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Title: Photorefractivity in polymer-stabilized nematic liquid crystals

Abstract

Polymer-stabilized liquid crystals, consisting of low concentrations of a polymeric electron acceptor, are shown to exhibit significantly enhanced photorefractive properties. The charge generation and transport properties of these composite systems are strongly modified from nematic liquid crystals doped with electron donors and acceptors. The new composites are produced by polymerizing a small quantity of a 1,4:5,8-naphthalenediimide electron acceptor functionalized with an acrylate group in an aligned nematic liquid crystal. Photopolymerization creates an anisotropic gel-like medium in which the liquid crystal is free to reorient in the presence of a space charge field, while maintaining charge trapping sites in the polymerized regions of the material. The presence of these trapping sites results in the observation of longer lived, higher resolution holographic gratings in the polymer-stabilized liquid crystals than observed in nematic liquid crystals alone. These gratings display Bragg regime diffraction. Asymmetric beam coupling, photo-conductivity, and four-wave mixing experiments are performed to characterize the photophysics of these novel materials.

Authors:
 [1];  [2]
  1. Argonne National Lab., IL (United States). Chemistry Div.
  2. Argonne National Lab., IL (United States). Chemistry Div.|[Northwestern Univ., Evanston, IL (United States). Dept. of Chemistry
Publication Date:
Research Org.:
Argonne National Lab., IL (United States)
Sponsoring Org.:
USDOE Office of Energy Research, Washington, DC (United States)
OSTI Identifier:
656737
Report Number(s):
ANL/CHM/CP-96854; CONF-980731-
ON: DE98057787; TRN: AHC29817%%295
DOE Contract Number:
W-31109-ENG-38
Resource Type:
Technical Report
Resource Relation:
Conference: 43. international symposium on optical science, engineering, and instrumentation, San Diego, CA (United States), 19-24 Jul 1998; Other Information: PBD: [1998]
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; LIQUID CRYSTALS; POLYMERIZATION; REFRACTIVE INDEX; COMPOSITE MATERIALS; PHOTOCONDUCTIVITY; TRAPPING; NONLINEAR OPTICS; EXPERIMENTAL DATA; DIFFRACTION GRATINGS

Citation Formats

Wiederrecht, G.P., and Wasielewski, M.R.. Photorefractivity in polymer-stabilized nematic liquid crystals. United States: N. p., 1998. Web. doi:10.2172/656737.
Wiederrecht, G.P., & Wasielewski, M.R.. Photorefractivity in polymer-stabilized nematic liquid crystals. United States. doi:10.2172/656737.
Wiederrecht, G.P., and Wasielewski, M.R.. Wed . "Photorefractivity in polymer-stabilized nematic liquid crystals". United States. doi:10.2172/656737. https://www.osti.gov/servlets/purl/656737.
@article{osti_656737,
title = {Photorefractivity in polymer-stabilized nematic liquid crystals},
author = {Wiederrecht, G.P. and Wasielewski, M.R.},
abstractNote = {Polymer-stabilized liquid crystals, consisting of low concentrations of a polymeric electron acceptor, are shown to exhibit significantly enhanced photorefractive properties. The charge generation and transport properties of these composite systems are strongly modified from nematic liquid crystals doped with electron donors and acceptors. The new composites are produced by polymerizing a small quantity of a 1,4:5,8-naphthalenediimide electron acceptor functionalized with an acrylate group in an aligned nematic liquid crystal. Photopolymerization creates an anisotropic gel-like medium in which the liquid crystal is free to reorient in the presence of a space charge field, while maintaining charge trapping sites in the polymerized regions of the material. The presence of these trapping sites results in the observation of longer lived, higher resolution holographic gratings in the polymer-stabilized liquid crystals than observed in nematic liquid crystals alone. These gratings display Bragg regime diffraction. Asymmetric beam coupling, photo-conductivity, and four-wave mixing experiments are performed to characterize the photophysics of these novel materials.},
doi = {10.2172/656737},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Jul 01 00:00:00 EDT 1998},
month = {Wed Jul 01 00:00:00 EDT 1998}
}

