Mesogen (or Liquid Crystal) is a compound that displays liquid crystal properties and "mesogenic" is the term given to a substance that induces the liquid crystalline state (also known as the liquid crystal phase).
Mesogens can be described as disordered solids or ordered liquids because they arise from a unique state of matter that exhibits both solid- and liquid-like properties called the liquid crystalline state. This liquid crystalline state (LC) is called the mesophase and occurs between the crystalline solid (Cr) state and the isotropic liquid (Iso) state at distinct temperature ranges.
The liquid crystal properties arise because mesogenic compounds are composed of rigid and flexible parts, which help characterize the order and mobility of its structure. The rigid components align mesogen moieties in one direction and have distinctive shapes that are typically found in the form of rod or disk shapes. The flexible segments provide mesogens with mobility because they are usually made up of alkyl chains, which hinder crystallization to a certain degree. The combination of rigid and flexible chains induce structural alignment and fluidity between liquid crystal moieties.
In doing so, varying degrees of order and mobility within mesogens results in different types of liquid crystal phases, Figure 1. The nematic phase (N) is the least ordered and most fluid liquid crystalline state or mesophase that is based on the rigid core of mesogen moieties. The nematic phase leads to long range orientational order and short range positional order of mesogens. The smectic (Sm) and columnar (CoI) phases are more ordered and less fluid than their nematic phases and demonstrate long range orientational order of rod-shaped and disk-shaped rigid cores, respectively.
Figure 1 – Organization of rod-like and disk-like rigid cores in liquid crystal phases of mesogens, where Iso is the isotropic liquid state; N is the nematic phase of the liquid crystal state; SmA is the smectic A phase; SmC is the smectic C phase; and CoI is the columnar phase.
An example of a discotic mesogen type rigid core is a triphenylene based disk molecule, where the hexagonal columnar liquid crystal phase exists between the 66℃ (crystalline solid phase) and 122℃ (isotropic liquid phase) temperature range, Figure 3.
Calamitic mesogens contain a rod-shaped rigid core and tend to organize in distinctive layers, forming lamellar or smectic liquid crystal phases (Sm) of long range positional order. Low order smectic phases, Figure 4, include smectic A (SmA) and smectic C (SmC) phases, while higher ordered smectic phases include smectic B, I, F, G and H (SmB/I/F/G/H) phases.
An example of a calamitic mesogen type rigid core is a benzyl cyanide based rod molecule, where the smectic A liquid crystal phase exists between the 60℃ (crystalline solid phase) and 62℃ (isotropic liquid phase) temperature range, Figure 5.
Bent-rod mesogens are special calamitic mesogens that contain a nonlinear rod-shaped or bent- rod shaped rigid core and organize to form ‘banana-phases.’ The rigid units of these phases pack in a way so that the highest density and polar order are achieved, typically with the apex of the bent rod pointing in one direction. When a layer of bent-rods points in the same polar direction as its adjacent layers the lamellar organization is known as the smectic PF (SmPF) phase, where the F subscript indicates ferroelectric switching, Figure 6. Smectic PA (SmPA) is the term given to a layer of bent-rods that points in the opposite polar direction as its neighbouring layers, where A stands for antiferroelectic switching, Figure 6.
Other variations of bent-rod liquid crystal phases include: antiferroelectric/ferroelectric smectic C (SmCPA/SmCPF) phases and antiferroelectric/ferroelectric smectic A (SmAPA/SmAPF) phases, which have distinctive tilt and orthogonal modes of lamellar organization.
The figure below illustrates an example of a nonlinear rod-shaped rigid core that gives rise to a bent-rod (calamitic) mesogen, where the antiferroelectric smectic C (Sm CPA) liquid crystal phase exists between the 145℃ (crystalline solid phase) and 162℃ (isotropic liquid phase) temperature range, Figure 7.
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