JPH0753953A - Organic electroluminescent element - Google Patents

Abstract

PURPOSE:To obtain the element stably emitting light over a long period of time by forming on a substrate a multilayer structure comprising an organic hole transport layer containing a specific compound, an organic luminescent layer, and an anode and a cathode between which the two layers are sandwiched. CONSTITUTION:The element comprises a substrate 1 and formed thereon a multilayer structure comprising an organic hole transport layer 3, an organic luminescent layer 4, and an anode and a cathode between which the layers 3 and 4 are sandwiched, the layer 3 containing a poly(vinyltriarylamine) (a) having repeating units represented by the formula (wherein Ar<1> is an optionally substituted arylene and Ar<2> and Ar<3> each is an optionally substituted aryl, the substituents each independently being an alkyl, an alkenyl, allyl, an alkoxycarbonyl, an alkoxy, or amino). The polymer (a) may further contain other units.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electroluminescence device, and more particularly to a thin film type device which emits light by applying an electric field to a light emitting layer made of an organic compound.

[0002]

2. Description of the Related Art Conventionally, as a thin film type electroluminescent (EL) element, Zn which is a II-VI group compound semiconductor of an inorganic material has been used.
It is general that S, CaS, SrS, etc. are doped with Mn or a rare earth element (Eu, Ce, Tb, Sm, etc.), which is the emission center, but the EL element made from the above inorganic material is 1). AC drive is required (50 to 1000 Hz), 2) high drive voltage (up to 200 V), 3) full colorization is difficult (especially blue is a problem), and 4) peripheral drive circuit costs are high. ing.

However, in recent years, in order to improve the above problems,
EL devices using organic thin films have been developed. In particular, the electrode type was optimized for the purpose of improving the efficiency of carrier injection from the electrode in order to increase the light emission efficiency, and the organic hole transport layer made of an aromatic diamine and the organic light emission made of an aluminum complex of 8-hydroxyquinoline. Of an organic electroluminescent device having a layer (Appl. Phy
s. Lett. , 51, p. 913, 1987), the luminous efficiency is greatly improved as compared with the conventional electroluminescent device using a single crystal such as anthracene, and is close to practical characteristics.

In addition to the above materials, poly (p-phenylene vinylene) (Natur is used as a material for the organic light emitting layer).
e, 347, 539, 1990; Appl. Ph
ys. Lett. , 61, 2793, 1992
Year), poly [2-methoxy, 5- (2'-ethylhexoxy) -1,4-phenylenevinylene] (Appl.P).
hys. Lett. , 58, 1982, 1991.
Year; Thin Solid Films, Volume 216, 9
6 1992; Nature, 357, 477.
P., 1992), poly (3-alkylthiophene)
(Jpn. J. Appl. Phys, 30 volumes, L193
8 pages, 1991; Appl. Phys. , 72
Vol., P. 564, 1992), etc.
A device in which a light emitting material and an electron transfer material are mixed with a polymer such as polyvinylcarbazole (Applied Physics, 61, 1044
Page, 1992) is also being developed.

[0005]

The biggest problem of the organic electroluminescent device is the service life during driving. Although there are several factors that shorten the life of the device, the deterioration of the thin film shape of the organic layer is dominant. This deterioration of the thin film shape is caused by crystallization (or aggregation) of the organic amorphous film due to heat generated when the device is driven.
It is believed that this is due to the above.

Many low molecular weight compounds (molecular weight of about 400 to 600) conventionally used as hole transport materials have a low glass transition temperature (Tg). For example, an aromatic diamine compound has a temperature of -23. ~ 82 ° C (U.S. Pat. No. 4,127,412), 39 to 78 ° C (No. 51)
The Japan Society of Applied Physics, 28a-PB-3, 1990) reported a glass transition temperature. In many organic amorphous thin films formed from such compounds, crystallization is accelerated by the temperature rise, and as a result, an island-like aggregate structure is exhibited. When such crystallization occurs, phenomena such as a decrease in light emitting efficiency, a non-light emitting portion called a dark spot, a short circuit, etc. appear as deterioration of the light emitting characteristics of the device, and finally the driving life is shortened. It is desirable that the Tg is 100 ° C. or higher because the temperature is expected to rise in the steps of vapor deposition, baking (annealing), wiring, encapsulation, etc. when the element is manufactured, other than when the element is driven.

