J. Phys. Chem. C 2008, 112, 15517–15525
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A Novel Deep Blue-Emitting ZnII Complex Based on Carbazole-Modified 2-(2-Hydroxyphenyl)benzimidazole: Synthesis, Bright Electroluminescence, and Substitution Effect on Photoluminescent, Thermal, and Electrochemical Properties Hui Xu,*,†,‡ Zhi-Feng Xu,† Zheng-Yu Yue,*,†,‡ Peng-Fei Yan,†,‡ Bin Wang,† Li-Wei Jia,† Guang-Ming Li,†,‡ Wen-Bin Sun,† and Ju-Wen Zhang† School of Chemistry and Materials, Heilongjiang UniVersity, 74 Xuefu Road, Harbin 150080, People’s Republic of China, and Heilongjiang ProVince Key Laboratory of Photoelectrical and Energy Materials, Heilongjiang UniVersity, 74 Xuefu Road, Harbin 150080, People’s Republic of China ReceiVed: April 17, 2008; ReVised Manuscript ReceiVed: July 18, 2008
A novel deep blue-emitting ZnII complex Zn(Lc)2 (Lc- ) 2-(1-(6-(9H-carbazol-9-yl)hexyl)-1H-benzo[d]imidazol-2-yl)phenolate) based on a carbazole-functionalized N^O ligand was synthesized by a modified method. Other two ZnII complexes (Zn(La)2, La- ) 2-(1H-benzo[d]imidazol-2-yl)phenolate; Zn(Lb)2, Lb- ) 2-(1ethyl-1H-benzo[d]imidazol-2-yl)phenolate) were also prepared for comparison. The remarkable substitution effect on the photoluminescent and thermal properties of the complexes was studied. The investigation indicated an unexpected amplifying hypsochromic effect of the substituents on the emission of the complex in the solid state: the larger substituent corresponded to the larger blue shift of the emission of the complex (Zn(Lc)2 has the shortest emission wavelength of 422 nm as the deep blue emission among these three complexes). The stronger steric effect induced by the bulky substitutions should be one of the most important factors. Among the three ZnII complexes, the temperature of decomposition of Zn(Lc)2 is the highest at 427 °C. Cyclic voltammetry (CV) of the complexes showed that the carbazole moieties remarkably improved the hole injection ability of Zn(Lc)2 with the HOMO energy level 0.6 eV higher than those of Zn(La)2 and Zn(Lb)2. The good hole injection and transporting ability of Zn(Lc)2 was further proved by its three-layer devices, in which the electroluminescent (EL) emission mainly originated from the electron-transporting Alq3 layer. Through the four-layer devices with the hole-blocking layer, the pure blue emission of Zn(Lc)2 at 452 nm was demonstrated. Zn(Lc)2 seems favorable among the blue-emitting ZnII complexes with a brightness more than 2000 cd m-2, a high efficiency stability, and an excellent EL spectra stability. Introduction Luminescent metal chelate complexes seem attractive for electroluminescent (EL) application on the basis of a precursory study by C. W. Tang and S. A. VanSlyke.1 Among these materials, Al3+ and Zn2+ complexes have received more attention because of their high thermal stability, excellent carrier transporting ability, and tunable emission color.2-9 Green-,5,10-13 blue-,14-31 red-,33-37 and white38-41-emitting ZnII complexes had been developed for organic light-emitting diodes (OLED). Because blue-emitting materials should have a large enough energy gap and many blue-emitting materials reported actually emitted blue-green light in their devices, which was induced by their wide emission ranges, bright and stable blue-emitting materials with high color purity are limited. As one of the most important types of blue-emitting materials, the blue-emitting ZnII complexes have been studied by many groups. After the first blue-emitting azomethine-ZnII complex reported by Y. Hamada,16 many blue-emitting ZnII complexes based on Schiff base ligands were developed.20,23,26,27,29 However, compared with the first report, the EL performance of these latter complexes was * To whom correspondence should be addressed. Tel: +86 (451) 8660 8610; fax: +86 (451) 86608042; e-mail:
[email protected] (Z.Y.Y.) and
[email protected] (H.X.). † School of Chemistry and Materials. ‡ Heilongjiang Province Key Laboratory of Photoelectrical and Energy Materials.
