RAPID COMMUNICATION A New Blue-Light Emitting Polymer: Synthesis and Photoinduced Electron Transfer Process YU CHEN,1 YING LIN,1 MOHAMED E. EI-KHOULY,2,3 NAN HE,1 AIXIA YAN,4 YING LIU,1 LIANGZHEN CAI,1 OSAMU ITO2 1

Lab for Advanced Materials, Department of Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China 2

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, CREST, JST, Katahira 2-1-1, Sendai 980-8577, Japan 3

Department of Chemistry, Faculty of Education, Tanta University, Egypt

4 State Key Laboratory of Chemical Resource Engineering, Department of Pharmaceutical Engineering, P.O. Box 53, Beijing University of Chemical Technology, 15 BeiSanHuan East Road, Beijing 100029, People’s Republic of China

Received 15 August 2007; accepted 22 March 2008 DOI: 10.1002/pola.22769 Published online in Wiley InterScience (www.interscience.wiley.com).

Keywords: conjugated polymers; fullerenes; photophysics; synthesis

INTRODUCTION Since the first electroluminescence from poly(phenylenevinylene) (PPV) was observed by Burroughes et al. in 1990,1 the dream of fabricating ultrathin, flexible, and larger-area active displays have stimulated extensive research activities across the world, and consequently the field of polymeric light-emitting diodes (PLED) has invested much effort into the design and synthesis of new polymeric functional materials, and the device optimization.2,3 In contrast to the red- and green-light emitting polymeric materials with high efficiency and good brightness,4 only a few blue-light emitting polymeric materials5–7 can meet the requirements of the practical applications. This has been a barrier to develop novel full-color polymeric displays because these materials can not only Correspondence to: Y. Chen (E-mail: [email protected] com); M.E. EI-Khouly (E-mail: [email protected] com); A. Yan (E-mail: [email protected]); and L. Cai (E-mail: [email protected]) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 4249–4253 (2008) C 2008 Wiley Periodicals, Inc. V

serve as blue light sources but also can be used to realize all visible emission colors though the internal color-conversion technique based on blue LEDs. Among the large number of blue-light emitting polymers5–7 that have been identified, polyfluorenes (PFs) have emerged as one of the most promising materials owing to their high photoluminescence quantum yields, good charge transport properties, better thermal and chemical stability, and the facile functionalization at the C-9 position of the fluorene unit which may offer an opportunity to reduce the interchain interactions thereby improving the optoelectronic properties of the resulting polymers.8 The biggest disadvantage of PFs lies in their poor spectral stability and low electroluminescence efficiency as a result of keto-defects generated by the oxidation of monoalkyfluorenes.9 As one of the useful hole transporting materials, triphenylamine (TPA) and its organic and polymeric derivatives have been widely used in OLEDs and PLEDs.10 It would be desirable to synthesize polytriphenylamine (PTPA) because in this case it can be directly used to fabricate the corresponding device. For these reasons, we designed and synthe4249

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Scheme 1. Synthesis of polymer Yu1.

