New Ambipolar Hosts Based on Carbazole and 4,5-Diazafluorene

Article pubs.acs.org/cm

New Ambipolar Hosts Based on Carbazole and 4,5-Diazafluorene Units for Highly Efficient Blue Phosphorescent OLEDs with Low Efficiency Roll-Off Cai-Jun Zheng,†,‡,∥ Jun Ye,†,§,∥ Ming-Fai Lo,‡ Man-Keung Fung,‡ Xue-Mei Ou,† Xiao-Hong Zhang,†,* and Chun-Sing Lee‡,* †

Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China ‡ Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China § Graduate University of Chinese Academy of Sciences, Beijing, 100039, China ABSTRACT: Two ambipolar host materials, 9,9-bis(9-methylcarbazol-3-yl)4,5-diazafluorene (MCAF) and 9,9-bis(9-phenylcarbazaol-3-yl)-4,5-diazafluorene (PCAF), comprising two electron-donating carbazole units and an electron-accepting 4,5-diazafluorene group, have been designed, synthesized, and characterized. Given the nonplanar structure of the sp3-hybridized C9 atom of the 4,5-diazafluorene unit, MCAF and PCAF exhibit high triplet energy levels of 2.82 and 2.83 eV, as well as high glass-transition temperatures of 187 and 188 °C, respectively. Equipped with ambipolar transport properties as well as suitable highest occupied and lowest unoccupied molecular orbital energy levels, the two compounds excellently perform in blue phosphorescent organic light-emitting devices (PHOLEDs). The MCAF-based blue PHOLED has a very low turn-on voltage of 2.6 V, a high current efficiency of 32.2 cd A−1, a high external quantum efficiency of 17.9%, a high power efficiency of 31.3 lm W−1, and a low efficiency roll-off with a high efficiency of 27.6 cd A−1 even at 10 000 cd m−2. These values are among the highest ever reported for devices doped with iridium(III) bis[2-(4′,6′difluorophenyl)pyridinato-N,C(2′)]-picolinate. KEYWORDS: ambipolar host, blue PHOLEDs, 4,5-diazafluorene, carbazole

1. INTRODUCTION Since the first report of phosphorescent organic light-emitting devices (PHOLEDs) by Forrest et al. in 1998,1 PHOLEDs have attracted considerable attention because they can approach a 100% internal quantum efficiency using both singlet and triplet excitons for emission.2 However, phosphorescent emitters typically have long exciton lifetimes and long diffusion lengths. Their performance can be compromised by concentration quenching and T1−T1 annihilation at high excitation densities.3 To reduce the excitation density, PHOLEDs always use a host− guest system by doping phosphorescent emitters into a suitable host material. Therefore, the development of high-performance host materials is extremely important for PHOLEDs. An efficient host material must have a suitable energy gap for effective energy transfer to the guest, and good carrier transport properties for a balanced recombination of carriers in the emitting layer (EML). The energy level of the material must match with neighboring layers for both effective charge injection and charge confinement. Decent thermal and morphological stabilities are also required from a host material for a long device operational lifetime. Traditional host materials usually have good transporting properties for only a single type © 2012 American Chemical Society

of charge carrier. For example, N,N′-dicarbazolyl-3,5-benzene (mCP)4,5 exhibits only good hole-transporting properties, whereas 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)1,2,4-triazole2,6 only has good electron-transporting capabilities. These unbalanced carrier-transporting properties are detrimental to the turn-on voltage and stability of OLEDs.5−8 Hence, ambipolar host materials that can balance carrier-transporting properties have recently attracted heightened interest.9−17 Ambipolar molecules often simultaneously contain both pand n-type groups. Consequently, conjugation caused by intramolecular donor−acceptor interactions lower the triplet energies of ambipolar molecules. Therefore, many reported ambipolar hosts such as o-CzOXD,10 DM-TIBN,11 and BUPH114 can only be used for red or green phosphorescent emitters. Bipolar hosts used in blue PHOLEDs have only been reported recently.9,13,15−17 One approach for designing bipolar hosts with high triplet energies is to use groups with extremely high triplet energies for constituting the molecules.9,13,16,17 On Received: June 28, 2011 Revised: January 21, 2012 Published: January 21, 2012 643