not improved. Only a few complexes have good EL properties, such as brightness more than 1000 cd m-2.14 Recently, ZnII complexes based on benzo-heterocyclic ligands have emerged.30,32,33,37,42,43 Among these complexes, Zn(BOX)2 was reported as blue-emitting material with EL brightness of 1039 cd m-2.13,16 The photoluminescent (PL) properties of another blue-emitting complex, Zn(pbm)2, was also reported.30,31 However, it should be noted that few studies have focused on multifunctionalization of the benzo-heterocyclic ligand-based ZnII complexes. It is believed that the multifunctionalization of the EL materials can endow the materials with more comprehensive capabilities and improve the performance of their devices.42-44 In this work we report a novel blue-emitting ZnII complex Zn(Lc)2 (Lc ) 2-(1-(6-(9H-carbazol-9-yl)hexyl)-1H-benzo[d]imidazol-2-yl)phenolate). A convenient method for functionalization of the ligand is also reported. Another two unfunctionalized complexes Zn(La)2 and Zn(Lb)2 (La ) 2-(1H-benzo[d]imidazol2-yl)phenolate, Lb ) 2-(1-ethyl-1H-benzo[d]imidazol-2-yl)phenolate) were also prepared for comparison (Scheme 1). The choice of the 2-(2-hydroxyphenyl)benzimidazole derivatives as the ligands was based on their bright blue emissions, flat conjugated plane, and the modifiable position at the benzimidazole ring. A hole-transporting carbazole group was introduced in HLc by a hexyl, while the substituent in HLb is an ethyl. HLa is unsubstituted. The effect of the substituents on PL properties of the complexes was investigated. It was found that
10.1021/jp803325g CCC: $40.75 2008 American Chemical Society Published on Web 09/06/2008
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SCHEME 1: Synthesis Procedure of the ZnII Complexes
SCHEME 2: Synthesis Procedure of Three O^N Ligands
SCHEME 3: The Common Route Used To Substitute N-H in Benzimidazole
the substituent with a larger volume can induce the emission of the complex in the solid state to be more hypsochromic. The improvement of the hole-injection ability of Zn(Lc)2 was proved by cyclic voltammetry (CV) analysis. The pure blue electroluminescence from the four-layer devices based on Zn(Lc)2 was demonstrated. With the brightness more than 2000 cd m-2, external quantum efficiency of 0.4%, excellent efficiency, and spectral stability and high color purity, Zn(Lc)2 seems favorable among the most efficient blue-emitting ZnII complexes.
Experimental Section Materials and Instruments. All the reagents and solvents used for the synthesis of the ligands and complexes were purchased from Aldrich and Acros and used without further purification. Infrared spectra were recorded using a Bruker-EQUINOX55 FT-IR spectrometer. 1H NMR spectra were recorded using a Varian Mercury plus 300NB spectrometer relative to tetram-
Deep Blue-Emitting ZnII Complex
Figure 1. Absorption and PL spectra of HLa (solid lines), HLb (dash lines), and HLc (dot lines) in CH2Cl2.
Figure 2. Absorption and PL spectra of HLa (solid lines), HLb (dash lines), and HLc (dot lines) in the solid state.
ethylsilane (TMS) as the internal standard. Molecular masses were determined by electrospray ionization mass spectrometry (ESI-MASS) (Finnigan LCQ or Aglient 6890-5973 GC-MASS). Elemental analyses were performed on a Vario EL III elemental analyzer. Absorption and photoluminescence (PL) emission spectra of the target compound were measured using a Shimadzu UV-3150 spectrophotometer and a Shimadzu RF-5301PC spectrophotometer, respectively. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on Shimadzu DSC-60A and DTG-60A thermal analyzers under nitrogen atmosphere at a heating rate of 10 °C min-1. Cyclic voltammetry (CV) of the complex films was performed on an Eco Chemie Autolab in a typical three-electrode cell with a platinum sheet working electrode, a platinum wire counter electrode, and a silver/silver nitrate (Ag/Ag+) reference electrode. All electrochemical experiments were carried out under a nitrogen atmosphere at room temperature in an electrolyte solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu4N+PF6-) in CH3CN at a sweep rate of 100 mV s-1. The complex films were prepared by coating their DMSO solutions on the electrode and volatilizing the solvent in a vacuum oven. The PL quantum yields of the ligands and complexes in the film were measured with an integrating-sphere photometer Labsphere URS-600 uniform source system with an optical power meter (model 1830-C). The films were formed by vacuum deposition under the device fabrication conditions. Preparation of the Ligands. 2-(1H-Benzo[d]imidazol-2yl)phenol (HLa). P2O5 (31.11 g, 0.22 mol) was added to H3PO4 (85%, 20 mL) in portions. Then the mixture was heated to 110
