Abstract: Derivatized malachite green leuco dyes for use in radio-chromic systems are described. The dyes have the following structure: in which Ar is a substituted phenyl or thiophene ring in which at least one substitution of the Ar ring is not ortho to the bond between the ring and the linking carbon, and in which R1, R2, R3, and R4 are independently selected from methyl, alkyl, or alkyl halide. The systems include the dyes in conjunction with an activator, e.g., a halogenated activator, and a carrier, e.g., a fluid carrier or an encapsulating polymeric matrix.

Abstract: A thermochromic pigment composition including: (A) at least one electron-donor organic dye compound, (B) at least one electron-acceptor compound, and (C) at least one compound corresponding to the following formula (I): in which: X is 0 or a single bond, Z is COO or OCO, R1 is O-alkyl or (CH2)mCH3—R2 is OCO(CH2)nCH3 or H, —y=0-3, —m=12-18; —p=8-18, and on the condition that, when X is 0, y is other than 0. The thermochromic pigment composition are also useable in thermochromic pigment microcapsules ink compositions including the thermochromic pigment microcapsules, and writing instruments including the ink compositions.

Abstract: There are described thermal imaging members and thermal imaging methods utilizing unsymmetrical rhodamine compounds. The rhodamine color-forming compounds exhibit a first color when in a crystalline form and a second color, different from the first color, when in an amorphous form.

Abstract: A method for using a rewritable thermal label of the non-contact type which comprises recording and erasing information repeatedly in accordance with a non-contact method on a rewitable thermal label which remains attached to an adherend. The label contains (a) an anchor coat layer comprising a crosslinked resin, (b) a heat-sensitive color development layer, (c) a light absorption and a photo-thermal conversion layer and (d) an adhesive layer. The layers (a), (b), (c) and (d) are laminated on a first face of a substrate successively. The anchor coat layer is placed next to the substrate. The layer (c) is an outermost layer. The adhesive layer is placed on a second face of the substrate.

Abstract: Compositions and methods for production of color images having increased light stability and reduced browning are described. The color forming composition can include a leuco dye, an infrared absorber, and at least one of a stabilizer and an anti-fade agent. The color forming compositions can be stabilized such that less than about a 30% decrease in optical density occurs over a three year period. The stabilizers can include chroman, thiolane-nickel complexes, spiroindanes, while suitable anti-fade agents can include vitamin E, vitamin E analogs, astaxanthin, chroman, ascorbic acid, carotene, and mixtures thereof. The color forming compositions are ambient light stable and are useful in forming images on a wide variety of substrates such as optical disks.

Abstract: A rewritable thermal label of the non-contact type which comprises an anchor coat layer comprising a crosslinked resin, a heat-sensitive color development layer and a light absorption and photo-thermal conversion layer which are laminated on one face of a substrate successively, the anchor coat layer being placed next to the substrate, and an adhesive layer placed on the other face of the substrate and allows recording and erasure of information repeatedly in accordance with the non-contact method; and a method for using a rewritable thermal label of the non-contact type which comprises recording and erasing information repeatedly in accordance with the non-contact method on the rewritable thermal label which remains attached to an adherend. Information can be recorded and erased repeatedly on the label which remains attached to the adherend and the label can be recycled together with the adherend.

Abstract: According to the present invention, a coloring property-improved novel leuco dye which can develop color by heat even with single molecule without requiring excess developer can be obtained, and an image recording medium having high sensitivity and excellent resolution effected by including the dye and a high sensitivity image recording medium further combined with an acid generator can also be obtained. Particularly, it relates to a leuco dye represented by a formula (1) (LD—(A—P1)n) and to an image recording medium which contains the leuco dye (wherein LD represents an acid-sensitive leuco dye mother nucleus, —A represents the residue of an acidic group (—AH) selected from —SO3H, —CO2H and —P(═O)(OH)xRy, R represents a hydrogen atom or a substituent group, x and y are x=1 and y=1 or x=2 and y=0, P1 represents a protecting group which can be removed by the action of light, heat or acid, and n is an integer of 1 or more).

