JPH0471891A - Recording material - Google Patents

Abstract

PURPOSE:To obtain a recording material having good color forming properties, imparting a developed color image having good stability and satisfying other conditions to be equipped, in a recording material using the color forming reaction due to the contact of an electron donating leuco dye with an electron acceptive compound, by using a specific compound as the electron donating leuco dye. CONSTITUTION:In a recording material utilizing such a phenomenon that a leuco dye comes into contact with an electron acceptive compound to form a color, a compound represented by formula 1 is used. In the formula 1, R1 - R4 are a hydrogen atom or a group which may be substituted selected from an alkyl group and an aryl group and the number of carbon atoms of each of R1 - R4 is 18 or less, especially pref., 8 or less, R3 and R4 are a hydrogen atom, an alkyl group, an aryl group, an aryloxy group, an alkoxy group, an aralkyl group, an acyl group or a halogen atom and R3 is pref. a hydrogen atom or a methyl group and R4 is prefl a hydrogen atom, an alkyl group or an acyl group and R3 is most pref. a methyl group and R4 is most pref. a hydrogen atom.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention relates to a recording material, and more particularly to a recording material that has significantly improved color development, dissolution stability, and stability of colored images.

(Prior art) Recording materials that use electron-donating colorless dyes (hereinafter referred to as colorless dyes) and electron-accepting compounds are already well-known as pressure-sensitive paper, thermal paper, light-sensitive pressure paper, current-carrying thermal recording paper, thermal transfer paper, etc. For example, in the UK special fFco, /≠O1μ≠2
No., U.S. Patent No. 10゜032, ≠, ≠37,2
．． No. 20, Special Publication No. 60-23,922, % Kai Sho 57-
17 de, t36, 40-/23.114, 6Q
-/Ko3゜I am familiar with No. 117.

As a recording material, in recent years (1) color density and color sensitivity (
Efforts are being made to improve the properties of 2) the fastness of the color former and (3) the oil solubility of the color former.

The present inventors have discovered that specific compounds are effective in improving these properties.

(Object of the Invention) Accordingly, an object of the present invention is to provide a recording material that has good color development properties and stability of color images, and also satisfies other necessary conditions.

(Structure of the Invention) An object of the present invention is to provide a recording material utilizing the phenomenon in which a colorless dye is colored when it comes into contact with an electron-accepting compound. This was achieved by developing a recording material characterized by using all of the compounds shown in .

selected from phenyl, phenyl, pyridylethyl, etc.

Also, R1 and R2II'S are bonded to each other! Rings which may contain 7- to 7-membered heteroatoms, such as bi, may form lysine, piperazine, morpholine, hexanonethyrenimine, inquinoline, and the like.

However, fL1 to R4 represent a hydrogen atom, an optionally substituted alkyl group, or an aryl group.

The number of carbon atoms in the substituents R1 to R4 is preferably at most it, particularly preferably at most t.

More details regarding R1 and R+2 include methyl, ethyl, inpropyl, butyl, inamyl, furfurylmethyl,
hexyl, coethylhexyl, benzyl, tolyl, methylbenzyl, cyclohexyl, dimethylanilinophenyl, phenoxyethyl N-furfurylmethyl-N-ethylamino-, diethylamine-dibutylamino-1N-
Particularly preferred are ethyl-N-ynylamino-1N-ethyl-N-hexylamino, ethylheindstylamino, and the like.

R3, R4U are removed from hydrogen atoms, alkyl groups, aryl groups, aralkyl groups, alkoxy groups, aralkyl groups, acyl groups, halogen atoms, etc.

3 is a hydrogen atom, a methyl group is R4t'li hydrogen atom,
Alkyl or acyl groups are preferred.

The most preferred case is when U or Ra is a methyl group and R4 is a hydrogen atom.

The compound of the present invention can be easily obtained, for example, by the methods (1) and (2) below.

m A compound of formula 11 is reacted with 3-inpropylphenyl halide.

The reaction is carried out in the presence of a total dehydrating agent of ruboxybenzophenone with the compound of formula (1) at a temperature of 20° C. to 00° C. for about 75 minutes to ≠ hours in the presence of a base and a catalyst.

As the base, oxides, hydroxides, and carbonates of Na, Mg, and Ca are preferable.

Copper, halogenated steel, and oxidized steel are preferred as catalysts.

