JP2004182740A - Compound for electroluminescent device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compound for an organic electroluminescent device having a hole transporting layer which excels in light-emission stability and storage stability. <P>SOLUTION: A tetraphenylbenzidine compound suited for the electroluminescent device consisting of an ITO electrode, a hole transport layer, an emissive layer, an electron transport layer, and a magnesium/silver electrode is provided. This tetraphenylbenzidine compound is used in the hole transport layer of the electroluminescent device or a heat-stable electroluminescent device or is suitably used in the emissive layer. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a light-emitting element having a hole-transporting layer, a light-emitting layer, and an electron-transporting layer, which is widely used as various display devices, and has a low applied voltage, high luminance, and excellent electric field stability. The present invention relates to a compound for a light emitting element (EL element).

  EL devices have been studied by many researchers for a long time because they are self-luminous and can display brighter and clearer images than liquid crystal devices. As a light emitting element which has reached a practical level at present, there is an element using ZnS which is an inorganic phosphor. However, such an inorganic EL element has not been widely used because it requires an applied voltage of 200 V or more for light emission.

On the other hand, a light emitting element using an organic material has been far from a practical level in the past, but in 1987 C.D. W. The characteristics have been drastically improved by the laminated structure element developed by Tang et al. They stacked a phosphor that can transport electrons with a stable structure of the deposited film and an organic substance that can transport holes, and succeeded in emitting light by injecting both carriers into the phosphor. did. As a result, the luminous efficiency of the organic electroluminescent device was improved, and luminescence of 1000 cd / m ² or more was obtained at a voltage of 10 V or less. Since then, many researchers have studied to improve the characteristics, and at present, luminescence characteristics of 10,000 cd / m ² or more are obtained in short-time light emission. As a conventional example of this type of light emitting element, there is one described in Patent Document 1, for example.
JP-A-4-220995

  The basic light-emitting characteristics of such an organic light-emitting device are already in a practically usable range, and the biggest cause currently hindering its practical use is firstly the lack of light-emission stability during driving, and Lack of storage stability. Insufficient light emission stability at the time of driving means a phenomenon in which the light emission luminance decreases when driven by applying an element current, a non-light emitting area called a dark spot occurs, or destruction occurs due to a short circuit of the element. In other words, the lack of storage stability refers to a phenomenon in which light emission characteristics are deteriorated only by storing the manufactured device.

  The present inventors have studied the mechanism of the deterioration of the EL element in order to solve the problems relating to the stability of light emission and the storage stability. As a result, it was found that one of the major causes of the property deterioration was the hole transport layer. That is, hole transporting materials such as (Chemical Formula 4: TPD) and (Chemical Formula 5: TPAC), which are generally used as a hole transporting layer, are (1) crystallized by humidity, temperature and current to form a thin film. Not uniform. (2) It was found that changes such as decomposition of the hole transport layer due to energization were caused, and the luminous property was significantly deteriorated.

本発明の目的は、このような知見に基づき、発光安定性、保存安定性に優れた正孔輸送層を有する有機ＥＬ素子用化合物を提供することにある。また、それらの正孔輸送層、発光層、電子輸送層の材料として有用なテトラフェニルベンジジン化合物を提供することにある。このような正孔輸送材料の具備しなければならない条件としては、（１）優れた正孔輸送能力を持つこと、（２）熱的に安定で、ガラス状態が安定であること、（３）薄膜を形成できること、（４）電気的、化学的に安定であること、等を挙げることができる。

An object of the present invention is to provide a compound for an organic EL device having a hole transport layer having excellent emission stability and storage stability based on such findings. Another object of the present invention is to provide a tetraphenylbenzidine compound useful as a material for the hole transport layer, the light emitting layer, and the electron transport layer. The conditions that such a hole transport material must have are (1) excellent hole transport ability, (2) thermally stable and stable glass state, and (3) A thin film can be formed; and (4) electrical and chemical stability.

  In order to achieve the above object, the present inventors prototyped an EL device comprising an ITO electrode, a hole transport layer, a light emitting layer, an electron transport layer, and a magnesium / silver electrode, and produced a large number of newly synthesized hole transports. The material was evaluated. As the light emitting layer, an aluminum quinoline trimer mainly serving also as an electron transport layer was used. As a material of the hole transport layer, at least a tetraphenylbenzidine compound represented by (Chemical Formula 6), or a tetraphenylbenzidine compound represented by (Chemical Formula 6) and triphenylamine trimeric represented by (Chemical Formula 7) Used one of the bodies.

