Photographic Color-Forming Development Reaction W. A. SCHMIDT’, VSEVOLOD TULAGIN, J. A. SPRUNG, R. C. GUNTHER, R. F. COLES, AND D. E. SARGENT Central Research Laboratory, General Aniline & Film Corp., Easton, Pa.
THE
amateur and professional photographers of today have available for use color films, in which three organic dyes are synthesized during the development of the conventional photographic silver image. The process was discovered by Fischer (6) in 1912 and was named by the inventor “the process of color-forming development.” In the conventional photographic development process, the silver ion in the exposed light-sensitive silver halide is reduced to the silver atom and an image consisting of metallic silver is produced. This reduction is brought about by so-called developers, mostly organic aromatic hydroxy or amino compounds, such as hydroquinone, p-aminophenol, or p-phenylenediamine and their substitution products. During the reduction of the silver ions, the organic developing substance becomes oxidized. In the process of color-forming development, the oxidized form of a photographic developing substance is condensed with a second substance, a n active coupler, to form a dye. The reaction can be represented by the equations given in Figure 1. Dye condensation takes place in situ and in direct proportion with the formation of the silver image. Color-forming development yields, therefore, a silver image intimately associated with a dye image. The silver image can be removed by standard bleaching processes, and pure dye images can be obtained. I n color photography, it has become accepted practice to use three suitably selected colors as primaries.
Fischer fully realized the possibilities that his new reaction had opened. It had always been necessary, in color photography, to obtain three (color separation) images and to give each of them individual treatment. Fischer’s process could be applied to a photographic film, which had the three images in three different emulsion layers on the same support in permanent superposition, The three images could be formed simultaneously in a single step of development and therefore i t was no longer necessary to separate the layers for individual treatment. The problem of registration of the three individual images was eliminated. A multilayer color film, a s envisaged by Fischer, was not produced until 1936, after Agfa chemists (16) had found a way to prepare couplers of high molecular weight, which could be dispersed in photographic emulsions but would not migrate from one layer to another. Kodachrome (4, 8) film, which wasintroduced in 1935, still required three steps of color development. R CO CH2
R t
co NHR
Present address, Ansoo Division, General Aniline & Film Corp., Binghamton, N. Y . 1
‘ZH5
co NHR
-
4% Y X W
CH3 N-C
Yellow. Green and red transmittant: blue absorbant: minus blue Magenta. Red and blue transmittant: green absorbant: minus green Cyan. Blue and green transmittant: red absorbant: minus red
By mixing these primaries in various proportions, a complete range of colors can be matched in a manner satisfactory to the average human observer. Fischer has suggested, as couplers, three classes of colorless compounds which form the three essential primary colors with one common developing substance. In Figure 1 are shown examples of dye; obtainable by colorforming development. A single developing substance, N ,N diethyl-p-phenylenediamine, is used in all three cases. With a n open-chain ketomethylene compound, such as acetoacetanilide, a yellow dye results. A magenta dye is obtained from 1phenyl-3-methyl-5-pyrasolone. The yellow and magenta dyes belong to the azomethine class. When a phenolic compound, such as l-hydroxy-2-naphthanilide, is used as the coupler, a cyan dye, which is of the quinoneimine class, is obtained. It was fortunate that Fischer was able t o select a class of dyes, ax, bx, cx, which possessed characteristic absorption bands in the three principal regions of the visible spectrum and which could be obtained by the condensation of a single oxidized developer, z, with one of three colorless couplers, a, b, c.
pf2H5
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t i 2 e
MAGENTA
Figure 1.
Dyes Obtainable by Color-Forming Development
The azomethine and quinoneimine dyes produced by Fischer’s process of color development leave much to be desired from the point of view of the chemist and the color photographer. Their stability is limited, and they deteriorate under the influence of light or certain chemicals, particularly acids. The spectral transmittance characteristics of the dyes, chiefly the magenta representatives, are not very satisfactory. Whereas the human eye accepts modern color photographs as satisfactory records of colored objects, more or less complicated procedures have to be introduced in order to make corrections for faulty absorption of the dyes, when an original photographic color image has to be reproduced without objectionable color degradation. AZOMETHINES QUINONEIMINES, KETO ANILES
In the field of dye chemistry, the szomethines, quinoneimines, or keto aniles are of interest chiefly as reactive intermediates, which can be converted to more stable compounds or dyes. 1726
.IN D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
August 1953
The aeomethine linkage is readily cleaved by mineral acids, azomethines are converted to keto compounds, and quinoneimines to quinones (6).
