ABSTRACT
Liquid crystals (LCs) are easily and strongly responsive to various external stimuli such as electric, magnetic, optical fields, and so on. Combining the unique electro-optical properties of LCs with the alignment feature with the help of rubbed polyimide films, electric response of LCs has been widely applied in production of commercially available flat-panel displays. Therefore, LCs have become a household name as a result of their widespread applications in television, smartphone, and computer displays. As shown in Fig. 3.1, a low applied voltage reorients the low-molecular-weight (LMW) LC molecules from the homogeneous state induced by the rubbed polyimide substrates to the vertical alignment state. By this way, a display element can be fabricated by changing the optical properties under an electric field, and bright and dark states are achieved accordingly. On removing the electric field, the LCs relaxes back to the substrate-induced ground state. The response can be as fast as microseconds and as strong enough to be applicable in a whole range of applications. Light is one of the most important clean energies, which can be regarded as limitless comparing with other energy forms. Furthermore, its easy control and processability enable it to deeply influence the development of LC science, especially when photochromic molecules are introduced into LC systems. During the past two decades, photochromic molecules are attracting the growing interest as guests in the field of materials science due to their ability
to change the properties of the host system, such as its electronic or ionic conductivity, fluorescence, magnetism, and shape, upon irradiation with light of the appropriate wavelength. Combining the photosensitivity of photochromic molecules and self-organization of LCs (Fig. 3.2), photoresponsive LCs show unique performance which will be introduced in this chapter. On the one hand, photoinert LCs can be controlled by the photochemical reactions of photosensitive materials. On the other hand, photoresponsive LCs can be directly orientated by light when photochromic groups act as mesogens.
