantum efficiency. External quantum efficiency refers to the efficiency with which light generated in the organic layer is extracted to the outside of the device; it is influenced by both internal quantum efficiency and light extraction efficiency.

[0003] Currently, a common approach to increase light extraction efficiency is to add a high-refractive-index capping layer (CPL) above the cathode. However, a single high-refractive-index CPL layer offers limited performance improvement, and the development of high-refractive-index materials faces bottlenecks, hindering further improvements in light extraction efficiency. Existing technologies utilize CPL layers with varying refractive indices to create a high-low refractive-index stacked film structure. This allows light emitted from within the device to pass through microcavities created by the high- and low-refractive-index layers, interfering with each other and thus emitting specific wavelengths of light emitted from the device. This narrows the emission spectrum, enhances the intensity of specific wavelengths, and improves the device's luminous efficiency. Therefore, developing a low-refractive-index CPL to complement a high-refractive-index CPL to improve device luminous efficiency while maintaining excellent overall performance in terms of device lifetime and driving voltage has significant practical application value. Summary of the Invention

[0004] The purpose of this invention is to provide a low refractive index capping layer material, its preparation method, and its application. The low refractive index capping layer material provided by this invention has a low refractive index, and when combined with a high refractive index capping layer material, it can further improve the luminous efficiency of organic electroluminescent devices by utilizing optical interference effects.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides a low refractive index capping layer material having a chemical structure as shown in Formula 1:

[0007]

[0008] In Equation 1, X is selected from O or NR, wherein R has the structure shown in Equation 2;

[0009]

[0010] In Equation 2, q is 2, 3, 4, or 5;

[0011] In Formula 2, R' is one or more of fluoro, cyano, tert-butyl, or trifluoromethyl.

[0012] In Formula 1, R1, R2, and R3 are each independently selected from fluorinated, cyaninated, unsubstituted C1-C8 alkyl, unsubstituted C3-C5 silyl, and C1-C8 alkyl completely substituted with fluorine, and at least one of R1, R2, and R3 is selected from fluorinated, cyaninated, and C1-C8 alkyl completely substituted with fluorine.

[0013] In Equation 1, L1 and L2 are each independently selected from the connecting bond, and unsubstituted C6 to C6. 18 The aryl group, unsubstituted C6-C 12 heteroaryl groups;

[0014] In Equation 1, m is 1, 2, 3, or 4;

[0015] In Equation 1, n and p are each independently 1, 2, 3, 4 or 5.

[0016] Preferably, in Formula 1, R1, R2, and R3 are each independently selected from fluorinated, cyaninated, unsubstituted C1-C4 alkyl, trimethylsilyl, and C1-C4 alkyl that is completely substituted with fluorine, and at least one of R1, R2, and R3 is selected from fluorinated, cyaninated, and C1-C4 alkyl that is completely substituted with fluorine.

[0017] In Equation 1, L1 and L2 are each independently selected from the connecting bond, and unsubstituted C6 to C6. 12 The aryl group, unsubstituted C6-C 10 Hybrid aryl groups.

[0018] Preferably, in Formula 1, R1, R2, and R3 are each independently selected from fluorinated, cyano, unsubstituted C1-C4 alkyl, trimethylsilyl, and C1-C4 alkyl completely substituted with fluorine, and at least one of R1, R2, and R3 is selected from fluorinated, cyano, and trifluoromethyl.

[0019] In Formula 1, L1 and L2 are each independently selected from the linking bond, unsubstituted phenylene, and unsubstituted naphthylene.

[0020] Preferably, in Formula 1, R1 is selected from fluorinated, cyano, unsubstituted C1-C4 alkyl, trimethylsilyl, and C1-C4 alkyl that has been completely fluorinated;

[0021] In Formula 1, R2 and R3 are simultaneously selected from fluorinated, cyaninated, unsubstituted C1-C4 alkyl, trimethylsilyl, and C1-C4 alkyl that has been completely substituted with fluorine, and R2 and R3 are the same.

[0022] In Formula 1, L1 and L2 are both selected from a linking bond, an unsubstituted phenylene, or an unsubstituted naphthylene, and L1 and L2 are the same.