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15519 °C for 2 h to form homogeneous polyphosphoric acid (PPA). Salicylic acid (1.39 g, 10 mmol) and benzene-1,2-diamine (1.08 g, 10 mmol) were added, and then the mixture was heated to 200 °C for 4 h and then poured into ice-water under stirring. The pH of the mixture was adjusted to 7.0 by aqeous NaOH (2 M). The precipitate was filtered and then recrystallized by ethanol as white needle crystals (1.05 g, 50%): mp 237-238 °C; 1H NMR (300 MHz, DMSO-d6, TMS): δ ) 13.21 (m, 2H), 8.08 (d, J ) 7.7 Hz, 1H), 7.67 (m, 2H), 7.39 (tr, J ) 7.5 Hz, 1H), 7.29 (m, 2H), 7.03 (m, 2H) ppm; FT-IR (KBr): ν bar ) 3322 (m), 3245 (s), 3054 (m), 1593 (s), 1492 (s), 1417 (s), 1319 (m), 1259 (s), 1130 (w), 1037 (w), 840 (s), 798 (w), 734 (s), 522 (m), 468 cm-1 (w); ESI-MS: m/z (%): 211 (100) [M+]; elemental analysis calcd (%) for C13H10N2O: C 74.27, H 4.79, N 13.33; found: C 74.22, H 4.81, N 13.49. 2-(1H-Benzo[d]imidazol-2-yl)phenyl 4-methylbenzenesulfonate (Ts-HLa). HLa (1.05 g, 5 mmol) and triethylamine (3.5 mL, 25 mmol) were dissolved in CH2Cl2 (10 mL). The mixture was cooled to 0 °C, and then 4-methylbenzenesulfonyl chloride (1.03 g, 5 mmol) in CH2Cl2 (10 mL) was added dropwise. After addition, the mixture was warmed to room temperature and stirred for 1 h. The reaction was quenched by water, and then the mixture was extracted by dichloromethane (3 × 30 mL). The organic layer was dried with MgSO4. The solvent was removed in vacuo, and the residue was purified by flash column chromatography using petroleum ether/ethyl acetate (2:1) as eluent as white powder (1.71 g, 90%): mp 133-135 °C; 1H NMR (300 MHz, DMSO-d6, TMS): δ ) 9.95 (br, 1H), 8.22 (d, J ) 7.2 Hz, 1H), 7.53 (m, 9H), 6.9 (d, J ) 8.1 Hz, 2H), 2.22 (s, 3H) ppm; FT-IR (KBr): ν bar ) 3058 (m), 2676 (w), 2360 (w), 1594 (m), 1376 (s), 1268 (w), 1197 (m), 1172 (s), 1081 (w), 968 (m), 860 (s), 808 (s), 775 (s), 728 (s), 651 (m), 547 cm-1 (s); ESI-MS: m/z (%): 365 (100) [M+], 210 (40) [La+]; elemental analysis calcd (%) for C20H16N2O3S: C 65.92, H 4.43, N 7.69, S 8.80; found: C 65.87, H 4.51, N 7.74, S 8.99. 2-(1-Ethyl-1H-benzo[d]imidazol-2-yl)phenol (HLb). Under N2, Ts-HLa (0.76 g, 2 mmol) and tetrabutylammonium bromide (TBAB, 64.5 mg, 0.2 mmol) were dissolved in DMSO (10 mL). Aqueous K2CO3 (2 M, 3.3 mL) was added in drops and stirred for 0.5 h. Then ethyl bromide (0.16 mL, 2.2 mmol) was added dropwise. The mixture was refluxed for 12 h at 70 °C. After being cooled to room temperature, the mixture was poured into water (100 mL) and extracted by dichloromethane (3 × 30 mL). The organic layer was dried with MgSO4. The solvent was removed in vacuo, and the residue was a pale yellow stick solid as crude Ts-HLb. This crude product was dissolved in methanol (20 mL). Aqueous NaOH (2 M, 5 mL) was added, and the mixture was refluxed for 2 h. Then the mixture was cooled to room temperature and neutralized by dilute aqueous HCl. The mixture was extracted by dichloromethane (3 × 30 mL). The organic layer was dried with MgSO4. The solvent was removed in vacuo, and the residue was purified by flash column chromatography using petroleum ether/ethyl acetate (8:1) as eluent to afford a white powder (0.38 g, 80%): mp 120-122 °C; 1H NMR (300 MHz, DMSO-d6, TMS): δ ) 10.94 (s, 1H), 7.67 (t, J ) 7.5 Hz, 2H),7.5 (d, J ) 7.8 Hz, 1H), 7.4 (tr, J ) 7.8 Hz, 1H), 7.27 (m, 2H), 7.02 (m, 2H), 4.3 (q, J ) 7.2 Hz, 2H), 1.28 ppm (t, J ) 7.2 Hz, 3H); FT-IR (KBr): ν bar ) 3446 (m), 3058 (w), 2939 (m), 1608 (m), 1589 (m), 1460 (s), 1405 (s), 1386 (m), 1278 (s), 1230 (w), 1132 (w), 1031 (w), 754 (s); ESI-MS: m/z (%): 237 (100) [M+]; elemental analysis calcd (%) for C15H14N2O: C 75.61, H 5.92, N 11.76; found: C 75.57, H 5.96, N 11.68.
