Emission color tuning of new fluorene-based alternating copolymers containing low band gap dyes

Synthetic Metals 155 (2005) 73–79

Emission color tuning of new fluorene-based alternating copolymers containing low band gap dyes Jonghee Lee, Nam Sung Cho, Jaemin Lee, Sang Kyu Lee, Hong-Ku Shim ∗ Center for Advanced Functional Polymers (CAFPoly), Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea Received 22 February 2005; received in revised form 1 June 2005; accepted 2 June 2005 Available online 13 October 2005

Abstract New fluorene-based alternating copolymers (PFPhPhCN, PFPhThCN, and PFThThCN) containing different comonomers have been designed and subsequently synthesized via Pd-catalyzed Suzuki polymerization. The synthesized polymers could be well characterized by 1 H NMR, FT-IR, and elemental analyses. These polymers were found to be thermally stable and readily soluble in common organic solvents. The UV–vis absorption maxima of PFPhPhCN, PFPhThCN, and PFThThCN were 399, 456 and 499 nm, and the PL maxima were 484, 539 and 620 nm, respectively. The emitting color of the homopolymer, poly(9,9-dioctylfluorene-2,7-diyl) (PDOF), could be tuned by incorporating various low band gap dyes into the polymer main-chain. The absorption and emission maxima of the copolymers were varied according to the type of incorporated aromatic group (thiophene or phenylene). In particular, PFThThCN exhibited almost pure red emission (chromaticity values x = 0.63, y = 0.37). © 2005 Elsevier B.V. All rights reserved. Keywords: Color tuning; Conjugated polymers; Light-emitting diodes (LED)

1. Introduction The development of electroluminescent polymers for the fabrication of full-color organic displays has given rise to intense academic and industrial research in this area. These polymers are of particular interest because their luminescence properties can be adjusted by manipulation of their chemical structures while their physical properties make them suitable for use in the spin coating and printing processes required to create large-area flatpanel displays [1–6]. Although polymer light-emitting diodes (PLEDs) have been developed to produce each of the three primary colors (red, green, and blue), to date only green and orange PLEDs meet the requirements of commercial use. A number of polyfluorene (PF) polymers and their derivatives are the preferred polymers in light-emitting applications because of their thermal and chemical stability, good solubility in common organic solvents, and high fluorescent quantum yields in a solid state [7–9]. However, problems have been encountered due to their tendency to exhibit excimer and/or aggregated formation ∗

Corresponding author. Tel.: +82 42 869 2827; fax: +82 42 869 2810. E-mail address: [email protected] (H.-K. Shim).

0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.06.005

as well as keto formation of the C-9 position [10], which lead to unwanted blue–green emission and fluorescence quenching. The study of PFs is ongoing, and numerous attempts have been made to improve their performance [11–13]. Although some attempts have been made to tune the color of blue-emitting PFs, appropriate red and green emitting materials that meet the requirements for display applications have not yet been obtained, and further improvements are necessary. Recently, manipulation of the emission wavelength and band-gap of PFs to emit the whole range of visible light has been achieved through copolymerisation [14]. In particular, alternative copolymerization is preferred because of two significant advantages over the blending of a dye chromophore into the polymer matrix: (1) aggregation is prevented and (2) efficient energy transfer from the polymer to the dye chromophore easily confines the singlet excitons [15]. Thus the introduction of narrower band gap comonomers into the fluorene monomer can be utilized for color tuning [16,17]. In this study, we have systematically prepared various fluorene-based copolymers introducing different narrow band gap moieties. We then determined the relationships between the structures of these copolymers and color tuning.

