Synthesis and Light-Emitting Properties of Disubstituted

J. Phys. Chem. B 2008, 112, 11227–11235

11227

Synthesis and Light-Emitting Properties of Disubstituted Polyacetylenes Carrying Chromophoric Naphthylethynylphenyl Pendants Jacky W. Y. Lam,† Anjun Qin,†,§ Yongqiang Dong,† Yuning Hong,† Cathy K. W. Jim,† Jianzhao Liu,† Yuping Dong,| Hoi Sing Kwok,‡ and Ben Zhong Tang*,†,‡,§ Department of Chemistry and Center for Display Research, The Hong Kong UniVersity of Science & Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China, Department of Polymer Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, China, and College of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China ReceiVed: March 07, 2008; ReVised Manuscript ReceiVed: July 02, 2008

Poly(1-phenyl-1-alkyne)s bearing chromophoric pendants and containing alkyl spacers (-{(C6H5)Cd C[(CH2)mOCOC6H4CtCNp]}n- [P1(m) (m ) 3, 4, 9); Np ) 1-naphthyl]) were synthesized, and the effects of structural variations on the optical properties, especially electroluminescence, of the polymers were investigated. The monomers were prepared in high yields by esterification and coupling reactions of n-phenyl(n - 1)-alkyn-1-ols. Selective polymerizations of the 1-phenyl-1-alkyne unit of the monomers were effected by WCl6-Ph4Sn catalyst, affording polymers with high molecular weights (Mw up to 63 000) in high yields (up to 83%). Structures and properties of the polymers were characterized and evaluated by IR, NMR, TGA, UV, PL, and EL analyses. All the polymers are thermally very stable, losing almost no weight when heated up to 400 °C. Photoexcitation of the polymer solutions induces strong blue light emission at 460 nm, with quantum yields up to 98%. No aggregation quenching was observed when the polymers were fabricated into solid films. Multilayer EL devices with the configuration of ITO/P1(m):PVK/BCP/Alq3/LiF/Al were fabricated, which emitted blue light with luminance up to 498 cd/m2. The device performance varied with the spacer length (m), with P1(4) giving the highest external quantum efficiency of 0.47%. The value was further enhanced to 0.86% by optimizing the layer thickness and inserting a hole-injection layer. Introduction Significant progress in the studies on polymer light-emitting diodes (PLEDs) has been made since the discovery of electroluminescence (EL) in poly(p-phenylenevinylene).1 PLEDs are advantageous over their low molecular mass congeners. Polymers, for example, can be readily fabricated into flexible, robust films by inexpensive spin-coating, screen printing, or ink-jet printing techniques. Their emission colors can be easily tuned by molecular engineering endeavors. A full-color display system requires red, green, and blue emissions. Synthesis of blue light emitting polymers is particularly interesting because blue light can be converted to green or red light using phosphors or color converters.2 Whereas red and green emitters with high luminance and efficiency have been developed,3-6 strong blue emitters are still rare. The most promising blue light emitting polymers are polyphenylenes and polyfluorenes, which are prepared by polycoupling or polycondensation reactions. The stoichiometric requirements in such polymerizations are practically difficult to meet, which often leads to the formation of polymers with low molecular weights. Polyacetylene is the best-known conjugated polymer and shows metallic conductivity upon doping.7 The polymer, however, does not emit visible light. Though blue EL has been observed in its substituted form such as poly(1-phenyl-1-octyne) (PPO; Chart 1), the luminance and efficiency of the PPO-based * Corresponding author. Phone: +852-2358-7375. Fax: +852-2358-1594. E-mail: [email protected].. † Department of Chemistry, HKUST. ‡ Center for Display Research, HKUST. § Zhejiang University. | Beijing Institute of Technology.

