Highly Efficient Phosphorescent Organic Light-Emitting Diodes Hosted

Nov 13, 2009 - Highly Efficient Phosphorescent Organic Light-Emitting Diodes Hosted by. 1,2,4-Triazole-Cored Triphenylamine Derivatives: Relationship ...
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J. Phys. Chem. C 2010, 114, 601–609

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Highly Efficient Phosphorescent Organic Light-Emitting Diodes Hosted by 1,2,4-Triazole-Cored Triphenylamine Derivatives: Relationship between Structure and Optoelectronic Properties Youtian Tao,† Qiang Wang,‡ Liang Ao,† Cheng Zhong,† Chuluo Yang,*,† Jingui Qin,† and Dongge Ma*,‡ Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan UniVersity, Wuhan 430072, People’s Republic of China, and State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ReceiVed: September 14, 2009; ReVised Manuscript ReceiVed: October 26, 2009

A series of new 1,2,4-triazole-cored triphenylamine derivatives with various linkages between triazole and triphenylamine (TPA) moieties were designed and synthesized through Suzuki cross-coupling reaction. The incorporation of rigid triazole moiety greatly improves their thermal and morphology stability, with Td and Tg in the ranges 480-531 °C and 106-155 °C, respectively. The o-TPA-linked 7 and 8 show much less intramolecular charge transfer and blue-shifted emission than their p-TPA linked analogues 1 and 3, respectively. The meta- and ortho-structured compounds display higher triplet energy and better electrophophorescent performances than their para-structured congeners. The significant improvement of electrophosphorescent performances can be achieved through subtle change of the host molecular structures, which could be attributed to the well-matched energy levels between the host and hole-transport layer, the high triplet energy of the host, and complete spatial separation of HOMO and LUMO energy levels. Devices hosted by structureoptimized o-TPA-m-PTAZ achieve the best EL performance, with the maximum current efficiencies and maximum external quantum efficiencies as high as 12.4 cd/A and 16.4% for deep red electrophosphorescence, and 50.7 cd/A and 14.2% for green electrophosphorescence. Introduction Organic light-emitting diodes (OLEDs) have attracted broad attention from both academia and industry over the past two decades, due to their applications in flat panel display and solid state lighting.1 Phosphorescent organic light-emitting diodes (PHOLEDs) based on heavy metal complexes as emitters have been considered to be crucial for high efficiency, since they can utilize both singlet and triplet excitons, thus approaching nearly 100% internal quantum efficiency.2-6 Generally, in order to achieve high electrophosphorescent efficiency, heavy metal complexes of phosphors have to be dispersed into host materials to reduce competitive factors such as concentration quenching and triplet-triplet annihilation.7 The host material in the emitting layer also serves as a recombination center for holes and electrons to generate the electronically excited states, followed by excitation energy transfer from the host to dopant, and hence, host material capable of transporting both holes and electrons can facilitate balanced carrier recombination, consequently benefitting the device performance. Very high efficiency PHOLEDs have been achieved by using double emissive layers with hole- or electron-transporting host in each emissive layer,8 respectively, or by using a single emissive layer with both hole- and electron-transporting hosts.9 However, the two strategies make the device fabrication complicated. In this regard, a single bipolar host material * Corresponding authors. E-mail: [email protected] (C.Y.); mdg1014@ ciac.jl.cn (D.M.). † Wuhan University. ‡ Chinese Academy of Sciences.

capable of transporting both holes and electrons is preferable to simplify device structure and generate broad recombination zones with balanced charge transport.4-6 It is well-known that electron-rich triarylamines, such as carbazole10,11 and triphenylamine (TPA),11,12 have been extensively used as hole-transporting materials due to their high hole mobilities. On the other hand, diverse electron-transporting materials,13 including tris(8-hydroxyquinoline)aluminum (Alq3),1a,14 4,7-diphenyl-1,10-phenanthroline (BPhen),15 1,3,5tris(N-phenylbenzimidazol-2-yl)benzene (TPBI),2e,16 2-(4biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD),2f,g,17 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4triazole (t-Bu-TAZ)6f,18 and pyridine derivatives,19 have been reported. For example, t-Bu-TAZ has mostly been used in blue phosphorescent OLEDs to serve as an efficient electrontransporting and hole-blocking layer for its high triplet energy level (2.75 eV) that would confine the triplet excitons within the emissive layer;20 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4triazole (TAZ) has also been employed as an efficient electron-transporting host for green PHOLEDs.2c,21 In this article, we design and synthesize a series of new TPA/ triazole hybrids by different connections between TPA and 3,5biphenyl-4-phenyl(or 4-naphthyl)-1,2,4-triazoles. We anticipate that the incorporation of the hole-transporting triphenylamine unit and the electron-transporting triazole moiety could impart them with bipolar charge transporting property. By investigating various properties, including thermal, photophysical, electrochemical, and electroluminescent properties, of the new series of bipolar host materials, we expect to understand the relation-

