Liquid Crystalline Phases

A material is defined as a crystalline solid (Cryst) when the structure has long-range order of the molecular positions in three dimensions. A fully ordered crystal will also have long-range orientational ordering of its constituent molecules. When a fully ordered molecular crystal is heated the thermal motions of the molecules within the lattice increase and eventually the vibrations become so intense that the regular arrangement of molecules is broken down with the loss of long-range orientational and positional order to give the disorganised isotropic liquid (Iso). The temperature at which this process occurs is called the melting point and the heat absorbed by the molecules is the latent heat of fusion. However, this process, which takes a compound from being very well ordered to being totally disordered in one step is a very destructive one, which is not universal for all compounds. For many compounds, this process occurs by way of one or more intermediate phases as the temperature is increased. These phases are called mesophases and some of these mesophases are liquid crystalline. Liquid crystalline phases have properties which are intermediate between those of the fully ordered crystalline solid and the isotropic liquid; liquid crystalline mesophases are fluids which, due to partial orientational ordering of the constituent molecules, have material properties such as permittivity, refractive index, elasticity and viscosity which are anisotropic (i.e., their magnitude will differ from one direction to another).


Mesogenic (i.e., mesophase-producing) compounds generally consist of long, narrow, lath-like and fairly rigid molecules (see Figure 1 and the examples on page 5). In the crystal state (Cryst), the molecules are held together by strong intermolecular forces of attraction which, due to the lath-like structure, are anisotropic. In simple terms, the smectic phase arises if the lateral intermolecular forces of attraction are stronger than the terminal forces and so, on heating, the terminal forces breakdown first, in-plane translational order is lost and this results in a lamellar arrangement of molecules in which the layers are not well defined (T2). Due to possible correlations within the layers and between the layers there are five true smectic modifications and a further six quasi-smectic disordered crystal mesophases. T3 illustrates the loss of both in-plane and out-of-plane translational order to leave a statistically parallel arrangement of molecules (orientational order) in the nematic phase. When the smectic phase is heated either out-of-plane translational ordering is lost (T4), which produces the nematic phase, or additionally orientational ordering is lost (T5), which gives the isotropic liquid. T6 represents the loss of orientational ordering of the nematic phase to give the isotropic liquid. No single liquid crystalline material exhibits all liquid crystal phase types but many compounds do exhibit two or three different types of liquid crystalline phase.

An important feature of mesophase-mesophase and mesophase-isotropic liquid phase transitions is that they are exactly reversible (to within ~0.5 ¡C), whereas crystal-crystal, crystal-mesophase and crystal-isotropic liquid phase transitions are not because supercooling occurs. This can be useful in determining the presence of a mesophase and since supercooling occurs on cooling to the crystal state then mesophases may be revealed on cooling and not on heating. Such mesophases are called monotropic and the temperatures at which they occur are given in round brackets (). Monotropic mesophases always occur below the melting point, whereas mesophases which occur above the melting point are formed on both heating and cooling and are called enantiotropic mesophases.

All of the compounds have the familiar long, lath-like structure but in many respects their structural composition is completely different and reflects the different applications for which the materials are intended. In the design of liquid crystal compounds the most important aspect is that they must exhibit the correct type of liquid crystalline phase over the desirable temperature, usually room temperature. Additionally liquid crystals must have a suitable combination of structural features to enable the generation of a rather subtle blend of physical properties. Clearly, to obtain everything from one material is not possible and so liquid crystals for commercial applications are all mixtures of appropriate materials that provide the best compromise of properties. A wide variety of synthetic organic chemistry techniques are used to prepare liquid crystal materials which must be of extremely high purity. An equally wide range of techniques are used to assess the materials both as single compounds and in mixtures. Compounds 2 and 3 were discovered at the University of Hull. Compound 3 was the first commercially viable, stable, room temperature liquid crystal that is suitable for use in watch and calculator type liquid crystal displays. Compound 2 is still the largest single component used in liquid crystal mixtures for displays. Compounds of types 4, 5 and 6 were also invented at the University of Hull and were designed to be used as ferroelectric host materials in fast-switching displays. Compound 9 is another compound from the wide variety synthesized at the University of Hull and this material has highly conjugated and polarizable structure; this material is used for specialist mixtures where a high birefringence is required. Compounds 10, 11 and 12 are chiral materials (see page 6) which are of great technological importance and are currently the subject of intense research. Many liquid crystalline materials are synthesized for fundamental reasons, i.e., to determine the relationship between the molecular structure and the physical properties; others are synthesized directly for applications (e.g., display devices and thermochromic devices).

