Synthesis, Photophysical Property, and Electroluminescent

Chapter 19

Downloaded by UNIV MASSACHUSETTS AMHERST on September 7, 2012 | http://pubs.acs.org Publication Date: August 17, 2004 | doi: 10.1021/bk-2005-0888.ch019

Synthesis, Photophysical Property, and Electroluminescent Applications of Silicon-Based Alternating Copolymers 1

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H. K. Kim , N . S. Baek , K. L. Paik , Y. Lee , and J. H. Lee 1

Center for Smart Light-Harvesting Materials and Department of Polymer Science and Engineering, Hannam University, Daejeon 306-791, Korea Dongbu Research Council, Deajeon 103-2, Korea SAIT, Daejeon 103-12, Korea 2

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Silicon-based alternating copolymers with a uniform πconjugated segment regulated by alkyl/aryl-substituted distyrylsilanes units were synthesized by the Heck coupling reaction to use as full color electroluminescent (EL) materials. The EL color of the resulting copolymers was tuned by controlling π-conjugated length as well as by introducing various aromatic units into the polymer backbone. Both single and multilayered light-emitting diodes were fabricated by vacuum deposition of the A1 or Ca onto a polymer film formed by spin-coating. Some of them exhibited a white EL color, due to the formation of a charge transfer complex. From photophysical and time-resolved transient decay studies, the formation of a charge separated complex was proposed. In this paper, the synthesis, characterization, photophysical properties and electroluminescence device (ELD) applications of siliconbased alternating copolymers are discussed.

© 2005 American Chemical Society In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Introduction Electroluminescent (EL) devices based on polymeric thin layers have attracted much attention because of their academic interest and wide variety of applications such as flat-panel displays, light-emitting diodes, and lasers (7-5). EL polymeric materials offer a number of advantages, such as low operating voltages, easy accessibility of three primary R/G/B colors with the control of ππ energy gap through the manipulation of the molecular structure, fast response time, high display quality, and ease of device processability compared to inorganic EL materials and organic dye molecules (6-9). Very recently, many efforts towards the main chain materials have been focused on developing blue and red light-emitting diodes capable of operating at ambient temperature, low voltages and easy processability. We also reported the development of a new type of processable silicon-based alternating copolymers having thiophene, carbazole, fluorene unit, etc in the polymer main chain by the well-known Pd-catalyzed Heck coupling reaction for blue and red light-emitting diodes (10,11). Instead of the Wittig method used in the early stage of this research (12,13), the Heck synthetic route to the preparation of silicon-based copolymers was used mainly for the following reasons. (1) The Heck route could overcome t he ρ roblem ο f low q uantum e fficiency d ue t ο the formation of t he triplet state arisingfromthe unreacted or remained aldehyde functional groups in the copolymers obtained from the Wittig reaction. (2) To obtain quantitatively fraw-double bond from cw-double bond, the Wittig reaction requires a further post-reaction of isomerization step, achieved by heating the crude polymers with a trace of iodine in toluene. However, the Heck reaction directly produces the desired polymers with trans configuration, which is important for the optimization of the luminescence efficiency and the emission wavelengths. Our results have shown that the introduction of organosilicon units with aromatic or flexible aliphatic group into π-conjugated systems could improve their processability and limit the π-conjugation length, resulting in blue light-emitting diodes (10-18). Surprisingly, the silicon-based copolymers with a relatively short π-conjugation length could be fabricated as blue light-emitting diodes operated at low voltages, due to the lowering of the LUMO level in luminescent polymers and therf-orbitalparticipation of silicon atoms. In this chapter, we describe the direct synthesis, photophysical and EL properties of the silicon-based copolymers by the Pd-catalyzed Heck reaction of the distyrylsilane monomer with various aromatic or heteroaromatic dibromides.

Synthesis of Silicon-based Alternating Copolymers The synthesis of monomers such as distyryl silane monomers, 3,6-dibromoN-(2-ethylhexyl)carbazole, 2,5-bis-(4-bromophenyl)-[ 1,3,4]-oxadiazole and 3,8-

In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

249 dibromo- 1,10-phenanthroline is described elsewhere (77-/5). Ru(II)-chelated complexes were prepared according to the method of Meyer et al. (19). The structural characteristics of the final monomers were provided by FT-IR, *H- & C-NMR, elecmental analysis, UV-Vis absorption and emission spectroscopies. The synthesis of silicon-based copolymers was carried out according to scheme 1 and 2 using the well-known Pd-catalyzed Heck coupling reaction as described previously (10,11,16-18). The polymerization results, thermal and photophysical properties of silicon-based copolymers are summarized in Table 1. All the copolymers have the glass transition temperature (Tg) in the range of 94 to 127 °C. They did not show any definite melting points, implying that the siliconbased copolymers are likely amorphous. All of the polymers showed good thermal stability up to 300 ~ 315 °C in a nitrogen atmosphere.


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Table 1. Summary of polymerization results, thermal and photophysical properties of silicon-based alternating copolymers. Polymers SiHMPPV SiPhPPV SiHMPVK SiPhPVK SiHMFPV SiPhFPV SiHMThV SiPhThV SiPhThThV SiHMOXD/Cz 10 SiHMOXD/Cz 91 SiHMOXD/Cz 55 SiHMOXD/Cz 19 Ru(II)-Chelated Polymer I Ru(II)-Chelated Polymer II

Yield (%) 52 48 51 43 70 77 50 55 50 52 48 57 65

Mx W 8.5 6.3 2.3 3.7 9.8 12.7 10.3 13.3 18.6 5.8 4.3 8.1 5.1

s CQ 94 127 106 102 96 109 94 112 116 105 115 116 108

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T

UV(A„J (nm) (nm) 356&365 470 35S&365 470 322A356 440 325&356 440 378 470 476 380 520 400 407 526 512 416 355 435 354 428 430 356 422&442 345 265,392 495,680 &460 &743 289,386 &465

(nm) 450 450 460 467&620 475 485

460

464&ö44

500, 678 &740

SOURCE: Reproduced from Refercence 1 la, 17 and 18.

In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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SiHMPPV

SiHMPVK

SiHMFPV

SiHMThV

Ru(I I)-Chela ted Polymer I (ligand : Ρ hen) Ru(II)-Chelated Polymer II (ligand : Bpy)

SiPhPPV

SiPhPVK

SiPhFPV

SiPhThV

SiPhThThV

Scheme 1. Synthesis of silicon-based copolymers withflexiblealiphatic and rigid aromatic group onto the organosilicon unit.

Pd(OAe),/TOP DMF/NBUj/100°C

OXD (x) : Cz (y) - 1 : 0 SIHMOXD/Cz 10 OXD (x) : Ca (y) » 0.9 : 0.1 SiHMOXD/Cz 91 OXD (x) : Cas

Synthesis, Photophysical Property, and Electroluminescent

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