Synthesis of Polyfluorene-Polytriarylamine Block Copolymer with Emitting Part at Junction Point for Light Emitting Applications

Mehdi Jahanfar, Kenta Suwa, Kousuke Tsuchiya, Kenji Ogino Ogino

Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, Tokyo, Japan.

DOI: 10.4236/ojopm.2013.32008                

Abstract

A block copolymer consisting of polyfluorene (PF) and polytriarylamine (PTAA) functionalized with green emitting phenoxazine moiety at the junction point of two blocks was designed and prepared for electroluminescent application. PF homopolymer was synthesized by Suzuki coupling polymerization, and was reacted with brominated phenoxazine. In the presence of the resulting PF functionalized with phenoxazine, C-N coupling polymerization of 4-(4’-bromophenyl)-4’’-butyldiphenylamine was carried out to afford a triblock copolymer, PTAA-phenoxazine-PF-phenoxazine-PTAA (PF-Ph-PTAA). Two types of random copolymers were also synthesized with fluorene and phenoxazine (PF2) by Suzuki coupling polymerization for comparison. All the polymers were soluble in common organic solvents and readily formed thin films by a solution processing. Prepared polymers exhibited similar UV absorption and PL emission in chloroform solutions. In a film state, the existence of phenoxazine unit drastically changed PL spectra. Although the content of phenoxazine unit in PF-Ph-PTAA was relatively high (13 mol%), it showed similar PL spectrum to that of PF2(phenoxazine content, 0.2 mol%) indicating that phenoxazine unit is isolated in single polymer chain nevertheless the high content. EL device based on PF-Ph-PTAA showed green-emission, suggesting that emission sites predominantly located in the vicinity of phenoxazine moiety because of its shallow HOMO level.

Keywords

Polyfluorene; Polytriarylamine; Block Copolymer; Junction Point; Polymer Light-Emitting Diode

1. Introduction

Polymer light-emitting diodes (PLEDs) have attracted much scientific and technological research interest since their first discovery in 1990 [1]. Utilization of electroluminescent polymeric materials shows several advantages over organic small molecules for use in PLEDs: better processability, and high flexibility [2]. Furthermore, inexpensive wet-processes such as spin-coating and ink-jet can be applied for the fabrication of PLED devices, which is essential in order to apply PLEDs to display and lighting technologies.

Unfortunately, it is generally recognized that polymeric devices show lower performance (efficiency, life time) compared with devices fabricated with a vacuum process based on low-molecular weight materials. Low efficiency in polymeric devices is partially due to difficulty in fabricating the devices with a layered-structure. To overcome the drawbacks, a breakthrough is necessary from the point of the molecular design and the control the morphology in the active layer. We have showed the advantage of block copolymers consisting of hole and electron transporting blocks as the host materials in phosphorescent devices [3-5]. Block copolymers were prepared via a nitroxide mediated living radical polymerization.

Block copolymers assemble into microor nano-phase separated structures with various domain shapes such as lamella, cylinder, or sphere. Exploiting nanostructures of block copolymers with appropriate designs can improve performance of applications due to allocation of functionality to each domain [6]. The other groups also reported several block copolymers for PLED applications [7,8].

More recently Tan et al. synthesized the different type of block copolymers for EL applications via the Suzuki coupling polymerization followed by the C-N coupling polymerization [9,10], which consisted of light emitting and electron transporting polyfluorene (PF) unit and hole transporting polytriarylamine (PTAA) unit. It was revealed that the introduction of PTAA increased emission efficiencies compared with PF homopolymer. This is due to the facile hole injection from the anode and/or the efficient electron block by PTAA moieties, which are located in the vicinity of the PEDOT/PSS coated on the anode through the hydrogen bonding of trioxyethylene group with PSS [10].

Here we proposed the novel methodology for the increase the efficiency in PLED. That is the novel molecular design of full functional polymers, which are block copolymers consisting of hole transporting unit and electron transporting unit with emitting moiety at the junction point. In the previous devices utilizing block copolymers as host polymers, emitting materials are low molecular weight phosphorescent dyes dispersed in host materials. In this case, emitting parts are randomly distributed in the active layer. Emission process is resulted from the recombination of holes and electrons, followed by the energy transfer from recombination centers to emitting moieties and/or the charge trap predominately occurred at the emitting sites. Therefore if the emitting parts are located in the vicinity of the interface between hole and electron transporting domains, more efficient energy transfer and carrier trap are anticipated. In order to attain the situation, a new molecular design is proposed. Target polymers are block copolymers consisting of hole transporting unit and electron transporting unit with emitting moiety at the junction point. If an ideal phase separation occurs, the emitting moiety exists at the interface between both domains.

In this study, the synthetic strategy we established for PF-b-PTAA (Suzuki coupling followed by C-N coupling polymerization) is modified to prepare a block copolymer consisting of PF and PTAA functionalized with green emitting phenoxazine moiety at the junction point of two blocks. Phenoxazine derivatives are known as a emitting dye [11,12], and have been utilized as the component of EL copolymers [13,14]. EL characteristics were preliminarily investigated for the comparison with random copolymer.

