Photoluminescence properties of copolymers with iridium-complex Ir(thq)2(dbm) units in the fluorene main chain

Available online at www.sciencedirect.com

Physics Procedia 14 (2011) 34–37

9th International Conference on Nano-Molecular Electronics

Photoluminescence properties of copolymers with iridium-complex Ir(thq)2(dbm) units in the fluorene main chain Taiju Tsuboia,*, Hui-Fang Shib Yosuke Nakaia, Shu-Juan Liub, Qiang Zhaob, Wei Huangb a Faculty of Engineering, Kyoto Sangyo University, Kamigamo, Kita-ku, Kyoto 603-8555, Japan. Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, Wenyuan Road 9, Nanjing 210046, China.

b

Abstract Copolyfluorenes with various concentrations (2, 4, 8, 12, 16, and 20 %) of red-light emitting Ir(III)-complex Ir(thq)2(dbm) units on the main chain are synthesized and the photoluminescence is investigated at 298-12 K. The intensity of the emission at about 620 nm increases with increasing the Ir-complex concentration from 2 % to 4 % but decreases with increasing the concentration from 4 % by the concentration quenching, while the blue emission from polyfluorene also decreases by energy transfer to Ircomplex. The two emission intensities increase with decreasing temperature from 298 K to 12 K. The red emission shows a red shift with increasing the concentration, by the solid state solvation effect. © 2010 Published by Elsevier B.V. Keywords: PFO copolymer; fluorene; Ir-complex; photoluminescence; energy transfer

1. Introduction Polymeric white light emitting diode has attracted considerable attention because of fabrication of large-area flat panel displays and lighting by low-cost solution processing. There are several methods to fabricate the white light emission devices, including the use of polymer blends and mixing polymer with phosphorescent complexes. The blend and mixing methods lead to phase separation and color instability. To avoid them, OLED with single polymer material has been investigated. White emission from single polymer is obtained from copolymer where green and red emitting chromophores are incorporated into the polymer main chain, side chain, or chain ends [1,2]. Polyfluorene (PFO) has been used as main chain for the white emissive copolymers since it has a wide band gap and emits intense blue light [3]. Efficient white-light electroluminescence from a single copolyfluorene has been realized with fluorescence- and phosphorescence-emitting units based on a conjugated polymer grafted with an iridium complex [4,5]. The Ir-complex concentration of 0.2 or 0.3 mole % has been usually chosen. High concentrations of Ir-complexes lead to very weak blue emission and intense red emission because of effective energy transfer from host to Ir-complex. Therefore most studies have been made for lightly-doped copolyfluorenes.

* Corresponding author. Tel.: +81-75-705-1899; fax: +81-75-705-1899. E-mail address: [email protected].

1875-3892 © 2011 Published by Elsevier Ltd. doi:10.1016/j.phpro.2011.05.008

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Very few studies have been made for highly-doped copolyfluorenes. The present work investigates the optical properties of not only lightly but also highly Ir-complex-doped copolyfluorenes. As the dopant we use Iridium (III)-complex Ir(thq)2(dbm) which is incorporated in the main chain, where Ir, thq, and dbm mean iridium, 2-(thiophen-2-yl) quinoline, and 1,3-diphenylpropane-1,3-dione, respectively. 2 Experimental results and discussion The PFO copolymers were synthesized through Suzuki polycondensation reaction from fluorene monomers and Ir-complex monomer Ir(thq)2(dbm) according to a previous report [6]. The feed ratios of the Ir(III) complex in the polycondensation reaction were 2, 4, 8, 12, 16, and 20 mole %, the corresponding fluorene copolymers are called PFO-Ir2, Ir4, Ir8, Ir12, Ir16, and Ir20, respectively. Molecular structure of the copolymer is shown in Fig. 1. The iridium-complex contents were confirmed by the 1H NMR spectra.

m

n O

O Ir

N S 2

m=100, PFO m=98,n=2, PFO-Ir2 m=96,n=4, PFO-Ir4 m=92,n=8, PFO-Ir8

Fig. 1. Chemical structure of the fluorene copolymers containing Ir(thq)2(dbm).

Thin films of PFO-Ir-complex copolymers and PFO were spin-coated on quartz plate after dissolving the copolymer powders in tetrahydofuran (THF) solution. Absorption and photoluminescence (PL) spectra were measured with a Shimadzu UV-3100 spectrophotometer and a Spex FluoroMax-3 spectrofluorometer, respectively. The absorption spectra of PFO-IrN (N=2,4,8,12,16,20) films consist of absorption due to PFO at λ<440 nm and weak absorption due to Ir(thq)2(dbm) at λ>440 nm. The same was obtained for the absorption spectra of PFO-IrN in THF solution. Fig. 2 shows the PL spectra of PFO-Ir2 film at 279-12 K. Red emission band due to Ir(thq)2(dbm) and blue emission due to fluorene are observed. Their intensities are comparable to each other. The red emission band has a vibronic structure with intense peak at 616 nm, shoulder at about 665 nm, and a very weak shoulder at about 730 nm. The red and blue emissions enhance with decreasing temperature from 298 K to 12 K. The red emission shows longer decay time than the blue emission, e.g., 1.15 μs for the red emission in PFO-Ir4 film, while 4 ns for the blue emission. This supports the assignment that the red emission is due to Ir(thq)2(dbm) phosphorescent molecule. 5

