**5. Photorefractive effect in photorefractive FLC blends containing photoconductive chiral compounds**

### **5.1. Photoconductive chiral dopants**

FLCs are more crystalline than liquid, in comparison with nematic LCs; therefore several sophisticated techniques are required to prepare fine FLC films. Preparation of a uniformly aligned, defect-free SS-FLC using a single FLC compound is very difficult. In most cases, mixtures of LC compounds are typically used to obtain fine SS-FLC films. The composition of FLC mixture contains a base LC, which is a mixture of SmC phase forming LC compounds, and a chiral dopant. The chiral dopant introduces a helical structure into the LC phase. Utilization of an FLC as a photorefractive material requires the addition of photoconductive compounds to the FLC. However, the introduction of such non-LC compounds to the FLC often hinders the formation of a uniformly aligned SS-state. Thus, appropriate design of the photoconductive compounds is crucial. The photorefractive effect of FLC blends containing photoconductive chiral dopants has been investigated [17-19]. Terthiophene was selected as a photoconductive chromophore because it is a well-known semiconductor compound and has a rod-like structure (Figure 16), which increases the solubility into the rod-like structured LC material. The structures of the LC compounds, the electron acceptor TNF, and the photocon‐ ductive chiral compounds are shown in Figure 17. A ternary mixture of LC compounds was selected as a base LC. The mixing ratio of 8PP8, 8PP10, and 8PP6 was 1:1:2 since the 1:1:2 mixture exhibits the SmC phase over the widest temperature range. The textures of the FLC blends in 10 μm gap cells were observed using polarizing optical microscopy. The alignment of the FLC molecules is dominated not only by the properties of the FLCs but also by the affinity of FLC molecules with the alignment layer (polyimide). A homogeneous, anisotropic film can be obtained through interactions between LC molecules and the alignment layer. The FLC cell is fabricated by the precise assembly of indium tin oxide glasses coated with polyimide alignment layer into a cell of 10 μm gap determined by the diameter of the spacer bead. The appropriate preparation conditions for the fabrication of the LC cell differ from FLC to FLC. The thickness of the polyimide coating (Hitachi Chemicals LX-1400) was 20 to 30 nm and the surface of the polyimide was rubbed with a polyester velvet roll under specific conditions. Typical examples of textures observed in the 3T-2MB and 3T-2OC samples under a polarizing microscope are shown in Figures 18 and 19. On increasing the concentration of the photocon‐ ductive chiral dopant, defects appeared in the texture. The uniformly aligned state with few defects was obtained for samples with 3T-2MB concentrations lower than 8 wt.% (Figure 18). However, the 3T-2OC sample retained the uniformly aligned state with few defects for 3T-2OC concentrations less than 6 wt.% (Figure 19). The spontaneous polarization of the 3T-2MB samples was less than 1 nC/cm2 . On the other hand, the spontaneous polarization of the 3T-2OC samples was approximately 5 nC/cm2 . The smaller spontaneous polarization (and thus smaller intermolecular interactions) of the 3T-2MB sample may be advantageous for the formation of the uniformly aligned SS-state. A texture with a pattern of strips was observed in the 3T-2MB samples (Figure 18). It indicates that a complete SS-state was not formed in the 3T-2MB sample in the 10 μm gap cell and the helical structure existed. Zig-zag defect, which is the typical defect in the SS-state (book shelf structure), was also observed in the texture (Figure 18). The evidence shows that the FLC mixture exhibited the SS-state in the area close to the glass surface and formed a helical structure around the center of the thickness of the 10 μm gap cell.
