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Origin of the methylene bonds in poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene] prepared according to Gilch's method: novel applications

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Origin of the methylene bonds in

poly[2-methoxy-5-(2



-ethyl-hexyloxy)-1,4-phenylenevinylene] prepared according to

Gilch’s method: novel applications

King-Fu Lin,

Yang-Liang Fan and Hsuan-Liang Chow

Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China

Abstract: Impurities containing methylene bridges between 2-((2-ethylhexyl)oxy)-5-methoxy-benzene molecules are inevitably formed during the synthesis of 1,4-bis(chloromethyl)-2-((2-ethylhexyl)oxy)-5-methoxy-benzene, the monomer used in the preparation of poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV), but they can be removed by double recrystallization of the monomer prior to polymerization. When impurities containing methylene bridges participate in a Gilch polymerization, the methylene bonds formed in the main chains are prone to break at 200◦C, that is, at least 150◦C below the major degradation temperature of defect-free MEH-PPV. Interestingly, the thermal treatment used to break the methylene bonds present reduces the chain aggregation of MEH-PPV during film formation and induces its blends with

poly(2,3-diphenyl-5-octyl-p-phenylene-vinylene) (DPO-PPV) to form a morphology similar to that of block copolymers. Both significantly

enhance the luminescence properties.  2006 Society of Chemical Industry

Keywords: conjugated polymers; luminescence; poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV); light-emitting diodes (LED)

INTRODUCTION

With the merits of low turn-on voltage, good solubility and high electroluminescent (EL) efficiency, poly[2-methoxy-5-(2 -ethyl-hexyloxy)-1,4-phenyleneviny-lene] (MEH-PPV) has attracted much attention for its potential applications in polymer light-emitting diodes (PLEDs).1 – 7 The preparation of MEH-PPV

is generally carried out through dehydrohalogena-tion of 1,4-bis(halo-methyl)-2-((2 -ethyl-hexyl)oxy)-5-methoxy-benzene) monomer, a method that was originally proposed by Gilch and Wheelwright.8Many researchers have attempted to improve its EL perfor-mance by thermal annealing above its glass transition temperature in order to reduce the imperfections of the film.9,10

Recently, PPV derivatives prepared via Gilch’s route were reported to have some structural defects, such as tolane-bisbenzyl (TBB) moieties,11non-eliminated

groups,12 and aldehyde.12 The presence of TBB in

MEH-PPV was shown to be detrimental to the lifetime of the prepared LEDs.13,14 Lately, we discovered the presence of methylene defects (or bonds) in MEH-PPV chains – defects that are prone to break at 200◦C, at least 150◦C lower than the major degradation temperature of defect-free MEH-PPV. This bond has been mistakenly assigned to the cis defect in our previous studies because of the similarity between

their1H-NMR spectra.15Through thermal annealing of MEH-PPV in order to break the methylene bonds at 200◦C, the extent of chain aggregation was significantly reduced, so that the maximum quantum yield of photoluminescence (PL) was increased to six times that of untreated MEH-PPV.16

Moreover, as MEH-PPV containing methylene bonds was blended with

poly(2,3-diphenyl-5-octyl-p-phenylenevinylene) (DPO-PPV) and then

submit-ted to a thermal treatment at 200◦C for 2 h, the broken MEH-PPV chain segments were able to chemically bond to the DPO-PPV chains by trans-vinylization.17,18 This chemical bonding turned the

immiscible blend of polymers into a morphology simi-lar to that of block copolymers. MEH-PPV has a solu-bility parameter δ= 20.38 (J cm−3)1/2, whereas DPO-PPV has δ= 22.93 (J cm−3)1/2, estimated by the group contribution method. These two polymers are basi-cally immiscible and toluene (δ= 18.21 (J cm−3)1/2) is

a better solvent for MEH-PPV than for DPO-PPV. As toluene was used as a co-solvent for MEH-PPV/DPO-PPV polyblend to spin-cast a film, the DPO-MEH-PPV/DPO-PPV phase formed a domain in the upper layer, which dis-persed into the more soluble MEH-PPV-rich phase in the wetting layer. Because both the lowest unoccu-pied molecular orbital (LUMO) and highest occuunoccu-pied molecular orbital (HOMO) levels of MEH-PPV in

Correspondence to: King-Fu Lin, Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China E-mail: kflin@ccms.ntu.edu.tw