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J. Lee et al. / Synthetic Metals 155 (2005) 73–79

2. Experimental 2.1. Measurements NMR spectra were recorded on a Bruker AVANCE 400 spectrometer with tertramethylsilane as an internal reference. Mass spectra were obtained using AUTOSPEC ULTIMA spectrometer. Elemental analysis was performed using EA-1110FISONS elemental analyzer. The number- and weight-average molecular weight of the polymer was determined by gel permeation chromatography (GPC) on Waters GPC-150C instrument using tetrahydrofuran (THF) as eluent and monodisperse polystyrene as standard. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the polymer were performed under a nitrogen atmosphere at a heating rate of 10 ◦ C/min with a Dupont 9900 analyzer. UV–vis spectra were measured by using a Jasco V-530 UV/vis spectrometer. PL spectra were measured by using Spex Fluorolog-3 spectrofluorometer. Cyclic voltammetry measurement was performed on an AUTOLAB/PGSTAT12 at room temperature with a threeelectrode cell in a solution of Bu4 NBF4 (0.10 M) in acetonitrile at a scan rate of 50 mV/s. Polymer films were prepared by dipping platinum working electrodes into the polymer solutions and then air-drying. A platinum wire was used as a counter electrode and Ag/Ag+ electrode as a reference electrode. Film thickness was measured with a TENCOR alpha-step 500 surface profiler. 2.2. Device fabrication and characterization A hole injection layer of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonic acid) (PSS) (PEDOT:PSS, Bayer Al 4083) was spin-coated onto the pre-patterned ITO anode and dried (40 nm). Polymer solution was then spin-coated onto the PEDOT:PSS layer and dried (80 nm). Ca/Al (500 nm/800 nm) cathode was vacuumdeposited onto the polymer film through shadow mask at a pressure below 1 × 10−6 Torr, yielding an active area of 0.04 cm2 . For PLED measurements, EL spectra were obtained with a Minolta CS-1000. The current–voltage–luminance characteristics were measured with a current–voltage source (Keithley 238) and a luminescence detector (Minolta LS100). 2.3. Materials 2,7-Dibromofluorene, thiophene-2-yl-acetonitrile, N-bromosuccinimide (NBS), 4-bromo-benzaldehyde, 5-bromo-thiophene-2-carbaldehyde, (4-bromo-phenyl)-acetonitrile, Aliquat® 336, bromobenzen, toluene (0.8%, anhydrous) were purchased from Aldrich Co. Tetrakis(triphenylphosphine)palladium(0) was purchased from DNF Solution Co., and all other reagents and solvents with analytical-grade were used during the whole experiments, and all chemicals were used without further purification. 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)-9,9 -dioctylfluorene (5) was synthesized according to the literature procedure [18].

