Novel yellow azo dyes and dichroic liquid crystal compositions made therewith

A method of improving the black color of dichroic dyes for use in black liquid crystal displays, comprising the steps of:

• 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;
• selecting at least one of the dichroic dyes of various colors to be a yellow azo dichroic dye having the general formula:
• ##STR40## wherein R and R' are individually alkyl or halogen, nitro, amino, hydroxy or cyano substituted alkyl and the alkyl has from about 4 to about 20 carbon atoms, 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 composition 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:
• ##STR41## wherein R and R' are individually alkyl or substituted alkyl, and the alkyl has halogen, nitro, amino, hydroxy, or cyano substituted alkyl, and the alkyl has from about 4 to about 20 carbon atoms, phenyl;
• phenylalkyl;
• halogen, nitro, cyano, hydroxy, dialkylamino, amino, alkyl, phenyloxy, phenyl, or alkoxy parasubstituted phenyl or phenylalkyl;
• or 3,5-dialkoxyphenyl radical.