Attempts have also been made to use a polymer material as an organic hole transport layer of an organic electroluminescence device instead of a low molecular weight compound. Polyvinylcarbazole (Technical report of IEICE, OME90-38, 1990)
Year), polysilane (Appl. Phys. Lett.,
59, 2760, 1991), Polyphosphazene (The 42nd Annual Meeting of the Polymer Society of Japan, I-8-07 and I)
-8-08, 1993) and the like have been reported, but polyvinylcarbazole has a high Tg of 200 ° C., but has a low durability due to problems such as traps, and polysilane has a short driving life of several seconds due to photodegradation and the like. Polyphosphazene has a high ionization potential and does not show the characteristics superior to those of conventional aromatic diamines. In addition to this, a hole transport layer in which an aromatic diamine compound is dispersed in polycarbonate or PMMA in an amount of 30 to 80% by weight has been studied (Jpn.
J. Appl. Phys. , 31 volumes, L960 pages, 19
1992), a low molecular weight compound acts as a plasticizer, lowering Tg, and the device characteristics are lower than those of the aromatic diamine compound.

For the above reasons, the practical situation of the organic electroluminescence device is that it has a serious problem in the driving life of the device.

[0009]

In view of the above situation, the inventors of the present invention have conducted extensive studies for the purpose of providing an organic electroluminescent device which exhibits stable emission characteristics over a long period of time.
It has been found that it is preferable that the organic hole transport layer contains polyvinyl arylamine, and the present invention has been completed.

That is, the gist of the present invention is an organic electroluminescent device comprising an organic hole transport layer and an organic light emitting layer sandwiched at least by an anode and a cathode on a substrate, wherein the organic hole transport layer is: General formula (I)

[0011]

[Chemical 2]

(In the formula, Ar ¹ represents an arylene group which may have a substituent, Ar ² and Ar ³ represent an aryl group which may have a substituent, and the substituents are each independently, An organic electroluminescent device characterized by containing a polyvinyltriarylamine having a repeating unit represented by an alkyl group, an alkenyl group, an allyl group, an alkoxycarbonyl group, an alkoxy group, an amino group).

The organic electroluminescent device of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a cross-sectional view schematically showing a structural example of the organic electroluminescent device of the present invention, in which 1 is a substrate, 2a and 2b are conductive layers, 3 is an organic hole transport layer, and 4 is an organic light emitting layer. Represent. The substrate 1 serves as a support for the organic electroluminescent element of the present invention, and is a quartz or glass plate,
A metal plate, a metal foil, a plastic film, a sheet, or the like is used, but a glass plate or a transparent synthetic resin substrate such as polyester, polymethacrylate, polycarbonate, or polysulfone is preferable.

A conductive layer 2a is provided on the substrate 1,
The conductive layer 2a is usually aluminum, gold,
Consists of a metal such as silver, nickel, palladium, tellurium, a metal oxide such as an oxide of indium and / or tin, copper iodide, carbon black, or a conductive polymer such as poly (3-methylthiophene) To be done. The conductive layer is usually formed by a sputtering method, a vacuum deposition method or the like, but metal fine particles such as silver or copper iodide, carbon black, conductive metal oxide fine particles, conductive polymer fine powder, etc. In this case, it can also be formed by dispersing it in an appropriate binder resin solution and applying it on a substrate. Further, in the case of a conductive polymer, a thin film can be directly formed on the substrate by electrolytic polymerization, or can be formed by coating on the substrate (Appl. Phy.
s. Lett. , 60, 2711, 1992).
The above conductive layers can be stacked with different materials. The thickness of the conductive layer 2a varies depending on the required transparency, but when the transparency is required, it is desirable that the visible light has a transmittance of 60% or more, preferably 80% or more. The thickness is usually 5 to 1000 nm, preferably 10 to 500 nm.

