Introduction
1.2 Liquid crystals
On heating any organic crystalline substance goes from crystal to fluid transition, and it loses the orientational and positional order at once. Some compound transitions occur through one or more intermediate states where the position or orientation order is gradually lost. These phases are called plastic crystals and mesomorphic states of matter. Liquid crystals are a state of matter with qualities that fall between liquids and solid crystals. Liquid crystals may flow like a liquid, and their molecules may orient in a crystal manner and are therefore defined as a mesophase (from the ancient Greek word "mesos," which means "intermediate"). It is sometimes referred to as the "fourth state of matter."[15–21] Mesomorphic compounds are materials that have LC characteristics. Mesomorphism is another term for LC behavior.
Mesogenic groups are molecular fragments that can be used to induce LC behavior in a structure. Several aromatic rings are frequently found in such groups. The high order characteristic of crystalline solids is partially diminished in an LC phase, giving the molecules some mobility and rendering the substance fluidic or plastic. The schematic representation of crystals to isotropic and intermediate phases is shown in Figure 1.1.
Figure 1.1: General schematic representation of crystal to isotropic and intermediate phases
Crystal
Liquid Crystal Plastic Crystal
Isotropic Liquids
Molecules are ordered positional (specific sites in lattices) and orientational in all directions.
Phase with positional order but
no orientational order Phase with more order than liquid but
maintain some orientational (and sometimes even positional) order.
Molecules are randomly diffused.
No positional and no orientational order
Page | 4 1.3 History of liquid crystals
It is the 134th year since liquid crystals were discovered.[22] Liquid crystals have found their way into so many products we use today that life would be incomprehensible without them.
Almost no other high-tech material has gained such widespread use so quickly. The millions of users who own a tablet computer are fascinated by its fantastic display, oblivious of the liquid crystals sandwiched between the glass plates and polarizers.
W.Heintz, a German biochemist, reported the synthesis of a triglyceride in 1850, which exhibited cloudy liquid on the heating before turning to clear liquid. Stearin melted from solid to cloudy liquid at 52°C and changed from 58°C to an opaque at 62.5°C to a clear liquid, but he could not understand what was happening. The structure of stearin is shown in Figure 1.2.[23]
Figure 1.2: Structure of Stearin.
The story behind liquid crystals is about Friedrich Reinitzer, 1888 botany and technical microscopy professor at Technical University in Prague. He was working on cholesterol derivatives.[24] In the case of Cholesterol and cholesterol benzoate, which he had extracted from carrot, and measured the melting point of the cholesterol benzoate was (Figure 1.3a) at 145.5
°C; however, he found a second melting point at 178.5 °C, and between the two transitions, a milky liquid phase was observed, and above 178.5 °C, the phase was clear. Under the polarised optical microscopy, he observed distinct violet and blue color phenomena at both phase transitions (double melting). However, cholesteryl acetate (Figure 1.3b) behaved similarly to cholesterol benzoate with a monotropic cholesteric phase. Later, he contacted physicist otto Lehmann, an expert in the physical isomerism of crystals. Lehmann had a polarizing microscope with a hot stage and was thus able to investigate more precisely than Reinitzer. The German physicist Otto Lehmann explained the existence of "double melting," a curious
Stearin
Page | 5 phenomenon. He referred to them as "soft crystals" first, then "crystalline fluids." He began to refer to the opaque phase as "liquid crystals" as he became more convinced that it was a homogeneous phase of matter with properties similar to liquids and solids. The discovery of liquid crystals, like today's science and technology of liquid crystals, also required an interdisciplinary approach. It's worth noting that researchers were working with liquid crystals as early as the 1850s, but they hadn't recognized how unique the phenomenon was. As a result of very lively correspondence between Lehmann and Reinitzer, In 1889, Lehmann had a publication on About flowing crystals. As a result, Reinitzer, a biologist, is credited with discovery of liquid crystals, while Lehmann, a physicist, is credited with finding liquid crystal research.[22]
Figure 1.3: Structure of Cholesterol benzoate.[25]
Afterward, in the early twentieth century, Daniel Vorländer, a chemistry professor at the University of Halle, began a systematic synthetic work in order to determine the structure- mesophase relation, and by 1935, he had synthesized over 1100 liquid crystalline substances in his laboratory alone.[26] He pointed out that all compounds with mesophases possessed elongated (rod-like) molecules, which are today known as calamitic molecules (Figure 1.4).