CHART 1: Chemical Structures of Poly(1-phenyl-1-alkyne) (PPA) Derivatives

PLED were very low, being merely 0.5 cd/m2 and 0.01%, respectively.8 The beauty of materials chemistry is that a subtle variation in the molecular structure of a polymeric material can change its property to a great extent. We have recently succeeded in the synthesis of a series of functional poly(1-phenyl-1-alkyne)s (PPAs) bearing different chromophoric pendants with high molecular weights by simple addition polymerizations.9 The chromophoric units were found to greatly affect the optical properties of the polymers. For example, whereas almost no light was emitted from an EL device of PPO,9f the PLEDs of

10.1021/jp802009d CCC: $40.75  2008 American Chemical Society Published on Web 08/16/2008

11228 J. Phys. Chem. B, Vol. 112, No. 36, 2008 SCHEME 1: Synthesis of Disubstituted Acetylene Monomers 1(m)

Lam et al. TABLE 1: Polymerizations of 5-{[4-(1-Naphthylethynyl)phenyl]carbonyloxy}-1-phenyl-1-pentyne [1(3)]a no.

catalyst

temp (°C)

yield (%)

1 2 3 4 5 6

NbCl5-Ph4Sn TaCl5-Ph4Sn WCl6-Ph4Sn WCl6-Ph4Sn WCl6-Ph4Sn MoCl5-Ph4Sn

60 60 RT 60 80 60

trace trace trace 59.0 66.6 0

Mwb

Mw/Mnb

51 000 26 400

2.3 2.0

a Carried out under nitrogen in toluene for 24 h; [M]0 ) 0.2 M, [catalyst] ) [cocatalyst] ) 10 mM. b Determined by GPC in THF on the basis of a polystyrene calibration.

TABLE 2: Polymerizations of 6-{[4-(1-Naphthylethynyl)phenyl]carbonyloxy}-1-phenyl-1-hexyne [1(4)]a

the PPA derivatives carrying naphthyl pendants [P2(m)] with the same device configuration emitted blue light in high luminance and efficiency (Chart 1). Since the attachment of chromophoric units to the PPA skeleton is beneficial in terms of enhancing the light-emission efficiency, in this work we designed and synthesized a group of acetylene monomers containing a chromophoric naphthylethynylphenyl (NEP) unit with different lengths of alkyl spacers [1(m); Scheme 1]. Compared with the naphthyl unit in 2(m), the NEP unit in 1(m) is more emissive. It is envisioned that this structural variation may change the emission behavior of the resultant polymers. Indeed, the photoluminescence (PL) quantum yields (ΦPL) of P1(m) are 2-fold higher than those of PPO and P2(m). Multilayer EL devices utilizing P1(m) as active layers emit blue light of 456 nm with a maximum external quantum efficiency (ηmax) of 0.47%. The value is further enhanced to 0.86% by optimizing the device configuration. Experimental Section General information about the materials and instrumentations, full experimental details about the syntheses of the monomers and polymers, and their numerical spectroscopic analysis data are all given in the Supporting Information. Results and Discussion Monomer Preparation. The NEP-containing disubstituted acetylene monomers 1(m) were prepared by a simple synthetic route given in Scheme 1. n-Phenyl-(n - 1)-alkyn-1-ols [3(m)] were transformed to n-phenyl-(n - 1)-alkynyl 4-ethynylbenzoates 5(m) by esterification with 4-ethynylbenzoic acid (4) in the presence of a mixture of 1,3-dicyclohexylcarbodiimine (DCC), p-toluenesulfonic acid (TsOH), and 4-(dimethylamino)pyridine (DMAP). The intermediates [5(m)] were coupled with 1-iodonaphthalene in the presence of Pd(PPh3)2Cl2 and CuI. ω-{[4-(1-Naphthylethynyl)phenyl]carbonyloxy}-1-phenyl-1alkynes [1(m)], the desired products, were obtained in high isolation yields (∼67-93%) after purification by column chromatography and recrystallization. All the acetylene monomers were characterized by spectroscopic techniques, from which satisfactory analysis data corresponding to their expected molecular structures were obtained (see Supporting Information for details). Polymer Synthesis. We first tried to polymerize 1(3) by transition metal catalysts. NbCl5- and TaCl5-Ph4Sn are known

no.

catalyst

temp (°C)

yield (%)

