Review of Crystalline Structures of Some Selected Homologous Series of Rod-Like Molecules Capable of Forming Liquid Crystalline Phases

Peter Zugenmaier

doi:10.3390/ijms12117360

Abstract

The crystal structures of four homologous series of rod-like molecules are reviewed, two of which form hydrogen bonds and two with a symmetric chemical constitution. Many of the compounds investigated turn into liquid crystalline phases upon temperature increase. It is of valuable interest to know possible conformations and possible packing arrangements as prerequisites to model liquid crystalline structures. The hydrogen bonds of homologous series of pure 4-(ω-hydroxyalkyloxy)-4′-hydroxybiphenyl (HnHBP, n the alkyloxy tail length) are realized through head to tail arrangements of the hydroxyl groups and crystallize except one compound in chiral space groups without the molecules containing any asymmetric carbon. The hydrogen bonds of the homologous series of 4-substituted benzoic acids with various lengths of the tail provide dimers through strong polar bonding of adjacent carboxyl groups and thus provide the stiff part of a mesogenic unit prerequisite for liquid crystalline phases. The homologous series of dialkanoyloxybiphenyls (BP-n, n = 1, 19), of which nine compounds could be crystallized, show liquid crystalline behavior for longer alkane chain lengths, despite the high mobility of the alkane chain ends already detectable in the crystal phase. A single molecule, half a molecule or two half molecules form the asymmetric unit in a centrosymmetric space group. The homologous series of 1,4-terephthalidene-bis-N-(4′-n-alkylaniline) (TBAA-n) exhibit a large variety of packing arrangements in the crystalline state, with or without relying on the symmetry center within the molecules.

Highlights

In this review, the crystal structures determined by single crystal X-ray evaluation are presented and compared for four selected homologous series at ambient temperature
Studying homologous series provides important information about possible changes of the conformation and packing arrangement of molecules by a variation of the molecular length. Such structural changes are caused by the influence of the interactions between molecules and may lead to unconventional packing of the molecules or symmetry changes by the space group to be considered for evaluating structural models
The b-dimension mirrors the lengths of the molecules. This is true for the dimensions of H3HBP exhibiting a one-molecule asymmetric unit with antiparallel packing arrangement in space group P212121 and β = 90° and surprisingly holds for H5HBP, if the a-dimension is reduced to 1⁄2 due the fact that the number of molecules is doubled as compared to the other compounds discussed

Summary

Introduction

The crystal structures determined by single crystal X-ray evaluation are presented and compared for four selected homologous series at ambient temperature. For two series of compounds, hydrogen bonding plays an important role and their influence on packing arrangements and symmetry of space groups will be considered. Studying homologous series provides important information about possible changes of the conformation and packing arrangement of molecules by a variation of the molecular length. Such structural changes are caused by the influence of the interactions between molecules and may lead to unconventional packing of the molecules or symmetry changes by the space group to be considered for evaluating structural models. Representation of the homologous series of 1,4-di-n-alkanoyloxy-biphenyl abbreviated BP-n

Discussion of the Crystallographic Structures

Crystal Structures of Some 4-Substituted Benzoic Acids

Experimental Section

Conclusions