10.1021/jp908886d  2010 American Chemical Society Published on Web 11/13/2009

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ship between molecular structure and device performance, and thus to provide an avenue to design effective host materials for PHOLEDs. Experimental Section General Information. 1H NMR and 13C NMR spectra were measured on a MECUYR-VX300 spectrometer. Elemental analysis of carbon, hydrogen, and nitrogen was performed on a Vario EL III microanalyzer. Mass spectra were measured on a ZAB 3F-HF mass spectrophotometer. UV-vis absorption spectra were recorded on a Shimadzu UV-2500 recording spectrophotometer. Photoluminescence (PL) spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer. Differential scanning calorimetry (DSC) was performed on a NETZSCH DSC 200 PC unit at a heating rate of 10 °C min-1 from 30 to 350 °C under argon. The glass transition temperature (Tg) was determined from the second heating scan. Thermogravimetric analysis (TGA) was undertaken with a NETZSCH STA 449C instrument. The thermal stability of the samples under a nitrogen atmosphere was determined by measuring their weight loss while heating at a rate of 15 °C min-1 from 25 to 600 °C. Cyclic voltammetry (CV) was carried out in nitrogenpurged anhydrous THF (reduction scan) and dichloromethane (oxidation scan) at room temperature with a CHI voltammetric analyzer. Tetrabutylammonium hexafluorophosphate (TBAPF6) (0.1 M) was used as the supporting electrolyte. The conventional three-electrode configuration consisted of a platinum working electrode, a platinum wire auxiliary electrode, and an Ag wire pseudoreference electrode with ferrocenium-ferrocene (Fc+/ Fc) as the internal standard. Cyclic voltammograms were obtained at a scan rate of 100 mV s-1. Formal potentials were calculated as the average of cyclic voltammetric anodic and cathodic peaks. The onset potential was determined from the intersection of two tangents drawn at the rising and background current of the cyclic voltammogram. Device Fabrication and Measurement. The hole-injection material MoO3, hole-transporting material NPB (1,4-bis(1naphthylphenylamino)biphenyl), hole-blocking material BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline), and electrontransporting material Alq3 (tris(8-hydroxyquinoline)aluminum) were commercially available. Commercial indium tin oxide (ITO) coated glass with sheet resistance of 10 Ω/square was used as the starting substrate. Before device fabrication, the ITO glass substrates were precleaned carefully and treated by O2 plasma for 2 min. Then the sample was transferred to the deposition system. Ten nanometers of MoO3 was first deposited to ITO substrate, followed by NPB, emissive layer, 10 nm of BCP, and 30 nm of Alq3. Finally, a cathode composed of 1 nm of lithium fluoride and 100 nm of aluminum were sequentially deposited onto the substrate in a vacuum of 10-6 Torr to construct the device. The L-V-J of electroluminescent (EL) devices was measured with a Keithley 2400 Source meter and a Keithley 2000 Source multimeter equipped with a calibrated silicon photodiode. The EL spectra were measured by a JY SPEX CCD3000 spectrometer. All measurements were carried out at room temperature under ambient conditions. Computational Details. The geometric and electronic properties of the compounds were performed with the Gaussian 03 program package. The calculation was optimized by means of the B3LYP (Becke three parameter hybrid functional with Lee-Yang-Perdew correlation functionals) with the 6-31G(d) atomic basis set.32. The triplet states ∆E(T1-S0) were calculated using time-dependent density functional theory (TD-DFT)

