Compound with bis-arylamine structure and organic electroluminescent device containing compound

Abstract

The invention discloses a compound with a bis-arylamine structure and an organic electroluminescent device containing the compound, and belongs to the technical field of semiconductors. The structure of the bis-arylamine compound is shown in a general formula (1), fluorenyl and phenyl serve as bridging groups, one arylamine is connected with a benzene ring on one side of the fluorenyl through phenylene, the arylamine and the fluorenyl must be located at the ortho-position of the phenylene, the other arylamine is directly connected to a benzene ring on the other side of the fluorenyl through a single bond, and the structure of the bis-arylamine compound is as shown in the general formula (1). Through the connection mode, the compound disclosed by the invention has an excellent hole transport rate, so that when the compound disclosed by the invention is applied as a hole transport material, the efficiency and the service life of a device can be remarkably improved.

Description

[0001] This invention application is based on the prior application number: 2022110292015, the application date: August 26, 2022, and the invention name: A compound with a diaromatic amine structure and an organic electroluminescent device containing the same. Technical Field

[0002] The present invention relates to the field of semiconductor technology, in particular to a compound with a diarylamine structure and an organic electroluminescent device containing the compound. Background Art

[0003] The carriers (holes and electrons) in an organic electroluminescent device (OLED) are injected into the device by the two electrodes of the device under the drive of an electric field, and meet in the organic light-emitting layer to recombine and emit light. High-performance organic electroluminescent devices require various organic functional materials to have good photoelectric properties. For example, as a charge transport material, it is required to have good carrier mobility. The injection and transport properties of the hole injection layer materials and hole transport layer materials used in existing organic electroluminescent devices are relatively weak, and the hole injection and transport rates do not match the electron injection and transport rates, resulting in a large offset in the recombination area, which is not conducive to the stability of the device. In addition, the reasonable energy level matching of the hole injection layer material and the hole transport layer material is an important factor in improving the efficiency and life of the device. Therefore, how to adjust the balance between holes and electrons and adjust the recombination area has always been an important topic in this field.

[0004] Blue organic electroluminescent devices have always been the weak point in the development of full-color OLEDs. Up to now, the efficiency and lifespan of blue light devices have been difficult to be comprehensively improved. Therefore, how to improve the performance of such devices remains a crucial issue and challenge facing this field. Most of the blue light host materials currently used in the market are electron-biased hosts. Therefore, in order to adjust the carrier balance of the light-emitting layer, hole transport materials need to have excellent hole transport properties. The better the hole injection and transport, the more the recombination area will be adjusted to shift away from the electron blocking layer, thereby emitting light away from the interface, which improves the performance of the device. At present, with the increase in the transmission rate of electron transport materials, the requirements for hole transport materials are getting higher and higher. However, the current industrialized hole transport materials can no longer meet the demand, which seriously hinders the improvement of the comprehensive performance of blue light devices. Therefore, there is an urgent need for hole transport materials with higher hole transport rates.

[0005] Because hole transport materials have a relatively thick film thickness, the heat resistance and amorphous nature of the material have a crucial impact on the life of the device. Materials with poor heat resistance are prone to decomposition during the evaporation process, contaminating the evaporation cavity and shortening the life of the device; materials with poor film phase stability will crystallize during the use of the device, reducing the service life of the device. Therefore, hole transport materials are required to have high film phase stability and decomposition temperature during use. However, the development of materials for stable and effective organic material layers for organic electroluminescent devices has not yet been fully realized. Therefore, there is a need to continuously develop new materials to better meet the performance requirements of organic electroluminescent devices. Summary of the Invention

[0006] In response to the above-mentioned problems existing in the prior art, the present invention proposes a compound with a diaromatic amine structure, with a fluorenyl group and a phenyl group as a bridging group, wherein one aromatic amine is connected to the benzene ring on one side of the fluorenyl group through a phenylene group, and this aromatic amine and the fluorenyl group must be in the ortho position of the phenylene group, and the other aromatic amine is directly connected to the benzene ring on the other side of the fluorenyl group through a single bond. This connection method enables the compound of the present invention to have an excellent hole transport rate, so that the compound of the present invention can be used as a hole transport material to significantly improve the efficiency and life of the device.

[0007] The technical solution of the present invention is as follows: a compound with a diarylamine structure, the compound structure is shown in the general formula (1):

[0008]

[0009] In the general formula (1), R1 to R4 are independently substituted or unsubstituted C 6-30 Aryl, substituted or unsubstituted C2-C 30 heteroaryl;

[0010] The L1 to L4 are independently a straight chain bond, a substituted or unsubstituted C 6-30 Arylene, substituted or unsubstituted C2-C 30 heteroarylene;

[0011] The substituents of the "substituted or unsubstituted" group are selected from deuterium atoms, C 1-10 Alkyl, C 6-30 Aryl, C2-C 30 one or more of heteroaryl groups;

[0012] In the general formula (1) Indicates whether there is a single bond connection between the two side groups or not;

[0013] The heteroatom is selected from one or more of an oxygen atom, a sulfur atom or a nitrogen atom.

[0014] The present invention also provides an organic electroluminescent device, which comprises an anode, a hole transport region, a light emitting region, an electron transport region and a cathode in sequence, wherein the hole transport region contains the diarylamine structural compound.

[0015] The beneficial technical effect of the present invention lies in: the technical core of the present invention lies in that the compound of the present invention has such structural characteristics that the biarylamine structure uses fluorenyl and phenyl as bridging groups, one of the arylamines is connected to the benzene ring on one side of the fluorenyl group through a phenylene group, and this arylamine and fluorenyl group must be in the ortho position of the phenylene group, and the other arylamine is directly connected to the benzene ring on the other side of the fluorenyl group through a single bond. This connection method gives the compound of the present invention the following advantages;

[0016] (1) This connection method enables the compound of the present invention to have excellent hole mobility. The improvement of hole mobility allows more holes to be injected into the main body, and the increase of exciton concentration in the main body increases the TTA efficiency and exciton utilization, thereby improving the device efficiency.

[0017] (2) This connection method enables the compound of the present invention to have a suitable energy level. The reasonable energy level matching makes the injection and conduction of carriers smoother, prevents accumulation at the interface, and is conducive to improving the interface stability. Excellent interface stability is conducive to improving the life of the device.

