Dicyanin (synthetic dye)

Dicyanin is a synthetic blue dye derived from coal tar that was used for sensitizing photographic plates, 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."

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 (for 1,1'-diethyl-2,4'-carbocyanine iodide).