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TABLE 1: PL Properties of the Ligands and Their ZnII Complexes emission peak/fwhm (nm) compound
absorption peak in CH2Cl2 (nm)
HLa HLb HLc Zn(La)2 Zn(Lb)2 Zn(Lc)2
240, 228, 229, 229, 228, 240,
309 238s, 292, 320 238, 265, 295, 320, 332s 251s, 292, 300, 336, 364b 252s, 292, 344b 264, 294, 332s, 346
9-(6-Bromohexyl)-9H-carbazole (Br-Cz). Carbazole (1.67 g, 10 mmol), 1,6-dibromohexane (2.37 mL, 15 mmol), TBAB (129 mg, 0.4 mmol), and aqueous NaOH (50%, 5 mL) were added to benzene (5 mL). The mixture was stirred in room temperature for 12 h. The organic phase was separated and washed by water (2 × 30 mL) and then dried with MgSO4. The solvent was removed in vacuo, and the residue was purified by flash column chromatography using petroleum ether as eluent to afford white needle crystals (1.98 g, 60%): mp 50-52 °C; 1H NMR (300 MHz, CDCl3, TMS): δ ) 8.12 (d, J ) 7.5 Hz, 2H), 7.44 (m, 4H), 7.23 (m, 2H), 4.32 (t, J ) 7.2 Hz, 2H), 3.37 (t, J ) 7.2 Hz, 2H), 1.90 (m, 2H), 1.81 (m, 2H), 1.46 (m, 4H) ppm; FTIR (KBr): ν bar ) 3046 (w), 2933 (m), 2856 (m), 1899 (w), 1592 (m), 1484 (s), 1458 (s), 1328 (s), 1274 (w), 1236 (m), 1211 (m), 1180 (m), 1149 (m), 1124 (w), 1060 (w), 997 (w), 848 (w), 754 (s), 727 (s), 638 cm-1 (m); GC-MS: m/z (%): 330 (100) [M+]; elemental analysis calcd (%) for C18H20BrN: C 65.46, H 6.10, N 4.24; found: C 65.37, H 6.11, N 4.35.
Figure 3. Absorption and PL spectra of Zn(La)2 (solid lines), Zn(Lb)2 (dash lines), and Zn(Lc)2 (dot lines) in CH2Cl2.
Figure 4. Absorption and PL spectra of Zn(La)2 (solid lines), Zn(Lb)2 (dash lines), and Zn(Lc)2 (dot lines) in the solid state.
in CH2Cl2
in the solid state
PL Q.Y. in film (%)
428/81 468/70 468/65 416/58 422/58 422/58
465/64 467/67 473/73 445/51 434/43 422/45
39.7 38.6 20.4 66.8 66.0 63.7
2-(1-(6-(9H-Carbazol-9-yl)hexyl)-1H-benzo[d]imidazol-2yl)phenol (HLc). The same procedure was followed as that used for HLb from Ts-HLa (760 mg, 2 mmol) except that Br-Cz (726 mg, 2.2 mmol) was used instead of ethyl bromide. The crude product was purified by flash column chromatography using petroleum ether/ethyl acetate (4:1) as eluent to afford a white powder (589 mg, 73%): mp 125-127 °C; 1H NMR (300 MHz, DMSO-d6, TMS): δ ) 10.77 (s, 1H); 8.13 (d, J ) 7.8 Hz, 2H), 7.49 (m, 8H), 7.25 (m, 2H), 7.18 (tr, J ) 7.5 Hz, 2H), 7.03 (d, J ) 7.8 Hz, 1H), 6.94 (tr, J ) 7.5 Hz, 1H), 4.27 (tr, J ) 7.2 Hz, 2H); 4.17 (tr, J ) 7.2 Hz, 2H); 1.60 (m, 4H); 1.14 (m, 4H) ppm; FT-IR (KBr): ν bar ) 3298 (m), 3048 (m), 2923 (s), 2858 (m), 1595 (m), 1484 (s), 1465 (s), 1452 (s), 1326 (s), 1232 (m), 1151 (m), 1058 (w), 1022 (w), 840 (w), 750 (s), 725 cm-1 (s); ESI-MS: m/z (%): 460 (100) [M+], 250 (25) [N-hexylcarbazole+], 211 (25) [La+], 180 (65) [N-methylcarbazole+]; elemental
Figure 5. DSC curves of Zn(La)2 (dot line), Zn(Lb)2 (dash lines), and Zn(Lc)2 (solid line).
Figure 6. TGA curves of Zn(La)2 (dot line), Zn(Lb)2 (dash lines), and Zn(Lc)2 (solid line).
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Figure 7. Comparison of computed frontier orbital energies of the ligands.
Figure 8. Redox curves of Zn(La)2 (dash dot line), Zn(Lb)2 (dash line), and Zn(Lc)2 (solid line).
