JP2593087B2 - Electrophotographic photoreceptor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electrophotographic photoreceptor, and more particularly, to a novel electrophotographic photoreceptor having a photosensitive layer containing an organic photoconductive compound as a main component.

(Prior art)

Conventionally, as an electrophotographic photosensitive member, an inorganic photosensitive member having a photosensitive layer mainly containing an inorganic photoconductive compound such as selenium, zinc oxide, cadmium sulfide, and silicon has been widely used. However, they were not always satisfactory in sensitivity, thermal stability, moisture resistance, durability and the like. For example, when selenium is crystallized, its properties as an electrophotographic photoreceptor are deteriorated, so that it is difficult to manufacture, and selenium is crystallized due to heat, fingerprints, and the like, and the performance as an electrophotographic photoreceptor is deteriorated. Cadmium sulfide and zinc oxide have problems in moisture resistance and durability.

For the purpose of overcoming the drawbacks of these inorganic photoconductors, research and development of organic photoconductors having a photosensitive layer containing various organic photoconductive compounds as main components have been actively conducted in recent years.

For example, Japanese Patent Publication No. Sho 50-10496 discloses an organic photoreceptor comprising a photosensitive layer containing poly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone.
However, this photoreceptor is not always satisfactory in sensitivity and durability.

In order to improve such drawbacks, attempts have been made to develop a high-performance organic photoreceptor by sharing the carrier generation function and the carrier transport function with different substances. Many studies have been made on such a so-called function-separated type photoreceptor because each material can be selected from a wide range and a photoreceptor having an arbitrary performance can be relatively easily produced.

As a result, various azo compounds have been developed and put to practical use as carrier-generating substances. On the other hand, carrier transport materials are also disclosed in, for example, JP-A-51-94829 and JP-A-52-7.
No. 2231, No. 53-27033, No. 55-52063, No. 58-65440
And a variety of substances have been proposed as disclosed in, for example, U.S. Pat.

[Problems to be solved by the invention]

Some of the electrophotographic photoreceptors using the carrier transport material as described above exhibit relatively excellent electrophotographic performance, but have poor durability against light, ozone or an electric load, and have poor performance when used repeatedly. It is not enough to satisfy practical requirements because it is stable and deteriorates. It is desired to develop a carrier transporting material that has a better carrier transporting function and shows stable performance for long-term use. I was

Furthermore, the demand for mounting an organic photoreceptor in an electrophotographic copying machine capable of copying at a higher speed has been increasing in recent years, and the development of a photoreceptor having higher sensitivity and higher durability has been desired.

The present invention has been made to solve such problems and to provide a photosensitive member having extremely high sensitivity and high durability.

[Means for Solving the Problems] The above-mentioned problem is caused by an electrophotographic photoreceptor having a photosensitive layer comprising a layer containing an ethylene derivative represented by the following general formula [I] on a conductive support. Solved by

General formula [I] In the formula, R ₁ represents a hydrogen atom, an alkyl group which may have a substituent, or an aryl group which may have a substituent, preferably a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, Represents a benzyl group, a phenyl group, a substituted phenyl group, or a naphthyl group. Ar ₁ represents an arylene group which may have a substituent, and preferably represents a phenylene group or a naphthylene group which may have a substituent. R ₂ and R ₃ represent an alkyl group, an aralkyl group or an aryl group, preferably an alkyl group having 1 to 8 carbon atoms, a benzyl group, a phenyl group or a naphthyl group, and these groups have a substituent. It may be.

That is, in the present invention, the carrier transport capability of the compound represented by the general formula [I] is utilized, and the carrier transport and the carrier transport of a so-called function-separated type electrophotographic photoreceptor in which generation and transport of the carrier are performed by separate substances are performed. By using it as a substance, it has excellent film properties, excellent electrophotographic properties such as charge retention, sensitivity, residual potential, etc., and has little fatigue deterioration even when repeatedly used, and is resistant to heat, ozone or light. Thus, an electrophotographic photosensitive member capable of exhibiting stable characteristics can be produced.

The carrier transporting material used in the present invention can be used alone or in combination of two or more of the compounds represented by the general formula [I], and can be used in combination with another carrier transporting material. Is also good.

Specific examples of the carrier transporting material effective for the present invention represented by the general formula [I] include, for example, those having the following structural formulas, which limit the carrier transporting material according to the present invention. Not something.

