Novel yellow azo dyes and dichroic liquid crystal composition made therewith

A method of improving the black color of dichroic dyes for use in black liquid crystal displays, comprising adding to a host liquid crystal material, a dye composition having a mixture of dichroic dyes of various colors in proper proportion to obtain a material which absorbs radiation in the spectral range of about 400-700 nm wherein at least one of the dichroic dyes of various colors is a yellow azo dichroic dye having the general formula:

• ##STR46## wherein R and R' are acyl groups and form an ester with the general formula, and R and R' have the formula:
• ##STR47## wherein R.sub.2 is alkyl or halogen, nitro, amino, hydroxy or cyano substituted alkyl and the alkyl has from about 4 to about 20 carbon atoms, or R.sub.2 is a phenyl;
• phenylalkyl;
• halogen, nitro, cyano, hydroxy, dialkylamino, amino, alkyl, phenyloxy, phenyl, or alkoxy parasubstituted phenyl or phenylalkyl;
• or 3,5-dialkoxyphenyl radical;
• and heating the composition to dissolve the dyes in the liquid crystal host material.

An improved black dichroic liquid crystal display comprising:

• a host liquid crystal material;
• and
• a dye composition dissolved in said host, the dye composition having a mixture of dichroic dyes of various colors in proper proportion to obtain a material which absorbs radiation in the spectral range of about 400-700 nm wherein at least one of the dichroic dyes of various colors is a yellow azo dichroic dye having the general formula:
• ##STR48## wherein R and R' are acyl groups and form an ester with the general formula, and R and R' have the formula:
• ##STR49## wherein R.sub.2 is alkyl or halogen, nitro, amino, hydroxy, or cyano substituted alkyl, and the alkyl has from about 4 to about 20 carbon atoms, or R.sub.2 is a phenyl;
• phenylalkyl;
• halogen, nitro, cyano, hydroxy, dialkylamino, amino, alkyl, phenyloxy, phenyl, or alkoxy parasubstituted phenyl or phenylalkyl;
• or 3, 5-dialkoxyphenyl radical.