2.3.1. 5-bromo-2-thiopheneacetonitrile (1) N-bromosuccinimide (7.58 g, 40.2 mmol) was added to a solution of 2-thiopheneacetonitrile (5 g, 40.0 mmol) in N,Ndimethylformamide (30 mL). The reaction mixture was then stirred for 5 h at room temperature. The resulting mixture was extracted with methylene chloride and brine and then dried with MgSO4 . After filtering, the solvent was evaporated and the pure liquid product was obtained by column chromatography. The product yield was 74.8% (6.11 g). 1 H NMR (CDCl3 , ppm) δ 6.90 (d, 1H), 6.78 (d, 1H), 3.80 (s, 2H). 13 C NMR (CDCl3 , ppm) δ 132.23, 129.92, 127.53, 116.19, 112.15, 18.62 2.3.2. 2,3-Bis-(4-bromo-phenyl)-acrylonitrile (2) To a solution of (4-bromo-phenyl) acetonitrile (3.0 g, 15.3 mmol) at RT, 150 mL of methanol and 2.83 g (15.3 mmol, 1 eq) of 4-bromo-benzaldehyde was added. The mixture was then stirred for 30 min. t-BuOK five spoons were added to the solution, and the resulting precipitate was stirred for 1 h. The mixture was purified by several recrystallizations in hexane to provide 3.06 g (55%) of the title product as a white solid. 1 H NMR (CDCl3 , ppm) δ 7.43 (s, 1H), 7.52 (d, 2H), 7.55 (d, 2H), 7.59 (d, 2H), 7.73 (d, 2H). 13 C NMR (CDCl3 , ppm) δ 111.4, 117.3, 123.7, 125.2, 127.5, 130.7, 132.2, 132.3, 133.1, 141.0. 2.3.3. 2-(4-Bromo-phenyl)-3-(5-bromo-thiophen-2-yl)acrylonitrile (3) The synthesis is analogous to that of 2, with 62 % yield: 1 H NMR (CDCl3 , ppm) δ 7.08 (s, 1H), 7.32 (d, 1H), 7.42 (d, 1H), 7.45–7.56 (m, 4H). 13 C NMR (CDCl3 , ppm) δ 107.4, 117.6, 118.9, 123.4, 127.1, 130.7, 132.3, 132.5, 133.3, 133.7, 139.2. 2.3.4. 2,3-Bis-(5-bromo-thiophene-2-yl)-acrylonitrile (4) The synthesis is analogous to that of 2, with 53 % yield: 1 H NMR (CDCl3 , ppm) δ 7.06 (s, 1H), 7.34 (d, 1H), 7.44 (s, 1H), 7.47–7.56 (m, 2H). 13 C NMR (CDCl3 , ppm) δ 107.1, 116.9, 119.0, 123.3, 126.9, 130.2, 132.2, 132.4, 133.8, 139.1. 2.3.5. 2,7-Bis(4,4,5,5,-tetramethyl-1,3,2-dioxaborolan2-yl)9,9 -dioctylfluorene (5) It was synthesized according to the procedures outlined in the literatures [18]. 2.3.6. Polymerization Into 100 mL two-neck flask were added 2.33 mmol of comonomers – 2 (0.846 g), 3 (0.860 g), and 4 (0.874 g) – and 2.28 mmol (1.5 g) of 2,7-bis(4,4,5,5-tetramethyl1,3,2-dioxabororan-2-yl)-9 ,9 -dioctylfluorene in 25 mL of anhydrous toluene. Into the mixture was transferred in a dry box 0.026 g of water soluble Pd(0) complex, tetrakis(triphenylphosphine)palladium (1 mol%). Subsequently, 2 M aqueous sodium carbonate (5.4 mL, 10.7 mmol) deaerated for 30 min and the phase transfer catalyst, Aliquat® 336 (0.092 g, 0.23 mmol) in toluene purged under nitrogen for 1 h was transferred via cannula. The reaction mixture was stirred at 100 ◦ C for 3 days and then the excess amount of bromobenzene (0.036 g, 0.23 mmol), the end capper, dissolved in 1 mL of anhydrous toluene was added and stirring continued for 12 h.

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The reaction mixture is cooled to about 50 ◦ C, added slowly to a vigorously stirred mixture of 200 mL of methanol and 13 mL of 1 N aqueous HCl. The polymer fibers are collected by filtration and re-precipitation from methanol and acetone. The polymers are purified further by washing for 2 days in a Soxhlet apparatus with acetone to remove oligomers and catalyst residues, and column chromatographied with a chloroform solution of the polymer. The re-precipitation procedure in chloroform/methanol is then repeated a several times. The final polymers were obtained after drying in vacuo at 40 ◦ C, yielding 65% (PFPhPhCN), 71% (PFPhThCN), and 63% (PFThThCN). PFPhPhCN 1 H NMR (CDCl3 , ppm) δ aromatic and vinylene; 8.03–7.58 (15H), aliphatic; 3.50 (4H), 1.51–0.73 (∼30H). Anal. Calcd. for (C44 H46 N1 )n: C, 89.29; H, 8.34; N, 2.37. Found: C, 84.95; H, 8.61; N, 2.14. PFPhThCN 1 H NMR (CDCl3 , ppm) δ aromatic and vinylene; 7.76–7.44 (13H), aliphatic; 3.50 (4H), 1.50–0.66 (∼30H). Anal. Calcd. for (C42 H44 N1 S1 )n: C, 84.37; H, 7.92; N, 2.34; S, 5.36. Found: C, 82.47; H, 7.90; N, 2.35; S, 5.54. PFThThCN 1 H NMR (CDCl3 , ppm) δ aromatic and vinylene; 7.79–7.35 (11H), aliphatic; 3.50 (4H), 1.57–0.62 (∼30H). Anal. Calcd. for (C40 H42 N1 S2 )n: C, 79.55; H, 7.51; N, 2.32; S, 10.62. Found: C, 9.18; H, 7.62; N, 2.38; S, 11.02. 3. Results and discussion 3.1. Synthesis and characterization All the copolymers were prepared using the well-known Suzuki polymerization method between the diboronic ester of fluorene (5) and the narrow-band-gap monomers (2, 3, and 4). The synthetic procedures for the monomer and the alternating copolymers are shown in Schemes 1 and 2. The molecular struc-