Illustrative compounds The carrier transporting material is easily synthesized by a known synthesis method. For example, reference is made to the method described in JP-A-58-198425.

Synthesis Example (Synthesis of Exemplified Compound 1.) 4.6 g (0.02 mol) of 1-pyrenecarboxaldehyde p
7.90 g (0.02 mol) of diethyl- (N, N-diphenylamino) benzylphosphonate was added to N, N-dimethylformamide 10
The mixture was dissolved in 0 ml, and sodium methoxide (2 g) was added. The mixture was reacted at room temperature for 5 hours, and then reacted at 100 ° C. for 1 hour. After cooling, the mixture was poured into 300 ml of a mixture of water, methanol and 1: 1 to precipitate crystals. The crystals were collected by filtration, washed with methanol, and then recrystallized from a mixed solvent of toluene and ethanol.

Yield 4.36 g (46.3%) From the FD mass spectrum, the molecular ion peak of 471 was observed, confirming that the target compound could be synthesized.

Elemental analysis value Calculated value C: 91.68 H: 5.35 N: 2.97 Actual value C: 91.76 H: 5.21 N: 2.79 Next, as a carrier-generating substance suitable for the present invention, an azo pigment is preferable. Both inorganic pigments and organic pigments can be used as long as they absorb light and generate free carriers. For example, inorganic pigments such as amorphous selenium, trigonal selenium, selenium-arsenic alloy, selenium-tellurium alloy, cadmium sulfide and the like, as well as organic pigments as shown in the following representative examples may be used.

(1) monoazo pigments, bisazo pigments, trisazo pigments,
Azo pigments such as metal complex salt azo pigments and the like (2) Perylene pigments such as perylene anhydride and perylene imide (3) Anthraquinone derivatives, anthantrone derivatives, dibenzopyrene quinone derivatives, pyranthrone derivatives, biolanthrone derivatives and isoviolanthrone Polycyclic quinone pigments such as derivatives (4) Indigo pigments such as indigo derivatives and thioindigo derivatives (5) Phthalocyanine pigments such as metal phthalocyanine and non-metal phthalocyanine (6) Diphenylmethane pigment, triphenylmethane pigment, xanthene pigment and acridine pigment (7) Quinone imine pigments such as azine pigments, oxazine pigments and thiazine pigments (8) Methine pigments such as cyanine pigments and azomethine pigments (9) Quinoline pigments (10) Benzoki (11) Naphthalimide-based pigments (12) Perinone-based pigments such as bisbenzimidazole derivatives The carrier transporting material used in the present invention has no film-forming ability by itself and can be combined with various binders. Thus, a photosensitive layer is formed.

As the binder used for the carrier generating layer and the carrier transporting layer, any one can be used, and it is preferable to use a hydrophobic electric insulating film-forming polymer. Examples of such a high-molecular polymer include the following, but are not limited thereto.

(1) Polycarbonate (2) Polyester (3) Methacrylic resin (4) Acrylic resin (5) Polyvinyl chloride (6) Polyvinylidene chloride (7) Polystyrene (8) Polyvinyl acetate (9) Styrene copolymer resin (for example, styrene) -Butadiene copolymer, styrene-methyl methacrylate copolymer, etc.) (10) Acrylonitrile copolymer resin (for example, vinylidene chloride-acrylonitrile copolymer, etc.) (11) Vinyl chloride-vinyl acetate copolymer (12) Vinyl chloride-vinyl acetate-maleic anhydride copolymer (13) Silicone resin (14) Silicone-alkyd resin (15) Phenol resin (for example, phenol-formaldehyde resin, cresol-formaldehyde resin,
(16) Styrene-alkyd resin (17) Poly-N-vinyl carbazole (18) Polyvinyl butyral (19) Polyvinyl formal (20) Polyhydroxystyrene These binders may be used alone or as a mixture of two or more. Can be used.