Bromine or iodine is particularly preferred as the aforementioned halide.

(2) Cohydroxy≠-substituted aminoco'-carboxylic acid As the dehydrating agent, sulfuric acid, fuming sulfuric acid, polyhydric acid, etc. are preferable.

R' is preferably a hydrogen atom, an alkyl group, methyl or ethyl.

Synthesis Example 1 mSulfuric acid rOxl, λ stirred into fuming sulfuric acid 01
-Hydroxy≠-dibutylaminobenzoylbenzoic acid fO, / 0 mole is weighed out.

This was mixed with o,io mol tmt of μm methoxy-λ-methyl-3'-inpropyldiphenylamine at room temperature for 72 hours. After neutralizing with water-cooled caustic soda water, it was purified by silica gel chromatography.

The developing solvent VC was used for a mixed solvent of nichloroform/acetonitrile.

After distilling off the distillation solvent to 2-37-isoprobylanilino-3-methyl-butylaminofluorane, hexa/
It showed a melting point of 17--/7J'''C.

In both cases it gave a deep red-black color on silica gel and on contact with bisphenol A gave a black color.

The synthesis method was developed by the present inventors in JP-A-Shoj/-! Reference was made to the disclosure in issue /31/.

Synthesis Example 2 2-Hydroxy-≠-diptylaminohenzoylbenzoic acid in Synthesis Example 1 was replaced with λ-hydroxyder diethylaminobenzoylbenzoic acid. The rest are 2-3'-
Isopropylanilino-3-methyl 6-diethylaminofluorane was obtained, exhibiting a black color on silica gel and in contact with BPA. The melting point was partially to λ100C.

In order to improve properties such as hue and solubility, the recording material of the present invention may contain, in addition to the above-mentioned colorless dyes, (known triphenylmethane phthalide compounds, fluoran compounds, and phenothiazine compounds). , indolyl 7-thallide compounds, leucoauramine compounds, rhodamine lactam compounds.

Triphenylmethane compounds, triazene compounds, fluorane compounds, phenothiazine compounds, indolyl 7-thallide compounds, leucoolamine compounds, rhodamine lactam compounds, triphenylmethane compounds, triazene compounds, spiropyran compounds, fluorene It is used in combination with one or more types of compounds.

For example, the specific example of phthalides is US reissue 1% revision 23.02 da, US patent 3.4 cll//
l, same 3. data/, //ko, same J, 4Ari/, iit
,same! , 10, /7≠, 3. Specific examples of fluorans.
62≠, 107, same 3. t27.717, same 3. t4t
i, oti, same 3d bow 2.121, same J, tl/, 39
0. Same 3. Octopus o, zio, same 3. Rijiri! t7/,
Same 4A, 2 pieces, 1λ, and 3 specific examples of spiropyrans.
．． ri'yi, rot, pyridine pyrazines are 3,7
71, ILL217. Same 3. IjJ, I49. Same≠, Ko4f:j, J/r, fluorans are patent application 1986-λ≠0
912, etc.

Specific examples include fluorans such as ni-anilino-3-methyl-t-diethylaminofluoran, no-anilino-3-methyl-j-N-fuclohexyl-methylami/fluoran, λ-p-chloroanilino-3-
Methyl-6-sibutylaminofluorane, λ-p-butylanilino-3-methyl-6-diethylaminofluorane, ni7nilino-3-chloro-6-diethylaminofluorane, 2-anilino-3-methyl-j- N-ethyl-ingylaminofluorane, λ-phenyl-6-dinithylaminofluorane, λ-anilino-3-methyl-4-N-ethyl-N-ingylaminofluoran, λ
-anilino 3-methyl-6-dinithylamino-7-methylfluoran, nianilino 3-methoxy 2-dibutylaminofluoran, λ-0-chloroanilino 4
-diethylaminofluorane, nianilino 3-pentadecyl-4-diethylaminofluorane, nianilino 3-ethyl-digitylaminofluorane, nianilino 3-methyl-3',! '-dichlorofluorane, ni-anilino-3-ethyl-+-N-ethyl-N-
Isoamylaminofluorane, ni-anilino-3-methyl-j-N-ethyl-N-r-pyridylpropylaminofluorane, λ-anilino-3-chloro-4-N-ethyl-he-ynylaminofluorane, or μrhodamine. B-anilinolactam, 'Z-min(p-nitroanilino)lactam, rhodamine B(p-chloroanilinonelactam, nianilino-2-diethylaminofluorane, etc.).