PL intensity (arb. units)

4 5 4 3

3 2

2

1

4 3 2

blue emission red emission

4

PL intensity (relative)

PFO-Ir2 1 279 K 2 202 3 127 12 4

3

2

1

1 1 0 400 440 480 520 560 600 640 680 720 760 wavelength (nm)

Fig. 2 Photoluminescence (PL) spectra of the PFO-Ir2 film at various temperatures. Excitation was made at 340 nm.

0 0

5

10

15

concentration of Ir-complex (%)

Fig.3 Intensities, at 12 K, of the 433 nm PFO and 620 nm Ir(thq)2(dbm) emissions plotted against Ir-concentration.

The red emission intensity increases with increasing the Ir(thq)2(dbm) concentration from 2 % to 4 % but decreases with increasing the concentration from 4 % together with the blue emission intensity from the host (Fig. 3). The intensity of the blue emission relative to the red emission decreases with increasing the concentration. The ratio

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of the 440 nm emission peak height to the 616 nm emission peak height is 1.42:1 in PFO-Ir2 at room temperature, indicating that white emission is obtained from the lightly doped PFO-Ir2 film. The blue emission is considerably weak in PFO-Ir16 film (Fig. 4). The blue emission disappears in PFO-Ir20 film and PFO-Ir20 powder (not shown). These results indicate that the energy transfer from fluorene host to Ir(thq)2(dbm) dopant enhances with increasing the concentration of Ir(thq)2(dbm) units. At the same time the intensity of the red emission decreases with increasing the concentration from 4 %, by the concentration quenching. Like the case of PFO-Ir2, the red emission increases with decreasing temperature from 298 to 12 K in PFO-Ir16 and PFO-Ir20 films (Fig. 5). 5 4

4 3

1.2 PFO-Ir16 1 293K 2 220 3 145 4 70 5 12

3

2 x100

1

2 5

1

0

PL intensity (normalized)

PL intensity (arb. units)

5

PFO-Ir16 625nm peak height PFO-Ir20 film red emission PFO-Ir20 powder Ir(thq)2dbm powder red emission

1.0 0.8 0.6 0.4 0.2 0.0

400 440 480 520 560 600 640 680 720 760 800

0

50

wavelength (nm)

Fig. 4 Photoluminescence (PL) spectra of the PFO-Ir16 at various temperatures, excited at 340 nm. The spectrum in a range of 400-600 nm at 12 K is enlarged by 100 times.

100 150 200 temperature (K)

250

300

Fig. 5 Temperature dependence of the photoluminescence intensities of the red emissions from PFO-Ir16, PFO-Ir20, and neat Ir(thq)2(dbm) films, and PFO-Ir20 powder.

Fig. 6 presents the PL excitation (PLE) spectra for red and blue emissions of PFO-Ir2 and PFO-Ir16 at 12 K. The PLE bands for the red emission in PFO-Ir2 appear at 602, 532, 434, 402, and 368 nm, where the last three bands coincide with the PLE bands for blue emission. The sharp band at 434 nm corresponds to the β-absorption band of PFO, while the 368 nm PLE band to the α-band of PFO [7,8]. The PLE spectrum for the red emission in PFO-Ir16 consists of broad bands at 602, about 532, about 430, 366, and about 340 nm. All the PLE bands correspond to the absorption bands of Ir(thq)2(dbm). Unlike the case of PFO-Ir2, there are no PLE bands which correspond to the absorption due to PFO. Therefore it is suggested that energy transfer from fluorene host to Ir(thq)2(dbm) occurs in the copolymer with small concentration of Ir(thq)2(dbm), but it does not occur in highly doped copolymer.

1.6

PLE 12 K 1 2 3 4

2

1.4 1.2

PFO-Ir2 for blue Em PFO-Ir2 for red Em PFO-Ir16 for blue Em PFO-Ir16 for red Em

1

1.0

2

0.8 0.6 0.4

4 3

0.2 0.0

280 320 360 400 440 480 520 560 600 640 wavelength (nm)

Fig. 6 Photoluminescence (PL) excitation spectra for blue and red emissions of PFO-Ir2 and PFO-Ir16 at 12 K.

2.5

6

neat Ir(thq)2dbm film

5 PL intensity (arb. units)

PL intensity (arb. units)

1.8

2.0

1 2 3 4 5 6

4

1.5 1.0

294 K 160 100 70 40 12

3 2

0.5

1 0.0 560

600

640

680

720

760

800

840

wavelength (nm)

Fig. 7 Photoluminescence (PL) spectra of the neat Ir(thq)2(dbm) film at various temperatures, excited at 460 nm.