[0018] (3) This connection method is conducive to improving the glass transition temperature of the molecules and at the same time helping to reduce the evaporation temperature of the molecules. That is to say, even if the molecular weight of the molecules is relatively high, it can ensure a lower evaporation temperature. This excellent performance is not only conducive to the thermal evaporation of materials, but also controls the thermal decomposition rate of materials, thereby improving the stability of the materials in device applications.

[0019] Furthermore, for the biarylamine molecular structure characterized by the present invention, optimizing the ligands attached to the aromatic amines can further enhance the material's performance. For example, selecting groups or group derivatives (containing parallel ring structures or substituted structures) with strong planarity or large structural radius, such as biphenyl, furanyl, fluorenyl, and phenanthrene, can further enhance the material's stability and mobility, while also facilitating precise control of the material's HOMO energy level, thereby achieving excellent device performance.

[0020] The organic functional materials that constitute OLED devices include not only hole injection conductive materials but also electron injection conductive materials and light-emitting layer materials. Good device application effects require a good carrier balance as a guarantee. Therefore, in order to obtain the best device application effects, the compounds of the diarylamine structure that match the characteristic structure of the present invention also require specific electronic materials to be matched. Based on the in-depth research of the inventors, the electronic materials are preferably materials containing nitrogen-containing benzene structural characteristics, such as triazine materials, pyridine materials, pyrazine materials, etc. or derivatives containing these characteristic groups. The compounds of the diarylamine structure of the present invention are combined with nitrogen-containing benzene ring electron transport materials to make it easy for electrons and holes to obtain an optimal balance state, and have high efficiency while also having excellent lifespan. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic cross-sectional view of the organic electroluminescent device of the present invention.

[0022] In the figure: 1 is a transparent substrate layer, 2 is an anode layer, 3 is a hole injection layer, 4 is a hole transport layer, 5 is an electron blocking layer, 6 is a light-emitting layer, 7 is a hole blocking layer, 8 is an electron transport layer, 9 is an electron injection layer, 10 is a cathode layer, and 11 is a CPL layer.

[0023] Figure 2 is the NMR spectrum of compound 4 of the present invention. DETAILED DESCRIPTION

[0024] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0025] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0026] In the present invention, unless otherwise specified, HOMO means the highest occupied molecular orbital of a molecule, and the HOMO energy level is expressed in absolute values.

[0027] In the present disclosure, when a layer or element is referred to as being "on" another layer or substrate, the layer or element may be directly on the other layer or substrate, or intervening layers may also be present. Furthermore, it will be understood that when a layer is referred to as being "between" two layers, the layer may be the only layer between the two layers, or one or more intervening layers may also be present. Like reference numerals refer to like elements throughout.

[0028] In the present invention, when describing electrodes, organic electroluminescent devices, and other structures, terms such as "upper," "lower," "top," and "bottom" that indicate orientation refer only to a particular state and do not imply that the structure can exist only in the described orientation. Conversely, if the structure can be repositioned, such as inverted, the orientation of the structure will change accordingly. Specifically, in the present invention, the "bottom" or "lower" side of an electrode refers to the side of the electrode closest to the substrate during fabrication, while the opposite side, farther from the substrate, is the "top" or "upper" side.

[0029] In this specification, the term "substituted" means that one or more hydrogen atoms on a designated atom or group are replaced by a designated group, provided that the normal valency of the designated atom is not exceeded under the existing circumstances.

[0030] In the present specification, the substituted or unsubstituted fluorenyl group refers to a substituted or unsubstituted dimethylfluorenyl group, a substituted or unsubstituted diphenylfluorenyl group, or a substituted or unsubstituted spirofluorenyl group.

[0031] More specifically, substituted or unsubstituted C6-C 30 Aryl and / or substituted or unsubstituted C2-C 30 The heterocyclic group is a substituted or unsubstituted phenyl group, a substituted or unsubstituted naphthyl group, a substituted or unsubstituted anthracenyl group, a substituted or unsubstituted phenanthrenyl group, a substituted or unsubstituted tetraphenyl group, a substituted or unsubstituted pyrenyl group, a substituted or unsubstituted biphenyl group, a substituted or unsubstituted p-terphenyl group, a substituted or unsubstituted m-terphenyl group, a substituted or unsubstituted substituted or unsubstituted triphenylene, substituted or unsubstituted perylene, substituted or unsubstituted indenyl, substituted or unsubstituted furyl, substituted or unsubstituted thienyl, substituted or unsubstituted pyrrolyl, substituted or unsubstituted pyrazolyl, substituted or unsubstituted imidazolyl, substituted or unsubstituted triazolyl, substituted or unsubstituted oxazolyl, substituted or unsubstituted thiazolyl, substituted or unsubstituted oxadiazolyl, substituted or unsubstituted thiadiazolyl, substituted or unsubstituted pyridinyl, substituted or unsubstituted pyrimidinyl, substituted or unsubstituted pyrazinyl, substituted or unsubstituted triazinyl, substituted or unsubstituted benzofuranyl, substituted or unsubstituted benzothiophenyl, substituted or unsubstituted benzimidazolyl, substituted or unsubstituted indolyl, substituted or unsubstituted quinolyl, substituted or unsubstituted isoquinolyl, substituted or unsubstituted quinazolinyl, substituted or unsubstituted quinoxalinyl, substituted or unsubstituted naphthyridinyl, substituted or unsubstituted benzoxazinyl, substituted or unsubstituted benzothiazinyl, substituted or unsubstituted acridinyl, substituted or unsubstituted phenanthrazinyl, substituted or unsubstituted phenathiazinyl, substituted or unsubstituted phenoxazinyl, substituted or unsubstituted fluorenyl, substituted or unsubstituted dibenzofuranyl, substituted or unsubstituted dibenzothiophenyl, substituted or unsubstituted carbazolyl, combinations thereof, or fused rings of combinations thereof, but are not limited thereto.

