Liquid Crystals: Role of Transition Metal Ions in the Design of Metallomesogens

What is meant by “liquid crystals”? There has been given so many definitions over the time, that introducing this subject seems discounted and repetitive. However, the world of liquid crystals is so fascinating that it is worth trying, hoping not to appear trivial to readers. Liquid crystals are compounds that adopt aggregation phases more ordered than the liquid phase (orientational ordered) but less ordered than the crystalline state (positional disordered). Therefore, liquid crystals are soft materials that combine order and fluidity with building components self-organized into a large variety of structures. Liquid crystal molecules are called mesogens and the phases they form are named mesophases.1–4 In the strict paradigm of molecules able to be mesogens, they all have the common structural feature to be anisotropic in shape, giving rise to anisotropic physical properties (optical, electrical, and magnetic properties). Moreover, so that a molecule can be a mesogen, it should combine within its structure moieties able to confer order and fluidity to the overall material. When anisotropy in shape is caused by a significant difference between one molecular axis and the other two, mesogenic compounds form mesophases by varying the temperature and these mesogens are called thermotropic liquid crystals. However, this geometrical anisotropy can be reached from two different overall molecular shapes: rod-like (axial portion more extended than the radial one) and disc-like (radial portion more extended than the axial one), giving rise to calamitic and discotic mesogens, respectively (Fig. 1). Anisotropy can be reached when different parts of the....