dx.doi.org/10.1021/cm2036647 | Chem. Mater. 2012, 24, 643−650

Chemistry of Materials

Article

Scheme 1. Synthesis of MCAF and PCAF

the other hand, it is also important to limit the extent of conjugation between groups.15 Nevertheless, groups with high triplet energies usually also have small steric volumes, and ambipolar molecules built with the first approach often do not possess good thermal and morphological stabilities.16 Thus, in this study, the second approach was used to design two new ambipolar blue hosts. Hole-transporting carbazole groups18,19 and an electron-transporting 4,5-diazafluorene group,20,21 which all have reasonably high triplet energies, were selected for the construction of two new ambipolar compounds, 9,9bis(9-methylcarbazol-3-yl)-4,5-diazafluorene (MCAF) and 9,9bis(9-phenylcarbazaol-3-yl)-4,5-diazafluorene (PCAF). As the carbazole groups are linked to the 4,5-diazafluorene segment via its sp3-hybridized C9 atom, the two resulting molecules are highly nonplanar and have minimal donor−acceptor interactions. As a result, MCAF and PCAF successfully keep high triplet energies of 2.82 and 2.83 eV, respectively, which enable their applications as hosts for blue phosphorescent dopants. Blue PHOLEDs with iridium(III) bis[2-(4′,6′-difluorophenyl)pyridinato-N,C(2′)]-picolinate (FIrpic)-doped MCAF and PCAF were fabricated and were shown to have the remarkable performances among all the FIrpic-doped devices ever fabricated.

Figure 1. ORTEP diagram of PCAF.

structure, the difference between the dihedral angles of the two carbazole groups appears in the crystal. Similar broken crystal symmetries have been reported in a number of systems.25,26 These twists in the molecular structure result in decreased intramolecular interactions between the carbazole and 4,5diazafluorene moieties that keep the high triplet energy of PCAF. MCAF, which has a similar molecular structure with PCAF, is also expected to have a high triplet energy. Figure 2 and Table 1 show that MCAF and PCAF exhibit similar photophysical properties because of their similar structures. The absorption and photoluminescence (PL) maxima of the two compounds differ only by 4 nm. The absorption at around 300 nm for two compounds is corresponding to the carbazole-centered n−π* transition,27 and the absorption at around 320 nm belongs to the 4,5diazafluorene core.28 Besides, intramolecular charge transfer (ICT) transition between carbazole and 4,5-diazafluorene is at around 350 nm. The triplet energies of MCAF and PCAF were determined as 2.82 and 2.83 eV, respectively, from the highest energy vibronic sub-band of their phosphorescence spectra at 77 K (Figure 2b). These energies are much higher than the

2. RESULTS AND DISCUSSION Scheme 1 shows the synthetic routes and structures of MCAF and PCAF. The intermediate 4,5-diazafluoren-9-one (1) was prepared by one-step oxidative ring contraction of phenanthroline.22 9-Methyl-9-carbazole (2) and 9-phenyl-9-carbazole (3) were obtained according to previously reported methods.23,24 Eaton’s reagent was used as the catalyst and condensing agent. Subsequently, the electron-rich positions of the carbazole rings efficiently reacted with the 9-positon electron-deficient carbon atom of 4,5-diazafluoren-9-one to give the target compounds MCAF and PCAF. The chemical structures of the intermediates and the final products are confirmed with 1H NMR spectroscopy, elemental analysis, and high-resolution mass spectrometry. The molecular structure of PCAF was further confirmed using X-ray crystallography. Figure 1 shows that the dihedral angle between the left carbazole and the 4,5diazafluorene unit is 65.5°. In contrast, the dihedral angle between the right carbazole and the 4,5-diazafluorene unit is 76.0°. Notably, although PCAF has a symmetrical chemical 644



