Chia-Hung Chou, So-Lin Hsu, Siao-Wei Yeh, Hsu-Shen Wang, and Kung-Hwa Wei*
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan 30049, R.O.C.
Received May 24, 2005; Revised Manuscript Received August 17, 2005
ABSTRACT: We have synthesized, using the Gilch polymerization method, a new series of high-brightness, soluble copolymers (POSS-PPV-co-MEHPPV) of poly(p-phenylenevinylene) (PPV) containing side-chain-tethered polyhedral oligomeric silsesquioxane (POSS) pendent units and poly(2-methoxy-5-[2-ethylhexyloxy]-1,4-phenylenevinylene) (MEHPPV). This particular molecular architecture of POSS-PPV-co-MEHPPV copolymers possesses not only a larger quantum yield (0.85 vs 0.19) but also higher degradation and glass transition temperatures relative to those of pure MEHPPV. The maximum brightness of a double-layered-configured light-emitting diode (ITO/PEDOT/emissive polymer/Ca/Al) incorporating a copolymer of MEHPPV and 10 mol % PPV-POSS was 5 times as large as that of a similar light-emitting diode incorporating pure MEHPPV (2196 vs 473 cd/m2).
Introduction
Emissive media based on electroluminescent (EL) polymers are currently under development for a number of display applications,1-4including large-area flat-panel displays that can be driven at low voltage. Polymer light-emitting diodes (PLEDs) are very promising can-didates for the development of low-cost, multicolored, large-area flat-panel displays because their molecular structures are readily modified and because they can be handled using a range of wet processing tech-niques.5,6 A number of issues remain to be resolved, however, before commercialization of these devices can occur. The formation of excimers resulting from the aggregation of their molecular chains in the solid state, and rather short operating lifetimes, owing to their low thermal stabilities. Several attempts have been made to reduce the formation of aggregates of polymer chains in the solid state. One such approach is the introduction of bulky organic units into the side chains of the polymer. This tactic has been used, for example, in the case of poly(p-phenylenevinylene) (PPV)-based alternat-ing copolymers containalternat-ing conjugated phenylenevi-nylene segments and nonconjugated spacers;7-9 these bulky side groups disrupt the packing of the polymer chains, which results in the formation of amorphous PPVs displaying reduced aggregation. Another approach to improving the efficiency of the devices is to blend hole-transporting and electron-hole-transporting materials to balance the injected charges.10-12aWhen such a device is operated for a long time, however, this method can
pendent groups as the side chains of the conjugated polymers.
The chemistry of polyhedral oligomeric silsesquioxane (POSS) covalently bonded to organic polymers has been developed recently. One set of members of the polyhe-dral oligomeric silsesquioxane family are octamers having the general formula (RSiO1.5)8; they consist of a rigid, cubic silica core having a nanopore diameter of ca. 0.3-0.4 nm.15The incorporation of (RSiO1.5)8 into some polymers leads to an enhancement of their ther-mal stability and mechanical properties.16-18
The first study in which POSS and conjugate poly-mers were combined involved tethering POSS to the chain ends of poly(2-methoxy-5-[2-ethylhexyloxy]-1,4-phenylenevinylene) (MEHPPV)19and the luminescent polymer poly(9,9′-dioctylfluorene) (PFO). The enhanced electroluminescence of these nanostructured polymers was attributed to POSS, imparting a reduction in the degree of either aggregation or excimer formation.13The approach of tethering POSS at the ends of a polymer’s chain, however, limits the number of POSS units that can be attached. In a previous study, we synthesized a series of side-chain-tethered POSS derivatives of poly-imide as a method of lowering its dielectric constant.20 More recently, side-chain-tethered PF-POSS copoly-mers have been synthesized; they display a stronger and purer blue electroluminescence.21a In this paper, we report a new series of asymmetric PPV derivatives presenting POSS units in their side chains. We synthe-9117 Macromolecules 2005, 38, 9117-9123
copolymers has not yet been described. We believe that developing POSS/poly(p-phenylenevinylene) copolymers having well-defined architectures will allow its lumi-nescence properties to be tailored more precisely through modifications of its molecular structure.
