JPH11160873A - Photosensitive composition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the photosensitive composition free from dropout of the hardened parts of a vinyl alcohol type polymer at the time of water development by incorporating a specified polyvinyl alcohol and a radically polymerizable unsaturated compound and a photopolymerization initiator. SOLUTION: The photosensitive composition contains the polyvinyl alcohol having an aromatic amino groups in the molecule and the additionally polymerizable ethylenically unsaturated compound and the photopolymerization initiator, and this polyvinyl alcohol having an aromatic amino groups has structural units represented by the formula in which each of R<1> and R<2> is an H atom or a <=8 C alkyl group; and R<3> is an H atom car a <=10 C alkyl group or such alkoxy or nitro, hydroxy, carboxy, or sulfo group. This photosensitive composition may contain a colorant, a pigment, a plasticizer, a fluorescent material, a photosensitizer, and the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a water-soluble or water-dispersible photosensitive composition in which PVA is hardly removed from a cured portion during washing and development.

[0002]

2. Description of the Related Art A photosensitive composition containing a vinyl alcohol polymer, a radically polymerizable unsaturated compound and a photopolymerization initiator is widely used, for example, as a relief material for printing. The above-mentioned photosensitive composition is prepared in the form of a sheet (referred to as a photosensitive resin plate) on a support such as a metal plate or a film plate, then exposed and developed to form a resin relief plate. A photosensitive composition using (hereinafter abbreviated as "PVA") has been awarded in the printing field because the uncured portion can be easily extracted with water after exposure and the plate making operation is easy.

[0003] However, the photosensitive composition using conventional PVA has a serious drawback that when the uncured portion is washed away with water after exposure, that is, at the time of development, even the PVA of the cured portion is removed. Therefore, there is a problem that the properties of the resulting resin letterpress are significantly impaired.

[0004] Various proposals have been made to solve such problems. For example, various modified PVAs having a radically polymerizable ethylenically unsaturated double bond have been developed. JP-A-03-192103 proposes a method of reacting a carboxylic acid-modified vinyl alcohol-based polymer as an anion-modified vinyl alcohol-based polymer with glycidyl methacrylate in hot water under acidic conditions. However, in this method, under acidic conditions for a long time,
It is necessary to carry out under severe reaction conditions such as hot water, it is not efficient, the reactivity of carboxylic acid and glycidyl group is low, a large amount of glycidyl methacrylate is required, and the introduction amount of ethylenically unsaturated double bond is low However, it is not enough to solve the above problems. Also, JP-A-04-303
842 proposes a method of reacting an amino group-modified vinyl alcohol polymer with a monomer having an epoxy group-containing ethylenically unsaturated double bond such as glycidyl methacrylate. Although the above problems are somewhat improved, it is considered that the amino group-modified PVA used in the reaction is difficult to be synthesized industrially, and it is considered that the amino group is directly bonded to the main chain of the PVA. The reactivity with the epoxy group is rather low and it takes a long time to complete the reaction, and it is not always efficient.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide the above-mentioned drawbacks of the conventional photosensitive composition using PVA, namely, that the PVA does not fall off from the cured portion at the time of development with water, is soluble in water, It is to provide a water-dispersible photosensitive composition.

[0006]

Means for Solving the Problems The present inventors have conducted intensive studies in view of the above problems, and as a result, have found that PVA (a) having an aromatic amino group in the molecule, a radically polymerizable unsaturated compound (b), It has been found that the above object is achieved by using the polymerization initiator (c), and the present invention has been completed.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a typical example of a PVA having an aromatic amino group in a molecule is a PVA having a structure represented by the following formula 3.

[0008]

Embedded image

Wherein R ¹ and R ² are hydrogen or a hydrocarbon group having 8 or less carbon atoms which may have a substituent, and R ³ is hydrogen, an alkyl group having 10 or less carbon atoms, or a hydrocarbon group having 10 or less carbon atoms. It represents a substituent selected from an alkoxy group, a nitro group, a hydroxy group, a carboxyl group, and a sulfonic acid group. NR ¹ R ² and R ³ may be in any of the ortho, meta and para positions.

