T H E PHOTOCHEMICAL OXIDATION OF LEUCO-BASES” BY B. 1%. CARROLL
Although the early division of the spectrum into actinic, visible, and heat radiation has been abandoned, cases of photochemical reactions which are primarily sensitive to the longer wave lengths, are rare enough to be of particular interest. These reactions are generally oxidations. It has been known for some time that the oxidation of leuco-bases to the corresponding dyes is accelerated by light; this photo-oxidation from the colorless base to the intensely colored dye obviously making possible a photographic process in which the leuco-base is the sensitive material, and the image is composed of dye instead of silver. Studies of the reaction by Grad, Konig2, and Edera demonstrated at least qualitatively, that the maximum of absorption of the dye coincides with that of sensitivity of the leuco-base. The process accordingly appeared to offer a possibility for photography of the red and short infra-red. A study of the literature, supplemented by experiment, makes it appear probable that only the dyes of the triphenylmethane and xanthene series form stable and photosensitive leuco-bases. Other series are excluded because of instability of the bases in the dark, as in the case of the indigoid dyes, or lack of photosensitivity, as in the case of the indamines. The malachite green series (derivatives of diparaaminotriphenylmethane) were finally selected because of their selective absorption of the red. Synthesis of the leuco-bases being found more satisfactory than reduction of the commercial dyes, four were prepared by the former method; leuco malachite green, leuco brilliant green, and two bases not in commercial use, obtained respectively by the condensation of benzylethylaniline with benzaldehyde, and of dimethylaniline with furfural. These will be designated as light green and furfural green respectively. Konig established that the incorporation of the bases in collodion films not only provides a practicable form for photographic use, but also accelerates the photooxidation. The absorption spectra were found to be only slightly displaced toward the longer wave lengths, the maxima being as follows: malachite green 624 mF, brilliant green 628 mp, light green 628 mp,furfural green 634 and 480 mp. As the sensitivity of the plates so prepared is about that of “printing-out” paper, the investigation resolved itself principally into search for a sensitizer. Solutions of the leuco-base and sensitizer were mixed with the collodion, and plates were coated so as to give approximately 0.6 mg. collodion and 0.3 mg. base per cm2. I n preliminary tests they were exposed as soon as *Published by permission of the Director of the Bureau of Standards of the U. S. Department of Commerce. IGros: Z . physik. Chem. 37 157 (1901). Iionig: Z. angew. Chem. 17 1633 (1904). Eder: Sitzungsber. Akad. Wiss. Wien, 128 IIa. April (1919).
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dry to a 250-watt gas-filled tungsten light at a distance of 30 cm. for 30 minutes. The plates being too insensitive for spectrograph exposures, the relative color sensitivity was tested by exposing part of the plate under Corning ruby glass, absorbing completely below 600 mp, and another part under a Wratten H filter, absorbing above 5 2 0 mp. The plates were k e d in changes of benzene, which was found to be quicker and less destructive to the film than the IO% aqueous solution of monochloracetic acid recommended by Konig, The extent of the reaction was measured in terms of density at 620-640 mp, using a Fabry-Buisson microphotometer and a monochromatic illuminator as source
FIG. I Characteristic curves of leuco-base plates. Density a t 620-40 mp, plotted against exposure in candle-meter-seconds times 1 0 6 . I . Unsensitized light green. 2 . Light green sensitized by addition of quinoline, 3 . Light green sensitized by addition of mannitol hexanitrate. 4. Light green sensitized by addition of quinoline and mannitol hexanitrate. j. Unsensitized malachite green.
of light. (Density, in the photographic sense, being the negative logarithm of transmittancy, is proportional to the dye per unit area, assuming Beer’s law). Altogether some forty compounds were tried as sensitizers of one or more of the four leuco-bases. As might be expected, all four were similarly affected by a given compound. I n general, the most photosensitive combinations oxidized the most rapidly in the dark, and while materials were found which accelerated the “light” reaction more than the “dark” reaction, nothing was discovered which gave promise of a stable plate of practicable speed. We therefore discontinued work on the process, although the reaction is worthy of further study for its scientific interest. The results may be briefly summarized: I. Bases accelerate the oxidation and free acids retard it, both in light and dark. Konig’s discovery of the sensitizing action of quinoline is a special case of this rule; the heterocylic and aliphatic amines are more efficient than the aromatic.
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2. The nitric acid esters were the only oxidizing agents found to accelerate the light reaction relatively more then the “dark” reaction. 3. For the four bases tested, the greater the molecular weight of the group attached to the amino nitrogen, the better the stability. The data in Table I indicate the relative spectral sensitivity and stability of three of the bases, sensitized with quinoline and mannitol hexanitrate; the leuco furfural green was too readily oxidized in the dark for quantitative measurements. Densities being additive, the density of the unexposed portion of the plate has been subtracted from the exposure values.
0.20
0.15
0.10
B 0.05 ,
O*O0b
74 JE
1
FIU.2 Characteristic curves of leuco light green plates, unsensitized. Lower curve, exposure started forty minutes after pouring; upper curve, exposure started one hour forty minutes after pouring.
TABLE I Density a t 620-40mp Dye
Malachite green Brilliant ” Light ,,
On Complete oxidation
3 1.9 1.45
After 3 Days in dark
Direct
After Exposure Red glass H filter
3
I .05
0.63
1.9
0.17
0.05
I .OI
0.62
0.59
0.13 0.06 0.13
The relation between exposure and density was tested with an optical wedge (an Eder-Hecht “Kopier-Photometer”). This gives an intensity scale of exposure; the results with a time scale would be complicated by the rapid oxidation in the dark. The curves in Figs, I and 2 all show that within the limits of error, the dye formed in a given time is proportional to the intensity of light. The sensitivity of these materials is measureable in terms of the slope of the density-exposure curve, rather than as the reciprocal of inertia, as in the case of the silver bromide-gelatine emulsion; all curves appear to pass through the origin, without indication of an induction period, The sensitivity of the plates increased for a time after drying. This was particularly noticeable with leuco light green, the most stable of the bases; the magnitude oi the effect is evident from Fig. 2.
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Discussion The reaction is interesting not only because of unusual sensitivity to the longer wave-lengths, but also as an illustration of the different mechanisms by which added materials may influence a photochemical reaction. The amino bases do n o t affect the absorption of radiant energy, but increase the reactivity of the system. The nitric acid esters undoubtedly increase the reactivity of the system, yet their superiority over other oxidizing agents is probably due to their known sensitivity to light of short wave-lengths, so that a t least part of the sensitivity of the plates to this region may be due to the leuco base acting as acceptor for the oxygen liberated in the photolysis of the collodion. The action of the dye formed by the oxidation of the base must, in the absence of a better understanding, be termed purely catalytic. Red light, absorbed by the dye but not by the base, is unquestionably effective in the oxidation of the latter. The experiments of Gros proved that the dye attains its maximum effect at very low concentrations; if this were not the case, the velocity of the oxidation would increase as it went on, and the observed proportionality between exposure and reaction (measured as density) would be impossible. We are therefore forced to ascribe the increase in sensitivity of the plates on standing to an unknown change in reactivity, rather than to the increase in dye resulting from the dark oxidation.
Summary The photochemical oxidation of leuco bases offers little hope of utility as a photographic process, but it may be followed quantitatively by opticalmethods of analysis, and is well adapted to study of photochemical catalysis.
