Synthesis and Characterization of Poly( N -alkyl-2,7-carbazole)s: Blue Light-Emitting Materials
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Synthesis and Characterization of Poly(N-alkyl-2,7-carbazole)s: Blue Light-Emitting Materials J.-F. Morin*, M. Leclerc*, I. Lévesque† and M. D’Iorio† *Département de Chimie, Université Laval, Cité Universitaire, Québec, Canada, G1K 7P4 † National Research Council of Canada, Institute for Microstructural Sciences, Montreal Road, Ottawa, Ontario, Canada, K1A OR6 ABSTRACT The synthesis of a new class of π-conjugated based on N-alkyl-2,7-carbazole is reported. Two different synthetic pathways were used to obtain 2,7-functionalized carbazole units. The first pathway leads to N-alkyl-2,7-dichlorocarbazole which undergoes a Yamamoto coupling to obtain the homopolymer. The second synthetic method provides N-alkyl-2,7-diiodocarbazole. This monomer allows the utilization of more versatile synthetic tools such as Suzuki and Stille couplings to obtain highly regioregular copolymers. Poly(N-(2-ethylhexyl)-2,7-carbazole) (PEHC) prepared using Yamamoto coupling shows strong pure blue photoluminescence (437 nm) and electroluminescence (432 nm, 225 cd/m2) with no excimer formation. INTRODUCTION Since the first polymeric light-emitting diode (PLED) [1], many classes of electroluminescent π-conjugated polymers have been synthesized and investigated in optoelectronic devices. Up to now, poly(p-phenylenevinylene)s [2,3] and polyfluorenes [4,5] have shown very promising properties for the development of efficient organic light-emitting diodes (OLEDs). Despite good results obtained with PPV and polyfluorene derivatives, the poor stability of the polymers under bias voltage in ambient condition remains a problem. Research is ongoing in this field to obtain improved materials. In this perspective, we have developed a new class of π-conjugated polymer that have never been investigated: poly(N-alkyl-2,7-carbazole)s. Many studies were achieved on polymers containing a carbazole unit. Poly(vinylcarbazole) (PVK) was found to be a very good hole transporting materials in LED [6]. Poly(N-alkyl-3,6-carbazole) shows very interesting spectroelectrochemical properties that allow fabrication of electrochromic windows [7]. Also, many statistical copolymers based on N-alkyl-3,6-carbazole have been studied for their electroluminescent properties [8,9]. However, these polymers are poorly conjugated because the π-electron delocalization is limited on two monomeric units. Consequently, optical properties in the visible range are poor. On the opposite, poly(N-alkyl-2,7-carbazole)s display high photoluminescent and electroluminescent efficiencies in the visible range due to a more delocalized structure. Thus, it shows very good properties as an emissive layer in light-emitting diodes. We wish to report here the synthesis and the optical features of the strong blue fluorescent poly(N-(2-ethylhexyl)2,7-carbazole) (PEHC). Also, we present the general properties of the first PEHC-based light-emitting diode. EXPERIMENTAL SECTION As shown in scheme 1, PEHC have been easily prepared in 4 straightforward steps, starting from two commercially available aromatic units,
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