Abstract: Disclosed are heat sensitive coatings and record materials that are environmentally resistant and therefore do not require a protective coating. One embodiment includes an environmentally resistant heat sensitive coating that includes an acrylate polymer of the formula: wherein R1 is a hydrogen or a methyl group, R2 is a hydrogen or a methyl group, and R3 is nitrile or chlorine or wherein, when c is 0, R4 is methyl, phenyl, or substituted phenyl, and when c is greater than 0, R4 is methyl, phenyl, substituted phenyl, halogen, nitrile or hydroxyl; m is greater than 1; n is greater than 1; b is greater than or equal to 0; and c is an integer from 0 to 38.

Abstract: A thermal sensitive recording medium which has a feature to change the color of recorded pattern slowly and spontaneously after development, and the recorded pattern which has several days passed can be easily distinguished from that of just after development. Said thermal sensitive recording medium contains 4-hydroxybenzoic acid ester represented by general formula (I) as a color developer and uses triphenylmethane-based leuco dye and reddish color developing leuco dye whose maximum absorption wave length is 450.about.560 nm as a dye precursor. ##STR1## In this formula, R represents unsubstituted or substituted lower alkyl group of carbon number 1-7 or benzyl group.

Abstract: Novel thermally imageable monochrome product compositions, elements, and processes are disclosed herein. These compositions and elements characteristically have high contrast and fast imaging speeds. The thermally imageable compositions of this invention comprise (a) at least one hexaarylbiimidazole (HABI) compound, (b) at least one leuco dye, (c) at least one acid-generating compound, (d) a polymeric binder, (e) optionally at least one UV stabilizer and/or at least one inhibitor of color formation, and, in certain embodiments, (f) at least one near IR-absorbing dye. These compositions have the propensity for affording, upon thermal imaging, highly colored images having high optical density values. At the same time, background color is low in preferred compositions even after extensive exposure to ambient light. These compositions can be imagewise heated to effect color formation (i.e.

Abstract: There is described a novel thermographic recording film, and more specifically, a novel image-forming system incorporated therein which employs both thiolactone and silver reduction chemistries.Thermographic recording films comprising the novel image-forming system of the present invention exhibit excellent thermal sensitivity and visual color saturation, enhanced image density, and desirable visual minimum optical densities.

Abstract: There is described a novel thermographic recording film, and more specifically, a novel image-forming system incorporated therein comprising at least one layer and including and a Lewis acid material, a di- or triarylmethane thiolactone dye precursor, an acidic organic material, a binder and a thermal stabilizer.Thermographic recording films comprising the novel image-forming system of the present invention exhibit excellent thermal stability, and desirable minimum optical densities, indicative of substantially less premature image development at elevated environmental temperatures.Embodiments of the invention wherein the thermographic recording films further include a light insensitive organic silver salt, a reducing agent, a binder and, preferably, a toning agent, show substantially enhanced image density.

Abstract: Particularly advantageous and environmentally compatible recording material comprises, in microencapsulated form, at least one 3,1-benzoxazine color donor dissolved in a solvent, and is characterized in that the solvent consists of one or more natural oils and/or one or more esters of fatty acids on which natural oils are based, or comprises such oils and/or esters.

Abstract: Disclosed is a heat-sensitive recording paper comprising (a) a support made of glassine paper, (b) a resin layer formed on one side or both sides of the support, and (c) a heat-sensitive recording layer containing a leuco dye and a color developing material which develops a color on contact with the leuco dye,the resin layer being formed by coating one side or both sides of the support with an organic solvent coating composition prepared by dissolving a resin in an organic solvent, followed by drying, andthe heat-sensitive recording layer being formed on the resin layer or on the side of the support not having the resin layer thereon,wherein the organic solvent coating composition may further contain a pigment in an amount of about 3 to about 25% by weight based on the amount of the resin.

Abstract: There is described, in thermographic recording films, the use of a compound containing at least two epoxide moieties in the protective layer and/or in a layer on top of the protective layer of thermographic recording films to reduce gouging and streaking of the printed image film and to reduce head build-up on the thermal printhead.

Abstract: This invention relates to thermographic recording films including both a topcoat and a washcoat. The washcoat comprises at least one quaternary ammonium salt including at least one hydroxyl functional group, and a compound containing at least two epoxide moieties. The subject washcoat, in conjunction with the topcoat, reduces printer head build-up and streaking of the printed image film while simultaneously increases the anti-stat and slip properties of the film.

