Antiferroelectric Liquid Crystals from Achiral Molecules And A Liquid Conglomerate
- PDF / 2,977,773 Bytes
- 12 Pages / 417.6 x 639 pts Page_size
- 25 Downloads / 250 Views
ABSTRACT Until recently, it was an empirical fact that creation of a chiral liquid crystal phase required enantiomerically enriched molecules. In addition, to date known ferroelectric and antiferroelectric smectics have also been composed of enantiomerically enriched molecules. Herein are described the first examples of the formation of chiral and antiferroelectric supermolecular liquid crystalline structures from achiral molecules. In one case (apparently metastable) the liquid crystal structure is macroscopically chiral, with samples composed of heterochiral macroscopic domains: a liquid conglomerate.
INTRODUCTION In the phase transition from isotropic liquid (or solution) to crystalline solid, spontaneous breaking of achiral symmetry (referred to here as "achiral symmetry breaking") is a common occurrence, first discovered in the middle of the nineteenth century by Pasteur. In this most famous case, a conglomerate (mixture of right handed and left handed macroscopic crystals) is produced upon recrystallization of racemic sodium ammonium tartrate under certain conditions.1 Another interesting phenomenon, quite relevant to the current discussion, occurs upon crystallization of the racemic 2-cyanoalkylphenylbenzoate 1 (Figure 1).2 In this case the chiral molecules undergo a kind of spontaneous nano-phase separation, resolving into enantiomerically
pure layers, with adjacent layers pairwise heterochiral (i.e. all adjacent pairs of layers are nonsuperposable mirror images). The molecules in these layers are also clearly tilted in the crystal, with molecules in adjacent layer pairs tilting in opposite directions (anticlinic). The combination of heterochiral and anticlinic layers affords a structure with net polar symmetry 3 in the bulk-thus these crystals should be pyroelectric. The macroscopic symmetry of the bulk crystal, however, is achiral. At the molecular level, chemical transformation of an achiral compound to a racemate (e.g. reduction of 2-butanone with sodium borohydride to produce racemic 2-butanol) is also an example of spontaneous achiral symmetry breaking, since chiral molecules are formed from achiral ones-quite analogous to the above-mentioned cases, except the chiral "domains" are single molecules. This, of course, is a trivial example-achiral symmetry breaking in this context typically refers to the formation of macroscopic chiral domains from ensembles of achiral or racemic molecules. Other interesting examples of spontaneous achiral symmetry breaking have been observed at interfaces. Thus, for example, McConnell and Weis first observed the formation of chiral crystalline monolayer domains in Langmuir films of enantiomerically pure lipids by epifluorescence optical microscopy. 4 Racemic functionalized amino acids form conglomerates of enantiomerically pure 2-D crystals at the air-water interface, 5 and achiral, 6 enantiomerically pure, and racemic 7 liquid crystal mesogens form chiral 2-D crystals on graphite, as evidenced by
Mat. Res. Soc. Symp. Proc. Vol. 559 01999 Materials Research Society
Data Loading...
