Mesophase design: II. Molecular structure and crystal packing of 4-alkyloxycyanobiphenyls
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Mesophase Design: II. Molecular Structure and Crystal Packing of 4Alkyloxycyanobiphenyls L. G. Kuz’mina, N. S. Kucherepa, and A. V. Churakov Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia email: [email protected] Received January 17, 2011
Abstract—A joint analysis of the molecular geometry and crystal packings of the members of a homologous series of 4alkyloxycyanobiphenyl CnH2n + 1–O–C6H4–C6H4–CN with n = 5–12 does not reveal a direct relationship between the molecular geometry and mesogenic properties of these compounds. The crystal packings of all of them are built from alternating loose aliphatic and dense aromatic regions. The closely packed aromatic regions of crystals exhibit weak intermolecular directed interactions of different natures which are responsible for the occurrence of the mesophase. The type of mesophase depends on the number s of structureforming elements, the ratio of their energies (if s > 1), and the melting temperature of the com pound. A graph system is used to describe the crystal packings and the mesophase. It is shown that, although the mesophase graph is similar to the crystal graph, they may differ somewhat, because some symmetric lim itations (for example, lattice) on the molecular associate structure are removed in the melt. DOI: 10.1134/S1063774512020137
INTRODUCTION Previously, to reveal the correlations between the structure and the liquidcrystal (LC) properties of the compounds of a homologous series of 4alkylcyanobi phenyl (ACBn; n is the number of carbon atoms in the alkyl chain) and establish the mechanism of phase transitions with the participation of the mesophase, the molecular structures and crystal packings of the representatives of this series were analyzed in [1]. In a continuation of these studies, we perform a similar analysis of the molecular and crystal structures of homologues of a series of 4alkyloxycyanobiphenyls with the general formula CnH2n + 1–О–C6H4–C6H4–CN (AOCBn). The homologues of this series with n = 5–12 belong to LC compounds; their mesophase is presented by either smectic–nematics or nematics. The conclusions, based on an analysis of the Xray structure of the compounds, called for differential scanning calorimetry (DSC) measurements for the compounds with n = 7 and 8. As in the previous paper, the solution to the prob lem being stated is based on the established existence of a genetic relationship between the mesophase and the crystal phase and the suggestion that the character istic properties of the mesophase are determined by its degree of structuring. According to concepts of supramolecular chemistry [2, 3], the formation of supramolecular ensembles in the liquid phase is deter mined to a large extent by a set of weak directed inter actions of different types. These supramolecular ensembles determine the degree of structuring of the
melt and set the LC properties of the system. Previ ously we successfully used this ideology
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