TABLE 2: The Onset Voltages and the Frontier Orbital Energy Levels of the Complexes complex Zn(La)2 Zn(Lb)2 Zn(Lc)2
oxy voltageonset (V), EHOMO (eV)
1.63, -6.37 1.60, -6.34 1.00, -5.74
red voltageonset (V), ELUMO (eV)
-2.04, -2.70 -2.14,-2.60 -2.25, -2.49
∆E (eV) 3.67 3.74 3.25
analysis calcd (%) for C31H29N3O: C 81.02, H 6.36, N 9.14; found: C 81.11, H 6.25, N 9.32. General Procedure for Preparation of ZnII Complexes. Under N2, the ligand (1 mmol) was dissolved in methanol (10 mL). Zn(OAc)2 · 2H2O (109.8 mg, 0.5 mmol) in methanol (5 mL) was added in drops. White precipitate was formed after refluxing for 3 h and then filtered and washed with 3 × 30 mL of methanol. Zn(La)2. White powder recrystallized from ethanol with a yield of 75%. 1H NMR (300 MHz, DMSO-d6, TMS): δ ) 13.46 (s, 2H), 8.01 (d, J ) 8.1 Hz, 2H), 7.6 (d, J ) 8.1 Hz, 2H), 7.28 (m, 4H), 7.05 (m, 4H), 6.80 (d, J ) 8.4 Hz, 2H), 6.69 (tr, J ) 7.4 Hz, 2H) ppm; FT-IR (KBr): ν bar ) 3058 (w), 1625 (w), 1604 (m), 1533 (m), 1477 (s), 1463 (s), 1446 (s), 1309 (s), 1253 (s), 1179 (m), 1028 (w), 862 (w), 804 (w), 738 (s), 563 (w), 505 cm-1 (w); ESI-MS: m/z (%): 481 (100) [M+], 210 (65) [La+]; elemental analysis calcd (%) for C26H18N4O2Zn: C 64.54, H 3.75, N 11.58; found: C 64.77, H 3.80, N 11.62. Zn(Lb)2. White powder purified by a train sublimation method with a yield of 72%. FT-IR (KBr): ν bar ) 3064 (w), 2997 (w), 2896 (w), 1594 (s), 1542 (m), 1465 (s), 1429 (s), 1380
(m), 1309 (s), 1261 (s), 1153 (m), 1132 (m), 1033 (w), 865 (m), 784 (s), 703 cm-1 (w); ESI-MS: m/z (%): 1080 (100) [2M+], 238 (70) [Lb+]; elemental analysis calcd (%) for C30H26N4O2Zn: C 66.73, H 4.85, N 10.38; found: C 66.85, H 4.87, N 10.36. Zn(Lc)2. White powder purified by a train sublimation method with yield of 55%. FT-IR (KBr): ν bar ) 3049 (w), 2929 (m), 2856 (w), 1599 (s), 1542 (m), 1475 (s), 1429 (s), 1402 (m), 1325 (s), 1263 (m), 1151 (m), 1126 (w), 860 (m), 750 (s), 723 cm-1 (s); ESI-MS: m/z (%): 1962 (100) [2M+], 459 (75) [Lc+]; elemental analysis calcd (%) for C62H56N6O2Zn: C 75.79, H 5.74, N 8.55; found: C 75.98, H 5.85, N 8.45. Device Fabrication and Testing. Three-layer and four-layer OLEDs were fabricated by vacuum deposition with configurations of ITO/NPB (60 nm)/Zn(Lc)2 (30 nm)/ Alq3 (40 nm)/LiF (1 nm)/Al (100 nm) and ITO/NPB (60 nm)/Zn(Lc)2 (30 nm)/ BCP (10 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (100 nm), respectively, wherein NPB is N,N-bis(naphthylphenyl)-4,4′-biphenyldiamine as the hole-transporting layer, BCP is 2,9-dimethyl4,7-diphenyl-1,10-phenanthroline as the electron transporting/ hole blocking layer, Alq3 was used as the electron transporting layer, and ITO and LiF/Al were used as the anode and cathode, respectively. Before loading into a deposition chamber, the ITO substrate was cleaned with detergents and deionized water, dried in an oven at 120 °C for 4 h, and treated with UV-ozone for 20 min. Devices were fabricated by evaporating organic layers at a rate of 0.1-0.3 nm s-1 onto the ITO substrate sequentially at a pressure below 1 × 10-6 mbar. Onto the Alq3 layer, a layer of LiF with 0.5 nm thickness was deposited at a rate of 0.1 nm s-1 to improve electron injection. Finally, a 100-nm-thick layer of Al was deposited at a rate of 0.6 nm s-1 as the cathode. The emission area of the devices was 0.14 cm2 as determined by the overlap area of the anode and the cathode. The EL spectra and CIE coordinates were measured using a PR650 spectra colorimeter. The current-density-voltage and brightnessvoltage curves of the devices were measured using a Keithley 2400/2000 source meter and a calibrated silicon photodiode. All the experiments and measurements were carried out at room temperature under ambient conditions. Results and Discussion Design and Synthesis. Multifunctionalization of the lightemitting materials is proved to be one of the most efficient methods to improve their EL performances. Although ZnII complexes based on O^N ligands were found to have very excellent PL and EL performances, there are still few studies focused on functionalization of the O^N ligands and their ZnII complexes. Our purpose was to design and synthesize blueemitting ZnII complexes and discover a convenient way to achieve multifunctionalization without making the emission shift red. Aryl benzimidazoles are known as one kind of important bright blue violet-emitting materials. So the blue emission from the ZnII complexes based on the derivatives of benzimidazole can be expected. Different from other azoles, imidazole can beconveniently functionalized by substituting NH with alkyls or aryls. Through linkage of imidazole and functional groups with flexible alkyls, the effect of modification may not induce the emissions of the corresponding ZnII complexes to shift red. Since the hydroxyl on the phenyl, which was used to coordinate with ZnII, has similar reactivity to that of NH in the imidazole ring, before alkylation the hydroxyl should be protected and then recovered before coordination. Following a common method, 2-(2-hydroxyphenyl)benzimidazole was first reacted with 2 equiv of alkyl bromide, which resulted in the
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Figure 9. EL spectra of device A at different voltages and the scheme of energy levels of device A.