sized a new blue-emitting polymer poly[5-(diphenylamino)-1,3-phenylenevinylene] (Yu1) by the McMurry condensation reaction11 of 5-(N,N-diphenylamino)benzene-1,3-dicarbaldehyde(Yu0)12 in the presence of zinc dust and TiCl4. This polymer exhibited a strong blue emission around 455 nm with a fluorescence lifetime of 7.12 ns in toluene. Photoinduced intermolecular electron transfer process of C60/Yu1 system was also investigated by steady-state, time resolved, and nanosecond transient absorption techniques. As a result, the efficiency of the electron transfer process (Fet) from Yu1 to 3C60* was evaluated as 0.76. In a typical McMurry condensation reaction, as shown in Scheme 1, to a mixture of zinc dust (1.43 g, 99.999%) and TiCl4 (2 mL, 17.9 mmol) 40 mL of anhydrous THF was added at 208C under highly purified nitrogen, and then stirred for 15 min at this temperature. After the temperature was allowed to rise to room temperature, the grayish green reaction mixture was heated and refluxed for 2 h. The color of the mixture turned from brownish red to bluish gray to deep brown and to deep cafe color gradually during heating and refluxing. 5-(N,N-diphenylamino)benzene-1,3-dicarbaldehyde (0.88 g)12 in dry THF (50 mL) was added dropwise to the mixture at room temperature with magnetic stirring, and further refluxed for 20 h. The polymerization was quenched with dilute HCl (50 mL, 2 M) at 0 8C, the color of the reaction solution changed from deep violet to yellowish brown quickly. The product was repeatedly extracted with chloroform; the combined organic layers were washed with brine and water, respectively, dried over MgSO4 and filtered. The filtration was concentrated to 10 mL under the reduced pressure, and then was poured into methanol to give Yu1 (0.24 g). The number average molecular weight (Mn) and the weight average molecular weight (Mw) for Yu1 were 1.71 3 103 and 2.10 3 103 g/mol, respectively, with a polydispersity index of 1.23. The aromatic and olefin proton signals of Yu1 appear at d ¼ 6.90–7.18 ppm in the 1H-NMR spectrum (in CDCl3). This polymer exhibits a single glass-transition temperature (157 8C) and high solubility in common organic solvents. Thermogravimetric analysis in N2 shows that Yu1 has good thermal stability (stable up to 310 8C). As shown in Figure 1, the main absorption bands in the UV/Vis absorption spectrum of Yu0 are located at 270, 296(sh), 343, and 419(br.) nm, which are

attributed to the p-p* transition of the monomer. After polymerization, a strong absorption band appears at 307 nm. As expected, the polymer Yu1 exhibits very strong blue light emission with maximum peak centered at 475 nm in dilute CH2Cl2 solution (at 455 nm in dilute toluene solution). In contrast to Yu1, only very week photoluminescence was observed in the fluorescence spectrum of the monomer Yu0 under the same measurement condition. For both Yu0 and Yu1, their fluorescence decay profiles (Fig. 2) show single exponential-decay processes giving the fluorescence lifetime of 4.76 ns for Yu0 and 7.12 ns for Yu1, respectively. The redox potentials of the studied samples have been evaluated using differential pulse voltammetry (DPV) technique by sweeping an applied voltage to the solutions with a suitable electrolyte. The first oxidation potentials (Eox) were evaluated as 920 and 720 mV versus Ag/Ag+ for Yu0 and Yu1, respectively, suggesting that the electron-donor ability of Yu1 is

Figure 1. UV/Vis absorption spectra and fluorescence spectra (kex ¼ 300 nm) of Yu0 and Yu1 in CH2Cl2. Concentration for each sample: 1 3 105 M. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.] Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola

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Figure 2. Fluorescence decay profiles of Yu0 and Yu1 in deoxygenated dry toluene monitored at 400– 600 nm; kex ¼ 400 nm.

higher than that of Yu0. On the other hand, the first reduction potential (Ered) of the C60 was located at 790 mV versus Ag/Ag+. By photo-excitation of C60 (0.1 mM) in the presence of Yu0 (0.2 mM) in Ar-saturated benzonitrile using 532 nm laser photolysis, the transient spectra (Fig. 3) show a characteristic band * at 760 nm. With the decay of 3C* , the concomof 3C60 60 itant slow rise was observed at 15 ls for the C60 radical anion (C2 60 ) at 1080 nm and the radical cation of the triphenylamine (TPA+) at 600–800 nm which * . The tranoverlaps with the absorption band of 3C60 sient spectra of C60 (0.10 mM) in the presence of Yu1 (0.20 mM), as shown in Figure 4, exhibit the characteristic peak of 3C*60 at 740 nm. With the decay of 3 * C60 , the concomitant rise of the C60 radical anion (C2 60 ) at 1080 nm was observed. Moreover, the absorption appearing in the visible and near IR regions with a maximum centered at 1300 nm was assigned to the