Abstract: A pressure sensitive recording material utilizing a color forming reaction between a colorless or light-colored basic dye and a color acceptor, and in which (1) a basic dye-containing layer and color acceptor-containing layer are formed on one surface of different substrates respectively, (2) a basic dye-containing layer and color acceptor-containing layer are formed on one surface and the other surface of a same substrate respectively, (3) a basic dye-containing layer and color acceptor-containing layer are superposed on one surface of a substrate or (4) a layer containing both basic dye and color acceptor is formed on one surface of a substrate, the pressure sensitive recording material being characterized in that the basic dye is at least one diarylmethane compound represented by the formula (1) given below, the basic dye being enclosed in synthetic high polymer microcapsules having a mean particle size of 3 to 15 .mu.m and an average film thickness of 0.1 to 0.7 .mu.

Abstract: Pressure-sensitive copying material utilizes a blend of rapid- and slow-developing chromogenic materials in vegetable oil solvent and an acid clay or synthetic inorganic color developer. The surface pH of the color developer is not more than 8.7, which gives rise to enhanced image intensity and fade resistance compared with the use of conventional higher color developer surface pH values.

Abstract: There are disclosed thermally-processable image recording materials comprising a support carrying thereon; a di- or triarylmethane thiolactone dye, an organic silver salt, a binder; and a select group of substituted purine compounds for reducing a post-processed formation of a yellowish-brown color (gilding) within such materials.

Abstract: Pressure or heat sensitive record materials with color-former sublimation problems reduced or eliminated whilst not reducing the color forming capability per unit weight of color former by providing what are in effect dimers of vinyl carbinols, with the vinyl carbinol residues, linked through the aryl groups of a diaryl amine.

Abstract: This invention relates to the use of a compound containing at least two epoxide moieties in the protective topcoat layer and/or in a layer on top of the protective topcoat layer of certain thermographic recording films to reduce gouging and streaking of the printed image film and to reduce head build-up on the thermal printhead.

Abstract: An excellent pressure-sensitive copying material which has an excellent color developing rate, little odor, and neither offset nor bleeding, said copying material comprising an electron-accepting developer and a solution of an electron-donating color former which produces colors upon contacting with said color developer, wherein it is characterized in that the solvent of said solution comprises a mixture of 20 to 80 weight % of sec-butyldiphenylmethane, 5 to 50 weight % of sec-butyldiphenylethane (1,1) and 5 to 50 weight % of sec-butyldiphenylethane (1,2).

Abstract: This invention provides transparent thermographic recording films which exhibit good anti-stick properties, are scratch resistant and substantailly craze-free. The thermographic recording films comprise a transparent support carrying (a) a dye image-forming system comprising a di- or triarylmethane thiolactone dye precursor, an organic silver salt, a heat-fusible organic acidic material, and polyvinylbutyral as the binder; and, (b) a protective topcoat layer positioned above said dye image-forming system and comprising a water-insoluble polymeric binder, a mixture of at least two colloidal silicas having different average particle diameters in the proportion, by weight, of 1 part of silica having an average diameter of 50 nm or smaller and 0.3 to 1 part of silica particles having an average diameter no more than 40% of the larger sized silica particles, the ratio of total silica to binder being at least 3 parts per weight silica to 1 part per weight binder.

Abstract: This invention relates to novel recording materials which employ color-forming di- and triarylmethane compounds possessing certain S-containing ring-closing moieties, namely, or thiolactone, dithiolactone or thioether ring-closing moiety and to a method of forming color by contacting these dye precursor compounds with a Lewis acid material capable of opening the thiolactone, dithiolactone or thioether ring-closing moiety whereby the compound is rendered colored, that is, converted to its chromophore color.

Abstract: Pressure- or heat-sensitive record material utilizes a chromogenic material of the formula (I) (Ia or Ib): ##STR1## in which: one of A.sub.1 and A.sub.3 is an optionally-substituted carbocyclic aryl group and the other of A.sub.1 and A.sub.3 is either an optionally-substituted aryl group which is the same as or different from A.sub.1, or an optionally-substituted nitrogen-containing aromatic heterocyclic group, with the proviso that if both A.sub.1 and A.sub.3 are aryl groups, then at least one of A.sub.1 and A.sub.3 has a substituted amino or -N-heterocyclic substituent in the 4- position (relative to the bond joining A.sub.1 or A.sub.3 respectively to the remainder of the molecule);A.sub.2 is hydrogen or an optionally-substituted aryl, alkyl or aralkyl group; andA.sub.4 is hydrogen or an optionally-substituted alkyl, aryl or aralkyl group.