[0032] In this specification, substituted or unsubstituted C6-C 30 Arylene or substituted or unsubstituted C2-C 30 Heteroarylene refers to a substituted or unsubstituted C6-C 30 Aryl or substituted or unsubstituted C5-C 30 Heterocyclic groups such as substituted or unsubstituted phenylene, substituted or unsubstituted naphthylene, substituted or unsubstituted anthrylene, substituted or unsubstituted phenanthrenylene, substituted or unsubstituted tetraphenylene, substituted or unsubstituted pyrenylene, substituted or unsubstituted biphenylene, substituted or unsubstituted terphenylene, substituted or unsubstituted metaterphenylene, substituted or unsubstituted terphenylene, substituted or unsubstituted triphenylene, substituted or unsubstituted perylene, substituted or unsubstituted indenylene, substituted or unsubstituted furylene, substituted or unsubstituted thienylene, substituted or unsubstituted pyrrolylene, substituted or unsubstituted pyrazolylene, substituted or unsubstituted imidazolylene, substituted or unsubstituted triazolylene, substituted or unsubstituted oxazolylene, substituted or unsubstituted thiazolylene, substituted or unsubstituted oxadiazolylene, substituted or unsubstituted thiadiazolylene, substituted or unsubstituted pyridinylene, substituted or unsubstituted pyrimidinylene, substituted or unsubstituted pyrazinylene, substituted or unsubstituted triazinylene, substituted or unsubstituted benzofuranylene, substituted or unsubstituted benzothiophenylene, substituted or unsubstituted benzimidazolylene, substituted or unsubstituted indolylene, substituted or unsubstituted quinolinylene, substituted or unsubstituted isoquinolinylene, substituted or unsubstituted quinazolinylene, substituted or unsubstituted quinoxalinylene, substituted or unsubstituted naphthyridinylene, substituted or unsubstituted benzoxazinylene, substituted or unsubstituted benzothiazinylene, substituted or unsubstituted acridinylene, substituted or unsubstituted phenazinylene, substituted or unsubstituted phenathiazinylene, substituted or unsubstituted phenoxazinylene, substituted or unsubstituted fluorenylene, substituted or unsubstituted dibenzofuranylene, substituted or unsubstituted dibenzothiophenylene, substituted or unsubstituted carbazolylene, combinations thereof, or fused rings of combinations thereof, but are not limited thereto.

[0033] Compounds of biphenylamine structure represented by general formula (1):

[0034]

[0035] In the general formula (1), R1 to R4 are independently substituted or unsubstituted C 6-30 Aryl, substituted or unsubstituted C2-C 30 heteroaryl;

[0036] The L1 to L4 are independently a straight chain bond, a substituted or unsubstituted C 6-30 Arylene, substituted or unsubstituted C2-C 30 heteroarylene;

[0037] The substituents of the "substituted or unsubstituted" group are selected from deuterium atoms, C 1-10 Alkyl, C 6-30 Aryl, C2-C 30 one or more of heteroaryl groups;

[0038] In the general formula (1) Indicates whether there is a single bond connection between the two side groups or not;

[0039] The heteroatom is selected from one or more of an oxygen atom, a sulfur atom or a nitrogen atom.

[0040] In a preferred embodiment, the compound structure is as shown in general formula (1-1) or general formula (1-2):

[0041]

[0042] In the general formula (1-1) and the general formula (1-2), R1 to R4 are independently substituted or unsubstituted phenyl, substituted or unsubstituted biphenyl, substituted or unsubstituted terphenyl, substituted or unsubstituted naphthyl, substituted or unsubstituted phenanthrenyl, substituted or unsubstituted furyl, substituted or unsubstituted fluorenyl, substituted or unsubstituted benzofuranyl, or substituted or unsubstituted dibenzofuranyl;

[0043] The L1 to L4 are independently a straight chain bond, a substituted or unsubstituted phenylene group, a substituted or unsubstituted biphenylene group, a substituted or unsubstituted terphenylene group, or a substituted or unsubstituted naphthylene group;

[0044] The substituent of the "substituted or unsubstituted" group is selected from one or more of a deuterium atom, a methyl group, an ethyl group, a tert-butyl group, a phenyl group, a naphthyl group, a biphenyl group, a furyl group, a benzofuranyl group, and a dibenzofuranyl group.

[0045] In a preferred embodiment, the compound structure is as shown in general formula (2-1), general formula (2-2), general formula (2-3), general formula (2-4) or general formula (2-5):

[0046]

[0047] In general formula (2-1) to general formula (2-5), R1 to R4 are independently substituted or unsubstituted C 6-30 Aryl, substituted or unsubstituted C2-C 30 heteroaryl;

[0048] The L1 to L4 are independently a straight chain bond, a substituted or unsubstituted C 6-30 Arylene, substituted or unsubstituted C2-C 30 heteroarylene;

[0049] The substituents of the "substituted or unsubstituted" group are selected from one or more of a deuterium atom, a methyl group, an ethyl group, a tert-butyl group, a phenyl group, a naphthyl group, a biphenyl group, a furyl group, a benzofuranyl group, and a dibenzofuranyl group;

[0050] The heteroatom is selected from one or more of an oxygen atom, a sulfur atom or a nitrogen atom.

[0051] In a preferred embodiment, the compound structure is as shown in formula (3-1), formula (3-2), formula (3-3), formula (3-4), formula (3-5) or formula (3-6):

[0052]

[0053] In general formulas (3-1) to (3-6), Indicates whether there is a single bond connection between the two side groups or not;

[0054] The R1, R2, R3, and R4 are independently represented by substituted or unsubstituted C 6-30 Aryl, substituted or unsubstituted C2-C 30 heteroaryl;

[0055] The L1 to L4 are independently a straight chain bond, a substituted or unsubstituted C 6-30 Arylene, substituted or unsubstituted C2-C 30 heteroarylene;

[0056] The substituents of the "substituted or unsubstituted" group are selected from one or more of a deuterium atom, a methyl group, an ethyl group, a tert-butyl group, a phenyl group, a naphthyl group, a biphenyl group, a furyl group, a benzofuranyl group, and a dibenzofuranyl group;

[0057] The heteroatom is selected from one or more of an oxygen atom, a sulfur atom or a nitrogen atom.

