The present invention relates to novel direct dyes and compositions comprising them for dyeing keratin fibers especially human hair. The novel dyes of the present invention have benzothiophene structure providing intensive and homogeneous dyeing on keratin fibers, especially human hair.

A mono-azo dichroic dye compound represented by the following Chemical Formula 1, a composition for polarizing plate, a polarizing plate, and an optical device. ##STR00001## In Chemical Formula 1, Z represents any one selected from the group consisting of CN, COOR.sub.11, indanone and 1,3-dioxoindane, wherein R.sub.11 is any one selected from the group consisting of alkyl groups, aryl groups, and substituted or non-substituted heteroaryl groups. The mono-azo dichroic dye compound shows excellent dichroic properties, and has excellent heat resistance and photo-resistance, and thus may be used advantageously for a dye-based polarizing plate.