Photoactive silane derivatives for liquid crystals

A silane compound of general formula I ##STR4## wherein X.sup.1, X.sup.2 and X.sup.3 are alkyl, alkoxy or halogen, at least one of which is either alkoxy or halogen;

• S.sup.1 is a straight-chain or branched alkylene grouping having a formula --(CH.sub.2).sub.r --, which is unsubstituted or substituted by one or more fluorine, chlorine or cyano substituents, or is a chain having a formula-(CH.sub.2).sub.r --L.sup.1 --(CH.sub.2).sub.s --L.sup.2 -- in which L.sup.1 and L.sup.2 each independently is a single bond or cross-linking functional group selected from the group consisting of O, COO, OOC, NR.sup.2, NR.sup.2 --CO, CO--NR.sup.2, NR.sup.2 --COO, O--CO--NR.sup.2, CH.dbd.CH and C.tbd.C wherein R.sup.2 is hydrogen or lower alkyl;
• r and s are each a whole number of 1 to 20 wherein r+s.ltoreq.20;
• ring A is 1,3- or 1,4-phenylene and is unsubstituted or substituted with fluorine, chlorine, cyano, alkyl or alkoxy, pyridine-2,5-diyl, pyrimidine-2,5-diyl, 1,3-dioxane-2,5-diyl, cyclohexane-1,4-diyl, piperidine-1,4-diyl or piperazine-1,4-diyl;
• ring B is 1,3- or 1,4-phenylene and is unsubstituted or substituted with fluorine, chlorine, cyano, alkyl or alkoxy, pyridine-2,5-diyl, pyrimidine-2,5-diyl, 1,4- or 2,6-naphthylene, 1,3-dioxane-2,5-diyl or cyclohexane-1,4-diyl;
• Y.sup.1, Y.sup.2 each independently is a single covalent bond, --(CH.sub.2).sub.t --, --O--, --CO--, --CO--O--, --O--OC--, --NR.sup.3 --, --CO--NR.sup.3 --, --R.sup.3 N--CO--, --(CH.sub.2).sub.u --O--, --O--(CH.sub.2).sub.u --, --(CH.sub.2).sub.u --NR.sup.3 -- or --NR.sup.3 --(CH.sub.2).sub.u --, wherein
• R.sup.3 is hydrogen or lower alkyl;
• t is a whole number of 1 to 4;
• u is a whole number of 1 to 3;
• m, n each independently is 0 or 1;
• ring C is 1,3- or 1,4-phenylene which is unsubstituted or substituted with fluorine, chlorine, cyano, alkyl or alkoxy, pyrimidine-2,5- or 3,5-diyl, pyridine-2,5- or -2,4-diyl or -2,6-diyl, 2,5-thiophenylene, 2,5-furanylene or 1,4- or 2,6-naphthylene;
• and
• Z is --O-- or --NR.sup.4 --, in which R.sup.4 is hydrogen or lower alkyl or a second group of formula D wherein
• D is straight-chain or branched alkyl with 1 to 20 carbon atoms and is unsubstituted or substituted with fluorine or chlorine or a cycloalkyl residue with 3 to 8 ring atoms which is unsubstituted or substituted with fluorine, chlorine, alkyl or alkoxy.