ただし、Ｒ１ 、Ｒ２ はターシャリーブチル基、フェニル基、低級アルキル基もしくは低級アルコキシ基を置換基として有するフェニル基、Ｒ３ は水素原子、メチル基またはメトキシ基を表す。

Here, R1 and R2 represent a tertiary butyl group, a phenyl group, a phenyl group having a lower alkyl group or a lower alkoxy group as a substituent, and R3 represents a hydrogen atom, a methyl group or a methoxy group.

ただし、Ｒ１ 、Ｒ２ 、Ｒ３ は水素原子、低級アルキル基、または低級アルコキシ基、Ｒ４ は水素原子、メチル基、メトキシ基、またはクロル原子を表す。

Here, R1, R2 and R3 represent a hydrogen atom, a lower alkyl group or a lower alkoxy group, and R4 represents a hydrogen atom, a methyl group, a methoxy group or a chloro atom.

  According to the present invention, as a result of using the above-described hole transport materials, they not only have excellent hole transport ability, but also form a good thin film and are thermally stable. I understood. As a result, it was clarified that an EL device having excellent luminescence stability and storage stability could be realized, and could be widely used as a display device.

  As described above, the present invention is an electroluminescent device characterized by using a tetraphenylbenzidine compound as a material of a hole transport layer. By using the material of the present invention, a conventional organic electroluminescent device is used. It is possible to realize an electroluminescent device in which luminescence stability and storage stability, which are the most serious problems of the device, are remarkably improved.

  Examples of the compound for an electroluminescent device of the present invention will be described with reference to examples.

  The tetraphenylbenzidine compound, which is one of the hole transporting materials of the present invention, is a condensation reaction of a corresponding 4,4′-dihalogenated biphenyl with a corresponding diphenylamine compound, or a corresponding benzidine compound and a corresponding aryl halide. Can be synthesized by a condensation reaction of These condensation reactions are methods known as Ullmann reactions.

  Further, a triphenylamine trimer, which is another hole transporting material of the present invention, is a triamine compound obtained by a condensation reaction of a corresponding aniline compound with a corresponding 4′-halogenated biphenylacetanilide compound and hydrolysis thereof. And an aryl halide. These condensation reactions are methods known as Ullmann reactions.

  These compounds were identified by elemental analysis and IR measurement, and further purified by a recrystallization method using a solvent and a vacuum sublimation method to have a purity of 99.8% or more. The purity was confirmed by TLC scanner, TG-DTA, and melting point measurement. The melting point and decomposition point serve as a measure of the thermal stability of the hole transport layer, and the glass transition point serves as a measure of the stability of the glassy state. The inventors have synthesized materials by changing the substituents of the above compounds in various ways. As a result, the melting point and the size of the decomposition point varied depending on the substituent, and in the case of some substituents, a material having a high melting point and a high decomposition point could be obtained. The following are some representative synthetic examples.

(Synthesis Reference Example 1)
60.0 g (0.47 mol) of p-isobutylaniline was dissolved in 126 ml of glacial acetic acid, and 59.9 g (0.58 mol) of acetic anhydride was added dropwise at 30 ° C. Allowed to react for hours. The reaction solution was poured into 300 ml of water, and the precipitated crystals were filtered, washed with water, and dried. The crystals were recrystallized from a mixed solution of 140 ml of toluene, 700 ml of n-hexane and 60.4 g of p-isobutylacetanilide (yield 67.3%). Melting point was 124.5-125.0 ° C.

  The above obtained p-isobutylacetanilide, 17.9 g (0.094 mol), bromobenzene 22.1 g (0.14 mol), anhydrous potassium carbonate, 16.9 g (0.12 mol), copper 0.89 g (0.014 mol) of the powder was mixed and reacted at 168 to 217 ° C for 14 hours. The reaction product was extracted with 100 ml of toluene, and the insolubles were removed by filtration, removed, and then concentrated to dryness. This was dissolved in 30 ml of isoamyl alcohol, 3.4 g of water, 11.8 g (0.18 mol) of 85% potassium hydroxide and hydrolyzed at 131 ° C. After isoamyl alcohol and excess bromobenzene were distilled off by steam distillation, the mixture was extracted with toluene (120 ml), washed with water, dried and concentrated. The concentrate was dried to obtain 17.6 g (86.8% yield) of N-4-isobutylphenylaniline.

