p-phenylenevinylene) Presenting Pendent
Pentaphenylene Dendron Groups for
Light-Emitting Diodes
YA-HSIEN TSENG, FANG-IY WU, PING-I SHIH, CHING-FONG SHU
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan
Received 6 May 2005; accepted 17 June 2005 DOI: 10.1002/pola.20992
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: We have synthesized, using the Gilch method, a novel poly(p-phenylenevi-nylene) derivative (PPV-PP) containing two pendent pentaphenylene dendritic wedges, and have characterized its structure and properties. The incorporated side chain pentaphenylene dendrons serve as solubilizing groups, prevent p-stacking interactions from occurring between the polymer main chains, and suppress the formation of excimers in the solid state. Photoluminescence studies indicate that effi-cient intramolecular energy transfer occurred from the photoexcited pentaphenylene groups to the poly(p-phenylenevinylene) backbone. The polymer film exhibits a maxi-mum emission at 510 nm and had a photoluminescence efficiency of 46%, which is similar to that measured in dilute solution. The photoluminescence spectra remained almost unchanged after thermal annealing at 1508C for 20 h, and displayed inhibited excimer formation. Polymer light-emitting diodes that we fabricated in the configura-tion ITO/PEDOT/PPV-PP/Mg:Ag/Ag exhibited a maximum emission peak at 513 nm, corresponding to the green region [x ¼ 0.30 and y ¼ 0.62 in the Commission Interna-tionale de L’Eclairage (CIE) chromaticity coordinates]. The maximum brightness and maximum luminance efficiency were 1562 cd/m2 and 1.93 cd/A, respectively. VVC 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5147–5155, 2005
Keywords: conjugated polymer; dendrimer; LED; photoluminescence; poly(p-phe-nylenevinylene)
INTRODUCTION
Ever since poly(phenylenevinylene) (PPV) was first employed in a polymer-based light-emitting diode (PLED) in 1990,1 organic light-emitting polymers have been subject to an intense amount of academic and industrial research because of their potential applications in flat-panel displays.2–5Organic luminescent polymers are attractive because of (a) the ability to fine-tune the luminescence properties of polymers by manipulating their chemical structure and (b)
the feasibility of using spin-coating and ink-jet printing processes for preparing large-area dis-play devices. PPV and its derivatives are among the leading candidates for light-emitting materi-als,6–10 because they offer several advantages associated with their good mechanical proper-ties, solution processability, thermal stability, and structural diversity; they have already been used in PLED applications. An ideal LED poly-mer, however, must possess a high photolumi-nescence (PL) quantum efficiency and the rela-tively low quantum efficiencies of PPVs in the solid state remains an issue that needs to be resolved.11 The major cause of the PPVs’ low PL quantum efficiencies is mutual cofacial stacking of their conjugated backbones through favorable Correspondence to: C.-F. Shu (E-mail: [email protected])
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 5147–5155 (2005)
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VC2005 Wiley Periodicals, Inc.
interchain p–p interactions, especially in the solid state, which leads to self-quenching arising from the formation of excimers.12,13 To overcome this problem, various bulky substituents, such as alkoxy, alkylsilyl, phenyl (aryl), and fluorenyl groups, have been attached to the PPV backbone to prevent its close packing and suppress the intermolecular inter-actions that lead to the formation of excimers.14–27
Dendronized polymers, which consist of a lin-ear, polymeric core appended with dendrons or dendrimers,28–30 are another class of macromole-cules that presently is receiving a considerable degree of attention.31–33 This dendronized poly-mer approach has been applied to polypoly-mers hav-ing conjugated backbones, such as poly(p-phenyl-ene),34,35 poly(p-phenylenevinylene),36–38 poly(p-phenyleneethynylene),39 poly(thiophene),40 and poly(fluorene).41–43 Recently, Mu¨llen et al. re-ported polyfluorenes appended with pentapheny-lene dendrons, anticipating that the incorpora-tion of the three-dimensional dendritic side chain to the light-emitting polymer would improve its luminescence activity.41 They demonstrated that the shielding effect provided by the dendritic side chains on the conjugated polyfluorene backbone prevents p-stacking and suppresses the forma-tion of aggregates/excimers. In this article, we report the synthesis of a new PPV derivative, PPV-PP, possessing pentaphenylene dendrons in-corporated as pendent units at the 2nd and 5th positions of the PPV backbone. The introduction of sterically hindered pentaphenylene groups in PPV not only enhances the thermal stability but also minimizes interchain interactions, and thus, high PL quantum yields are achieved. In addi-tion, the side substituents present two flexible 2-ethylhexyloxy chains that improve the solubility of the polymer. We prepared PPV-PP through the macromonomer route, using the Gilch method to polymerize dendronized 1,4-bis(bromomethyl)ben-zene, and also investigated its thermal, photo-physical, and electroluminescence (EL) properties.