1 2 3 4 5 6 7

NbCl5-Ph4Sn TaCl5-Ph4Sn WCl6-Ph4Sn WCl6-Ph4Sn WCl6-Ph4Sn WCl6-Ph4Sn MoCl5-Ph4Sn

60 60 RT 40 60 80 60

trace 0 trace 83.6 37.9 37.1 0

Mwb

Mw/Mnb

5 900 21 900 33 000

3.3 2.4 2.7


to effect the polymerizations of 1-phenyl-1-alkynes,10 but we found that none of them was capable of polymerizing the monomer (Table 1, nos. 1 and 2). Comparing 1-phenyl-1pentyne [(C6H5)CtC(CH2)2CH3] with 1(3) [(C6H5)Ct C(CH2)3OCOC6H4CtCsNp], one can immediately recognize their structural difference: 1(3) contains a ester group. The poisoning effect of the polar ester functionality on the catalysts is thus responsible for the failure of the polymerizations by the NbCl5- and TaCl5-Ph4Sn mixtures. We then checked whether there was any hope for the monomer to be polymerized by other transition metal initiators. Whereas WCl6-Ph4Sn gives no polymeric product in toluene at room temperature (RT), raising the temperature to 60 °C dramatically activates the catalyst and a pale-yellow powdery product is isolated in 59% yield after precipitating the polymer solution into acetone. The obtained polymer product is completely soluble in common organic solvents, indicative of its non-cross-linked nature. Although the polymer yield is increased slightly at 80 °C, the molecular weight of the polymer is decreased. The monomer does not undergo polymerization in the presence of MoCl5-Ph4Sn. Similar to 1(3), monomer 1(4) could not be polymerized by NbCl5- and TaCl5-Ph4Sn (Table 2, nos. 1 and 2). On the other hand, WCl6-Ph4Sn catalyst works well for the polymerization at 40 °C, affording a completely soluble polymer in a high yield. Raising the temperature increases the molecular weight of the polymer at the expense of its yield. Like 1(3), monomer 1(4) undergoes sluggish polymerization when MoCl5-Ph4Sn is used. Table 3 summarizes the polymerization behaviors of 1(9). Like 1(3) and 1(4), monomer 1(9) fails to be polymerized by NbCl5- and TaCl5-Ph4Sn. Different from its congeners with shorter spacer lengths, the monomer could be polymerized by WCl6-Ph4Sn at room temperature, although the yield is low. Both the polymer yield and molecular weight are boosted at elevated temperatures, and a high molecular weight polymer is isolated in an impressively high yield (81%) at 80 °C. In contrast, MoCl5-Ph4Sn is completely ineffective. Structural Characterization. Molecular structures of the polymeric products were characterized by spectroscopic methods

Light-Emitting Polyacetylenes

J. Phys. Chem. B, Vol. 112, No. 36, 2008 11229

TABLE 3: Polymerizations of 11-{[4-(1-Naphthylethynyl)phenyl]carbonyloxy}-1-phenyl-1-undecyne [1(9)]a no.

catalyst

temp (°C)

yield (%)

1 2 3 4 5 6 7

NbCl5-Ph4Sn TaCl5-Ph4Sn WCl6-Ph4Sn WCl6-Ph4Sn WCl6-Ph4Sn WCl6-Ph4Sn MoCl5-Ph4Sn

60 60 RT 40 60 80 60

trace 0 16.0 26.6 62.5 81.0 0

Mwb

Mw/Mnb

24 500 21 000 63 300 63 000

1.9 1.9 3.2 2.9


Figure 1. IR spectra of (A) monomer 1(9) and (B) its polymer P1(9) (sample taken from Table 3, no. 5).

with satisfactory analytical data obtained. An example of the IR spectrum of P1(9) is shown in Figure 1; for comparison, the spectrum of its monomer 1(9) is also shown in the same figure. The CtC stretching vibration band of 1-phenyl-1-alkyne unit is normally very weak and can only be occasionally observed at 2230 cm-1,9d while that of diphenylacetylene unit is strong enough to be visible at 2200 cm-1.11 Monomer 1(9) exhibits a readily identifiable peak at 2209 cm-1, which is presumably due to CtC stretching of its NEP group. This absorption band is still observed in the spectrum of P1(9), suggesting that WCl6-Ph4Sn can selectively polymerize the 1-phenyl-1-undecyne unit of 1(9) with no harm to other functional groups including the ester group and the diphenylacetylene unit. Figure 2 shows the 1H NMR spectra of monomer 1(9) and its polymer P1(9). The protons of the phenyl group of the 1-pheny1-undecyne unit of 1(9) resonate at δ 7.38 and 7.25, which disappear and shift downfield to δ 6.98 upon polymerization. The resonance of the methylene protons next to the triple bond (δ 2.40) is weakened after polymerization because the allenic protons (dCCH2) are attached to a rigid polyene backbone. The resonance peaks of the NEP unit of 1(9) are found at similar chemical shifts in P1(9), indicating that the obtained polymeric product is indeed P1(9), as shown in Chart 1. The 13C NMR spectra of monomer 1(9) and its polymer P1(9) are shown in Figure 3. Whereas the peaks at δ 90.38 and 80.60 in 1(9) are associated with the resonances of acetylene carbon atoms of its 1-phenyl-1-undecyne unit, those at δ 93.5 and 90.43 stem from the resonances of the NEP group. After polymerization, the resonance peaks of the 1-phenyl-1-undecyne triple bond disappear, and that of the propargyl carbon (d) shifts downfield due to its transformation to the allenic structure. The resonances of the pendant triple bond are still observed in P1(9),

