J. Phys. Chem. C 2007, 111, 6883-6888
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Fluorene-Centered, Ethynylene-Linked Carbazole Oligomers: Synthesis, Photoluminescence, and Electroluminescence Zujin Zhao,† Yixin Zhao,‡ Ping Lu,*,† and Wenjin Tian*,‡ Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, and Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: January 10, 2007; In Final Form: March 5, 2007
Two monodisperse fluorene-centered, ethynylene-linked carbazole oligomers (Cz4F and Cz6F) were synthesized and fully characterized. They were highly fluorescent and emitted bright blue in solutions and in films. Single- and double-layer devices based on them were fabricated to investigate their electroluminescence (EL) as well as their hole-transport properties. Cz4F, doped with PVK, in a double-layer device exhibited a rather broad emission spectrum with a maximum brightness of 926.6 cd m-2 at 17.0 V, which indicated that it might have potential use in white organic light-emitting diodes (WOLEDs). Moreover, Cz6F exhibited a bifunctional property, blue- or white-light-emitting property, and hole-transport ability, which qualified it as a light-emitter as well as hole-transport material.
Introduction Since Friend et al.1 first used poly(p-phenylenevinylene) (PPV) as the light-emitting material in an organic light-emitting diode (OLED) in 1990, a great number of conjugated polymers based on different aromatic building blocks have been designed and synthesized. For example, alkylated polyfluorenes developed by Dow Chemical in the early 1990s were introduced as blue emitters for OLEDs.2 In the past decade, fluorene-based conjugated polymers have attracted intensive attention due to their high photoluminescence (PL) and electroluminescence (EL) quantum yields, good thermal stabilities, solubilities, and facile functionalization at the C-9 position of fluorene. Moreover, some fluorene homo- and copolymers exhibit thermotropic liquidcrystalline behavior and can be aligned and applied in polarized OLEDs3 as well as in organic field effect transistors (FETs) with high mobility.4 However, a stable blue-light-emitting OLED based on polyfluorene is still a challenge because of the appearance of undesired, tailed, green emission during device fabrication and operation, which result in a color instability and reduced efficiency. Earlier reports ascribed this troublesome green band to aggregation and/or excimer formation through chain stacking.2b,5 However, recent experimental results indicate that the green emission originates from keto defects in the polyfluorene backbone formed via the oxidative degradation of 9,9-dialkylated fluorene segments.6 Another hindrance for polyfluorenebased LEDs to achieve high efficiency is large band gaps between the LUMO and HOMO energy levels of polyfluorene, which make it hard to balance charge injection. In this regard, one attempt to suppress the green emission and improve the device efficiency is to introduce electron-donating moieties into the polyfluorenes. For instance, stable EL emissions and improved hole-transport ability could be achieved by incorporat* Corresponding author. Telephone: + 86-571-879-52543. E-mail:
[email protected] (P.L.). † Zhejiang University. ‡ Jilin University.
