Light-controlled phase transition

No matter what kind of molecules is used, the purpose is to efficiently control the properties of CLCs by light. In the following sections, the different types of light-induced behavior in CLCs such as light-induced phase transition, handedness inversion, pitch length change, and the related reflection color control are introduced. 

This chapter highlights the photomodulation of CLCs, including light-induced phase transition related to cholesteric mesophase, helix inversion, reflection color control, and creation of mechanical motion in photoresponsive CLCs. The material systems introduced in this chapter are classified into two main groups based on chiral meso-gens and induced CLCs with several subsystems in each. Although the cholesteric phase was initially found with pure mesogenic compounds and the early quest for... 

Phospholipid molecules form bilayer films or membranes about 5 nm in thickness as illustrated in Fig. XV-10. Vesicles or liposomes are closed bilayer shells in the 100-1000-nm size range formed on sonication of bilayer forming amphiphiles. Vesicles find use as controlled release and delivery vehicles in cosmetic lotions, agrochemicals, and, potentially, drugs. The advances in cryoelec-tron microscopy (see Section VIII-2A) in recent years have aided their characterization [70-72]. Additional light and x-ray scattering measurements reveal bilayer thickness and phase transitions [70, 71]. Differential thermal analysis... 

Sensitized for blue-green or red light, photoconductive polyimides and liquid crystal mixtures of cyanobiphenyls and azoxybenzene have been used in spatial light modulators [255-261]. Modulation procedure was achieved by means of the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering and light scattering in polymer-dispersed liquid crystals. 

Irradiation at X>460 nm gave a fast cis-trans isomerization, with a concomitant increase in Tg. The sol-gel phase transition could be controlled in this way by light, while read-out of the states could be performed by measuring the modulation of the transmission or CD. For instance, the traras-azobenzene gelators displayed a CD effect, presumably due to the formation of helical aggregates, whereas the cis-isomers did not. 

Figure 11.21. Schematic iliustration of the use of azobenzene-containing side-chain iiquid crystalline polymers to record light-controlled dynamic grating on the basis of photochemical phase transition. Source Yamamoto et al., 2001. Reprinted with permission.

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