Liquid Crystal Research

Chromonic Liquid Crystals

Certain dyes naturally aggregate in aqueous solution. When these aggregates are anisotropic in shape, for example, stack in columns, a liquid crystal phase can form when the concentration is high enough. Instead of the conventional liquid crystal phase in which the orientationally ordered units are molecules, in these chromonic liquid crystals, it is the aggregates that are orientationally ordered.

The higher the dye concentration and the lower the temperature, the larger the aggregates are. This means that the liquid crystal phase will be sensitive to both concentration and temperature. Below is the phase diagram for Bordeaux dye, an aggregating dye derived from naphthalenecarboxylic acid. The wide coexistence region between the liquid crystal and isotropic liquid phases is due to the fact that the aggregates have a wide distribution of sizes.

The change in free energy when a molecule joins an aggregate can be estimated from x-ray experiments and from absorption experiments. Using absorption data from researchers working on many different aggregating dye systems, this change in free energy (denoted a and having units of kT) is clearly related to how large the concentration must be for the liquid crystal phase to form at room temperature. This correlation can be seen in the chart below, where compounds with the same color background have the same number of molecules in the cross-sectional area of the aggregate.

In the food dye Sunset Yellow FCF, the most important optical group interacts with light polarized in the plane of the molecule. Since the molecules stack with these planes perpendicular to the long axis of the stack, the direction with the highest index of refraction is perpendicular to the average orientation of the stacks. This makes the birefringence negative, as can be seen below.

Since absorption takes place for light polarized along this same molecular direction, the order parameter of the bond responsible for the absorption can be measured by investigating the absorption for light polarized parallel and perpendicular to the average orientation of the stacks. If it is assumed that this molecular direction is on average perpendicular to the average orientation of the stacks, the order parameter of the aggregates can be determined. This is shown below, where it is clear that the behavior of the order parameter is similar for chromonic and conventional liquid crystals.

Chromonic liquid crystals sometimes create very interesting textures when viewed through a polarizing microscope. Below are two such images. One shows birefringence colors and defect loops. The other displays the growth of the liquid crystal phase as filaments.

Trans-Cis Isomerization in an Azoxybenzene Liquid Crystal

When a liquid crystal with an azo- linkage is irradiated with ultraviolet light, the transition from the linear trans conformer to the bend cis conformer has a strong effect on the liquid crystal properties. The transition back from the cis to the trans conformer can be induced by irradiation with short wavelength visible light. Such effects in azobenzene liquid crystals have been studied at length, both to understand the underlying photophysics and possibly to apply them in photo-addressable devices.

Much less is known about trans-cis isomerization in azoxybenzene liquid crystals. These molecules also respond to ultraviolet and short wavelength visible light in much the same way that azobenzene-based systems do. A moderate amount of uv light drives roughly half of the molecules from the trans to cis conformer. This changes the absorption spectrum significantly, and with additional experiments, the absorption spectrum of the cis conformer can be calculated (see below).

The rate constants for the thermal relaxation from the cis conformer to the trans conformer are quite low, meaning that at room temperature it takes approximately 24 hours for relaxation to occur. The rate constant is temperature dependent, showing Arrhenius behavior with an excitation energy of about 63 kJ/mole, a value slightly less than for azobenzene.

Chirality in Liquid Crystals

The introduction of handedness to liquid crystal phases due to the presence of chiral molecules affects both the stability of the various liquid crystal phases and the nature of the phase transitions between them. A good example of this is what happens in the chiral nematic phase of a liquid crystal that possesses a helix inversion, i.e., a temperature at which the helix of the chiral nematic phase changes from one handedness to the other. In a mixture of two liquid crystals, the helix inversion line in the temperature - concentration plane depends on both variables. But in the isotropic phase, the helix inversion line depends only on concentration. This is illustrated in the following diagram.

Another example of the unique effects of chirality in liquid crystal is the stability of exotic phases when the chirality of a system is sufficiently high. Instead of a simple transition from the chiral nematic to the isotropic phase, as the temperature is raised up to three “blue phases” become stable over small temperature intervals. These blue phases represent a fluid phase of matter in which defects in the orientational order form either a cubic lattice (for two of the blue phases) or a disordered structure (for one of the blue phases). The growth characteristics of the cubic blue phases are analogous to what occurs in solid crystals, as demonstrated by the picture of single crystals of one of the fluid blue phases shown below.

Since the overall symmetry of the amorphous blue phase and the isotropic phase is the same, it is possible that the transition line between these two phases ends at a critical point. Such a critical point was discovered and studied through both optical and calorimetric techniques. The nature of this critical point is analogous to the liquid – gas critical point.

Smectic Phases with No Layer Contraction

Shown below are depictions of the smectic A and smectic C phases of a liquid crystal. When most liquid crystals undergo a transition from the non-tilted smectic A phase to the tilted smectic C phase, the layer spacing decreases as expected.

Recently materials have been found in which there is little to no layer contraction at the transition, a situation hypothesized by deVries many years ago. The exact structure of the smectic A phases of these materials is not fully understood, but the experimental data can be explained by assuming the molecules are tilted in the smectic A phase, but there is azimuthal disorder of the tilt direction. The phase transition to the smectic C phase is therefore simply an ordering of the azimuthal direction with little or no change in the tilt angle. This ordering of the azimuthal direction of the tilt angle can also be induced by applying an electric field in the plane of the layers (see diagram below).

The size and speed of this response makes such phases a candidate for the next generation of liquid crystal displays. Measurements of the orientational order of dissolved dyes in conventional smectic A phases (such as KN125) and deVries smectic phases (such as TSiKN65) reveal that something is very different about the local orientational order in these two types of smectic A phases.