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THE ACTION OF OPTICAL SENSITIZERS ON THE PHOTOGRAPHIC PLATER \ G. KORNFELD Rodak Research Laboratories, Rochester, New York Received May 25, 1988 In this paper a survey is given of the published work dealing with the mechanism of optical sensitizing of photographic emulsions, the connection between sensitizing and other properties, and the mechanism of desensi- tizing. There are some facts available concerning the first point, and a close connection has been established between light absorption and sensi- tizing power. Up to the present time, however, only fragmentary infor- mation has been published on the relation between sensitizing power and other properties and on the nature.of desensitizing. I. INTRODUCTION The spectral range of photographic action Since photochemical action in any system is necessarily connected with its light absorption, the light absorption in silver halide emulsions is of fundamental importance for determining the spectral range of photographic action. The absorption of pure silver bromide in a microcrystalline state was measured by Slade and Toy (1) in 1920. They found absorption beginning at about 450 mu which strongly increased towards the violet and ultra- violet regions. Eggert and Noddack (2), in 1923, found the limit towards the long wave length region at 465 m,u. In 1928 Eggert and Schmidt (3) undertook a very careful investigation of thin layers of microcrystalline silver bromide and silver chloride, for which they found the absorption limits towards the longer wave lengths, at 480 mu and 400 mu, respectively. There is no limit towards the shorter wave lengths in pure silver bromide. Absorption and photochemical action occur throughout the whole ultra- violet region and beyond it. It is well known that the photographic plate was instrumental in Roentgen's discovery of x-rays. Photographic action, however, in this region of very large quanta is essentially different from that in visible and ultraviolet light. Glocker and Traub (4) found that, with x-rays, there was no threshold of sensitivity for small intensities, ~ Contribution No. 10 to the Third Report of the Committee on Photochemistry, National Research Council. 97 .
98 G. EORNFELD and Bunsen's reciprocity law was found to be valid over a long range. It is not to be expected that the law of photochemical equivalence will hold in the region of x-rays, where the photochemical action corresponds to the amount of energy transferred to secondary electrons. This was the con- clusion from Glocker's (5) investigation in 1927. Eggert and Noddack (6) some time before had found that one quantum of x-rays absorbed corresponds to about 103 silver atoms released. That the law of equiva- lence does not hold in the x-ray region was also confirmed by Gunther and Tittel (7~. Between 0.245 A. and 1.54 i. the number of silver atoms per quantum absorbed changed from 920 to 148, but the ratio between the energy of the secondary electrons and the amount of reduced silver was found to be fairly constant. The absorption of gelatin is negligible in the x-ray region, but, in the visible and ultraviolet regions, the presence of gelatin in the photographic emulsion distorts the close correlation between absorption and photo- graphic action which can be observed in pure silver halides. Eggert and Noddack (2) determined the absorption of emulsion-coated plates in the region of the longer wave lengths and found more than 20 per cent absorp- tion, even at 615 ma. This absorption is of no use in photography, how- ever, since gelatin can not act as a sensitizer. In the far ultraviolet region gelatin actually impedes photographic action by its own strong absorption. For the region below 200 mu, therefore, Schumann plates, which do not contain any gelatin, are used. Absorption is not so well defined in a photographic emulsion as it is in pure silver bromide. It is dependent, to some extent, on the mode of preparation. In an investigation with Frankenburger, Fajans (8) was the first to point out the deforming influence of adsorbed ions on the crystal forces which naturally must result in a spectral shift. In several succeed- ing investigations this spectral shift was thoroughly examined by him and his coworkers (9, 10, 11, 12, 13~. Recently de Boer (14) has quantitatively connected the spectral shift with the heat of adsorption in the normal and excited state. There is another factor which influences the absorption even more during exposure, that is, photolytically developed silver in colloidal form. The phenomenon was discovered as early as 1840 by E. Becquerel (15, 16) with a Daguerreotype plate (silver iodide on silver) which could be made sensi- tive to the yellow and even to the red region by long exposure. He observed the same effect with silver bromide and silverer chloride papers. He therefore called the blue end of the spectrum "exciting radiation" and the yellow and red regions "continuing radiation". Luppo-Cramer (17) and Eder (18), in 1909, explained the phenomenon as optical sensitizing by colloidal silver, and Luppo-Cramer was able to reproduce the phe- nomenon by adding colloidal silver to silver chloride. The great extension .