thus spectroscopically proving that only the 1-phenyl-1-undecyne triple bond of 1(9) has been consumed by the polymerization reaction. A broad peak is found at δ 141.5, due to the resonance of the olefinic carbon (b) of the polyene backbone. Thermal Stability. Notwithstanding the high conductivity of its doped form, polyacetylene has found few practical applications because of its notorious intractability and instability. Attachment of aromatic pendants to the polyacetylene backbone has solved the problem and has generated readily processable and thermally stable substituted polyacetylenes.12,13 For example, a 5% weight loss of poly(1-phenyl-1-propyne) occurs at a temperature of 330 °C.10 Since P1(m) are PPA derivatives with chromophoric pendants, it is expected that the polymers would show high thermal stability. Indeed, the polymers do not lose any weight when heated up to ∼400 °C (Figure 4). Clearly, the attachment of the NEP pendants to the PPA skeleton has enhanced the resistance of the polymers to thermolytic attack, thanks to the protective jacket effect contributed by the aromatic appendages.14 It is noteworthy that the amounts of residue left (>40%) are rather high for all the polymers. Thermally induced acetylene polymerizations15 of the triple bonds of the NEP pendants of P1(m) further cross-link the polymers. The resultant thermosets may have graphitized upon pyrolysis, resulting in the high weight residue at the high temperatures. Electronic Absorption. The absorption spectra of THF solutions of 1(4), P1(m), and PPO are shown in Figure 5. The chromophoric pendant of 1(4) absorbs strongly at 333 nm. This absorption peak is also observed in P1(4), revealing that the polymer bears the NEP appendages, or in other words, the triple bond in the NEP unit has remained intact during the polymerization reaction. Since the monomer does not absorb at wavelengths longer than 380 nm, the absorption of P1(4) in the long wavelength region is obviously from its PPA skeleton. This assignment is supported by the observation that PPO, a parent form of P1(4), absorbs in the long wavelength region. The ground-state electronic transitions are insensitive to the change in the spacer length: the spectra of P1(3) and P1(9) are similar to that of P1(4). Photoluminescence. As mentioned in the Introduction, polyacetylene is nonemissive. Its substituted forms can, however, be emissive,9a,16,17 with disubstituted polyacetylenes being generally more emissive than their monosubstituted congeners.18 PPO is an example, which emits at 460 nm with a ΦPL of 43% when photoexcited.18f Would the attachment of the NEP pendants to the PPA skeleton further enhance the PL efficiency? The answer is “yes”, as can be seen from the PL spectra of P1(m) given in Figure 6A. When the THF solution of P1(4) is excited at 370 nm, it emits a strong blue light of 460 nm, whose intensity is much higher than that of PPO. Its ΦPL value (94%) is more than 2-fold higher than that of PPO. Almost no emission from the NEP pendant of P1(4) is detected at ∼400 nm. As the PL spectrum of the NEP pendant overlaps with the absorption spectrum of the PPA skeleton, the light emitted by the former is absorbed by the latter, which pumps the polymer strand to its excited state. When the excitons decay back to their ground state, a strong, blue light is emitted from the PPA chain. Polymers P1(3) and P1(9) also emit at similar wavelengths with slightly different efficiencies. This result is in contrast to the early observation that longer alkyl chains favor stronger PL and EL in PPAs.8 The longer spacer length may have better alleviated the interaction between the polymer chains, which enhances the chances for the confined excitons to recombine radiatively. On the other hand, it may reduce the efficiency of energy transfer. The competition between the constructive and

11230 J. Phys. Chem. B, Vol. 112, No. 36, 2008

Lam et al.