The PVA used in the present invention is a compound represented by the formula
It is essential to have an amino group (hereinafter abbreviated as “aromatic amino group”) bonded to an aromatic ring as shown in the following. Even when the aromatic amino group is bonded to a side chain of PVA, It may be bonded to the terminal, but usually, one bonded to the side chain is used. The aromatic amino group may be directly bonded to the main chain of PVA.
PVA having the structure shown in (1) is preferably used.

[0011]

Embedded image

Here, R ¹ and R ^{2 each} represent hydrogen or a hydrocarbon group having 8 or less carbon atoms which may have a substituent, and R ³ represents hydrogen, an alkyl group having 10 or less carbon atoms, or a hydrocarbon group having 10 or less carbon atoms. It represents a substituent selected from an alkoxy group, a nitro group, a hydroxy group, a carboxyl group, and a sulfonic acid group. Also, R ⁴ ~
R ⁶ represents hydrogen or a hydrocarbon group having 8 or less carbon atoms which may have a substituent. Further, typically, PVA having a structural unit represented by the following formula 5 is preferably used.

[0013]

Embedded image

Here, R ¹ and R ^{2 each} represent hydrogen or a hydrocarbon group having 8 or less carbon atoms which may have a substituent, and R ³ represents hydrogen, an alkyl group having 10 or less carbon atoms, or a hydrocarbon group having 10 or less carbon atoms. It represents a substituent selected from an alkoxy group, a nitro group, a hydroxy group, a carboxyl group, and a sulfonic acid group. R ⁴
To R ⁸ represent hydrogen or a hydrocarbon group having 8 or less carbon atoms which may have a substituent, and S represents a sulfur atom, respectively.
A is not particularly limited in its structure as long as A is a divalent hydrocarbon group or a divalent hydrocarbon group containing at least one kind of nitrogen atom, oxygen atom and sulfur atom. For example, -CH2OCH2
-, -OCH2-, -CONH-φ-OCH2-, -C
ONHCH2-, -CONHCH2OCH2-, -CO
NHCH2OCH2CH2-, -CONHCH2OCH
2CH2CH2-, -CONHCH2OCH2CH2C
H2CH2-, -CONHCH2-φ (CH3) 2-C
H2- and the like are exemplified. In addition, a plurality of amino groups bonded to the aromatic ring may be present in particular. In addition, there is no particular problem even if there are a plurality of substituents R ³ bonded to the aromatic ring. Further, as shown in Chemical Formula 2, Chemical Formula 4 and Chemical Formula 5, generally, a benzene ring is suitably used as an aromatic ring.
For example, a naphthalene ring or a pyrene ring may be used. In the above formula, as the alkyl group having 10 or less carbon atoms of R ³ , a methyl group, an ethyl group, a propyl group, a butyl group and the like can be mentioned as typical examples. As the alkoxy group, a methoxy group and an ethoxy group can be mentioned. A representative example is given.

The content of the aromatic amino group of PVA used in the present invention is usually 0.01 to 30 mol%, preferably 0.1 to 30 mol%.
1 to 20 mol%, more preferably 0.5 to 10 mol%
It is. When the content of the aromatic amino group is extremely low, the object of the present invention cannot be sufficiently achieved, and when the content is extremely high, characteristics such as water solubility inherently possessed by the vinyl alcohol polymer are lost.

Although the molecular weight of PVA used in the present invention is not particularly limited, 4% dimethyl sulfoxide (DM
SO) The viscosity of the solution at 20 ° C (measured with a Brookfield viscometer) is preferably in the range of 3 to 10,000 mPa · S, more preferably in the range of 3 to 5000 mPa · S, and in the range of 5 to 1000 mPa · S. More preferred.

The PVA used in the present invention, except for the unit containing an aromatic amino group or the structural unit represented by Chemical formula 5, is
It consists essentially of vinyl alcohol units or vinyl alcohol units and vinyl ester units. Vinyl ester units include vinyl formate, vinyl acetate, vinyl propionate, vinyl pivalate, vinyl trifluoroacetate,
-Vinyl ethylhexanoate and the like, and usually vinyl acetate is used. The degree of saponification of the vinyl ester component of the PVA of the present invention is not particularly limited as long as it is water-soluble or water-dispersible, but is usually at least 65 mol%, preferably at least 80 mol%, and optimally at least 95 mol%. .