Contract/grant sponsor: National Science Council of Taiwan, Republic of China; contract/grant number: NSC93-ET-7-002-009-ET Contract/grant sponsor: National Economic Ministry of Taiwan, Republic of China; contract/grant number: 92-EC-17-A-08-S1-0015 (Received 30 November 2005; revised version received 18 January 2006; accepted 31 January 2006)

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the wetting layer are higher than those of DPO-PPV in the upper layer, the vertically segregated morphology fits the category of so-called ‘type II’ heterojunction.19 – 21 As a result, the turn-on voltage

of the polymer light-emitting diodes (PLEDs) pre-pared by the thermally treated polyblend decreased to ∼0.6 V, and their EL emission intensities and quan-tum efficiencies increased to about four times those of the untreated polyblend.17,18

In this contribution, we will describe how the methylene bonds form in the MEH-PPV chains and their degradation behavior. Novel applications to enhance the luminescence properties of MEH-PPV and its blends with DPO-MEH-PPV through thermal treatment were also elucidated.

EXPERIMENTAL Materials

All reagents were commercially available and used as received unless otherwise stated. Tetrahydrofuran (THF) was freshly distilled over sodium before it was used. All the reactions were carried out under nitrogen atmosphere. The monomer synthesis and polymerization of DPO-PPV can be found elsewhere.6 The monomer used to prepare MEH-PPV, 1,4-bis(chloromethyl)-2-((2 -ethylhexyl)oxy)-5-methoxy-benzene, was synthesized according to Ref. 22 with some modifications, which are detailed as follows.

Synthesis of 2-((2-ethylhexyl)oxy)-5-methoxy-benzene22

A mixture of 4-methoxyphenol (31 g, 0.25 mol) and KOH (19.8 g, 0.35 mol) in methanol was refluxed for 0.5 h. After cooling to room temperature, 2-ethylhexyl bromide (53 g, 0.275 mol) was added dropwise and further refluxed for 48 h until the brownish solution turned light yellow. After the methanol had been removed with a rotary evaporator, the remaining mixture was combined with 250 mL of ether, washed several times with water, and dried over MgSO4. After

removing the solvent, 30.5 g (51.7% yield) of clear liquid product was obtained.

Synthesis of 1,4-bis(chloromethyl)-2-((2-ethyl hexyl)oxy)-5-methoxy-benzene

The compound (10.62 g, 0.045 mol) prepared above was dissolved in 60 mL dioxane. While the result-ing solution was cooled down to 0◦C, 45 mL of concentrated HCl and 35 mL of 39% aqueous for-malin solution were added. The solution was fur-ther saturated with HCl by bubbling gaseous HCl for 20 min before warming up to room tempera-ture. Afterwards, the mixture was stirred at room temperature for 3 days and then refluxed for 3.5 h. After cooling, concentrating, and storing the reaction mixture in a refrigerator overnight, a precipitate of light-yellow crude product was obtained. This crude product was dissolved in a minimum amount of warm hexane and precipitated with methanol again

to afford a white crystalline product (11.5 g, ∼76% yield). To distinguish this product (which contained a small amount of impurities having methylene bridges between 2-((2-ethylhexyl)oxy)-5-methoxy-benzene)s from the further purified monomers, we designated it as MEH-PPV-monomer I. To remove the impuri-ties, further purification was done by dissolving 1 g of the above product in 50 mL of isopropanol at refluxing temperature (∼85◦C). After complete dis-solution, the solution was maintained at 80◦C for 0.5 h and then cooled down to room temperature, and a white crystalline product precipitated. After filtra-tion, it was redissolved in chloroform and precipitated with methanol again to obtain MEH-PPV-monomer II (0.3 g,∼30% yield).

Polymerization

Polymerization of MEH-PPV and DPO-PPV mono-mers were carried out by the Gilch route.17,18 In

general, 2 g of the monomer was first dissolved in 100 mL of THF. Then,∼4.5 equiv. of potassium tert-butoxide (Lancaster) dissolved in 150 mL of THF was added slowly to the stirred monomer solution under nitrogen atmosphere. After complete addition of the base, the reaction proceeded with stirring at room temperature (∼25◦C) for a further 3 days. A large quantity of THF was used to prevent the formation of a gel in the polymerization system. At the end of the reaction, the solution was poured into rapidly stirred methanol. The precipitate was then collected by filtration. The resulting polymer was obtained after drying in vacuo. MEH-PPVs resulting from monomers I and II are designated MEH-PPV(I) and MEH-PPV(II), respectively. The molecular weights of MEH-PPVs and DPO-PPVs were determined by gel permeation chromatography (GPC) (Table 1).

