Combinatorial synthesis and screening for blue luminescent π-conjugated polymer thin film

Applied Surface Science 189 (2002) 319±326

Combinatorial synthesis and screening for blue luminescent p-conjugated polymer thin ®lm Yukiko Muramatsua,b,*, Takakazu Yamamotob, Tomohiro Hayakawac, Hideomi Koinumaa,c a

CRESTÐJapan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan b Chemical Resources Laboratory, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan c Ceramics Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

Abstract Two series of random copolymers (poly(PP-ran-MP)s which consist of p-phenylene, PP, and m-phenylene, MP, units and poly(PPy-ran-MPy)s which consist of p-pyridine, PPy, and m-pyridine, MPy, units) with various monomeric unit ratios were prepared. Thin ®lms of poly(PP-ran-MP)s were combinatorially deposited by vacuum evaporation with a ®xed mask and slit masks on a quartz glass, and poly(PPy-ran-MPy)s were superposed on the poly(PP-ran-MP)s layer. The thin ®lm of poly(PP-ranMP) containing the PP and MP units in a 5:5 ratio, poly(PP-ran-MP-5/5), showed 7.6 times stronger blue photoluminescence (PL), compared with the thin ®lms of poly(p-phenylene), PPP, and poly(m-phenylene), PMP, homopolymers. The PL intensity of the ®lm of poly(PP-ran-MP-5/5) was much stronger than the sum of the PL intensities of the ®lms of PPP and PMP. Furthermore, [poly(m-pyridine), PMPy/poly(PP-ran-MP-5/5)] bi-layer ®lm emitted blue light of about 3 times stronger intensity than the poly(PP-ran-MP-5/5) monolayer ®lm. An alternating copolymer of p-phenylene and m-phenylene, poly(PP-alt-MP-5/5) was prepared by a Stille coupling reaction and its PL peak was observed at about 50 nm shorter wavelength than that of poly(PP-ranMP-5/5). # 2002 Elsevier Science B.V. All rights reserved. PACS: 36:20 (Macromolecules and polymer molecules) Keywords: Combinatorial method; Polyphenylene; Polypyridine; Luminescence

1. Introduction Blue luminecsent materials are important material for full color ¯at panel display. Many studies on organic light emitting diode (LED) were recently published [1,2], and full color displays with organic compounds are already produced manufactually. LEDs using polymers are considered to have several *

Corresponding author. Present address: CRESTÐJapan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan.

advantages since the polymer has mechanical toughness, and large area LEDs are expected to be manufactured at low cost by using polymers. p-Conjugated polymers have attracted attentions as candidate materials for the polymer LED [3,4]. However, in order to obtain a blue emitting diode, it is necessary to obtain a p-conjugated polymer with a short effective p-conjugation length by introducing a bulky substituent [5] or non-conjugation unit in the polymer [6]. We have synthesized copolymers constituted of p-conjugated units and non-conjugated units, and investigated their luminescent properties.

0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 1 0 1 0 - 8

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2. Experimental 2.1. Measurements IR spectra were recorded on a JASCO IR-810 spectrometer or PERKIN ELMER Spectrun One FT-IR spectrometer equipped with an Auto IMAGE system. UV±visible and photoluminescence (PL) spectra in solution were measured with a Shimadzu UV-3100PC and a Hitachi F4010 spectrometers, respectively. UV±visible and PL spectra of vacuum deposited ®lms were measured with a JASCO microspot photospectrometer. TAG curves were obtained with a Shimadzu thermoanalyzer DT-30. Thickness of the vacuum deposited ®lm was measured by a stylus method using a Veeco Dectak3 ST surface pro®le measuring system. 2.2. Preparation of poly(PP-ran-MP)s Magnesium grains were activated in a Schlenk tube by stirring with a magnetic bar under argon. A mixture of 1,4-dibromobenzene and 1,3-dibromobenzene (1,4-dibromobenzene/1,3-dibromobenzene: 10/0, 8/2, 6/4, 5/5, 4/6, 2/8 or 0/10) in THF was added into the Schlenk tube containing magnesium (Mg/dibromobenzene: 1:1.2 molar ratio), and stirred for several hours at room temperature to obtain Grignard reagents. Dichloro(2,20 -bipyridine)nickel(II), NiCl2(bpy), was added as a catalyst, and the reaction mixture was stirred at 75 8C for 40 h under argon. The reaction mixtures were poured into diluted hydrochloric acid, and washed repeatedly with diluted hydrochloric acid, diluted with aqueous NH3 solution, water and methanol. The polymer was collected by ®ltration and dried. The obtained polymer was further treated with LiAlH4 in THF to convert terminal C±Br bonds into C±H bonds. 2.3. Preparation of poly(PPy-ran-MPy)s Two THF solutions containing 2,5-dibromopyridine and 3,5-dibromopyridine, both in 0.50 M, were prepared under argon. Two THF solutions at various volume ratios (2,5-dibromopyridine solution (ml)/3,5-dibromopyridine solution (ml): 10/0, 8/2, 6/4, 5/5, 4/6, 2/8 or 0/10) were added to anhydrous DMF solutions (10 ml) containing bis(1,5-cyclooctadiene)nickel(0) Ni(cod)2