Figure 10. Brightness-current density-voltage curve of device B.
Figure 11. Current efficiency and external quantum efficiency curves of device B (9: current efficiency, b: external quantum efficiency).
substitution of both NH and OH, and then the hydroxyl was recovered by heating the intermediate with pyridine hydrochlorate or BBr3 (Scheme 3).31 Obviously, since half of the alkyl bromide was wasted, this method is not suitable for introducing functional groups. We tried to protect the hydroxyl of salicylic acid with acetyl. Unfortunately, we found a strong steric effect in 2-acetoxybenzoic acid that precluded reaction with benzene1,2-diamine. For another attempt the hydroxyl was protected by etherification, but the yield of the next condensation reaction was small since many byproducts were formed and when exposed to the ambient conditions the product was denatured with the color changing from white to dark red. So it seemed
difficult to protect the hydroxyl of salicylic acid without influencing the condensation reaction. We found that through adjusting the pH value of the reaction system, 4-methylbenzenesulfonyl chloride can react with the hydroxyl in 2-(2hydroxyphenyl)benzimidazole selectively (Scheme 2). The intermediate could not be hydrolyzed by weak base so that during alkylation only NH reacted with the bromide. Then the hydroxyl was recovered by hydrolysis with strong base. By this method a number of functional groups can be conveniently introduced as flexible alkyls with a favorable yield. Optical Properties. The absorption spectra of HLa, HLb, and HLc in CH2Cl2 (10-6 mol L-1) were measured (Figure 1). The peak originated from the imidazole ring in HLa was at 309 nm. The peak from its phenol moiety was at 240 nm. However, in HLb the peak from the imidazole ring was a duplicate peak at 292 and 320 nm. The peak of its phenol moiety at 228 nm was also accompanied with a shoulder peak at 238 nm. It indicated that although in the dilute solution the hydrogen bond interaction between the unsubstituted imidazole rings from different HLas still exists, which induced the absorption peaks of HLa become broader and smoother than those of HLb. For HLc the peaks from carbazole moiety were recognized at 265 and 295 nm. The peak originated from its imidazole was at 320 nm accompanied by a shoulder peak at 332 nm. The peaks of its phenol moiety were at 229 and 238 nm. Compared with HLa, the absorption edges of HLb and HLc remarkably shift red. Simultaneously, in CH2Cl2, the emission wavelengths of substituted HLb and HLc were 40 nm longer than that of HLa. This red-shift of the absorption edges and PL emissions may be induced by the electron-donating effect of the alkyls. Notably, the PL emissions of HLb and HLc were nearly the same. It means that in the dilute solution the carbazole moiety in HLc hardly influenced the emission of the ligand. The absorption and PL spectra of the ligands in the solid state were also measured (Figure 2). The absorption peaks of the ligands in the solid state became much broader and smoother than those in solution. Compared with their solutions, the absorption edge of HLa in the solid state shifted red for about 60 nm, but the absorption edges of HLb and HLc almost remained. Simultaneously, the PL emission wavelength of HLa in the solid state was 37 nm longer than that in its solution. The emission wavelengths of HLb as a solid and in solution were almost the same. However, compared with its solution the emission of HLc in the solid state shifted red for 5 nm (Table 1). Obviously, the hydrogen bonds formed between HLas worsened the aggregation in the solid state, which induced the
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Figure 12. EL spectra of device B at different voltages (solid lines), PL spectrum of the film prepared by vacuum evaporation (dash line), and the scheme of energy levels of device B.