Article

exotherms at about 283 °C. The almost identical values are attributed to the similar nonplanar molecular structures of the two compounds. The high Tg and Td values of MCAF and PCAF are beneficial to the morphological stability of their films and are expected to decrease the phase separation rate of the guest−host system. Quantum calculations were performed for MCAF and PCAF at the B3LYP/6-31G theoretical level.29 Figure 4 shows the electron density distributions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the two compounds. Their HOMO and LUMO are localized predominantly on the electron-rich carbazole and the electron-deficient 4,5-diazafluoren fragments, respectively. Considering three subunits are interconnected via neutral C−C bond which nullifies inductive effects of the subunits through a sp3-hybridized C atom,30,31 this finding confirms that there are minimal intramolecular interactions between the two moieties. The electrochemical properties of MCAF and PCAF were also investigated with cyclic voltammetry (CV). Figure 5 shows that both compounds have reversible reduction and oxidation behaviors, which indicate their potential in ambipolar carrier transport.10,32,33 The HOMO and the LUMO energy levels of MCAF and PCAF were estimated from the half-wave potentials of the redox curves relative to the vacuum level (Table 1).18 The HOMO energy levels of MCAF and PCAF are −5.99 and −6.07 eV, respectively. The difference between two HOMO energy levels should be ascribed to the inductive electrondonating effect of methyl and π-electron delocalization effect of phenyl.18,34 Due to the small interaction between carbazole and 4,5-diazafluoren, the LUMO energy levels of two compounds are nearly the same. Besides, the HOMO and LUMO energy levels of the two compounds are similar to the HOMO of carbazole18 and the LUMO of 4,5-diazafluoren,21 respectively. On the basis of the CV results, the energy gaps in MCAF and PCAF were estimated as 3.22 and 3.29 eV, respectively, which are similar to the results estimated from the optical absorption edges of solid films on quartz substrates. The charge-carrier properties of two compounds were investigated in hole-only and electron-only device. The holeonly device had the structure of ITO/MoO3 (12 nm)/MCAF or PCAF (60 nm)/MoO3 (12 nm)/Al, while the electron-only device had the structure of ITO/Al (60 nm)/MCAF or PCAF (60 nm)/Al. It can be assumed that only single carriers are injected and transported in the devices due to the work function of MoO3 (or Al) being high (or low) enough to block electron (or hole) injection.35,36 As shown in Figure 6, the remarkable current densities are clearly obvious to prove the ambipolar properties of two compounds. Furthermore, MCAF exhibits the nearly same hole- and electron-current densities in

Figure 2. (a) Absorption and PL spectra of MCAF and PCAF in dilute toluene solution at room temperature. (b) The phosphorescence spectra of MCAF and PCAF in 2-MeTHF glass matrix at 77 K.

triplet energy of the common blue phosphorescent dopant, FIrpic (2.62 eV).5 Hence, MCAF and PCAF can be appropriate hosts for blue PHOLEDs. The thermal properties of MCAF and PCAF were investigated using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere. Both compounds exhibit good thermal stabilities (Figure 3). The decomposition temperatures (Td), which correspond to a 5% weight loss upon heating during TGA, were measured as 395 and 416 °C for MCAF and PCAF, respectively. High glass transition temperatures (Tg) of 187 and 188 °C were determined for MCAF and PCAF, respectively, by DCS measurements during second-heating scans. Both compounds also exhibit the recrystallization

Table 1. Summary of Physical Measurements of MCAF and PCAF compound MCAF PCAF

λmax,absa (nm) 352, 323, 301 348, 320, 299

λmax,fa (nm)

λpb (nm)

ET (eV)

Tgc (°C)

T mc (°C)

Tdc (°C)

E1/2oxd (V)

HOMOe (eV)

E1/2redd (V)

LUMOe (eV)

Egf (eV)

Egg (eV)

380, 365

440

2.82

187

384

395

1.25

−5.99

−1.97

−2.77

3.22

3.27

376, 362

438

2.83

188

362

416

1.33

−6.07

−1.96

−2.78

3.29

3.34

a

Measured in toluene solution at room temperature. bMeasured in 2-MeTHF glass matrix at 77 K. cTg: glass transition temperatures, Tm: melting temperatures, Td: decomposition temperatures. dE1/2ox: half-wave oxidation potentials vs SCE; E1/2red: half-wave reduction potentials vs SCE. e HOMO = −E1/2ox (vs SCE) − ESCE and LUMO = E1/2red (vs SCE) − ESCE; the energy level of SCE (ESCE) is 4.74 eV vs vacuum level.18 fEg: The band gap energies were estimated from CV. gEg: The band gap energies were estimated from the optical absorption edges of solid films on quartz substrates. 645