[0058] In a preferred embodiment, R1 to R4 are independently substituted or unsubstituted phenyl, substituted or unsubstituted biphenyl, substituted or unsubstituted terphenyl, substituted or unsubstituted naphthyl, substituted or unsubstituted anthracenyl, substituted or unsubstituted phenanthrenyl, substituted or unsubstituted furyl, substituted or unsubstituted benzofuranyl, substituted or unsubstituted dibenzofuranyl, substituted or unsubstituted thienyl, substituted or unsubstituted benzothienyl, substituted or unsubstituted dibenzothienyl, substituted or unsubstituted carbazolyl, substituted or unsubstituted N-phenylcarbazolyl, substituted or unsubstituted 9,9-dimethylfluorenyl, substituted or unsubstituted 9,9-diphenylfluorenyl, or substituted or unsubstituted spirofluorenyl;

[0059] The L1 to L4 are independently a straight chain bond, a substituted or unsubstituted phenylene group, a substituted or unsubstituted biphenylene group, a substituted or unsubstituted terphenylene group, or a substituted or unsubstituted naphthylene group;

[0060] The substituent of the "substituted or unsubstituted" group is selected from one or more of a deuterium atom, a methyl group, an ethyl group, a tert-butyl group, a phenyl group, a naphthyl group, a biphenyl group, a furyl group, a benzofuranyl group, and a dibenzofuranyl group.

[0061] In a preferred embodiment, R1 to R4 are independently represented by: substituted or unsubstituted phenyl, substituted or unsubstituted biphenyl, substituted or unsubstituted terphenyl, substituted or unsubstituted naphthyl, substituted or unsubstituted phenanthrenyl, substituted or unsubstituted furyl;

[0062] The L1 to L4 are independently a straight chain bond, a substituted or unsubstituted phenylene group, a substituted or unsubstituted biphenylene group, a substituted or unsubstituted terphenylene group, or a substituted or unsubstituted naphthylene group;

[0063] The substituent of the "substituted or unsubstituted" group is selected from one or more of a deuterium atom, a methyl group, an ethyl group, a tert-butyl group, a phenyl group, a naphthyl group, a biphenyl group, a furyl group, a benzofuranyl group, and a dibenzofuranyl group.

[0064] In a preferred embodiment, the specific structure of the compound is any one of the following structures:

[0065]

[0066]

[0067]

[0068]

[0069]

[0070]

[0071]

[0072]

[0073] organic electroluminescent devices

[0074] The present invention provides an organic electroluminescent device, which comprises an anode, a hole transport region, a light emitting region, an electron transport region and a cathode in sequence, wherein the hole transport region comprises a diarylamine structural compound represented by general formula (1).

[0075] In a preferred embodiment, the hole transport region includes a hole injection layer, a hole transport layer and an electron blocking layer, and the hole transport layer and the hole injection layer contain a diarylamine structural compound represented by general formula (1).

[0076] More preferably, the hole transport layer comprises a diarylamine structural compound represented by the general formula (1), and the hole injection layer comprises a diarylamine structural compound represented by the general formula (1) and other doping materials.

[0077] In an exemplary embodiment of the present invention, an organic electroluminescent device may include an anode, a hole transport region, a light emitting region, an electron transport region, and a cathode.

[0078] The organic electroluminescent device of the present invention may be a bottom-emitting organic electroluminescent device, a top-emitting organic electroluminescent device, or a stacked organic electroluminescent device, without particular limitation.

[0079] Any substrate commonly used in organic electroluminescent devices can be used in the organic electroluminescent device of the present invention. Examples include transparent substrates such as glass or transparent plastic substrates; opaque substrates such as silicon substrates; and flexible polyimide (PI) film substrates. Different substrates have varying mechanical strength, thermal stability, transparency, surface smoothness, and water resistance. Depending on the properties of the substrate, its use varies. In the present invention, a transparent substrate is preferably used. The thickness of the substrate is not particularly limited.

[0080] anode

[0081] Preferably, an anode can be formed on a substrate. In the present invention, the anode and the cathode are opposite to each other. The anode can be made of a conductor with a higher work function to facilitate hole injection, and can be, for example, a metal such as nickel, platinum, copper, zinc, silver, or an alloy thereof; a metal oxide such as zinc oxide, indium oxide, indium tin oxide (ITO), and indium zinc oxide (IZO); a combination of a metal and a metal oxide such as ZnO and Al or ITO and Ag; a conductive polymer such as poly(3-methylthiophene), poly(3,4-(ethylene-1,2-dioxy)thiophene) and polyaniline, but is not limited thereto. The thickness of the anode depends on the material used, and is typically 50-500 nm, preferably 70-300 nm, and more preferably 100-200 nm. In the present invention, a combination of a metal and a metal oxide, ITO and Ag, is preferably used.

[0082] cathode

[0083] The cathode can be made of a conductor with a low work function to facilitate electron injection. Examples include metals or alloys thereof, such as magnesium, calcium, sodium, potassium, titanium, indium, aluminum, silver, tin, and combinations thereof; and multilayer structures such as, but not limited to, LiF / Al, Li2O / Al, and BaF2 / Ca. The thickness of the cathode depends on the material used, but is typically 10-50 nm, preferably 15-20 nm.

[0084] Luminous area

[0085] In the present invention, the light-emitting region may be arranged between the anode and the cathode, and may comprise at least one host material and at least one guest material. As the host material and guest material of the light-emitting region of the organic electroluminescent device of the present invention, light-emitting layer materials for organic electroluminescent devices known in the prior art may be used. The host material may be, for example, a thiazole derivative, a benzimidazole derivative, a polydialkylfluorene derivative or 4,4'-bis(9-carbazolyl)biphenyl (CBP). The host material may use a compound comprising an anthracene group. The guest material may be, for example, quinacridone, coumarin, rubrene, perylene and its derivatives, benzopyran derivatives, rhodamine derivatives or aminostyrene derivatives.

[0086] In a preferred embodiment of the present invention, the light emitting region comprises one or two host material compounds.

[0087] In a preferred embodiment of the present invention, the light-emitting region comprises two host material compounds, and the two host material compounds can form an exciplex.