EXPERIMENTAL
General Directions
1,2-Bis(4-hydroxyphenyl)-1,2-ethanedione (2)44 and 1,4-diethynyl-2,5-dimethylbenzene45were prepared according to reported procedures. Solvents were dried using standard procedures. All other re-agents were used as received from commercial sources, unless otherwise stated.1H and13C NMR spectra were recorded on either a Varian Unity
Yinova 500 MHz or a Bruker DRX 300 MHz spectrometer. Mass spectra were obtained on a JEOL JMS-SX/SX 110 mass spectrometer. IR spectra were obtained on a Nicolet 360 FT-IR spectrometer. Size exclusion chromatography (SEC) was performed using a Waters chromatog-raphy unit interfaced to a Waters 410 differential refractometer. Three 5-lm Waters styragel col-umns (300 7.8 mm2) connected in series in decreasing order of pore size (104, 103, and 102 A˚ ) were used in conjunction with THF as the elu-ent. Standard polystyrene samples were used for calibration. Differential scanning calorimetry (DSC) was performed on a SEIKO EXSTAR 6000DSC unit, at a heating rate of 10 8C min–1 and a cooling rate of 40 8C min–1. Samples were scanned from 30 to 300 8C, cooled to 0 8C, and scanned again from 30 to 300 8C. Thermogravi-metric analysis (TGA) was done on a DuPont TGA 2950 instrument. The thermal stability of each sample under a nitrogen atmosphere was determined by measuring its weight loss while heating at a rate of 208C min–1. UV–vis spectra were measured using an HP 8453 diode-array spectrophotometer. PL spectra were obtained on a Hitachi F-4500 luminescence spectrometer. 1,2-Bis[4-(2-ethylhexyloxy)phenyl]-1,
2-ethanedione (3)
A stirred mixture of 2 (6.50 g, 26.83 mmol), 2-ethylhexyl bromide (13.68 g, 69.77 mmol), K2CO3 (9.62 g, 69.76 mmol), and DMF (65 mL) was heated at 120 8C for 48 h under nitrogen. The resulting mixture was poured into water (250 mL) and extracted with EtOAc (3 100 mL). The combined extracts were dried using MgSO4, and the solvent was evaporated in vacuo. The crude product was purified by column chroma-tography using hexane to afford 3 (9.42 g, 75.3%) as a yellow oil. 1H NMR (CDCl3): d 7.90 (dt, J ¼ 9.5, 2.4 Hz, 4H), 6.93 (dt, J ¼ 9.5, 2.4 Hz, 4H), 3.89 (d, J ¼ 5.7 Hz, 4H), 1.76–1.67 (m, 2H), 1.50–1.23 (m, 16H), 0.92–0.85 (m, 12H).13C NMR (CDCl3): 193.6, 164.7, 132.3, 126.0, 114.7, 70.8, 39.2, 30.4, 29.0, 23.7, 23.0, 14.0, 11.0. HRFAB–MS (m/z): [M þ H]þcalcd for C30H43O4, 467.3161; found 467.3161. Anal. Calcd (%) for (C30H42O4): C, 77.21; H, 9.07. Found: C, 77.04; H, 9.11.