Tao et al. calculations with B3LYP/6-311+g(d). Molecular orbitals were visualized using Gaussview. Synthesis of Materials. Bromo-substituted 1,2,4-triazole,22 2-(or 4)-triphenylamine boronic acid,23a and triphenylamine boronate23b were synthesized according to reported procedures. 4′,4′′-(4-Phenyl-4H-1,2,4-triazole-3,5-diyl)bis(N,N-diphenylbiphenyl-4-amine) (p-TPA-p-PTAZ, 1). A mixture of N,Ndiphenyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (0.816 g, 2.2 mmol), 3,5-bis(4-bromophenyl)-4-phenyl-4H-1,2,4triazole (0.455 g, 1 mmol), Pd(PPh3)4 (3% mmol, 35 mg), and Na2CO3 (10 mmol, 1.06 g) in 15 mL of THF and 3 mL of distilled water was refluxed under argon for 24 h. After the reaction, the resulting mixture was cooled to room temperature and then poured into water and extracted with dichloromethane. The organic extracts were collected and dried with anhydrous Na2SO4. After removal of the solvent, the crude product was purified by column chromatography on silica gel using 7:1 (v/ v) dichloromethane/ethyl acetate as eluent. Pure compound as a light yellow powder was obtained by recrystallization from CHCl3/C2H5OH. Yield: 81%. 1H NMR (CDCl3, 300 MHz) δ [ppm]: 7.52-7.44 (m, 16H), 7.29-7.22 (m, 9H), 7.12 (d, J ) 7.8 Hz, 12H), 7.04 (t, J ) 7.5 Hz, 4H). 13C NMR (CDCl3, 75 MHz) δ [ppm]: 153.9, 147.0, 146.7, 140.9, 132.6, 129.5, 128.7, 128.4, 127.0, 125.7, 124.5, 123.8, 122.8, 122.6. Anal. Calcd for C56H41N5 (%): C 85.80, H 5.27, N 8.93; Found: C 85.66, H 5.43, N 8.60. MS (ESI): m/z 783.9 (M+). 4′,4′′-(4-(Naphthalen-1-yl)-4H-1,2,4-triazole-3,5-diyl)bis(N,Ndiphenylbiphenyl-4-amine) (p-TPA-p-NTAZ, 2). 2 was obtained as a white powder with a yield of 86% by the same procedure as that for 1. 1H NMR (CDCl3, 300 MHz) δ [ppm]: 8.04 (d, J ) 7.8 Hz, 1H), 7.96 (d, J ) 8.4, Hz 1H), 7.55 (t, J ) 7.8 Hz, 2H), 7.48 (d, J ) 7.8 Hz, 2H), 7.42-40 (m, 10H), 7.35 (d, J ) 8.1 Hz, 3H), 7.27-7.22, (m, 8H), 7.08 (d, J ) 8.1 Hz, 12H), 7.05-7.00 (m, 4H). 13C NMR (CDCl3, 75 MHz) δ [ppm]: 155.5, 147.9, 147.7, 141.8, 134.4, 133.6, 132.1, 130.9, 130.4, 129.5, 128.9, 128.6, 127.8, 127.6, 127.0, 126.6, 125.8, 125.3, 124.8, 123.8, 123.3, 122.3. Anal. Calcd for C60H43N5 (%): C 86.41, H 5.20, N 8.40; Found: C 86.05, H 5.08, N 8.31. MS (ESI): m/z 834.9 (M+). 3′,3′′-(4-Phenyl-4H-1,2,4-triazole-3,5-diyl)bis(N,N-diphenylbiphenyl-4-amine) (p-TPA-m-PTAZ, 3). 3 was prepared as a white powder with a yield of 83% by the same procedure as that for 1. 1H NMR (CDCl3, 300 MHz) δ [ppm]: 7.60-7.56 (m, 4H), 7.51-7.49 (m, 3H), 7.43 (d, J ) 7.8 Hz, 2H), 7.37 (d, J ) 7.8 Hz, 2H), 7.30-7.21 (m, 16H), 7.13-7.02 (m, 14H). 13 C NMR (CDCl3, 75 MHz) δ [ppm]: 155.3, 148.1, 141.2, 136.0, 134.3, 130.7, 129.9, 129.5, 128.5, 128.3, 128.2, 127.7, 125.0, 124.1, 123.6. Anal. Calcd for C56H41N5 (%): C 85.80, H 5.27, N 8.93; Found: C 85.60, H 5.03, N 8.82. MS (ESI): m/z 783.4 (M+). 3′,3′′-(4-(Naphthalen-1-yl)-4H-1,2,4-triazole-3,5-diyl)bis(N,Ndiphenylbiphenyl-4-amine) (p-TPA-m-NTAZ, 4). 4 was prepared as a white powder with a yield of 78% by the same procedure as that for 1. 1H NMR (CDCl3, 300 MHz) δ [ppm]: 8.12 (d, J ) 8.1 Hz, 1H), 8.03 (d, J ) 8.1 Hz, 1H), 7.67 (s, 2H), 7.62 (d, J ) 8.1 Hz, 2H), 7.58-7.53 (m, 5H), 7.46 (d, J ) 7.2 Hz, 2H), 7.38-7.30, (m, 10H), 7.17 (d, J ) 8.1 Hz, 8H), 7.13-7.06 (m, 12H). 13C NMR (CDCl3, 75 MHz) δ [ppm]: 156.8, 148.6, 141.6, 135.3, 134.8, 131.6, 130.4, 130.0, 129.7, 128.8, 128.6, 128.0, 127.5, 127.4, 126.7, 125.5, 124.7, 124.1, 123.2. Anal. Calcd for C60H43N5 (%): C 86.41, H 5.20, N 8.40; Found: C 86.03, H 5.05, N 7.99. MS (ESI): m/z 834.3 (M+).