emission to shift red. The planar carbazole moiety improves the intermolecular π-π interaction of HLc, which also enhances the interaction between imidazole rings at the other end of the flexible chain. These enhanced interactions also worsened the aggregation of HLc in the solid state. The absorption spectra of the complexes in CH2Cl2 (10-6 mol L-1) are similar with those of the ligands, but the spectra shift red for about 40 nm (Figure 3). Notably, the fine structures of the absorption spectrum of Zn(La)2 were much clearer than its ligand HLa. It means that in their dilute solutions the intermolecular interaction between Zn(La)2 is much weaker than that between HLa. The chelate structure may decrease the intermolecular interaction. Compared with their ligands, the emissions of the complexes shifted blue remarkably, and their full widths at half-maximum (fwhm) were much smaller than those of their ligands (Table 1). All of the complexes have deep blue emissions and the emissions and fwhm of Zn(Lb)2 and Zn(Lc)2 were nearly the same. It was proved that introduction of the functional groups, such as carbazole, through flexible alkyls can preserve the emission color and color purity of the complexes. The absorption spectra of the complexes in the solid state were similar to those of their ligands (Figure 4), but in the absorption spectra of Zn(Lb)2 and Zn(Lc)2 in the solid state, compared with their ligands, the relative intensity of the peaks around 330 nm remarkably decrease. It means that after forming the chelates the π-π* transitions between their benzimidazole or carbazole moieties are reduced. In the solid state all of the complexes give out pure blue emissions rather than the bluegreen emissions of their ligands. Compared with their dilute solutions, the emissions of the complexes in the solid state also shifted red. The red-shifts of Zn(La)2, Zn(Lb)2, and Zn(Lc)2 were 29, 12, and 0 nm, respectively (Table 1). The strong hydrogen bond interaction between Zn(La)2 was the main reason for its biggest red-shift. For Zn(Lc)2, no red-shift was observed. It indicated that the bulky substituent can limit aggregation in the solid state. Compared with their corresponding ligands, the emissions of the complexes in the solid state shifted blue remarkably (20 nm for Zn(La)2 to HLa, 33 nm for Zn(Lb)2 to HLb, and 51 nm for Zn(Lc)2 to HLc). Unexpectedly, it was shown that the substitution facilitated this hypsochromic effect: the larger substituent induced the larger blue shift of the emission of the complex in the solid state. From Zn(La)2, Zn(Lb)2, to Zn(Lc)2, the emissions of the complexes in the solid state shift blue, sequentially. The peak wavelength of Zn(Lc)2 is the shortest
(only 422 nm) as a deep blue emission, which is 23 nm shorter than that of Zn(La)2. This trend was much different with their ligands. It was shown that the emissions of the complexes in dilute solution were nearly the same if the substituent was alkyl. So, the effect of the substituents on the emissions of the complexes in solution was limited. On the other hand, since the worst aggregation of HLa occurred in the solid state, the difference between the emissions of the ligands in the solid state was also small. Hence, from solution to solid state, the variation of the emission of the complex became the key factor for these hypsochromic shifts. The smaller red-shift of the emission of the complex in the solid state would make the hypsochromic effect more remarkable. In the solid state the emissions of the complexes were influenced directly by their coherent conditions. It is believed that the strong intermolecular interaction can make the emission bathochromic. The aggregation in the solid state is one of the main factors having the effect on the emissions of the complexes. So, compared with its ligand, the blue-shift of the emission of the complex becomes more remarkable when the complex can reduce the aggregation in the solid state more efficiently. Obviously, the bulky substituent with the stronger steric effect can reduce the aggregation. Accompanied by the chelate structure, the steric effect of the substituents can be further amplified. Although carbazole moieties in HLc induced the intermolecular π-π interaction, the chelate structure of Zn(Lc)2 can reduce this interaction between the carbazole rings. The steric effect of the hexyl carbazole groups is more effective on the emission of Zn(Lc)2. The same emission wavelengths in solution and solid of Zn(Lc)2 proved that the intermolecular aggregation was efficiently prevented. Consequently, Zn(Lc)2 with the largest substituent has the most blue emission. With the fwhm as short as 45 nm, Zn(Lc)2 seems favorable among the blue-emitting materials with high color purity. It is clear that introducing the functional group in imidazole through a flexible chain cannot induce the emission of the complex to shift red but improves the PL performance of the complex with a shorter emission wavelength and a higher color purity. The PL quantum yields (PLQY) of the ligands and complexes in films were measured (Table 1). HLa and HLb have the same PLQY, while the PLQY of HLc is only the half of those of HLa and HLb. The smaller PLQY of HLc may be induced by the interaction between carbazole and benzimidazole moieties. Compared with their ligands, the PLQY of the complexes are multiplied several times, which indicates that the formation of a chelate structure can improve their PL performances. Notably, different from its ligand, the PLQY of Zn(Lc)2 is comparable