ing carbazole units into the polymer backbone.7 Here, in contrast, we introduced fluorene into the backbone of the carbazole oligomers to investigate the effects of chemical structure on the electronic, photonic, and morphological properties. These monodisperse, fluorene-centered oligocarbazoles (Cz4F and Cz6F) were characterized as well-defined and uniform structures and possessed superior chemical purity; hence they could serve as models for studying the structure-property relationship of their corresponding polymers or be applied in OLEDs as emitters as well as hole-transport materials directly. Results and Discussion Synthetic procedures of the intermediates and final compounds are outlined in Scheme 1. A Pd/Cu-catalyzed Sonogashira reaction was used to construct the ethynylene-linked oligomers. The syntheses of 1 and 7 have been reported in our previous work.8 Reaction of 7 with 2 equiv of 3 in the presence of CuI/Pd(PPh3)2Cl2/Ph3P/NEt3 afforded Cz4F in 84% yield after separation by column chromatography. Analogous treatment was used to get Cz6F in 76% yield. The thermal properties of Cz4F and Cz6F were examined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) in N2 at a heating rate of 10 °C/min, and the results are listed in Table 1. Observed glass-transition temperatures (Tg’s) for Cz4F and Cz6F were 107 and 126 °C, respectively, which were higher than that of 1,4-bis(1-naphthylphenylamino)biphenyl (NPB, 96 °C).9 No melting points (Tm’s) were detected for either compound. Moreover, they exhibited high decomposition temperatures (Td’s, corresponding to a 5% weight loss) over 443 °C. Figure 1 shows the TGA charts of the two compounds. Cz4F and Cz6F were highly fluorescent and their quantum yields were measured as 0.79 and 0.68, respectively, in dilute THF solutions using 9,10-diphenylanthrecene (DPA, Φ ) 0.95 in cyclohexane)10 as a standard. The absorption spectra of Cz4F and Cz6F in dilute THF solutions are shown in Figure 2a. Both absorption spectra covered a broad range from 250 to 400 nm with maximum absorption peaks at 393 and 394 nm, respec-
10.1021/jp0701946 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/14/2007
6884 J. Phys. Chem. C, Vol. 111, No. 18, 2007 SCHEME 1
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a
a Conditions: (a) CuI, Pd(PPh3)2Cl2, Ph3P, NEt3, N2, reflux; (b) 3-methyl-1-butyn-3-ol, CuI, Pd(PPh3)2Cl2, Ph3P, NEt3, N2, reflux; (c) KOH, 2-propanol, reflux.
TABLE 1: Optical and Thermal Properties of Oligomers absa (nm) Cz4F Cz6F
em (nm)
THF
film
THF
methanol
nanosuspensionb
film
Φc
Egd (ev)
393 394
399 400
409 (430) 409 (431)
438 (461) 477
436 (461) 476
436 (463) 474
0.79 0.68
2.88 2.91
Tg/Tde (°C) 107/449 126/443
a First absorption peak. b Measured in THF/H2O (10/90 v/v) solutions. c Measured in THF solutions using DPA as a standard. d Optical energy gap calculated from the edge of the electronic absorption band. e Obtained from DSC and TGA measurements in N2 at a heating rate of 10 °C/min; Tm, not detected.
Figure 1. TGA charts of Cz4F and Cz6F in N2 at a heating rate of 10 °C/min.
tively. The absorption spectra of both oligomers in thin neat films were similar to those in solutions except for slight red shifts (Figure 2b). In dilute THF solutions, the PL emission spectra of Cz4F and Cz6F were almost identical, with the main peaks at 409 nm as well as shoulder peaks at about 430 nm (Figure 3a), which was ascribed to the similar efficient conjugation length.8,11 In solid states, the PL emissions of Cz4F both in the form of nanoparticles in THF/H2O (10/90 v/v) solution and in thin neat film (fabricated by spin-coating from its toluene solution) were narrow and almost identical. The main emission peaks were at 436 nm and the shoulder peaks were at 461 and 463 nm, respectively. Also, the emission spectra were similar to that in THF solutions except for a red shift of about 28 nm and a decrease of PL intensity. However, Cz6F exhibited broader and strongly red-shifted (65 nm) emission spectra in solid states with respect to that in THF solution. The main
emission peaks were located at 474 nm in thin neat film, and 476 nm in nanosuspension, with a fwhm (full width at halfmaximum) of 81 nm, respectively. Also, the fine structure of the emission spectra disappeared. Similar red-shifted emission spectra could also be observed when both compounds were dispersed in a poor solvent, such as methanol. Moreover, in solid states, Cz4F exhibited much better fluorescence properties than Cz6F, such as quantum yield (not listed because of potential error). We ascribed the strongly red-shifted emission and relatively lower quantum yield of Cz6F in solid states to enhanced molecular packing of Cz6F due to its larger size of the chain aromatic system with respect to that of Cz4F.12 Due to aggregation, the strong π-π interaction (i.e., electronic coupling) resulted in quenching of emission from individual molecules and the appearance of new emission band at longer wavelength from a π-stacked molecular aggregate.13 Furthermore, to investigate the luminescent stability, the oligomers in thin neat films were annealed at 140 °C under N2 for 12 h. As illustrated in Figure 3c, annealing of both oligomer films resulted in no significant change in PL emission spectra, in which only a small red shift (4 nm) for Cz4F and a slight broadening at about 550 nm for Cz6F were observed. These spectral changes should be due to further aggregation of molecules at high temperatures. To investigate the hole-transport properties of Cz4F and Cz6F, single-layer devices (type 1, indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene) (PEDOT)/Cz4F or Cz6F/Al; type 2, ITO/PEDOT/poly(N-vinylcarbazole) (PVK):Cz4F (1:1 w/w) or PVK:Cz6F (1:1 w/w)/Al) were fabricated and inspected. The doped devices were fabricated to compare the hole-transport
Synthesis of Carbazole Oligomers Cz4F and Cz6F
J. Phys. Chem. C, Vol. 111, No. 18, 2007 6885
Figure 2. UV-vis absorption spectra of Cz4F and Cz6F in THF solutions (a) and in thin neat films (b).
