Optically active liquid crystal mixtures and liquid crystal devices containing them

A liquid crystal composition which comprises at least two optically active compounds selected from the group consisting of compounds having the general formulae ##STR41## where R is a branched alkyl chain group which contains a chiral center and includes up to 12 carbon atoms, and a and b are 0 to 1 and a+b = O or 1:

• ##STR42## where R is alkyl, which may be straight chain, branched chain and may or may not contain a chiral center and includes up to 12 carbon atoms, c = d = 0 when ##STR43## and c and d may be 0 or 1 when ##STR44## when d = , Y is selected from hydrogen, ##STR45## and when d = 1, Y is selected from hydrogen, R.sup.1, ##STR46## where R.sup.1 an alkyl group as defined for R in group member (2);
• ##STR47## where R is defined as for group member (2), f and g may have values 0, or 1 such that f+g = 1 and where A is R.sup.1, OR.sup.1 or CN where R.sup.1 is an alkyl group as defined for R in group member (2);
• and ##STR48## where R is defined as for group member 2, h is 1 or 0, E is an ester linkage --OCO-- or --COO--, and B is R.sup.1, OR.sup.1, or CN where R.sup.1 is an alkyl group as defined for R in group member (2);
• wherein each of the at least two said optically active compounds has as an alkyl end group R or R.sup.1 a chiral alkyl group AlkCH(AlK.sup.1) (CH.sub.2).sub.n -- where Alk and Alk.sup.1 are different alkyl groups and n is an integer from 1 to 5 inclusive, and wherein at least one of the at least two said optically active compounds is selected from group members (2), (3) and (4), and wherein all of the optically active compounds in the composition have the same optical rotation angle sense and the liquid crystal composition has a helical structure having a pitch of the order of the wavelength of visible light and which composition is temperature dependent such that the composition is thermochromic.