Figure 2. 1H NMR spectra of chloroform solutions of (A) monomer 1(9) and (B) its polymer P1(9) (sample taken from Table 3, no. 5). The solvent peak is marked with an asterisk.

destructive effects may have led to the lower ΦPL values in P1(4) and P1(9) than in P1(3). Many fluorogenic polymers are highly emissive in dilute solutions but become weakly luminescent when fabricated into solid films.19 This is believed to be caused by strong interchain interactions in the solid state: the polymer strands cluster together to form less emissive species, resulting in weaker emissions. Would our polymers become weaker emitters when fabricated into thin solid films? Upon excitation, all the polymer films emit blue light at wavelengths similar to those in the solutions (Figure 6B), suggesting that aggregation exerts little effect on the PL processes of the polymers. Although we have technical difficulty in measuring the ΦPL values of the polymers in the solid state, our recent studies show that the emissions of PPO and its derivatives with chromophoric pendants are enhanced by aggregate formation due to the restrictions of intramolecular rotations of the phenyl rings.20 Similar behavior is expected for polymers P1(m): their films are probably more emissive than their solutions. Electroluminescence. The high PL efficiencies of the polymers prompted us to investigate their EL performance. We

first fabricated a single-layer PLED with a device configuration of ITO/P1(m)/Al, in which P1(4) was used as the active layer. The device emits a blue light of 456 nm with a ηmax value of ∼0.01%. Such a poor EL result contradicts its PL efficiency and may be caused by the unbalanced injection and transport of the charge carriers. To solve this problem, we built a multilayer PLED with a configuration of ITO/P1(4):PVK (60 nm)/BCP(20 nm)/Alq3(30 nm)/LiF(0.8 nm)/Al (device A; Chart 2), where ITO ) indium tin oxide, PVK ) poly(N-vinylcarbazole), BCP ) 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, and Alq3 ) tris(8-hydroxyquinolinolato)aluminum. In the device, the LiF and Alq3 layers help enhance electron injection and transport, while the BCP layer prevents holes from traveling through to reach the cathode. The polymer is blended with PVK at a weight ratio of 1:4 to improve the hole transport. With these engineering modifications, the device performance was greatly enhanced. The device is turned on at ∼10 V and emits a blue light of 456 nm (Figure 7). The EL spectrum is single-peaked and resembles the PL spectrum, indicating that the EL is truly from P1(4) and stems from the radiative decay of its singlet excitons



Figure 3. 13C NMR spectra of chloroform solutions of (A) monomer 1(9) and (B) its polymer P1(9) (sample taken from Table 3, no. 5). The solvent peaks are marked with asterisks.

Figure 4. TGA thermograms of polymers P1(3) (sample taken from Table 1, no. 4), P1(4) (Table 2, no. 5), and P1(9) (Table 3, no. 5) recorded under nitrogen at a heating rate of 20 °C/min.

Figure 5. Absorption spectra of THF solutions of polymers P1(3) (sample from Table 1, no. 4), P1(4) (Table 2, no. 5), and P1(9) (Table 3, no. 5). The spectra of 1(4) and PPO are shown for comparison.

formed by charge recombination. At an applied voltage of 21 V, the luminance reaches ∼500 cd/m2, which is bright enough for use in a flat-panel display system. The ηmax value is 0.47%,

which is 47-fold higher than that in the single-layer PLED (Figure 8). No light, however, is emitted when P1(4) is replaced by PPO. This demonstrates that the good EL performance of


Figure 6. PL spectra of P1(3) (sample taken from Table 1, no. 4), P1(4) (Table 2, no. 5), P1(9) (Table 3, no. 5), and PPO in (A) THF solution (0.05 mM) and (B) solid state (thin film). The spectrum of a THF solution of monomer 1(4) is given in (A) for comparison. Excitation wavelength (nm): 370 [P1(3), P1(4), and P1(9)], 355 (PPO), and 350 [1(4)].