[0023] In Equation 1, m is 1 or 2;

[0024] In Equation 1, n and p can both be represented as 2 or 3;

[0025] Preferably, the low-refractive-index capping layer material has the chemical structure shown in Formulas 1-1 to 1-18:

[0026]

[0027]

[0028] The present invention also provides a method for preparing the low refractive index capping layer material described in the above technical solution, characterized by comprising the following steps:

[0029] The preparation route of the low-refractive-index capping layer material is shown below:

[0030]

[0031]

[0032] Among them, R” and R”' are each independently selected from

[0033] R1, R2, R3, n, p, m, L1, L2, X are the same as the ranges mentioned above;

[0034] Hal, Hal1, and Hal2 are each independently selected from -F, -Cl, -Br, or -I;

[0035] (1) Under nitrogen protection, compound b (1.0 eq), compound a (1.0-1.5 eq), base (2.0-2.5 eq), palladium catalyst (0.01-0.02 eq), toluene, ethanol, and water (V) were added sequentially to a round-bottom flask. 甲苯 V 乙醇 :V 水 =2-4:1:1), heat to 60-90℃, reflux for 8-12 hours, cool to room temperature, add water, filter after the solid has precipitated completely, dry the filter cake, purify by column chromatography, remove the solvent from the filtrate using a rotary evaporator, dry the obtained solid to obtain intermediate 1.

[0036] (2) Under nitrogen protection, compound C (1.0 eq), intermediate 1 (1.0-1.5 eq), base (2.0-2.5 eq), palladium catalyst (0.01-0.02 eq), toluene, ethanol, and water (V) were added sequentially to a round-bottom flask.甲苯 V 乙醇 :V 水 =2-4:1:1), heat to 60-90℃, reflux for 8-12 hours, cool to room temperature, add water, filter after the solid has precipitated completely, dry the filter cake, purify by column chromatography, remove the solvent from the filtrate with a rotary evaporator, dry the obtained solid to obtain intermediate 2.

[0037] (3) Under nitrogen protection, intermediate 2 (1.0 eq) and compound d (1.1-1.5 eq) were completely dissolved in xylene in a round-bottom flask. Then, alkali (2.0-2.5 eq), palladium catalyst (0.01-0.05 eq), and phosphine ligand (0.02-0.15 eq) were added. The resulting product was then heated to 130-140 °C and stirred for 8-12 hours. The mixture was filtered hot using diatomaceous earth. After cooling the filtrate to room temperature, water was added to the filtrate for washing. The organic phase was separated, and the aqueous phase was extracted with ethyl acetate. The combined organic layers were then dried with magnesium sulfate and purified by column chromatography to obtain a low-refractive-index capping layer material with the chemical structure shown in Formula 1.

[0038] Preferably, the palladium catalyst is: Pd2(dba)3, Pd(PPh3)4, PdCl2, PdCl2(dppf), Pd(OAc)2, Pd(PPh3)2Cl2, or NiCl2(dppf).

[0039] Preferably, the phosphine ligand is: P(t-Bu)3, X-phos, PET3, PMe3, PPh3, KPPh2, or P(t-Bu)2Cl.

[0040] Preferably, the alkali is: K2CO3, K3PO4, Na2CO3, CsF, Cs2CO3, or t-BuONa.

[0041] This invention provides a low-refractive-index capping layer material, using a substituted carbazole-oxazole derivative benzene as the core, and introducing multiple steric hindrance groups and electron-withdrawing groups to regulate the molecular packing density and electron density, thereby adjusting the molecular polarizability. This results in a series of low-refractive-index capping layer materials with the chemical structure shown in Formula 1. By combining these materials with high-refractive-index materials, the light extraction efficiency of organic electroluminescent devices can be further improved using optical interference effects. The results of the examples show that the compounds prepared in Examples 1-155 of this invention have refractive indices of 1.411-1.519 in each color gamut, lower than the refractive indices (1.507-2.410) of the comparative examples (Ref 1-Ref 11), and the extinction coefficient k is almost zero in each color gamut, not affecting the luminescence of the light-emitting layer. Furthermore, the devices prepared by combining the compounds prepared in Examples 1-155 of this invention with high-refractive-index CPL exhibit better performance than devices prepared using only high-refractive-index CPL. Compared to existing low-refractive-index materials, the compound provided by this invention exhibits higher luminous efficiency under the same device conditions, indicating that the low-refractive-index capping layer material and high-refractive-index CPL provided by this invention can better concentrate light and further reduce light loss. Attached Figure Description