3,4-Bis[4-(2-ethylhexyloxy)phenyl]-2, 5-diphenylcyclopenta-2,4-dienone (4)
Tetrabutylammonium hydroxide (Bu4NOH, 35 mL) was added dropwise to a stirred solution of
3 (8.00 g, 17.14 mmol) and 1,3-diphenyl-2-propa-none (3.96 g, 18.85 mmol) in 1,4-dioxane (36 mL) at 80 8C. The mixture was heated at 80 8C for 1 h before being poured into water (300 mL) and extracted with EtOAc (3 100 mL). The com-bined extracts were dried using MgSO4, and the solvent was evaporated in vacuo. The crude product was purified by column chromatography (EtOAc/hexane, 1:100) to yield 4 (6.08 g, 55.3%) as a brown syrup.1H NMR (CDCl3): d 7.27–7.22 (m, 10H), 6.86 (dt, J ¼ 9.2, 2.3 Hz, 4H), 6.72 (dt, J ¼ 9.2, 2.3 Hz, 4H), 3.82 (d, J ¼ 5.8 Hz, 4H), 1.72–1.58 (m, 2H), 1.49–1.30 (m, 16H), 0.97–0.90 (m, 12H). 13C NMR (CDCl3): 200.4, 159.7, 154.2, 131.4, 131.2, 130.2, 128.1, 127.2, 125.0, 124.7, 114.0, 70.6, 39.4, 30.6, 29.2, 23.9, 23.1, 14.2, 11.2. HRFAB–MS (m/z): [M þ H]þ calcd for C45H53O3, 641.3995; found 641.3994. Anal. Calcd (%) for (C45H52O3): C, 84.33; H, 8.18. Found: C, 84.38; H, 8.42.
1,4-Dimethyl-2,5-bis{3,4-bis[4-(2-ethylhexyloxy)-phenyl]-2,5-diphenylphenyl}benzene (5)
A solution of 4 (10.30 g, 16.07 mmol) and 1,4-diethynyl-2,5-dimethyl-benzene (0.86 g, 5.6 mmol) in o-xylene (80 mL) was heated under reflux for 12 h. The reaction mixture was concentrated, and the residue was recrystallized from acetone to give 5 (6.20 g, 80.3%, mp 190–192 8C) as a white solid. 1H NMR (CDCl3): d 7.33 (s, 2H), 7.20–7.11 (m, 10H), 6.89–6.57 (m, 20H), 6.48– 6.38 (m, 8H), 3.66 (d, J ¼ 6.1 Hz, 4H), 3.63 (d, J ¼ 6.3 Hz, 4H), 1.90 (s, 6H), 1.63–1.55 (m, 4H), 1.45–1.26 (m, 32H), 0.86 (t, J ¼ 7.4 Hz, 12H), 0.85 (t, J ¼ 7.1 Hz, 12H). 13C NMR (CDCl3): 157.1, 156.8, 142.3, 142.2, 141.1, 140.4, 140.3, 140.2, 140.1, 139.9, 139.7, 138.7, 132.8, 132.6, 132.44, 132.37, 132.1, 131.9, 131.7, 131.4, 131.2, 130.8, 130.0, 127.5, 126.6, 125.9, 125.2, 125.1, 113.2, 113.0, 70.5, 39.3, 39.27, 30.5, 29.1, 23.8, 23.0, 19.8, 14.1, 11.1. HRFAB–MS (m/z): [M þ H]þ calcd for C100H115O4, 1379. 8796; found 1379.8793. Anal. Calcd (%) for (C100H114O4): C, 87.04; H, 8.33. Found: C, 87.13; H, 8.50.
1,4-Dibromomethyl-2,5-bis{3,4-bis[4-(2-ethylhexy-loxy)phenyl]-2,5-diphenylphenyl}benzene (6) N-Bromosuccinimide (NBS, 180 mg, 1.02 mmol) and azobis(isobutyronitrile) (AIBN, 10 mg) were added to a solution of 5 (700 mg, 507 lmol) in CCl4 (20 mL). The mixture was heated under
reflux for 8 h before being cooled and filtered. The filtrate was concentrated and purified by column chromatography (EtOAc/hexane, 1:200) to yield 6 (200 mg, 25.5%, mp 89–92 8C) as a white solid. 1H NMR (CDCl3): d 7.39 (s, 2H), 7.30 (s, 2H), 7.17–7.08 (m, 10H), 6.89–6.59 (m, 18H), 6.47 (d, J ¼ 8.4 Hz, 4H), 6.39 (d, J ¼ 8.5 Hz, 4H), 4.27 (d, J ¼ 10.1 Hz, 1H), 4.21 (d, J ¼ 10.3 Hz, 1H), 4.17 (d, J ¼ 10.2 Hz, 1H), 4.11 (d, J ¼ 10.4 Hz, 1H), 3.66 (d, J ¼ 6.0 Hz, 4H), 3.62 (d, J ¼ 5.7 Hz, 4H), 1.63–1.55 (m, 4H), 1.44–1.23 (m, 32H), 0.85 (t, J ¼ 7.6 Hz, 12H), 0.83 (t, J ¼ 7.6 Hz, 12H). 13C NMR (CDCl3): 157.9, 157.6, 142.7, 142.5, 142.3, 142.1, 141.14, 141.07, 140.93, 140.87, 140.5, 140.4, 140.1, 138.3, 138.1, 135.3, 134.3, 134.1, 133.2, 133.1, 133.05, 132.7, 132.3, 132.0, 131.4, 131.2, 130.7, 128.2, 127.6, 126.8, 126.3, 126.2, 114.0, 113.7, 71.2, 40.0, 39.96, 32.6, 32.5, 31.2, 31.1, 29.8, 24.5, 23.7, 14.8, 11.8. [M þ H]þ calcd for C100H113Br2O4, 1535.7005; found 1535.7008. Anal. Calcd (%) for (C100H112Br2O4): C, 78.19; H, 7.35. Found: C, 78.40; H, 7.51.