ACTION OF OPTICAL SENSITIZERS 99 of the absorption region pointed clearly to the absorption of light by the colloidal silver and to its sensitizing property. This sensitizing property of colloidal silver, as well as of colloidal silver sulfide, in analogy to the organic sensitizing dyes was extensively considered by Sheppard (19~. In 1873 H. W. Vogel (20) discovered that some of the strongly absorbing organic dyes could be used as sensitizers for the photographic plate. Coral- lin was the first dye used by him to sensitize the plate for the yellow and green regions. He saw at once the significant correspondence between the absorption of the dye in solution and the sensitized region, although both regions do not coincide exactly, and he foresaw its practical importance. The value of this discovery can hardly be overemphasized, since panchro- matic and orthochromatic plates were developed as a result, and the exten- sion of photography during recent years far into the infrared region can be traced to its influence. Most of the dyes which were employed as sensitizers in the beginning are now rarely used. At present nearly all the sensitizers used belong to the polymethine group, many of them being cyanine dyes. It is not intended here to describe in detail all the work that has been done on this subject. It ought to be mentioned, however, that close connections were found between the constitution of the dyes and their spectral range of absorption in solution. Once these connections were established, sensitizing dyes were synthesized for the whole visible region of the spectrum and through the infrared as far as 1356 me (21~. The systematic connection, however, has been confined so far to the constitution of a dye, on the one hand, and the intensity as well as the region of its absorption, on the other. As yet no evidence has been found to connect its sensitizing property with its chemical constitution or with any other quality. A great number of empirical facts on optical sensitizers have been found, but it seems hardly possible to fit them into a consistent picture. The conflicting results of earlier experiments can be explained partly by the fact that the sensitizers used were often not chemically pure and in some cases were even combined with various admixtures, but, even with pure materials, the complex nature of the phenomena makes it difficult to arrive at simple laws. Desensitizing was frequently regarded as optical sensitiz- ing with a negative sign, and the difference in constitution was connected with this antagonistic behavior. It is only recently that desensitizing has been recognized as a property common to all sensitizers. It has been fairly well established, moreover, that sensitizing and desensitizing belong to different stages in the formation of the latent image and probably take place at different spots on the silver bromide grain. As early as 1907 Sheppard and Mees (22) made a very clear statement regarding the differ- ent stages for the action of sensitizers and desensitizers in the primary
100 G. :EORNFELD process, but it seems generally to have been overlooked by other investi- gators. Then, in 1925, Sheppard (23) emphasized the fact that desensitiz- ing does not produce any detectable shift in the spectral sensitivity. One year later von Hubl (24) stated quite clearly that, for a special spectral region, a desensitizer might as well be considered also an optical sensitizer, since both actions were not connected with each other, optical sensitizing depending on color, whereas desensitizing as a chemical reaction can be produced by colorless substances. In 1931, writing on "Sensitizing by Desensitizers", Luppo-Cramer (25) stated that it seemed futile to divide the dyes into sensitizers and desensitizers, since the same dye might belong to either class, according to the conditions under which it was used. He supported this statement by several facts: (~) Many dyes known as opti- cal sensitizers in silver bromide emulsions act as desensitizers in silver iodide emulsions. (2) If an acceptor for halogen is added to a silver bromide emulsion, many dyes known as desensitizers then act only as sensitizers. (~) To unripened emulsions some desensitizing dyes (Capri blue, Janus green) act as sensitizers when used in low concentrations. A very cogent argument in favor of the above statement is the existence of optimum conditions for the sensitizing baths regarding concentration and time of bathing. This fact was discovered by Sheppard (57) as early as 1908 with a solution of an isocyanine dye. In 1933 Heisenberg (26) showed for three dyes, namely, pinacyanol, thiocarbocyanine, and seleno- carbocyanine, that there is an optimal concentration for the adsorption of a sensitizing dye, at which nearly all the dye is adsorbed. With concentra- tions increasing beyond this optimal concentration, desensitizing sets in, and Heisenberg proved that this desensitizing effect is real, since it is much too large to be accounted for merely by the increased light absorption of the dye. This desensitizing effect of sensitizing dyes when used in larger concentrations seems to be of a general nature. A very extensive and careful recent investigation by Leermakers, Carroll, and Stand (27) showed invariably the same result. The desensitizing action, then, is to be considered a property common to all optical sensitizers and, although this statement is not reversible, be- cause desensitizing is not exclusively a property of optical sensitizers. a study of desensitizing should contribute something to the knowledge of optical sensitizers. The connection, however, between the specific char- acter of a sensitizer and its desensitizing property if used in excess is a little more remote than that with optical sensitizing which will, therefore, be dealt with first. Il. SENSITIZING ACTION A. The sensitizing process The sensitizing action has always been believed to be closely connected with the primary act of absorption, and quite recently Leermakers (28) has