Blending and thermal treatment

MEH-PPV/DPO-PPV blends in 1:1 weight ratio were prepared by dissolving the individual polymers in THF, mixing the two solutions thoroughly, and then drying in vacuo. For thermal treatment,∼0.1 g of the dried blend was tightly wrapped with aluminum foil to prevent contact with air. The thermal treatment was conducted at 200◦C for 2 h under vacuum. The thermally treated samples were then dissolved in 50 mL THF to remove the undissolved portion by filtration through a 0.4µm filter. After precipitation from the solution by addition of methanol, the

Table 1. Molecular weight and polydispersity of MEH-PPV(I),

MEH-PPV(II) and DPO-PPVa

Sample Mw(gmol−1) Mn(gmol−1) Mw/Mn

MEH-PPV(I) 280 000 43 000 6.51

MEH-PPV(II) 333 000 42 900 7.76

DPO-PPV 281 000 72 500 3.87

aMw, weight-average molecular weight; Mn, number-average molecu-lar weight.

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thermally treated polyblend was obtained through filtration and drying in vacuo.

Characterization

Gel permeation chromatography measurements on MEH-PPV and DPO-PPV were carried out at 40◦C with a Testhigh series III pump and a Testhigh UV detector. One PhenolGEL 550A column and two PhenolGEL MXL columns in series were used with THF as the mobile phase (1 mL min−1). The molecular weights were estimated by reference to polystyrene standards with a narrow molecular weight distribution. 1H- and 13C-NMR spectra of all the

samples were recorded on a Bruker Avance 500 spectrometer at frequencies of 500 MHz for 1H and

125 MHz for 13C. All spectra were obtained in

CDCl3 at room temperature. Chemical shifts refer

to tetramethylsilane for 1H and to chloroform-d 1

for 13C. Thermogravimetric analysis (TGA) of the

polymer samples was conducted in a TA model TGA-51 thermogravimetric analyzer at a heating rate of 10◦C min−1under nitrogen.

The photoluminescence spectrum was recorded on a SPEX 1403 fluorescence spectrophotometer using a xenon lamp as an excitation source. Two-photon excitation microscopy of thermally treated polyblends

in film form was carried out with a Leica TCS SP2 confocal spectral microscope imaging system. The wavelength of excitation source was 488 nm, whereas that of the emitting light was collected in the range 500∼ 700 nm. Film samples were prepared by dissolving the thermally treated polyblend in toluene at room temperature, and then spin-casting onto a glass plate. The preparation procedure of PLED device has been given elsewhere.16The EL spectrum of thermally

treated polyblend was recorded on a SPEX1403 fluorescence spectrophotometer at a forward bias of 15 V.

RESULTS AND DISCUSSION Origin of the methylene bonds

Figure 1 shows the 1H-NMR spectrum of

MEH-PPV-monomer I. Interestingly, a tiny split peak appeared at δ= 6.6 ∼ 6.7 ppm, which can only be observed by magnification of the spectrum as shown in the top of the figure. Using column chromatography (silica gel, hexane:ethylacetate = 90:10 by volume) to separate this species from monomer I, and then measuring its heteronuclear multiple quantum correlation (HMQC) spectrum, yielded the result shown in Fig. 2(a). By referring

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to the reported chemical shift values of bis(4-methyl-2,5-dimethoxyphenyl) methane (Fig. 2(b)) which has a methylene bridge between its 2,5-dimethoxy-benzenes,23 we confirmed that the impurities in

