Banana-shaped or rod-like mesogens? Molecular structure, crystal structureand mesophase behaviour of 4,6-dichloro-1,3-phenylene bis[4-(4-n-subst.-phenyliminomethyl)benzoates]

Terminally alkyl- and alkyloxysubstituted 4,6-dichloro-1,3-phenylene bis[4- (phenyliminomethyl)benzoates] have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy, X-ray diff raction and dielectric measurements. Surprisingly, nematic and smectic phas es are observed, although comparable bisesters of resorcinol and monochloro resorcinol exhibit B phases typical for banana-shaped mesogens. It could be proved by NMR investigations in the liquid crystallinen state that the mol ecules have a nearly rod-like shape. Dielectric measurements indicate a rot ation around the molecular long axis as known from calamitic mesogens, whic h is clearly distinguished from the behaviour of bent molecules. Obviously, substitution of the central phenyl ring of the bent 1,3-phenylene bis[4-(4 -n-alkyloxyphenyliminamethyl)benzoates] by two chlorine atoms causes a mark edly change of the molecular shape resulting in nearly calamitic mesogens. These results are supported by single crystal structure studies, which anew the precise determination of the molecular structure and packing in the so lid state.