2. Experimental

2.1. Materials

Figure 1 illustrates the synthetic route of targeted polymers. n-Butylphenylphenoxazine (1) [14], 2,7-dibromo- 9-(4-methylphenyl)-9-(4-octylphenyl)-fluorene (4) [9], 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9- (4-methylphenyl)-9-(4-octylphenyl)-fluorene (5) [9], 4-(4’-bromophenyl)-4”-butyl-diphenylamine (6) [15] were synthesized according to reported procedures. All reagents and solvents were used without further purification unless stated otherwise. Tetrahydrofuran (THF) was distilled over sodium and benzophenone, and stored under nitrogen atmosphere. Toluene was distilled over calcium hydrine, and stored under nitrogen atmosphere. The other regents and solvents were obtained commercially

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, “Light-Emitting Diodes Based on Conjugated Polymers,” Nature, Vol. 347, No. 6293, 1990, pp. 539-541. doi:10.1038/347539a0
[2]	L. Akcelrud, “Electroluminescent Polymers,” Progress in Polymer Science, Vol. 18, No. 6, 2003, pp. 875-962. doi:10.1016/S0079-6700(02)00140-5
[3]	K. Tsuchiya, K. Sakaguchi, H. Kasuga, A. Kawakami, H. Taka, H. Kita and K. Ogino, “Synthesis of Charge Transporting Block Copolymers Containing 2,7-Dimethoxycarbazole Units for Light Emitting Device,” Polymer, Vol. 50, No. 3, 2010, pp. 616-622. doi:10.1016/j.polymer.2009.12.024
[4]	K. Tsuchiya, H. Kasuga, A. Kawakami, H. Taka, H. Kita and K. Ogino, “Synthesis of Bipolar Charge Transporting Block Copolymers and Characterization for Organic Light-Emitting Diode,” Journal of Polymer Science Part A: Polymer Chemistry, Vol. 48, No. 7, 2010, pp. 1461-1468. doi:10.1002/pola.23853
[5]	K. Tsuchiya, K. Sakaguchi, A. Kawakami, H. Taka, H. Kita, T. Shimomura and K. Ogino, “Charge Transporting Block Copolymer for Morphological Control in Light Emitting Device Based on Polymer Blends,” Synthetic Metals, Vol. 160, No. 15-16, 2010, pp. 1679-1682. doi:10.1016/j.synthmet.2010.05.040
[6]	J. K. Kim, S. Y. Yang, Y. Lee and Y. Kim, “Functional Nanomaterials Based on Block Copolymer Self-Assembly,” Progress in Polymer Science, Vol. 35, No. 11, 2010, pp. 1325-1349. doi:10.1016/j.progpolymsci.2010.06.002
[7]	L. Deng, P. T. Furuta, S. Garon, J. Li, D. Kavulak, M. E. Thompson and J. M. J. Fréchet, “Living Radical Polymerization of Bipolar Transport Materials for Highly Efficient Light Emitting Diodes,” Chemistry of Materials, Vol. 18, No. 2, 2006, pp. 386-395. doi:10.1021/cm051922+
[8]	B. Ma, B. J. Kim, L. Deng, D. A. Poulsen, M. E. Thompson and J. M. J. Fréchet, “Bipolar Copolymers as Host for Electroluminescent Devices: Effects of Molecular Structure on Film Morphology and Device Performance,” Macromolecules, Vol. 40, No. 23, 2007, pp. 8156-8161. doi:10.1021/ma0715526
[9]	Y. Tan, Z. Gu, K. Tsuchiya and K. Ogino, “Synthesis and Luminescent Properties of Block copolymers Based on Polyfluorene and Polytriphenylamine,” Polymer, Vol. 53, No. 7, 2012, pp. 1444-1452. doi:10.1016/j.polymer.2012.02.021
[10]	Y. Tan, K. Tsuchiya and K. Ogino, “Synthesis of Polyfluorene Block Copolymers and Effect of Side Chain Group on Electroluminescent Device Performance,” Chemistry Letters, Vol. 41, No. 3, 2012, pp. 257-259. doi:10.1246/cl.2012.257
[11]	A. Nowakowska-Oleksy, J. Sooducho and J. Cabaj, “Phenoxazine Based Units-Synthesis, Photophysics and Electrochemistry,” Journal of Fluorescence, Vol. 21, No. 1, 2011, pp. 169-178. doi:10.1007/s10895-010-0701-6
[12]	H. Tanaka, K. Shizu, H. Miyazaki and C. Adachi, “Efficient Green Thermally Activated Delayed Fluorescence (TADF) from a Phenoxazine-Triphenyltriazine (PXZ-TRZ) Derivative,” Chemical Communications, Vol. 48, No. 93, 2012, pp. 11392-11394. doi:10.1039/c2cc36237f
[13]	J.-H Park, N. S. Cho, Y. K. Jung, H.-J. Cho, H.-K. Shim, H. Kim and Y. S. Lee, “Polymeric Light Emitting Properties and Structural Relationships of Fluorene-Based Conjugated Copolymers Containing Various Hole Transporting Derivatives,” Organic Electronics, Vol. 8, No. 2-3, 2007, pp. 272-285. doi:10.1016/j.orgel.2006.08.002
[14]	Y. Pei, M. Otake, M. Vacha and H. Sato, “Synthesis and Characterization of a Novel Electroluminescent Polymer Based on Phenoxazine and Fluorene Derivatives,” React. Funct. Polym., Vol. 67, No. 11, 2007, pp. 1211-1217. doi:10.1016/j.reactfunctpolym.2007.07.011
[15]	K. Tsuchiya, T. Shimomura and K. Ogino, “Preparation of Diblock Copolymer Based on Poly(4-n-butyltriphenyl amine) via Palladium Coupling Polymerization,” Polymer, Vol. 50, No. 1, 2009, pp. 95-101. doi:10.1016/j.polymer.2008.10.057