Further, other monomer units may be contained as long as the gist of the present invention is not impaired.
Examples of such units are ethylene, propylene, 1
Olefins such as butene and isobutene; acrylic acid, methacrylic acid, crotonic acid, (phthalic anhydride) phthalic acid,
Unsaturated acids such as (anhydride) maleic acid and (anhydride) itaconic acid or salts thereof or mono- or dialkyl esters having 1 to 18 carbon atoms; acrylamide, 1 to 1 carbon atoms
18 acrylamides such as N-alkylacrylamide, N, N-dimethylacrylamide, 2-acrylamidopropanesulfonic acid or a salt thereof, acrylamidopropyldimethylamine or an acid salt thereof or a quaternary salt thereof; methacrylamide, having 1 to 18 carbon atoms N-
Methacrylamides such as alkyl methacrylamide, N, N-dimethylmethacrylamide, 2-methacrylamidopropanesulfonic acid or a salt thereof, methacrylamidopropyldimethylamine or an acid salt thereof or a quaternary salt thereof; N-vinylpyrrolidone, N- N-vinyl amides such as vinyl formamide and N-vinyl acetamide; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl ethers such as alkyl vinyl ether having 1 to 18 carbon atoms, hydroxyalkyl vinyl ether and alkoxy alkyl vinyl ether; Vinyl halides such as vinyl, vinylidene chloride, vinyl fluoride, vinylidene fluoride, and vinyl bromide; vinyl silanes such as trimethoxyvinyl silane; allyl acetate; allyl chloride; Alcohol, although dimethylallyl alcohol and the like are not limited thereto. The content of these monomer units is not limited as in the case of the degree of saponification, and it is sufficient that the obtained polymer is water-soluble or water-dispersible.

The aromatic amino group-containing PVA used in the present invention
Is obtained, for example, by reacting a copolymer of a vinyl ester monomer and an unsaturated monomer having an epoxy group with a thiol or a thioester having an aromatic amino group and then hydrolyzing the reaction.

The method of copolymerizing the vinyl ester monomer and the unsaturated monomer having an epoxy group includes known methods such as bulk polymerization, solution polymerization, suspension polymerization and emulsion polymerization. Among them, a bulk polymerization method or a solution polymerization method in which polymerization is performed in the absence of a solvent or in a solution of alcohol or the like is usually employed, and when a polymer having a high degree of polymerization is obtained, an emulsion polymerization method is employed. Examples of the alcohol used as a solvent during the solution polymerization include lower alcohols such as methyl alcohol, ethyl alcohol, and propyl alcohol. As the initiator used for the copolymerization, 2,2′-
Known initiators such as azo-based initiators such as azobisisobutyronitrile, 2,2′-azobis (2,4-dimethylvaleronitrile), benzoyl peroxide and n-propylperoxycarbonate or peroxide-based initiators Agents. The polymerization temperature is not particularly limited.
A range of from 150C to 150C is suitable.

Examples of the unsaturated monomer having an epoxy group used for copolymerization with a vinyl ester monomer include allyl glycidyl ether, methallyl glycidyl ether, butadiene monoepoxide, 1,2-epoxy-5-hexene, and 1,2. -Epoxy-7-octene, 1,2-epoxy-9-decene, 8-hydroxy-6,7-epoxy-1-octene, 8-acetoxy-6,7-epoxy-
1-octene, N- (2,3-epoxy) propyl acrylamide, N- (2,3-epoxy) propyl methacrylamide, 4-acrylamidophenyl glycidyl ether, 3-acrylamidophenyl glycidyl ether, 4-methacrylamidophenyl glycidyl ether , 3-methacrylamide phenyl glycidyl ether, N-glycidoxymethyl acrylamide, N-glycidoxymethyl methacrylamide, N-glycidoxyethyl acrylamide, N-glycidoxyethyl methacrylamide, N-glycidoxypropyl acrylamide ,
N-glycidoxypropyl methacrylamide, N-glycidoxybutyl acrylamide, N-glycidoxybutyl methacrylamide, 4-acrylamidomethyl-2,
5-dimethyl-phenylglycidyl ether, 4-methacrylamidomethyl-2,5-dimethyl-phenylglycidyl ether, acrylamidopropyldimethyl (2,3-epoxy) propylammonium chloride,
Methacrylamidopropyldimethyl (2,3-epoxy) propylammonium chloride, glycidyl methacrylate and the like are used, but are not limited thereto.