Figure 1.4: Representation of a calamitic liquid crystal.
Cholesterol benzoate Cholesteryl acetate
a) b)
Page | 6 In 1923, Vorlander, a pioneer in liquid crystal research, proposed disc-shaped molecules with packing arrangements comparable to voltas columns while examining triphenylene and perylene.[27] Because of the lack of flexible chains, he could not detect any mesomorphic fingerprints. His article also stated that leaf-shaped molecules do not form any liquid crystals.
Later, however, triphenylenes and perylenes with flexible periphery were essential reports in discotic literature. However, in 1977, Chandrasekhar and his colleagues at Raman Research Institute discovered for the first time that flat disc-shaped molecules (Figure 1.5) do form liquid crystalline phases. They synthesized a series of benzene hexaesters and described a novel class of LCs, using optical, thermal, and X-ray diffraction techniques.[28] This finding caught the attention of chemists all over the world, resulting in the birth of a fascinating new branch of discotic liquid crystal study.
Figure 1.5: Representation of a disc-shaped liquid crystal.
Banana-shaped molecules (Figure 1.6)[29] are the most recent addition to the liquid crystal family. An angular central unit, two stiff linear cores, and terminal chains make up their molecular structure. The finding of ferroelectricity in non-chiral banana-shaped molecules prompted a surge of study. So far, over a thousand bent molecular form compounds have been created. Mesophases with polar order and supramolecular chirality can be reached using bent- shaped molecules.
Figure 1.6: Representation of a bent-shaped liquid crystal.
Page | 7 1.4 Classification of liquid crystals.
A variety of factors can be used to categorize liquid crystals. For example, based on the molecular weight of constituent molecules, such as low molecular mass liquid crystals (monomers and oligomers) and high molecular mass liquid crystals (polymers). Based on how mesophase forms, such as by adding a specific solvent (lyotropic) and changing the temperature (thermotropic). Based on constituent molecules' nature (organic, inorganic, and organometallic) and also based on constituent molecule's shape (calamitic, discotic & bent- core ). The flow chart in Figure 1.7 depicts the classification of liquid crystals. However, the most popular variety of LCs can be divided into three groups:
(i) Thermotropic liquid crystals – mesophase formation is temperature-dependent.
(ii) Lyotropic liquid crystals – mesophase formation is solvent and concentration-dependent.
(iii) Amphotropic liquid crystals exist when the component molecules exhibit thermotropic and lyotropic liquid crystalline phases.
Despite the fact that this thesis is only concerned with thermotropic liquid crystals, a small introduction to lyotropic liquid crystals is provided below due to their importance in living systems.[30]
Figure 1.7: Classification of liquid crystals.
Page | 8 1.5 Lyotropic liquid crystals:
Amphiphiles are the molecules that contain both hydrophilic and hydrophobic segments because of the incompatibility of these different segments with the surrounding medium.
Amphiphiles are well known to self-assemble into diverse nanostructures using supramolecular interactions such as hydrogen bonding, Van der Waals interactions, hydrophobic interactions, etc. These nanostructures may exhibit mesophase properties. Concentration and/or temperature control the formation of mesophases in lyotropic liquid crystals.[31] Typical examples of lyotropic liquid crystals are soaps in water and various phospholipids. One of the well-reviewed amphiphilic self-assembling systems is the self-assembly of phospholipids with phosphate as the hydrophilic domain and a fatty acid chain attached to it as the hydrophobic domain to form a bilayer. The polar head group is exposed to the aqueous medium, while hydrophobic interactions between the alkyl chain drive self-assembly to form a bilayer with a hydrophobic segment embedded inside (Figure 1.8). Numerous amphiphilic systems self-assemble into soft nanomaterials with tailored functional properties which have been reported in the literature.[32]
Below are the descriptions of a few representative systems. According to their molecular architecture, a series of amphiphiles with rod-coil topology are self-assembled into diverse nanostructures such as lamellar, vesicles, micelles, and cylindrical micelles[33], hexagonal columnar, and the cubic phases. X-ray diffraction techniques have classified these mesophase structures.[34,35] The schematic representation of lyotropic liquid crystalline self-assembly is shown in Figure 1.9.
Figure 1.8: Schematic showing the self-assembly of phospholipids in an aqueous medium into a bilayer.
Hydrophilic head (polar)
Hydrophobic tail (Non polar)
Self assembly In water
=
Page | 9
Figure 1.9: Lyotropic liquid crystalline self-assembly.