Synthesis and Characterization of a Novel Light-Emitting Polymer Containing Highly Efficient Hole-Transporting Aromatic Diamine

Abstract

Recently, polymeric light-emitting materials have attracted much attention for their potential applications in large-area displays. 1 Following the discovery of poly-(p-phenylene vinylene) (PPV), 2 a wide range of polymers, for example, polythiophene, 3,4 poly(p-phenylene), 5 poly(vinylcarbazole), 6,7 and polyfluorene, 8 have been reported in the literature as potential candidates for polymeric light-emitting diodes (LEDs). One of the greatest advantages of these devices over molecular LEDs is the ease of processing of the polymers into high-quality thin films. In particular, organic-soluble conjugated polymers, such as poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV), have received considerable attention. 9-12 Although MEH-PPV can be fabricated into a LED possessing a high quantum efficiency (η ext ) 1%), there are several deficiencies, such as the polymer's synthesis. The polymer uses the 2-(2′-ethylhexyloxy)-5-methoxy-1,4-bis(chloromethyl)benzene monomer, which often results in a significant amount of insoluble polymer gel due to the cross-linking of the polymer chains. In addition, an inherent thermal property, its low glass-transition temperature (T g ) 65°C ), may prohibit the long-term use of the device. In this paper, we report the synthesis and characterization of a novel copolymer (TPD-MEH-PPV) consisting of tetraphenyldiaminobiphenyl (TPD) and MEH-PPV moieties. The design of the polymer was based on the motivation (1) to improve both the hole-transporting property and oxidative stability of the MEH-PPV by inserting into the polymer chain an easily p-dopable and electrochemically reversible TPD moiety; (2) to raise the T g of the MEH-PPV by inserting the TPD moiety which has more rigid biphenyl structure; and (3) to improve simultaneously both the synthetic procedure and the solublity of the polymer 13 by using the Wittig-Horner condensation reaction to avoid potential cross-linking problems and to enhance the solubility of the polymer in common organic solvents by attaching two n-butyl groups onto the benzene rings of the TPD.The method for preparing the copolymer is outlined in Scheme 1. Monomer 2 was synthesized from 4,4-dibromobiphenyl by a three-step reaction 14-16 (overall yield, 33%). Alkylation of 4-methoxyphenol, followed by chloromethylation of 3, gave 2-(2′-ethylhexyloxy)-5-methoxy-1,4-bis(chloromethyl)benzene 4 in 60% yield. The condensation reaction between 4 and triethyl phosphite afforded the diphosphonate 5 in quantitative yield. 17 The TPD-MEH-PPV was prepared by the Wittig-Horner condensation reaction 18 between the dialdehyde 2 and 2-(2′-ethylhexyloxy)-5-methoxy-1,4-xylene tetraethyl diphosphonate 5 in tetrahydrofuran (THF). 19 The chemical structure of the resulting polymer was confirmed by 1 H NMR and elemental analysis. It is worth pointing out that polymer gels were not formed during this polymerization process.TPD-MEH-PPV was a yellow powder which dissolved easily in common organic solvents such as chloroform, tetrahydrofuran, and xylene. The molecular weigh...

Probing interchain interactions in emissive blends of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] with polystyrene and poly(styrene-co-2-ethylhexyl acrylate) by fluorescence spectroscopy

“…[1][2][3][4] It is well known that the photoexcitation of the MEH-PPV chains in dilute solution creates a singlet intrachain 1001 Probing Interchain Interactions in Emissive Blends Vol. 17, No.…”

“…1 Within the class of conjugated polymers, one widely studied is poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) ( Figure 1a), which exhibits relatively high photoluminescence (PL) and electroluminescence efficiencies. [1][2][3][4] It is well known that the photoexcitation of the MEH-PPV chains in dilute solution creates a singlet intrachain exciton with emission at λ em = ca. 580 nm, 6,7 while in films the photoexcitation results in interchain excitons with redshift emissions λ em > 640 nm or higher.…”

“…580 nm, 6,7 while in films the photoexcitation results in interchain excitons with redshift emissions λ em > 640 nm or higher. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] MEH-PPV electroluminescence is in general observed at > 640 nm and is coincident with the photoluminescence spectrum. Because of the formation of the interchain species, the efficiencies of electroluminescence conjugated polymer devices are remarkably reduced.…”

“…Because of the formation of the interchain species, the efficiencies of electroluminescence conjugated polymer devices are remarkably reduced. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] To overcome the lack of efficiency, several experiments using MEH-PPV blended with other polymers have been reported. [17][18][19][20][21][22][23][24][25][26][27][28][29][30] Nevertheless, there are several other reasons to prepare blends with electroluminescent materials: to isolate the electroluminescent macromolecules for performing single molecule spectroscopy and understanding the electron energy transfer processes; 25,31,32 to change the emission colors; 23,28,33 by decreasing the aggregation with enhancement of the electron mobility; 34 to combining electron and hole transport properties; 35 to produce white lightemitting LEDs etc.…”

Synthesis and Photoluminescence of Novel Organo-Soluble Polyarylates Bearing (N-Carbazolyl)triphenylamine Moieties

“…[11][12][13][14] The feasibility of using spin-coating and ink-jet-printing processes for large-area luminescent devices and the possibility of various chemical modifications (to improve emission efficiencies and allow patterning) make polymeric materials containing triarylamine units very attractive. [15][16][17][18][19][20][21][22][23] To enhance the hole injection ability of polymeric emissive materials such as poly(1,4-phenylenevinylene)s (PPVs) and polyfluorenes (PFs), there have been several reports on PPV and PF derivatives involving hole-transporting units such as triphenylamine or carbazole groups in the emissive -conjugated core/main chains, [24][25][26][27][28][29] grafting them as side chains in a polymer, [30][31][32] or attaching them onto the polymer chain ends or outer surface of dendritic wedges. 33,34 Carbazole is another well-known hole-transporting and light emitting unit.…”