Figure 3. PL emission spectra of Cz4F and Cz6F in THF solutions (a), in THF/H2O (10/90 v/v) solutions and methanol solutions (b), and in thin neat films before and after annealing at 140 °C under N2 for 12 h (c).
Figure 4. Current density-voltage spectra of Cz4F in type 1-4 devices (a) and Cz6F in type 1-4 devices (b).
abilities of oligomers with PVK. The single-layer devices based on Cz4F exhibited poor performances and the EL spectra could not be detected, which was ascribed to the poor film-forming properties of Cz4F with short molecular length. However, good current density-voltage characteristics were observed from both devices based on Cz4F (Figure 4a), which indicated that strong π-π interactions existed between Cz4F molecules in solid states. Cz6F exhibited improved EL emission at 452 and 460 nm, respectively, in both types of devices. Also, devices (type 1 and 2) based on Cz6F shown onset voltage at 5.5 and 7.5 V as well as maximum brightness of 124.6 cd m-2 at 8.5 V and 170.7 cd m-2 at 11.5 V, respectively. Moreover, the “hole-only”
device based on Cz6F only (type 1) exhibited great current density (637 mA/cm2 at 9 V), which indicated its good holetransport ability (Figure 4b). Its low onset voltage and great current density could be ascribed to the close packing and thus strong π-π interaction between molecules, which could be beneficial to improving the charge carrier mobility.14 To get better EL performances with these materials, doublelayer devices were fabricated (type 3, ITO/PEDOT/Cz4F or Cz6F/aluminum tris(8-hydroxyquinoline) (Alq3)/Al; type 4, ITO/PEDOT/PVK:Cz4F (1:1 w/w) or PVK:Cz6F (1:1 w/w)/ Alq3/Al) with a incorporation of an electron-transport layer of Alq3. Cz4F in a device (type 3) exhibited an EL emission peak
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Figure 5. EL emission spectra of Cz4F in type 1-4 devices (a) and Cz6F in type 1-4 devices (b).
Figure 7. EL spectra changes of Cz6F with increase of voltage.
Figure 6. CIE diagram for the emission color (0.20, 0.24) of Cz4F in type 4 device at 10 V.