Article

Figure 3. DSC (a and b) and TGA (c and d) curves of MCAF and PCAF.

diazafluorene core, which markedly weakens the electrontransporting capacity and strengthens the hole-transporting capacity. To assess the potentials of MCAF and PCAF as ambipolar host materials in blue PHOLEDs, FIrpic-doped devices were fabricated with a simple configuration of ITO/NPB (30 nm)/ TCTA (10 nm)/host: (x) wt% FIrpic (30 nm)/TPBI (30 nm)/LiF (1.5 nm)/Al. In these devices, ITO (indium tin oxide) and LiF/Al (lithium fluoride/aluminum) are the anode and cathode, respectively. NPB (4,4′-bis[N-(1-naphthyl)-Nphenyl amino]biphenyl) is the hole-transporting layer and TCTA (4,4′,4″-tris(N-carbazolyl)triphenylamine) is the electron-blocking layer. TPBI (1,3,5-tris(N-phenylbenzimidazol-2yl)benzene) is both the electron-transporting and hole-blocking layer. The EML is the newly prepared FIrpic-doped MCAF or PCAF. The best electroluminescence (EL) performance was achieved with 8 wt % FIrpic for both hosts. The current density−luminance−voltage characteristics of the devices are shown in Figure 7, and the key device performance parameters are summarized in Table 2. Both MCAF- and PCAF-based devices have low turn-on voltages of 2.6 and 2.7 V, respectively, at a brightness of 1 cd m−2. Considering that the triplet energy of FIrpic is about 2.62 eV, these turn-on voltage values have hence already reached the limit of the FIrpic-based blue PHOLEDs. One important reason for such low turn-on voltages is the ambipolar transporting property of the hosts. Another important reason is the suitable HOMO and LUMO energy levels of the two

Figure 4. Calculated spatial distributions of the HOMO and LUMO energy densities of MCAF and PCAF.

the devices, proving MCAF could be a better ambipolar host material compared with PCAF. Considering the similar physical properties of two compounds, the different carrier-transporting properties between MCAF and PCAF are probably due to their different steric configurations. Compared with MCAF, the larger outside 9-phenylcarbazole fragments in PCAF might have the stronger blocking effect on the middle 4,5646



Article

Figure 7. Current density−voltage and luminance−voltage characteristics of the PHOLEDs with MCAF and PCAF.

current densities. Therefore, both devices exhibit very high and stable efficiencies, as shown in Figure 9 and Table 2. The MCAF-based device has a maximum current efficiency of 32.2 cd A−1 and a maximum external quantum efficiency (EQE) of 17.9% at a luminance of 691.8 cd m−2. A high efficiency of 27.6 cd A−1 even at 10 000 cd m−2, which signifies only a mild efficiency roll-off, is also exhibited. As a result of the low driving voltages, the MCAF device shows high power efficiencies of 31.3 and 14.5 lm W−1 at 35 and 10 000 cd m−2, respectively. These values are among the highest ever reported for FIrpicdoped blue PHOLEDs.5,9,13,15−17,19 On the other hand, the efficiency of the PCAF-based device is slightly lower than the MCAF-based device. This result can be understood by considering the hole- and electron-transporting properties of two compounds. More balanced carriers in the MCAF-based device could lead to a higher utilization ratio of the carriers. For comparison, a similarly structured device was fabricated using a conventional blue host (mCP). Table 2 shows that the mCP-based device has a lower performance, having a high turnon voltage of 5.5 V and a low maximum efficiency of 10.9 cd A−1.5 The huge discrepancies between the performances of the MCAF- and mCP-based devices further confirm the advantage of our newly prepared ambipolar blue host materials.

Figure 5. Cyclic voltammograms of MCAF and PCAF in DMF with 0.1 M TBAPF6 as supporting electrolyte.

