Anomalous odd-even effects in columnar and smectic phases of discotic tetraphenylenes.

Rather uncommon but pronounced odd-even effects in the clearing temperatures of liquid crystals are found for three new homologous series of discotic tetraphenylenes [figure: see text]. Detailed similarities to the well-known odd-even effects in calmitic mesogens point towards a more general understanding of odd-even effects in liquid crystals.The phase transition temperatures in many homologous series of liquid crystals show odd-even effects similar to the well-known odd-even effects in non-mesogenic organic compounds like carboxylic acids. For discotic liquid crystals only a few series with a sufficient number of consecutive homologues are available and only a small part of these series show odd-even effects at all. We now synthesized and investigated three series of discotic tetraphenylenes each ranging from the n=7 to the n=16 homologues. For these compounds an anomalous odd-even effect is found: The ascending and descending transition temperatures with increasing number of methylene groups in the side chains exhibit an inversion of this alternation which is always found between the n=12 and n=14 homologues. An overview of odd-even effects in liquid crystals is presented and we discuss the nature and the origin of odd-even effect in discotic mesogens.