Photochromic photoinduced anisotropy

Besides azobenzene, spirobenzopyran, and salicylidene derivatives, other photochromic dyes have been used in LBK films. For example, anthocyanine dyes 5 (see Figure 6.5) have been used to obtain monolayers that change the area at constant pressure upon irradiation. Diphenyldiacetylerie chromophores 6 (Figure 6.5) have shown photoinduced anisotropy upon polar-... 

A PAP may be either amorphous or liquid crystalline and usually comprises suitably functionalized photochromic azobenzene and mesogenic moieties as side groups. The latter play an important cooperative or synergistic role with the photo-chromic group enhancing the overall birefringence (photoinduced anisotropy). 

Figure 20 Schematic representation of (a) the appearance of photoinduced anisotropy in films of azobenzene-containing polymers leading to orientation of photochromic and mesogenic groups (b) trans-cis-trans isomerization of azobenzene groups and (c) change in the direction of director n relative to the electric vector of the light wavelength, E, during photo-orientation.

Liquid-crystalline phases with two-dimensional order (Table 1) influence photochromic processes because their molecular orientation is reversibly altered by photochromic reactions owing to the marked structural changes of the photoactive molecules.3 Intensive interest has been focused on the photoinduced alteration of mesophases admixed with photochromic compounds, since photochromism can be transformed to the reversible modification of the optical anisotropy of liquid-crystalline states. This phenomenon is applicable to optical storage of information as nondestructive readout can be performed with light of wavelengths far from those causing the photochromic reactions. 

T. Fischer, L. Lasker, J. Stumpe, and S. G. Kostromin, Photoinduced optical anisotropy in films of photochromic liquid crystalline polymers, Photochem. Photobiol. A 80, 453 159... 

L. Lasker, T. Fischer, J. Stumpe, S. Kostromin, S. Ivanov, V. Shivaev, and R. Ruhmann, Photoinduced optical anisotropy in thin films of amorphous photochromic side chain polymers. Mol. Cryst. Liq. Cryst. 246, 347-350 (1994). 

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