INDEX Azobenzene

Photorefractivity is a property exhibited by some materials in which the redistribution in space of photogenerated charges will induce a nonuniform electric space-charge field which can, in turn, affect the refractive index of the material. In a new material the active species is a highly efficient cyclopalladated molecule97,98 shown in Figure 5. The palladium-bonded azobenzene molecule is conformationally locked, and gratings derived from cis—trans isomerizations can be safely excluded. 

Liquid crystalline azobenzene polymers can undergo photoisomerization on irradiation at 360 or 450 nm on gold films. This permits changing the refractive index of the film depending upon the wavelength used136. A study has examined the photopolymerization of p-styrene sulphonate counteranions associated with monolayers of disulphide amphiphiles as self-assembled monolayers on gold137. 

The absorption coefficient k and the refractive index n (and their spectral variation) depend on the material and can be varied by introducing different substituents into the periphery of the azobenzene chromophore. 

Fig. 26 Maximum refractive index modulation (n max) and azobenzene-content normalized maximum refractive index modulation (n max/c) of different types of cyano-azobenzene-containing polymers...

Besides the absolute value of the refractive index modulation, the stability is an important parameter, particularly for the development of a storage material. In Fig. 27 the temporal variation of the diffraction efficiency of volume gratings is plotted for films of a statistical copolymer 6 (azobenzene content 29.5 wt%) and... 

It was demonstrated above that both the value and the stability of the refractive index modulation are far better for block copolymers than for statistical copolymers with comparable azobenzene content. But the influence of the morphology and the domain size on the refractive index modulation remains unclear. To investigate this we chose a block copolymer series with three different morphologies lamellar, cylindrical, and spherical. In Fig. 28 the growth and decay for different morphologies of the block copolymers 10 (Afn 45,900gmoP1 azobenzene content 31 wt%) and 11 (Mn 42,400gmon1 azobenzene content 25 wt%), which are based on a PMMA-PHEMA backbone, are shown [66, 113]. Both block copolymers exhibit a similar... 

The refractive index modulation of the block copolymer lb carrying cyano-azobenzene side-groups is more than twice that of the methoxy-substituted block copolymer 14. In addition, lb exhibits a much higher sensitivity than 14. This is just a consequence of (7), in which the sensitivity increases with the refractive index modulation. Furthermore, 14 exhibits a continuous decay after the writing laser is turned off, similar to that shown in Fig. 27. Block copolymer lb, in contrast, even shows an increase by 15% within a few hours, and the refractive index modulation is stable for years. In this case, the so-called post-development is not due to a different morphology as discussed above. Rather, the liquid crystalline character of the... 

A refractive index change on the order of 10-2 could be induced in the polymer on irradiation with blue light. This technology can also be utilized in the fabrication of multilayer or thick films. By alternately coating urethane-urea copolymer with azobenzene and nonphotosensitive polyvinylalcohol multilayer films could be obtained. In this case, the bitmaps are recorded as images on individual layers. 

As mentioned previously, photoisomerization of azobenzenes leads to refractive index or optical thickness changes that can be probed by means of evanescent wave optics. 

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