Reactive Mesogenes: Synthesis and Application in Optoelectronic Devices
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Reactive Mesogenes: Synthesis and Application in Optoelectronic Devices Peter Strohriegl, Doris Hanft, Markus Jandke and Thomas Pfeuffer Bayreuth University, Macromolecular Chemistry I and Bayreuth Institute of Macromolecular Research (BIMF), D-95440 Bayreuth, GERMANY e-mail: [email protected]
ABSTRACT Reactive mesogenes posess polymerizable groups attached to a rigid, liquid crystalline core. From such molecules, densely crosslinked networks in which the liquid crystalline order is permanentely fixed can be formed by photopolymerization. Our major synthetic goal was the formation of glass forming reactive mesogenes. Such compounds do not crystallize upon cooling but vitrify and form supercooled LC-phases ('LCglasses'). They exhibit broad LC-phases and enable us to carry out photopolymerization in a broad range of temperatures. We have systematically investigated how the topology of the reactive mesogenes influences the stability of the resulting glasses. Comparing twin molecules with three- and four-armed stars we found that the supercooled LC-phase in the three-armed stars has a stability superior to both twin molecules and four-armed stars. In the three-armed star Triple-4 with a suitable substituent pattern the supercooled LC-phase is stable at room temperature for at least nine months. Doped with suitable chiral molecules the glass forming nematics form cholesteric phases which were used for cholesteric polymer networks and for polarization holograms with one s-and one p-polarized writing beam. Furthermore we have extended our synthetic efforts to reactive mesogenes with three or five conjugated fluorene units as LC-core. After orientation, the mesogenes were photocrosslinked and used as active layer in OLEDs that emit highly polarized blue light.
INTRODUCTION In the first part of this article photopolymerizable nematic materials for cholesteric polymer networks are described. In the last decade the use of polymer films with a stable cholesteric order has attracted a lot of interest in optical applications, e.g. color filters in projection systems [1], reflective polarizers [2] or color-flop pigments for cars [3]. Such films can be produced either by cooling vitrifying cholesteric materials below their glass transition temperature or by in-situ photopolymerization in the LC state. Glass-forming LC-materials can be divided into polymers [4] and low molar mass compounds [5,6]. The main advantage of low molecular weight compounds is the lower melt viscosity [7] and therefore the faster and more effective orientation. An alternative method to produce polymer films with a cholesteric order is the photopolymerization of LC monomers [8]. Hereby small molecules can be oriented fast and almost defect-free in their LC-phase. After crosslinking via photopolymerization a polymer network is achieved which is thermally stable up to its decomposition temperature. CC3.2.1
Our aim was to synthesize vitrifying materials with photopolymerizable groups in order to prevent crystallization and to allow photopolyme
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