The Nomenclature and Structure of Liquid Crystalline Phases

Liquid crystalline phases are named according to their degree of molecular ordering. For smectic phases there are various possible ways in which the constituent molecules can order and this enables the generation of six quasi-smectic crystal mesophases and five true smectic liquid crystal phases. The quasi-smectic crystal phases are given the letters B, J, G and E, K, H which are rather arbitrary but relate to the degree of ordering of the constituent molecules. True smectic liquid crystals are given the symbol S and a subscript which indicates the phase type (SA, SC and SB, SI, SF), again the nomenclature is rather arbitrary. The nematic phase is given the symbol N and there is only one nematic phase.

In smectic phases the molecules are arranged in layers which are not well defined and the different smectic phase types arise because of the different possibilities of molecular ordering within the phase structure. For example, in the B phase (disordered crystal) the molecules are arranged in a hexagonal arrangement and the positions of the hexagonal nets within each layer repeat in a regular manner throughout the phase, however, where the hexagonal nets do not regularly repeat throughout the structure a SB phase is generated. When the hexagonal ordering of the molecules becomes disrupted but the layer-like structure remains then the least ordered smectic liquid crystal phase (SA) is generated. In some of the phases (e.g., SC and SI) the molecules are tilted within the layers. Accordingly, the SC phase is the tilted analogue of the SA phase and the SI and SF phases are the tilted analogues of the SB phase. The layer ordering of the smectic liquid crystal phases confers a viscous nature but the material does flow. The nematic phase does not have a layered structure and the only degree of ordering is the statistically parallel arrangement of the molecules in one direction (director). Accordingly, the nematic phase is very fluid and much like a conventional liquid in nature except that a bulk example appears opaque. The opaque appearance is because of director fluctuations within a bulk sample; a fully aligned nematic sample would appear transparent.

Chirality (Handedness) in Liquid Crystals

In certain cases, organic molecules can be chiral or handed, i.e., there are two possible isomers associated with a particular structure and they are object and mirror image; rather like a left hand and a right hand. The isolation of one particular isomer gives a chiral material and this type of chirality is often described as molecular chirality. However, liquid crystals are ordered phases that are also fluid and when certain liquid crystal phases are composed of chiral molecules then the whole phase becomes chiral or handed; this type of chirality is often called form chirality where some macroscopic feature of the bulk phase has a handed structural feature. The most common liquid crystalline phase that exhibits form chirality is the chiral nematic (N*) phase. When composed of chiral molecules the nematic phase is called the chiral nematic (N*) phase and is also commonly referred to as the cholesteric phase (Ch). The term cholesteric is historical in that all early chiral nematic materials were derivatives of cholesterol. In the chiral nematic phase the chirality manifests itself in the form of a helical arrangement of molecules (see Figure 4a). The molecular alignment is identical to that found in the nematic phase except that the molecular chirality causes a slight, sequential change in the direction of the rod-like molecules through a section of material. This gradual change in molecular direction scribes a helix and the length over which this occurs is temperature dependent and is called the pitch. The pitch becomes shorter as the temperature is increased (see page 10). For a particular chiral material the direction of the helix is opposite for each isomer but the pitch length is the same.

The chiral smectic C (SC*) phase also exhibits form chirality as a helical macrostructure. In the achiral smectic C (SC) phase the constituent molecules are tilted with respect to the layer normal and the helix in the chiral variant is generated by a slight, sequential change in the direction of the tilt within each layer (see Figure 4b). The pitch of the helix of the SC* phase is also temperature dependent but at high temperatures the pitch is long and at low temperatures the pitch is short. The combination of chirality and tilt in the chiral smectic C phase reduces the symmetry of the system and the constituent molecules are spontaneously polarized; this phenomenon has great technological implications for very fast-switching ferroelectric light shutters and display devices.

http://www.hull.ac.uk/chemistry/research/LChistory.html#phases



Electrochromic devices

Self-contained, hermetically sealed, two-electrode electrolytic cells that change their ability to transmit (or reflect) light in response to a small bias (typically 1–2 V) applied across the two electrodes. The operation of electrochromic devices relies upon their electrochromic material content. These materials are organic or inorganic substances that are able to interconvert between two or more color states upon oxidation or reduction, that is, upon electrolytic loss or gain of electrons. The electrochromic materials that are appropriate for most practical applications are strong light absorbers in one redox state but colorless in another.