The conductive layer 2a may be the same as the substrate 1 if it is opaque. Further, the above conductive layers can be stacked with different materials. In the example of FIG. 1, the conductive layer 2a plays a role of hole injection as an anode. On the other hand, the conductive layer 2b functions as a cathode to inject electrons into the organic light emitting layer 4. As the material used for the conductive layer 2b, the material for the conductive layer 2a can be used, but a metal having a low work function is preferable for efficient electron injection, and tin, magnesium, indium, aluminum, A suitable metal such as silver or an alloy thereof is used. The thickness of the conductive layer 2b is usually the same as that of the conductive layer 2a. Although not shown in FIG. 1, a substrate similar to the substrate 1 may be further provided on the conductive layer 2b. However, it is necessary for the EL element that at least one of the conductive layers 2a and 2b has good transparency. From this, one of the conductive layers 2a and 2b preferably has a film thickness of 10 to 500 nm, and is desired to have good transparency.

The organic hole transport layer 3 is provided on the conductive layer 2a. As the hole transport material, the hole injection efficiency from the conductive layer 2a is high and the injected holes are efficiently injected. It must be a material that can be transported. For that purpose, it is required that the ionization potential is small, the hole mobility is large, the stability is excellent, and impurities that serve as traps are hard to be generated during manufacturing or use.

The present invention is characterized in that the organic hole transport layer contains a polyvinyltriarylamine having a repeating unit represented by the general formula (I).
In the general formula (I), Ar ¹ is preferably a phenylene group which may have a substituent, and Ar ² and Ar ³ are preferably a phenyl group which may have a substituent, a naphthyl group and The condensed aromatic ring group which may have a substituent such as an anthryl group is shown. As the above-mentioned substituents, each independently, an alkyl group having 1 to 6 carbon atoms such as a methyl group and an ethyl group; an alkenyl group such as a vinyl group; an allyl group; a carbon number 1 to 1 such as a methoxycarbonyl group and an ethoxycarbonyl group. 6 alkoxycarbonyl group; C1-6 alkoxy group such as methoxy group and ethoxy group; amino group such as dimethylamino group and diethylamino group.

The number average molecular weight of the polyvinyltriarylamine having the repeating unit represented by the general formula (I) is preferably 500 to 2,000,000, and particularly preferably 1,000 to 1,000,000. In the present invention, by containing a polyvinylarylamine into which a triarylamine has been introduced as a side chain of polyvinyl in the organic hole transport layer, the hole mobility can be increased and at the same time, the Tg is 100 ° C. or higher. It becomes possible to do.

The polyvinyl triarylamine having the repeating unit represented by the general formula (I) is synthesized by the method disclosed in, for example, Japanese Patent Application Laid-Open No. 1-105954. Specific preferred examples of the polyvinyltriarylamine having the repeating unit represented by the general formula (I) are shown in Table 1 below, but the invention is not limited thereto.

[0020]

[Table 1]

[0021]

[Table 2]

Further, the polyvinyltriarylamine used in the present invention may have a copolymerization part other than the polyvinyltriarylamine having the repeating unit represented by the general formula (I). Polyvinyltriarylamine is applied to the conductive layer 2 by a coating method or a vacuum deposition method.
The organic hole transport layer 3 can be formed by stacking on the a.

In the case of the coating method, it is dissolved in an organic solvent such as chloroform, dichloroethane, tetrahydrofuran, toluene or the like, coated on the conductive layer 2a by a method such as a spin coating method or a dipping method, and dried by heating to transport organic holes. Form layer 3. It is desirable not to use a binder resin or the like. At this time, the spin coating method is preferable in order to form a uniform thin film of submicron order without defects such as pinholes.

In the case of the vacuum vapor deposition method, polyvinyltriarylamine is placed in a crucible installed in a vacuum container,
After evacuating the vacuum vessel to 10 ⁻⁶ Torr with a suitable vacuum pump, the crucible is heated to evaporate the polymer and form a layer on the substrate placed facing the crucible. When forming the organic hole transport layer, a metal complex and / or a metal salt of an aromatic carboxylic acid (JP-A-4-320484), a benzophenone derivative and a thiobenzophenone derivative (Japanese Patent Application No. 4-204) are further used as an acceptor.
106977), fullerenes (Japanese Patent Application No. 4-1444)
No. 79) at a concentration of 10 ⁻³ to 10% by weight,
It is possible to generate holes as free carriers and drive at a low voltage.

