Photorefractive polymer chromophore

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). 

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). 

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.

In the photorefractive polymer composites described above, the value of the glass transition temperature is of great importance because it needs to be in a range where the electro-active chromophores can be reoriented by the photorefractive space-charge charge field. Adjustment of Tg is done though the addition of a plasticizer. Examples of plasticizers are shown in Fig. 23. As for the polymer matrixes, plasticizers can be photoconducting or inert. ECZ (molecule (a) in Fig. 23) is widely used with PVK. Inert plasticizers such as BBP (molecule (c) in Fig. 23) have also been combined with PVK [86]. 

Very recently, the first metathesis reaction was utilized to synthesize a new type of photorefractive polymers, based on poly(1.6-heptadiyne) derivatives, that contain both a carbazole moiety as a hole transporter and NLO chromophores. attached to TT-conjugated backbones. Photorefractive polymers based on the previous works for the photoconductivity of poly(1.6-heptadiyne) derivatives containing a carbazole moiety and electrooptic activity of poly(1.6-heptadiyne) derivatives containing NLO chromophores were developed.Herein, all functional groups are covalently linked to the polymer backbone. 

Full-held, retrorehective holographic imaging through turbid media has been achieved using a photorefractive polymer composite as a coherence gate The photorefractive devices used, are based on PVK and TNFDM which is doped with the chromophore l-(2 -ethylhexyloxy)-2,5-dimethyl-4-(4 -nitrophenylazo)benzene. There is certain evidence that TNFDM interacts with chromophores by complexation. ... 

A recording technique of holograms and the non-destructive readout in a photorefractive polymer, utilizes two-photon absorption. The holograms are formed through the photorefractive effect. The technique uses the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into a PVK matrix. The holograms can be fully erased with a pulsed laser beam. However, they are insensitive to continuous wave laser beams with the same wavelength. 

The following seems to be a general rule The response times of photorefractive polymer composites are strongly dependent on both the glass transition temperature and the electro-optical chromophore. ... 

D. Wright, U. Gubler, Y. Roh, W. E. Moemer, M. He, and R. J. Twieg. High-performance photorefractive polymer composite with 2-dicyano-methylen-3-cyano-2,5-dihydrofuran chromophore. Appl. Phys. Lett., 79 (26) 4274 276, December 2001. 

E. Hendrickx, Y. D. Zhang, K. B. Ferrio, J. A. Herlocker, J. Anderson, N. R. Armstrong, E. A. Mash, A. P. Persoons, N. Peyghambarian, and B. Kip-pelen. Photoconductive properties of PVK-based photorefractive polymer composites doped with fluorinated styrene chromophores. J. Mater. Chem., 9(9) 2251-2258, September 1999. 

R. Bittner, C. Brauchle, and K. Meerholz. Influence of the glass-transition temperature and the chromophore content on the grating buildup dynamics of poly(A-vinylcarbazole)-based photorefractive polymers. App/. Opt., 37 (14) 2843-2851, May 1998. 

For several decades, the fields of photoconducting (75) and purely electrooptic polymers (74) have been very active but had almost no direct overlap. With the development of photorefractive polymers in the early nineties, the knowledge of these two research areas could be combined and has led to a rapid improvement of the performance of existing photorefractive polymers. The photorefractive polymer composite DMNPAA PVK ECZ TNF (DMNPAA 2,5 -dimethyl-4-(p-nitrophenyl-azo)anisole PVK poly(N-vinylcarbazole) ECZ N-ethylcarbazole TNF 2,4,7-trinitrofluorenone) we developed recently (P) has reached a level of performance that competes with that of the best inorganic photorefractive crystals (77,72). With the recent progress achieved in the development of new chromophores for electro-optic applications (75), the efficiency of these new materials is expected to be significantly further improved. 

Jung GB, Honda K, Mutai T, Matoba O, Ashihara S, Shimura T, et al. Structural design of nonlinear optical chromophores for high-performance photorefractive polymers. Jpn J Appl Phys Part 1 2003 42(5A) 2699-704. 

Hendrickx E, Zhang YD, Ferrio KB, Herlocker JA, Anderson J, Armstrong NR, et al. Photo-conductive properties of PVK-based photorefractive polymer composites doped with fluo-rinated styrene chromophores. J Mater Chem 1999 9(9) 2251-8. 

It was pointed out that the performance of photorefractive polymer composites is too large to be explained by the simple electrooptic photorefractive effect alone. A theoretical model was offered where both the birefringence and electrooptic coefficient are periodically modulated by the space-charge field due to the orientational mobility of the nonlinear chromophores at ambient temperatures. 

Marder et a/./ describe the synthesis of several new photorefractive polymers, which are composed of a new type of nonlinear optical chromophore attached to conjugated polymer, poly(p-phenylene-thiophene). Since the NLO chromophore is labile in many reaction conditions, the Stille coupling reaction was used to prepare these polymers. The resulting polymers exhibit high PR performances. An optical gain coefficient of 158 cm at a field of 50 V/ m and a diffraction efficiency of 68% at a field of 46 V/ m for polymer PI were obtained, which are among the best values for fully functionalized PR polymers to date. 

You et al. describes the synthesis and physical study of several new photorefractive polymers that consist of a nonlinear optical chromophore attached to conjugated poly(p-phenylene-thiophene)s backbones. A Stille coupling reaction was used to prepare these materials. 

Because of the versatility of the polyurethane system it is possible to introduce comonomers which can affect the physical properties of the derived polymers. For example, photo cross-linkable polyurethanes are formulated using 2,5-dimethoxy-2,4 -diisocyanato stilbene as a monomer (76). Comonomers, having an azoaromatic chromophore, are used in optical bleaching applications (77), or in the formation of photorefractive polymers (78). The latter random poljnners have second-order nonlinear optical (NLO) properties. Linear poljnners are also obtained from HDI/PTMG and diacetylenic diols. These polymers can be cross-linked through the acetylenic linkages producing a network polymer with properties similar to poly(diacetylenes) (79). 

A number of composite systems have been reported in which a polymer possessing one of the requisite functionalities, e.g., covalently attached NLO chromophores, is doped with the others, e.g., CG and CT dopants, or a photoconductive polymer, e.g., poly(N-vinylcarbazole), is doped with the sensitizer and NLO dopants (1, 4-7). The high dopant loading levels necessary (up to 50 wt%) result in severe limitations of doped systems, including diffusion, volatilization, and/or phase separation (crystallization) of the dopants. In addition, plasticizers and compatibilizers are often used to lower the glass transition temperature (Tg) of the polymer and increase solubility of the dopants in the host polymer, respectively. This, in turn, dilutes the effective concentration of CT, CG, and NLO moieties, diminishing the efficiency and sensitivity of the photorefractive polymer composite. 

J. Hwang, J. Sohn, S.Y. Park, Synthesis and structural effect of multifunctional photorefractive polymers containing monolithic chromophores. Macromolecules 36, 7970-7976 (2003)... 

Choi, C.S., Moon, I.K., and Kim, N. (2009) New photorefractive polymer composites doped with liquid nonlinear optical chromophores. Macromol. Res.,... 

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