3. CONCLUSIONS We have constructed two ambipolar host materials, MCAF and PCAF, using two electron-donor carbazole units and an electron-acceptor 4,5-diazafluorene group. Given the nonplanar structure of the sp3-hybridized C9 atom of the 4,5-diazafluorene unit, both MCAF and PCAF successfully keep high triplet energies of 2.82 and 2.83 eV and high glass-transition temperatures of 187 and 188 °C, respectively. The prepared host materials have ambipolar transporting properties as well as suitable HOMO and LUMO energy levels. Hence, fabricated blue PHOLEDs based on the two compounds exhibit much better performances than a similarly structured device based on the traditional mCP host. The MCAF-based blue PHOLED has a very low turn-on voltage of 2.6 V, a high current efficiency of 32.2 cd A−1, a high EQE of 17.9%, and a high power efficiency of 31.3 lm W−1. A low efficiency roll-off rate with a high efficiency of 27.6 cd A−1 (even at 10 000 cd m−2) is also exhibited. These values are among the highest ever reported for FIrpic-doped devices5,9,13,15−17,19 and can provide a useful strategy for designing new blue ambipolar host materials.

Figure 6. Current density−voltage characteristics of the hole-only and electron-only devices with MCAF and PCAF.

compounds. The schematic energy level diagrams of the two devices are shown in Figure 8.37,38 The slight barriers against a hole injection from TCTA into MCAF or PCAF and against an electron injection from TPBI into MCAF or PCAF are evident. Furthermore, there are injection barriers of approximately 0.5 eV for the electron at the TCTA/EML and about 0.3 eV for the hole at the EML/TPBI interfaces. This finding implies that the carriers are confined after an efficient injection into the EML, resulting in high exciton formation efficiencies even under high 647



Article

Table 2. EL Data of the Blue PHOLEDs

a

host

turn-on voltagea (V)

max CEb (cd A−1)

max PEc (lm W−1)

max EQEd (%)

CE @ 10000 cd m−2 (cd A−1)

PE @ 10000 cd/m−2 (lm W−1)

EQE @ 10000 cd m−2 (%)

MCAF PCAF mCP

2.6 2.7 5.5

32.2 23.8 10.9

31.3 21.3 4.22

17.9 12.8 5.38

27.6 19.5 7.04

14.5 10.2 1.93

15.4 10.5 4.54

The voltage at 1 cd m−2. bCurrent efficiency. cPower efficiency. dExternal quantum efficiency.

Figure 8. Relative energy level alignments of the PHOLEDs. (5.0 g, 92%). 1H NMR (400 MHz, CDCl3): δ = 8.11 (d, J = 7.7 Hz, 2 H), 7.48 (t, J = 8.0 Hz, 2 H), 7.41 (d, J = 8 Hz, 2 H), 7.24 (t, J = 8 Hz, 2 H), 3.87 (s, 3 H). HRMS calcd, 181.2332; found, 181. 2338. 9-Phenyl-9-carbazole (3). A mixture of iodobenzene (2.0 g, 10 mmol), carbazole (1.67 g, 10 mmol), CuI (190 mg, 1.0 mmol), Lproline (115 mg, 1.0 mmol), K2CO3 (2.8 g, 20.0 mmol), and dimethyl sulfoxide (DMSO, 20 mL) was heated at 110 °C for 36 h under argon. The mixture was cooled to room temperature, and the reaction was quenched with water. The mixture was extracted with CH2Cl2 and dried over Na2SO4. After the solvent was removed, the residue was purified by column chromatography on silica gel using petroleum as the eluent. The product was obtained as a white solid (2.23 g, 92%). 1 H NMR (DMSO-d6, 400 Hz): δ = 8.25 (d, J = 7.76 Hz, 2 H), 7.69 (t, J = 8.0 Hz, 2 H), 7.64−7.61 (m, 2 H), 7.55 (t, J = 7.30, 1 H), 7.44 (t, J = 7.4 Hz, 2 H), 7.38 (d, J = 8.0, 2 H), 7.29 (t, J = 7.32, 2 H). HRMS calcd, 243.3026; found, 243.3023. MCAF. Eaton’s reagent (800 μL) was added under a nitrogen flow to a solution of 2 (1.1 g, 6.1 mmol) and 1 (0.5 g, 2.8 mmol) in CH2Cl2 (5.0 mL). The reaction mixture was heated at 100 °C for 1 h, and the escaping CH2Cl2 was collected in a cold trap. After cooling, the reaction mixture was quenched with water and neutralized with potassium carbonate. The mixture was extracted with CH2Cl2 and dried over Na2SO4. The solvent was evaporated under reduced pressure. The final product was obtained through column chromatography on silica gel (using CH2Cl2/acetone, 2:1) as a white solid (1.25 g, 85%). 1H NMR (CDCl3): δ = 8.8 (s, 2 H), 7.96 (s, 2 H), 7.89 (d, J = 10.4 Hz, 4 H), 7.47 (t, J = 8.0 Hz, 2 H), 7.43−7.30 (m, 8 H), 7.16 (t, J = 7.28 Hz, 2 H), 3.84 (s, 6 H). HRMS calcd, 526.6293; found, 526.6288. Anal. Calcd for C37H26N4: C, 84.38; H, 4.98; N, 10.64. Found: C, 84.35; H, 5.01; N, 10.55. Purity: 99%. PCAF. Eaton’s reagent (800 μL) was added under a nitrogen flow to a solution of 3 (1.5 g, 6.1 mmol) and 1 (0.5 g, 2.8 mmol) in CH2Cl2 (5.0 mL). After the addition of Eaton’s reagent, the reaction mixture was heated at 100 °C for 1 h, and the escaping CH2Cl2 was collected in a cold trap. After cooling, the reaction mixture was quenched with water and neutralized with potassium carbonate. The mixture was extracted with CH2Cl2 and dried over Na2SO4. Evaporation of the solvent under reduced pressure followed. The final product was obtained using column chromatography on silica gel (using CH2Cl2/ acetone, 2:1) as a white solid (1.65 g, 91%). 1H NMR (CDCl3): δ = 8.79 (d, 2 H), 7.94 (d, J = 8.6 Hz, 6 H), 7.76−7.52 (m, 8 H), 7.45 (t, J = 7 0.16 Hz, 2 H), 7.38 (d, J = 3.6 Hz, 4 H), 7.34−7.31 (m, 6 H), 7.21 (m, 2 H). HRMS calcd, 650.7681; found, 650.7676. Anal. Calcd for