  Further, N-4-isobutylphenylaniline, 17.6 g (0.078 mol), 4,4′-jodobiphenyl, 12.6 g (0.031 mol), anhydrous potassium carbonate, 12.9 g (0.093 mol) ) And 0.89 g (0.014 mol) of copper powder were mixed and reacted at 190 to 220 ° C. for 12 hours. The reaction product was extracted with 70 ml of toluene, the insolubles were removed by filtration, removed, and then concentrated to an oil. The obtained crude product was purified by column chromatography (carrier: silica gel, eluent: toluene / n-hexane = 1/6) to give N, N′-bis (p-isobutylphenyl) -N, N′-. 8.5 g (yield 45.7%) of diphenylbenzidine was obtained. Melting point was 133.8-135.3 ° C. The product was identified by elemental analysis and IR measurement. The elemental analysis values are as follows. Carbon: measured 87.77%, theoretical: 87.96%, hydrogen: measured 7.43%, theoretical 7.38%, nitrogen: measured 4.51%, theoretical 4,66%.

(Synthesis Reference Example 2)
2. Acetanilide, 23.0 g (0.17 mol), 4,4'-diiodobiphenyl, 85.3 g (0.21 mol), anhydrous potassium carbonate, 24.9 g (0.18 mol), copper powder, 48 g (0.039 mol), nitrobenzene and 40 ml were mixed and reacted at 190 to 205 ° C. for 10 hours. The reaction product was extracted with 200 ml of toluene, the insoluble matter was separated by filtration, removed, and then concentrated to dryness. This was purified by column chromatography (carrier: silica gel, eluent: toluene / n-hexane = 1/6), N- (4′-iodo-4-biphenylyl) acetanilide, 45.5 g (yield 64.8). %). The melting point was 135.0-136.0 ° C.

  Subsequently, N- (4′-iodo-4-biphenylyl) acetanilide, 18.2 g (0.044 mol), aniline, 1.84 g (0.020 mol), anhydrous potassium carbonate, 6.91 g (0.050 mol) ) And copper powder, 0.64 g (0.010 mol), nitrobenzene, and 10 ml were mixed and reacted at 190 to 215 ° C. for 15 hours. The reaction product was extracted with 100 ml of toluene, and the insolubles were removed by filtration, removed, and concentrated to an oil. The oily substance was dissolved in 50 ml of isoamyl alcohol, 1 ml of water, 2.64 g (0.040 mol) of 85% potassium hydroxide was added, and the mixture was hydrolyzed at 130 ° C. After isoamyl alcohol was distilled off by steam distillation, the mixture was extracted with 250 ml of toluene, washed with water, dried and concentrated. The concentrate was purified by column chromatography (carrier: silica gel, eluent: toluene / n-hexane = 3/1), and N, N′-bis (4′-phenylamino-4-biphenylyl) aniline, 8.85 g (Yield: 76.3%).

  Further, N, N'-bis (4'-phenylamino-4-biphenylyl) aniline, 8.70 g (0.015 mol), iodobenzene, 6.74 g (0.033 mol), anhydrous potassium carbonate, 4.56 g (0.33 mol), copper powder, 0.48 g (0.0075 mol), nitrobenzene and 10 ml were mixed and reacted at 195 to 205 ° C for 16 hours. The reaction product was extracted with 100 ml of toluene, the insoluble matter was separated by filtration, and after concentration, n-hexane was added to take out crude crystals. The crude crystals were purified by column chromatography to obtain 5.50 g (yield: 50.1%) of N, N'-bis (4'-diphenylamino-4-biphenylyl) aniline. No clear melting point was found. The product was identified by elemental analysis and IR measurement. Elemental analysis values are as follows. Carbon: measured 88.80%, theoretical: 88.61%, hydrogen: measured 5.77%, theoretical 5.65%, nitrogen: measured 5.62%, theoretical 5.74%.

