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 &mdash; Beacall, Martin, et. al. &mdash; 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).


 * &mdash; 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&middot;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&deg; for thirty minutes. The dye produced was tested by the Bureau of Standards and found to be an efficient sensitiser."
 * &mdash; 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&middot;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&deg; for thirty minutes. The dye produced was tested by the Bureau of Standards and found to be an efficient sensitiser."
 * &mdash; 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&middot;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&deg; for thirty minutes. The dye produced was tested by the Bureau of Standards and found to be an efficient sensitiser."