(1.5 g), 1,5-cyclooctadiene (0.60 ml), and 2,20 -bipyridyl (0.80 g) in a Schlenk tube under argon, and the reaction mixture was stirred at 60 8C for 48 h. The obtained polymers were poured into diluted hydrobromic acid, and washed with diluted hydrobromic acid, NH3 (aq.), a hot water of ethylenediaminetetraacetic acid disodium salt, water and methanol, repeatedly, and dried under vacuum. 2.4. Preparation of poly(PP-alt-MP-5/5) A hydrous DMF solution containing 2.02 g (5 mmol) of 1,4-bis(trimethylstannyl)benzene, 1.18 g (5 mmol) of 1,3-dibromobenzene and 5 mol% of tetrakis(triphenylphosphine)palladium(0) Pd(PPh3)4 were stirred at 75 8C under argon. After 48 h, the reaction mixture was poured into diluted hydrochloric acid. The obtained polymer was washed with diluted hydrochloric acid, diluted aqueous NH3 solution, water and methanol, repeatedly, and dried in vacuo. 3. Results and discussion 3.1. Preparation of copolymers Poly(PP-ran-MP)s and poly(PPy-ran-MPy)s which have various para/meta unit ratios (para/meta: 10/0, 8/ 2, 6/4, 5/5, 4/6, 2/8 and 0/10) were prepared by organometallic dehalogenation polycondensations of the corresponding dibromoaromatic compounds. The obtained copolymer was treated with LiAlH4 in order to convert the terminal C±Br bond into a C±H bond as shown in Scheme 1. An alternative copolymer, poly(PP-alt-MP-5/5) was also obtained by Stille coupling of 1,4-bis(trimethylstannyl) benzene and 1,3-dibromobenzene in the presence of Pd(PPh3)4 as depicted in Scheme 1. The obtained polymers were characterized by IR spectra and elemental analysis. 3.2. Combinatorial thin ®lm preparation and characterization Poly(PP-ran-MP)s and poly(PPy-ran-MPy)s were combinatorially vacuum evaporated on a quartz glass with 8  8 hatched ®xed mask and slit masks as exhibited in Fig. 1. At ®rst, poly(PP-ran-MP) was

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Scheme 1. Preparation of poly(PP-ran-MP)s, poly(PPy-ran-MPy)s and poly(PP-alt-MP).

Fig. 1. Combinatorial process using a set of slit mask systems.

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Table 1 Film thickness of the vacuum deposited poly(PP-ran-MP)s and poly(PPy-ran-MPy)s library and lmax data of poly(PP-ran-MP)s and poly(PPy-ran-MPy)s Ê) Film thickness (A

p/m

lmax (nm) Solutiona

Film

Poly(PP-ran-MP-p/m) 10/0 950 8/2 950 6/4 1100 5/5 1000 4/6 1250 2/8 1100 0/10 1300

± ± 301 285 282 279 266

311 303 291 280 258 251 250

Poly(PPy-ran-MPy-p/m) 10/0 1500 8/2 600 6/4 1200 5/5 850 4/6 650 2/8 900 0/10 650

333 313 311 301 300 290 281

314 311 309 300 298 270 265

a

UV±visible absorption spectra of poly(PP-ran-MP)s were measured in THF, whereas those of poly(PPy-ran-MPy)s were measured in 1,1,1,3,3,3-hexa¯uoro-2-propanol.

deposited on areas separated with the slit mask. Then, poly(PPy-ran-MPy) was superposed on the poly(PP-ran-MP) layer using an acrossed direction slit mask. The obtained library had 14 kinds of monolayer ®lms and 49 kinds of bi-layered ®lms. Thickness of each ®lm was measured with Dektak and the obtained values are summarized in Table 1. Chemical structures of the vacuum evaporated polymers were con®rmed by FT-IR spectroscopy, which revealed that vacuum evaporated polymers kept original chemical structures. 3.3. UV±visible spectrum of the monolayer ®lm Poly(PP-ran-MP) was soluble in organic solvents, such as THF, CHCl3, and CH2Cl2, when the PP/MP (p-phenylene/m-phenylene) ratio was 6/4, 5/5, 4/6, 2/8 and 0/10. UV±visible absorption spectra of poly(PP-ran-MP)s in the vacuum deposited thin ®lms were measured normally, and shown in Fig. 2. lmax data of poly(PP-ran-MP)s in THF and the vacuum evaporated ®lms are summarized in Table 1. As exhibited in the inset of Fig. 2, the lmax position of poly(PP-ran-MP)s shifts to a longer wavelength with increase in the PP unit.