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Table 1 Physical properties of the polymers Polymer

Yield (%)

Mna (×104 )

Mwa (×104 )

PDI

Tg (◦ C)

Td (◦ C)b

PDOF PFPhPhCN PFPhThCN PFThThCN

67 65 71 63

4.00 0.70 1.05 2.11

8.20 1.32 4.23 4.32

2.1 1.9 4.0 2.0

78 73 80 65

426 406 410 406

a b

Determined by GPC, relative to polystyrene standards. Temperature resulting in 5% weight loss based on initial weight.

tures of the monomers and polymers were confirmed with NMR, FT-IR, and elemental analyses. The polymers were found to be readily soluble in common organic solvents, such as chloroform, chlorobenzene, tetrahydrofuran, and toluene without any gel formation. The copolymers were spin-coated onto an ITO substrate and found to produce transparent and homogeneous thin films. The molecular weights of the polymers were determined with GPC, using tetrahydrofuran (THF) as an eluent and monodisperse polystyrene as a standard. The number-average molecular weights (Mn ) and the weight-average molecular weight (Mw ) of PFPhPhCN, PFPhThCN and PFThThCN ranged from 7000 to 21,000 (Mn ) and from 13,000 to 36,000 (Mw ), respectively, and their polydispersity indices (PDI) ranged from 1.9 to 4.0. The yields of the copolymers were from 63 to 71%. The polymerization results for the synthesized copolymers are summarized in Table 1. The thermal properties of the polymers were determined by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). All the polymers exhibited very good thermal stability, losing less than 5% of their weight on heating to approximately 400 ◦ C in TGA runs under a nitrogen atmosphere.

Scheme 1. Synthetic scheme of the monomers.

J. Lee et al. / Synthetic Metals 155 (2005) 73–79

76

Scheme 2. Synthetic scheme of the polymers.

Their glass transition temperatures (Tg ) were 65–80 ◦ C, respectively (Table 1). 3.2. Optical properties of the polymers The UV–vis absorption and photoluminescence (PL) spectra of the copolymers (PFPhPhCN, PFPhThCN, and PFThThCN) in solution and on thin films on quartz plates are shown in Figs. 1 and 2, and the results are summarized in Table 2. In dilute solution, the absorptions and emissions of the copolymers containing a thiophene unit are red-shifted to a greater extent

than those of the corresponding copolymers containing a phenylene unit. In particular, the absorption maximum of PFThThCN, which appears at 499 nm, is greatly red-shifted over 100 nm compared to that of PFPhPhCN because the thiophene unit of PFThThCN has a narrower energy band-gap than the phenylene unit of PFPhPhCN. With introduction of the narrower band-gap comonomers, the PL emissions around 420 and 440 nm, which are assigned to the emission of PDOF homopolymer [11,12], disappeared, whereas new emissions from the low energy band-gap comonomer units of the copolymer (PFArArCNs) appeared with the peak