Abstract: A recording material containing as a color forming system (a) a colorless or light-colored dye precursor, and (b) an isocyanate compound having aromaticity, and if necessary (c) a color developer is excellent in image retention rates in terms of light resistance and plasticizer resistance and has a large degree of freedom of hues.

Abstract: An imaging medium has at least one color-forming layer comprising a thermal color-forming composition adapted to undergo a change of color upon increase in the temperature of the color-forming layer above a color-forming temperature for a color-forming time, the color-forming layer further comprising a color stabilizer which is a hindered amine light stabilizer or a nitrones. The color stabilizer prevents development of color in the medium during storage before or after imaging, thereby reducing the minimum optical density of the imaged medium and/or color distortion in the image.

Abstract: Color-forming agents of the present formula ##STR1## wherein X denotes hydroxyl, alkoxy, alkenyloxy, aralkoxy, cycloalkoxy, aryloxy, acyloxy, alkylamino, dialkylamino, acylamino, aralkylamino or arylamino,R denotes alkyl, alkenyl or aralkyl,R.sub.1 denotes hydrogen, halogen or alkyl,R.sub.2 denotes alkoxy, aralkoxy, aryloxy or a radical of the formula ##STR2## Y.sub.1 and Y.sub.2 independently of one another denote alkyl, aryl, cycloalkyl or aralkyl or R.sub.2, with A in the o-position, forms a heterocyclic ring,a benzene ring can be fused onto the ring A, in the o-/m-position relative to R.sub.2, and the ring A and its fused-on rings and the cyclic and acyclic radicals can carry further non-ionic substitutents customary in dyestuffs chemistry,are used in thermoreactive papers and pressure-sensitive recording materials and for the preparation of highly concentrated solutions in organic solvents.

Abstract: The object of the present invention is to provide a pressure sensitive reproducing material which allows reproduced letters or patterns to fade away automatically with the passage of time.The present invention accomplished the object by a pressure sensitive reproducing material composed of a colored material supported on a substrate, in which the colored material comprises a phthalein group indicator which is color-developed by an inorganic weakly basic substance jointly used with an organic amine and contains hygroscopic material or drying inhibitor.

Abstract: A leuco dye of the formula (I): ##STR1## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7, and R.sup.8 each represent a lower alkyl group; A represents ##STR2## in which R.sup.9 and R.sup.10 each represent hydrogen, provided that both R.sup.9 and R.sup.10 may not be hydrogen, --CN or --COR.sup.14 in which R.sup.14 represents a phenyl group which is unsubstituted or is substituted by an alkyl group having 1 to 4 carbon atoms, an alkoxyl group having 1 to 4 carbon atoms, or a halogen, a naphthyl group which is unsubstituted or is substituted by an alkyl group having 1 to 4 carbon atoms, an alkoxyl group having 1 to 4 carbon atoms or a halogen, or a lower alkoxyl group, ##STR3## in which ##STR4## represents a phenyl group or a naphthyl group, R.sup.11 represents hydrogen, a lower alkyl group, a halogen, an amino group, which is unsubstituted or is substituted by an alkyl group having 1 to 4 carbon atoms, or a nitro group, or ##STR5## in which R.sup.

Abstract: A recording material comprising at least one thermodecoloring type dye represented by formula (I) or formula (II) ##STR1## wherein Ar.sup.1 and AR.sup.2 each represents an aryl group or heteroaryl group; R.sup.1 represents an alkyl group, alkenyl group, aralkyl group, aryl group, or heteroaryl group; A represents an atomic group forming a 5 to 6 member ring; and X.crclbar. represents a residue with an electric charge value of -1; or Ar.sup.1 and Ar.sup.2 bond together to form a ring.

