In a thermal transfer recording sheet including a base material and a colorant layer, the colorant layer includes a yellow dye layer containing a yellow dye, a magenta dye layer containing a magenta dye, and a cyan dye layer containing a cyan dye, and the cyan dye includes a compound represented by Formula (1a) or Formula (1b): ##STR00001##
where R.sub.1 to R.sub.7 are substituents, X.sup. is an anion, and the compound of Formula (1b) has at least one anionic substituent.

In a thermal transfer recording sheet including a base material and a colorant layer, the colorant layer includes a yellow dye layer containing a yellow dye, a magenta dye layer containing a magenta dye, and a cyan dye layer containing a cyan dye, and the cyan dye includes a compound represented by Formula (Ia) or Formula (Ib):

##STR00001##

where R.sub.1 to R.sub.7 are substituents, X.sup. is an anion, and the compound of Formula (Ib) has at least one anionic substituent.

Disclosed is 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, R.sub.3 represents a substituted or non-substituted heterocyclic group having 1-20 carbon atoms. 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.

The present invention refers to dyes of the formula (I) and mixtures thereof ##STR00001##
a process for their production and to their use.

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.