[0042] Figure 1 The proton NMR spectrum of intermediate 1-268 prepared in Example 1 of this invention;

[0043] Figure 2 The image shows the hydrogen nuclear magnetic resonance spectrum of compound 268 in Example 1 of this invention. Detailed Implementation

[0044] In this invention, the low-refractive-index capping layer material preferably has one or more of the following compounds: compound 1 to compound 459:

[0045]

[0046]

[0047]

[0048]

[0049]

[0050]

[0051]

[0052]

[0053]

[0054]

[0055]

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[0060]

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[0065]

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[0068]

[0069]

[0070]

[0071]

[0072]

[0073]

[0074]

[0075]

[0076] Unless otherwise specified, all raw materials used in this invention are commercially available products in the art.

[0077] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0078] This invention involves a series of palladium-catalyzed coupling reactions. On one hand, it utilizes the difference in reactivity between Br and F, Cl, and I, which are greater than F, Cl, and Br. On the other hand, it controls the reaction sites by regulating reaction conditions. Furthermore, the reaction is purified using column chromatography or a silica gel funnel to remove byproducts, yielding the target compound. The following general knowledge is referenced:

[0079] Organometallic Chemistry (6th Edition), Robert H. Crabtree, published by East China University of Science and Technology Press, Shanghai, September 00, 2017, ISBN: 978-7-5628-5111-0, page 388.

[0080] Organic Chemistry and Optoelectronic Materials Experiment Tutorial, Chen Runfeng, Publisher: Southeast University Press, Publication Date: 2019-11-00, ISBN: 9787564184230, Page 174.

[0081] Example 1

[0082]

[0083] (1) Under nitrogen protection, compound b-268 (50 mmol, CAS: 2097253-68-2), compound a-268 (75 mmol, CAS: 200290-11-5), K2CO3 (110 mmol), Pd(PPh3)4 (0.5 mmol), toluene, ethanol, and water (V) were added sequentially to a round-bottom flask. 甲苯 V 乙醇 :V 水 =150mL:50mL:50mL), heated to 65℃, refluxed for 8 hours, cooled to room temperature, water was added, and after the solid precipitated completely, it was filtered. The filter cake was dried, purified by column chromatography, and the solvent was removed from the filtrate using a rotary evaporator. The obtained solid was dried to give intermediate 1-268 (16.39 g, yield: 67%, MS (ESI, m / Z): [M+H)). + =489.19, mass spectrometer model is Waters XEVO TQD, low precision, tested using ESI source, the 1H NMR spectrum of intermediate 1-268 is as follows Figure 1 (As shown).

[0084] (2) Under nitrogen protection, compound c-268 (50 mmol, CAS: 73852-19-4), intermediate 1-268 (70 mmol), K2CO3 (110 mmol), Pd(PPh3)4 (0.5 mmol), toluene, ethanol, and water (V) were added sequentially to a round-bottom flask. 甲苯 V 乙醇 :V 水 =150mL:50mL:50mL), heated to 65℃, refluxed for 8 hours, cooled to room temperature, water was added, and after the solid precipitated completely, it was filtered. The filter cake was dried and purified by column chromatography. The filtrate was used to remove the solvent using a rotary evaporator, and the obtained solid was dried to give intermediate 2-268 (20.69 g, yield: 62%, MS (ESI, m / Z): [M+H)). + =667.28, mass spectrometer model is Waters XEVO TQD, low precision, tested using ESI source).

[0085] (3) Under nitrogen protection, intermediate 2-268 (50 mmol) and compound d-268 (60 mmol, CAS: 114997-92-1) were completely dissolved in xylene in a round-bottom flask. Then, t-BuONa (100 mmol), Pd(OAc)2 (0.8 mmol), and X-phos (1.0 mmol) were added. The resulting product was heated to 130 °C and stirred for 10 hours. The mixture was filtered hot using diatomaceous earth. After cooling the filtrate to room temperature, water was added to wash the filtrate. The organic phase was separated, and the aqueous phase was extracted with ethyl acetate. The combined organic layers were then dried with magnesium sulfate and purified by column chromatography to give compound 268 (23.06 g, yield: 57%, MS (ESI, m / Z): [M+H)). + =809.27, mass spectrometer model is Waters XEVOTQD, low precision, ESI source is used for testing).

