Poly(Binaphthalenevinylene- alt -Phenylenevinylene) Derivatives: Novel Luminescent Polymers for Light-Emitting Devices
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Lixin Zheng, Xuezhong Jiang, Michelle S. Liu, and Alex K-Y. Jen* Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, [email protected]. Abstract Novel conjugated light-emitting polymers were synthesized via a Wittig-Horner condensation reaction between a binaphthyl dicarbaldehyde and a series of electron-rich or electron-deficient aryl diphosphonates. After comparing these materials with the model compound, 4c, it was revealed that the introduction of a twisted, non-coplanar binaphthyl structure provided an effective approach for tailoring the spectral characteristics of the polymers and improving their solubility and thermal stability (glass transition temperature ,T g, >200oC). Furthermore, the band structures of the polymers could be fine-tuned by changing the electronic properties of the bridging aromatic units. The electroluninescence (EL) and device characteristics were also reported in this paper. Introduction Polyphenylenevinylene (PPV) derivatives have received a considerable amount of attention due to their applications in EL devices.[1,2] One of the greatest advantages of these devices over molecular light-emitting diodes (LEDs) is the ease of processing of the polymers into high-quality thin films. Although organic-soluble PPV derivatives such as poly[2-(2’-ethylhexyloxy)-5-methoxy -1,4-phentlenevinylene], MEH-PPV, have provided devices with a rather simple fabrication process and high luminescent efficiency, [3-5] most challenging tasks still exist in improving thermal properties and optimizing photophysical behavior of PPVs to achieve high device performance. In this paper, our recent results on a series of poly(binaphthalenevinylene-alt-phenylenevinylene)s will be reported. The structural design of these polymers, i.e. the introduction of a binaphthyl unit into the main chain of the PPV derivatives, is based on the idea (1) of manipulating polymer's conjugation length (emission color) through the large dihedral angle in binaphthyl unit; (2) of increasing T g of polymers by the virtue of restricting rotation at the bond that joins the naphthalene rings; (3) of increasing the solubility and suppress interchain interactions by the twist structure of the binaphthyl unit. [6,7] Moreover, the synthesis of the polymers using a Wittig-Horner condensation reaction may avoid the potential crosslinking problem in the synthesis of organic-soluble PPVs. Synthesis and Characterization The routes for synthesis of the polymers(3a-c) and the model compounds (4a-c) are outlined in scheme 1. The binaphthyl dialdehyde 1 was made via the corresponding dibromide upon metallation with 4 equ. n-BuLi and subsequently quenched with N,Ndimethylformamide. Tetraethyl xylylenebisphosphonates (2a-c) were synthesized according to the literature procedure.[8] The polymers were synthesized via a Wittig-
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b: c:
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