CHART 2: Configurations of EL Devices A and B

P1(4) is not due to the better external engineering control in the multilayer PLED but is mainly due to the improved intrinsic emission efficiency of the polymer. Polymer P1(9), a structural conger of P1(4) with five more methylene spacers, also emits a blue EL at ∼460 nm when the applied voltage exceeds 10 V. At the same voltage, its luminance is similar to that of P1(4) but its efficiency is lower. The PLED performance of P1(3) is even poorer, whose ηmax value is only about half that of P1(4). Although the change in the spacer

Lam et al. length seems to be a subtle structural variation, it does affect the EL of polyacetylenes to a great extent, offering an opportunity to tune their optical properties by further molecular engineering endeavors. Device Performance Optimization. Our results show that the polymers can emit strong EL through judicious design of polymer structure and proper modification of device configuration. We envisioned that there was still room for further enhancing the PLED performance. With such anticipation, we tried to optimize the device configuration, using P1(4), the best EL emitter, as the emitting material. We first varied the blend ratio of the polymer with PVK. When the ratio is increased progressively from 1:1 to 1:32, the turn-on voltage shifts from ∼13 to 11 V because of better hole transport in the blend (Table 4). The EL spectrum is also moved to a lower wavelength region. For example, at a blend ratio of 1:32, the EL peak is located at 424 nm, which is 60 nm blue shifted from that at 1:1. A similar phenomenon has also been observed in a PLED device of TPA-MEHPPV blended with PVK.21 As PVK emits an EL at ∼420 nm, it explains why the spectrum is shifted progressively to the bluer region with an increase in the PVK fraction in the polymer blend. The luminance and efficiency of the EL device are increased when more PVK is used, and they reach their maximums at a blend ratio of 1:4 (the values here are different from those in Figures 7 and 8 because the layer thicknesses are different). Further increase in the blend ratio lowers both values, suggesting that P1(4) is a better EL emitter than PVK. We then investigated the EL from the polymer blend layers with different thicknesses. The results are summarized in Table 5. All the devices emit at similar wavelengths. Upon layer thickening, the turn-on voltage is increased from 11 to 14 V, which is understandable because the resistance for the charges to pass though the polymer blend layer will be increased. The highest luminance and EL efficiency are obtained at an optimal thickness of 40 nm, being 945 cd/m2 and 0.46%, respectively. Strong dependences of EL efficiencies on the thicknesses of Alq3 layers have been observed in several systems.22 We wonder whether such a dependence exists in our system. Figure 9A shows the change in the current density with the applied voltage in the multilayer PLEDs of P1(4) with different thicknesses of Alq3 layers. When the Alq3 layer becomes thinner, the devices become operable at lower voltages with higher current densities. The luminance-voltage profiles resemble those of the current density-voltage curves. The maximum luminance varies little with the Alq3 thickness but the PLEDs with thinner Alq3 layers can attain the same luminance at lower voltages than those with thicker ones. Although the EL intensity is not enhanced, the ηmax value is increased from 0.42 to 0.65% when the Alq3 thickness is increased from 7.5 to 22.5 nm. At a value of 30 nm, the efficiency drops to 0.58% but is still higher than that at 7.5 nm. After optimizing the blend ratio and the thicknesses of emitting and Alq3 layers, the EL efficiency is increased to 0.65%, which is 1.5-fold higher than that shown in Figure 8. Even better performance is achieved if a hole-injection layer of PEDOT:PSS or PAN:PSS [where PEDOT ) poly(3,4ethylene-dioxythiophene), PSS ) poly(styrenesulfonate), and PAN ) polyanilinium] is inserted between the ITO anode and the polymer blend layer. The modified device structure is shown in Chart 2 (device B), and the results are summarized in Table 6. Insertion of the PAN:PSS layer dramatically enhances the current efficiency of the PLED to 1.05 cd/A, corresponding to



Figure 7. Plots of (A) current density and (B) luminance versus applied voltage in the multilayer PLEDs of P1(m) with a device configuration of ITO/P1(m):PVK(60 nm)/BCP(20 nm)/Alq3(30 nm)/LiF(0.8 nm)/Al. Inset: EL spectrum of P1(4).

TABLE 5: Effect of Thickness of Polymer Blend Layer on PLED Performancea no.

thickness (nm)

λmax (nm)

Vonb (V)

Lmax (cd/m2)

CEmax (cd/A)

ηmax (%)

1 2 3 4

26 33 40 52

464 464 464 476

11.9 11.0 11.3 13.0

414 596 945 862

0.62 0.69 0.80 0.75

0.43 0.43 0.46 0.39

a With a device configuration of ITO/P1(4):PVK(1:4 by wt)/ BCP(20 nm)/Alq3(20 nm)/LiF(1 nm)/Al. Abbreviations: λmax ) EL peak maximum, Von ) turn-on voltage, Lmax ) maximum luminance, CEmax ) maximum current efficiency, and ηmax ) maximum external quantum efficiency. b At 1 cd/m2.