at 434 nm with a fwhm of 64 nm, while in a device (type 4) doped with PVK it exhibited a rather broad EL emission band, which had a peak at 431 nm and a fwhm of 134 nm, more than twice that in a type 3 device, and almost covered a range from 400 to 600 nm (Figure 5a). Though the Commission Internationale de L’Eclairage (CIE) coordinates for this EL spectrum at 10 V was (0.20, 0.24) (sky blue, Figure 6), such a broad EL emission band was rarely reported from carbazole/fluorene copolymers or cooligomers. To the best of our knowledge, similar wide EL emission bands were observed from carbazole derivatives, which were applied to fabricate single-emitting white OLEDs.15 Thus, we inferred that Cz4F might also have potential use in white OLEDs as an emitter, and our further research will focus on optimization of a device structure. However, the reason for such a strong tendency to emit white light was not very clear yet due to the absence of EL emission data in single-layer device, which might be the formation of an electromer or the participating emission of Alq3. On the other hand, the EL emission of Cz6F in double-layer devices (types 3 and 4) was similar to that in single-layer devices except for a smaller fwhm and a slightly blue-shifted peak (Figure 5b). The results indicated that incorporation of a PVK layer into the device had little influence on the EL emission of Cz6F in devices. Moreover, Cz6F also trended to emit white light as voltage increased (Figure 7) and white-light emission (CIE: 0.30, 0.33) was achieved at 19 V, which could be attributed to the formation of an electromer after investigating EL emission in both single- and double-layer devices. Moreover, by corporation of Alq3, the performances of double-layer devices (types 3 and 4) were greatly improved with respect to those of single-layer devices (Table 2). On the other hand, though type 3 and 4 devices based on Cz4F exhibited low onset voltages of 4.0 and 4.5 V as well as a similar current density, respectively (Figure 4a), the current efficiency of a type
4 device was 0.16 cd A-1, which was 4 times that of a type 3 device. Furthermore, the maximum brightness of a type 4 device was 926.6 cd m-2 at 17.0 V, while that of a type 3 device was 377.8 cd m-2 at 18.0 V (Figure 8a). The obvious improvement of current efficiency and brightness of a type 4 device based on Cz4F should be due to the balance of holes and electrons by doping of the hole-transport material PVK and incorporation of an Alq3 layer which acted as an electron transporter and a hole blocker and the good film-forming property of PVK. Cz4F in this device should mainly act as an emitter. With regard to Cz6F, the maximum brightness of both type 3 and 4 devices were 462.3 cd m-2 at 13.0 V and 401.9 cd m-2 at 16.0V, respectively. However, from Figure 4b and Figure 8b, a nondoped device (type 3) exhibited better current densityvoltage-brightness characteristics as well as a higher current efficiency (Table 2) than a doped device with PVK (type 4), which indicated that the incorporation of the hole-transport material PVK had no obvious help to improve the device performances. According to the performances of four devices based on Cz6F, we inferred it might be a promising holetransport as well as blue- or white-light-emitting material for OLEDs, while Cz4F might be a candidate for a light emitter in doped devices. Table 2 summarizes the performances of all devices. In conclusion, two monodisperse fluorene-centered, ethynylene-linked carbazole oligomers were successfully synthesized and fully characterized. They were highly fluorescent, and their films emitted stable light after annealing at high temperature. Single- and double-layer devices based on both oligomers were fabricated and investigated. Both oligomers exhibited a tendency to emit white light as the voltage increased. Particularly Cz4F and Cz6F in a doped double-layer device exhibited rather broad EL emission and good performances, which indicated that they might be good emitters and have potential use in white OLEDs. Meanwhile, Cz6F exhibited bifunctional properties, which qualified it as a light-emitter as well as a hole-transport material.
Synthesis of Carbazole Oligomers Cz4F and Cz6F
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TABLE 2: Summary of Devicea Performances Cz4F
Cz6F
device
EL (nm)
fwhm (nm)
onset voltage (V)
ηmax,L (cd A-1)
Lmax (cd m-2, V)
type 1 type 2 type 3 type 4 type 1 type 2 type 3 type 4
NA NA 434 431 468 476 464 470
NA NA 64 134 111 121 100 101
5.0 6.0 4.0 4.5 5.5 7.5 5.5 6.0
NA NA 0.04 0.16 0.013 0.032 0.103 0.120
11.3, 11.0 2.0, 11.5 377.8, 18.0 926.6, 17.0 124.6, 8.5 170.7, 11.5 462.3, 13.0 401.9, 16.0
a Type 1, ITO/PEDOT/Cz4F or Cz6F/Al; type 2, ITO/PEDOT/PVK:Cz4F (1:1 w/w) or PVK:Cz6F (1:1 w/w)/Al; type 3, ITO/PEDOT/Cz4F or Cz6F/Alq3/Al; type 4, ITO/PEDOT/PVK:Cz4F (1:1 w/w) or PVK:Cz6F (1:1 w/w)/Alq3/Al.
Figure 8. Brightness-voltage spectra of Cz4F in type 3 and 4 devices (a) and Cz6F in type 1-4 devices (b).