[0086] The proton NMR spectrum of compound 268 is as follows: Figure 2 As shown.

[0087] Characterization:

[0088] HPLC purity: >99.8%.

[0089] Elemental analysis:

[0090] Theoretical values: C, 62.31; H, 2.37; F, 28.16; N, 5.19; O, 1.98

[0091] Test values: C, 62.21; H, 2.43; F, 28.17; N, 5.21; O, 2.02

[0092] Examples 2 to 155

[0093] Following the synthesis method of Example 1, the corresponding raw materials were replaced, and the tests were performed using a WatersXEVOTQD mass spectrometer. Due to its low precision, an ESI source was used for testing. The corresponding compounds prepared in Examples 2 to 155 and their corresponding mass spectrometry test values ​​are shown in Table 1 below.

[0094] Table 1. Mass spectrometry data of the compounds prepared in Examples 2 to 155.

[0095]

[0096]

[0097]

[0098]

[0099] Preparation of monolayer films for optical property evaluation

[0100] Evaporated films with a thickness of 80 nm were fabricated on silicon substrates using Examples 1 to 155 and compounds Ref1 to Ref11. The refractive index n and extinction coefficient k at wavelengths of 460 nm, 530 nm, and 620 nm were measured. The measured data are shown in Table 2 below. The structural formulas of compounds Ref1 to Ref11 are shown below.

[0101]

[0102]

[0103] Table 2. Test results of refractive index n and extinction coefficient k of compounds prepared in Examples 1 to 155 and compounds Ref1 to Ref11.

[0104]

[0105]

[0106]

[0107]

[0108]

[0109]

[0110] As shown in Table 2, the compounds prepared in Examples 1 to 155 of this invention have a refractive index of 1.411 to 1.519 in each color gamut, which is lower than the refractive index (1.507 to 2.410) of the comparative examples (Ref1 to Ref11), and the extinction coefficient k is almost 0 in each color gamut, which does not affect the luminescence of the light-emitting layer.

[0111] Among them, Ref11 and compound 353 are parallel comparative examples. The only structural difference is whether there is a cyano group substitution on the benzene ring fused to the oxazole. As can be seen from the data in Table 2, the introduction of the cyano group reduces the electron density, resulting in a decrease in the refractive index of compound 353 in all color gamuts.

[0112] Device Example 1:

[0113] a. ITO anode: The ITO (indium tin oxide)-Ag-ITO (indium tin oxide) glass substrate with a coating thickness of 150nm is cleaned twice in distilled water and ultrasonically washed for 30 minutes. Then it is cleaned twice more in distilled water and ultrasonically washed for 10 minutes. After washing, it is baked in a vacuum oven at 220℃ for 2 hours. After baking, it is cooled down and can be used. The obtained substrate is used as the ITO anode. The device process is carried out by evaporation deposition machine. Other functional layers are deposited on it in sequence.

[0114] b. HIL (Hole Injection Layer): The evaporation rate is such that hole injection layer materials HT and P-dopant are vacuum evaporated onto the ITO anode surface obtained in step a, wherein the evaporation rate ratio of HT and P-dopant is 97:3 and the thickness is 10nm.

[0115] c. HTL (Hole Transport Layer): The evaporation rate is such that a 130 nm HT layer is vacuum-deposited on the hole injection layer obtained in step b as a hole transport layer.

[0116] d. Prime (Emitting Assist Layer): with The evaporation rate is such that a 5 nm Prime layer is vacuum-deposited on the hole transport layer obtained in step c as a light-emitting auxiliary layer.

[0117] e. EML (Emitting Layer): Then, on the emitting auxiliary layer obtained in step d, with... The evaporation rate is such that a host material and a dopant material with a thickness of 20 nm are vacuum-evaporated as the light-emitting layer, and the evaporation rate ratio of the host to the dopant is 98:2.