[0088] In a preferred embodiment of the present invention, the host material of the light-emitting region is selected from one or more of the following compounds BH-1 to BH-11:

[0089]

[0090] In the present invention, the light-emitting region may contain a phosphorescent or fluorescent guest material to improve the fluorescent or phosphorescent properties of the organic electroluminescent device. Specific examples of phosphorescent guest materials include metal complexes of iridium, platinum, etc. For fluorescent guest materials, those commonly used in the art can be used. In a preferred embodiment of the present invention, the guest material of the light-emitting film layer used is selected from one of the following compounds BD-1 to BD-10:

[0091]

[0092]

[0093] In the light-emitting region of the present invention, the ratio of the host material to the guest material used is 99:1 to 70:30, preferably 99:1 to 85:15 and more preferably 97:3 to 87:13, based on mass.

[0094] The thickness of the light emitting region may be 10-50 nm, preferably 15-30 nm, but the thickness is not limited to this range.

[0095] hole transport region

[0096] In the organic electroluminescent device of the present invention, the hole transport region is disposed between the anode and the light emitting region, and includes a hole injection layer, a hole transport layer and an electron blocking layer.

[0097] hole injection layer

[0098] The hole injection material used in the hole injection layer (also called the anode interface buffer layer) is a material that can fully accept holes from the anode at a low voltage, and the highest occupied molecular orbital (HOMO) of the hole injection material is preferably a value between the work function of the anode material and the HOMO of the adjacent organic material layer. In a preferred embodiment of the present invention, the hole injection layer is a mixed film layer of a main organic material and a P-type dopant material. In order to smoothly inject holes from the anode into the organic film layer, the HOMO energy level of the main organic material must have certain characteristics with the P-type dopant material, so that it is possible to realize the charge transfer state between the main material and the dopant material, realize the ohmic contact between the hole injection layer and the anode, and thus realize the efficient injection of holes from the electrode to the hole injection layer. This feature is summarized as: the difference between the HOMO energy level of the main material and the LUMO energy level of the P-type dopant material is ≤0.4eV. Therefore, for hole-type main materials with different HOMO energy levels, different P-type dopant materials need to be selected to match them in order to achieve ohmic contact at the interface and improve the hole injection effect.

[0099] Specific examples of the host organic material preferably include, but are not limited to, metalloporphyrins, oligothiophenes, aromatic amine organic materials, hexanitrile hexaazatriphenylene, quinacridone organic materials, perylene organic materials, anthraquinone, and polyaniline and polythiophene conductive polymers. Preferably, the host organic material is an aromatic amine organic material.

[0100] Preferably, the P-type doping material is a compound having charge conductivity selected from the following: quinone derivatives or metal oxides, such as tungsten oxide and molybdenum oxide, but not limited thereto.

[0101] In a preferred embodiment of the present invention, the P-type dopant material used is selected from any one of the following compounds HI-1 to HI-8:

[0102]

[0103]

[0104] In one embodiment of the present invention, the ratio of the host organic material to the P-type dopant material is 99:1-95:5, preferably 99:1-97:3, based on mass.

[0105] In a preferred embodiment of the present invention, the hole injection layer is a mixed film layer of an aromatic amine compound and a P-type dopant material, and the aromatic amine compound is a diaromatic amine compound represented by general formula (1).

[0106] The thickness of the hole injection layer of the present invention may be 5-20 nm, preferably 8-15 nm, but the thickness is not limited to this range.

[0107] hole transport layer

[0108] In the organic electroluminescent device of the present invention, a hole transport layer may be provided above the hole injection layer. The hole transport material is a suitable material having high hole mobility that can accept holes from the anode or the hole injection layer and transport the holes to the light-emitting layer. In a preferred embodiment, the hole transport layer comprises a compound having the same general formula (1) as the hole injection layer and having a biphenylamine structure.

[0109] The thickness of the hole transport layer of the present invention may be 80-200 nm, preferably 100-150 nm, but the thickness is not limited to this range.

[0110] electron blocking layer

[0111] In the organic electroluminescent device of the present invention, an electron-blocking layer may be disposed between the hole-transporting layer and the light-emitting layer, and in particular, may be in contact with the light-emitting layer. The electron-blocking layer is disposed in contact with the light-emitting layer, thereby enabling precise control of hole transfer at the interface between the light-emitting layer and the hole-transporting layer. In one embodiment of the present invention, the electron-blocking layer material is selected from carbazole aromatic amine derivatives. The thickness of the electron-blocking layer may be 5-20 nm, preferably 8-15 nm, but is not limited to this range.

[0112] This invention does not negate the principles of traditional hole-injection material substrate pairing. Instead, it builds upon the physical properties of traditional material screening parameters. This acknowledges the influence of factors such as the HOMO energy level, carrier mobility, film phase stability, and thermal stability on the hole injection efficiency of organic electroluminescent devices. This approach further enhances material selection criteria, thereby selecting superior organic electroluminescent materials for device pairing, thereby improving the precision of material selection for the fabrication of high-performance organic electroluminescent devices.

[0113] Electron transport region

[0114] In the organic electroluminescent device of the present invention, the electron transport region is disposed between the light emitting region and the cathode, and includes an electron transport layer and an electron injection layer, but is not limited thereto.

[0115] electron injection layer

[0116] The electron injection layer can be arranged between the electron transport layer and the cathode. The electron injection layer material is generally preferably a material with a low work function, so that electrons are easily injected into the organic functional material layer. Preferably, the electron injection layer material is an N-type metal material. As the electron injection layer material of the organic electroluminescent device of the present invention, the electron injection layer materials for organic electroluminescent devices known in the prior art can be used, for example, lithium; lithium salts, such as 8-hydroxyquinoline lithium, lithium fluoride, lithium carbonate or lithium azide; or cesium salts, cesium fluoride, cesium carbonate or cesium azide. The thickness of the electron injection layer of the present invention can be 0.1-5nm, preferably 0.5-3nm and more preferably 0.8-1.5nm, but the thickness is not limited to this range.

[0117] electron transport layer

[0118] The electron transport layer can be disposed on the light-emitting film layer or (if present) the hole blocking layer. The electron transport layer material is a material that easily accepts electrons from the cathode and transfers the received electrons to the light-emitting layer. Preferably, the material has a high electron mobility. As the electron transport layer of the organic electroluminescent device of the present invention, electron transport layer materials for organic electroluminescent devices known in the prior art can be used, for example, metal complexes of hydroxyquinoline derivatives represented by Alq3, BAlq and LiQ, various rare earth metal complexes, triazole derivatives, triazine derivatives such as 2,4-bis(9,9-dimethyl-9H-fluoren-2-yl)-6-(naphthalene-2-yl)-1,3,5-triazine (CAS No.: 1459162-51-6), imidazole derivatives such as 2-(4-(9,10-di(naphthalene-2-yl)anthracene-2-yl)phenyl)-1-phenyl-1H-benzo[d]imidazole (CAS No.: 561064-11-7, commonly known as LG201), oxadiazole derivatives, etc.