Some examples of other colorless dyes include triarylmethane compounds such as 3,3-bis(p-dimethylaminophenyl)-1-dimethylaminophthalide (i.e., crystal violet lactone); There are J-bis(p-dimethylaminophenyl phthalide, etc.), and diphenylmethane-based compounds such as H, μ'-bis-dimethylaminobenzhydrin benzyl ether, N-herophenyl-a icoolamine, N-2, μ,! -trichlorophenylleucoolamine, etc. Thiazine compounds include benzoylleucomethylene blue, p-nitrobenzoylleucomethylene blue, etc. Spiro compounds include 3-methyl-spiro-dinaphthopyran,
3-Ethyl-spiro-dinaphthopyran, 3,3'-cyclolosespiro-dinaphthopyran, 3-benzylspiro-
Examples include dinaphthopyran, 3-methyl-naphtho-(3-methoxy-benzylspiropyran, 3-propyl-spiro-dibenzobilane), and it is desirable to use two or more of them in combination.

Electron-accepting compounds that provide color upon contact with colorless dyes include inorganic and organic Lewis and Brønsted acids. For example, compounds having an acidic water atom such as a phenolic hydroxyl group, a carboxyl group, or an imide group, or metal complexes are useful. For example, phenol derivatives, salicylic acid derivatives, metal salts of aromatic carboxylic acids, acid clay, bentonite, novolac resins, metal-treated novolac resins, such as cochloro≠-phenylphenol, co-corbis(≠-hydroxyphenyl)propane, chloride, chloride, etc. -isopropylidene bis(2-methylphenol), /, /-bis-(3-chloro-≠-hydroxyphenyl)cyclohexane, co, --bis(j-
Chloro-tei-hydroxyphenyl)-3-methylbutane, N-ethyl-N- p-hydroxybenzenesulfonate
β-(p-methoxyphenoxyethylamide, +.

μ′-yne is thilidenediphenol, μ′-methylchlorohexylidenediphenol, ll≠-bis≠
'-Hydroquincumylbenzene, l.

Gubis-triple'-hydroxybenzoylbenzene, ≠
pl-thiobis(3,6-dimethylphenol), ≠
4c/-dihydroxydiphenylsulfone, μm hydroxybenzophenone, 2.4t-dihydroxy-sulfone'
-ctoxybenzophenone, 3,3'-methylenebis-
≠-Hydroxybenziic acid benzyl ester, polyvinylphenol, co, λ', ≠, 3'-tetrahydroxydiphenyl sulfone, ≠-dimethyl hydroxy hepthalate, ≠-ingtyl hydroxybenzoate, co, μ, 3'-tri Hydroxyco'-benzyloxydiphenylsulfone, /, j-bis-p-hydroxyphenylpentane, i, g-bis-p-hydroquinphenoxyhexane, ≠-hydroxybenzoic acid tolyl, ≠-hydroxybenzoic acid α-7 ale Benzyl ester, m-gysylylenebis-Hydroxybenzoic acid ester, μm
phenethyl hydroxybenzoate, g-hydroxybenzoic acid H-p-1 milbenzyl, ≠-hydroxybenzoic acid-cinnamyl ester, μm hydroxybenzoic acid benzyl ester, ≠-hydroxybenzoic acid-m-70a benzyl ester, l-h) -0-xybenzoic acid-cumyl ester, +, 3'-hydroxy-λ', 2'-diptylphenyl sulfone, β-phenyl orseinate, cinnamyl orseinate, orselic acid-0-chlorophenoxyethyl ester,! ,! '-methylenebisu2. Reference-Dipydihydroxybenzoic acid benzyl ester phenylphenoxyethyl orcelinate, m-
Phenylphenoquinoethyl orcelinate, 2.4t-
Dihydroxy-7benzoic acid-β,-3'-1-butyl-μ'
- Hydroquine phenoxyethyl ester, hisphenol A bis-p-hydroxybenzoic acid ester,! −β−
p-methoxyphenoxyethquine-co-siloxybenzoic acid, co.

