Chiral azobenzenes photochemical change

Azobenzene systems represent very attractive phototriggers in Ch LCs, thanks to their resistance to photofatigue, the simplicity of the molecules, and the ease of modification of their molecular structures. This chapter first describes helical twisting ability of chiral azobenzene compounds by focusing the structural effects on HTP as well as photochemical change in HTP, and then phototuning of helical structures of Ch LCs through the trans-cis photoisomerization of the chiral azobenzene compounds for applications to optical devices. 

This result indicates that the photochemical change in HTP is related not only to the perturbation effect of the trans-cis photoisomerization on the molecular orientation but also to photochemical change in the intermolecular interaction between the trans or the cis chiral azobenzene molecule and the host LC molecule. 

In binary Ch LC systems consisting of the chiral compound and the host nematic LC, the intermolecular interaction between the chiral azobenzene molecule and LC molecule contributes to HTP of the chiral azobenzene compounds. In this section, photoswitching behavior of some chiral azobenzene compounds (Fig. 10.7) is examined in terms of the influence of the chiral groups on the photochemical change in HTP. 

As mentioned in the preceding section, the anisotropy in the molecular shape influences significantly HTP of chiral compounds. Thus, to discuss the structural effect on the photochemical change in HTP, the molecular aspect ratio, LjD, was estimated with MOPAC/PM3 method. The molecular aspect ratios of the chiral azobenzene compounds in their isomeric states are given in Table 10.3. The data reveal that HTP of the chiral azobenzene compounds in both the isomeric states... 

The photochemical control of phase change of antiferroelectric LCs wi achieved either by using azobenzenes possessing chiral side chains as d< (A-6) in an antiferroelectric LC or by using azobenzene-containing chiral roelectric LCs (A-7, A-8) [63,64]. 

---