Dicyanin (synthetic dye)

Dicyanin (C₂₅H₂₅IN₂) is a synthetic blue dye derived from coal tar that was intended for use in the sensitizing of photographic plates, being first manufactured in the early twentieth century by the dye works of Meister, Lucius & Brüning at Höchst (near Frankfurt), Germany. It became associated with auric research thanks to the experiments of Walter J. Kilner, who used the dye (which he also referred to as spectauranine) in the manufacture of his Kilner screens.

Preparation

In Dyestuffs & Coal-Tar Products (1915), the authors — Beacall, Martin, et. al. — relate that the dicyanin is a quinoline dye of the cyanine type, reporting it is "prepared by action of KOH [potassium hydroxide] + atmospheric O [oxygen] on α-γ-Dimethylquinolinium salts," for intended use as a sensitiser of "silver bromide gelatine plates up to the red line α, with a strong minimal effect between E and F," specifically noting it is "used as a red sensitiser for scientific work, but the prepared plates are not very sensitive."^[B]

Later, in Constitution of the Dicyanines (1924), Mills and Odams demonstrated dicyanine is a carbocyanin, giving a formula of 2,4-dimethylquinoline iodoethylate and sodium methylate in methanol^[M] (for 1,1'-diethyl-2,4'-carbocyanine iodide).

• Beacall, Thomas; Sand, H. Julius; Martin, Geoffrey; Challenger, F. (1915), "Photographic Chemicals: Quinoline Dyes", Dyestuffs & Coal-Tar Products: Their Chemistry, Manufacture and Application; including Chapters on Modern Inks, Photographic Chemicals, Synthetic Drugs, Sweetening Chemicals, and Other Products Derived from Coal Tar, London: C. Lockwood & Son, p. 139, https://babel.hathitrust.org/cgi/pt?id=uc1.$b71225&view=1up&seq=157 
• Mikeska, Louis A.; Haller, Herbert I.; Adams, Elliott Q. (1920), "Synthesis of Photosensitizing Dyes (II), Dicyanine A.", Journal of the American Chemical Society 42: 2392, https://books.google.com/books?id=-yBAAQAAMAAJ&pg=PA2392 
• Palkin, Samuel (1923), "Synthesis of Dicyanine-A", Industrial and Engineering Chemistry 15: 379-381, https://books.google.com/books?id=bGg2AQAAMAAJ&pg=PA379  — per the Jour. Chem. Soc.: "The conditions affecting the synthesis of dicyanine-A from 6-ethoxy-2:4-dimethylquinoline ethiodide were studied and variations tried in the kind of solvent used, concentration of hydroxyl·ion, type of alkali, catalysts, water, oxygen, time, and temperature. No solvent was found superior to ethyl alcohol, but excellent results were obtained from sodium sulphide as the alkali salt in 95% alcohol. Chloroform acts as a catalyst in the presence of sodium ethoxide, the yield of dye obtained being about twelve times that obtained by Mikeska, Haller, and Adams. The optimum conditions were: 1.5 cc of chloroform per 1 g. of intermediate, 25 g. of sodium sulphide (Na₂S,9H₂O) in 100 c.c. of 95% alcohol, 1 g. of intermediate to 25 c.c. of alcohol and a temperature of about 50° for thirty minutes. The dye produced was tested by the Bureau of Standards and found to be an efficient sensitiser."
• Mills, William Hobson; Odams, Ronald Charles (1924), "The Cyanine Dyes, Part VIII. Synthesis of a 2:4'-Carbocyanine. Constitution of the Dicyanines", Journal of the Chemical Society, Transactions 125: 1913-1921, https://pubs.rsc.org/en/content/articlelanding/1924/ct/ct9242501913 

Studies

• Kenneth, C. E.; Wratten, S. H. (January 1908), "Plates Sensitised with Dicyanin and Photography of the Infra Red", The Photographic Journal (London: Royal Photographic Society of Great Britain) 32 (1): 25-37, https://babel.hathitrust.org/cgi/pt?id=coo.31924078270059;view=1up;seq=37 
• Kilner, Walter J. (February 1912), "The Shifting of the Chromatic Focus of the Eye by Use of a Dicyanin Screen", Archives of the Roentgen Ray (London: Rebman) 16 (9): 344-346, 351-353, https://babel.hathitrust.org/cgi/pt?id=uc1.c3312416&view=1up&seq=356 
• Merrill, Paul W. (April 1919), "Application of Dicyanin to the Photography of Stellar Spectra", Bulletin of the Bureau of Standards (Washington, D.C.: U.S. Dept. of Commerce) 14 (4): 487-505, https://catalog.hathitrust.org/Record/009487742 
• Walters, Jr., Francis M.; Davis, Raymond (1922), Dicyanin, "Studies in Color Sensitive Photographic Plates and Methods of Sensitizing by Bathing", Scientific Papers of the Bureau of Standards (Washington, D.C.: U.S. Dept. of Commerce) 17 (422): 366-367, https://babel.hathitrust.org/cgi/pt?id=mdp.39015010797002;view=1up;seq=472