Technical Report:

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  • The observation of photorefractive gratings in new crosslinked polymer-stabilized liquid crystals (PSLCs) is discussed and compared to previous PSLCs. The PSLCs easily incorporate reduced or oxidized molecules that are present in a nematic liquid crystal at a concentration of 2 mol%. The PSLCs that are crosslinked provide improved photo-refractive grating resolution, due to their improved functionality as an immobile electron trap. These materials are capable of functioning well into the Bragg diffraction regime. Photoconductivity experiments that support the photorefractive mechanism and a different charge transport mechanism than neat liquid crystals are also performed.
  • New organic materials that exhibit photorefractive effects are of wide interest for potential optical signal processing applications. The authors report on a photorefractive nematic liquid crystal composite containing the conjugated polymer poly(2,5-bis(2{prime}-ethylhexyloxy)-1,4-phenylenevinylene), BEH-PPV that exhibits a novel fringe spacing dependent inversion of the polarity of the space-charge field due to competition between the ionic diffusion and charge drift transport mechanisms. A eutectic mixture of 35% (wt %) 4{prime}-(n-octyloxy)-4-cyanobiphenyl, 8OCB, and 65% 4{prime}(n-pentyl)-4-cyanobiphenyl, 5CB, was doped with 10{sup {minus}5} M BEH-PPV (200 kD by GPC), as the electron donor. The molecular weight of the BEH-PPV polymer implies that 500 repeat unitsmore » of the monomer are present with an extended chain length of 0.35 {micro}m. N,N{prime}-Dioctyl-1,4:5,8-naphthalenediimide, NI, 8 {times} 10{sup {minus}3} M, was added as the electron acceptor. The free energy change for the photoinduced electron-transfer reaction, (BEH-PPV) + NI {yields} (BEH-PPV){sup +} + NI{sup {minus}}, is {minus}1.0 eV. Two other liquid-crystal composites were also studied as controls.« less
  • We have shown that PSLCs are capable of forming photorefractive gratings that operate in the thick grating regime. Polymer stabilization alters the charge transport and trapping characteristics of LCs, resulting in longer lived gratings, while maintaining the advantages of high orientational birefringence within LCs. Furthermore, very low applied electric fields (800 V/cm) and low optical intensities (100 mW/cm{sup 2}) are required to create large photorefractive effects in these materials. It is expected that optimization of the redox potentials of the chromophores within the PSLCs will continue to improve the performance of these materials.
  • We describe the detailed study of a polymer stabilized liquid crystal compound, which was created by using a reactive (monofunctional) azobenzene mesogenic guest and a nematic liquid crystal host. The resonant interaction of light with the azobenzene segment of the guest and the mesogenic nature of the latter enable the optical alignment of host molecules and the permanent fixing of that orientation by means of UV polymerization of the guest. We use dynamic spectral, polarimetric, and scattering techniques to study the orientational ordering and interaction of the guest-host system. We show that the uniform UV polymerization of this compound resultsmore » in a low scattering material system with dielectric and elastic properties that are relatively close to those of the host, while still providing the capacity for optical configuration of its morphology.« less
  • We report photorefractivity in nematic liquid crystals doped with electron donor-acceptor molecules that undergo intramolecular photoinduced charge separation. We show that subsequent intermolecular electron transfer from the intramolecular ion pairs to neutral donor-acceptor molecules is responsible for the charge migration over macroscopic distances that is required to produce photorefractivity. The results are compared to nematic liquid crystals doped with identical unlinked donors and acceptors that can achieve charge separation only through intermolecular electron transfer. We find that the liquid crystals doped with molecules that first undergo intramolecular charge separation exhibit superior photorefractivity when compared to the same liquid crystals dopedmore » with unlinked donors and acceptors. The differing mechanisms for charge generation and charge transport in these liquid crystal composites are analyzed. 43 refs., 7 figs., 2 tabs.« less