Temperature dependence of PL spectra of neat Ir(thq)2(dbm) film is shown in Fig. 7. Like the cases of PFOIrN(N=2,16,20) films and of PFO-Ir20 powder (Fig. 4), the red emission intensity increases with decreasing

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temperature. The peak of the intense band is observed at 638 nm at 12 K, which is red shifted from the 616 nm peak of PFO-Ir2 film. The shift enhances with increasing concentration of Ir-complex as summarized in Table I. This table also shows the blue emission due to fluorene in these copolymers. Unlike the red emission, the blue α- and βphase PFO emission bands locate at almost the same position for the variation of the concentration. It was found that the two phases are present in PFO-Ir2 film, but the α phase disappears in PFO-Ir2 powder. This indicates that the α phase with disordered and twisted PFO structure is unstable in the ordered PFO-Ir2 crystalline state. Table I. Peak positions (in nm) of emission bands of PFO-IrN (N= 2, 4, 8, 12, 16, 20) films, PFO-Ir20 powder, and neat Ir(thq)2(dbm) film, neat non-doped PFO film, and non-doped PFO powder at 12 K.

PFO-Ir2 film PFO-Ir4 film PFO-Ir8 film PFO-Ir12 film PFO-Ir16 film PFO-Ir20 film PFO-Ir20 powder neat Ir(thq)2(dbm) film neat PFO film PFO powder

α-PFO 423 419 420 420

β-PFO 442 442 441 441 441

424 424

444 444

Ir(thq)2(dbm) 616, 671 623, 678 623, 678 623, 677 629, 684 631, 686 635, 689 638, 695

The red shift of the Ir(thq)2dbm emission band is observed with increasing the Ir(thq)2dbm concentration. This is explained by the solid state solvation effect [9]. When the Ir(thq)2dbm concentration is increased, the distance between nearest-neighbor Ir(thq)2dbm molecules decreases, resulting in increase of the local polarization field and then leading to red shift by the solvation effect. It is noteworthy that, in neat Ir(thq)2dbm film, an intense emission peak appears at 650 nm in a temperature range of 220-50 K in addition to the 638 nm emission. The 638 nm emission is understood the red shift of the 616 nm emission, which is observed in lightly-doped PFO-Ir2 film, by the solid solvation effect. On the other hand, the 650 nm emission is suggested to arise from aggregation of Ir(thq)2dbm like dimer because of 100 % doped film. 3. Summary The intensity of the red emission in PFO-IrN (N=2,4,8,12,16,20) films increases with increasing Ir(thq)2(dbm) concentration from 2 % to 4 % but decreases with increasing the concentration from 4 % together with the blue emission due to PFO host. No blue emission appears in the film with concentration of 20 %. From the observation that the emissions from both PFO host and Ir(thq)2(dbm) dopant decrease rapidly with increasing the concentration from 4 %, it is concluded that concentration quenching occurs above 4 %. The PLE spectra for the red emission indicate that energy transfer from PFO host to Ir(thq)2(dbm) occurs in the copolymer films with small concentration of Ir(thq)2(dbm), but it does not occur in highly 16 % doped copolymer. Although PFO-IrN films and powders show similar PL spectra, the peak shift appears between the films and powders, e.g., PFO-Ir20 powder shows red shift by 4 nm from PFO-Ir20 film. The red shift indicates presence of intermolecular interaction in PFO-IrN microcrystals. References: [1] C.F. Liao, B.Y. Hsieh, Y. Chen, J. Polymer Sci. A: Polymer Chem. 47 (2009) 149. [2] L.-R. Tsai, C.-W. Li, Y. Chen, J. Polym. Sci. Part A: Polym. Chem 46 (2008) 5945. [3] J. Liu, Q.G. Zhou, Y.X. Cheng, D.G. Ma, X.B. Jing, F.S. Wang, Adv. Funct. Mater. 16 (2006) 957. [4] J. X. Jiang, Y. H. Xu, W. Yang, R. Guan, Z. Q. Liu, H. Y. Zhen, Y. Cao, Adv. Mater. 18 (2006) 1769. [5] X. Chen, J. L. Liao, Y. Liang, M. O. Ahmed, H.-E. Tseng, S.-A. Chen, J. Am. Chem. Soc. 125 (2003) 636. [6] H.-F. Shi, S.-J. Liu, H.-B. Sun, W.-J. Xu, Z.-F. An, J. Chen, S. Sun, X.-M. Lu, Q. Zhao, W. Huang, Chem. Eur. J. 16 (2010) 12158. [7] F.B. Dias, J. Morgado, A.L. Macanita, F.P. da Costa, H.D. Burrows, A.P. Monkman, Macromol. 39 (2006) 5854. [8] R. Zhu, J.M. Lin, C. Zheng, W. Wei, W. Huang, Y.H. Xu, J.B. Peng, Y. Cao, J. Phys. Chem. B 112 (2008) 1611. [9] V. Bulovic R. Deshpande, M.E. Thompson, S.R. Forrest, Chem. Phys. Lett. 308 (1999) 317.