Liquid Crystals: A Brief Introduction
Liquid crystals inhabit the middle ground between "ordinary" liquids and crystalline solids. There are several kinds of liquid crystal phases, which are characterized by differing degrees and types of ordering..
The least ordered of the liquid crystals are the nematic phases. Typically formed from rod-shaped molecules that align in a uniform direction, these phases have low viscosity and form the basis for the LCD industry.
Like the nematics, smectic phases are usually formed by rod-shaped molecules that align parallel to each other. In addition, these molecules arrange themselves in layers. Smectic phases are categorized according to the degree of molecular order, and whether the molecules are tilted with respect to the layers. In the smectic A phase, the molecules orient perpendicular to the layer.
Smectic C phases are similar to smectic A phases, but the molecules are tilted with respect to the layers. Note that molecules within each layer tend to be quite disordered. Other smectic liquid crystals, such as the smectic B and G phases, more ordering within the layers.
Not all liquid crystal-forming molecules (or "mesogens") are rod-shaped. Disc-shaped molecules ("discotic mesogens") also assemble into into columnar liquid crystal phases (right). These materials have garnered interest as organic semiconductors for OLEDs and photovoltaics.
Designing Discotic Mesogens
If discotic mesogens are to be successfully deployed as liquid crystalline semiconductors, we first need to understand the relationship between molecular structure and LC behavior. Over the course of many years, we have been studying the effects of molecular structure on the stability of columnar phases and on the types of phases formed. Much of this work has focused on the dibenzo[a,c]phenazines, which allow us to systematically vary the substituents attached to the aromatic core.
Our studies have uncovered several key features that can be used in the design of new materials: the presence of heteroatoms promotes LC formation, as does increasing the size of the aromatic core. We have also shown that electron-withdrawing groups stabilize columnar phases, which we rationalize in terms of dipole-dipole interactions that favor an anti-parallel arrangement of the discs.
Molecular Symmetry Effects
A number of factors control the stability of liquid crystal phases; functional groups, the length of side chains, the nature of the core and shape anisotropy being amongst the most important. It has long been recognized that molecular symmetry can often impact phase behavior; however, few systematic studies into symmetry effects on the phase behavior of liquid crystals have been carried out.
Collaborative LC Studies
In addition to the research described above, we also work with several groups to understand the LC phase behavior of compounds prepared in their labs. These collaborations include ongoing efforts with Ken Maly and his group at Wilfrid Laurier University and with C. C. Ling at the University of Calgary.