Fig. 2. UV±visble spectra of poly(PP-ran-MP)s in solid state. The para/meta ratio: 10/0 (±), 6/4 (- -), 5/5 (Ð
Ð), 4/6 (Ð Ð Ð) and 0/10 (Ð

Ð), respectively. The inset shows the lmax data of poly(PP-ran-MP)s.

Poly(PPy-ran-MPy)s with various PP/MP ratios were soluble in formic acid, tri¯uoroacetic acid and 1,1,1,3,3,3-hexa¯uoro-2-propanol. The lmax data of poly(PPy-ran-MPy)s in 1,1,1,3,3,3-hexa¯uoro-2-propanol and vacuum deposited thin ®lms are also given in Table 1. The lmax position of poly(PPy-ran-MPy)s also shifts to a longer wavelength with an increase in the PPy unit. 3.4. PL spectra of the monolayer ®lm The PL spectra of the poly(PP-ran-MP) monolayer ®lms and their relative intensities are depicted in Fig. 3(a). The maximum emission wavelength, lem, of poly(PP-ran-MP)s is shifted to a longer wavelength with increase in proportion of the PP unit and the PL intensity becomes maximum at a PP=MP ˆ 5=5 ratio. Poly(PP-ran-MP-5/5) indicates 7.6 times stronger PL intensity than poly(p-phenylene) (PPP). Both PPP and poly(pyridine-2,5-diyl) take a rigid rod-like linear structures [7]. Random poly(PP-ranMP-5/5) will take a random structure and will not assume an aligned structure. In this sense, poly(PPran-MP-5/5) is considered to take a zero-dimensional structure in molecular level. Its strong PL may be due to effective con®nement of excited electronic state(s) (e.g., exciton state) in segments with appropriate p-conjugation lengths. PL spectral data of poly(PPy-ran-MPy)s are given in Fig. 3(b). The lem position of poly(PPy-ran-MPy)s shifts to a longer wavelength with increase in the content of the PPy unit. PL intensity of the

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Fig. 5. (a) UV±visble spectra and (b) PL spectra of poly(PP-ranMP-5/5) and poly(PP-alt-MP-5/5) in the vacuum deposited ®lm. Excited with 350 nm Xe light. Fig. 3. PL spectra of (a) poly(PP-ran-MP)s and (b) poly(PPy-ranMPy)s. The para/meta ratio: 10/0 (±), 8/2 (- -), 5/5 (Ð
Ð), 2/8 (Ð Ð Ð), and 0/10 (Ð

Ð), respectively. The inset of (a) shows the PL intensity of poly(PP-ran-MP) vs. mol% of the PP unit in the copolymer. The ®lm was irradiated with 350 nm Xe light.

poly(PPy-ran-MPy) ®lms are weaker than that of poly(PP-ran-MP) ®lms. In (CF3)2COOH, the PL intensity of poly(PPy-ran-MPy)s varies analogously with the change of the composition of the polymer as shown in Fig. 4. The PL intensity in (CF3)2COOH indicates that poly(PPy-ran-MPy) itself gives a strong

Fig. 4. PL intensity of poly(PPy-ran-MPy)s in 1,1,1,3,3,3-hexa¯uoro-2-propanol vs. mol% of the PPy unit in the copolymer.

emission in (CF3)2COOH, however, a strong quenching of PL occurs in the solid state. PMPy shows narrow and weak PL at 326 nm. 3.5. Comparison of poly(PP-ran-MP-5/5) and poly(PP-alt-MP-5/5) Fig. 5 exhibits UV±visible and PL spectra of vacuum deposited poly(PP-alt-MP-5/5) and poly(PP-ran-MP5/5) monolayer ®lms. Poly(PP-alt-MP-5/5) gives lmax at 281 nm; the peak position locates at a considerably shorter wavelength, compared with the lmax position of poly(PP-ran-MP-5/5). It is reported that PP trimer H(p-C6H4)3H exhibits lmax at 280 nm in chloroform solution [8,9]. These data suggest that poly(PP-altMP-5/5) has an effective p-conjugation length similar to that of the PP trimer, whereas poly(PP-ran-MP-5/5) contains units with longer effective p-conjugation lengths. Similar difference was observed in the absorption edge between poly(PP-ran-MP-5/5) and poly(PP-alt-MP-5/5). The absorption band edge of poly(PP-alt-MP-5/5) appears at a shorter wavelength by about 70 nm than that of poly(PP-ran-MP-5/5), supporting the assumption that poly(PP-ran-MP-5/5)