[0118] f. HB (hole blocking layer): with The evaporation rate is such that a hole blocking layer with a thickness of 5 nm is vacuum-deposited on the light-emitting layer obtained in step e.

[0119] g. ETL (Electron Transport Layer): The deposition rate is such that ET and Liq with a thickness of 35 nm are vacuum-deposited on the hole blocking layer obtained in step f as an electron transport layer. The deposition rate ratio of ET to Liq is 50:50.

[0120] h, EIL (Electron Injection Layer): with At a certain evaporation rate, a Yb film layer of 1.0 nm is deposited on the electron transport layer obtained in step g to form an electron injection layer;

[0121] i. Cathode: with The evaporation rate ratio is 1:9. Magnesium and silver are evaporated at 18 nm on the electron injection layer obtained in step h to obtain an OLED device, i.e., a substrate with complete evaporation.

[0122] j. CPL-1 (low refractive index layer): with The evaporation rate was such that compound 268 of the present invention with a thickness of 20 nm was vacuum-deposited on the cathode as a low refractive index CPL.

[0123] k, CPL-2 (high refractive index layer): with The evaporation rate is such that a CPL with a thickness of 50 nm is vacuum-deposited on the low-refractive-index CPL obtained in step j, which is then used as a high-refractive-index CPL.

[0124] l. Encapsulate the vapor-deposited substrate obtained in step i: First, use a coating equipment to coat the cleaned cover plate with UV adhesive. Then, move the coated cover plate to the pressing section, place the vapor-deposited substrate on the top of the cover plate, and finally bond the substrate and cover plate together under the action of the bonding equipment, while simultaneously completing the UV adhesive photocuring.

[0125] The structural formulas of the compounds p-dopant, HT, Host, Dopant, Prime, HB, ET, and CPL used in Example 1 of the above device are shown below:

[0126]

[0127] Device Examples 2-155:

[0128] Referring to the method provided in Device Example 1 above, compounds 2 to 155 in Examples 2 to 155 were used to replace compound 268 in Device Example 1, and corresponding organic electroluminescent devices were prepared, which are respectively referred to as Device Examples 2 to 155.

[0129] Comparative Examples 1-3, 5-6, and 10-11 were prepared by replacing compound 268 in Device Example 1 with Ref1, Ref2, Ref3, Ref5, Ref6, Ref10, and Ref11 respectively, using the method provided in Device Example 1. These were respectively designated as Comparative Examples 1, 2, 3, 5, 6, 10, and 11.

[0130] Comparative Examples 4 and 7-9 follow the method provided in Device Example 1, replacing compound 268 in Device Example 1 with Ref4, Ref7, Ref8, and Ref9 respectively, and replacing CPL in CPL-2 with Ref4, Ref7, Ref8, and Ref9 respectively. These are designated as Comparative Examples 4, 7, 8, and 9. That is, the CPL-1 and CPL-2 layer materials are the same in Comparative Examples 4 and 7-9.

[0131] The driving voltage, luminous efficiency, BI value and lifetime of the organic electroluminescent devices obtained in Examples 1 to 155 and Comparative Examples 1 to 11 were characterized at a brightness of 1000 nits. The test results are shown in Table 3 below.

[0132] Table 3. Test results of luminescence characteristics of devices in Examples 1-155 and Comparative Examples 1-11

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[0140] As shown in Table 3, the devices prepared by combining the compounds prepared in Examples 1 to 155 of this invention with high-refractive-index CPL exhibit better performance than devices prepared using only high-refractive-index CPL. Compared to existing low-refractive-index materials, the compounds provided by this invention show higher luminous efficiency under the same device conditions, indicating that the low-refractive-index capping layer material and high-refractive-index CPL provided by this invention can better concentrate light and further reduce light loss.

[0141] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A low-refractive-index capping material having a chemical structure as shown in Formula 1: In Equation 1, X is selected from O or NR, where, The R has the structure shown in Equation 2; In Equation 2, q is 2, 3, 4, or 5; In Formula 2, R' is one of fluorine, cyano, tert-butyl or trifluoromethyl; In Formula 1, R1, R2, and R3 are each independently selected from fluorine, cyano, unsubstituted C1-C8 alkyl, unsubstituted C3-C5 silyl, and C1-C8 alkyl completely substituted with fluorine, and at least one of R1, R2, and R3 is selected from fluorine, cyano, and C1-C8 alkyl completely substituted with fluorine. In Equation 1, L1 and L2 are each independently selected from the connecting bond, and unsubstituted C6~C. 18 The aryl group, unsubstituted C6~C 12 heteroaryl; In Equation 1, m is 1, 2, 3, or 4; In Equation 1, n and p are each independently 1, 2, 3, 4 or 5.