[0119] In a preferred organic electroluminescent device of the present invention, the electron transport region comprises a nitrogen heterocyclic compound represented by the following general formula (4):

[0120]

[0121] In the general formula (4), Ar1, Ar2, and Ar3 are each independently a substituted or unsubstituted C6-C 30 Aryl, substituted or unsubstituted C2-C 30 One of the heterocyclic groups;

[0122] L is selected from a single bond, a substituted or unsubstituted C6-C 30 Arylene, substituted or unsubstituted C2-C 30 A heterocyclylene group;

[0123] X1, X2, and X3 independently represent N or C-(H), and at least one of X1, X2, and X3 represents N;

[0124] The substituents of the "substituted or unsubstituted" group are optionally selected from deuterium, tritium, C 1-10 Alkyl, C6-C 30 Aryl, C2-C 30 one or more of heteroaryl groups;

[0125] The heteroatom is selected from one or more of an oxygen atom, a sulfur atom or a nitrogen atom.

[0126] In a preferred embodiment of the present invention, the electron transport layer comprises any one of the following compounds:

[0127]

[0128]

[0129] In a more preferred embodiment of the present invention, the electron transport layer comprises any one of the following compounds:

[0130]

[0131] In a preferred embodiment of the present invention, in addition to the compound of general formula (3), the electron transport layer further comprises other compounds conventionally used in electron transport layers, for example, Alq3, Liq, preferably Liq. In a more preferred embodiment of the present invention, the electron transport layer consists of one of the compounds of general formula (3) and one of the other compounds conventionally used in electron transport layers (preferably Liq).

[0132] The hole injection and transport rates of the hole transport region containing the compound of the present invention can be well matched to the electron injection and transport rates. Preferably, the hole injection and transport rates of the hole transport region containing the compound of the present invention can be better matched to the electron injection and transport rates of the electron transport region containing the nitrogen heterocyclic derivative of general formula (3).

[0133] Therefore, in a particular embodiment of the present invention, an electron transport region comprising one or more nitrogen heterocyclic derivatives of the general formula (4) or consisting thereof is used in combination with a hole transport region comprising the compound of the present invention to achieve relatively better technical effects.

[0134] The thickness of the electron transport layer of the present invention may be 10-80 nm, preferably 20-60 nm, and more preferably 25-45 nm, but the thickness is not limited to this range.

[0135] Covering

[0136] In order to improve the light extraction efficiency of the organic electroluminescent device, a light extraction layer (i.e., CPL layer, also called cover layer) can be added to the cathode of the device. According to the principles of optical absorption and refraction, the refractive index of the CPL cover layer material should be as high as possible, and the absorption coefficient should be as small as possible. Any material known in the art can be used as the CPL layer material, such as Alq3, or N4,N4'-diphenyl-N4,N4'-di(9-phenyl-3-carbazolyl)biphenyl-4,4'-diamine. The thickness of the CPL cover layer is generally 5-300 nm, preferably 20-100 nm and more preferably 40-80 nm.

[0137] The organic electroluminescent device of the present invention may further include an encapsulation structure. The encapsulation structure may be a protective structure that prevents foreign substances, such as moisture and oxygen, from entering the organic layer of the organic electroluminescent device. The encapsulation structure may be, for example, a can, such as a glass or metal can, or a thin film covering the entire surface of the organic layer.

[0138] Hereinafter, an organic electroluminescent device according to an embodiment of the present invention is described.

[0139] In the accompanying drawings, the thickness of layers, films, substrates, regions, etc., is exaggerated for clarity. Throughout the specification, like reference numerals denote like elements. It should be understood that when an element, such as a layer, film, region, or substrate, is referred to as being "on" another element, it can be directly on the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly on" another element, there are no intervening elements present.

[0140] Figure 1 is a schematic cross-sectional view of an organic electroluminescent device according to an embodiment of the present invention.

[0141] See Figure 1 , 1 is a transparent substrate layer, 2 is an anode layer, 3 is a hole injection layer, 4 is a hole transport layer, 5 is an electron blocking layer, 6 is a light emitting layer, 7 is a hole blocking layer, 8 is an electron transport layer, 9 is an electron injection layer, 10 is a cathode layer, and 11 is a CPL layer.

[0142] The present invention also relates to a method for preparing an organic electroluminescent device, comprising sequentially laminating an anode, a hole injection layer, a hole transport layer, an electron blocking layer, an organic film layer, an electron transport layer, an electron injection layer, and a cathode, and optionally a capping layer, on a substrate. In this regard, vacuum deposition, vacuum evaporation, spin coating, casting, LB method, inkjet printing, laser printing, or LITI methods may be used, but are not limited thereto. In the present invention, vacuum evaporation is preferably used to form the various layers. Those skilled in the art can conventionally select the various process conditions in the vacuum evaporation method according to actual needs.

[0143] In addition, it should be noted that the materials used to form each layer described in the present invention can be formed into a film alone and used as a single layer, or can be mixed with other materials to form a film and used as a single layer. It can also be a stacked structure between layers formed into films alone, a stacked structure between layers formed into films after mixing, or a stacked structure between layers formed into films alone and layers formed into films after mixing.

[0144] Exemplary embodiments have been disclosed herein, and although specific terms are used therein, these terms are used and interpreted only in a general and descriptive sense and not for purposes of limitation. In some cases, as will become apparent to one of ordinary skill in the art upon filing this application, unless specifically indicated otherwise, features, characteristics, and / or elements described in conjunction with a particular embodiment may be used alone or in combination with features, characteristics, and / or elements described in conjunction with other embodiments. Accordingly, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention.

[0145] The following examples are intended to better explain the present invention, but the scope of the present invention is not limited thereto.

[0146] Example

[0147] Unless otherwise specified, various materials used in the following examples and comparative examples are commercially available or can be obtained by methods known to those skilled in the art.