≠-dihydroxybenzoic acid-p-methylbenzyl ether, 2. II-dihydroxyben & fragrant 1M-β-7 di/ki/ethyl ester, J, ≠-dihydroquine-6-methylbenzoic acid-0-methylbenzyl ester, bis-guhydroquine phenyl acid-resistant phenoxyethyl ester, ≠,
-Diacetyldiphenylthiourea, 3-phenylsalicylic acid, t-p-α-methylbenzyl-α-methylbenzylsalicylic acid, t-p-methoxyphenox7ethyloxysalicylic acid,! -Fenoki 7 etquin salicyl H,! -p-benzyl-α-methylbenzylsalicylic acid, 3-xylyl-β-(α,α-dimethylbenzyl)salicylic acid, 3. ! -di(α-methylbenzyl)salicylic acid, co-hydroxyl-α-ethylbenzyl-3-
3. Aromatic carboxylic acids such as 7-toic acid; j-Dicyclo is an organic color developer such as phenolic resins such as ntadienylsalicylic acid and para-phenylphenol-formalin resin. Complexes of zinc rhodan and molybdic acid, such as imidazole complexes of zinc rhodan, cophenylimidazole complexes, picoline complexes, 2-benzylimidazole complexes, benzimidanol complexes, 2. J-dimethyl-
/-phenyl-3-pyrazoline-y-one complex, /-phenyl-cornyl-3-benzyl-3-pyrazoline-
! -one complex, /-phenyl-comethyl-3-(2-
ethylhexyl)-3-pyrazoline-! -on complex, /
-7x Nyl-1-methyl-3-isopropyl-3-pyrazoline-! -one complex, /-phenyluco, 3-
Dibenzylupyrazoline! -one complex, /-phenyl-cobenzyl-3-methyl-hilazoline-! -one complexes, β-diketone molybdate complexes, for example as ligands acetylacetone, benzoylacetone, thenoyl trifluoroacetone/, acetylacetylacetone,
Examples include EDTA, ethyl benzoylacetate, bipyridyl, and oxine. Furthermore, these organic color developers such as zinc, magnesium, aluminum, and calcium.

Salts with polyvalent metals such as Particularly preferred is the case where one or more of bisphenols or their derivatives, salicylic acid derivatives, and zinc salts are used. When the specific compounds of the formula, colorless dyes, and electron-accepting compounds are applied to recording materials, they are used in the form of fine dispersions or fine droplets, as described above.

When used in thermal paper, the compound of the present invention, the colorless dye and the electron-accepting compound used in combination are 3μ or less in the dispersion medium,
Good luck/! It is used after being crushed and dispersed to a particle size of μ or less. As a dispersion medium, a water-soluble polymer aqueous solution having a concentration of about 0.2 to io% is generally used, and dispersion is carried out using a ball mill, a sand mill, a horizontal sand mill, an attritor, a colloid mill, or the like.

The ratio of colorless dye and electron-accepting compound used is preferably between /:/Q and i:o, i in weight ratio, and further rx
Particularly preferred is between i:j and 2:3.

The compound of the present invention is used in an amount of 10 wt % or more, preferably about J O wt % or more, based on the total colorless dye.

In this case, it is preferable to use a sensitizer such as an aromatic ether, ester, and/or aliphatic amide or ureide having a melting point of 70° C. to 13° C. in combination.

Examples of aromatic ethers or esters include penzylokina 7-talene, di-m-tolyloxyethane,
β-phenoquine ethoxy7aninecy, l-7dinoxyJp-ethylphenoxyethane, bis-β-(p-methoxyphenoxy)ethyl ether, /-tolyloxy-2-p-methylphenoquinethane, /,2-di Phenoxyethane, /,! -diphenoxybutane, /, 3-bis(≠-methoxyphenoxy)propane, / -(
4t-methoxyphenoxy)-1-phenoxypro/ξ
/-p-methylphenoquine-2p /fluorophenoquinethane, /,2-bis-p-methoxyphenylthioethoxyethane, /-phenoxy-J-p-methoxyphenylthioethyl ether, /#-hisu p-methquinphenylthioethane, β-su7thi-p-methoxyphenoxyacetate, p-methoxyphenyl=p' -
Methoxyphenoxyacetate, /-Hydroxyconaphthoic acid phenyl ester, /-Benzyloxybenzo rH benzyl ester, β-hue/xyethoxy7benzoic acid ditylamide, β-naphthylthiobenzyl ether, ≠-allylox7biphenyl, /-f-phthyl (U)
-Oxy-λ-phenokimp a puff and the like.