Figure 8. Plots of external quantum efficiency versus applied voltage in the multilayer PLEDs of P1(m) with a device configuration of ITO/ P1(m):PVK(60 nm)/BCP(20 nm)/Alq3(30 nm)/LiF(0.8 nm)/Al.

TABLE 4: Effect of Blend Ratio of P1(4) with PVK on PLED Performancea no.

P1(4):PVK (by wt)

λmax (nm)

Vonb (V)

Lmax (cd/m2)

CEmax (cd/A)

ηmax (%)

1 2 3 4 5 6

1:0 1:1 1:4 1:8 1:16 1:32

492 484 464 448 444 424

13.8 12.8 11.9 11.7 12.0 11.0

86 259 414 315 273 243

0.18 0.39 0.62 0.41 0.34 0.29

0.08 0.26 0.43 0.31 0.28 0.26

a With a device configuration of ITO/P1(4):PVK(26 nm)/BCP(20 nm)/Alq3(20 nm)/LiF(1 nm)/Al. Abbreviations: λmax ) EL peak maximum, Von ) turn-on voltage, Lmax ) maximum luminance, CEmax ) maximum current efficiency, and ηmax ) maximum external quantum efficiency. b At 1 cd/m2.

an ηmax value of 0.74% (Table 6). Replacing the PAN:PSS layer by a PEDOT:PSS layer further increases the EL efficiency to 0.86%, which is comparable to some of the best results reported by other research groups for other blue PLEDs.23 We also tested the hole-transport abilities of other materials. We replaced PVK with TPD [N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine] and fabricated a PLED with a device configuration of ITO/P1(4):TPD/BCP/Alq3/LiF/Al. Although the ηmax value is moderate, the device starts to emit light at a low voltage with a high luminance, demonstrative of the importance of the device configuration in the EL of P1(4).

Spectral Stability. Polyfluorenes are the best-known blue light emitting polymers, but their poor spectral stability has limited the scope of their electrooptical applications.24 The formation of fluorene units in their PLEDs during the device operation changes the emission color from blue to blue-green. In sharp contrast, the PLEDs of our polymers enjoy excellent spectral stability. For example, the PLED of P1(4) with a PEDOT:PSS hole-injection layer shows an EL spectrum with a peak at 464 nm when a voltage of 11 V is applied (Figure 10A). When the applied voltage is increased to 14 V, the EL spectrum experiences little change. Even up to 20 V, practically no change is recognizable in the EL spectrum. The PLED with or without PAN:PSS layer also shows good spectral stability (Figure 10B). The EL peak practically does not shift with an increase in the applied voltage, revealing that the spectral stability of the PLEDs of our polymers is outstanding in comparison to those of polyfluorene-based devices. Conclusions In this work, we have synthesized new chromophorecontaining PPAs and investigated the effects of structural variations on their optical properties. Our findings can be summarized as follows: (1) Selective polymerization of 1-phenyl-1-alkyne unit in monomers 1(m) was effected by WCl6-Ph4Sn, affording polymers P1(m) with high molecular weights (Mw up to 63 000) in high yields (up to 83%). (2) All the polymers are thermally very stable, thanks to the protective jacket effect contributed by their phenyl and NEP pendants. (3) Upon photoexcitation, the THF solutions of the polymers emit strong blue lights of 460 nm, with PL quantum yields up to 98%.


Lam et al.

Figure 10. (A) Spectral stability of P1(4) against applied voltage in a PLED with a device configuration of ITO/PEDOT:PSS(60 nm)/P1(4): PVK(1:4 by weight) (26 nm)/BCP(20 nm)/Alq3(20 nm)/LiF(1 nm)/Al. (B) Plots of peak maximum (λmax) versus applied bias in a PLED of P1(4) with device configurations of ITO/P1(4):PVK(1:4 by weight) (26 nm)/BCP(20 nm)/Alq3(20 nm)/LiF(1 nm)/Al and ITO/PAN:PSS(60 nm)/P1(4):PVK(1:4 by weight) (26 nm)/BCP(20 nm)/Alq3(20 nm)/ LiF(1 nm)/Al.