PHOLEDs Hosted by 1,2,4-Triazole/TPA

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SCHEME 1: Structures of 1-8

2′,2′′-(4-Phenyl-4H-1,2,4-triazole-3,5-diyl)bis(N,N-diphenylbiphenyl-4-amine) (p-TPA-o-PTAZ, 5). A mixture of 4-(diphenylamino)phenylboronic acid (0.58 g, 2 mmol), 3,5-bis(2bromophenyl)-4-phenyl-4H-1,2,4-triazole (0.36 g, 0.8 mmol), Pd(PPh3)4 (30 mg, 3% mmol), and KOH (0.448 g, 10 mmol) in 15 mL of THF and 1.5 mL of distilled water was refluxed for 24 h under argon atmosphere. After the reaction, the resulting mixture was cooled to room temperature and then poured into water and extracted with dichloromethane. The organic extracts were collected and dried with anhydrous Na2SO4. After removal of the solvent, the crude product was purified by column chromatography on silica gel using 7:1 (v/v) dichloromethane/ ethyl acetate as eluent. Pure compound as a white powder was obtained by recrystallization from CHCl3/C2H5OH. Yield: 92%. 1 H NMR (CDCl3, 300 MHz) δ [ppm]: 7.74-7.71 (m, 2H), 7.51-7.48 (m, 4H), 7.23-7.16 (m, 10H), 7.05-6.98 (m, 12H), 6.86 (t, J ) 7.2 Hz, 1H), 6.74-6.70 (m, 6H), 6.66 (d, J ) 8.1

Hz, 2H), 6.49 (d, J ) 8.1 Hz, 4H). 13C NMR (CDCl3, 75 MHz) δ [ppm]: 149.9, 142.9, 142.3, 136.7, 129.0, 128.4, 127.2, 126.0, 124.9, 124.7, 124.6, 123.3, 122.7, 122.2, 121.4, 120.3, 119.7, 118.8, 118.5. Anal. Calcd for C56H41N5 (%): C 85.80, H 5.27, N 8.93; Found: C 85.90, H 5.50, N 8.64. MS (ESI): m/z 784.2 (M+). 4′,4′′,4′′′-(4H-1,2,4-Triazole-3,4,5-triyl)tris(N,N-diphenylbiphenyl-4-amine) (t-p-TPA-p-PTAZ, 6). 6 was obtained with a procedure similar to that for 1 with a yield of 75%, except with the use of 3,4,5-tris(4-bromophenyl)-4H-1,2,4-triazole (0.43 g, 0.8 mmol) and N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (1.07 g, 2.9 mmol) as the reactants. 1H NMR (CDCl3, 300 MHz) δ [ppm]: 7.66 (d, J ) 7.5 Hz, 2H), 7.52 (m, 10H), 7.45 (d, J ) 7.2 Hz, 4H), 7.26 (m, 14H), 7.16-7.04 (m, 24H). 13C NMR (CDCl3, 75 MHz) δ [ppm]: 154.9, 148.7, 147.9, 147.6, 142.0, 141.9, 133.7, 132.6, 129.6, 129.5, 129.4, 128.4, 127.9, 127.8, 126.7, 125.3, 124.9, 124.8,

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SCHEME 2: Synthetic Routes of 1-8

TABLE 1: Physical Data of 1-8 HOMO/ HOMO/ compd Tg/Tm/Td (°C) λabsa (nm) λem,maxa (nm) λem,maxb (nm) LUMOexp c (eV) LUMOcal d (eV) ETexp e (eV) ∆E(T1-S0)cal d (eV) µd (D) 1 2 3 4 5 6 7 8 e

134/287/505 146/311/531 124/181/518 130/NA/529 124/259/480 155/246/521 119/274/509 106/257/483

354 359 336 336 332 354 304/343 307/352

448 451 418 420 419 448 447 422

413 416 395 395 397 413 427 413

5.23/2.30 5.20/2.29 5.22/NA 5.22/2.28 5.26/2.31 5.23/2.31 5.33/2.30 5.31/NA

5.24/1.66 5.22/1.96 5.22/1.49 5.21/1.99 5.11/1.3 5.22/1.7 5.29/1.63 5.14/1.45

2.39 2.37 2.50 2.50 2.52 2.41 2.48 2.57

a Measured in DCM. b Measured in toluene. c Determined from the onset of the oxidation/reduction. Measured from phosphorescence spectra in film state at 77 K.