MEH-PPV-monomer I contained a similar methylene bridge in between 2-((2 -ethylhexyl)oxy)-5-methoxy-benzenes. The original tiny split peak coupled to the carbons at δ= 113.4 and 114 ppm in the HMQC spectrum (Fig. 2(a)) was attributed to the phenylene protons of these 2-((2 -ethylhexyl)oxy)-5-methoxy-benzenes. The resonance peak of methylene protons at δ= 3.9 ppm was also coupled to the carbon at δ= 29.6 ppm. Because the reaction scheme to prepare MEH-PPV monomer is similar to that of phenolformaldehyde (except for its higher content in HCl) it ought to have a chance to form the methylene bridge during the early stage of monomer synthesis. The species also carried methylene chloride groups on the other side of 2-((2 -ethylhexyl)oxy)-5-methoxy-benzenes as indicated by the presence of the peak

at δ= 4.7 ppm (Fig. 2(a)) so that it performed like a monomer. The impurities reached ∼10 mol% of the MEH-PPV monomer I as estimated from the ratio of peak area at δ= 6.6–6.7 ppm to its area plus that at δ= 6.95 ppm (Fig. 1). However, it could be completely removed through double recrystallization of monomer I. We designated the fully purified monomers as MEH-PPV monomer II.

As the MEH-PPV monomer I was subjected to Gilch polymerization, the tiny peak still appeared at

δ= 6.6–6.7 ppm in the 1H-NMR spectrum of the

resulting MEH-PPV(I) (Fig. 3(a)). However, most of the PPV derivatives prepared via the Gilch route also contain tolane-bisbenzyl (TBB) due to head-to-head or tail-to-tail reactions.11 The phenylene protons of bisphenzyl groups happen to resonate at

δ= 6.6–6.7 ppm, which can be seen in the1H-NMR

spectrum of MEH-PPV(II) (Fig. 3(b)). Dividing the peak intensity at δ= 6.6 ∼ 6.7 ppm by that at δ = 7.5 ppm contributed by the trans-vinylene bonds for

Figure 2. (a) HMQC spectrum of the species containing methylene bridge separated from monomer I through column chromatography and (b) reported chemical shift values of protons and carbons (bold) for the methylene bridge and phenylene groups of

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Figure 3. 1H-NMR spectra of (a) MEH-PPV(I) and (b) MEH-PPV(II).

both polymers, the obtained ratio for the MEH-PPV (I) after subtracting the ratio for the MEH-PPV(II) is also ∼10%, which is almost equal to the content of the impurities having a methylene bridge in monomer I. The result indicates that most of the impurities containing methylene bridges participated in the Gilch polymerization.

Degradation of methylene bonds and its novel applications

It is generally agreed that cis-vinylene protons also resonate at δ= 6.5–6.8 ppm for the PPV derivatives prepared by the Wittig reactions.24 – 26In our previous study, we have mistakenly attributed the peak at

δ= 6.6–6.7 ppm for the MEH-PPV prepared from

monomer I to the formation of cis-vinylene bonds during Gilch polymerization.15 As the resulting polymer was subjected to thermal treatment at 200◦C for 1 – 2 h, half of the methylene bonds were broken as indicated by the decrease of the peak at

δ= 6.6–6.7 ppm.16In this study, we have also found that the MEH-PPV(I) began to degrade at ∼200◦C, but that the major degradation took place at 367◦C (Fig. 4). In contrast to MEH-PPV(I), MEH-PPV(II) started degrading at ∼350◦C but with its major degradation at 395◦C. For MEH-PPV(I), the broken polymer segments during thermal treatment at 200◦C were highly reactive and tended to chemically bond to the adjacent polymer chains, provoking a reduction of chain aggregation. As a result, the PL quantum efficiency in film form was increased to∼6 times that of untreated MEH-PPV.16

The copolymerization of MEH-PPV and DPO-PPV monomers through the Gilch route tended to form an alternate copolymer with reactivity ratios of MEH-PPV and DPO-MEH-PPV monomers equal to 0.16 and 0.19, respectively.6 As MEH-PPV(I)/DPO-PPV blend in

Structure 1.

1:1 weight ratio was subjected to thermal treatment at 200◦C for 2 h, the broken MEH-PPV chain segments had a tendency to bond to the DPO-PPV chains by trans-vinylization; this is at the origin of the small peak at δ= 7.35 ppm in1H-NMR, which stems from