Figure 9. Current efficiency−luminance and power efficiency− luminance plots of the MCAF and PCAF based devices.

4. EXPERIMENTAL SECTION 4,5-Diazafluoren-9-one (1). A solution of phenanthroline monohydrate (2.2 g, 11.1 mmol) and KOH (2 g, 35.5 mmol) in water (130 mL) was boiled. A hot solution of KMnO4 (5 g, 31.5 mmol) in water (80 mL) was added dropwise over ca. 1 h to the boiling solution. The mixture was refluxed for another 2 h and was filtered hot. The orange filtrate was cooled and extracted with chloroform, and the combined organic extracts were dried over anhydrous Na2SO4. After solvent removal, the crude product was further purified by column chromatography on silica gel using acetone/petroleum ether (2:1) as the eluent. The product was isolated as a yellow solid (980 mg, 48%). 1H NMR (acetone-d6, 400 MHz): δ = 8.80 (d, J = 5.0 Hz, 2 H), 8.06 (d, J = 7.5 Hz, 2 H), 7.50 (dd, J1 = 7.5 Hz, J2 = 5.0 Hz, 2 H). HRMS calcd, 182.1782; found, 182.1787. 9-Methyl-9-carbazole (2). Sodium hydride (1.15 g, 0.03 mmol) was added portion-wise to a solution of carbazole (5 g, 0.03 mmol) in dimethylformamide (50 mL). The resulting suspension was stirred at room temperature for 0.5 h, and iodomethane (1.43 mL, 0.03 mmol) was added dropwise. The mixture was stirred for 10 h, and ice water was added cautiously until precipitation was complete. The precipitate was collected by filtration under a vacuum, washed with water, and dried in a vacuum oven. The product was obtained as a white solid 648