123.8, 123.6, 123.4. Anal. Calcd for C74H54N6 (%): C 86.52, H 5.30, N 8.18; Found: C 86.28, H 5.42, N 8.02. 4′,4′′-(4-Phenyl-4H-1,2,4-triazole-3,5-diyl)bis(N,N-diphenylbiphenyl-2-amine) (o-TPA-p-PTAZ, 7). 7 was obtained as a white powder with a yield of 77% by the same procedure as that for 5. 1H NMR (CDCl3, 300 MHz) δ [ppm]: 7.42-7.40 (m, 4H), 7.34-7.30 (m, 6H), 7.13 (m, 9H), 7.06 (t, J ) 7.5 Hz, 8H), 6.98 (d, J ) 6.9 Hz, 2H), 6.83 (t, J ) 7.5 Hz, 12H). 13 C NMR (CDCl3, 75 MHz) δ [ppm]: 154.8, 147.6, 145.1, 141.3, 139.2, 135.3, 131.9, 130.1, 129.7, 129.6, 129.4, 128.9, 128.7, 128.5, 127.9, 126.0, 125.3, 122.4, 121.7. Anal. Calcd for C56H41N5 (%): C 85.80, H 5.27, N 8.93; Found: C 86.01, H 5.62, N 8.84. MS (ESI): m/z 783.6 (M+). 3′,3′′-(4-Phenyl-4H-1,2,4-triazole-3,5-diyl)bis(N,N-diphenylbiphenyl-2-amine) (o-TPA-m-PTAZ, 8). 8 was obtained as a white powder with a yield of 84% by the same procedure as that for 5. 1H NMR (CDCl3, 300 MHz) δ [ppm]: 7.42 (t, J ) 7.2 Hz, 1H), 7.36-7.20 (m, 12H), 7.13 (d, J ) 7.8 Hz, 2H), 7.06 (t, J ) 7.8 Hz, 8H), 7.01-6.92 (m, 4H), 6.82 (t, J ) 7.5 Hz, 14H). 13C NMR (CDCl3, 75 MHz) δ [ppm]: 154.8, 147.6, 145.0, 140.3, 139.2, 135.3, 132.0, 130.1, 130.0, 129.5, 129.4, 129.0, 128.0, 127.8, 127.4, 126.7, 126.0, 122.4, 121.7. Anal. Calcd for C56H41N5 (%): C 85.80, H 5.27, N 8.93; Found: C 86.06, H 5.20, N 8.71. MS (ESI): m/z 783.5 (M+). Results and Discussion Synthesis and Characterization. The structures and synthetic routes of the new compounds 1-8 are depicted in Schemes 1 and 2, respectively. All the compounds were readily prepared

2.59 2.64 2.75 2.64 2.82 2.67 2.82 2.91 d

6.12 6.15 5.81 5.96 5.06 7.63 6.39 6.06

Values from DFT calculation.

through Suzuki cross-coupling reactions of dibromo-substituted 1,2,4-triazole with triphenylamine boronate or boronic acid under different base conditions. It is notable that 1-4 and 6 were prepared with the Na2CO3 as base, while 5, 7, and 8 were synthesized under the condition of strong basic KOH owing to the sterically hindered ortho-dibromide or ortho-linked triphenylamine boronic acid.24 The structures of the new compounds were fully characterized by 1H NMR and 13C NMR spectroscopies, mass spectrometry, and elemental analysis (see Experimental Section). Thermal Properties. The thermal stabilities of the new compounds were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Table 1). Their decomposition temperatures, as indicated by 5% weight loss, range from 481 to 531 °C. The high Td values suggest that they could be capable of enduring vacuum thermal sublimation during OLED fabrication. Their glass transition temperatures (Tg) are in the range 106-155 °C, which are much higher than those of usual host materials, such as 4,4′-bis(9carbazolyl)-2,2′-biphenyl (CBP, 62 °C) and 1,3-bis(9-carbazolyl)benzene (m-CP, 60 °C).25 The high Tg values suggest that the new compounds could form morphologically stable and uniform amorphous films, an essential property for OLEDs upon thermal evaporation.26 The high Td and Tg of the compounds may be attributed to their rigid triazole core. Photophysical Properties. Figure 1a shows the UV-vis absorption spectra of 1-8. The absorptions around 300 nm for 7 and 8 with o-TPA linkage could be assigned to triphenylamine-centered n-π* transition, which appears as a sub-band

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Figure 2. Cyclic voltammograms for oxidation curves of 1-8.