As shown in FIGS. 1 and 2, the photoreceptor of the present invention comprises, on a conductive support 1, a carrier generating layer 2 mainly composed of a carrier generating substance and a compound of the present invention composed mainly of a carrier transporting substance. A photosensitive layer 4 composed of a laminate with a carrier transport layer 3 is provided. As shown in FIGS. 3 and 4, the photosensitive layer 4 is formed of an intermediate layer 5 provided on the conductive support 1.
May be provided. Thus, when the photosensitive layer 4 has a two-layer structure, an electrophotographic photosensitive member having the best electrophotographic characteristics can be obtained. In the present invention, as shown in FIGS. 5 and 6, the photosensitive layer 4 in which the carrier generating substance 7 in the form of fine particles is dispersed in the layer 6 containing the carrier transporting substance as a main component is used as a conductive support. 1 may be provided directly or via an intermediate layer 5.

Further, a protective layer 8 may be provided on the photosensitive layer 4 if necessary.

Here, when the photosensitive layer 4 has a two-layer structure, which of the carrier generating layer 2 and the carrier transporting layer 3 is the upper layer is determined by whether the charging polarity is selected to be positive or negative.
That is, in the case of a negatively charged photosensitive layer, the carrier transport layer 3 is advantageously formed as an upper layer, because the carrier transport material in the carrier transport layer 3 is a substance having a high transport ability for holes. Because it is.

The carrier generation layer 2 constituting the two-layered photosensitive layer 4 is formed on the conductive support 1 or the carrier transport layer 3 directly or on an intermediate layer such as an adhesive layer or a barrier layer as required. Can be formed by the following method.

(1) Vacuum evaporation method (2) Method of applying a solution in which a carrier-generating substance is dissolved in an appropriate solvent (3) The carrier-generating substance is made into fine particles in a dispersion medium by a ball mill, a sand grinder, or the like.
A method of applying a dispersion obtained by mixing and dispersing with a binder.

The thickness of the carrier generation layer 2 thus formed is preferably 0.01 μm to 5 μm, and more preferably 0.05 μm to 3 μm.

The thickness of the carrier transport layer 3 can be changed as necessary, but is preferably 5 μm to 30 μm. The composition ratio of the carrier transporting layer 3 is preferably 0.8 to 10 parts by weight with respect to 1 part by weight of the carrier transporting material of the present invention. When forming 4, the binder is preferably used in a range of 5 parts by weight or less based on 1 part by weight of the carrier generating substance.

When the carrier generating layer is formed as a dispersion type using a binder, the binder is preferably used in an amount of 5 parts by weight or less based on 1 part by weight of the carrier generating substance.

The layer structure of the photoreceptor of the present invention has a laminated structure and a single-layer structure as described above, and any one or more of a carrier transport layer, a carrier generation layer, a single-layer photosensitive layer, and a protective layer that are surface layers. The layer may contain one or more kinds of electron-accepting substances for the purpose of improving sensitivity, reducing residual potential or reducing fatigue upon repeated use.

The intermediate layer functions as an adhesive layer or a barrier layer, and in addition to the binder resin, for example, polyvinyl alcohol, ethyl cellulose, carboxymethyl cellulose, casein, or the like is used.

As the conductive support 1 used for the configuration of the electrophotographic photoreceptor of the present invention, the following are mainly used,
It is not limited by these.

(1) A metal plate such as an aluminum plate or a stainless steel plate, and a drum-shaped plate.

(2) A thin metal layer of aluminum, palladium, gold or the like provided on a support such as paper or plastic film by lamination or vapor deposition.

(3) A substrate in which a layer of a conductive compound such as a conductive polymer, indium oxide, or tin oxide is provided by coating or vapor deposition on a support such as paper or a plastic film.

For forming constituent layers such as the carrier transport layer and the carrier generation layer according to the present invention, vacuum deposition, sputtering, CV
A vapor deposition method such as D or a coating method such as dipping, spray, blade, or roll method is optionally used.

The photoreceptor of the present invention has the above-described configuration, and has excellent charging characteristics, sensitivity characteristics, and image forming characteristics, as will be apparent from the examples described later. In particular, even when subjected to a repetitive transfer type electrophotographic system, it has little fatigue deterioration and excellent durability.

An unexpected effect of the present invention is that the sensitivity of the carrier transporting material of the present invention in the carrier transporting layer is not significantly reduced even when the concentration thereof is extremely low. 20 parts per 100 parts by weight of binder resin
If it is contained in an amount of not less than part by weight, it can function sufficiently as an electrophotographic photosensitive member.

〔Example〕

Examples of the present invention will be specifically described below, but the embodiments of the present invention are not limited thereto.