PPV-PP
A solution of t-BuOK (86 mg, 0.78 mmol) in anhydrous THF (1.6 mL) was added dropwise to a stirred solution of monomer 6 (100 mg, 65.1 lmol) in anhydrous THF (1.0 mL) at 55 8C. The mixture was then heated at 55 8C for 12 h. The end groups were capped by heating with ben-zyl bromide (11 mg, 60 lmol) at 55 8C for another 2 h. The reaction mixture was cooled to room tem-perature and precipitated into a mixture of MeOH and H2O (1:1 v/v). The crude polymer was collected and washed with excess MeOH. The resulting polymer was dissolved in chloroform and reprecipitated into MeOH. The precipitate was then washed for 48 h with acetone, using a Soxhlet apparatus before being dried under vac-uum to give PPV-PP (70 mg, 77%). 1H NMR (CDCl3): d 7.45–6.60 (m, 42H), 3.61 (br, 8H), 1.57 (br, 4H), 1.26 (br, 32H), 0.86 (br, 24H).
Light-Emitting Devices
Polymer LED devices were fabricated in the config-uration ITO/poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT) (35 nm)/light-emitting layer (160 nm)/Mg:Ag (100 nm)/Ag (100 nm). To improve hole injection and substrate smoothness, the PEDOT was spin-coated onto the ITO glass and dried at 80 8C for 12 h under
vac-uum. The light-emitting layer was spin-coated on top of the PEDOT layer, using chlorobenzene as the solvent and then dried for 3 h under vacuum at 60 8C. Before film casting, the polymer solution was filtered through a Teflon filter (0.45 lm). Sub-sequently, the cathodic Mg:Ag (10:1, 100 nm) alloy was deposited by coevaporation onto the emitting layer; an additional layer of Ag (100 nm) deposited onto top of the device to act as a protection layer. The current–voltage–luminance characteristics were measured under ambient conditions, using a Keithley 2400 source meter and a Newport 1835C optical meter equipped with an 818ST sili-con photodiode.
RESULTS AND DISCUSSION
Polymer Synthesis and Characterization
Scheme 1 presents the synthesis of the macro-monomer containing pentaphenylene pendent groups. The dendritic side chains used here are similar to those developed by Mu¨llen et al.41 4,40-Dimethoxybenzil1 was demethylated in HBr/AcOH and then treated with 2-ethylhexyl bromide in the presence of K2CO3 to yield the alkoxy derivative 3. Subsequent Knoevenagel condensation of the dialkoxybenzil 3 with 1,3-diphenyl-2-propanone in 1,4-dioxane, using Bu4NOH as base, afforded 3,4-bis[4-(2-ethylhex-
yloxy)phenyl]-2,5-diphenylcyclopenta-2,4-dien-one.4 The Diels-Alder reaction of 4 with 1,4-di-ethynyl-2,5-dimethyl-benzene in refluxing o-xy-lene gave the pentaphenyo-xy-lene dendrimer 5 pos-sessing a p-xylene core, which was then bromi-nated with NBS to furnish the corresponding 1,4-bis(bromomethyl)benzene 6.
As indicated in Scheme 2, we performed the homopolymerization of monomer 6 to the poly-(p-phenylenevinylene) bearing pentaphenylene dendron side chains in THF, according to the Gilch method, using excess potassium tert-but-oxide as both a condensing agent and a dehydro-brominating agent. During the polymerization, the viscosity of the reaction mixture increased without precipitation, and we observed an
in-Scheme 1. Synthesis of macromonomer containing polyphenylene dendrons.
