Quinone-imine dyes have para-quinone-di-imine as their theoretical precursor. This compound does not actually exist, but gives the name for the dye class.
There are several subgroups of quinone-imine dyes. Three are important in histotechnology, two are not. The two unimportant subgroups are the indamins and the indophenols. The three important subgroups are the azins, the oxazins and the thiazins, incorporating 2 nitrogens, nitrogen and oxygen, and nitrogen and sulphur atoms respectively.
Azins have the general formula shown to the right. The group is further subdivided into three subgroups: eurhodins, safranins and indulins. The eurhodins are the simplest, with attached amino groups, illustrated by neutral red, a dye of considerable importance. Similarly the safranins, which have an aryl ring attached to a central nitrogen, include safranin O. The indulins are not commonly used, but nigrosin has value for negative staining.
Oxazins have the general formula shown to the right. The oxazins include gallocyanin, gallamin blue and celestin blue B. These three dyes have been recommended as hematoxylin substitutes in H&E staining when used in solution with iron alum.
Thiazins have the general formula shown to the right. Thiazins are particularly important as they include several very commonly used dyes, including some that are strongly metachromatic. These include the methylene blue homologues.