Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Polarized Light Emission from Uniaxially Oriented and PolymerStabilized AIE Luminogen Thin Films Jahyeon Koo,† Seok-In Lim,† Seung Hee Lee,† Jin Soo Kim,‡ Yeon-Tae Yu,‡ Cheul-Ro Lee,‡ Dae-Yoon Kim,*,§ and Kwang-Un Jeong*,†
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BK21 Plus Haptic Polymer Composite Research Team, Department of Polymer-Nano Science and Technology, Department of BIN Convergence Technology, and ‡Division of Advanced Materials Engineering, Chonbuk National University, Jeonju 54896, Republic of Korea § Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: Organic materials with linearly polarized luminescence (LPL) properties and good processability are critical to the development of advanced optical devices. To fabricate polymer-stabilized thin films with LPL properties, dicyanodistyrylbenzene-based reactive luminogens (abbreviated as DRL) containing both vinyl and thiol units were newly synthesized. On the basis of morphological observations combined with thermal and scattering analyses, it was found that DRL has a nematic liquid crystal mesophase. DRL molecules were uniaxially oriented by surface alignment layer and stabilized by thiol−ene click reaction. Uniaxially oriented and polymer-stabilized DRL thin films showed excellent chemical resistance as well as mechanical and thermal stability due to the chemically cross-linked polymer network. The DRL polymer film exhibited strong emission at 564 nm with remarkable LPL property. The polymer-stabilized films obtained from reactive luminogens with aggregation-induced enhanced emission (AIEE) properties have great potential for the application of advanced optical devices.
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INTRODUCTION Research fields of display industries have changed rapidly according to the requirements for highly efficient energy-saving devices. With this trend, many researchers in the display field have tried to develop highly efficient energy-saving devices in various ways. Among them, highly efficient light-emitting materials have naturally attracted great attention.1,2 However, it has reached the limit to increase the quantum efficiency of organic luminogens. In view of the structure of display devices, linearly polarized luminescence can be a meaningful solution for high light efficiency. It is well-known that liquid crystal displays (LCDs) must contain two crossed-polarizers due to their operating principles. Organic light-emitting diode (OLED) devices should also include an antireflective film consisting of a polarizer and a retarder to block the reflected light from the metal electrode.3,4 As a result, the serious loss of emitted light is caused by a polarizer, and the increase of power consumption is inevitable in both LCD and OLED. In this situation, there is a great need for research on organic materials with linearly polarized luminescence (LPL) property. The linearly polarized optical properties of organic materials are strongly related to the uniaxial orientation of anisotropic molecules. To achieve the linearly polarized light emission, the alignment of various types of materials including organometallic dyes and conjugated polymers can be realized in © XXXX American Chemical Society
different ways. Of the various materials, it is very attractive to use liquid crystal (LC) materials for the development of linearly polarized light-emitting films, since the LC materials exhibit both the translational motions and the molecular orientational orders.5,6 To reiterate, because of the translational motions and the molecular orientational orders of LC materials, LC molecules can be uniaxially aligned over macroscopic area by applying external stimuli such as surface anchoring force of alignment layer, shear stress, and electric and magnetic fields.7−9 To stabilize the oriented LC molecules, it is preferred to use a polymerizable LC monomer, the socalled reactive mesogen (RM). Because RM has both LC anisotropy and polymerizability, the oriented polymer film can be easily fabricated by polymerization of the uniaxially oriented RM.10,11 The chemical cross-linking of anisotropically oriented RM networks can significantly improve thermal, chemical, and mechanical stability. Because of these advantages, RM is often used for the fabrication of optical films.12 Recently, a few researchers have adopted the RM system to fabricate polymer films with LPL properties.13,14 Baliyan and co-workers polymerized the mixture of fluorescent RM and Received: November 26, 2018 Revised: January 27, 2019
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DOI: 10.1021/acs.macromol.8b02513 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
antiparallelly rubbed optical cells. Based on the thermal, microscopic, and scattering analyses, phase behaviors of DRL and its mixtures were first investigated. The polymerization conditions of uniaxially oriented thin film were optimized by taking into account thiol−ene click chemistry as well as phase behaviors of DRL and its mixtures.17,18 The molecular arrangement in the polymerized film was estimated by polarized optical microscopy (POM), field emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). Polarized emission property of the fabricated DRL polymer film was investigated by the fluorophotometer with a polarizer. Additionally, the robustness of polymer-stabilized DRL film was confirmed by thermal, mechanical, and chemical stability analyses.
nematic (N) diacrylate monomers which was uniaxially oriented in a LC medium. However, the low solubility limits the concentration of luminogens in the mixture to