Article

C47H30N4: C, 86.74; H, 4.65; N, 8.61. Found: C, 86.69; H, 4.63; N, 8.63. Purity: 99%. General Measurements. The NMR spectra were recorded on VaRIAN-GEMINI-300 and Bruker Avance Π-400 spectrometer at room temperature. Low- and high-resolution mass spectra were recorded using a BIFLEXIII MALDI-TOF mass spectrometer in MALDI mode or in EI mode. Two hosts used for EL devices were further purified by vacuum sublimation. The electronic absorption spectra were measured using a Hitachi UV−vis spectrophotometer U3010. The fluorescence spectra were recorded using a Hitachi fluorescence spectrometer F-4500. The phosphorescence spectra of the compounds were measured in a 2-MeTHF glass matrix at 77 K using a Hitachi fluorescence spectrometer F-4500. The cyclic voltammetry (CV) experiments were performed using a Potentiostal/Galvanostat Model 283 electrochemical analyzer with scan rate of 100 mV s−1. All of the measurements were carried out at room temperature with a conventional three-electrode configuration consisting of a glassy carbon working electrode, a platinum wire counter electrode, and a SCE (saturated calomel electrode) reference electrode, calibrated against a ferrocene/ferrocenium couple. The solvent in all of the experiments was DMF, and the supporting electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6). The DSC measurements were carried out using a TA Instruments DSC 2910 thermal analyzer at a heating rate of 10 °C min−1. The TGA measurements were performed on a TA Instruments TGA 2050 thermal analyzer with a heating rate of 10 °C min−1 in a nitrogen atmosphere. OLED Fabrication. An ITO-coated glass with a sheet resistance of 30 Ω square−1 was used as the substrate. Before the device fabrication, the ITO glass substrates were cleaned with isopropyl alcohol and deionized water, dried in an oven at 120 °C, treated with UV-ozone, and transferred to a vacuum deposition system with a base pressure above 1 × 10−6 Torr for organic and metal deposition. The devices were fabricated by evaporating the organic layers at a rate of 1−2 Å s−1. The cathode was completed through the thermal deposition of LiF at a rate of 0.1 Å s−1. The cathode was then capped with Al metal through thermal evaporation at a rate of 10 Å s−1. EL spectra and International Commission on Illumination color coordinates were measured using a Spectrascan PR650 photometer. The current− voltage characteristics were measured using a computer-controlled Keithley 2400 SourceMeter under ambient atmosphere.

■

(5) Holmes, R. J.; Forrest, S. R.; Tung, Y. J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E. Appl. Phys. Lett. 2003, 82, 2422. (6) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E.. Appl. Phys. Lett. 2000, 77, 904. (7) Lundin, N. J.; Blackman, A. G.; Gordon, K. C.; Officer, D. L. Angew. Chem., Int. Ed. 2006, 45, 2582. (8) Chen, C. T.; Wei, Y.; Lin, J. S.; Moturu, M.; Chao, W. S.; Tao, Y. T.; Chien, C. H. J. Am. Chem. Soc. 2006, 128, 10992. (9) Cai, X. Y.; Padmaperuma, A. B.; Sapochak, L. S.; Vecchi, P. A.; Burrows, P. E. Appl. Phys. Lett. 2008, 92, 083308. (10) Tao, Y. T.; Wang, Q.; Yang, C. L.; Zhang, Z. Q.; Zou, T. T.; Qin, J. G.; Ma, D. G. Angew. Chem. Int. Ed.. 2008, 47, 8104. (11) Ge, Z. Y.; Hayakawa, T.; Ando, S.; Ueda, M.; Akiike, T.; Miyamoto, H.; Kajita, T.; Kakimoto, M. A. Adv. Funct. Mater. 2008, 18, 584. (12) Lai, M. Y.; Chen, C. H.; Huang, W. S.; Lin, J. T.; Ke, T. H.; Chen, L. Y.; Tsai, M. H.; Wu, C. C. Angew. Chem., Int. Ed. 2008, 47, 581. (13) Su, S. J.; Sasabe, H.; Takeda, T.; Kido, J. Chem. Mater. 2008, 20, 1691. (14) Gao, Z. Q.; Luo, M.; Sun, X. H.; Tam, H. L.; Wong, M. S.; Mi, B. X.; Xia, P. F.; Cheah, K. W.; Chen, C. H. Adv. Mater. 2009, 21, 688. (15) Hsu, F. M.; Chien, C. H.; Shu, C. F.; Lai, C. H.; Hsieh, C. C.; Wang, K. W.; Chou, P. T. Adv. Funct. Mater. 2009, 19, 2834. (16) Jeon, S. O.; Yook, K. S.; Joo, C. W.; Lee, J. Y. es. Adv. Funct. Mater. 2009, 19, 3644. (17) Chou, H. H.; Cheng, C. H.. Adv. Mater. 2010, 22, 2468. (18) Brunner, K.; van Dijken, A.; Borner, H.; Bastiaansen, J.; Kiggen, N. M. M.; Langeveld, B. M. W. J. Am. Chem. Soc. 2004, 126, 6035. (19) Tsai, M. H.; Hong, Y. H.; Chang, C. H.; Su, H. C.; Wu, C. C.; Matoliukstyte, A.; Simokaitiene, J.; Grigalevicius, S.; Grazulevicius, J. V.; Hsu, C. P. Adv. Mater. 2007, 19, 862. (20) Wong, K. T.; Chen, R. T.; Fang, F. C.; Wu, C. C.; Lin, Y. T. Org. Lett. 2005, 7, 1979. (21) Wong, K. T.; Chen, H. F.; Fang, F. C. Org. Lett. 2006, 8, 3501. (22) Plater, M. J.; Kemp, S.; Lattmann, E. J. Chem. Soc., Perkin Trans. 1 2000, 971. (23) Alisi, M. A.; Dragone, P.; Furlotti, G.; Cazzolla, N.; Russo, V.; Mangano, G.; Coletta, I.; Polenzani, L. U. S. Patent 20080207727, 2008. (24) Ma, D. W.; Cai, Q.; Zhang, H. Org. Lett. 2003, 5, 2453. (25) Tonzola, C. J.; Kulkarni, A. P.; Gifford, A. P.; Kaminsky, W.; Jenekhe, S. A. Adv. Funct. Mater. 2007, 17, 863. (26) Zhang, X. J.; Yuan, G. D.; Li, Q. S.; Wang, B.; Zhang, X. H.; Zhang, R. Q.; Chang, J. C.; Lee, C. S.; Lee, S. T. Chem. Mater. 2008, 20, 6945. (27) Ge, Z.; Hayakawa, T.; Ando, S.; Ueda, M.; Akiike, T.; Miyamoto, H.; Kajita, T.; Kakimoto, M. A. Org. Lett. 2008, 10, 421. (28) Thummel, R. P.; Lefoulon, F.; Mahadevan, R. J. Org. Chem. 1985, 50, 3824. (29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision A1; Gaussian Inc.: Pittsburgh, PA, 2003.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant No. 50825304, 51033007, 51103169, 51128301) and Beijing Natural Science Foundation (Grant No. 2111002), P. R. China. Mr. Wei-Ming Zhao is gratefully acknowledged for his valuable suggestions and help of the revised manuscript.