2. The low refractive index capping layer material according to claim 1, characterized in that, In Formula 1, R1, R2, and R3 are each independently selected from fluorine, cyano, unsubstituted C1-C4 alkyl, trimethylsilyl, and C1-C4 alkyl that is completely substituted with fluorine, and at least one of R1, R2, and R3 is selected from fluorine, cyano, and C1-C4 alkyl that is completely substituted with fluorine. In Equation 1, L1 and L2 are each independently selected from the connecting bond, and unsubstituted C6~C. 12 The aryl group, unsubstituted C6~C 10 Hybrid aryl groups.

3. The low refractive index capping layer material according to claim 1 or 2, characterized in that, In Formula 1, R1, R2, and R3 are each independently selected from fluorine, cyano, unsubstituted C1-C4 alkyl, trimethylsilyl, and C1-C4 alkyl that is completely substituted with fluorine, and at least one of R1, R2, and R3 is selected from fluorine, cyano, and trifluoromethyl. In Formula 1, L1 and L2 are each independently selected from the linking bond, unsubstituted phenylene, and unsubstituted naphthylene.

4. The low refractive index capping layer material according to claim 2, characterized in that, In Formula 1, R1 is selected from fluorine, cyano, unsubstituted C1-C4 alkyl, trimethylsilyl, and C1-C4 alkyl that has been completely substituted with fluorine; In Formula 1, R2 and R3 are simultaneously selected from fluorine, cyano, unsubstituted C1-C4 alkyl, trimethylsilyl, and C1-C4 alkyl completely substituted with fluorine, and R2 and R3 are the same. In Formula 1, L1 and L2 are both selected from a linking bond, an unsubstituted phenylene, or an unsubstituted naphthylene, and L1 and L2 are the same. In Equation 1, m is 1 or 2; In Equation 1, n and p are each independently 2 or 3.

5. The low refractive index capping layer material according to claim 1, characterized in that, The low-refractive-index capping material has the chemical structures shown in Formulas 1-1 to 1-18: 。 6. A method for preparing the low refractive index capping layer material according to any one of claims 1 to 5, characterized in that, Includes the following steps: The preparation route of the low-refractive-index capping layer material is shown below: Among them, R” and R”' are each independently selected from R1, R2, R3, n, p, m, L1, L2, X as described in any one of claims 1-5; Hal, Hal1, and Hal2 are each independently selected from -F, -Cl, -Br, or -I; (1) Under nitrogen protection, compound b, compound a, base, palladium catalyst, toluene, ethanol and water were mixed and heated under reflux to obtain intermediate 1. (2) Under nitrogen protection, compound c, intermediate 1 obtained in step (1), base, palladium catalyst, toluene, ethanol and water are heated and refluxed in sequence to obtain intermediate 2; (3) Under nitrogen protection, after the intermediate 2 and compound d obtained in step (2) are completely dissolved in xylene, an alkali, a palladium catalyst and a phosphine ligand are added, and then a substitution reaction is carried out in sequence to obtain a low refractive index capping material with the chemical structure shown in Formula 1.

7. The preparation method according to claim 6, characterized in that, The alkali is K2CO3, K3PO4, Na2CO3, CsF, Cs2CO3 or t-BuONa.

8. The preparation method according to claim 6, characterized in that, The palladium catalyst is Pd2(dba)3, Pd(PPh3)4, PdCl2, PdCl2(dppf), Pd(OAc)2, Pd(PPh3)2Cl2 or NiCl2(dppf).

9. The preparation method according to claim 6, characterized in that, In step (3), the phosphine ligand is P(t-Bu)3, X-phos, PET3, PMe3, PPh3, KPPh2 or P(t-Bu)2Cl.

10. The use of the low refractive index capping material according to any one of claims 1 to 5 in organic electroluminescent devices.

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