Experimental Section General. 1H and 13C NMR spectra are obtained on a Bruker AVANCE DMX500 spectrometer operating in the Fourier transform (FT) mode. Five percent w/v solutions in chloroform-d are used to obtain NMR spectra. TMS is used as an internal standard. IonSpec HiResMALDI is used to obtain mass spectra. Fluorescence measurements are made with a RF-5301pc spectrofluorometer (Shimadzu, Kyoto, Japan) equipped with a xenon lamp. UV-vis absorption spectra are recorded on a Shimadzu UV-2450 spectrophotometer. NETZSCH STA 409 PG/PC is used to measure the thermal performances of the compounds. The solvents are distilled before used. Commercially available reagents are used without further purification unless otherwise mentioned. Synthesis. 3 (1.36 g, 2 mmol) or 6 (1.93 g, 2 mmol), 7 (0.41 g, 1 mmol), cuprous iodide (10 mg, 0.05 mmol), dichlorobis(triphenylphosphine)palladium(II) (3.5 mg, 0.005 mmol), triphenylphosphine (5 mg, 0.02 mmol), and dry triethylamine (100 mL) were placed in a 150 mL round-bottomed flask equipped with a Teflon-covered magnetic stir bar. After the solution was purged with nitrogen for 1/2 h, it was refluxed under nitrogen for 4 h. The reaction mixture was filtered, and the filtrate was evaporated under reduced pressure. The residue was purified through column chromatography (silica gel, hexane/methylene chloride as eluent). In this way, 1.27 g (84% yield) of Cz4F or 1.58 g (76% yield) of Cz6F was obtained. 9-Heptyl-3-(2-(9,9-diheptyl-2-(2-(9-heptyl-3-(2-(9-heptyl-9Hcarbazol-6-yl)ethynyl)-9H-carbazol-6-yl)ethynyl)-9H-fluoren-7yl)ethynyl)-6-(2-(9-heptyl-9H-carbazol-3-yl)ethynyl)-9H-carbazole (Cz4F). 1H NMR δ CDCl3: 0.68 (br, 4H), 0.80-0.83 (m, 6H), 0.85-0.89 (m, 12H), 1.09 (br, 12H), 1.18-1.20 (m, 4H), 1.24-1.28 (m, 16H), 1.30-1.38 (m, 16H), 1.86-1.88 (m, 8H), 2.02-2.05 (m, 4H), 4.27-4.30 (m, 8H), 7.25-7.28 (m, 2H), 7.37-7.41 (m, 8H), 7.47-7.50 (m, 2H), 7.58-7.60 (m, 4H), 7.69-7.71 (m, 8H), 8.12 (d, 2H, J ) 8.0 Hz), 8.33-8.35 (m, 6H) ppm. 13C NMR δ CDCl3: 14.30, 22.83, 22.85, 24.05, 27.51, 29.23, 29.29, 29.30, 30.32, 31.97, 32.11, 40.75, 43.44, 43.60,
55.48, 88.78, 89.15, 89.38, 91.25, 108.97, 109.13, 109.22, 113.95, 114.07, 114.83, 119.47, 120.14, 120.77, 122.62, 122.76, 122.84, 123.11, 124.08, 124.14, 124.39, 126.08, 126.23, 129.46, 129.86, 129.92, 130.83, 140.18, 140.45, 140.67, 140.68, 141.04, 151.33 ppm. MALDI-TOF MS (m/z): 1511.2 (M+). 9-Heptyl-3-(2-(9,9-diheptyl-2-(2-(9-heptyl-3-(2-(9-heptyl-3-(2(9-heptyl-9H-carbazol-6-yl)ethynyl)-9H-carbazol-6-yl)ethynyl)9H-carbazol-6-yl)ethynyl)-9H-fluoren-7-yl)ethynyl)-6-(2-(9-heptyl3-(2-(9-heptyl-9H-carbazol-3-yl)ethynyl)-9H-carbazol-6yl)ethynyl)-9H-carbazole (Cz6F). 