Experimental Section
Chlorobenzylcyclopentyl-POSS19awas synthesized according to literature procedures. THF was distilled under nitrogen from sodium benzophenone ketyl; other solvents were dried using standard procedures. All other reagents were used as received from commercial sources, unless otherwise stated.
Chlorobenzylcyclopentyl-POSS. 1H NMR (300 MHz, CDCl3): δ 7.64 (d, J ) 8.1 Hz, 2H), 7.37 (d, J ) 8.1 Hz, 2H), 4.57 (s, 2H), 2.26-1.21 (m, 56H), 1.16-0.81 ppm (m, 7H).29Si NMR (600 MHz, THF): -67.8, -68.2, -79.6 ppm.
Synthesis of POSS-CH3(1). 2,5-Dimethylphenol (238 mg, 1.95 mmol) was stirred with K2CO3(4.58 g, 33.18 mmol) and KI (1.57 g, 9.48 mmol) in DMF (30 mL) and THF (15 mL) at room temperature for 1 h. A small amount of Cl-POSS (2.0 g, 1.95 mmol) was added, and then the whole mixture was heated at 70 °C for 3 h. The reaction mixture was then slowly poured into water (300 mL) and extracted with chloroform (3× 50 mL). The combined extracts were dried (MgSO4), the solvents were evaporated, and the residue was purified by column chromatography (hexane/chloroform, 1:10) to afford 1 (1.86 g, 81%).1H NMR (300 MHz, CDCl3): δ 7.70 (d, J ) 8.1 Hz, 2H),
7.51-7.31 (b, 3H), 7.03 (d, J ) 6.9 Hz, 1H), 6.67 (s, 1H), 5.09 (s, 2H), 2.37 (s, 3H), 2.32 (s, 3H), 2.27-1.21 (m, 56H), 1.17-0.81 ppm (m, 7H).
Synthesis of POSS-CH2Br (2). A mixture of POSS-CH3
(1; 600 mg, 0.510 mmol), NBS (198.6 mg, 1.02 mmol), and AIBN (8.0 mg) was heated under reflux in carbon tetrachloride under nitrogen for 3 h. The reaction mixture was filtered to remove succinimide, the solvent was evaporated, and the residue was purified by column chromatography (hexane/
chloroform, 1:10).1H NMR (300 MHz, CDCl3) δ: 7.73 (d, J ) 8.1 Hz, 2H), 7.59 (d, J ) 6.9 Hz, 1H), 7.38 (d, J ) 8.1 Hz, 2H), 7.09 (d, J ) 6.9 Hz, 1H), 7.01 (s, 1H), 5.18 (s, 2H), 4.73 (s, 2H), 4.57 (s, 2H), 2.26-1.21 (m, 56H), 1.16-0.81 ppm (m, 7H).
Anal. Calcd for C50H76Br2O13Si8 (%): C, 47.30; H, 6.03.
Found: C, 47.18; H, 6.09.
General Procedure for the Synthesis of Copolymers POSS-PPV-co-MEHPPV. A solution of potassium tert-butoxide (1 M in THF) was added to a solution of the monomer in dry THF, and then the mixture was stirred for 4 h. End-group capping was performed by heating the solution under reflux for 1 h in the presence of tetrabutylbenzyl bromide.
Addition of the THF solution to methanol precipitated the polymer, which was collected, washed with methanol, and stirred in a mixture of methanol and water (1:1) for 1 h. The polymer was collected, washed with methanol, filtered, and dried at 50 °C for 24 h. After cooling, the polymer was recovered by precipitating it into a mixture of methanol and acetone (4:1). The crude polymer was collected, purified twice Scheme 1.a Synthesis of POSS-PPV-co-MEHPPV Copolymers
aReagents and conditions: i, trichloro[4-(chloromethyl)phenyl]silane, HNEt3Cl; ii, K2CO3, DMF/THF; iii, N-bromosuccinimide (NBS)/AIBN/CCl4; iv, tert-BuOK/THF.
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by reprecipitation from THF into methanol, and subsequently dried under vacuum at 50 °C for 24 h. The1H and13C NMR spectra of MEHPPV and PPV-POSS appear to be identical because of the low content of POSS in the latter polymer.