SCHEME 3: Labeled φ1-φ5 with 6 as a Representative

Figure 1. UV-vis absorption spectra in CH2Cl2 (a) and fluorescence spectra in toluene (b) and CH2Cl2 (c).

or shoulders for 1-6 with p-TPA linkage. On the other hand, the longer wavelength absorptions ranging from 336 to 359 nm could be attributed to π-π* transition from the electrondonating triphenylamine moiety to the electron-withdrawing triazole core. Notably, the intensities of π-π* transitions are sharply decreased for 7 and 8 with o-TPA linkage as compared to 1-6 with p-TPA linkage. This could be explained by the suppression of intramolecular charge transfer owing to the poor π conjugation between TPA and triazole moieties for o-TPA linkage. Figure 1b and 1c show the photoluminescent (PL) spectra of 1-8 in toluene and dichloromethane solution, respectively. The peak emissions are in the range 395-427 nm in toluene solution, whereas they exhibit significant red shifts to the range 418-451 nm in CH2Cl2 solution due to the increasing solvent polarity. The emission wavelengths and the degree of bathochromic shifts correlate with the linkage modes between the TPA and PTAZ or NTAZ, because the linkage modes determine the degrees of conjugation and intramolecular charge transfer between donor and acceptor. For

example, 1, 2, and 6 with the linkage of p-TPA and p-PTAZ or NTAZ show peak emissions at 413-416 nm in toluene solution, and ca. 35 nm red shift in dichloromethane solution due to their easy intramolecular charge transfer, whereas 8 with the linkage of twisted o-TPA and m-TAZ exhibits only 9 nm red shift due to the decreasing π conjugation between donor and acceptor. To be an appropriate host material, the host should have a proper triplet energy (ET) higher than the phosphorescent guest emitter to prevent reverse energy transfer from the guest back to the host. The triplet energies of the compounds are determined to be in the range 2.37-2.57 eV by the highestenergy vibronic sub-band of the phosphorescence spectra at 77 K. Generally, the triplet energies are inverse of the π-conjugation degree between TPA and triazole moieties. For example, 7 and 8 with the twisted o-TPA linkage exhibit higher ET values (2.48 eV for 7, 2.57 eV for 8) than their p-TPA analogues of 1 and 3 (2.39 eV for 1, 2.50 eV for 3), respectively, whereas 1 and 2 with the linkage of p-TPA and p-PTAZ or NTAZ show lower triplet energy levels (2.39 eV for 1, 2.37 eV for 2). Electrochemical Properties. The electrochemical properties of the compounds were probed by cyclic voltammetry (CV); see Figure 2. During the oxidation scan in CH2Cl2, all the compounds show reversible oxidation processes. Furthermore, an additional peak at ca. 0.3 V was found, most likely

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Figure 3. Spatial distributions of the calculated HOMO and LUMO energy levels of 1-8.

Figure 4. (a) Current density-voltage-luminance characteristics. (b) Current efficiency versus current density curves for deep red electrophosphorescent devices A-H.

resulting from electrochemical polymerization due to the existence of the electrochemically active sites in triphenylamine units.3f,27 Interestingly, this peak is not obvious for 5, presumably owing to its twisted conformation. The reduction processes in THF solution for the compounds are quasireversible or irreversible like most triazole derivatives.28 The HOMO/LUMO energy levels were determined from the onsets of the oxidation and reduction potentials with reference to the energy level of ferrocene (4.8 eV) (relative to vacuum level), respectively (Table 1). The compounds exhibit close LUMO levels (2.28-2.31 eV), whereas their HOMO levels