ACTION OF OPTICAL SENSITIZERS 101 confirmed very accurately the close correspondence between sensitivity and absorption in a sensitized emulsion. The conclusion thus seems in- evitable that sensitizing is connected with the primary act of light absorp- tion. Now, according to the present state of the theory of the latent image, into which Webb (29) has introduced quantum mechanics, absorp- tion in unsensitized silver bromide results first in the raising of an electron to a higher level. In this level its mobility is not blocked by other elec- trons and, accordingly, photoconductance results. This photoconduct- ance was measured by Toy and Harrison (30) and was found within 0.07 sec. to reach a stationary state which was proportional to the intensity. This would be expected if the majority of the electrons left the upper level again and dropped to a lower level. It has been assumed that such lower levels are produced by impurities and that in catching the electrons they give rise to the color centers, which are called F centers, in Pohl's terminol- ogy. Hilsch and Pohl (31) found in crystals of alkali halides that these F color centers are characterized by sharp absorption bands; in silver halide crystals, the absorption bands are more diffuse and broader, overlapping the proper absorption of the silver halides. Hilsch and Pohl assume that it consists of various bands belonging to different states of aggregation, since there is evidence that, in the silver halides, the photoproduct collects into specks of varying dispersity immediately after it is formed. In this connection it is perhaps of some interest to point to the results of Wagner and Beyer (32~. They found that the lattice defect in silver bromide crystals is produced by interstitial silver ions which have left their normal places in the lattice, whereas in alkali halides the ions which have left their places are not present in the lattice any more. For the photographic emulsion Sheppard (23) stressed the necessity of dividing the formation of the latent image into two stages: (~) the primary act of absorption and (2) the formation of concentration specks around the sensitivity specks. He was able to show that the relative spectral sensitivity was scarcely affected by the formation of sensitivity nuclei (such as from thiocarba- mide), which greatly increased the absolute sensitivity to any wave length, and Carroll and Hubbard (33) confirmed this statement. Webb has pointed out that the results obtained by Pohl and his school, in crystals, fit very well into the concentration-speck hypothesis suggested by Shep- pard, Trivelli, and Loveland (34) for the photographic emulsion. Re- cently experimental evidence has been found for the existence of such concentration centers in photographic emulsions. van Kreveld and Jurriens (35) devised an extremely sensitive method for measuring the absorption of a photographic plate after exposure. They found that the characteristic absorption between 5900 A. and 7000 A. was proportional to the time of exposure (36~. The intensity of absorption taken as a function of the time of exposure gave a straight line passing through zero. Moreover, the method was so sensitive that they could include very short
102 G. EORNFELD exposures in their measurements. The first point was produced by an energy of 9 X Hi quanta per cm.2 on an Ilford Process plate. This point actually lies at the beginning of the overexposure region of the H. and D. curve, but is still so near to the region of the latent image that there is a high probability that the latent image is characterized by the same ab- sorption.2 Now the same absorption, only increased in intensity, has also been found for the print-out region, in which Eggert and Noddack (37) found the amount of photolytically produced silver to be that required by the law of photochemical equivalence. They also extrapolated their re- sults to the region of the latent image, and the investigational of van Kreveld and Jurriens confirmed their statement. It can be assumed, therefore, with a probability very near to certainty, that the latent image consists of silver which has been produced by light according to the law of photochemical equivalence, so that, for each electron released, one atom of silver is formed. In a sensitized emulsion the same primary process is supposed to take place, but with one exception. The original level from which the electron is raised is higher than the original level in the unsensitized emulsion and, accordingly, the energy required for raising it is less. Sheppard and Crouch (38) attributed this electron level to the sensitizing dye, and this assumption seems amply justified by the close resemblance between the absorption spectrum of the sensitized.emulsion and that of the sensitizing dye itself. Thus, in the sensitized region, the electron should originate from the adsorbed dye, whereas, in the absorption region of silver bromide, it should originate from the bromide ion in the lattice. One more fact should be mentioned as an argument in favor of this theory, i.e., the photo- conductance of sensitizing dyes. As early as 1905 Joly (40) pointed out the parallelism between photoconductance and absorption in sensitizing dyes, and in 1923 Zchodro (41), in comparing the photoconductance of three sensitizing dyes (cyanine, pinaverdol, and pinachrome) in dry col- lodion, with their absorption, found complete correlation. This result, however, is not conclusive. The photoconductance of the dyes should be investigated in the same specific molecular state in which they are adsorbed in the silver bromide emulsions. From the theory just outlined it should be concluded that not more than one silver atom could be produced by the light-action of one dye molecule unless this molecule could be restored to its former state, rather than be decomposed after the release of an electron. This assumption seems far-fetched, although one fact points in this direction, the strong fluorescence which is found in sensitizing dyes. It is to be regarded, how- ever, as one of the possible solutions for ~ puzzling experimental result 2 It must be borne in mind, however, that it would hardly be possible to discover a slight deviation of the straight line in the region of the latent image.