Characterization. 1H, 13C, and 29Si nuclear magnetic resonance (NMR) spectra of the compounds were obtained using a Bruker DRX 300 MHz spectrometer. Mass spectra of the samples were obtained on a JEOL JMS-SX 102A spec-trometer. Fourier transform infrared (FTIR) spectra of the synthesized materials were acquired using a Nicolet 360 FT-IR spectrometer. Gel permeation chromatographic analyses were performed on a Waters 410 differential refractometer and a Waters 600 controller (Waters styragel column). All GPC analyses of polymers in THF solutions were performed at a flow rate of 1 mL/min at 40 °C; the samples were calibrated using polystyrene standards. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measure-ments were performed under a nitrogen atmosphere at heating rates of 20 and 10 °C/min, respectively, using Du Pont TGA-2950 and TA-2000 instruments, respectively. UV-vis absorp-tion and photoluminescence (PL) spectra were recorded on a HP 8453 spectrophotometer and a Hitachi F-4500 lumines-cence spectrometer, respectively. Before investigating the thermal stability of the synthesized polymers, their polymer films were annealed in air at 200 °C for 2 h.
Device Fabrication and Testing. The electroluminescent (EL) devices were fabricated on an ITO-coated glass substrate that was precleaned and then treated with oxygen plasma before use. A layer of poly(ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS, Baytron P from Bayer Co.;
ca. 40 nm thick) was formed by spin-coating it from an aqueous solution (1.3 wt %). The EL layer was spin-coated, at 1500 rpm from the corresponding toluene solution (15 mg mL-1), on top of the vacuum-dried PEDOT:PSS layer. The nominal thickness of the EL layer was 65 nm. Using a base pressure below 1× 10-6Torr, a layer of Ca (35 nm) was vacuum-deposited as the cathode, and a thick layer of Al was deposited subsequently as the protecting layer. The current-voltage characteristics were measured using a Hewlett-Packard 4155B semiconductor parameter analyzer. The power of the EL emission was measured using a Newport 2835-C multifunction optical meter.
The brightness was calculated using the forward output power and the EL spectra of the devices; a Lambertian distribution of the EL emission was assumed.
Results and Discussion
Figure 1 displays the 1H NMR spectra of Cl-POSS,
POSS-CH3(1), and POSS-CH2Br (2). The CH2peak of the tethered bulky POSS enhanced the thermal stability
Table 1. Physical Properties of the POSS-PPV-co-MEHPPV Copolymers
Tg(°C) Tda(°C) Mn Mw PDI yield (%)
MEHPPV 71 370 61 000 111 000 1.82 78
POSS-PPV1-co-MEHPPV 90 381 55 000 107 000 1.95 66
POSS-PPV3-co-MEHPPV 96 395 52 000 98 000 1.88 64
POSS-PPV5-co-MEHPPV b 417 48 000 91 000 1.90 69
POSS-PPV10-co-MEHPPV b 453 39 000 73 000 1.87 60
aTemperature at which 5% weight loss occurred, based on the initial weight.bThe glass transition disappeared because of the steric hindrance imposed on the main molecular chains by the POSS units.
Figure 1. 1H NMR spectra of (a) Cl-POSS, (b) POSS-CH3, and (c) POSS-CH2Br.
Figure 2. FTIR spectra of (a) MEHPPV, (b) POSS-PPV1-co-MEHPPV, (c) POSS-PPV3-co-POSS-PPV1-co-MEHPPV, (d) POSS-PPV5-co-MEHPPV, and (e) POSS-PPV10-co-MEHPPV.
