Triarylmethane Compounds as Redox Indicators in the Schoenemann Reaction: Iii. The Dyes and Their Spectra

Abstract

A series of triaryl~nethane dyes were made by varying the substituents in the thiophene ring of the dye [4-(p-dimethylamino-a-2-thienylbenzylidene)-2,5-cyclohexadiene-l-ylide~~e]-dimethylamlnonium chloride. The spectra of the zinc salts of these dyes were shown to be similar t o that of the dye formed in the Schoenemann reaction when the parent triarylmethane compound was used as a n indicator. Neither of the absorption bands occurring in the 475 mp or 625 mp regions is affected by pH or buffer salts in the pH range 2.4 to 6.4 but both bands are hypsochro~nically shifted when the pH is-lowered to 0.5. The substitution of 5-nitro and 5-acetamido groups in the thiophcne ring results in the largest bathochromic and hypsochromic shifts respectively. Interpretation of the effect of substitution and pH was attempted employing existing theories on the relation between color and constitution. INTRODUCTIOXThe preceding papers in this series (4, 14) described the preparation and use of some triarylmethane compounds as indicators in the Schoenemann reaction with a view to improving the method for detecting the phosphonofluoridate type of anticholinesterase compounds. As the success of the method depended greatly on the color of the dye produced it was of interest to ascertain the effect of various substituents attached to the thiophene ring on the spectra of the dyes. The effect of the replacement of the benzene nucleus by thiophene and a number of 5-substituted thiophene nuclei on triarylmethane dyes has been reported by Ailason and Nord (11,12). However, the present investig a t' ion enlarges on the previous studies by including a number of new mono-and di-substituted thienyl derivatives. The effect of pH on the spectra is also reported.I t was of interest to checlc the assumption that the oxidized compound produced in the Schoenernann reaction was the same as that of an authentic dye cation. Therefore spectral conlparisons have been made of a number of zinc chloride salts of prepared dyes with the dyes formed in the Schoenemann reaction. E X P E R I M E N T A L Preparation of DyesThe preparation and p~lrification of the triarylmethane compouilds from which the dyes were made have been described previously (4).The zinc chloride salts of the dyes were prepared from the correspoilding triarylmethane compounds according to the procedure of Mason and Nord (11) except that identical molar concentratioils of the solutions were always used in an attempt to produce salts of the same composition. The method of Davies and Hodgsoil (2) was investigated but was not found t o be superior to the method outlined by Ailason and Nord (11) or Levi (7).Briefly, the leuco compouilds (0.04 mole) were dissolved in 2 N HC1 (80 ml.) and the solutions were diluted with 800 ml. of water. Freshly prepared lead dioxide (0.04 mole) was added with stirring and cooling. After 2 hours sodium sulphate (0.05 mole) was 1Manz~script received J u l y 24, 1957.