Table 2 Optical properties and energy levels of the polymers Polymer

PDOF PFPhPhCN PFPhThCN PFThThCN a b c d e

Solution, λmax (nm)a

Film, λmax (nm)b

Absorption

PL emission

Absorption

PL emission

386 395 456 499

414 465 518 558

383 399 456 499

423 484 539 620

Eonset,ox (vs. SCE, V)

HOMOc /LUMOd (Eg e ) (eV)

1.41 1.43 1.28 1.12

−5.80/−2.87 (2.93) −5.83/−3.18 (2.67) −5.68/−3.33 (2.35) −5.52/−3.38 (2.14)

Measured in chloroform solution. Measured in thin film onto fused quartz plates. Calculated using the empirical equation: Ip (HOMO) = −(Eonset,ox + 4.39) (eV). Calculated from the HOMO level and optical band gap. The optical band gap, Eg , is taken as the absorption onset (value in parentheses) of the UV–vis spectrum of the polymer film.

J. Lee et al. / Synthetic Metals 155 (2005) 73–79

77

Fig. 1. (a) UV–vis absorption spectra and (b) PL spectra of the polymer in the chloroform solutions.

Fig. 2. (a) UV–vis absorption spectra and (b) PL spectra of the polymer coated onto fused quartz plates.

positions at 484, 539 and 620 nm for PFPhPhCN, PFPhThCN, and PFThThCN, respectively (Fig. 2). This appears to indicate that the energy transfer from the fluorene segment to the narrow band-gap comonomer occurred mainly along the polymer chain. In other words, the intramolecular energy transfer via efficient F¨oster energy transfer or a trapping mechanism along the polymer chain must be very efficient. Notably, Fig. 2 illustrates that the largest red-shift of the PL emission maximum of the copolymer with respect to that of PDOF is obtained when increasing thiophene aromatic units. The comonomer containing both cyano vinylene and two thiophene units, ThThCN, has the narrowest band gap energy among the comonomers, and hence the emission from copolymers containing ThThCN is even further red-shifted with respect to that of the homopolymer [19]. Of particular note is the PL emission maximum of PFThThCN at 620 nm, which suggests the strong possibility of pure red EL emission. The electrochemical properties of the polymers including the HOMO and LUMO energy levels were investigated to characterize and compare the electronic properties of the polymers through cyclic voltammetry (CV). The measurements were calibrated using ferrocene (4.8 eV below the vacuum level) [20] as the standard and are listed in Table 2. From the first oxida-

tion process, the HOMO energy levels of PFPhPhCN, PFPhThCN, and PFThThCN were estimated to be −5.83, −5.68 and −5.52 eV, respectively. Unfortunately, a reduction wave was hardly obtained. Hence the LUMO energy levels of the polymers were estimated from the onset of the absorption spectra of the copolymer films as this is the common method of obtaining the LUMO energy level of polyfluorene derivatives. The LUMO energy levels of PFPhPhCN, PFPhThCN, and PFThThCN were estimated as −3.18, −3.33 and −3.38 eV, respectively. The band gap energy of PFPhPhCN, PFPhThCN, and PFThThCN, obtained from the onset of the UV spectrum of each polymer, was 2.67, 2.35 and 2.14 eV, respectively, as shown in Fig. 3. Overall, the incorporation of the electron-rich thiophene and cyano moieties leads to higher HOMO and lower LUMO levels in all of the copolymers relative to those found for the PDOF homopolymer [21]. PFThThCN is found to have the smallest energy band gap, as anticipated by the PL spectra. A single layer PLED device with a configuration of ITO/PEDOT:PSS (40 nm)/polymer (80 nm)/Ca (500 nm)/Al (800 nm) was fabricated. The electroluminescence (EL) spectra of the copolymers are similar to their PL spectra, as shown in Fig. 4. The EL emission maximum is red-shifted to the greatest extent for PFThThCN, which was synthesized from a monomer

J. Lee et al. / Synthetic Metals 155 (2005) 73–79

78 Table 3 Performance of the devices with the polymers Polymer

EL emission λmax (nm)