Scheme 2. Synthesis of PPV with polyphenylene pendents.
tense fluorescence. In the 1H NMR spectrum of the PPV-PP obtained, the characteristic signal of the benzylic protons of monomer 6 (at4.20 ppm) disappeared entirely, which confirmed that plete polymerization had occurred. We also com-pared the IR spectrum of PPV-PP with that of compound 5. PPV-PP displayed an additional absorption at 970 cm–1, which we assign to the out-of-plane deformations of the trans vinylene moieties, but we observed no signal at873 cm–1 corresponding to any cis vinylene groups. These results indicate that the vinylene group formed under Gilch conditions existed predominantly in the trans configuration.
We determined the molecular weights of the PPV-PP by using gel permeation chromatography (GPC), with THF as the eluent, calibrated against polystyrene standards. GPC analysis indicated that the weight-average molecular weight (Mw) and polydispersity (Mw/Mn) of this polymer are 3.6 105 g/mol and 2.4, respec-tively; these values are merely indicative because the rigid structure of this rod-type poly(p-phenyl-enevinylene) may deviate strongly from that of the coil-like polystyrene standards. The GPC data do reveal, however, that the poly(p-phenyle-nevinylene) prepared using the Gilch method possesses a relatively high molecular weight. PPV-PP is readily soluble in common organic sol-vents, such as THF, toluene, chloroform, and chlorobenzene. We attribute the high solubility of this polymer to the presence of the highly branched dendrons, along with the two flexible 2-ethylhexyloxy chains attached to the side sub-stituents. We investigated the thermal properties of PPV-PP through TGA and DSC. As revealed by TGA, the polymer exhibited good thermal stability, with its 5% weight loss occurring at 327 8C. DSC was performed in the temperature range from 30 to 300 8C. We did not detect any possible phase transition signals for PPV-PP when we repeated the heating/cooling cycles dur-ing DSC. This finddur-ing occurs probably as a result of the rigidity of the dendronized polyphenyl sub-stituents and the stiffness of this polymer chain; we believe that practically no segmental motion of the polymer backbone is possible, even at an elevated temperature.
Photophysical Properties
Figure 1 displays the UV–vis absorption and PL spectra of a dilute PPV-PP solution; Table 1
Figure 1. (a) UV–vis absorption (solid line) and PL (solid and dotted lines: excited at 444 and 260 nm, respectively) spectra of PPV-PP in THF solution. (b) UV–vis absorption and PL (excited at 260 nm) spectra of compound 5 in THF.
Table 1. Optical Properties and Quantum Yields of PPV-PP, 5, and DO-PPV
Solutiona Filmb Abs (nm) PL (nm) Ffc Abs (nm) PL (nm) Ffd PPV-PP 256,444 503,532 (sh) 0.51 257,450 510,545 (sh) 0.46 5 260 366 DO-PPVe 445 507 0.32 435 546 0.10 aEvaluated in THF.
bPrepared by spin-coating from a CHCl
3solution. c
Relative to quinine sulfate in 0.1 N H2SO4(Ff¼ 0.53).
dRelative to 9,10-diphenylanthracene dispersed in poly(methyl methacrylate) (PMMA) film
(Ff¼ 0.83). eData from 19.
summarizes the spectral data. The absorption and PL spectra of compound 5, which serves as a model compound for studying the optical prop-erties of the dendritic pentaphenylene moieties, are also presented in Figure 1. In THF solution, PPV-PP exhibits two major absorptions: one below 350 nm and the other over a longer wave-length region with kmax at 444 nm. By compar-ing with the absorption spectrum of compound 5, the first absorption of PPV-PP (in the short-wavelength region) was ascribed to p-electron transitions that originate predominately from the pentaphenylene dendritic pendent units and the second was assigned to a p–p* transition derived from the conjugated poly(p-phenylenevi-nylene) (PPV) backbone. On excitation of the PPV main chain at 444 nm, the PL displays an emission band at 503 nm combined with a shoulder at 532 nm. When the PL of PPV-PP was compared with that of MEH-PPV, the PL spectrum of PPV-PP was relatively blue-shifted.27 This was attributed to the presence of bulky pendent groups, which may disrupt the coplanarity of the main chain and cause a par-tial destruction of the p-delocalization along the polymer backbone. We measured the PL quan-tum yield (Ff) of PPV-PP in THF to be 0.51, relative to a quinine sulfate standard (Ff ¼ 0.53 in 0.1 N H2SO4 at 365 nm excitation).46
Moreover, on irradiation at 260 nm—a wave-length we attribute to the absorptions of the pendent pentaphenylene units—the PL spec-trum remains almost identical to that obtained under excitation of the PPV backbone at 444 nm. We did not detect any luminescence arising from the dendritic polypenylene side chains at 366 nm. This result suggests that efficient Fo¨rster energy transfer occurred as a consequence of the moderate spectral overlap between the emission spectrum of the dendritic PP side chains and the absorption spectrum of the PPV polymer backbone (Fig. 1).47 Most of the excitons formed in the PP pendent groups on direct photoexcita-tion are more likely to migrate to the lower PPV backbone, from which the emission occurs. The fact that we observed complete quenching of the dendritic pentaphenylene emission even in a very dilute solution (10–6 M) indicates that effi-cient intramolecular energy transfer takes place in the PPV-PP polymer and may contribute sig-nificantly to the emission intensity of the PPV chain.
