Anomalous Odd–Even Effects in Columnar and Smectic Phases of Discotic Tetraphenylenes

Abstract

Rather uncommon but pronounced odd–even effects in the clearing temperatures of liquid crystals are found for three new homologous series of discotic tetraphenylenes (see figure). 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.