Pleochroic dyes

Claim 1

A liquid crystal electro-optic device comprising two dielectric substrates at least one of which is optically transparent, at least one electrode on the inner surface of each substrate, and a layer of material sandwiched between the substrates, characterized in that said material is a liquid crystal twist-type nematic or cholesteric-to-nematic system which comprises a mixture of compounds each having a generalized formula ##STR43## wherein R is an alkyl or alkoxy group and m = 0 or 1, at least one of said compounds being cyanobiphenyl wherein m = 0, said material including in solution a dye having a formula selected from one of the following: (I) ##STR44## (II) a derivative of formula (I) wherein any of the lateral positions on any one of the benzene rings is substituted by one of the following substituents (i) a halogen atom (ii) a methyl group (iii) a halogen-substituted methyl group (iv) a methoxy group; (III) a derivative of formula (I) wherein any two of the lateral positions on any one of the benzene rings is bridged by a fused aromatic ring forming a naphthalene structure; wherein A is an azo or azoxy linkage group n 1 and n 2 are integers in the inclusive range 0 to 4 and Y 1 and Y 2 are each one of the following (i) hydrogen (ii) OR 1 where R 1 is an alkyl or arylalkyl group (iii) ##STR45## wherein R 2 and R 3 are each one of the following: a simple alkyl or substituted alkyl group or an alkylene group forming a reduced hetero-aromatic system with the terminal aromatic ring.