Figure 9. Effects of Alq3 thickness on (A) current density, (B) luminance, and (C) external quantum efficiency (EQE) in a multilayer PLED of P1(4) with a device configuration of ITO/P1(4):PVK(1:4 by weight) (40 nm)/BCP(20 nm)/Alq3/LiF(1 nm)/Al.

TABLE 6: Effects of Hole-Injection and -Transport Layers on PLED Performance of P1(4)a no.

λmax (nm)

Vonb (V)

Lmax (cd/m2)

CEmax (cd/A)

ηmax (%)

1c 2d 3e

464 464 484

10 9 5

133 455 927

1.05 1.25 0.80

0.74 0.86 0.43

Abbreviations: λmax ) peak maximum, Von ) turn-on voltage, Lmax ) maximum luminance, CEmax ) maximum current efficiency, and ηmax ) maximum external quantum efficiency. b At 1 cd/m2. c Device configuration: ITO/PAN:PSS(60 nm)/P1(4):PVK(26 nm)/ BCP(20 nm)/Alq3(20 nm)/LiF(1 nm)/Al. d Device configuration: ITO/PEDOT:PSS(60 nm)/P1(4):PVK(26 nm)/BCP(20 nm)/Alq3(20 nm)/LiF(1 nm)/Al. e Device configuration: ITO/PEDOT:PSS(60 nm)/P1(4):TPD(20 nm)/BCP(20 nm)/Alq3(30 nm)/LiF(0.8 nm)Al. The ratio between P1(4) and PVK or TPD for all the devices was kept at 1:4 by weight. a

(4) The polymer thin films emit blue lights at wavelengths similar to those in the solutions, suggesting that the polymer emission is not quenched by aggregate formation. (5) The multilayer PLEDs of the polymers emitted blue light, whose external quantum efficiency was enhanced to 0.86% by optimizing the layer thickness and inserting a hole-injection layer. Acknowledgment. The work reported in this paper was partially supported by the Research Grants Council of Hong

Kong (602707, 602706, and HKU2/05C), the Ministry of Science & Technology of China (2002CB613401), and the National Natural Science Foundation of China (20634020). B.Z.T. thanks the Cao Guangbiao Foundation of Zhejiang University for support. Supporting Information Available: Detailed experimental procedures for the monomer preparation, polymer synthesis, and device fabrication. Full list of spectroscopic analysis data for the monomers and polymers. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539–543. (b) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, C.; Heeger, A. J. Nature 1992, 357, 477–479. (c) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628–630. (2) (a) Rothberg, L. J.; Lovinger, A. J. J. Mater. Res. 1996, 11 (12), 3174–3187. (b) Virgili, T.; Lidzey, D. G.; Bradley, D. D. C. AdV. Mater. 2000, 12 (1), 58–62. (c) Qiu, C. F.; Wang, L. D.; Chen, H. Y.; Wong, M.; Kwok, H. S. Appl. Phys. Lett. 2001, 74, 2276–2278. (3) (a) Friend, R. H.; Gymer, R. W.; Holmes, J. H.; Burroughs, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121–128. (b) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402–428. (c) Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.; Moon, R.; Roitman, D.; Stocking, A. Science 1996, 273, 884– 888. (d) Bradley, D. D. C. Synth. Met. 1993, 54, 401–415. (e) Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471–1507. (f) Segura, J. L. Acta Polym. 1998, 49, 319–344. (g) Lovinger, A. J.; Rothberg, L. J. J. Mater. Res. 1996, 11, 1581–1592. (h) Grell, M; Bradley, D. D. C. AdV. Mater. 1999, 11, 895–905. (i) Conwell, E. Trends Polym. Sci. 1997, 5, 218–222. (j) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99–117. (k) Scherf, U.; List, E. J. W. AdV. Mater. 2002, 14, 477–487. (l) Kim, D. Y.; Cho, H. N.; Kim, C. Y. Prog. Polym. Sci. 2000, 25, 1089– 1139. (m) Neher, D. Macromol. Rapid Commun. 2001, 22, 1366–1385. (n) Dai, L.; Winkler, B.; Dong, L.; Tong, L.; Mau, A. W. H. AdV. Mater. 2001, 13, 915–925.

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