ACTION OF OPTICAL SENSITIZERS 103 obtained by Leszynski (42), by Tollert (43), and recently by Bokinik and Iljina (44). All these authors found that, in a sensitized emulsion, the number of silverer atoms present after exposure was many times greater than the number of dye molecules. Leszynski found that, after the exposure of an emulsion sensitized by erythrosin, the ratio of silver atoms to dye mole- cules was of the order of 20. Tollert repeated Leszynski's investigation with a very fine-grained emulsion, taking care to consider only those dye molecules which had actually been adsorbed to the silver bromide, and arrived at the same result. With the dye concentrations and exposure times which were used in his experiments, a ratio of 64 was actually found. Both authors used the same analytical methods and there is a possibility, of course, that a systematic error could have been made which is responsible for the anomaly. The method used was the same as that of Eggert and Noddack (37), i.e., first dissolving the silver bromide in thiosulfate and then determining the amount of silver In the precipitate. The thiosulfate might have given rise to some silver sulfide which would increase the amount of silver in the precipitate. There is always a tendency for thio- sulfate to give silver sulfide, especially in the presence of gelatin, and this might be increased by the presence of the dye. There is, however, additional evidence for a high, although not equally high, efficiency of the sensitizing dyes, namely, the work of Bokinik and Iljina (44~. These authors investigated the sensitizing action of ery- throsin in silver bromide sols without gelatin and with an excess of bromide ions, with the result that the ratio of silver atoms to adsorbed dye mole- cules was found to lie between 4 and 15, rising with increasing alkalinity. It appears, however, that some other spectral region than the green light of the sensitizing region enhanced the edect on the silver bromide sol, for, if their curves (amount of silver against adsorbed dye) are extrapolated to a dye concentration of zero, a large and varying amount of silver seems to have been formed In every case. Thus, although the results of all three experiments are not quite conclu- sive, still they are not to be considered wrong.3 Accordingly an explana- tion for the results should be sought. Three different explanations have been offered, one of which has already been given, i.e., that the dye mole- cule is restored after the release of an electron. This restoration of the molecule could easily occur if the remaining part did not decompose in the meantime. In this connection, a study of phosphorescence in adsorbed dyes would be of interest. Another explanation is the possibility that the absorbing dye molecule 3 These results were strongly supported by an experiment recently reported by Dr. Sheppard at the September, 1937, Meeting of the American Chemical Society, which will be referred to later in this paper.
104 G. KORNFELD imparts its energy somehow to the crystal lattice, so that an electron is released from the lattice and the dye molecule is not affected at all. This is the explanation preferred by Leermakers (28), but it does not explain why the energy required to release an electron in the silver halide lattice should be reduced to such an unusual degree by the adsorbed dye, nor why this energy should correspond so closely to the absorption of the dye itself. Recently Scheibe (44a) has tried to find a solution for this question by assuming that the dye in an aggregated state should be able to absorb more than one quantum in one elementary act. There is a third explanation connecting the excess of silver atoms over dye molecules with a chemical reaction of these latter molecules. Shep- pard and Crouch (38) suggested an explosion of the dye molecule after the release of an electron, and they produced experimental evidence for the occurring of a chemical reaction between cyanine dyes and silver halide in the presence of light. Other contributions to this question have re- cently been made by Semerano (45, 46) and by Mecke and Semerano (47~. The most recent results of Sheppard, Lambert, and Walker (46a), however, referred to already in this paper, rule out this explanation, for these authors were able to show that in the presence of an acceptor for halogen the sensi- tizing action proceeds without any decomposition of the dye. Thus the decomposition of the sensitizing dye can not be connected with the actual sensitizing. The part played by the sensitizing dyes in the photographic process seems, on the whole, to be fairly well established, although some questions remain still to be answered. The main problem, however, concerns the properties which are required to make a sensitizer out of a dye, and unfortunately very little is known about them. In the following sections a survey will be given of the at- tempts which have been made to correlate the various properties of dyes with their sensitizing characteristics. B. Adsorption of sensitizers The fact that adsorption is a necessary though not a sufficient condition for sensitizing has been known for some time (22~. As early as 1904 Kieser (48) studied a great number of sensitizing dyes in their relations to the surface of silver halide grains, and found that saturation was reached at very low concentrations of the dye, but it was only recently that quanti- tative measurements were started of the adsorption of sensitizing dyes by silver halide emulsions. In 1925 Sheppard and Crouch (38) measured the adsorption of Ortho- chrome T. dissolved in water, on a silver bromide emulsion with only 1 per cent gelatin at 50°C. The adsorption was measured in two ways: (~) by extracting the aqueous solution of the remaining dye with chloroform and determining the dye concentration with a spectrophotometer, and (2)