The thickness of the organic hole transport layer 3 formed as described above is usually 10 to 300 nm, preferably 30.
-100 nm. The organic light emitting layer 4 is provided on the organic hole transport layer 3, and the organic light emitting layer 4 efficiently transports electrons from the cathode between the electrodes to which an electric field is applied in the direction of the organic hole transport layer. Is formed from a compound capable of.

The compound used for the organic light emitting layer 4 needs to be a compound having a high electron injection efficiency from the conductive layer 2b and capable of efficiently transporting the injected electrons. For that purpose, it is required that the compound has a high electron affinity, a high electron mobility, excellent stability, and an impurity that becomes a trap and is less likely to be generated at the time of production or use. In addition, a role of causing light emission upon recombination of holes and electrons is also required. Further, it is important to provide a uniform thin film shape from the viewpoint of device stability.

As a material for the organic light emitting layer, an aromatic compound such as tetraphenyl butadiene (JP-A-57-517).
No. 81), a metal complex such as an aluminum complex of 8-hydroxyquinoline (JP-A-59-194393, US Pat. No. 5,151,629, US Pat. No. 5,).
141, 671), a cyclopentadiene derivative (JP-A-2-289675), a perinone derivative (JP-A-2-289676), an oxadiazole derivative (JP-A-2-216791), a bisstyrylbenzene derivative ( Japanese Patent Laid-Open No. 1-245087 and 2-2
No. 22484), perylene derivatives (JP-A-2-18)
No. 9890, No. 3-791), coumarin compounds (JP-A Nos. 2-191694 and 3-792), rare earth complexes (JP-A 1-256584), and distyrylpyrazine derivatives (JP-A No. 2-25984). No. 252793), p-phenylene compounds (JP-A-3-33183), thiadiazolopyridine derivatives (JP-A-3-37).
292), a pyrrolopyridine derivative (JP-A-3-3).
7293), a naphthyridine derivative (JP-A-3-2).
No. 03982). These compounds may be used alone or, if necessary, may be mixed and used.

The thickness of the organic light emitting layer 4 is usually 10 to 20.
It is 0 nm, preferably 30 to 100 nm. For the purpose of improving the luminous efficiency of the device and changing the luminescent color, for example, by doping an aluminum complex of 8-hydroxyquinoline as a host material with a fluorescent dye for laser such as coumarin (J. Appl. Phys., Vol. 65). , 36
10 pages, 1989). Also in the present invention, the emission characteristics of the device can be further improved by further doping the organic light emitting layer 4 with an organic phosphor such as a laser dye in an amount of 10 ⁻³ to 10 mol%.

The structure of the organic electroluminescent element of the present invention includes the following layered structures.

[0030]

[Table 3] Anode / organic hole transport layer / organic light emitting layer / cathode Anode / organic hole transport layer / organic light emitting layer / electron transport layer / cathode Anode / organic hole transport layer / organic light emitting layer / interface layer / cathode Anode / organic hole transport layer / organic light emitting layer / electron transport layer / interface layer / cathode In the above layer structure, the electron transport layer is for further improving the efficiency of the device, and is laminated on the organic light emitting layer. To be done. The compound used for the electron transport layer is required to be capable of easily injecting electrons from the cathode and have a further high electron transporting ability. As such an electron transport material,

[0031]

[Chemical 3]

[0032]

[Chemical 4]

Oxadiazole derivatives such as (App
l. Phys. Lett. , 55, 1489, 19
1989; Jpn. J. Appl. Phys. , 31 volumes,
1812, 1992) or a system in which they are dispersed in a resin such as PMMA (Appl. Phys. Lett., 61).
Vol., P. 2793, 1992), or n-type hydrogenated amorphous silicon carbide, n-type zinc sulfide, n-type zinc selenide and the like. The thickness of the electron transport layer is usually 5 to 200.
nm, preferably 10 to 100 nm.

In the above layer structure, the interface layer is for improving the contact between the cathode and the organic layer,
Aromatic diamine compound (Japanese Patent Application No. 5-48075), quinacridone compound (Japanese Patent Application No. 5-116204), naphthacene derivative (Japanese Patent Application No. 5-116205), organic silicon compound (Japanese Patent Application No. 5-116206). , An organic phosphorus compound (Japanese Patent Application No. 5-116207) and the like. The thickness of the interface layer is usually 2 to 100 nm, preferably 5 to
It is 30 nm. Instead of providing an interfacial layer, the interfacial layer material is added in the vicinity of the cathode interface of the organic light emitting layer and the electron transport layer by 50
A region containing mol% or more may be provided.