vary in a range from 5.20 to 5.33 eV, depending on the linkage modes of electronic donor and acceptor moieties. Compared to t-Bu-TAZ (HOMO/LUMO: 6.3/2.7 eV)6f or TAZ (6.6/2.6 eV),21 the introduction of the triphenylamine moiety raises both HOMO and LUMO energy levels of the compounds. Theoretical Calculations. To understand the structure-property relationships of the compounds at the molecular level, the geometric and electronic properties of the compounds were studied by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations using B3LYP hybrid functional (for details, see the Experimental Section). The calculated HOMO and LUMO energy levels as well as triplet energy gaps ∆E(T1-S1) for 1-8 are summarized in Table 1. The dihedral angles (φ) estimated through DFT calculations are listed in Table S1 in the Supporting Information, and the locations of relevant φ1-φ5 are labeled in Scheme 3 with 6 as a representative. The φ1 (the dihedral angle between 1,2,4-triazole ring and 4-aryl ring) values for 1 (66.65°) and 3 (66.52°) are less than those of 2 (75.69°) and 4 (74.55°) with bulky 4-naphthyl, respectively. For the ortho-structured 5, all the φ1-φ5 values are larger than those for para- and meta-structured 1 and 3, attributing to its twisted conformation. Similarly, the φ2 and φ5 values (the dihedral angles between the adjacent phenyl ring) for o-TPA linked 7 and 8, are also larger than those for 1-4 with p-TPA linkage. Figure 3 illustrates the spatial distributions of the calculated HOMO and LUMO energy levels of 1-8. The HOMO levels of all the compounds are mainly located on electron-donating triphenylamine moiety. The LUMO levels of 1, 3, and 5-8 with 4-phenyl-1,2,4-triazole moiety are mainly located on the electron-withdrawing 1,2,4-triazole moiety, whereas those of 2 and 4 are mostly distributed on the naphthyl ring. The spacial distributions between HOMO and LUMO levels are as follows: (i) naphthyl-based 2 and 4 are larger than phenyl-based 1 and 3, respectively; (ii) ortho- and meta-structured compounds are larger than their para-structured analogues, such as 4 > 2, 3 (5)

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TABLE 2: EL Data of Devices A-H device

host

Vona (V)

Lmaxb (cd/m2), voltages (V)

ηc,maxc (cd/A)

ηp,maxd (lm/W)

ηext,maxe (%)

A B C D E F G H

1 2 3 4 5 6 7 8

2.9 3.3 2.9 2.9 3.1 3.9 2.9 3.1

2796, 13.7 10 059, 15.5 29 483, 12.1 20 230, 13.5 10 210, 14.3 7251, 14.3 21 357, 16.9 10 539, 17.1

7.1 7.6 8.4 8.1 9.8 4.7 11.8 12.4

6.3 4.1 7.4 6.6 9.3 3.4 10.4 9.8

11.5 10.0 10.1 10.5 14.1 6.5 15.4 16.4

a Turn-on voltage. efficiency.

b

Maximum luminance.

c

Maximum current efficiency.

Figure 5. (a) Current density-voltage-luminance characteristics. (b) Current efficiency versus current density curves for green electrophosphorescent devices I-M.

> 1, 8 > 3, etc. The complete separation between HOMO and LUMO levels is favorable to the efficient hole- and electrontransporting properties and the prevention of reverse energy transfer, and thus benefits the EL performance of the PHOLEDs.4 Electroluminescence. In view of the triplet energies of 1-8 (2.37-2.57 eV), we first fabricated the deep red organic lightemitting devices A-H by using red phosphorescent emitter bis(1-phenylisoquinolinato-N,C2′)iridium(acetylacetonate)[(piq)2Ir(acac)] (ET ) 2.0 eV)29 as guest and 1-8 as hosts, respectively. The configurations of devices are as follows: ITO/MoO3(10 nm)/1, 4-bis[(1-naphthylphenyl)amino]biphenyl (NPB, 80 nm)/host 1-8; 6 wt % [(piq)2Ir(acac)] (20 nm)/

d

Maximum power efficiency.

e

Maximum external quantum

2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, 10 nm)/ tris(8-hydroxyquinoline)aluminum (Alq3, 30 nm)/LiF (1 nm)/ Al (100 nm). NPB and Alq3 are used as the hole- and electron-transporting materials, respectively; BCP is used as hole and exciton blocking layer; MoO3 and LiF serve as holeand electron-injecting layer, respectively. The current density-voltage-luminance characteristics and efficiency versus current density curves are shown in Figure 4, and the EL data are summarized in Table 2. Almost all devices emit deep red light with 1931 Commission International de L’Eclairage x, y coordinates of (0.68, 0.32). These devices except device F display low turn-on voltages of 2.9-3.3 V (corresponding to a brightness of 1 cd/m2), which are significantly lower than that of CBP host (5.3 V) under identical device structures.6a The low turn-on voltages can be attributed to the well-matched HOMO energy levels between the triphenylamine/triazole host (5.20-5.33 eV) and the hole-transport NPB layer (5.4 eV).30 Comparing the device performance (Table 2), the following trends can be found: (i) The naphthyl-based 2 and 4 exhibit performances similar to those of their phenyl analogues 1 and 3, respectively; (ii) device A with p-TPA-p-PTAZ (1) as host shows a maximum current efficiency (ηc,max) of 7.1 cd/A and a power efficiency (ηp,max) of 6.3 lm/W, respectively. These values for meta- and ortho-structured 3 and 5 are enhanced to 8.4 cd/A and 7.4 lm/W (device C) and 9.8 cd/A and 9.3 lm/W (device E), respectively. EL efficiencies for 1, 3, and 5 follow the sequence of 5 (ortho) > 3 (meta) > 1 (para); (iii) devices G and H hosted by 7 and 8 with o-TPA linkage display significantly better EL performance than their p-TPA analogues 1 and 3. The best performance is achieved in device H hosted by 8 with the linkage of o-TPA and m-PTAZ, with a maximum current efficiency of 12.4 cd/A and a maximum external quantum efficiency of 16.4%, which are close to the best results for deep red electrophosphorescence.3g,6a The above-mentioned trends could be explained by the two aspects. First, the EL performances correlate with the spatial separation between HOMO and LUMO levels at hole- and electron-transport moieties, as the complete separation is advantageous to the efficient charge carrier transport. Second,