■

REFERENCES

(1) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (2) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048. (3) Baldo, M. A.; Adachi, C.; Forrest, S. R. Phys. Rev. B 2000, 62, 10967. (4) Thoms, T.; Okada, S.; Chen, J. P.; Furugori, M. Thin Solid Films 2003, 436, 264. 649



Article

(30) Kim, D. W.; Coropceanu, V.; Bredas, J. L. J. Am. Chem. Soc. 2011, 133, 17895. (31) Salman, S.; Kim, D. W.; Coropceanu, V.; Bredas, J. L. Chem. Mater. 2011, 23, 5223. (32) Liao, Y. L.; Lin, C. Y.; Wong, K. T.; Hou, T. H.; Hung, W. Y. Org. Lett. 2007, 9, 4511. (33) Shirota, Y.; Kageyama, H. Chem. Rev. 2007, 107, 953. (34) Thomas, K. R. J.; Lin, J. T.; Tao, Y. T.; Ko, C. W. J. Am. Chem. Soc. 2001, 123, 9404. (35) Kao, P. C.; Lin, J. H.; Wang, J. Y.; Yang, C. H.; Chen, S. H. J. Appl. Phys. 2011, 109, 094505. (36) Jiang, W.; Duan, L.; Qiao, J.; Dong, G. F.; Wang, L. D.; Qiu, Y. Org. Lett. 2011, 13, 3146. (37) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.; Leo, K. Nature 2009, 459, 234. (38) Zheng, C. J.; Zhao, W. M.; Wang, Z. Q.; Huang, D.; Ye, J.; Ou, X. M.; Zhang, X. H.; Lee, C. S.; Lee, S. T. J. Mater. Chem. 2010, 20, 1560.

650


New Ambipolar Hosts Based on Carbazole and 4,5-Diazafluorene

Recommend Documents