Molecular Twisting Power chiraler Azobenzole

The Molecular Twisting Power (MTP) of the trans- and cis-azocompounds 1 - 4, each substituted either with para- or orthostanding chiral side chains, of the azoxyanalogues 5, 6 and of the chiral nitrobenzenes 7, 8 dissolved in the nematic liquid crystal NP 1132 are investigated. For all various series, althoug there is a distinct change in the geometry of the aromatic part and the substitution pattern, the helical screw sense of the induced cholesteric phase alternates with the number of bonds between the chiral center and the aromatic ring system. For the orthosubstituted azobenzenes 3, 4 a reversible inversion of the helix during the cis-trans-isomerisation is observed. Most of the compounds with the chiral center in the a-position to the aromatic ring system show anomalous behaviour of the MTP.