To investigate the effects of the dendritic side chains on the packing of the polymer chains, we
investigated the photophysical properties of PPV-PP in the solid state. Figure 2 displays the absorption and PL spectra of PPV-PP films spin-coated from CHCl3 solution onto quartz plates. In comparison with the PL spectrum of the corresponding dilute solutions, the signals of the polymer film are only slightly red-shifted (7 nm) and do not provide any signs of aggre-gate or excimer formation. The PL quantum effi-ciency of the film was 0.46; we determined this value by comparing it with the PL of 9,10-diphe-nylanthracene dispersed in poly(methyl metha-crylate) (PMMA) film (Ff ¼ 0.83).
48
We believe that both the PL spectrum and quantum effi-ciency in the film state are nearly identical to those obtained from dilute solution because of the presence of the bulky pentaphenylene den-drimer substituents. Generally, the lumines-cence efficiency of PPV in the solid state is sub-stantially lower than that measured for isolated molecules because of interchain interactions that produce lower-energy excited states that are not strongly radiatively coupled to the ground state.11–13 The large dendritic frame-works are more likely to encapsulate the conju-gated backbone and prevent p-stacking inter-actions between the polymer main chains, which has the effect of suppressing the formation of aggregates/excimers in the solid state.41–43 In contrast, the film of a PPV polymer [poly (2,5-dioctyloxy-1,4-phenylenevinylene) (DO-PPV)] that possesses two flexible alkoxy substituents per monomer unit, but does not present den-dritic side chains, exhibits a nearly 40-nm
red-Figure 2. UV–vis absorption and PL (excited at 450 nm) spectra of PPV-PP film before and after anneal-ing at 1508C for 20 h under a nitrogen atmosphere.
shift in its PL spectrum and a much lower quan-tum efficiency than it does in dilute solution.19
Conjugated polymers generally are rigid-chain molecules that possess relatively planar geometries and have a great tendency to pack cofacially in the solid state through molecular chain diffusion.12 The reduced emission inten-sity that occurs on prolonged heating could arise from enhanced interchromophoric interactions, which result in a long tail appearing in the emission spectrum and quenching through the formation of nonemissive excimers. We heated the polymer film on a hot plate at 150 8C under a nitrogen atmosphere for 20 h to examine the effect that incorporating the PP pendent groups has on the thermal stability of PPV-PP; we recorded the PL spectrum once the film had cooled to room temperature. After this thermal treatment, the PL spectra remained almost unchanged, as indicated in Figure 2, and we did not observe a long-wavelength emission band caused by excimer formation. Because of the presence of the bulky dendrimer pendent groups, the steric demand of which restricts the close packing of the polymer chains and reduces the probability of interchain interactions, the tendency for formation of aggregates and exci-mers, even on thermal treatment, is suppressed. Electroluminescence Properties of LED Device To evaluate the potential of using PPV-PP as an emissive material in polymer LED applications, we fabricated a double-layer LED having the configuration ITO/PEDOT/PPV-PP/Mg:Ag/Ag for
our preliminary investigations. We used PEDOT as a hole-injection layer and deposited a thick layer of Ag on the top of the device to act as a protection layer. As indicated in Figure 3, the EL spectrum exhibits a maximum at 513 nm with a shoulder at 549 nm, which corresponds to a green light having Commission Internatio-nale de L’Eclairage (CIE) color coordinates of (0.30, 0.62). In addition, the EL spectrum is quite similar to the PL spectrum. This result indicates that both the EL and PL originate from the same radiative decay process of the singlet exciton. Figure 4 presents plots of the luminance efficiency and brightness vs. current density for the nonoptimized device. The lumi-nance reached 1562 cd/m2 at a current density of 89.3 mA/cm2, corresponding to an efficiency of 1.75 cd/A. The maximum luminance efficiency was 1.93 cd/A at 824 cd/m2, with a current den-sity of 42.6 mA/cm2. Further improvements in the EL performance may be possible by optimiz-ing the device structure.