Amide compounds include stearic acid amide, methylene bis stearamide, methylol stearamide, stearyl urea, cyclohexyl urea, stearic acid anisidide, benzoylstearylamine, phenoxyacetobenzylamide, phenylacetylbenzylamide, futoquinethylbenzylamide , Tsuquik benzylamide, etc. are effective.

It is used after being finely dispersed in these to a particle size of 3 μm or less.

It is used by being finely dispersed in these at the same time as a colorless dye or an electron-accepting compound. In particular, it is preferable to disperse the colorless dye at the same time from the viewpoint of preventing capri. These are added at a weight ratio of 0% or more and 300% or less relative to the electron-accepting compound, particularly 10% or more/jO (preferably 4 or less).

Furthermore, the particle size is 0. / to 10μ pigment, such as kaolin,
Calcined kaolin, Merck, calcium carbonate, aluminum hydroxide, magnesium hydroxide, calcined gypsum, silica, magnesium carbonate, zinc oxide, alumina, barium carbonate,
It is preferable to use barium sulfate, mica, microballoons, urea-formalin filler polyethylene particles, cellulose fillers, and the like. Furthermore, it is also preferable to use hindered phenol in combination. As the hindered phenol, phenol derivatives in which at least i@ or more of the 2nd or 4th positions are substituted with a branched alkyl group, such as /, /-bis(cornyl-μmhydroxy-
s -t -7'tylphenyl)butane, /, /, J-
Tris(3-methyl-≠-hydroxy-z-t-i-tylphenyl)butane, bis(co-hydroxy-j-t-butyl-!-methylphenyl)methane, bis(co-methyl-Hiro-hydroxy-j-t-nia”tylphenyl) ) Sulfide group β-methoxyphenoxalicylic acid, bisphenol S monoinpropyl ether i; Yes.

Examples of ultraviolet absorbers include cinnamic acid derivatives, benzophenone derivatives, benzotriazolyl phenol derivatives, etc., such as butyl α-cyano-β-phenylcinnamate, 0-benzotriazolyl-p-chloro, Phenol, 0-benzotriazolyl-
j, 4'-diptylphenol, 0-benzotriaglyl-p-chlorophenol, and the like. Or 1HAL
A small amount of a hindered amine compound called s can also be used together.

The above-mentioned patents also have more information about these materials.

Additives are further added to the coating liquid thus obtained in order to meet various requirements.

Examples of additives include completely dispersing oil-absorbing substances such as polyurea filler in the binder to prevent the recording head from becoming dirty during recording, and metal additives to improve the releasability of the head. Soap etc. are added. Therefore, in addition to colorless dyes, electron-accepting compounds, etc. that contribute to color development, waxes, antistatic agents, ultraviolet absorbers, antifoaming agents, conductive agents, fluorescent dyes, surfactants, etc. are used in combination.
It is then applied to form a recording material.

Waxes include paraffin wax, carboxy-modified paraffin wax, polyethylene wax,
Examples include higher fatty acid esters.

Examples of metal minerals include polyvalent metal salts of higher fatty acids, ie, zinc stearate, zinc oleate, and the like.

These are dispersed in a binder and applied.

There are water-soluble binders such as polyvinyl alcohol, hydroxy-7ethyl cellulose, hydroxib cellulose, epichlorohydrin-modified polyamide,
Examples include maleic anhydride copolymers, acrylic acid copolymers, modified polyacrylamide, starch derivatives, casein, and gelatin. Also, add 7JO of water-resistant agents (gelling agents, cross-linking agents) to these binders, or use hydrophobic polymer emulsions, styrene-butadiene latex, acrylic resin emulsions, etc. You can also add

Furthermore, a layer consisting of a water-soluble polymer compound such as polyvinyl alcohol, hydroxyethyl starch, or epoxy-modified polyacrylamide and a gelling agent (hardening agent) is provided on the surface of the coating layer for the purpose of imparting chemical resistance. You can also do it.

The coating fluid is most commonly applied onto base paper, acid-free paper, wood-free paper or synthetic paper, preferably acid-free paper coated with a white pigment such as synthetic calcium carbonate.