Figure 3. Transient absorption spectra obtained by 532 nm laser light of C60 (0.1 mM) in the presence of Yu0 (0.2 mM) in Ar-saturated benzonitrile. Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola

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Figure 4. Transient absorption spectra obtained by 532 nm laser light of C60 in the presence of Yu1 (0.2 mM) in Ar-saturated benzonitrile. radical cation of the Yu1. Interestingly, this wavelength is apparently much longer than that of Yu0 revealing the effect of the attached polymer moieties on the delocalization of the radical cation over 600– 1500 nm. These observations indicate that the electron transfer process takes place via 3C*60 as shown in Scheme 2. This is supported by adding oxygen into the solutions where an intermolecular energy transfer from 3C*60 to oxygen emerges, suppressing the electron transfer events between 3C*60 and TPA derivatives. The kO2 was evaluated to be 8.0 3 109 M1 s1 on assuming [O2] ¼ 1 3 103 mol dm3. The rate constants of the bimolecular quenching (kq) were evaluated under the pseudofirst-order condi-

Scheme 2. Schematic diagram showing the electron transfer process of C60/Yu system in benzonitrile.

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J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 46 (2008)  molar extinction coefficient value of C60 at 1080 nm, the rate of the back electron transfer process (kbet)  from C60 to Yu0+ was evaluated as 6.40 3 109 M1 s1. Compared to Yu0/C60 mixture, the kbet value is significantly slower for Yu1/C60 mixture (2.28 3 109 M1 s1). In summary, a new blue-emitting polymer Yu1 was prepared via McMurry condensation reaction in the presence of zinc dust and TiCl4. This polymer displays good thermal stability and highly solubility in common organic solvents. In Ar-saturated toluene, the fluorescence decay profile of this blue-emitting polymer exhibits single exponential decay with lifetime of 7.12 ns. The electron transfer process from Yu1 to 3 * * . The C60 is more efficient than that from Yu0 to 3C60 efficiencies of the electron transfer process were eval* /Yu1, uated as 0.07 and 0.76 for 3C*60 /Yu0 and 3C60 respectively.

We are grateful for the financial support of National Natural Science Foundation of China (20676034), ECUST (YJ0142124), New Century Excellent Talents in University (NCET-050413), and Shanghai Municipal Educational Commission (SMEC-05SG35), respectively.

REFERENCES AND NOTES

Figure 5. Slow decay of C2 60 at long time-scale in C60/Yu0 (upper panel) and C60/Yu1 (lower panel) in Ar-saturated benzonitrile. * as a function of tions by monitoring the decay of 3C60 concentration of Yu0. The kq values were evaluated */ as 1.4 3 109 M1 s1 and 2.68 3 109 M1 s1 for 3C60 Yu0 and 3C*60 /Yu1, respectively. The efficiencies of the electron transfer process (Fet) were evaluated, from the ratio of the maximal concentration of the generated radical ions of C60 to the initial concentration of triplet C60, as 0.07 and 0.76 for 3C*60 /Yu0 and 3C*60 / Yu1, respectively. On the basis of the kq and Fet, the rates of the electron transfer process were evaluated as 9.8 3 107 M1 s1 for 3C*60 /Yu0 and 2.05 3 109 and 3 * C60 /Yu1, respectively. From these findings, it is clearly observed that the electron transfer process from Yu1 to 3C*60 is more efficient than that from Yu0 to 3C*60 . After reaching maximal concentration, Yu0+ and  C60 begin to decay as shown in Figure 5. The decay  time profile of C60 observed on the longer timescale obeys second-order kinetics in benzonitrile (inset of  Fig. 5), indicating that Yu0+ and C60 recombine after being solvated as free ions or as a solvent-separated ion pair. The slope of the second-order plot was evaluated as 5.35 3 105 M1 s1. On substituting the

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RAPID COMMUNICATION. A New Blue-Light ... 3Department of Chemistry, Faculty of Education, Tanta University, Egypt ... Beijing University of Chemical Technology, 15 BeiSanHuan East Road, Beijing 100029, People's Republic of China.

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