Banana-Shaped or Rod-Like Mesogens? Molecular Structure, Crystal Structure and Mesophase Behaviour of 4,6-Dichloro-1,3-Phenylene Bis[4-(4-n-Subst.-Phenyliminomethyl) Benzoates]

Abstract

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 diffraction and dielectric measurements. Surprisingly, nematic and smectic phases are observed, although comparable bisesters of resorcinol and monochlororesorcinol exhibit B phases typical for banana-shaped mesogens. It could be proved by NMR investigations in the liquid crystallinen state that the molecules have a nearly rod-like shape. Dielectric measurements indicate a rotation around the molecular long axis as known from calamitic mesogens, which 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-alkyloxyphenyliminomethyl)benzoates] by two chlorine atomas causes a markedly change of the molecular shape resulting in nearly calamitic mesogens. These results are supported by single crystal structure studies, which allow the precise determination of the molecular structure and packing in the solid state.

Key Words:

• liquid crystal
• banana-shaped mesogens
• NMR
• X-ray diffraction
• dielectric measurements
• molecular structure
• crystal structure