ACTION OF OPTICAL SENSITIZERS . 105 by determining directly the amount of dye adsorbed at the silver bromide grains after dissolving them in thiosulfate. The results of both methods agreed fairly well. The adsorption curves at first showed increasing ad- sorption with increasing concentration. Then, over a large part, the curves were parallel to the concentration axis until finally they rose steeply again with further increase in concentration. The parallel parts of the curves were interpreted by the authors as representing saturation in a single layer, and the steep rise afterwards was assumed to be due to the agglomeration of multilayers. The size-frequency of the silver bromide grains was measured, and the surface area was calculated and compared with the amount of adsorbed dye in a saturated single layer. This latter value varies according to the alkalinity. For a pH of 5.5 there were found 5 X 10-~° gram-moles of dye per square centimeter of surface area. According to a statement of Trivelli and Sheppard (49), the authors as- sumed the surface area to consist mainly of bromide ions, since excess bromide was present. By calculating the number of bromide ions in the surface, they were able to establish the ratio of bromide ions to dye mole- cules as 2.3. In connection with the adsorption at bromide ions, the in- crease of adsorption in alkaline solutions presented some difficulty. For, although it would have seemed natural to assume that the dye cations were adsorbed at the bromide ions, the increase in adsorption in alkaline solu- tion pointed to the adsorption of the molecular form. There were two forms of the dye found in a solution in water, a dissociated uncolored form which was soluble in water, and an undissociated colored form which tended to be dispersed in colloidal solution in water (50), whereas, in alcohol and other organic polar solvents, the colored form was more soluble. The equilibrium between these two forms in water depended on the acidity. The dissociated colorless form prevailed in acid solution and the colored form in alkaline solution. Thus it seemed certain that the colored form, although not actually dissociated, was still exclusively adsorbed at the bromide ions. A parallel may be drawn, perhaps, to the statement of Franck and Eucken (51), according to which energy exchange is facilitated between molecules which can react with each other, even though they can not react under the special conditions under consideration. The connection between the basic or acid nature of a dye and the place of adsorption in silver bromide was confirmed in an investigation by Shep- pard, Lambert, and Keenan (52~. The acid dye dichlorofluorescein was found to be adsorbed only in the presence of an excess of silver ions, where- as, in alkaline solution, the basic dye pinacyanol was adsorbed only at the bromide ions. It should be mentioned here that the experimenters of Leermakers, Carroll, and Staud (39) gave the same results. All the basic cyanine dyes were adsorbed exclusively by the bromide ions. An investi- gation of the adsorption of pinacyanol (52), carried out parallel with the
106 G. EORNFELD investigation of the adsorption of Orthochrome T. yielded a value between 1.69 and 2.78 for the ratio of bromide ions to dye molecules. (The range of variation depends on the assumption of octahedral or of cubic faces at the surface.) This value was found to be roughly in agreement with the results of other investigations, on dye adsorption at silver bromide sur- faces, which had been carried on in the meantime. Having found eight bromide ions for one molecule of adsorbed methylene blue, Wulff and Seidel (53) collected other data on adsorption at the surface of salts of heavy metals. They gave the ratio 3 as the result of an experiment on the adsorption of erythrosin at silver bromide surfaces carried out by O. J. Walker and K. Fajans. Since that time other investigators have very definitely confirmed the ratio Br > 1. Bokinik (54) gave the ratio 10 for pinacyanol, and Leer- dye makers, Carroll, and Stand (27) found the ratio 10 for two different thio- carbocyanines and 20 for a thiodicarbocyanine. Their saturation value was defined as the amount of dye adsorbed at optimal sensitization, whereas saturation in Sheppard's experiment was defined by the parallel part of the adsorption curves. Since both methods yielded different results (the ratios being 2 and 10, respectively), the authors concluded that there exists a difference between saturation considered from the point of view of adsorp- tion, and saturation considered from the point of view of sensitizing. Their investigation was a systematic study of the correlation between the optimal concentration of a sensitizer and the available grain area, and it was carried out