Abstract: A leuco dye of the formula (I): ##STR1## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7, and R.sup.8 each represent a lower alkyl group; A represents ##STR2## in which R.sup.9 and R.sup.10 each represent hydrogen, provided that both R.sup.9 and R.sup.10 may not be hydrogen, --CN or --COR.sup.14 in which R.sup.14 represents a phenyl group which is unsubstituted or is substituted by an alkyl group having 1 to 4 carbon atoms, an alkoxyl group having 1 to 4 carbon atoms, or a halogen, a naphthyl group which is unsubstituted or is substituted by an alkyl group having 1 to 4 carbon atoms, an alkoxyl group having 1 to 4 carbon atoms or a halogen, or a lower alkoxyl group, ##STR3## in which ##STR4## represents a phenyl group or a naphthyl group, R.sup.11 represents hydrogen, a lower alkyl group, a halogen, an amino group, which is unsubstituted or is substituted by an alkyl group having 1 to 4 carbon atoms, or a nitro group, or ##STR5## in which R.sup.

Abstract: Disclosed is a class of dyes that can be rendered colorless by the application of heat in a silverless imaging system and which have utility in information recording and display, color hard copy and photographic systems. Certain dye ylides, for example, xanthane dye ylides are disclosed which when incorporated in a thin film can be imagewise heated to develop an image by reason of the ylide dye disassociating in the areas where heated and thereby being rendered colorless.

Abstract: Bis{[1-(2-A-2-B)ethenyl]}[2-R.sup.2 -4-X-phenyl-or 1-(2-R.sup.2 -4-X-phenyl) ethenyl]methanes useful as color formers, particularly in transfer imaging, pressure sensitive and thermal-responsive carbonless duplicating systems, are prepared by the interaction of two molecular proportions of the corresponding 1-A-1-B-ethene with the appropriate 2-R.sup.2 -4-X-phenyl-(CH.dbd.CH).sub.n --CHO presence of an acidic catalyst.

Abstract: A pressure-sensitive recording material is disclosed, having microcapsules containing a triphenylmethane leuco-dye dissolved in a solvent composed at least 80% of plant, animal or paraffin oils, the microcapsule walls being formed of a wall material which is water-soluble or water-dispersible before the formation of the microcapsules.

Abstract: A recording material is disclosed, which comprises a support having thereon a recording layer containing an at least one electron-donating colorless dye and at least one electron-accepting compound, wherein said electron-donating colorless dye is a copolymer of at least one polymerizable colorless dye monomer and at least one comonomer. The recording material has improved color forming property, shelf life, and stability of colored images.

Abstract: A heat responsive recording element having a recording layer containing a colorless di- or tri-aryl methane compound having a closed ring moiety incorporating the meso carbon atom and containing a nitrogen atom directly bonded to the meso carbon. The nitrogen atom is also bonded to a heterocyclic or carbocyclic ring substituted with an alkylating group. Upon heating the recording layer imagewise the alkylating group effects intramolecular alkylation of the nitrogen atom with irreversible breaking of the meso carbon atom-nitrogen atom bond rendering the compound colored in the imagewise heating pattern.

Abstract: A heat responsive recording element having a recording layer containing a colorless di- or tri- aryl compound having a closed ring moiety incorporating the meso carbon atom and containing a nitrogen atom directly bonded to the meso carbon. The nitrogen atom is also bonded to an arylating group that has a displaceable entity. Upon heating the recording layer imagewise the arylating group effects intramolecular arylation of the nitrogen atom with irreversible breaking of the meso carbon atom-nitrogen atom bond rendering the compound colored in the imagewise heating pattern.

Abstract: Disclosed herein is a heat-sensitive recording material in which a chromogenic dye-procursor comprises:(A) a mixture comprising at least two divinyl phthalide compounds represented by the formula (I): ##STR1## wherein R.sup.1 and R.sup.2 represent respectively alkyl group of 1 to 6 carbon atoms, alkyl group of 1 to 4 carbon atoms having an alkoxy group of 1 to 4 carbon atoms or cycloalkyl group of 5 to 7 carbon atoms, X.sup.1 and X.sup.2 represent respectively hydrogen atom, alkyl group of 1 to 8 carbon atoms, alkoxy group of 1 to 8 carbon atoms, alkoxy group of 1 to 4 carbon atoms having an alkoxy group of 1 to 4 carbon atoms, cycloalkoxy group of 5 to 7 carbon atoms, alkenyloxy group of 3 to 8 carbon atoms, benzyloxy group, substituted benzyloxy group, furfurloxy group, tetrahydrofurfuryloxy group, phenyl group, substituted phenyl group, phenoxy group, substituted phenoxy group, fluorine atom, chlorine atom or bromine atom, m and n represent 0 or integer of 1 to 4, m+n=4 and R.sup.1 and R.sup.

