Chiral azobenzenes

Similar behavior has been observed in polyisocyanates, which have been shown to possess a helical structure. Unlike polypeptides, polyisocyanates have no stereocenter in their backbone they therefore form a racemic mixture of left-handed and right-handed helices. 1221 Incorporation of chiral azobenzene dyes into the side... 

Similar results were reported for the combination of chiral azobenzenes doped in nematic liquid crystals [59,60]. 

A large number of azobenzene-based amorphous and liquid crystalline polymers, particularly polyacrylates and polymethacrylates with chiral azobenzene pendants, have been prepared for the development of data storage and photonic devices [1-3,11-14]. For instance, the introduction of optically active mesogenic azobenzene residues into the side groups of the polymers produces chiral nematic and cholesteric phases, which are regulated by photoisomerization of the azobenzene units [10,14]. In most cases, however, the optical activity and chiroptical... 

Bioengineering has more recently been broadened by expanding the natural protein alphabet with artificial amino acids. This enables novel and nonnatural protein sequences to be created, while still exploiting the highly efficient natural synthesis machinery. Chiral azobenzene amino acids have been synthesized and incorporated into protein sequences (Wang and Schultz, 2004). The introduction of artificial photoactive residues opens the possibility of... 

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. 

Figure 10.3. Changes in helical pitch as a function of the chiral azobenzene compounds (azo-1, azo-2, and azo-3) in E44 before (o) and after (A) UV irradiation at 40°C.

To clarify the effect of the trans-cis photoisomerization of the chiral azobenzene compounds on the change in HTP, the change in the hehcal pitches of Ch LCs containing nonchiral azobenzene compound, p-azo-7-dl (Fig. 10.7), which was derived with racemic alcohol, was examined. Ch LCs were prepared by adding R811 and chiral or nonchiral azobenzene compounds with the same structure, p-azo-7-dl or p-azo-7, in E44. In the case of the (R811/p-azo-7/E44) sample, the helical pitch was increased by the trans-cis photoisomerization of... 

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. 

HTP of some chiral azobenzene compounds in E44 are given in Table 10.1. As described in the preceding section, HTPs of the chiral compounds increased as the distance between azobenzene group and chiral group became short. Therefore, the... 

Figure 10.7. Structures of chiral azobenzene compounds and a nonphotochromic chiral compound.

Figure 10.9 shows the snapshot after 100 ps of mixture of 5CB/p-azo-9 (98/ 2 mol%) obtained by MD simulation. It seems likely that ti—ti stacking interaction between 5CB molecules is enhanced through the intermolecular interaction between the chiral azobenzene molecule and 5CB molecule. As a result, the order parameter, S, was increased by mixing trans-p-azo-9 in 5CB. However, S of 5CB mixed with cis-p-a.zo-9 was almost constant. The molecular orientation of the LC molecules doped with the trans-azohenzene is stable relatively to that doped with... 

TABLE 10.1. HTPs [( x 108m mol g-E44), Experimental], the Length of the Long Axis, L, and the Diameter, D [(A), Measured from Molecular Orbital Calculation with PM3 Method], Data of Chiral Azobenzene Compounds... 

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. 

The photoisomerization ratios determined by comparing H NMR spectra of azobenzene compounds before and after UV irradiation are given in Table 10.3. The yields of cis form of all chiral azobenzene compounds at the photostationary state were 90% (Alam et al., 2007). Therefore, it is assumed that there is no effect of the yield of cis form at photostationary state on the change in HTP under UV light. 

The photochromic chiral azobenzene compounds, azo-1, azo-4, azo-5, azo-6, azo-7, and azo-10, only differ in their chiral substituents as can be seen in Fig. 10.7. Their HTPs were decreased upon UV irradiation because of the trans-cis photoisomerization of the chiral azobenzene molecules (Table 10.3). 

TABLE 10.3. Characterization of Chiral Azobenzene Compounds and Chiral... 

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... 

This section discusses the photochemical switching of selective reflection, transparency, helical sense, and lasing through the phototuning of helical structure of the Ch LCs by photoisomerization of chiral azobenzene compounds. 

Azobenzenes have simple chemical structures and exhibit simple photoisomerization between the trans- and cis form. However, they can really provide various photofunctions by their combination with sophisticated systems such as Ch LCs. The Ch LCs have a good potential for display applications, because they show interesting optical properties depending on the helical structures and the textures. Many types of optical materials can be created by the use of the photoisomerization of chiral azobenzenes in the Ch LCs. New ideas, including design of molecules and systems, may allow us for future materials not only in the field of optics but also in other fields. 

Alam MZ, Yoshioka T, Ogata T, Nonaka T, Kurihara S. 2007. The influence of molecular structure on helical twisting power of chiral azobenzene compounds. Liq Cryst 470 63 70. Bahr C, Escher C, Fliegner D, Heppke G, Molsen H. 1991. Behavior of helical pitch in cholesteric and chiral smectic C phases. Ber Bunsen Ges 95(10) 1233 1237. 

Kurihara S, Nomiyama S, Nonaka T. 2000. Photochemical switching between a compen sated nematic phase and a twisted nematic phase by photoisomerization of chiral azobenzene molecules. Chem Mater 12(1) 9 12. 

Sanpei H. 2005. Studies on design and synthesis of chiral azobenzene derivatives, Master Thesis. Kumamoto University, Kumamoto, Japan. 

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