Example 1 On a conductive support formed by evaporating aluminum on a polyester film, a 0.1 μm-thick film made of a vinyl chloride-vinyl acetate-maleic anhydride copolymer “ESREC MF-10” (manufactured by Sekisui Chemical Co., Ltd.). Was provided.

On top of this, 1 part by weight of dibromance anthrone "Monolight Red 2Y" (manufactured by ICI) and 0.5 part by weight of polycarbonate resin "Panlite L-1250" (manufactured by Teijin Chemicals Ltd.) represented by the following structural formula were added to 1, A solution mixed with 100 parts by weight of 2-dichloroethane and dispersed in a ball mill for 24 hours was applied so that the film thickness after drying was 1 μm to form a carrier generation layer.

On top of this, the exemplary compound (1) 7.
5 parts by weight and polycarbonate resin "PANLITE L-125
0 '' and 10 parts by weight of 1,2-dichloroethane dissolved in 80 parts by weight were applied so that the film thickness after drying was 16 μm to form a carrier transport layer, thereby preparing an electrophotographic photoreceptor of the present invention. .

The electrophotographic photoreceptor was measured for electrophotographic characteristics by a dynamic method using an electrostatic copying paper test apparatus “EPA-8100” (Kawaguchi Electric Works).

That is, the surface potential VA when the photosensitive layer surface of the photoreceptor was charged at a charged voltage of −6 KV for 5 seconds, and then the light of a tungsten lamp was applied so that the illuminance of the photoreceptor surface became 3.5 lux, and the surface potential was applied. Exposure (sensitivity) ▲ E ⁶⁰⁰ ₁₀₀ ▼ required to attenuate from −600 V to −100 V, and 30 lux
・ Surface potential (residual potential) VR after irradiation with exposure amount of sec
Was asked respectively.

The same measurement was repeated 100 times. Result is,
It is as shown in Table 1.

Comparative Example (1) A comparative photoconductor (1) was prepared in the same manner as in Example 1 except that a stilbene derivative represented by the following structural formula was used as the carrier transporting substance, and the same measurement as in Example 1 was performed. Was.

The results are as shown in Table 1.

As is clear from the above results, the electrophotographic photoreceptor of the present invention of Example 1 has a higher sensitivity and a lower sensitivity than the photoreceptor of Comparative Example (1).
It is remarkably excellent in residual potential characteristics and repetition stability.

Examples 2 to 10 Exemplified compounds (2), (4),
(6), (8), (10), (14), (18), (20),
An electrophotographic photoreceptor of the present invention was prepared in the same manner as in Example 1 except that (26) was used. The measurement was performed on these electrophotographic photosensitive members in the same manner as in Example 1. The results are shown in Table 2.

Example 11 On a conductive support formed by laminating an aluminum foil on a polyester film, 1 part by weight of a bisazo pigment represented by the following structural formula and a polymethyl methacrylate resin "Dianal BR-80" (Mitsubishi Rayon) Was added to 100 parts by weight of 1,2-dichloroethane and dispersed by a sand grinder for 4 hours to apply a liquid having a thickness of 0.4 μm after drying to form a carrier generating layer.

Next, the exemplary compound (3) 7.5 as a carrier transporting substance
Parts by weight and polycarbonate resin "Panlite K-1300"
10 parts by weight (manufactured by Teijin Chemicals Limited) and 1,2-dichloroethane 100
The solution dissolved in parts by weight was applied so that the film thickness after drying became 15 μm, and a carrier transport layer was formed to prepare an electrophotographic photoreceptor of the present invention.

This electrophotographic photosensitive member was measured in the same manner as in Example 1, and the results shown in Table 3 were obtained.

Comparative Example (2) A comparative photoconductor (2) was prepared in the same manner as in Example 11, except that a stilbene derivative represented by the following structural formula was used as a carrier transporting substance. This comparative photoreceptor was measured in the same manner as in Example 1, and the results shown in Table 3 were obtained.

As is clear from the above results, the electrophotographic photoreceptor of the present invention of Example 11 is remarkably superior in sensitivity, residual potential and repetition stability as compared with the comparative photoreceptor.

Example 12 On a conductive support obtained by evaporating aluminum on a polyester film, a thickness 0.1 of a vinyl chloride-vinyl acetate-maleic anhydride copolymer "ESREC MF-10" was used.
A μm intermediate layer was provided.