CONCLUSIONS
Using the macromonomer approach, we have synthesized a dendronized polymer, PPV-PP, which consists of a conjugated poly(p-phenylene-vinylene) backbone on which are appended pen-taphenylene dendritic wedges. The dendritic monomer 6 was prepared through Diels–Alder cycloaddition of 1,4-diethynyl-2,5-dimethyl-ben-zene with tetraphenylcyclopentadienone deriva-tive 4; it was polymerized using the Gilch
Figure 3. PL spectrum of PPV-PP film and EL spec-trum of ITO/PEDOT/PPV-PP/Mg:Ag/Ag device.
Figure 4. Plots of the luminance efficiency and brightness versus current density of the PPV-PP-based device.
method to afford PPV-PP. As a result of incorpo-rating the dendritic pentaphenylene pendent groups, which reduce interchain interactions and suppress the formation of excimers, this PPV-PP is soluble in common organic solvents and it exhibits a high PL efficiency in the solid state. Even after being annealed at 150 8C for 20 h, the PL spectra of the polymer film remained almost unchanged with no excimer being formed. Using this dendritic polymer, we fabricated a double-layer LED device having the configuration ITO/PEDOT/PPV-PP/Mg:Ag/ Ag. The EL of the device exhibited a green light having CIE color coordinates of (0.30, 0.62). The maximum luminance efficiency was 1.93 cd/A at 824 cd/m2, with a current density 42.6 mA/cm2.
We thank the National Science Council for financial support. Our special thanks to Professor C.-H. Cheng for his support during the preparation and character-ization of the light-emitting devices.
REFERENCES AND NOTES
1. Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539.
2. Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew Chem Int Ed 1998, 37, 402.
3. Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121.
4. Bernius, M. T.; Inbasekaran, M.; O’Brien, J.; Wu, W. Adv Mater 2000, 12, 1737.
5. Mitschke, U.; Ba¨urele, P. J Mater Chem 2000, 10, 1471.
6. Jin, S.-H.; Kim, M.-Y.; Kim, J. Y.; Lee, K.; Gal, Y.-S. J Am Chem Soc 2004, 126, 2474.
7. Yu, L.-S.; Chen, S.-A. Adv Mater 2004, 16, 744. 8. Liao, L.; Pang, Y.; Ding, L.; Karasz, F. E.; Smith,
P. R.; Meador, M. A. J Polym Sci Part A: Polym Chem 2004, 42, 5853.
9. Mikroyannidis, J. A.; Spiliopoulos, I. K.; Kasimis, T. S.; Kulkarni, A. P.; Jenekhe, S. A. J Polym Sci Part A: Polym Chem 2004, 42, 2112.
10. Mikroyannidis, J. A.; Spiliopoulos, I. K. J Polym Sci Part A: Polym Chem 2004, 42, 1768.
11. Kim, Y.; Zhu, Z.; Swager, T. M. J Am Chem Soc 2004, 126, 452.
12. Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765.
13. Cornil, J.; dos Santos, D. A.; Crispin, X.; Silbey, R.; Bredas, J. L. J Am Chem Soc 1998, 120, 1289.
14. Chuah, B. S.; Cacialli, F.; dos Santos, D. A.; Feeder, N.; Davies, J. E.; Moratti, S. C.; Holmes, A. B.; Friend, R. H.; Bre´das, J. L. Synth Met 1999, 102, 935.
15. Hwang, D.-H.; Kim, S. T.; Shim, H.-K.; Holmes, A. B.; Moratti, S. C.; Friend, R. H. Chem Com-mun 1996, 2241.
16. Ahn, T.; Song, S.-Y.; Shim, H.-K. Macromolecules 2000, 33, 6764.
17. Martin, R. E.; Geneste, F.; Riehn, R.; Chuah, B. S.; Cacialli, F.; Friend, R. H.; Holmes, A. B. Chem Commun 2000, 291.