[0148] Synthesis of intermediates

[0149] Synthesis of intermediate B-3:

[0150]

[0151] In a three-necked flask, under nitrogen protection, 0.02 mol of raw material D-1, 0.022 mol of raw material E-1, and 250 ml of toluene were added and stirred, and then 1.1×10 -4 molPd2(dba)3,1.1×10 -4 1 mol P(t-Bu)₃ and 0.05 mol sodium tert-butoxide were heated to 110°C and refluxed for 21 hours. The mixture was cooled to room temperature, filtered, and the filtrate was evaporated under reduced pressure. The crude product was purified by silica gel column chromatography (eluent: hexane: CHCl = 5:1) to obtain the desired intermediate B-3. LC-MS: Measured: 236.18 ([M+H]⁺), Required: 235.10.

[0152] The following intermediates were prepared using the same method as intermediate B-3, except that different raw materials D and E were used or different equivalents of raw materials D and E were used. The raw materials used in the synthesis process are shown in the following table:

[0153]

[0154]

[0155] Synthesis of intermediate A-3:

[0156]

[0157] In a three-necked flask, under nitrogen protection, 0.01 mol of intermediate B-3, 0.012 mol of raw material F-1, and 150 ml of toluene were added and stirred, and then 5×10 -5 mol Pd2(dba)3,5×10 -5 mol P(t-Bu)₃ and 0.03 mol sodium tert-butoxide were heated to 110°C and refluxed for 28 hours. The mixture was cooled to room temperature, filtered, and the filtrate was evaporated under reduced pressure. The crude product was purified by silica gel column chromatography (eluent: hexane: CHCl = 5:1) to obtain the target intermediate A-3. LC-MS: Measured: 438.07 ([M+H]⁺), Required: 437.22.

[0158] The following intermediate A was prepared using the same method as intermediate A-3, except that different intermediate B or raw material F was used. The intermediates and raw materials used in the synthesis process are shown in the following table:

[0159]

[0160] Synthesis Example

[0161] Example 1: Synthesis of Compound 1:

[0162] (1)

[0163] (2)

[0164] (1) In a three-necked flask, under nitrogen protection, add 0.01 mol of intermediate B-1, 0.012 mol of raw material M-1, and 150 ml of toluene, stir and mix, then add 5×10 -5 mol Pd2(dba)3,5×10 -5 mol P(t-Bu)₃ and 0.03 mol sodium tert-butoxide were heated to 110°C and refluxed for 24 hours. The mixture was cooled to room temperature, filtered, and the filtrate was evaporated under reduced pressure. The crude product was purified by silica gel column chromatography (eluent: hexane: CHCl = 5:1) to obtain the target intermediate C-1. LC-MS: Measured: 518.35 ([M+H]⁺), Required: 517.16.

[0165] (2) 0.01 mol of intermediate C-1 and 0.012 mol of intermediate A-1 were added to a three-necked flask, dissolved in a mixed solvent (60 mL toluene, 30 mL ethanol), and then 1×10 -4 molPd(PPh3)4, 3mol / L K2CO3 aqueous solution 15mL, heated to reflux under nitrogen protection for 12 hours. Sampling point plate, confirm that the reaction is complete. After cooling to room temperature, the reaction mixture is filtered through a diatomaceous earth pad, rinsed with chloroform, and the resulting filtrate is evaporated in vacuo. The obtained residue is purified by column chromatography on silica gel using hexane / toluene as eluent to obtain compound 1.

[0166] The compounds in the following Examples 2 to 17 were prepared using the same method as Example 1, except that different intermediates A, intermediates B and raw materials M were used. The intermediates and raw materials used in the synthesis process are shown in Table 1 below.

[0167] Table 1

[0168]

[0169]

[0170]

[0171] Preparation of organic electroluminescent devices

[0172] The molecular structures of the materials involved in the following preparation process are shown below:

[0173]

[0174] Device Comparative Example 1

[0175] The organic electroluminescent device was prepared by the following steps:

[0176] a) using transparent glass as a substrate, washing the anode layer 2 (Ag (100 nm)) thereon, i.e., sequentially performing alkali washing, pure water washing, drying, and then performing ultraviolet-ozone washing to remove organic residues on the surface of the anode layer;

[0177] b) depositing HT-1 and p-type doped HI-1 with a thickness of 10 nm as a hole injection layer 3 on the washed anode layer 2 using a vacuum evaporation apparatus, wherein the mass ratio of HT-1 to HI-1 is 97:3;

[0178] c) On the hole injection layer, a hole transport layer 4 was deposited by vacuum evaporation. The hole transport layer material was HT-1 and had a thickness of 117 nm.

[0179] d) depositing an electron blocking layer 5 on the hole transport layer by vacuum evaporation. The electron blocking layer material is EB-1 and has a thickness of 10 nm.

[0180] e) fabricating a light-emitting layer 6 of an OLED light-emitting device on the electron blocking layer. The light-emitting layer 6 comprises BH-1 as a host material, BD-1 as a dopant material, a doping ratio of 3% by weight, and a thickness of 20 nm.

[0181] f) On the light-emitting layer, HB-1 was further evaporated as a hole-blocking layer 7 with a thickness of 8 nm;

[0182] g) Vacuum evaporation of ET-1 and Liq on the hole-blocking layer with a mass ratio of 50:50 and a thickness of 30 nm. This layer serves as the electron transport layer 8.

[0183] h) LiF is deposited on the electron transport layer by vacuum evaporation to a thickness of 1 nm. This layer serves as the electron injection layer 9;

[0184] i) vacuum evaporating a Mg:Ag (1:9) electrode layer with a thickness of 16 nm on the electron injection layer, which serves as the cathode layer 10;

[0185] j) On the cathode layer, 70 nm of CPL-1 was vacuum-deposited as the CPL layer 11.

[0186] Device Comparative Examples 2-5 and Device Comparative Example 9 were carried out according to the method of Device Comparative Example 1, except that the organic materials in steps b) / c) were replaced with the organic materials shown in Table 3. Device Comparative Examples 6-8 were carried out according to the method of Device Comparative Example 1, except that the organic materials in steps b) / c) / g) were replaced with the organic materials shown in Table 3. Device Preparation Examples 1-18 were carried out according to the method of Device Comparative Example 1, except that the organic materials in steps b) / c) were replaced with the organic materials shown in Table 3. Device Preparation Examples 19-24 were carried out according to the method of Device Comparative Example 1, except that the organic materials in steps b) / c) / g) were replaced with the organic materials shown in Table 3.