A typical electrochromic device is a sandwichlike structure with two glass plates and an electrolyte (see illustration). Each glass plate is coated on the inside with a transparent electrically conducting layer of indium-tin oxide, which operates as an electrode. Electrochromic mirrors include an additional reflective coating (for example, aluminum) on the outside of one of the glass plates. The electrolyte carries the ionic current inside the cell between the two electrodes, and it can be as simple as a salt (for example, sodium chloride, NaCl) dissolved in a dissociating solvent such as water. However, development has focused on gel and solid electrolytes, because they offer several advantages: they are easier to confine in the space between the electrodes; they function as laminators holding the two glass plates together; and their use minimizes the hydrostatic pressure that can cause substrate deformation and leakage problems, particularly in large-area devices such as smart windows.





State-of-the-technology electrochromic devices utilize two electrochromic materials with complementary properties: the first electrochromic material is normally reduced (ECM1red) and undergoes a colorless-to-colored transition upon oxidation (loss of electrons), while the second electrochromic material is normally oxidized (ECM2ox) and undergoes a similar transition upon reduction (gain of electrons). The electrochromic materials ECM1red and ECM2ox are selected so that they do not react with each other. The oxidation of ECM1red and the reduction of ECM2ox then are forced by the external power source (see illustration), which operates as an electron pump consuming energy in order to transfer electrons from one electrode to the other. Oxidation of ECM1red occurs at the positive electrode (anode) and is a source of electrons, while reduction of ECM2ox occurs at the negative electrode (cathode) and is a sink of electrons. This approach, known as complementary counterelectrode technology, has two distinct advantages. First, the long-term operating stability of the electrochromic cell is greatly enhanced, because providing both a source and a complementary sink of electrons within the same cell prevents any electrolytic decomposition of the electrolyte. Second, the reinforcing effect of two electrochromic materials changing color simultaneously enhances the contrast difference between the color states per unit charge consumed. Depending on the location of the two electrochromic materials within the electrochromic devices, three main types of such devices exist: solution, precipitation, and thin-film. See also Electrode; Electrolyte; Oxidation-reduction.

Electrochromic devices are analogous to liquid-crystal devices in that they do not generate their own light but modulate the ambient light. Liquid-crystal devices require use of polarizers; consequently, their viewing angle is limited, and lateral size limitations are imposed because the spacing between the electrodes (thickness) must be controlled within a few micrometers over the entire device area. Electrochromic devices do not require polarizers, thereby allowing a viewing angle approaching 180°, and contrast ratios similar to black ink on white paper (20:1 or better); moreover, control of the thickness is not important. Other desirable features of electrochromic devices include inherent color, continuous gray scale, and low average power consumption for the thin-film-type devices. Furthermore, it has been shown that electrochromic thin films can be patterned with a 2–5-μm resolution to form a large number of display elements that can be matrix-addressed. Nevertheless, even though there is no apparent intrinsic limitation, the best cycling lifetimes claimed for electrochromic materials are of the order of 10–20 million cycles, while the lifetime of liquid-crystal devices is of the order of several hundred million cycles. This long lifetime has made liquid-crystal devices a very successful technology in matrix-addressed, flat-panel displays.

The larger tolerance in thickness variation for electrochromic devices renders them better suited than liquid-crystal devices for large-area light modulation applications, such as smart windows, space dividers, and smart mirrors. Another possible application is in large-area displays that do not need frequent refreshing, such as signs and announcement boards. Reconfigurable optical recording devices (for example, disks) have been proposed as a high-resolution application that is within the presently available lifetimes of electrochromic materials.

http://www.answers.com/topic/electrochromic-device