R R
SNP
'N'
1727
by a solution of sodium carbonate of less than 10% concentration. The reaction proceeds in a n elegant manner, when a suitable amino substituent is provided in the indamine in a n ortho position t o the nitrogen linkage between the two nuclei. An indamine of this nature can be obtained by two principal methods: 1. A coupler having an amino substituent ortho to the active cou ling position can be chosen. This coupler can be condensed witE an oxidized form of the conventional N,N-diethyl-p-phenylenediamine developer (la). 2. A conventional coupler can be used and a p-phenylenediamine carrying an amino substituent in ortho position t o the primary amino group can function as the oxidizable developer (10, 11, is,14).
*
%
Condensation products from p-nitrosodiethylaniline and active methylene compounds, generally named anils, are identical with the azomethines derived from color development with N,Ndiethyl-p-phenylenediamine. They can react with another active methylene compound with the elimination of the p-phenylene diamine. This reaction is utilized commercially on a large scale in the synthesis of asymmetrical thioindigo and other dyes (9). V N R 2
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ORANGE
MAGENTA
R
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+
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a
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A somewhat similar reaction mechanism has been described by Vittum and Duennebier (16),by which magenta azomethine dyes from pyrazolone and N,N-diethyl-p-phenylenediaminereact with active pyrazolone couplers t o form colorless bispyrazolone condensation products with regeneration of the p-phenylenediamine. Quinoneimine, indamine, and indophenol dyes undergo addition reactions such as are typical for 1,4 conjugated systems present in quinones. Indamines can be converted to azine and thiazine dyes (1,8).
X = NC6H5
S
Because mo$t of these side reactions require rather drastic conditions and are not likely to take place in the gelatin layers of developed color films, there is no need t o be unduly alarmed about the permanence of present-day color pictures. I n the Central Research Laboratory of General Aniline & Film Corp., investigations have been carried out on the application of other classes of dyes t o color photography, and on the formation of dyes, other than azomethines, by color-forming development. The reaction described in this paper yields azine dyes by color-forming development. AZINES
A suggestion to convert indamine dyes, in photographic layers, to azine or safranine dyes was made by Friedman (7). He proposed t o oxidize the indamine in the presence of another amine, such as aniline hydrochloride, and assumed that a safranine dye would be formed. This interesting speculation has not yet been successfully reduced to practice. Azine dyes readily form under the extremely mild conditions that prevail when a photographic layer is developed in a dilute aqueous solution at room temperature a t the alkalinity provided
Figure 2. Dye Development from Azine Coupler and Conventional Developer Figure 3. Dye Development from Conventional Coupler and Azine Developer
When 3'-(diethylamino) methanesulfonanilide is used as a coupler and N,N-diethyl-p-phenylenediamineas the developer (Figures 2 and 3), an exposed photographic silver halide emulsion can be developed to yield, in addition to the normal silver image, a transient blue indamine dye image. As ring closure occurs, the indamine dye changes gradually into an orange-brown and finally into a n orange-yellow azine image in the alkaline developer solution. Upon acidification, the azine dye is converted into its brilliant bluish red or magenta form. MECHANISM OF AZINE RING FORMATION
The mechanism of azine ring formation can be considered analogous t o addition of a n electronegative substituent t o a 1,4 conjugated or a quinoid system. The primary indamine or X
X
X
quinoneimine condensation product can be written in three resonance forms (Figure 4). Of these, form 1 possesses the quinoid structure, to which the proposed mechanism can readily be applied. The chemical nature of X and Y and the electronic character of substituents in the nuclei are significant factors which influence the contributions of the possible resonance forms in the system. A few practical examples, given in Figure 5, should serve t o illustrate these conditions. The structure of the dyes is written so that the upper and lower aromatic nuclei originated from the coupler and developer, respectively.
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Cases 1 and 2 were derived from p-phenylenediamine developers. I n cases 3 and 4 the o-phenylenediamine structure was present in the developer. Likewise the couplers in cases 1 and 2 possess an amino substituent ortho to the active coupling
Figure 4. Resonance Forms of Primary Indamine or Quinoneimine Condensation Product
positions. In cases 3 and 4 conventional types of couplers are represented. Because the quinoneimine, shown in case 1, is stable, azine ring closure does not occur under conditions prevailing during photographic development. Form 3 (Figure 4) is predominant in the resonance equilibrium, and form 1, essential for azine dye formation, is sufficiently suppressed. In case 2, the free energies of forms 1 and 3 (Figure 4) are nearly equal and differ from 2. Becording to the theory of resonance, this would lead t o a predominance of resonance forms 1 and 3 over form 2. The negative character of the substituent on the nitrogen atom ortho to the nitrogen linkage tends to suppress the orthoquinoid form 2. The blue indamine can be observed momentarily, but azine ring formation follows promptly as the secondary reaction goes to completion.
