Arylmethane dyes are derived from methane:
Diarylmethanes have the general formula:
Triarylmethanes have the general formula:
Arylmethane dyes are so called because they are derived from methane, but in which some of the hydrogen atoms are replaced with aryl rings. Since a synonym for aryl is phenyl, they are also referred to phenylmethane dyes. Aryl rings are often referred to as benzene rings. However, benzene is a specific chemical compound and this usage is technically incorrect, although very common.
Diarylmethane (or diphenylmethane) dyes have two aryl rings. Only one dye in this subgroup has gained much use, and that is auramine O, a fluorescent dye used for the demonstration of acid fast bacteria and to make a fluorescent Schiff reagent. Its structural formula is shown below. Note the alkylamino groups attached to the aryl rings. This dye is subclassified as an aminodiarylmethane as a consequence. The chromophore is the carbon to nitrogen double bond.
Triarylmethane (or triphenylmethane) dyes contain three aryl rings. These are much more common and numerous examples can be given. The pararosanin homologues are good examples. Pararosanilin itself is used to make Schiff's reagent, and basic fuchsin is used to make carbol fuchsin, important for the identification of tuberculosis.These dyes also contain amino groups, and for that reason are sometimes identified as aminotriarylmethane dyes, and are often basic. In pararosanilin, shown below, each of the three aryl rings has an amino group. The quinoid ring is the chromophore.
Not all aminotriarylmethane dyes are basic. Some are acid dyes. These have strongly anionic sulphonic groups as well, making the overall charge on the molecule negative. Examples are acid fuchsin (shown below) and methyl blue.
Another subgroup of the arylmethanes has hydroxyl groups instead of amino groups. These dyes are termed hydroxytriarylmethane dyes, and are acid dyes. An example is mordant blue 3, shown below, which has been recommended as a hematoxylin substitute. Dyes in this subgroup are not widely used.
There are other subgroups of arylmethane dyes, but they are rarely used in histotechnology.
R. D. Lillie.
Conn's Biological Stains
Williams & Wilkins, Baltimore, MD., U.S.A.