Photoluminescent Polymer Films for Display Applications
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INTRODUCTION Despite their dominant position in flat panel display technology, the efficiency of liquidcrystal displays (LCDs) remains limited due to the use of absorbing polarizers and color filters. Recently, we have introduced photoluminescent (PL) polymer films into these LCDs to act as polarizer and as color filter 1' 2 which resulted in a substantial amelioration of the brightness of the displays. To overcome the drawback that these PL polarizers -in which the luminescent species are highly oriented- not only emit light anisotropically, but also absorb incident light in a highly polarized 3fashion, an improved concept based on a polarizing energy transfer effect has been
proposed. These PL polarizers are based on uniaxially oriented blends of a matrix polymer
(ultra-high molecular weight polyethylene, UHMW PE) which comprise a form-isotropic sensitizer and a rigid, conjugated, luminescent polymer. The unoriented sensitizer (i.e. the donor) absorbs light isotropically and subsequently transfers the energy to the oriented conjugated polymer (i.e. the acceptor) which subsequently emits highly polarized light. This concept proved to be extremely efficient when a coumarin sensitizer was used as the donor, and a poly(pphenylene ethynylene) (PPE) derivative as acceptor.3 As a result of the high efficiency of the polarizing energy transfer in this system, the performance of the new polarizers was improved dramatically. For a generalization of this approach, however, problems resulting from the required physical proximity of the donor and the acceptor and difficulties related to the delicate 4 phase behavior in multi-component systems have to be overcome. In order to circumvent the above discussed problems, we now propose to chemically attach the sensitizer moieties to the conjugated polymer backbone (Figure 1).5 In order to examine the broader applicability of our approach, different PPE polymers, ANT-OPPE and COU-OPPE, with anthracene and coumarin moieties covalently attached via a flexible spacer were prepared (Figure 1) and processed into oriented blend films with UHMW PE as the matrix material. 2 For comparison, model compounds 4-methyl-7-octyloxycoumarin (MOC) and propyl 9anthracenecarboxylate (PAC) (Figure 1) were also synthesized and processed together with unsubstituted EHO-OPPE and UHMW PE in a similar fashion.
293 Mat. Res. Soc. Symp. Proc. Vol. 558 ©2000 Materials Research Society
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Figure 1: Novel polymers ANT-OPPE and COU-OPPE and reference compounds EHO-OPPE, MOC and PAC.
RESULTS AND DISCUSSION The novel PPEs, ANT-OPPE and COU-OPPE, were synthesized by a cross-coupling polymerization under Heck conditions of a derivatized diiodoaryl monomer with a diethynyl monomer as discussed elsewhere. 5 Key intermediate was a derivatized diiodoaryl monomer which comprised a terminal hydroxy function and could easily be coupled to the sensitizer precurso
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