with three different dyes and seven emulsions. The curves obtained by plotting the sensitivity in various spectral regions against the logarithm of the dye concentration show a great increase in sensitivity towards the optimal concentration in the red region, a slighter increase in the green part of the spectrum, and no change in the blue part. At higher concentrations a decrease in sensitivity occurs throughout the whole spec- trum. At the optimal concentration, practically the whole amount, i.e., 99 per cent, of the dye was found to be adsorbed. For concentrations beyond this the percentage of unabsorbed dye increased rapidly. (For pinacyanol, thiocarbococyanine, and selenocarbocyanine, Heisenberg (26) obtained exactly the same results.) The surface area for each emulsion was determined from the projective area of the average grain, the number of grains per cm.3, and the volume of the average grain (obtained by finding the silverer content of the emulsion, the density of silver bromide, and the number of grains). The results are shown in table 1 (taken from the paper by Leermakers, Carroll, and Staud (271~. The fifth column shows a remarkably constant value for each dye. This constant ratio of adsorbed dye to available surface is especially remark- able, since the emulsions were prepared in various ways, both from neutral a
ACTION OF OPTICAL SENSITIZERS 107 silver nitrate and from ammoniacal silver oxide. Discussing the difference in the ratio, dye to surface, for dye I and dye II, on the one hand, and dye III, on the other, Leermakers, Carroll, and Staud point out that the devia- tion is in the right direction, since dye III as a thiodicarbocyanine has a larger area than dye I or dye II, both of which are thiocarbocyanines. They point out at the same time, however, that the deviation is greater than could be accounted for by the difference in size. The data of table 1 are used to estimate the amount of surface saturation at the optimal con- centration. For dye II, an 8-alkylthiocarbocyanine, the surface is assumed TABLE 1 Optimal surface concentration of sensitizing dyes (From Leermakers, Carrot], and Staud (27) ~ EMCL`SION 2 3 4 5 6 7 3 4 6 7 3 4 6 7 S-tJRFACE cm.2 per cm.3 400 480 580 660 840 950 1080 400 580 660 950 1080 400 580 660 950 1080 . OPTIMAL DYE CONCENTRATION _ moles per cm.3 of emulsion X 108 I 3.6 3.8 5.2 6.0 7.2 9.0 12.0 II 4.4 6.3 8.0 11.2 12.6 III 1.9 2.3 2.9 4.8 4.8 MOLES OF DYE PER CM.2 OF SURFACE X 10 9.0 7.9 9.0 9.1 8.6 9.4 11.1 11.0 10.9 12.1 11.8 11.6 4.8 4.0 4.4 5.0 4.4 to be 150 A., according to the values given recently for atomic radii. If the molecules are assumed to lie flat on the crystal surface, they will then cover 11.5 X 10-~i X 6 X 1023 X 150 X 10-~6 cm.2 per square centimeter of surface, or 1 cm.2 per square centimeter, i.e., they will form a unimolecular layer. This result seems fairly convincing, although the authors themselves are willing to assume that it may be a coincidence, because there are other facts (39) which seem to require the assumption of an agglomeration of the adsorbed dye. These data are connected with some spectral character- istics which will be reported later in the present paper.
108 G. 1iORNFELD Two more investigations should be mentioned in connection with ad- sorption measurements, one by Bagdassarjan and Rabinovitch (55), and the other by S{etinkina (56), which was reported by Rabinovitch. Bag- dassarjan and Rabinovitch, studying the adsorption isotherms of silver bromide suspended in water, of erythrosin, eosin, rhodamine B. pyronine G. phloxine, acid rhodamine, and Bordeaux B. and their sensitizing power in silver bromide emulsions, found again that adsorption was necessary for sensitizing. In addition, they discovered that dyes (pyronine G) with adsorption isotherms which do not show any flat portions pointing to saturation are extremely weak sensitizers. Scetinkina (56) measured the dependence of sensitizing action on the concentration of the sensitizer, and found a maximum for erythrosin and phloxine at relatively low con- centrations. At very high concentrations the sensitivity decreases. These results are in good agreement with the results of the other investi- gators. C. Some special spectroscopic properties of sensitizers That the absorption spectra of dyes vary according to the solvent has been known for a long time. In 1908 Sheppard (57) investigated the absorption of some sensitizing dyes in alcoholic solutions and in water and found a very marked difference between them. The sensitivity curves of emulsions sensitized by these dyes resembled to some extent the absorption curves in water. In another investigation Sheppard (50) obtained some ultramicroscopic evidence of the colloidal state of the dyes dissolved in water. Recent investigations of this were carried on by Scheibe (58), by Scheibe, Kandler, and Ecker (59), by Scheibe, Mareis, and Ecker (60), by Jelley (61, 62), and by Leermakers, Carroll, and Stand (39~. A peculiar spectroscopic phenomenon, discovered by Jelley for a special cyanine dye (1, l'-diethylpseudocyanine chloride), was observed when the dye was changing from true solution in alcohol or a similar solvent to the crystallized state. During the change it passed through a transitory state characterized by a sharp absorption band at about 575 mu. This absorption band was associated with a strong resonance fluorescence, an unusual phenomenon in the liquid or solid state. This transitory state of the crystal was found to be relatively stable in solutions of some salts and in the case of adsorption of the dye on various substances (599. In investigating silver bromide emulsions sensitized by cyanine dyes, Leermakers, Carroll, and Stand found similar sharp characteristic absorp- tion bands for many adsorbed dyes; It was this fact which made them doubtful of the unimolecular layer, since Scheibe, Kandler, and Ecker attributed these spectral characteristics to a polymerized state, and Scheibe, Mareis, and Ecker confirmed this statement. Jelley, however, assumed a nematic state, i.e., a liquid crystal with orientation along one