  Next, these were actually evaluated as EL elements, and the emission characteristics, stability of the emission characteristics, and storage stability of the elements were examined. As shown in FIG. 1, the EL element has a structure in which an ITO electrode is formed in advance as a transparent electrode 2 on a glass substrate 1, and a hole transport layer 3, an electron transport layer / light emitting layer 4, and a Mg / Ag electrode 5 are formed on the glass substrate 1. It was prepared by vapor deposition in order. First, a sufficiently washed glass substrate (the ITO electrode was formed into a film), a hole transporting material, and an aluminum quinoline trimer purified as an electron transporting luminescent material were set in a vapor deposition apparatus. A hole transport layer was deposited at a rate of 0.1 nm / sec, and a sample having a different film thickness was prepared to determine a thickness at which optimal light emission was obtained. Although the film thickness varies depending on the material, the optimum film thickness was between 40 and 60 nm. The film thickness was monitored using a quartz oscillator. The deposition of the aluminum quinoline trimer was also performed at a rate of 0.1 nm / sec, and the film thickness was 50 nm. The Mg / Ag electrode was operated at a speed of 0.4 nm / sec, and the thickness was set to 100 nm. All of these depositions were performed continuously without breaking vacuum. Immediately after the device was manufactured, the electrode was taken out in dry nitrogen, and the characteristics were measured.

The light emission characteristics of the obtained device were defined as the light emission luminance when a current of 100 mA / cm ² was applied. As for the stability of light emission, a change in light emission luminance at that time was measured by continuously applying a current at which light emission of 200 cd / m ² was obtained. The lifetime of light emission was defined as the time until the luminance was reduced to half, that is, 100 cd / m ² . The storage stability was defined as the time required for the device to stand at room temperature in dry air for a certain period of time, then applying a current of 20 mA / cm ² until the luminance became half of the initial light emission characteristics.

  In order to evaluate the hole transporting material of the present invention, an aluminum quinoline trimer was used as the electron transporting layer and the light emitting layer 4. Of course, in the present invention, various rare earth complexes, oxazole derivatives, polyparaffins, etc. Various materials such as phenylene vinylene can be used. Further, by adding a dopant such as quinacridone or coumarin to the light emitting layer, a higher performance EL can be manufactured. Furthermore, an electroluminescent element including three layers of an electron transport layer, a light emitting layer, and a hole transport layer can be provided. Further, by combining the hole transporting material of the present invention with a suitable electron transporting material, the hole transporting layer can be used as a light emitting layer.

  As a result of such studies, it has been found that when the hole transporting material has a melting point of 130 ° C. or higher and a decomposition point of 300 ° C. or higher, excellent luminescence stability and storage stability can be obtained. Therefore, the substituent of the above compound is not limited to the substituent of the present invention, and any compound having the above melting point and decomposition point can be used.

  The hole transporting material according to the present invention can be used alone, but two or more types can be used in a mixed state by forming a film by co-evaporation or the like. Further, the hole transport material of the present invention can be used by co-evaporation with a conventional hole transport material such as TPAC or TPD. By using two or more kinds by simultaneous vapor deposition, an effect of making crystallization hardly occurs is often exhibited.

(Reference Example 1)
A well-washed ITO electrode, a tetraphenylbenzidine compound (1) (R1 = pn-Bu, R2 = H, R3 = H, mp = 132.9 ° C.) as a hole transporting material, an electron transporting luminescent material The purified aluminum quinoline trimer was set in a vapor deposition apparatus. Compound (1) was deposited at a rate of 0.1 nm / sec to a thickness of 50 nm. The film thickness was monitored using a quartz oscillator. Alkyminoline was also deposited at a rate of 0.1 nm / sec, and the film thickness was 50 nm. The Mg / Ag electrode was operated at a speed of 0.4 nm / sec, and the thickness was set to 100 nm. All of these depositions were performed continuously without breaking vacuum. Immediately after the device was manufactured, the electrode was taken out in dry nitrogen, and the characteristics were measured. The light emission characteristics were 2500 cd / m ² , the light emission life was 620 hr, and the storage stability was 2200 hr.

(Comparative example)
For comparison, an EL element was manufactured under the same conditions using (Chemical Formula 4: TPD for short) and (Chemical formula 5: TPAC for short) as a hole transport material, and the characteristics thereof were examined. The light emission characteristics, light emission lifetime, and storage stability of the TPD were 2200 cd / m ² , 220 Hr, and 460 Hr, respectively. On the other hand, the light emission property, lifespan of light emission, and storage stability in TPAC were 2500 cd / m ² , 280 Hr, and 560 Hr, respectively.

(Element Example 1)
Tetraphenylbenzidine compound (5) (R1 = tBu, R2 = tBu, R3 = H), (7) (R1 = C6H5, R2 = C6H5, R3 = H) in the same manner as in Reference Example 1. , (8) (R1 = C6H5, R2 = C6H5, R3 = CH3), (10) (R1 = p-CH3-C6H4, R2 = p-CH3-C6H4, R3 = H) An EL element used as a material was produced, and its characteristics were evaluated.