The reaction between a copolymer of a vinyl ester monomer and an unsaturated monomer having an epoxy group with a thiol or thioester having an aromatic amino group utilizes the reaction between a thiol group or a thioester group and an epoxy group. However, the reaction is carried out without a solvent or in a solvent that dissolves or swells a compound having a thiol group or a thioester group and a polymer having an epoxy group. Examples of such a solvent include alcohols such as methanol, ethanol, n-propyl alcohol and n-butyl alcohol; aromatics such as benzene, toluene and xylene; ethers such as tetrahydrofuran, dioxane and diethyl ether; and n-hexane. And the like, and used alone or as a mixture. The reaction conditions vary depending on the structure of the epoxy group and the structure of the thiol or thioester. However, in general, when a solvent is used, the polymer concentration is 5 to 90%, and the concentration of the thiol group or the thioester group / the concentration of the epoxy group is 1.0 to 5%. 0.0 (molar ratio), reaction temperature 0 to 250 ° C, reaction time 0.01 to 2
0 hours. Here, in the reaction with thiol, basic compounds such as tertiary amines (eg, triethylamine, pyridine, etc.), phosphines (eg, tributylphosphine, triphenylphosphine, etc.), sodium hydroxide, tetraethylammonium hydroxide, sodium methylate, and acetic acid In the reaction of a weakly basic compound such as sodium or sodium carbonate with a thioester, a quaternary ammonium salt such as tributylammonium chloride or tributylammonium bromide is effective as a reaction catalyst. In order to prevent thiol oxidation, the reaction system may be degassed or replaced with nitrogen, or an antioxidant may be added.

The thiol having an aromatic amino group to be reacted with a copolymer of a vinyl ester monomer and a vinyl monomer having an epoxy group includes 2-aminothiophenol, 3-aminothiophenol, 4-aminothiophenol, and 2-aminothiophenol. -(N-methyl) aminothiophenol,
3- (N-methyl) aminothiophenol, 4- (N-
Methyl) aminothiophenol, 2- (N, N-dimethyl) aminothiophenol, 3- (N, N-dimethyl)
Aminothiophenol, 4- (N, N-dimethyl) aminothiophenol, 2-amino-3-methoxythiophenol, 2-amino-4-methoxythiophenol, 4
-Amino-2-methoxythiophenol, 4-amino-
3-methoxythiophenol, 2-amino-3-hydroxythiophenol, 2-amino-4-hydroxythiophenol, 4-amino-2-hydroxythiophenol, 4-amino-3-hydroxythiophenol, 2-
Amino-3-nitrothiophenol, 2-amino-4-
Nitrothiophenol, 4-amino-2-nitrothiophenol, 4-amino-3-nitrothiophenol,
2,3-diaminothiophenol, 2,4-diaminothiophenol, 3,4-diaminothiophenol, 2-
Examples include, but are not limited to, aminothionaphthol, sodium 3-amino-p-thiophenolsulfonate, sodium 3-amino-p-thiophenolcarboxylate, sodium 4-amino-2-sulfonate thionaphthol, and the like. Not something. Further, esters such as these acetates and benzoates can also be used.

For the hydrolysis subsequent to the reaction between the thiol group or thioester group and the epoxy group, a saponification reaction using a basic catalyst or an acid catalyst, which is commonly used in saponification of polyvinyl ester, can be applied as it is. That is, using a basic catalyst such as sodium hydroxide, potassium hydroxide or sodium methylate or an acidic catalyst such as p-toluenesulfonic acid, the reaction is carried out using an alcohol or glycol such as methanol, ethanol, propanol, butanol or ethylene glycol as a solvent. Is performed. In this case, a solvent such as tetrahydrofuran, dioxane, dimethyl sulfoxide, diethylene glycol dimethyl ether, toluene, acetone, or water is appropriately mixed and used in order to improve the solubility of the vinyl ester polymer and the catalyst. The conditions of the saponification reaction are appropriately adjusted depending on the structure of the vinyl ester polymer to be used and the desired degree of saponification of the vinyl alcohol polymer. Usually, the saponification is carried out at a catalyst concentration / vinyl ester unit concentration (molar ratio) = 0.00
1-1.2, reaction temperature: 20-180 ° C, reaction time:
It is performed in the range of 0.1 to 20 hours.