TABLE 3: EL Data of Devices I-M

a

device

host

Vona (V)

Lmaxb (cd/m2), voltages (V)

ηc,maxc (cd/A)

ηp,maxd (lm/W)

ηext,maxe (%)

I J K L M

3 4 5 7 8

2.7 2.7 2.7 2.7 2.9

29 483, 12.1 48 908, 13.1 48 616, 13.7 48 534, 16.1 29 968, 16.5

29.0 38.7 49.0 42.8 50.7

24.6 26.5 52.7 40.5 38.0

8.2 10.5 13.6 12.0 14.2

Turn-on voltage. efficiency.

b

Maximum luminance.

c

Maximum current efficiency.

d

Maximum power efficiency.

e

Maximum external quantum

608

J. Phys. Chem. C, Vol. 114, No. 1, 2010

the high triplet energies of the hosts could efficiently suppress adverse energy back transfer from guest to host, consequently resulting in good device performances. We have also fabricated green electrophosphorescent devices by using 3, 4, 5, 7, and 8 as the hosts because of their ET values higher than 2.4 eV of the green phosphorescent emitter Ir(ppy)3. The device structures are identical with the red PHOLEDs, except the doping concentration of 9 wt %. The current density-voltage-brightness characteristics and efficiency versus current density curves are shown in Figure 5, and the EL data are summarized in Table 3. All the green emissive devices exibit turn-on voltages as low as 2.7-2.9 V. The device performances have trends similar to those of the deep red emissive devices. Device K hosted by 5 with twisted conformation exhibits an Lmax of 48 618 cd/ m2, ηc,max of 49.0 cd/A, and ηp,max of 52.7 lm/W, while these values are 29 968 cd/m2, 50.7 cd/A, and 38 lm/W for device M hosted by 8 with the linkage of o-TPA and m-PTAZ. Conclusion In conclusion, we have developed a series of new 1,2,4triazole-cored triphenylamine derivatives through Suzuki cross-coupling reaction. The photophysical and electrochemical properties of the hybrids can be tuned through the different linkages between the electronic donor and acceptor components. The EL performances of the devices hosted by the hybrids correlate with their molecular structures, and substantial gains in the EL performances can be made by subtle changes in the structural design of the host material. Devices hosted by o-TPA-m-PTAZ (8) achieve the best EL performance, with a maximum current efficiency of 12.4 cd/A and maximum external quantum efficiency of 16.4% for deep red electrophosphorescence and 50.7 cd/A and 14.2% for green electrophosphorescence. The high EL performance can be attributed to the well-matched energy levels between the host and hole-transport layer, the high triplet energy of the host, and complete spatial separation of HOMO and LUMO energy levels. The design principles for the host molecules in this work could be a guide to finding new efficient bipolar host materials for phosphorescent emitters. Acknowledgment. We thank the National Natural Science Foundation of China (Project Nos. 50773057, 90922020, and 20621401) and the National Basic Research Program of China (973 Program; 2009CB623602, 2009CB930603) for financial support. Supporting Information Available: Dihedral angles (φ) estimated through DFT calculations for 1-8 and EL spectra for the devices. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (b) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (c) Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R. Nature 2006, 440, 908. (d) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (e) Service, R. F. Science 1996, 273, 878. (2) (a) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lu¨ssem, B.; Leo, K. Nature 2009, 459, 234. (b) Baldo, M. A.; Lamansky, S. P.; Burrows, E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (c) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904. (d) Su, Y. J.; Huang, H. L.; Li, C. L.; Chien, C. H.; Tao, Y. T.; Chou, P. T.; Datta, S.; Liu,

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