18. Hsieh, B. R.; Yu, Y.; Forsythe, E. W.; Schaaf, G. M.; Feld, W. A. J Am Chem Soc 1998, 120, 231. 19. Peng, Z.; Zhang, J.; Xu, B. Macromolecules 1999,
32, 5162.
20. Jin, S.-H.; Jang, M.-S.; Suh, H.-S.; Cho, H.-N.; Lee, J.-H.; Gal, Y.-S. Chem Mater 2002, 14, 643. 21. Jin, S.-H.; Kang, S.-Y.; Yeom, I.-S.; Kim, J. Y.;
Park, S. H.; Lee, K.; Gal, Y.-S.; Cho, H.-N. Chem Mater 2002, 14, 5090.
22. Mikroyannidis, J. A. Macromolecules 2002, 35, 9289.
23. Mikroyannidis, J. A. Chem Mater 2003, 15, 1865. 24. Chen, Z.-H.; Lee, N. H. S.; Huang, W.; Xu, Y.-S.;
Cao, Y. Macromolecules 2003, 36, 1009.
25. Lee, S. H.; Jang, B.-B.; Tsutsui, T. Macromole-cules 2002, 35, 1356.
26. Sohn, B.-H.; Kim, K.; Choi, D. S.; Kim, Y. K.; Jeoung, S. C.; Jin, J.-I. Macromolecules 2002, 35, 2876.
27. Shin, D.-C.; Kim, Y.-H.; You, H.; Kwon, S.-K. Macromolecules 2003, 36, 3222.
28. Hecht, S. J Polym Sci Part A: Polym Chem 2003, 41, 1047.
29. Fre´chet, J. M. J. J Polym Sci Part A: Polym Chem 2003, 41, 3713.
30. Kim, C.; Kim, H.; Park, K. J Polym Sci Part A: Polym Chem 2004, 42, 2155.
31. Frey, H. Angew Chem Int Ed 1998, 37, 2193. 32. Schlu¨ter, A. D.; Rabe, J. P. Angew Chem Int Ed
2000, 39, 864.
33. Lee, C. C.; Grayson, S. M.; Fre´chet, J. M. J. J Polym Sci Part A: Polym Chem 2004, 42, 3563. 34. Stocker, W.; Karakaya, B.; Schu¨rmann, B. L.;
Rabe, J. P.; Schlu¨ter, A. D. J Am Chem Soc 1998, 120, 7691.
35. Bo, Z.; Zhang, C.; Severin, N.; Rabe, J. P.; Schlu¨ter A. D. Macromolecules 2000, 33, 2688. 36. Bao, Z.; Amundson, K. R.; Lovinger, A. J.
Macro-molecules 1998, 31, 8647.
37. Jakubiak, R.; Bao, Z.; Rothberg, L. Synth Met 2000, 114, 61.
38. Tang, R.; Chuai, Y.; Cheng, C.; Xi, F.; Zou, D. J Polym Sci Part A: Polym Chem 2005, 43, 3126. 39. Sato, T.; Jiang, D.-L.; Aida, T. J Am Chem Soc
1999, 121, 10658.
40. Malenfant, P. R. L.; Fre´chet, J. M. Macromole-cules 2000, 33, 3634.
41. Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Mu¨llen, K.; Meghdadi, F.; List, E. J. W.; Leising, G. J Am Chem Soc 2001, 123, 946.
42. Marsitzky, D.; Vestberg, R.; Blainey, P.; Tang, B. T.; Hawker, C. J.; Carter, K. R. J Am Chem Soc 2001, 123, 6965.
43. Chou, C.-H.; Shu, C.-F. Macromolecules 2002, 35, 9673.
44. Gilman, H.; Broadbent, H. S. J Am Chem Soc 1948, 70, 2619.
45. Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980, 8, 627.
46. Demas, J. N.; Grosby, G. A. J Phys Chem 1971, 75, 991.
47. Liu, D.; De Feyter, S.; Cotlet, M.; Stefan, A.; Wiesler, U.-M.; Herrmann, A.; Grebel-Koehler, D.; Qu, J.; Mu¨llen, K.; De Schryver, F. C. Macromole-cules 2003, 36, 5918.
48. Geng, Y.; Katsis, D.; Culligan, S. W.; Ou, J. J.; Chen, S. H.; Rothberg, L. J Chem Mater 2002, 14, 463.