1H NMR δ CDCl3: 0.67 (br, 4H), 0.78-0.83 (m, 6H), 0.84-0.89 (m, 18H), 1.08 (br, 12H), 1.18-1.20 (m, 4H), 1.26-1.29 (m, 24H), 1.36-1.37 (m, 24H), 1.87-1.88 (m, 12H), 2.01-2.03 (m, 4H), 4.27-4.29 (m, 12H), 7.24-7.26 (m, 2H), 7.36-7.40 (m, 10H), 7.46-7.49 (m, 2H), 7.53-7.54 (m, 2H), 7.58-7.59 (m, 4H), 7.68-7.73 (m, 12H), 8.11 (d, 2H, J ) 7.5 Hz), 8.35 (br, 10H) ppm. 13C NMR δ CDCl3: 14.30, 22.82, 24.03, 27.50, 29.23, 29.26, 29.29, 30.25, 30.31, 31.97, 32.07, 32.09, 40.74, 43.44, 43.59, 55.50, 88.82, 89.07, 89.18, 89.23, 89.34, 91.26, 108.96, 109.13, 109.19, 109.24, 114.00, 114.12, 114.56, 114.75, 119.46, 120.14, 120.77, 122.64, 122.78, 122.79, 122.82, 122.85, 122.87, 123.12, 124.07, 124.18, 124.42, 126.08, 126.22, 129.46, 129.86, 129.98, 130.84, 140.17, 140.45, 140.50, 140.69, 141.04, 151.35 ppm. MALDITOF MS (m/z): 2085.2 (M+). LED Fabrication and Measurement. A single-layer device was fabricated on an indium tin oxide (ITO) substrate covered with a 30 nm thick layer of poly(3,4-ethylenedioxythiophene) (PEDOT) deposited by the usual spin-coating technique and cured at 80 °C for about 1 h. The emitting layer (Cz4F, Cz6F, PVK:Cz4F (1:1 w/w), or PVK:Cz6F (1:1 w/w)) (100 nm) was spin-coated onto the PEDOT layer. Finally, an aluminum electrode (200 nm) was prepared by vacuum evaporation. To improve device performance, an additional electron-transport layer (30 nm) of Alq3 was deposited under a reduced pressure of 2 × 10-4 Pa on the emitting layer to fabricate a doublelayer device. The emitting area of the EL devices was 4 mm2. The layer thickness of the deposited material was monitored in
6888 J. Phys. Chem. C, Vol. 111, No. 18, 2007 situ using an oscillating quartz thickness monitor. Finally, the EL spectra of these devices were measured by a PR650 Spectroscan spectrometer. The luminance-current-voltage characteristics were recorded simultaneously with the EL spectra by combining the spectrometer with a Keithley Model 2400 programmable voltage-current source. All measurements were performed under ambient atmosphere at room temperature. Acknowledgment. P.L.thanks the National Natural Science Foundation of China (20374045, 20674070), Natural Science Foundation of Zhejiang Province (R404109), and Ministry of Education of China for financial support. W.T. thanks the Major State Basic Research Development Program (973) (Grant 2002CB613401), the Natural Science Foundation of China (Grant 20474023, 50673035), and the Research Project of Jilin Province (20050504, 20060702). Supporting Information Available: NMR and mass spectra of Cz4F and Cz6F. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (2) (a) Bernius, M. T.; Inbasekaran, M.; O’Brien, J.; Wu, W. AdV. Mater. 2000, 12, 1737. (b) Bliznyuk, V. N.; Carter, S. A.; Scott, J. C.; Klarner, G.; Miller, R. D.; Miller, D. C. Macromolecules 1999, 32, 361. (c) Kla¨rner, G.; Lee, J.-I.; Lee, V. Y.; Chan, E.; Chen, J. P.; Nelson, A.; Markiewicz, D.; Siemens, R.; Scott, J. C.; Miller, R. D. Chem. Mater. 1999, 11, 1800. (3) (a) O’Neill, M.; Kelly, S. M. AdV. Mater. 2003, 15, 1135. (b) Whitehead, K. S.; Grell, M.; Bradley, D. D. C.; Jandke, M.; Strohrigl, P. Appl. Phys. Lett. 2000, 76, 2946. (c) Yang, X. H.; Neher, D.; Lucht, S.; Nothofer, H.; Gunter, R.; Scherf, U.; Hagen, R.; Kostromine, S. Appl. Phys. Lett. 2002, 81, 2319.
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