Epitaxygens: mesomorphic properties of triptycene derivatives

The structure of mesophases derived from a triptycene subunit substituted with five paraffinic chains has been determined by X-ray diffraction at small angles. They all show a smectic ordering. This is the first example of a smectogen whose rigid core belongs to the D_3h symmetry. A hexagonal order has been observed in the X-ray diffraction patterns; it has been associated with a p31m packing of the rigid cores within the layers. The intralamellar cell parameter is in agreement with the molecular dimensions determined from CPK models. The calculated molecular length is in accordance with the experimental interlamellar distance. The difference in mesomorphic behaviour between the five and six chain derivatives is discussed.