Macromolecules, Vol. 38, No. 22, 2005 Poly(phenylenevinylene) Copolymer 9119
peak maxima of MEHPPV occurred at 499 and 553 nm, respectively; these values are close to those reported in the literature.19 We observed no aggregation band in these spectra because THF is a good solvent for MEH-PPV. The absortion and emission peaks for MEHPPV and POSS-PPV-co-MEHPPV are almost identical. For
each polymer, the absorption peak maximum in solution (THF) is located between 499 and 494 nm, with a slight blue shift caused by the presence of POSS; the PL maxima occur at similar wavelengths for all of the polymers. Figure 3A presents the normalized absorption and PL emission spectra of the MEHPPV-POSS
co-Table 2. Optical Properties of the POSS-PPV-co-MEHPPV Nanocomposites
λmax(UV, nm) λmax(PL, nm)a QY
solutionb film solutionb film filmc
MEHPPV 499 517 553 (592) 591 (634) 0.19
POSS-PPV1-co-MEHPPV 499 512 552 (591) 588 (633) 0.43
POSS-PPV3-co-MEHPPV 498 512 552 (591) 586 (632) 0.62
POSS-PPV5-co-MEHPPV 497 511 552 (591) 585 (631) 0.84
POSS-PPV10-co-MEHPPV 494 505 551 (590) 584 (631) 0.87
aThe data in parentheses are the wavelengths of the shoulders and subpeaks.bThe absorption and emission were measured in THF.
cPL quantum yield estimated relative to a sample of Rhodamine 6G (Φr) 0.95).23-25
Figure 3. (A) UV-vis absorption and PL spectra of (a) MEHPPV, (b) POSS-PPV1-co-MEHPPV, (c) POSS-PPV3-co-MEHPPV, (d) POSS-PPV5-co-MEHPPV, and (e) POSS-PPV10-co-MEHPPV recorded in the solid state. (B) Normalized (relative to their maximum wavelengths) PL spectra of (a) MEHPPV and (b) POSS-PPV10-co-MEHPPV annealed at room temperature and 150 °C in the solid state. (C) X-ray diffraction spectra of (a) MEHPPV, (b) POSS-PPV1-co-MEHPPV, (c) POSS-PPV3-co-MEHPPV, (d) POSS-PPV5-co-MEHPPV, (e) POSS-PPV10-co-MEHPPV, and (f) Cl-POSS. (D) Deconvoluted X-ray diffraction spectra of (a) MEHPPV and (b) POSS-PPV10-co-MEHPPV.
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Macromolecules, Vol. 38, No. 22, 2005 Poly(phenylenevinylene) Copolymer 9121
polymer films. The quantum yields of the POSS-PPV-co-MEHPPV copolymers increased substantially as the amount of tethered POSS increased.30In particular, the quantum yield of MEHPPV containing 10% POSS was 4 times higher than that of pure MEHPPV (0.87 vs 0.19). We attribute this finding to the steric hindrance caused by the POSS units in preventing aggregation of the PPV main chains, which, in turn, reduces the degree of dimer formation after excitation. This improved quantum yield, which has not been reported in any previous studies of POSS/PPV copolymers, is a direct result of employing this particular side-chain-tethered POSS architecture.
For fresh MEH-PPV film, the yellowish peak at∼590 nm originates from single chain exciton emissions,26 whereas the reddish peak at 630 nm is related to emissions from interchain species, such as aggregates or excimers.26aThe difference in the PL spectra of the fresh MEH-PPV and POSSPPV-co-MEHPPV film is small because both polymer chains have not reached equilibrium states by their preparation process (i.e., spin-coating). By annealing the polymer films at higher temperatures as in the work by Schwartz27and Yang Yang28et al., one could see some more profound changes in their PL spectra. For instance, Figure 3B shows that after annealing MEHPPV film at 150 °C for 2 h under air its PL spectrum displays a red-shifted broad main peak at 630 nm, indicating formation of aggregates.
While the main emission peak for annealed POSS-PPV10-co-MEHPPV film remain near 590 nm, with a weak peak at 630 nm. Therefore, at the equilibrium states, there is a distinct difference in their PL spectra, due to the bulky and rigid POSS groups tethered to the side of MEHPPV chains. The same phenomena are also observed in EL spectra (Figure 5).
