siloxane compounds having a smectic liquid crystal phase and the general formula ## str1 ## wherein each r = alkyl , alkenyl or aryl , q represents a monovalent group , for example alkyl , -- n om &# 39 ;, a chiral organic group , a dye group , a non - linear optic group or the group -- n l , in which l represents a siloxane group , m and m &# 39 ; each represent ## str2 ## wherein a is carboxyl , t is cn , f or cl and p = 0 or 1 , provided that when t is f or cl x has a value of at least 2 .