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1
Figure 5.
2
3
4
Examples of Azine Ring Formation
I n case 3, in which a 1-naphthol derivative is used as a coupler, the resulting quinoneimine would have, as chief contributor, the resonance form involving the neutral, nonpolar form 1 (Figure 4) characterized by the double-bonded oxygen and the stable 1,4naphthoquinone configuration. Azine ring closure, with elimination of the sulfinic acid radical, occurs rapidly. 8-Quinolinol (case 4) presents an interesting case and requires consideration of various factors: The nega’ M S e H 3 tive character of the heterocyclic nitrogen atom makes the electron on W N H - c s H s the oxygen _ _ less available and would L favor forms 2 and 3 in Figure 4. HQ Chelate ring formation would also favor forms 2 and 3. However, in the example shown, the combined effect of two electronegative phenyl groups and dNk the attraction of electrons by the SUIfonic acid group suppresses quinone formation in the nucleus contributed by the developer, and favor form 1 (Figure 4). Because several opposing forces are present, ring closure takes place only slowly, and, in the case shown, incompletely. In the example given in Figure 6, again the accumulative effects of two electronegative phenyl groups and the
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attraction of the electrons by the sulfonic acid radical in the peri position are sufficient to overcome the strong tendency to form a 1,4-naphthoquinone. The chief resonance contribution corresponds to form 3 and azine ring closure takes place rapidly. In order to apply the new azine color-forming reaction to color photography, it appeared desirable to select the second alternative mentioned above. This method would require couplers of the conventional type, such as had been incorporated in photcgraphic emulsions for some time, and would offer the possibility of making significant changes in the developer molecule during the required search for a yellow, magenta, and cyan representative in the class of azine dyes.
Azine color developers having the required amino substituents in ortho position to the primary amino group could be conveniently prepared from 2,4-dichloro-5-nitrobenzenesulfonic acid (2). The two active chlorine atoms could be replaced by secondary or tertiary amino groups, and the nitro group could be reduced to the primary amino group as a last step. The spwtral transmittance of the final dye is significantly affected by substituents in the developer molecule. Couplers carrying a n amino group as the activating substituent are, as a rule, not sufficiently inert and cause undesirable effects on photographic emulsions. Therefore, phenolic or enolic couplers are preferable in color photography. Most of the azine dyes have a strong absorption band in the green region of the spectrum and are of red, purple, or bluish hue. There was no difficulty in selecting couplers for the magenta image, as 8-quinolinol and 1-naphthol derivatives gave very brilliant magenta azine dyes (Figure 4,cases 3 and 4). In the search for yellow couplers, the open-chain ketomethylene compounds were considered. Condensation between the couplers and the oxidized azine developing substances occurred, but the resulting yellow dyes were indicators, and changed from yellow to magenta upon acidification. It was found that the yellow form could be stabilized in photographic gelatin, when substituents in the coupler a s well as in the developer molecule were properly chosen. Stable yellow images were obtained by use of an aroyl acetoarylide as a coupler and 5-amino-2-arylamino-4 alkylaminobenzenesulfonic acid as the developer. The constitution of the yellow dye has not been proved conclusively. The
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Figure 6 . Azine Ring Formation
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
August 1953
I
400
500
600
500
700
Figure 7.
C.
Yellow a w m e t h i n e Magenta awmethine
A.
B.
600
D.
Cyan q u i n o n e i m i n e Yellow a z i n e
formula given is in best agreement with a n analysis of a representative of this class. For the isolation, purification, and analysis of this compound, the authors are indebted to C. W. Gould.
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500
100
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Spectral Transmittance Curves E. F.
Magenta a z i n e Cyan azine
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application. It is possible to manufacture azine color couplers, which can be incorporated in photographic emulsions, and which do not have a detrimental influence on the critically balanced photographic characteristic of such photographic emulsion layers. Multilayer color films, employing the azine color-forming reaction, can be exposed, developed, and viewed or projected, exactly like the currently available color films, such a s Ansco color or Ektachrome. The new field offers many opportunities for selecting new couplers and even new developers, which should eventually replace the compounds t h a t have been selected a s a first acceptable choice. I n Figure 7 are given spectral transmittance curves of conventional dyes obtained by color development with N,N-diethyl-pphenylenediamine
a. Yellow from a benzoylacetanilide derivative b. Magenta from a 1-phenyl-3-methyI-5-pyrazolone c.