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contains units with various effective p-conjugation lengths. In accord to the difference in the absorption edge, poly(PP-alt-MP-5/5) emits light at about 60 nm shorter wavelength than poly(PP-ran-MP-5/5). The intensity of PL is weaker in poly(PP-alt-MP-5/5). 3.6. PL data of bi-layer ®lms PL spectra of 49 bi-layer ®lms (cf. experimental) of [poly(PPy-ran-MPy)/poly(PP-ran-MP)] were also measured. These bi-layer ®lms emit bright blue to green light, and the PL intensity data are summarized in Fig. 6. A [PMPy/poly(PP-ran-MP-5/5)] bi-layer gives strong blue emission, in spite of the weak PL of the PMPy thin ®lm. The PL pro®le of [PMPy/ poly(PP-ran-MP-5/5)] bi-layer ®lm is similar to that of poly(PP-ran-MP-5/5), however, the emission is considerably stronger than that of the poly(PP-ran-MP-5/5) monolayer thin ®lm. Poly(PP-ran-MP-5/5) layer seems to work as an emission layer in this bi-layer ®lms, whereas PMPy seems to capture the irradiated light and transfers the captured energy to the poly(PPran-MP-5/5) layer. Eventually, a strong blue emission was obtained from the bi-layer system. It is reported that the pyridine unit behaves as an antenna unit to gather photo-energy [10]. In this study, PMPy layer

Fig. 6. (Top) PL spectra of poly(PP-ran-MP-5/5) monolayer ®lm and [PMPy/poly(PP-ran-MP-5/5)] bi-layer ®lm. (Bottom) PL intensity of the [poly(PPy-ran-MPy)/poly(PP-ran-MP)] bi-layer ®lm depending on the composition of the two polymers.

Fig. 7. Combinatorial process using a set of slide mask system for an optimization of ®lm thickness.

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Fig. 8. PL intensity of [PMPy/poly(PP-ran-MP-5/5)] bi-layer ®lms. Exited at 350 nm.

also seems to behave as an antenna layer to gather the photo-energy. 3.7. Combinatorial optimization of PL of [PMPy/ poly(PP-ran-MP-5/5)] bi-layer ®lm Since [PMPy/poly(PP-ran-MP-5/5)] bi-layer shows a strong blue PL, we have optimized the PL intensity by changing the ®lm thickness combinatorially as depicted in Fig. 7. The thickness of vacuum evaporated poly(PP-ran-MP-5/5) and PMPy ®lms were controlled with acrossed two slide masks and the ®lms were formed on a quartz glass substrate with thickness Ê. ranging from 220 to 2500 A Table 2 Film thickness of the [PMPy/poly(PP-ran-MP-5/5)] library Pixel

Ê) Film thickness (A

Poly(PP-ran-MP-5/5) 1G 2G 3G 4G 5G 6G

2100 1900 1350 920 480 220

PMPy 7A 7B 7C 7D 7E 7F

2500 1600 1000 750 500 250

The obtained library has six monolayer ®lms of poly(PP-ran-MP-5/5), six monolayer ®lms of PMPy, and 36 bi-layer ®lms. The thickness of obtained ®lms was measured with Dektak and the data are given in Table 2. PL intensity of these monolayer and bi-layer ®lms is depicted in Fig. 8. Pixel 1C, which has a Ê )/poly(PP-ran-MP-5/5) (ca. 2000 A Ê )] [PMPy(1000 A structure, indicates the strongest PL intensity in the library. The PL intensity of the this bi-layer ®lm is stronger by about 3 times than that of poly(PP-ran-MP-5/5) monolayer ®lm. 4. Conclusion Strong blue luminescent polymer poly(PP-ran-MP5/5) has been prepared. Combinatorial optimization indicates that the combination of [PMPy/poly(PP-ranMP-5/5)] gives the strongest blue luminescent bi-layer ®lm. By the combinatorial optimization of the ®lm Ê )/poly(PP-ran-MP-5/5) (ca. thickness [PMPy(1000 A Ê 2000 A)], bi-layer ®lm has been found to emit blue light most strongly. References [1] S. Miyata, H.S. Nalwa (Eds.), Organic Electroluminescent Materials and Devices, Gordon and Breach, Amsterdam, 1997. [2] C.H. Chen, C.W. Tang, Macromol. Symp. 125 (1997) 1.

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[7] T. Yamamoto, T. Maruyama, Z.-H. Zhou, T. Ito, T. Fukuda, Y. Yoneda, F. Begum, T. Ikeda, S. Sasaki, H. Takezoe, A. Fukuda, K. Kubota, J. Am. Chem. Soc. 116 (1994) 4832. [8] A.E. Gillam, P.H. Hey, J. Chem. Soc. (1939) 1170. [9] A. Wenzel, J. Chem. Soc. 21 (1953) 403. [10] T. Yamamoto, Y. Yoneda, K.-I. Kizu, Macromol. Rapid Commun. 16 (1995) 549.