Any radically polymerizable unsaturated compound (hereinafter abbreviated as "polymerizable monomer") used in the photosensitive composition of the present invention, typically any compound which can be radically polymerized by a photoinitiator can be used. It is. Such examples include 2-hydroxyethyl (meth) acrylate, ethylene glycol (meth) acrylate, ethylene glycol di (meth) acrylate, triethylene glycol (meth) acrylate, triethylene glycol di (meth) acrylate, polyethylene glycol (Meth) acrylate, polyethylene glycol di (meth) acrylate, β-oxypropyl (meth) acrylate, β
-Oxyethyl (meth) acrylate, and N-β-
Oxyethyl acrylamide, N-oxymethyl acrylamide, N-γ-oxypropyl acrylamide, N
-P-oxyphenylacrylamide, N-β-oxyethyl N′-methylacrylamide, and (meth) acrylamide derivatives corresponding thereto, and acrylamide, methacrylamide, m-phenylenebisacrylamide, ethylenediaminebisacrylamide, hexamethylene Examples include, but are not limited to, diamine bismethacrylamide.

As the photopolymerization initiator used in the photosensitive composition of the present invention, for example, benzophenone, benzoin, p,
p'-dimethylbenzoin, benzoin methyl ether, benzoin isopropyl ether, acyloin or nuclei-substituted aromatic acyloin or acyloin alkyl ether, diacetyl, benzyl, ketoaldonyl compound, 1,1'-azocyclohexanecarbonitrile, azonitrile, 9,10- Examples include, but are not limited to, anthroquinone, chloranthraquinone, 1,4-naphthoquinone, and 9,10-phenanthroquinone.

The photosensitive composition of the present invention may contain additives such as a coloring agent, a pigment, a plasticizer, a fluorescent dye and a photosensitizer, if necessary.

[0028]

The present invention will be described in detail below with reference to examples and comparative examples. In the following Examples and Comparative Examples, "parts" and "%" mean by weight unless otherwise specified.

Synthesis Example of Aromatic Amino Group-Containing Vinyl Alcohol Polymer Polymer 1 A vinyl acetate monomer (VAc) 1000 was placed in a reactor equipped with a stirrer, a reflux condenser, a nitrogen inlet and a thermometer.
Parts, 13.4 parts of allyl glycidyl ether and 400 parts of methanol were charged, and the mixture was degassed by bubbling nitrogen gas for 15 minutes. Separately, an initiator solution prepared by dissolving 4.3 parts of 2,2′-azobisisobutyronitrile in 29 parts of methanol was prepared, and nitrogen was replaced by bubbling with nitrogen gas.
The temperature of the reactor was raised, and when the internal temperature reached 60 ° C., a separately adjusted initiator solution was added to initiate polymerization.
After polymerization at 60 ° C. for 5 hours, the polymerization was stopped by cooling.
The solid content at this time was 45.1%. Then 3
Unreacted vinyl acetate monomer was removed while occasionally adding methanol at 0 ° C. under reduced pressure to obtain a methanol solution of polyvinyl acetate copolymer (concentration: 44.5%). A part of this methanol solution was put into ether to recover the polymer, which was purified by reprecipitation twice with acetone-ether, and then dried at 40 ° C. under reduced pressure. Proton NMR (JEOL) of this purified polymer using CDCl3 as a solvent
The viscosity average molecular weight was calculated by measuring the intrinsic viscosity in acetone (JIS G-270) and measuring the intrinsic viscosity in acetone (JIS). The viscosity average molecular weight was 82% containing 1.0 mol% of allyl glycidyl ether units (epoxy group). × 10 ³ vinyl acetate copolymer. 100 parts of the obtained vinyl acetate copolymer is weighed and measured in a reactor equipped with a stirrer, a reflux condenser, a nitrogen inlet tube and a thermometer. After bubbling nitrogen gas for 15 minutes, 2-aminothiophenol 2. 2
And a solution of 0.07 part of sodium acetate dissolved in 32 parts of methanol. After reacting at 50 ° C. for 2 hours with stirring, the mixture was cooled to 40 ° C. and saponified by adding 9.4 parts of a 10% strength methanol solution of sodium hydroxide. After leaving at 40 ° C. for 1 hour, pulverized, neutralized with acetic acid, washed with a Soxhlet extractor for 48 hours or more, and dried at 60 ° C. for 20 hours or more to obtain an aromatic amino group-modified PVA. . When the IR and proton NMR (d6-DMSO) of the PVA were measured, the epoxy group had completely disappeared, the introduction of 1.0 mol% aniline group was confirmed, and the vinyl alcohol content was 97.4.
Mole%. A 4% DMSO solution of the PVA was prepared, and its viscosity (using a Brookfield viscometer) measured at 20 ° C. was 64.2 mPa · S. The PVA is referred to as a polymer A.