. ACTION OF OPTICAL SENSITIZERS 109 axis only, and the question of the "aggregated" state was left open.4 Whatever this state be called, it will still be tempting to link it with the sensitizing power. But some of the results of Leermakers, Carroll, and Stand are not in agreement with the assumption that the sensitizing action is exclusively a property of this "aggregated" state. They found paral- lelism between sensitivity and absorption in all spectral regions, not merely for the bands belonging to the nematic state, but also for bands in other spectral regions which were obtained in emulsions dyed from alcoholic solutions. The sensitivity, however, was always proportional to the absorption. Another connection seems worth investigating, namely, that between fluorescence and sensitizing, especially with regard to the possibility (men- tioned in the discussion of the sensitizing process) that the dye molecule could be quickly restored after the release of an electron. Fluorescence in solutions is known to occur very frequently in cyanine dyes (63), but it is important to know whether the dyes adsorbed to silver bromide will show fluorescence. An investigation of the fluorescence of sensitized emulsions would be especially interesting in view of some remarkable results obtained with isomers by Brooker and Keyes (64) and by Leermakers, Carroll, and Stand (39~. These isomers have the following constitution: $~ ACHE ~ J C2H5 A U ~CHJ~ ~ J \'N/ $i ~ ;=CH\~ /\ C2H5 I C2~5 N \/ /\ C2H5 I B MUCH \\N/~ C2H5 I N \/ 2H5 C2H5 I C C2H5 D 4 At a recent meeting (September, 1937) of the American Chemical Society Dr. Sheppard suggested a unimolecular layer in which the molecules are standing on edge, so that aggregation could take place in one dimension. .
. 110 G. 1`ORNFELD The only difference between A and B. or between C and D, lies in the position of the benzene ring marked X. Each of the dyes showed strong adsorption to silver bromide emulsions and a sharp absorption band, but only A and C were found capable of sensitizing the emulsions; B and D did not act as sensitizers. It would be interesting in this connection to know whether the isomers show any differences in fluorescence or photocon- ductance. There are at present very few data which can be attributed to the sensi- tizing property. Bokinik (65) points out, in discussing the various ex- planations offered for the sensitizing action, that, of all the existing dyes, only those related to the phthalein or the cyanine groups were found to be sensitizers, but this statement is more applicable to the properties desirable for a practically useful sensitizer than to the sensitizing property itself. Chibisoff (66), on the other hand, calls attention to some secondary effects of optical sensitizers whereby they act simultaneously as chemical sensitizers. These effects are especially marked when the dyes are added to the emulsions before ripening. They can then influence the growth of the crystal and the ripening process. Bancroft, Ackerman, and Gallagher (67) have connected the sensitizing power of a dye with its reducing action as an acceptor for halogen, and accordingly define an optical sensitizer as a dye which absorbs light in a special region and which is capable of reduc- ing silver salts. IlI. DESENSITIZING ACTION The desensitizing action, as a property common to all sensitizers at high concentrations, should reveal some information on their characteristics. Unfortunately, the phenomenon itself seems complex, and the variation of terminology with author and time has helped to confuse the picture still more. There is, however, one fact which seems to be fairly well estab- lished: namely, that desensitizing takes place at the developing centers and not at the places of primary absorption (68~. In addition to many facts which lead to this conclusion, there is an experiment reported by Luppo-Cramer that definitely proves this point. Pinakryptol yellow is a very strong desensitizer and is even capable of dissolving the silver of the print-out image (70, 719. Accordingly, no darkening was observed in developing a plate which had been desensitized with pinakryptol yellow before exposure, but after dissolving the silver bromide in thiosulfate, the plate could be developed physically. This demonstrates clearly that the primary act of absorption during exposure was not impeded by the de- sensitizer adsorbed at the surface. According to this result Weber's theory (72, 73) can be rejected, in so far as optical sensitizers are concerned. His theory assumed that desensi- .