In this case, it is preferable to use a pigment and a thermoplastic or thermofusible substance in combination.

Generally, the amount of coating is approximately 7 m2 of solids.

In addition, when used for thermal paper, OLS 2λl 1/,
Same λ/10! ! ≠, etc. may be taken.

(Example) Examples are given below, but the present invention is not limited to these only.

Example 1 (1) Preparation of sample λ-AniIJ/-j-Mef-N-ethyl-n-propylaminofluorane 29, 2g of the compound of Synthesis Example 2, f3.3/% polyvinyl alcohol, respectively The kumquats were sand-milled together with an aqueous solution (Saponity value Tata %) and dispersed to an average particle size of 2.9 μm.

-10,000, 13g in bisphenol, β-naphtholbenzyl ether/19.3% polyvinyl alcohol aqueous solution t
Disperse the solution in a ball mill with oy for one day. Furthermore,/
/, 3-tris-2'-methylg'-hydroxy-t
' -t-glylphenylbutane 0, / gf! %
1) Disperse overnight with Hinyl Alcohol Aqueous Solution G and 209.

After mixing this well, add all 6 g of Georgia Caorinko O1 fine particles (disperse), and then add paraffin wax emulsion jO% dispersion + 9: (Middle East oil cellosol #≠λt)≠, !g to make a coating liquid. .

The support was prepared with 397 m of synthetic Cal/Rum carbonate and pennyloxynaphthalene at a ratio of 20:/(wt).
2 coated acid-free paper was used.

The coating liquid is coated on this support with a solid coating amount of j9/m
2, and after drying at to °C for 7 minutes,
The recording material #+ obtained by applying a super calender to the coated paper at a linear pressure of tOKyW/cm had no cuffing during raw storage and had excellent stability over time.

The coated paper was heated using a facsimile machine, and the resulting colored image was a clear dark black, showing good resistance to chemicals and sunlight.

Example 2 λ-anilino 3-methyl-+-N-ethyl- of Example 1
n-brobylaminofluorane - anilino 3-methyl-4-N-ethyl-in place of amylaminofluoran, β-7tol benzyl ether gold/-phenoquine-Jp-nitylphenoquine Did you get good results by using the same method instead of ethane? Obtained.

Example 3 2-3′-inpropylanilino-3-methyl-4-N,N-digtylaminofluorane as an electron-donating colorless dye, bisphenol A, /-( as an electron-accepting compound)
≠-methquinofenoquine)-cofenoquinopro/min, each 20j; dispersed with ifloog's j% polyvinyl alcohol (Cran PVA-10) aqueous solution in a ball mill overnight to reduce the average particle size to /, 5 μm or less, and A dispersion was obtained. Also, calcium carbonate? Og was dispersed with a homogenizer together with hexametaphosphate cedar O6j% solution/AO9 to obtain a pigment dispersion at.

Each of the dispersions prepared as above, total electron-donating colorless dye dispersion ry, and child-accepting compound dispersion 10! ! ,/−
(≠-methoxy/phenoquine)--Fenoquine/propane dispersion iog and calcium carbonate dispersion/jy were mixed in the ratio, and further 27% zinc stearate emulsion 39 was added to obtain a heat-sensitive coating liquid.

11 Styrene-Acrylic Copolymer Hollow Fine Particle Dispersion (Mitsui Toatsu Chemical MU XV)
P-/1o) 2/, tg7m2 was applied and dried, and 5.
257 m2 was coated, the surface was treated with a calender, and the head voltage was set at 2 h using a thermal printing experimental device equipped with a Kyocera thermal head (KLT-2/&-fMPD/) and a pressure of 20 Ky 7 Cm2. While using a pressure roll under the conditions of V, pulse cycle/OmS, print with e Lux width f/, 0 and reflect the print density using Macbeth reflection! The measurement was carried out once at a total of one titer.

The concentration was /6.

In addition, when ordinary high-quality paper was used, there was a slight decrease in density and dot reproducibility.

Claims (1)

[Claims] A recording material that utilizes color development due to contact between an electron-donating colorless dye and an electron-accepting compound, characterized in that a compound represented by formula I is used as the electron-donating colorless dye. Recording materials. ▲There are numerical formulas, chemical formulas, tables, etc.▼Formula I However, R_1 to R_4 represent a hydrogen atom, an optionally substituted alkyl group, or an aryl group.