(Element Reference Example 2)
The tetraphenylbenzidine compound (2) (R1 = iBu, R2 = H, R3 = H), (3) (R1 = iBu, R2 = H, R3 = CH3), (3) 4) (R1 = tBu, R2 = H, R3 = H), (6) (R1 = C6H5, R2 = H, R3 = H), (9) (R1 = p-CH3-C6H4, R2 = H) , R3 = OCH3) as a hole transport material, and the characteristics thereof were evaluated. The result is shown in FIG. The substitution positions of R1 and R2 in the above tetraphenylbenzidine compounds (2) to (10) all indicate the p-position. From this, it was found that the tetraphenylbenzidine compounds (..5), (7), (8), and (10) according to the present invention were excellent in luminescence life and storage stability.

(Element Reference Example 3)
As the triphenylamine trimer compound usable for the hole transport material, the following compounds can be mentioned. Triphenylamine trimer compound (11) (R1 = H, R2 = H, R3 = H, R4 = H), (12) (R1 = H, R2 = H, R3 = H, R4 = CH3), ( 13) (R1 = tBu, R2 = p-CH3, R3 = p-CH3, R4 = H), (14) (R1 = H, R2 = H, R3 = H, R4 = OCH3), (15) (R1 = H, R2 = m-CH3, R3 = m-CH3, R4 = H), (16) (R1 = H, R2 = p-OCH3, R3 = p-OCH3, R4 = H), (17) (R1 = P-CH3, R2 = H, R3 = H, R4 = CH3.), (18) (R1 = p-CH3, R2 = p-iBu, R3 = p-iBu, R4 = H), (19) ( R1 = p-nBu, R2 = m-CH3, R3 = H, R4 = Cl) (20) (R1 = -OC2 H5, R2 = p-CH3, R3 = p-CH3, R4 = H).

(Reference Example 4)
The triphenylamine trimer compound (11) (R1 = H, R2 = H, R3 = H, R4 = H) and the tetraphenylbenzidine compound (4) (R1 = p −tBu, R2 = H, R3 = H) were co-evaporated to produce an EL element used as a hole transport material, and its characteristics were evaluated. The light emission characteristics were 3300 cd / m ² , the light emission life was 720 hr, and the storage stability was 2900 hr.

  The present invention provides a tetraphenylbenzidine compound suitable as a material for a hole transport layer or a light-emitting layer of an organic electroluminescent device. It is possible to realize an electroluminescent device in which luminescence stability and storage stability, which have been major problems, are significantly improved.

FIG. 1 is an enlarged perspective view of a partial cross section showing the configuration of an electroluminescent device according to an embodiment of the present invention. List showing characteristics of an electroluminescent device using a tetraphenylbenzidine compound as a hole transport layer in one embodiment of the present invention.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Glass substrate 2 Transparent electrode 3 Hole transport layer 4 Electron transport layer and light emitting layer 5 Mg / Ag electrode

Claims (7)

A tetraphenylbenzidine compound represented by the following general formula.

Here, R1 and R2 represent a tertiary butyl group, a phenyl group, a phenyl group having a lower alkyl group or a lower alkoxy group as a substituent, and R3 represents a hydrogen atom, a methyl group or a methoxy group. A compound for an electroluminescent device, which is a tetraphenylbenzidine compound represented by the following general formula.

Here, R1 and R2 represent a tertiary butyl group, a phenyl group, a phenyl group having a lower alkyl group or a lower alkoxy group as a substituent, and R3 represents a hydrogen atom, a methyl group or a methoxy group. A compound for a thermostable electroluminescent device, which is a tetraphenylbenzidine compound represented by the following general formula.

Here, R1 and R2 represent a tertiary butyl group, a phenyl group, a phenyl group having a lower alkyl group or a lower alkoxy group as a substituent, and R3 represents a hydrogen atom, a methyl group or a methoxy group. The compound for an electroluminescent device according to claim 2, wherein the compound for an electroluminescent device is used for a hole transport layer. The compound for a thermostable electroluminescent device according to claim 3, wherein the compound for a thermostable electroluminescent device is used for a hole transport layer. The compound for an electroluminescent device according to claim 2, wherein the compound for an electroluminescent device is used for a light emitting layer. The compound for a thermostable electroluminescent device according to claim 3, wherein the compound for a thermostable electroluminescent device is used for a light emitting layer.

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