Further, 1 part by weight of a bisazo pigment represented by the following structural formula as a carrier-generating substance and 0.5 part by weight of a polycarbonate resin “PANLITE K-1300” were added to tetrahydrofuran 1
The carrier dispersion layer was formed by applying a liquid dispersed in a ball mill for 24 hours in addition to 00 parts by weight so that the film thickness after drying was 0.4 μm.

Next, a solution prepared by dissolving 7.5 parts by weight of the exemplary compound (5) as a carrier transporting substance and 10 parts by weight of a polycarbonate resin “PANLITE K-1300” in 100 parts by weight of tetrahydrofuran is dried so that the film thickness becomes 17 μm. To prepare an electrophotographic photoreceptor of the present invention.

The electrophotographic photosensitive member was measured in the same manner as in Example 1, and the results shown in Table 4 were obtained.

Example 13 An electrophotographic photoreceptor of the present invention was prepared in the same manner as in Example 12, except that a bisazo pigment represented by the following structural formula was used as a carrier-generating substance.

This electrophotographic photosensitive member was measured in the same manner as in Example 1, and the results shown in Table 4 were obtained.

Example 14 On a cylindrical aluminum drum having a diameter of 80 mm, a vinyl chloride-vinyl acetate-maleic anhydride copolymer "ESLEK
An intermediate layer having a thickness of 0.1 μm and comprising “MF-10” was applied by dipping.

On top of that, 1 part by weight of dibromance anthrone "Monolight Red 2Y" and polycarbonate resin "Panlite L"
-1250 "and 1 part by weight of 1,2-dichloroethane were mixed with 100 parts by weight, and a liquid dispersed in a ball mill for 24 hours was applied by a dipping method to form a carrier generating layer having a thickness of 1 μm.

Further, 7.5 parts by weight of the exemplary compound (2) as a carrier transporting substance and a polycarbonate resin “Z-200” represented by the following structural formula (Mitsubishi Gas Chemical Co., Ltd.) 10 parts by weight) and a solution prepared by dissolving 10 parts by weight in 80 parts by weight of 1,2-dichloroethane were applied by the same dipping method to form a film having a thickness of 17%.
A μm carrier transport layer was formed to prepare an electrophotographic photoreceptor of the present invention.

This electrophotographic photoreceptor is transferred to an electrophotographic copying machine "U-Bix1550M".
R ", and when the image was copied, a copied image that was faithful to the original, had high contrast, and was excellent in gradation was obtained. Even when this was repeated 50,000 times, a copied image similar to the initial one was obtained.

Comparative Example (3) A comparative photoconductor (3) was prepared in the same manner as in Example 14, except that a hydrazone derivative represented by the following structural formula was used as a carrier transporting substance.

When an image was copied in the same manner as in Example 14 for this comparative photoreceptor, a good copied image was initially obtained as in Example 14. However, fogging became noticeable at around 20,000 copies, and at 20,000 copies, Only images with much fog and reduced contrast were obtained.

Example 15 Thickness Containing Vinyl Chloride-Vinyl Acetate-Maleic Anhydride Copolymer "Esrec MF-10" on Conductive Support Laminated with Aluminum Foil on Polyester Film
An intermediate layer of 0.1 μm was provided.

Then, a liquid obtained by dissolving 7.5 parts by weight of the exemplary compound (7) as a carrier transporting substance and 10 parts by weight of a polycarbonate resin “Panlite L-1250” in 80 parts by weight of 1,2-dichloroethane has a film thickness after drying. It was applied to a thickness of 15 μm to form a carrier transport layer.

Further thereon, 1 part by weight of a carrier-generating substance represented by the following structural formula and an exemplary compound (I
-7) 1.5 parts by weight, polycarbonate 2 parts by weight of resin "PANLITE L-1250" was added to 100 parts by weight of 1,2-dichloroethane, and a solution dispersed in a ball mill was applied for 24 hours, and a carrier generating layer having a thickness of 3 μm after drying was provided. Was prepared.

This electrophotographic photosensitive member was measured in the same manner as in Example 1 except that the charging voltage was changed to +6 kv, and the results shown in Table 5 were obtained.