Polymer 2 to Polymer 9 In the same manner as in Polymer 1, each polymer was obtained by using the copolymerized monomers and thiols shown in Table 1.

[0031]

[Table 1]

Comparative Polymers 1 to 5 The polymers shown in Table 2 were synthesized by known methods.

[0033]

[Table 2]

Example 1 70 parts of Polymer 1 was added to 200 parts of water, dissolved under heating at 90 ° C., and 100 parts of 2-hydroxyethyl methacrylate and 3 parts of benzoin isopropyl ether were added. After defoaming this solution, it is applied to an aluminum plate,
After drying for 0.5 minutes, a 0.5 mm film was obtained. Subsequently, a high-pressure mercury lamp was irradiated from a distance of 75 cm for 10 minutes to obtain a cured film.
The obtained cured film was immersed in hot water at 90 ° C. for 2 hours, extracted in hot water, the extraction rate was determined by the following formula, and evaluated by the following symbols. Table 3 shows the results.

[0035]

(Equation 1)

：: Extraction rate of less than 2% Δ: Extraction rate of 2% or more and less than 5% ×: Extraction rate of 5% or more The lower the value of the above-mentioned extraction rate, the lower the P of the hardened portion during washing and development.
It shows that the loss of VA is small.

Examples 2 to 9 In the same manner as in Example 1 using Polymers 2 to 9, cured films were synthesized by the method shown in Table 3, and an extraction test with hot water was performed.

Comparative Examples 1 to 5 Cured films were synthesized in the same manner as in Example 1 using Comparative Polymers 1 to 5 by the method shown in Table 3, and then subjected to an extraction test with hot water.

[0039]

[Table 3]

[0040]

According to the present invention, the photosensitive composition of the present invention can be prepared by using conventional PVA.
Compared to those using
There is little dropout of A.

 ──────────────────────────────────────────────────続 き Continued from the front page (72) Inventor Naoki Fujiwara 1621 Sakurazu, Kurashiki-shi, Okayama Prefecture Inside Kuraray Co., Ltd.

Claims (3)

[Claims] 1. A photosensitive composition comprising a vinyl alcohol polymer having an aromatic amino group in a molecule (a), a radically polymerizable unsaturated compound (b) and a photopolymerization initiator (c). 2. The photosensitive composition according to claim 1, wherein the vinyl alcohol polymer having an aromatic amino group in the molecule (a) is a vinyl alcohol polymer having a structure represented by the following chemical formula 1. Embedded image However, R ¹ and R ² represent hydrogen or a hydrocarbon group having 8 or less carbon atoms which may have a substituent, R ³ represents hydrogen, an alkyl group having 10 or less carbon atoms, an alkoxy group having 10 or less carbon atoms, It represents a substituent selected from a nitro group, a hydroxy group, a carboxyl group, and a sulfonic acid group. 3. The photosensitive composition according to claim 1, wherein the vinyl alcohol polymer having an aromatic amino group in the molecule (a) is a vinyl alcohol polymer having a structure represented by the following chemical formula 2. . Embedded image However, R ¹ and R ² represent hydrogen or a hydrocarbon group having 8 or less carbon atoms which may have a substituent, R ³ represents hydrogen, an alkyl group having 10 or less carbon atoms, an alkoxy group having 10 or less carbon atoms, It represents a substituent selected from a nitro group, a hydroxy group, a carboxyl group, and a sulfonic acid group. R ^{4 to} R ⁶ represent hydrogen or a hydrocarbon group having 8 or less carbon atoms which may have a substituent.