Optical element and compounds used in its manufacture

A non-linear optical compound of the formula ##STR11## wherein:

• X.sup.1, X.sup.2 :
• one of which is --NO.sub.2 and the other is --NR.sup.6 R.sup.7 in which R.sup.6 and R.sup.7 each independently is H or lower alkyl;
• R.sup.3, R.sup.4 :
• each independently is H, F, Cl, Br, NH.sub.2, NO.sub.2, CN, lower alkyl or lower alkoxy, with the proviso that X.sup.1 is different from R.sup.4 and X.sup.2 is different from R.sup.3 ;
• R.sup.1 is:
• chiral alkyl or cycloalkyl;
• chiral alkyl or cycloalkyl in which, independently of each other, one or two non-adjacent --CH.sub.2 groups are replaced by O, S, --COO--, --OOC--, or --HC.dbd.CH-- or in an aliphatic residue also by --C.tbd.C--;
• chiral alkyl or cycloalkyl substituted by at least one of F, Cl, CN, lower alkyl or lower alkoxy;
• or
• chiral alkyl or cycloalkyl in which, independently of each other, one or two non-adjacent --CH.sub.2 groups are replaced by O, S, --COO--, --OOC--, or --HC.dbd.CH--, or in aliphatic residue also by --C.tbd.C--, which chiral alkyl or cycloalkyl is substituted by at least one of F, Cl, CN, lower alkyl or lower alkoxy;
• with the proviso that the center of chirality is situated in an alkyl chain on the first or second C and on a carbon atom in the case of a cycloalkyl residue,
• A.sup.1 is:
• 2, 3, or 4 six-membered rings which are bonded to one another or to the p-nitro-aniline ring by a single covalent bond;
• 2.