Absorption and Luminescence Properties of Sequentially Random- and Defined Copolymers Based on Poly(fluorene-benzothiadi
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Absorption and Luminescence Properties of Sequentially Random- and Defined Copolymers Based on Poly(fluorene-benzothiadiazole) Petra Herguth, Joo Hyun Kim, Xuezhong Jiang, Michelle S. Liu and Alex K-Y. Jen Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195-2120, U.S.A. ABSTRACT The emission wavelength in conjugated copolymers is determined by the various monomers used, their sequence in the polymer backbone, the effective conjugation length of the material, and intra- and intermolecular interactions of the different units. This paper will discuss the influence of conjugation length as well as the influence of intramolecular charge-transfer on the absorption and emission properties. Fluorene, benzothiadiazole and a third comonomer (fluorene, xylene or triphenylamine) were used as building blocks. The influence of monomer sequence is probed as well by comparing sequentially random copolymers to their sequentially defined ones with identical monomer ratios. Model oligomers were also made for comparison. INTRODUCTION Recently, fluorene-based copolymers have emerged as promising materials for polymer light-emitting diodes (PLEDs) [1]. Polyfluorene-based materials exhibit many desirable properties, such as high fluorescence quantum yields, good film forming properties and holetransport properties. Furthermore emission covering the whole visible spectrum can be achieved by incorporating low band-gap monomers into the polymer backbone [2]. Benzothiadiazole (BT) especially has been used extensively for this purpose. Its integration into the polymer main chain allows for color tuning to the green region of the visible spectrum. A BT ratio of as little as 10 % is sufficient to exhibit sole emission around 540 nm even so the polymer has its main absorption maximum at 377 nm [3]. The large stokes shift can be attributed to an efficient energy/charge transfer to the BT-containing segments of the polymer backbone. The incorporation of additional monomers into the polymer chain does not affect the luminescence wavelength [4]. In fact triscopolymers with fluorene, BT and a third comonomer show similar emission colors. However the addition of the third comonomer does affect the charge transporting properties and energy levels of the material [3]. Thus it is possible to alter the properties of the material in favor of better device performance without changing its emission properties. As of today the structure of the emissive segment has not been investigated. This paper will discuss the emission properties of three different systems based on fluorene, BT and a third comonomer such as fluorene (F), xylene (M) or triphenylamine (A). The systems were designed to incorporate 50 % of fluorene, 25 % of BT and 25 % of the third comonomer. Synthesis was conducted as to incorporate the third comonomer either in a sequentially random or defined fashion. Oligomer were also made for comparison purposes (see Figure 1).
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