In the present invention, by containing polyvinyltriarylamine in the organic hole transport layer, it is possible to obtain an element having heat resistance and exhibiting stable light emission characteristics even after long-term driving. Incidentally, it is also possible to have a structure opposite to that of FIG. 1, that is, the conductive layer 2b, the organic light emitting layer 4, the organic hole transport layer 3, and the conductive layer 2a may be laminated in this order on the substrate, and at least as described above. It is also possible to provide the organic electroluminescent element of the present invention between two substrates, one of which is highly transparent. Similarly, it is also possible to stack in a structure opposite to the above-mentioned layer structure.

[0036]

EXAMPLES Next, the present invention will be described more specifically by way of examples, but the present invention is not limited to the description of the following examples unless it exceeds the gist. Example 1 Polyvinyltriphenylamine (No. (1) in Table 1) represented by the following structural formula was synthesized by the method disclosed in JP-A-1-105954.

[0037]

[Chemical 5]

The number average molecular weight of this polymer is 9,300.
The weight average molecular weight was 20,200. When DSC measurement (using Seiko DSC-20) was performed, T
The g was 116 ° C. The above polymer under the following conditions,
It was spin-coated on a glass substrate.

[0039]

[Table 4] Solvent Dichloroethane Coating solution concentration 21.6 [mg / ml] Spinner rotation speed 3000 [rpm] Spinner rotation time 20 [second] Drying condition 70 ° C-30 minutes A uniform film thickness of 63 nm was obtained by the above spin coating. A thin film was formed. The ionization potential of this thin film sample was measured with an ultraviolet electron analyzer (AC-1) manufactured by Riken Keiki Co., Ltd., and it was 5.55 eV. Also,
Even when this thin film sample was stored in the atmosphere for 2 months, the shape of the film remained stable and stable.

Example 2 A laminated film of an organic hole transport layer and an organic light emitting layer was prepared by the following method. Indium tin oxide (IT
O) A transparent conductive film deposited to a thickness of 120 nm was subjected to ultrasonic cleaning with acetone, pure water, ultrasonic cleaning with isopropyl alcohol, drying with dry nitrogen, and UV / ozone cleaning, and then in the same manner as in Example 1. An organic hole transport layer made of polyvinyl triphenylamine was formed to a thickness of 65 nm.

The above sample was placed in a vacuum vapor deposition apparatus and evacuated using an oil diffusion pump equipped with a liquid nitrogen trap until the degree of vacuum in the apparatus became 2 × 10 ^-6 Torr or less. As a material for the organic light emitting layer, an aluminum 8-hydroxyquinoline complex Al (C ₉
H ₆ NO) ₃

[0042]

[Chemical 6]

[0043] was placed in a ceramic crucible and heated by a tantalum wire heater around the crucible for vapor deposition. The temperature of the crucible at this time was controlled in the range of 230 to 270 ° C., the vacuum degree during vapor deposition was 2 × 10 ⁻⁶ Torr, and the vapor deposition time was 3 minutes 3
It was 0 seconds. In this manner, an organic light emitting layer having a film thickness of 75 nm was laminated on the organic hole transport layer. After the above laminated film on the ITO glass substrate was vapor-deposited, it was taken out from the vacuum vapor deposition apparatus and observed by an electron microscope (SEM) (20,000 times). As a result, it was a film having excellent uniformity and no defects.

This laminated film was placed in a vacuum electric furnace and
After heating at 83 ° C for 1 hour at ^-4 Torr vacuum, S again
When EM observation was performed, the uniform and defect-free film structure was not changed.

Example 3 The substrate was heated to 73 during vacuum deposition of the organic light emitting layer.
A laminated film consisting of an organic hole transport layer / organic light emitting layer was formed on an ITO glass substrate in the same manner as in Example 2 except that the temperature was set to be ° C. SEM observation (20,000 times) of the above laminated film revealed that the film had excellent uniformity and no defects. This laminated film was placed in a vacuum electric furnace and heated at 83 ° C. for 1 hour at a vacuum degree of 10 ⁻⁴ Torr, and then SEM observation was performed again, but the uniform and defect-free film structure was not changed. .