We have carried out X-ray diffraction studies for confirming that interchain distance of PPV was in-creased by side-chain-tethered POSS. Figure 3C dis-plays the X-ray diffraction curves of Cl-POSS, MEHP-PV, and PPV-POSS. There are three distinct diffraction peaks at 2θ ) 8.3°, 19.1°, and 26.1° for Cl-POSS (Figure 3f), which correspond to d-spacings of 10.5, 4.6, and 3.3 Å, respectively. The d-spacing of 10.5 Å reflects the size of the Cl-POSS molecules; the other two spacings are owing to the rhombohedral crystal structure of POSS molecules.29a For pure MEHPPV and PPV-POSS co-polymers, it is evident that they are amorphous and
have no side chain alignment. The nearly amorphous structure of pure MEHPPV (Figure 3C-a) originates from the two highly asymmetric substituents: the methoxy and 2-methoxy 5-(2′-ethoxyhexyloxy) for MEH-PPV.29bWhen POSS molecules were tethered to PPV, the average interchain distance increased appreciably from X-ray diffraction results. For instance, in the presence of 10% POSS (PPV-POSS-10%), the interchain distance increases to 4.6 Å (19.5°) from 4.0 Å (22.0°) for MEHPPV after deconvolution of the X-ray diffraction curve.
Figure 4A presents a transmission electron micros-copy (TEM) image of our POSS-PPV10-co-MEHPPV sample.31This image reveals that no large aggregates were formed, but small domains of POSS are present;
these POSS domains are dispersed evenly in the poly-mer matrixsa situation that we confirmed by analyzing this sample’s energy dispersive spectrum (EDS; Figure 4A-c). The topology of POSS-PPV-co-MEHPPV copoly-mers films were studied with atomic force microscopy (AFM). Figure 4B shows the height image and the phase image of MEH-PPV and POSS-PPV10-co-MEHPPV polymer films prepared with chlorobenzene as the solvent. The surface roughness of PPV-POSS film is larger than that of pure MEHPPV (0.496 nm vs 0.206 nm) due to the presence of POSS. The phase images also show that POSS groups are dispersed homogeneously in the whole system.
Electroluminescence Characteristics. Figure 5 displays the electroluminescence (EL) spectra of POSS-PPV-co-MEHPPV devices. The EL device prepared from MEHPPV emits a strong peak at 590 nm and a vibronic signal in the range 610-620 nm, which is due presum-ably to the aggregation of MEHPPV and its excimer formation, as discussed earlier. The introduction of bulky siloxane units into the PPV side chains presum-ably increases the interchain distance, thereby retarding interchain interactions and leading to a reduction in the degree of exciton migration to defect sites.22Figure 6 displays the variations in the current density and brightness of the EL devices. The turn-on voltage decreased to 2.5 V for PPV containing 10% POSS from 3.5 V for the pure-MEHPPV EL device. As indicated in Figure 6, a more than 4-fold increase in the maximum brightness of the POSS-PPV10-co-MEHPPV device oc-curs with respect to that of the pure-MEHPPV EL device (2196 vs 473 cd/m2) at a driving voltage of 9.5 V.
The efficiency of POSS-PPV10-co-MEHPPV is 1.83 cd/A at a current density of 1221 A/m2.
Figure 5. Electroluminescence spectra of the devices pre-pared from POSS-PPV-co-MEHPPV in the configuration ITO/
PEDOT/polymer/Ca/Al.
Figure 6. I-L-V curves of the devices prepared from POSS-PPV-co-MEHPPV and MEHPPV in the configuration ITO/
PEDOT/polymer/Ca/Al.
9122 Chou et al. Macromolecules, Vol. 38, No. 22, 2005
These improvements might be due to a decreased degree of aggregation upon the incorporation of the POSS units into the PPV chains.
Summary
We have synthesized a novel poly(p-phenylenevi-nylene) side-chain-tethered polyhedral silsesquioxane (POSS-PPV-co-MEHPPV) that possesses a well-defined architecture. This particular molecular architecture of PPV-POSS increases the quantum yield of MEHPPV significantly by reducing the degree of interchain ag-gregation; it also results in a much brighter red light from the EL device by decreasing the degree of aggrega-tion between the polymer chains.
Acknowledgment. We acknowledge Chunghwa Pic-ture Tubes, Ltd., and the National Science Council, Taiwan, for funding this work through Grant NSC 94-2120-M-009-001. Mr. Chen-Ping Chen and Mr. Yao-Te Chang are also acknowledged for experimental as-sistance and helpful discussions.
Supporting Information Available: Normalized UV-vis absorption and photoluminescence spectra and DSC and TGA traces of MEHPPV and PPV-POSS. This material is available free of charge via the Internet at http://pubs.acs.org.
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