ACTION OF OPTICAL SENSITIZERS 111 sizing has only an indirect influence on silver bromide by removing the optical and chemical sensitizers. This was supported by the following facts: Luppo-Cramer (74) showed with many examples that emulsions of silver bromide in collodion were not susceptible to the action of desensitizers unless they contained a sensitizer. Mudrovcic observed that cyanine dyes and the desensitizer methylene blue reacted with each other. In water and in collodion the cyanine dye was bleached out in the presence of methylene blue. (Since, however, in gelatin the process was reversed, the methylene blue being destroyed, this effect could as well be used as an argument against Weber's theory.) Finally, Weber himself observed that, in a sensitized plate, desensitizing showed more strongly in the sensitized spec- tral region than the absorption region of silver bromide. Blau and Wam- bacher (76) strongly objected to accepting this as a general statement and pointed out that, wherever this secondary effect was found, it could be easily explained by preferential absorption of the desensitizer. It would indeed be difficult to explain by this theory the desensitizing effect of optical sensitizers at high concentrations. With regard to the assumed edect of desensitizers on chemical sensitizers contained in gelatin, by which Weber explained the difference between gelatin and collodion in regard to the susceptibility to desensitizing, Blau and Wambacher pointed out that this explanation could not be valid. For, in the meantime, Ollendorfl and Rhodius (77) had succeeded in coating plates with a silver bromide emulsion without any protective colloid which could be desens;- tized with methylene blue. Luppo-Cramer confirmed (78) this result, although he did find that the number of dyes capable of acting as deser~si- tizers is much more limited in such colloid-free emulsions. Even pina- krypto1 yellow, one of the strongest desensitizers, is ineffective with them. The exceptional susceptibility to desensitizing was explained for the gelatin emulsion by Luppo-Cramer as being related to the high degree of dispersion of the latent image in gelatin. In a special case there would, of course, always be the possibility of a complication by some of the secondary effects which have just been dis- cussed, but desensitizing as a general phenomenon can safely be taken as a direct action on the developing centers. Even so, the range of possible reactions is still very wide and many questions remain to be answered. There is the Herschel effect, i.e., the simple regression phenomenon produced by infrared radiation (the name, "Herschel effect," is used here for the direct regression, as it was used originally, i.e., the "visible" Herschel effect, as named by Trivelli (809~. Does desensitizing mean sensitizing of the Herschel effect? Or is it merely an oxidation of the silver in the latent image? Is it essentially an isolation of the developing nuclei (68~? Does it merely prevent the formation of the latent image,
112 G. EORNFELD or does it also destroy it? Is the desensitizing action merely a "narcosis" (81) of the centers lasting through development? influenced by light absorption? And are these reactions Some of these questions can be answered from the results of various investigators. Some significant results have been obtained by Sheppard (82) concerning the mechanism of reaction on the silver bromide grain. By measuring the electromotive force in concentration cells with silver salts, he found a strong tendency to form complex silver salts in several compounds used in photography either as sensitizers, desensitizers, or antifogging agents. It is conceivable that complexes are formed with silver either in state nascendi or in the very fine dispersion of the latent image when oxygen is present. In experiments by Blau and Wambacher it was found that, for desensitizing at least, oxygen was necessary, unless there was an excess of chloride ions (83, 84, 859. Pinakryptol yellow, induline scarlet, antilu- mine, and phenosafranine did not act as desensitizers when oxygen was completely removed, either by evacuation or by substituting nitrogen. The strong influence of halide ions on the desensitizing action had pre- viously been observed by Carroll and Kretchman (86) With safranine either the sensitizing or the desensitizing effect was found, depending upon the concentration of bromide ions. The investigation of Carroll and Kretchman rendered quantitative results on the influence of light on de- sensitizing, and they were able to show the good correlation between reversal and energy absorption. This reversal phenomenon is, of course, not related in any way to the well-known photographic reversal, the be- ginning of polarization. The reversal by desensitizers can be identified with the "latent Herschel effect" as Trivelli named it, contrasting it to the "visible Herschel effect", which is a direct regression of the print-out image and which was, in fact, the effect actually discovered by Herschel. Where the regions of absorption by silver bromide and the dye were sufficiently separated in wave length, maxima were found corresponding to each region. Desensitizing can, therefore, be considered as a reaction in the developing centers which can be photosensitized by the light absorbed either by silver bromide or by the desensitizing dye. The photosensitizing is not the only reaction produced by a desensitizer. Loss of developability occurs also in the dark, but at a much lower rate. This was demonstrated by Carroll (87) and confirmed by Mauz (88~. There still remains the question of whether desensitizing involves de- stroying the silver centers or merely making them unfit for development. Or rather, it should be asked whether there are any desensitizing dyes which impede the development of the silver centers without destroying them. For, quite aside from the dyes (such as methylene blue, Janus green, etc.) which are known to be capable of dissolving silver (89), there is pinakryptol
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