[0187] Table 3

[0188]

[0189]

[0190] In the table above, taking Example 1 as an example, "1:HI-1 (3% 10 nm)" in the second column indicates that the hole injection layer is composed of Compound 1 of the present invention and P-type dopant material HI-1, where 3% refers to the weight ratio of P-type dopant material HI-1 to the total weight of the hole injection layer. 10 nm indicates the thickness of the layer. "1 (117 nm)" in the third column indicates that Compound 1 of the present invention is used, and the thickness of the layer is 117 nm. Similarly, the meanings in the other tables are deduced accordingly.

[0191] After the OLED light-emitting device is prepared as described above, the cathode and anode are connected using a known driving circuit, and various performances of the device are measured.

[0192] The device performance measurement results of Examples 1-24 and Comparative Examples 1-9 are shown in Table 4.

[0193] Table 4

[0194]

[0195]

[0196] Note: Voltage, current efficiency and color coordinates were tested using an IVL (current-voltage-luminance) test system (Suzhou Fushida Scientific Instrument Co., Ltd.) at a current density of 10 mA / cm 2 The lifespan test system is the EAS-62C OLED device lifespan tester from Japan System Engineering Co., Ltd.; LT95 refers to the time it takes for the device brightness to decay to 95% at a specific brightness; Index = current efficiency / CIEy.

[0197] In Table 4, the voltage represents the driving voltage of the device, that is, the device voltage. From the results of Comparative Examples 1-5, Comparative Example 9, and Device Examples 1-18 in Table 4, it can be seen that the use of the biphenylamine compounds of the present invention as hole injection and hole transport layer materials has improved device efficiency and extended device life due to their high carrier transport rate.

[0198] From the results of Comparative Examples 6-8 and Device Examples 19-24 in Table 4, it can be seen that the compounds of the diarylamine structure of the present invention are used in combination with specific electron transport layer materials. This combination effectively reduces the voltage of the device and further extends the service life of the device.

[0199] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A compound having a biphenylamine structure, characterized in that: The compound structure is shown in general formula (2-2), general formula (2-4) or general formula (2-5): In the general formula (2-2), the general formula (2-4) and the general formula (2-5), R1 to R4 are independently substituted or unsubstituted phenyl, substituted or unsubstituted biphenyl, substituted or unsubstituted naphthyl, substituted or unsubstituted furyl, or substituted or unsubstituted thienyl; The L1 to L4 are independently a straight chain bond, a substituted or unsubstituted phenylene group, a substituted or unsubstituted biphenylene group, or a substituted or unsubstituted naphthylene group; The substituents of the "substituted or unsubstituted" group are selected from deuterium atoms and phenyl groups.

2. The compound of the bisarylamine structure according to claim 1, characterized in that The compound structure is shown in general formula (3-1), general formula (3-2) or general formula (3-3): In general formula (3-1) to general formula (3-3), Indicates that there is a single bond between the two side groups; R1, R2, R3, and R4 are independently substituted or unsubstituted phenyl, substituted or unsubstituted biphenyl, substituted or unsubstituted naphthyl, substituted or unsubstituted furyl, or substituted or unsubstituted thienyl; The L1 to L4 are independently a straight chain bond, a substituted or unsubstituted phenylene group, a substituted or unsubstituted biphenylene group, or a substituted or unsubstituted naphthylene group; The substituent of the "substituted or unsubstituted" group is selected from one or more of a deuterium atom and a phenyl group.

3. The compound of the bisarylamine structure according to any one of claims 1 to 2, characterized in that: R1 to R4 are independently substituted or unsubstituted phenyl, substituted or unsubstituted biphenyl, substituted or unsubstituted naphthyl, substituted or unsubstituted furyl, substituted or unsubstituted thienyl; The L1 to L4 are independently a straight chain bond, a substituted or unsubstituted phenylene group; The substituent of the "substituted or unsubstituted" group is selected from one or more of a deuterium atom and a phenyl group.

4. The compound of the bisarylamine structure according to any one of claims 1 to 2, characterized in that: R1 to R4 are independently represented by: phenyl, biphenyl, naphthyl, furyl; The L1 to L4 are independently a straight chain bond, a phenylene group, a biphenylene group, or a naphthylene group.

5. The compound of the bisarylamine structure according to claim 1, characterized in that The specific structure of the compound is any one of the following structures:

6. An organic electroluminescent device comprising, in sequence, an anode, a hole transport region, a light emitting region, an electron transport region, and a cathode, characterized in that: The hole transport region comprises the bisarylamine structural compound according to any one of claims 1 to 5.

7. The organic electroluminescent device according to claim 6, characterized in that: The hole transport region includes a hole injection layer, a hole transport layer and an electron blocking layer, and the hole transport layer and the hole injection layer contain the diarylamine structural compound according to any one of claims 1 to 5.

8. The organic electroluminescent device according to claim 7, characterized in that: The hole transport layer comprises the diarylamine structure compound according to any one of claims 1 to 5, and the hole injection layer comprises the diarylamine structure compound according to any one of claims 1 to 5 and other doping materials.

9. The organic electroluminescent device according to claim 6, characterized in that: The electron transport region comprises a nitrogen heterocyclic compound represented by the following general formula (4): In the general formula (4), Ar1, Ar2, and Ar3 are each independently a substituted or unsubstituted C6-C 30 Aryl, substituted or unsubstituted C2-C 30 One of the heterocyclic groups; L is selected from a single bond, a substituted or unsubstituted C6-C 30 Arylene, substituted or unsubstituted C2-C 30 A heterocyclylene group; X1, X2, and X3 independently represent N or C-(H), and at least one of X1, X2, and X3 represents N; The substituents of the "substituted or unsubstituted" group are optionally selected from deuterium, tritium, C 1-10 Alkyl, C6-C 30 Aryl, C2-C 30 one or more of heteroaryl groups; The heteroatom is selected from one or more of an oxygen atom, a sulfur atom or a nitrogen atom.

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