Abstract: A light-sensitive material comprising a light-sensitive layer containing silver halide, a reducing agent, a polymerizable compound and a leuco dye provided on a support, characterized in that the leuco dye is a compound having the following formula (I): ##STR1## in which A is --O-- or ##STR2## and R is a substituent having a positive substituent constant (.sigma.); one of X and Y is nitrogen and the other is carbon; each of R.sup.1 and R.sup.2 is an alkyl group, a cycloalkyl group or an aralkyl group; each of R.sup.3 and R.sup.4 is an alkyl group, an alkoxy group, a cycloalkyl group, an aralkyl group or an aryloxy group; each of l, m and n is 1 or 2.

Abstract: The invention relates to a process for encapsulating solutions of reactants of color reaction systems by means of conventional encapsulation processes, the reactant initially being dissolved in a solvent with a good dissolving power and immediately prior to emulsification or encapsulation, a non-solvent, which only dissolves the reactant to an insignificant extent, is admixed with the fresh solution obtained in a quantity which adjusts a supersaturated system, the capsules obtainable as a result thereof, as well as their use with an encapsulated solution of a basic color former, which supplies a color by the reaction with acid reactants, in non-carbon paper.

Abstract: A recording material comprising a support having provided thereon, microcapsules containing a basic leuco dye dissolved in an organic solvent is disclosed. The microcapsules further contains at least one compound selected from the group consisting of 2-mercaptobenzothiazoles, guanidines, thioureas, 2-mercaptoimidazolines and thiurams. The recording material is free from coloration due to exposure to light and provides a recorded image of high density and high contrast.

Abstract: A thermal imaging method is provided which employs as color-forming co-reactants, (a) a substantially colorless di- or triarylmethane compound possessing on the meso carbon atom within its triarylmethane structure an aryl group substituted in the ortho position with a nucleophilic moiety which is ring-closed on the meso carbon atom and (b) an electrophilic reagent which upon heating and contacting said di- or triarylmethane compound undergoes a bimolecular nucleophilic substitution reaction with the nucleophilic moiety to form a colored, ring-opened di- or triarylmethane compound.

Abstract: A heat-sensitive recording material is disclosed, comprising a support and heat-sensitive recording layer on the support, said layer containing (1) microcapsules containing a basic dye precursor and an organic solvent as the core thereof and (2) a developer capable of forming color on reacting with the basic dye precursor, wherein said microcapsules are produced by the use of a wall-forming substance comprising xylylene diisocyanate or an adduct thereof such that the microcapsule walls are impermeable to both the basic dye precursor and developer at room temperature, but at a temperature of a thermal head during a recording process, become permeable to at least one of the basic dye precursor and developer, thereby permitting formation of a colored image as a result of the reaction between the basic dye precursor and the developer.

Abstract: Disclosed is a colorless dye for use in recording materials comprising a compound represented by the general formula: ##STR1## Said recording materials are of the types which utilize a color developing reaction between an electron-donating colorless dye and an electron-accepting color developer, such as, carbonless pressure-sensitive recording materials, thermosensitive recording materials, etc.

Abstract: A new reversible heat sensitive recording composition for erasable recording is disclosed which is used as a recording or marking material that is capable of erasure at a specific temperature so that coloring or decoloring can be controlled to occur at desired temperature ranges. This range or the hysteresis value of the composition is determined by the .DELTA.T value of the contained ester compound. In addition, the composition can provide a wide variety of hues and can readily be prepared. The marking or recording drawn in this composition can be maintained at room temperature or less.

Abstract: A heat-sensitive recording material excellent in stability of recorded images is disclosed, which comprises a support having formed thereon a heat-sensitive recording layer comprising a colorless or pale-colored basic dye and a color developer capable of forming a color upon coming into contact with the basic dye, wherein said heat-sensitive recording layer further comprises at least one compound selected from the compounds represented by following formulae (I) and (II): ##STR1## wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, and R.sub.8 independently represent a hydrogen atom or an alkyl group having from 1 to 4 carbon atoms.