Comparative Example (4) A comparative photoconductor was prepared in the same manner as in Example 15, except that a stilbene derivative represented by the following structural formula was used as a carrier transporting substance.

The comparative photoreceptor was measured in the same manner as in Example 15, and the results shown in Table 5 were obtained.

As is apparent from the above results, the electrophotographic photoreceptor of the present invention has extremely excellent characteristics in sensitivity and repetition stability as compared with the comparative photoreceptor.

Example 16 An intermediate layer having a thickness of 0.1 μm and comprising a vinyl chloride-vinyl acetate-maleic anhydride copolymer “Eslec MF-10” was provided on a polyester film on which aluminum was deposited.

Then, 1 part by weight of a bisazo pigment represented by the following structural formula and 0.5 part by weight of a polycarbonate resin "PANLITE L-1250" as a carrier transporting substance were added to 1,2-dichloroethane.
A liquid dispersed in a ball mill for 24 hours in addition to 100 parts by weight was applied so that the film thickness after drying was 0.3 μm to form a carrier generation layer.

Further, a liquid obtained by dissolving 7.5 parts by weight of the exemplary compound (9) as a carrier transporting substance and 10 parts by weight of a polycarbonate resin “PANLITE K-1300” in 100 parts by weight of 1,2-dichloroethane has a film thickness after drying. It was applied to a thickness of 16 μm to form a carrier transporting layer, and an electrophotographic photoreceptor of the present invention was prepared.

This electrophotographic photosensitive member was measured in the same manner as in Example 1, and the results shown in Table 6 were obtained.

Example 17 1 part by weight of a carrier-generating substance represented by the following structural formula and a polyester resin "Vylon 200" (Toyobo Co., Ltd.) were formed on a polyester film on which aluminum was deposited. 0.5 parts by weight) was added to 100 parts by weight of 1,2-dichloroethane, and a liquid dispersed in a ball mill for 24 hours was applied.
A μm carrier generation layer was formed.

In addition, the example compound (1
1) 4.0 parts by weight of polycarbonate resin "PANLITE K-
A solution prepared by dissolving 10 parts by weight of “1300” in 100 parts by weight of 1,2-dichloroethane was applied to form a 15 μm-thick carrier transport layer to prepare an electrophotographic photoreceptor of the present invention.

This electrophotographic photosensitive member was measured in the same manner as in Example 1, and the results shown in Table 7 were obtained.

Comparative Example (5) A comparative photoconductor (5) was prepared in the same manner as in Example 17, except that 4.0 parts by weight of the stilbene derivative (A) represented by the following structural formula was used as the carrier transporting substance.

The comparative photoreceptor was measured in the same manner as in the example, and the results shown in Table 7 were obtained.

Stilbene derivative (A) As is clear from the above results, the electrophotographic photoreceptor of the present invention is remarkably excellent in sensitivity, residual potential characteristics and repetition stability even at a low CTM concentration, as compared with the comparative photoreceptor.

Example 18 A carrier generating layer was formed in the same manner as in Example 11, except that metal-free τ-phthalocyanine was used as a carrier generating substance.

Next, exemplary compound (2) 4.0 as a carrier transporting substance
A carrier transporting layer was formed in the same manner as in Example 11 except that parts by weight were used, to obtain a photoreceptor of the present invention.

The electrophotographic photosensitive member was measured in the same manner as in Example 1, and the results shown in Table 8 were obtained.

[Brief description of the drawings]

1 to 6 are cross-sectional views each showing an example of the mechanical configuration of the photoconductor of the present invention. DESCRIPTION OF SYMBOLS 1 ... Conductive support 2 ... Carrier generating layer 3 ... Carrier transporting layer 4 ... Photosensitive layer 5 ... Intermediate layer 6 ... Layer containing carrier transporting substance 7 ... Carrier generating substance 8 ... Protective layer

Claims (1)

(57) [Claims] 1. An electrophotographic photoreceptor comprising a conductive support and a photosensitive layer containing at least one ethylene derivative represented by the following general formula [I]. General formula [I] (In the formula, R ₁ represents a hydrogen atom, an alkyl group which may have a substituent, an aryl group which may have a substituent, Ar ₁ represents an arylene group which may have a substituent, R ₂ and R ₃ represent an alkyl group, an aralkyl group, or an aryl group, and these groups may have a substituent. ]