Side Chain Influence on Main Chain Orientation of PPV-Type Oligomers
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Side chain influence on main chain orientation of PPV-type oligomers
Ruth Müllner(a), Laurence Noirez(b) , Egbert Zojer(c) , Franz Stelzer(a) , Günther Leising(c) (a) Institute for Chemistry and Technology of Organic Materials, TU-Graz, Stremayrg. 16/I, A-8010 Graz (b) Laboratoire Léon Brillouin, Ce-Saclay, F-91191 Gif-sur-Yvette Cedex (c) Institute for Solid State Physics, TU-Graz, Petersgasse 12, A-8010 Graz
ABSTRACT Oligo (p-phenylene vinylene)s OPV were synthesized having cyanobiphenyloxy-groups bonded to the vinylene double bonds via alkylene spacers. A Pd(0)-reaction was used to synthesize the oligomers starting with the dibromo-substituted benzene and the alkenyloxy-biphenyl-nitrile. The orientation of the mesogens by a magnetic field and their influence on the orientation of the main chain was investigated by means of neutron diffraction ND. Data on the photophysics of the oligomers were gained using several spectroscopic methods such as UV/VIS, polarized optical microscopy POM, SEC, 1H-NMR, FT-IR.
INTRODUCTION Organic conjugated materials have been in the center of interest for applications in opto-electronic devices for more than 10 years by now [1, 2]. Their big advantages, low weight and tunability of their properties and thus the possibility to adjust to a specific problem or device, are what make them most interesting. Among the most investigated materials in this research field are poly (p-phenylene vinylene) PPV and its derivatives. A combination of a material of this kind with liquid crystals gives a compound that can be used for polarized light emitting devices [3]. The conjugated main chain can be oriented via the mesogenic units which themselves are macroscopically oriented by introducing a magnetic field. The conjugated backbone of a PPV chain provides the charge transport and the mesogens of a liquid crystal attached to the back bone via a spacer provide the orientation to produce polarized light.
EXPERIMENTAL DETAILS Using the Pd(0) catalyzed Heck reaction [4] we synthesized three different OPV with liquid crystalline side chains off the vinylene double bond. The materials were characterized by 1H-NMR, FT-IR, POM, DSC, SEC (calibrated with polystyrene standards) and photoluminescence / absorption spectra in solution (dichloromethane).
2
For the neutron diffraction experiments carried out in Saclay, we synthesized the analogue materials with a deuterated backbone. The measurements were performed using a two-dimensional detector PAXY with a distance of 1,25m and a wavelength λ = 4Å. The magnetic field of one Tesla flux density was applied to the sample in a heating cell.
DISCUSSION The general structure of the molecules synthesized is given in Fig. 1.
n (CH2) x O
C N
n = 3-5 x = 3 (Spacer A), 4 (Spacer B), 9 (Spacer C)
Fig. 1: OPV synthesized with three different liquid crystalline side chains By varying the spacer length we wanted to see how the orientation of the main chain is influenced relative to the mesogen. Concerning the synthesis of these materials, the amount of repetitive
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