Effects of Substituents on Color

Like most dyes, substituents on di- and triphenylmethane dyes have a significant effect on the absorption spectra. Acetylation of the imino nitrogen of auramine O (32) results in shift to longer wavelength (Eq. 3). A 

Diphenylmethanes that have two identical phenyl groups are synthesized by the condensation of formaldehyde or its equivalent with an arylamine in the presence of concentrated hydrochloric acid.47-51 However, it is usually difficult to stop the reaction at the diphenylmethane stage. 

Magnesium phenolates react with triethylorthoformate regiospecifically at the ortho position of the phenoxy group (normally phenols give alkyl ethers) giving diarylmethanes. This reaction is complex and the product composition depends on the phenol and the reaction conditions.52 

Reaction of 4-hydroxymethylaniline 37 with a variety of arylamines in the presence of an acid catalyst gives both symmetrical and unsymmetrical diphenylmethanes.53-55 This reaction proceeds via intermediate 38. 

A general experimental procedure57 for a diarylmethane leuco compound via a benzotriazole To a stirred solution of the corresponding (benzotriazol-l-yl-methyl)aniline (5 mmol) in methanol (30 ml) under reflux was added a solution of the appropriate aromatic compound (5 mmol) and concentrated hydrochloric acid (1 ml) in water (30 ml). The resulting mixture was heated under reflux followed by the addition of aqueous KOH (1 M, 50ml). The product was isolated by filtration or by extraction with ether, and further purified by recrystallization or by column chromatography. 

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The effect of substituents on colors associated with conjugated systems is of particular interest in the study of dyes, because most dyes have relatively short conjugated systems and would not be intensely colored in the absence of substituent groups. (The plant pigments /3-carotene, Section 2-1, and lycopene, often used as food coloring, are exceptions.)... 

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