Versatile Syntheses of Various Homo- and Copolymers of Poly(1,4-Arylene Vinylene)S
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Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA U.K. ** Cavendish Laboratory, Department of Physics, University of Cambridge, Madingley Road, Cambridge, CB3 0HE U.K. ABSTRACT The synthesis of poly(1,4-phenylene vinylene)s (PPVs) containing a 2,3-dialkoxy substitution pattern has been developed. Poly[2,3-bis(2-ethylhexyloxy)-1,4-phenylene vinylene)] (BEH-PPV, 4) was prepared and its optical properties compared with the recently discussed poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV, 1), which showed in contrast to classical 2,5-substituted PPV derivatives a notable blue-shift of the longest-wavelength absorption (λmax) and a considerable increase of the solid-state photoluminescence (PL) efficiency. The two statistical PPV copolymers 5 and 6 comprising 2,3-dibutoxy and dimethyloctylsilyl- or 2,5bis(dimethyloctylsilyl) side-chains, respectively, were synthesized via the Gilch dehydrohalogenation route. Double-layer light-emitting devices (LEDs) were demonstrated to combine high electroluminescence efficiencies with low turn-on voltages. INTRODUCTION Over the past ten years a variety of conjugated polymers have been studied as the active emissive layer in thin film polymer light-emitting devices (LEDs) [1-5]. Poly(1,4-arylene vinylene)s were soon identified as a particularly promising class of conjugated materials owing to their large synthetic flexibility, which allows fine-tuning of physical and optical properties [6]. For example using up to three different 1,4-bis(halomethyl) precursors, the Aventis-Covion-Philips group has developed statistical copolymeric poly(arylene vinylene)s that exhibit electroluminescence (EL) efficiencies in excess of 16 lm W-1 at realistic brightnesses (100 Cd m2) [7]. In addition, PPV polymers allow processing either through precursor polymers or directly in their fully conjugated form. H 9C 4O
OC 4H 9
Si (CH3 ) 2C 8H1 7
Si( CH3 )2 C8 H17
n
n
n C 8H 17 (CH 3) 2Si
DB-PPV, 1
DMOS-PPV, 2
BDMOS-PPV, 3
Recently, we have been interested in the investigation of PPVs with side-chains at the positions 2 and 3 of the phenylene ring, which led to the synthesis and characterization of poly(2,3-
dibutoxy-1,4-phenylene vinylene) (DB-PPV, 1) [8,9]. This polymer exhibited a considerable hypsochromic shift of the longest wavelength absorption (λmax) and a substantial increase of the solid-state photoluminescence (PL) efficiency in comparison with 2,5-substituted PPV derivatives. For instance, solid films of polymer 1 revealed a PL efficiency of 40%, whereas for the widely investigated poly[(2-methoxy-5-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) values between 15-20% have been reported [10,11]. More recently, a new class of PPV based materials containing silyl side-chains attracted much interest as they exhibit extremely high PL efficiencies of up 60% in the solid-state [12,13]. However, devices fabricated with typical layer thicknesses of ca. 100 nm of poly(2-dimethyloctylsilyl-1,4-phenylene vinylene) (DMOS-PP
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