CIE coordinate (x, y)

Turn-on voltagea (V)

Max. efficiency (cd/A) (%)

Max. brightness (cd/m2 )

PFPhPhCN PFPhThCN PFThThCN

474 537 627

(0.24, 0.39) (0.44, 0.55) (0.63, 0.37)

4.4 4.1 3.0

(0.46, 0.19) (0.19, 0.07) (0.05, 0.02)

1220 1640 480

a

Voltage needed for brightness of 1 cd/m2 .

containing cyano vinylene and two thiophene units. Moreover, as shown in Fig. 5, PFThThCN exhibited pure red emission; its chromaticity value is (x = 0.63, y = 0.37), which is almost identical to the standard red (x = 0.66, y = 0.34) demanded by the NTSC [22]. Further, its system also has low turn-on voltage, 3.0 V. The voltage–luminance (V–L) characteristics of the devices are shown in Fig. 6, and their related performances are summarized in Table 3. The maximum brightness of all the devices was in a range of approximately 500–1700 cd/m2 . The PFPhThCN device had the highest performance, with a maximum brightness of 1640 cd/m2 at 8.4 V. Studies to improve efficiency with better charge injection and more efficient charge recombination and further research into new applications in organic solar cells [23]

Fig. 5. CIE coordinates (x, y) of PFPhPhCN, PFPhThCN, and PFThThCN (NTSC-solid line).

Fig. 3. Energy band diagram of PFPhPhCN, PFPhThCN, and PFThThCN.

Fig. 6. Current–voltage–luminance curves of the PLED devices.

of utilizing the narrow energy band gap polymer-PFThThCN are currently underway. 4. Conclusion

Fig. 4. Electroluminescence (EL) spectra of the polymers.

We have developed various fluorene-based copolymers (PFPhPhCN, PFPhThCN, and PFThThCN) with the aim of achieving pure red-light emission. The PL and EL maxima of the copolymers were red-shifted with respect to those of the

J. Lee et al. / Synthetic Metals 155 (2005) 73–79

homopolymer, poly(9,9-diocylfluorene-2,7-diyl) (PDOF), upon introduction of cyano vinylene and aromatic groups from phenylene to thiophene. In particular, the PFThThCN device exhibited pure red emission with chromaticity values of (x = 0.63, y = 0.37), which are almost identical to those of standard red (x = 0.66, y = 0.34) demanded by the NTSC, and it also exhibited a low turn-on voltage. Acknowledgements We gratefully acknowledge financial support from the Center for Advanced Functional Polymers (CAFPoly) from the Korea Science and Engineering Foundation, as well as from BK21. References [1] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature (London) 394 (1999) 121. [2] A. Kraft, A.C. Grimsdale, A.B. Homes, Angew. Chem. Int. Ed. 37 (1998) 402. [3] M.T. Bernius, M. Inbasekan, J. O’Brien, W. Wu, Adv. Mater. 12 (2000) 1737. [4] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature (London) 357 (1992) 477. [5] A.J. Heeger, Solid State Commun. 107 (1998) 673. [6] (a) M.M. Alam, S.A. Jenekhe, Chem. Mater. 14 (2002) 4775; (b) J.A. Milkroyannidis, I.K. Spiliopoulos, T.S. Kasimis, A.P. Kulkarni, S.A. Jenekhe, Macromolecules 36 (2003) 9295; (c) J.H. Lee, D.-H. Hwang, Chem. Commun. 22 (2003) 2836; (d) H.K. Shim, J.I. Jin, Adv. Polym. Sci. 158 (2002) 194. [7] Q. Pei, Y. Yang, J. Am. Chem. Soc. 118 (1996) 7614. [8] W.-L. Yu, J. Pei, Y. Cao, W. Huang, A.J. Heeger, Chem. Commun. (1999) 1837. [9] J. Pei, W.-L. Yu, W. Huang, A.J. Heeger, Chem. Commun. (2000) 1631.

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