Cyan from a 1-hydroxy-2-naphthoic acid anilide
and corresponding azine dyes obtained from color development with 5-amino-2-arylamino-4-alkyIaminobenzenesulfonic acid.
NH
9
d.
Yellow from a benzoylacetanilide derivative
e. Magenta from a n 8-quinolinol derivative f. Cyan from 6-anilino-1-naphthol-3-sulfonic acid derivatives This work was carried out in a n industrial research laboratory, 50p
90;
-
MAGENTA
YELLOW
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It proved difficult to find a suitable cyan azine dye which could be obtained from the same developer and a t the same time as the yellow and magenta dye. A solution was found when 6anilino-I-naphthol-3-sulfonicacid (J acid) was used as a coupler. The formation of the bluish green dye is outlined here.
and the problem of developing the new reaction for early commercial application was given preference over the study of the many fundamental aspects and theoretically interesting phases. A color film utilizing this new reaction and yielding images composed of azine dyes is a t present in the development stage and will be introduced commercially when necessary arrangements for large scale manufacture have been completed. ACKNOWLEDGMENT
The following members of the staff made valuable contributions during the early stages of this investigation: R. T. Olsen, A. C. Starke, C. H. Stratton, and R. S. Johnson. LITERATURE CITED
The J acid derivative is a phenolic coupler. I n contrast to the color-forming development reaction which takes place usually in 4 position of aromatic compounds, J acid couples in 2 position or ortho to the hydroxyl group. During azine ring closure, the hydroxyl group is eliminated and the nitrogen atom of the 6-anilino group assumes the role of a resonance terminal in the final dye. The ring closure results in a n extension of the number of conjugated bonds between resonance terminals, and produces the desired shift of the absorption band from the green to the red portion of the spectrum. A 2,6-quinoid configuration in the naphthalene ring is one of the contributing forms of resonance. COiMMERCIAL APPLICATION
With a suitable representative available of each of the three primary colors, yellow (minus blue), magenta (minus green), and cyan (minus red), the new reaction was ready for commercial
(1) Andresen, M., Ber., 19,2214 (1886). (2) Badische Anilin und Soda Fabrik, Ger. Patent 120,345 (1901); Beilstein's Handbuch, Vol. 11, p. 74,Berlin, Julius Springer, 1928. (3) Bernthsen, A.,Ber., 19,2690(1886). (4) Davies, E. R., Phot. J., 76,248 (1936). (5) Ehrlich, P., and Sachs, F., Ber., 32,2341 (1899). (6) Fischer, Rudolph, and Siegrist, H., Phot. Korr., 51, 18 (1914): U. S.Patent 102,028(June 30,1914). (7) Friedman, J., Am. Phot., 35,224 (1941). ( 8 ) Mees, C.E. K., J . FrankZinInst., 233,41-50 (1942). (9) Pummerer, R.,Ber., 43, 1373 (1910). (10) Sargent, D.E., and Gunther, R. C. (to General ilniline & Film Corp.), U..S. Patent 2,522,802(Sept. 19,1950). (11) Schmidt, W.A., and Gunther, R. C. (to General Aniline & Film Corp.), Ibid., 2,527,379(Oct. 24,1950). (12) Schmidt, W.A.,and Sprung, J. A. (to General Aniline & Film Corp.),Ibid.,2,536,010 (Dec.26,1950),2,543,338 (Feb.27,1951). (13) Schmidt, W.A. and Tulagin, V. (to General Aniline & Film Corp.),Ibid., 2,486,440(Nov. 1, 1949). (14) Tulagin, V. (to General Aniline & Film Corp.), Ibid., 2,414,491 (Jan. 21, 1947). (15) Vittum, P. W., and Duennebier, F. C., J . Am. Chem. Soc., 72, 1536 (1950). (16) Wilmanns, G.,Kumetat, K., Froehlich, A,, Schneider, W., and Brodersen, R. (to Agfa Ansco Corp.), U. S. Patent 2,186,849 (Jan. 9,1940).
RECEIVED for review January 29, 1953. ACCBPTEDJune 2, 1953. Presented before Section 12, Organic Chemistry, at the XIIth International Congress of Pure and Applied Chemistry, New York, N. Y., September 10 t o 13, 1951.
