History of color photography (1945)

PRIMARY COLOR DEVELOPMENT 363 This last is very interesting in that it is apparently an exception to the rules established above. It would merit some study. All the other substances mentioned also appear to be outside the limits of the established rules. They all have a common grouping however, of the type 0-c*° /Vc-OH >. — Q > Keto form Enol form These exist in a type of isomerism known as keto-enol tautomers. It is interesting to note that in the enol form the two carbon atoms have the conjugated linkage that is also present in the ring itself. In indoxyl an — {NH) — group is attached to the end carbon, and also to the benzene nucleus to form a fused indole ring. Now again we have a conjugated system in which adjacent carbons have a hydroxyl and a substituted amino group present. In thio-indoxyl, it is a substituted thiol group (SR) which is attached to the end carbon. This would mean that a thiol group, in which the sulphur atom is firmly attached and in no danger of splitting off, also can endow a substance with developing properties. As a rule such compounds give up the (SH) group very readily, forming hydrogen sulphide, which promptly reacts to form silver sulphide. In oxy-iso-carbo-styril, a similar situation exists. An (NR) group is attached to the end carbon, endowing the molecule with developing properties. If this were not sufficient, then we can consider the molecule from another point of view. It has two hydroxyl groups separated from each other by four atoms, linked together by means of a conjugated chain, hence presenting a hydroquinone type of grouping. There is one other property common to all of these substances. The dyes which they form are all well-known. They all consist of two identical groups linked together by a double bond. Indoxyl, for instance, yields the dye indigo H Thio-indoxyl yields the dye thio-indigo *CH — Oxy-iso-carbostyril gives the dye karbindigo s * OH