Comparative Example 1 As the organic hole transport layer material, N, represented by the following structural formula,
N'-diphenyl-N, N '-(3-methylphenyl)
-1,1'-biphenyl-4,4'-diamine

[0047]

[Chemical 7]

A laminated film comprising an organic hole transport layer / organic light emitting layer was formed on an ITO glass substrate in the same manner as in Example 2 except that was formed by vacuum vapor deposition to have a film thickness of 60 nm. When SEM observation (20,000 times) was performed on this laminated film, void-like defects having a size of about 100 nm were observed at a number density of 15 per 84 μm ² . The laminated film was treated in the same manner as in Example 2 in a vacuum electric furnace at 83 ° C -1.
When heated under the condition of time, the film became cloudy and agglomeration occurred vigorously to the extent that it was clearly visible.

Example 4 An organic electroluminescent device having the structure shown in FIG. 1 was produced by the following method. Ultrasonic cleaning with acetone, ultrasonic cleaning with pure water, ultrasonic cleaning with isopropyl alcohol, drying with dry nitrogen, UV / ozone cleaning was performed on a glass substrate with a transparent conductive film of indium tin oxide (ITO) deposited to a thickness of 120 nm. After that, it was installed in a vacuum vapor deposition apparatus and evacuated using an oil diffusion pump equipped with a liquid nitrogen trap until the degree of vacuum in the apparatus became 2 × 10 ⁻⁶ Torr or less.

The concentration of the polyvinyl triphenylamine coating solution was 2.6, 5.3, 11.4, 21.6 [mg / m
1], 16.5, 35, 50,
An organic hole transport layer having a film thickness of 63 nm was formed. Next, in the same manner as in Example 2, an organic light emitting layer made of an 8-hydroxyquinolinol complex of aluminum was laminated in a thickness of 75 nm on each of the organic hole / hole transport layers described above.

Finally, as a cathode, an alloy electrode of magnesium and silver was vapor-deposited with a film thickness of 150 nm by the binary simultaneous vapor deposition method. Vapor deposition uses a molybdenum boat and the degree of vacuum is 4
A glossy film was obtained at x10 ^-6 Torr and vapor deposition time of 4 minutes and 20 seconds. The atomic ratio of magnesium to silver is 10: 1.
It was 5. When a positive DC voltage is applied to the ITO electrode (anode) of the organic electroluminescent device thus manufactured and a negative DC voltage is applied to the magnesium-silver alloy electrode (cathode), the device emits uniform green light. The peak wavelength of light emission was 530 nm. The light emission characteristics of the device are shown in Table 2.

Example 5 An organic electroluminescent device was prepared in the same manner as in Example 4, except that polyvinyltriphenylamine was placed in a molybdenum boat and the boat was heated to form an organic hole transport layer of 40 nm in 6 minutes. It was made. The light emission characteristics of this device are shown in Table 2.

[0053]

[Table 5]

[0054]

According to the present invention, it is possible to obtain an organic electroluminescent device having a thermally stable thin film structure and exhibiting excellent light emitting characteristics. Therefore, the organic electroluminescent device according to the present invention is a light source (for example, a light source of a copying machine, a liquid crystal display or a backlight of a meter, etc.) that makes use of the characteristics of a flat panel display (for example, for OA computers or wall-mounted televisions) or a surface light emitter. It can be applied to light sources), display boards, and marker lights, and its technical value is great.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an example of an organic electroluminescent device according to the present invention.

[Explanation of symbols]

 1 Substrate 2a, 2b Conductive Layer 3 Organic Hole Transport Layer 4 Organic Light Emitting Layer

Claims (1)

[Claims] 1. An organic electroluminescent device comprising an organic hole transport layer and an organic light emitting layer sandwiched at least by an anode and a cathode on a substrate, wherein the organic hole transport layer has the following general formula (I): Chemical 1] (In the formula, Ar ¹ represents an arylene group which may have a substituent, Ar ² and Ar ³ represent an aryl group which may have a substituent, and the substituents are each independently an alkyl group, An organic electroluminescence device comprising a polyvinyltriarylamine having a repeating unit represented by an alkenyl group, an allyl group, an alkoxycarbonyl group, an alkoxy group, an amino group).