Chromophores photorefractive

A composition comprising a Ceo fullerene-terminated poly(A-[(propylphenyl]-AA, A -triphenyl-(l,l - biphenyl)-4,4 -diamine)methacrylate has been prepared by living radical polymerization. When blended with a plasticizer and a nonlinear chromophore, photorefractive efficiencies were improved by up to 9% at abiased voltage of 60 V/pm. 

Recently photorefractivity in photoconductive polymers has been demonstrated (92—94). The second-order nonlinearity is obtained by poling the polymer doped with a nonlinear chromophore. Such a polymer may or may not be a good photoconductor. Usually sensitizers have to be added to enhance the charge-generation efficiency. The sensitizer function of fuUerene in a photorefractive polymer has been demonstrated (93). 

Chromophores with a rather high optical anisotropy are the merocyanines (77), especially in the cyanine limit with equal contributions of the apolar and zwitterionic resonance structures [319]. Thus, they also have been proposed as promising candidates for photorefractive systems based on molecular glasses. For 77, doped with a photosensitizer, a refractive index modulation of 0.01 at an electrical field of 22 V/pm was reported. 

The dominant methodology is that based on covalent bonding of the chromophore to the polymer, as this shows much improved long term stability over simple guest-host systems. Guest-host polymers using NLO guests have been extensively studied in the area of photorefractive polymers (see section 5.6.3). 

We have reported further improvements in the photorefractive sensitivity of liquid crystals by doping with chromophores that undergo efficient photoinduced intramolecular charge transfer [24], The magnitude of the observed photorefrac-... 

Furthermore, selected push-pull chromophores 84 were investigated by differential scanning calorimetry (DSC), revealing relatively low glass transition temperatures, Tg, a favorable property for composites in photorefractive materials [83],... 

In earlier investigations by the author [1,2] an additional nonlinear optical chromophore, (IV), and charge transport agent, (V), respectively, were prepared and used in photorefractive applications. 

S.R. Marder, B. Kippelen, A.K.Y. Jen, N. Peyghambarian, Design and Synthesis of Chromophores and Polymers for Electro-Optic and Photorefractive Applications , Nature, 388, 845 (1997)... 

Figure 10. Top liquid-phase absorbance spectrum of each component of a typical photorefractive polymer composite. Each component, A -vinylcarbazole (PVK), 2,4,7-trinitro-9-fluorenone (TNF), and typical chromophore (EHDNPB), is diluted in dichloromethane separately. (Absorption due to the solvent has been subtracted.) Bottom the absorbance of light in a solid sample due to the charge transfer complexation between PVK and TNF. The sample was prepared from a 9 1 ratio of PVK/ TNF. The extension of absorption to longer wavelengths is clear.

Figure 11. Possible relative positions of the energy levels in a typical guest-host photorefractive polymer composite containing PVK, TNF, and an azo-chromophore. A possible sequence of events leading to mobile charge generation is also included.

Figure 19. A typical HTOF signal from the photorefractive polymer composite PVK/0.1 wt.% TNF and 40 wt.% 4-(hexyloxy)nitrobenzene electro-optic chromophore, reproduced with permission from [42]. A clear peak is observed which represents the transit time of holes across half a grating period. Eventually a steady state is reached due to the spatial hole distribution being smeared out by dispersive transport and hole recombination.

Figure 20. A selection of chromophores used in guest-host organic photorefractive materials.

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