the vinylene protons of the moiety (Structure 1), which is the sole vinylene peak for the alternative copolymer prepared from 1:1 molar ratio of MEH-PPV and DPO-MEH-PPV monomers.17,18 MEH-PPV and DPO-PPV are immiscible. It was also interesting to observe that each DPO-PPV domain (green) of the thermally treated MEH-PPV(I)/DPO-PPV blend was surrounded by a thin layer of red MEH-PPV domains (Fig. 5). As the thermally treated blend was employed as a light-emitting layer for PLED, the turn-on voltage was 0.6 eV – much lower than for that prepared from the untreated polyblend. Its maximum EL intensity was ∼2700 cd m−2, about four times that of the untreated blend. The EL quantum yield was 2 cd A−1, which barely changed with the applied current density, whereas that of the untreated blend decreased from 0.7 to 0.4 cd A−1by increasing the current density.17,18 The heterojunction formed between MEH-PPV and DPO-PPV is shown schematically in Fig. 6. MEH-PPV acts like a hole-transport polymer and DPO-MEH-PPV like an electron-transport polymer. The energy gap of the heterojunction is only 1.98 eV – lower than the band gap of pristine MEH-PPV (2.14 eV). For the thermally treated blend, the maximum emission wavelength of EL spectrum was at 577 nm (2.15 eV), while that of the PL was at 544 nm (2.28 eV) as indicated in Fig. 7. The difference is 0.13 eV, close to the energy barrier of 0.16 eV between their LUMOs for the excitons to jump from DPO-PPV to MEH-DPO-PPV in order to cross the LUMO of the heterojunction (Fig. 6). On the contrary, for the untreated MEH-PPV/DPO-PPV blend, the

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Figure 4. TGA plots of (a) MEH-PPV(I) and (b) MEH-PPV(II).

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(b)

Figure 5. Photo-excitation micrographs of (a) thermally treated and (b) untreated MEH-PPV(I)/DPO-PPV blend.17,18

maximum emission wavelength of PL spectrum is similar to that of the EL spectrum.17,18 Therefore, it

can be concluded that the thermally treated MEH-PPV(I)/DPO-PPV blend has more heterojunction regions to generate exciplexes than the untreated blend, so that the EL emission is dominated by exciplexes. The EL quantum efficiency was thus significantly increased.

CONCLUSIONS

We discovered methylene defects in MEH-PPV chains, which could only be eliminated by double

Figure 6. Scheme for heterojunction formed between MEH-PPV and DPO-PPV. Electrons from DPO-PPV and holes from MEH-PPV are trapped and form the exciplexes at the heterojunction during EL emission.

Figure 7. EL and PL spectra of thermally treated MEH-PPV(I)/DPO-PPV blend.

recrystallization of the monomers. Because the methy-lene bonds are prone to break at 200◦C, a thermal treatment could reduce chain aggregation of MEH-PPV in film form, and also turn MEH- MEH-PPV(I)/DPO-PPV blends into block copolymer type structures. Both substantially enhanced the luminescence properties.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support of the National Science Council of Taiwan, Republic of China, through grant NSC93-ET-7-002-009-ET, and the National Economic Ministry of Taiwan, Republic of China, through grant 92-EC-17-A-08-S1-0015.

REFERENCES

1 Braun D and Heeger AJ, Appl Phys Lett 58:1982 (1991). 2 Parker ID, J Appl Phys 75:1656 (1994).

3 Padmanaban G and Ramakrishnan S, J Am Chem Soc 122:2244 (2000).

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6 Chiu C-C, Lin K-F and Chou H-L, J Polym Sci Part A: Polym

Chem 41:2180 (2003).

7 Parekh BP, Tangonan AA, Newaz SS, Sanduja SK, Ashraf AQ, Krishnamoorti R, et al, Macromolecules 37:8883 (2004). 8 Gilch HG and Wheelwright WL, J Polym Sci Part A: Polym

Chem 4:1337 (1966).

9 Lee TW and Park OO, Adv Mater 12:801 (2000).

10 Lee TW, Park OO, Do LM and Zyung T, Synth Met 117:249 (2001).

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數據

Table 1. Molecular weight and polydispersity of MEH-PPV(I), MEH-PPV(II) and DPO-PPV a
Figure 1 shows the 1 H-NMR spectrum of MEH- MEH-PPV-monomer I. Interestingly, a tiny split peak appeared at δ = 6.6 ∼ 6.7 ppm, which can only be observed by magnification of the spectrum as shown in the top of the figure
Figure 2. (a) HMQC spectrum of the species containing methylene bridge separated from monomer I through column chromatography and (b) reported chemical shift values of protons and carbons (bold) for the methylene bridge and phenylene groups of
Figure 3. 1 H-NMR spectra of (a) MEH-PPV(I) and (b) MEH-PPV(II).
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