interactions of the chiral molecule with its nematogenic neighbors, the anisotropy of the nematic host phase, and on the order parameter (Goossens, 1971). HTP is related to the helical pitch by the following equation (De Gennes and Prost, 1993): HTP 1 pc 10:2

where c is the concentration of the dopant. Thus, higher HTP values require less chiral dopant to yield the same l value. In addition, it is reported that HTP of the chiral dopants depends on their structures, and the chiral dopants with LC-like structure possess relatively higher HTP. Generally, anisotropic molecular shapes, such as rod shape, are required for liquid crystallinity and the appearance of LC phases (Demus, 1998; De Gennes and Prost, 1993). Therefore, if the shapes of molecules in LCs can be changed photochemically, LC properties including helical pitch length can be switched photochemically. Many studies have been done on photocontrolling of helical strcuture of Ch LCs through photoisomerization of various types of photochromic compounds, with/without chiral groups in molecules, such as azobenzenes (Bobrovsky and Shibaev, 2005; Kumaresan et al., 2005; Pieraccini et al., 2004; van Delden et al., 2004; Mallia and Tamaoki, 2003; Tamaoki et al., 2003; Bobrovsky and Shibaev, 2002; Ruslim and Ichimura, 2002; Moriyama and Tamaoki, 2001; Ruslim and Ichimura, 2001; Lee et al., 2000; Ruslim and Ichimura, 2000, Tamaoki et al., 2000;

Sackmann, 1971) , stilbenes (Haas et al., 1974), menthones (Van de Witte et al., 1999, 1998; Brehmer et al., 1998), diarylethenes (Yamaguchi et al., 2001; Denekamp and Feringa, 1998), overcrowded alkenes (Eelkema et al., 2006; van Delden et al., 2003; Feringa et al., 1995), and ugides (Sagisaka and Yokoyama, 2000). When the photochromic compounds with chiral groups are dissolved in a nematic LC, phototuning of the induced helical structure can be easily achieved by the use of suitable light, leading to a difference in HTP between photoisomers through photochemical change in their molecular shapes. Azobenzene molecules undergo reversible trans cis photoisomerization by irradiation of light with appropriate wavelength (Ross and Blank, 1971), and when a chiral azobenzene molecule is doped in Ch LCs, its HTP can be changed by reversible photoisomerization. Thus, according to Eq. 10.2, the helical pitch can be controlled through reversible photoisomerization. Azobenzene systems represent very attractive phototriggers in Ch LCs, thanks to their resistance to photofatigue, the simplicity of the molecules, and the ease of modi cation of their molecular structures. This chapter rst 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.

Figure 10.2 shows chiral and nonchiral azobenzene compounds. A Ch phase was induced by mixing each chiral azobenzene compound in a host nemaic LC. In such a binary system consisting of a nonchiral host nematic LC and a chiral compound, the reciprocal of the helical pitch (1/p) is known to increase linearly with the concentration (c) of the chiral compound at lower c, and HTP of the chiral compound can be de ned as the slope of the 1/p versus c (Eq. 10.2). Figure 10.3 shows changes in the 1/p of the induced Ch LCs as a function of c in the dark and under ultraviolet (UV) light (365 nm) to cause trans cis photoisomerization of the chiral azobenzene compounds. The helical pitch was determined by Cano s method (Bahr et al., 1991). The 1/p increased almost linearly with the increase of c. HTP of the chiral azobenzenes was on the order of azo-1Wazo-2Wazo-3: 23 102 for azo-1, 6.6 102 for azo-2, and 2.6 102 (mm 1 mol 1 g of E44) for azo-3, respectively (Kurihara et al., 2000). HTP was decreased by increasing the distance between azobenzene group and chiral center. The distance between the azo group and the chiral center is one of the factors in uencing the twisting ability. A decrease in the 1/p was brought about photochemically by UV irradiation. This result means that the trans to cis photoisomerization of these azobenzene compounds caused the decrease in the helical pitch length.