Liquid crystal compounds

A compound of formula (I)

• where R1 is alkyl or alkenyl, Y1 and Y2 are independently selected from oxygen or sulphur, n is an integer of from 1 to 5, A is an optionally substituted phenyl or an optionally substituted cycloalkyl ring, X is a direct bond, a C1-4alkylene, a C2-4alkenylene, an acetylene, —CO(O)— or a group of sub-formula (i)
• where X1 and X2 are independently selected from a direct bond, a C1-4alkylene, a C2-4alkenylene, an acetylene or —CO(O)— and R2, R3 and R4 are independently selected from hydrogen, halo or cyano, provided that no more than one of R2, R3 and R4 is hydrogen and that where A is unsubstituted phenyl and Y1 and Y2 are both oxygen, then:
• (a) where X is a direct bond, R2—R4 together represent other than either two cyano groups or two halogen atoms when R1 is alkyl;
• (b) where X is —CO(O)—, R2—R4 together represent other than one halogen and one cyano group, and
• (c) where X is an acetylene group, R2—R4 together represent other than one cyano group and two halogen atoms.