15524 J. Phys. Chem. C, Vol. 112, No. 39, 2008 to those of Zn(La)2 and Zn(Lb)2. It means that the chelate structure of Zn(Lc)2 could reduce the interaction between carbazole and benzimidazole moieties and that the substituents remarkably do not influence the PLQY of the ZnII complexes. Thermal Properties. The melting point temperature (Tm) of Zn(Lb)2 is 311 °C, which is 70 °C higher than that of Zn(Lc)2 (Tm ) 241 °C). Tm of Zn(La)2 was not observed since its decomposition occurred before melting (Figure 5). The hydrogen on the imidazole in Zn(La)2 facilitates the formation of an intermolecular hydrogen bond, which enhances its intermolecular interaction. Obviously, after alkyl substitution the hydrogen bond is eliminated and the intermolecular interaction in Zn(Lb)2 and Zn(Lc)2 becomes much weaker, which induces a lower Tm. The longer flexible chains in Zn(Lc)2 further decreases the Tm. The lowest Tm also correlated to the weakest intermolecular interaction of Zn(Lc)2 among these three ZnII complexes. The thermal stability of the complexes was determined by TGA (Figure 6). The temperature of decomposition (Td) of Zn(La)2, Zn(Lb)2, and Zn(Lc)2 is 356, 378, and 427 °C, respectively. The substituted complexes have stronger thermal stability, which was exhibited in their improved structure stability. Notably, the Td of Zn(Lc)2 is 50 °C higher than that of Zn(Lb)2 and 70 °C higher than that of Zn(La)2. The excellent thermal stability of Zn(Lc)2 makes its device fabrication by vacuum deposition more feasible. Computational Results. The geometry optimization and the frontier orbitals (the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) population analysis of the ligands were performed at the B3LYP/ 6-31G* level. All calculations were carried out by using the GAUSSIAN03 program package (Figure 7). It was shown that compared with HLa after substitution LUMO energy levels of HLb and HLc are slightly elevated. All of the LUMO electron cloud densities of the ligands are located on phenylbenzimidazole moieties. The HOMO energy level of HLb is equivalent to that of HLa, and their HOMO electron cloud densities are also located on phenylbenzimidazole moieties. It indicated that the alkyl substituent has no effect on the hole injection ability of the ligand. Different from HLa and HLb, the HOMO electron cloud density of HLc is located on the carbazole group rather than the phenylbenzimidazole, and the HOMO energy level of HLc is elevated. Electrochemical Properties. The redox behaviors of the complex films were investigated by CV analysis at room temperature in acetonitrile measured against an Ag/Ag+ electrode (0.1 M tetrabutylammonium hexafluorophosphate as the electrolyte) (Figure 8). The frontier orbital energy levels of the complexes were calculated according to the onset voltages of their redox peaks.45 The results are listed in Table 2 for comparison. The oxidation behaviors of Zn(La)2 and Zn(Lb)2 are similar. Their oxidation curves consisted of two irreversible peaks. The only difference between them is the first oxidation peak of Zn(Lb)2 which is much stronger than that of Zn(La)2, which should originate from the difference of their substituted or unsubstituted benzimidazoles. The oxidation peak of Zn(Lc)2 is reversible with an onset voltage of 1.00 V, which is attributed to the carbazole groups. HOMO energy levels of Zn(La)2 and Zn(Lb)2 are equivalent, which indicates that only when modified with alkyl groups is the hole injection ability of the ZnII complexes unable to be improved. However, compared with Zn(La)2 and Zn(Lb)2, the HOMO energy level of Zn(Lc)2 is elevated for 0.6 eV. It is clear that the introduction of carbazole groups by alkyl linking groups can enhance the hole injection
Xu et al. ability of the complex. The reduction peaks of these three complexes are irreversible. From Zn(La)2, Zn(Lb)2, to Zn(Lc)2, the LUMO energy levels rise successively. In general, the order of the frontier energy levels of the complexes is in accord with the Gaussian simulation results of the ligands. By introducing the selected functional groups through flexible chains, the carrier injection ability of the corresponding complexes can be tuned conveniently. Electroluminescent Performance. Three-layer devices of Zn(Lc)2 (Device A) were fabricated with a configuration of ITO/ NPB (60nm)/Zn(Lc)2 (30nm)/Alq3 (40nm)/LiF (1nm)/Al. At low operation voltages, the EL emission of the device consisted of 520 nm from Alq3 and 450 nm from Zn(Lc)2 (Figure 9). Since at low operation voltages the amount of holes injected was limited, the recombination of holes and electrons may occurr at the interface of Zn(Lc)2 and Alq3, which induced the emissions from both layers. Along with the voltage increase, more and more holes were injected. The recombination zone gradually shifted into the layer of Alq3, which induces the decrease in blue proportion, and the emission became pure green at 520 nm. This indicated that Zn(Lc)2 has good hole injection and transporting ability, and in device A the hole transporting speed was much faster than the electron transporting speed. Because of the unbalance in carrier injection and transporting, the major carrier in device A was a hole and the recombination zone was closer to the cathode. Because of the unbalanced transporting speeds of holes and electrons, the excitons should be limited in the layer of Zn(Lc)2 by the hole-blocking layer so that the electroluminescence from the ZnII complex can be realized. A thin layer of hole-blocking BCP was inserted to fabricate device B with a configuration of ITO/NPB (60nm)/Zn(Lc)2 (30nm)/BCP (10nm)/Alq3 (40nm)/ LiF (1nm)/Al. The devices gave off the pure blue emission of Zn(Lc)2 at 452 nm, which demonstrated the recombination of the carriers in the layer of the complex. The brightness-current density-voltage curve of device B was measured (Figure 10). Its turn-on voltage was as low as 4.5 V at 1 cd m-2, and its highest brightness of 2648 cd m-2 was achieved at 13 V with the current density of 538.92 mA cm-2. Zn(Lc)2 is one of few blue-emitting ZnII complexes with EL luminance more than 2000 cd m-2.22 The efficiencies of device B were calculated (Figure 11). The maximum current efficiency of 0.54 cd A-1 and the maximum external quantum efficiency (E.Q.E.) of 0.4% was achieved at 12 V with a current density of 302.56 mA cm-2. It was shown that along with the elevation of the current density, the efficiencies were also increasing until they reached their maximum value. It is noteworthy that the EL efficiencies of the device did not obviously drop when current density further increased, and when the current density reached its maximum, the device still had maintained the efficiencies of 0.49 cd A-1 and 0.36%, which indicates that the device was still stable under extreme operating conditions. Although the EL efficiencies of Zn(Lc)2 were not very high among the blue-emitting ZnII complexes, during the operation voltage increase, Zn(Lc)2 exhibited an excellent efficiency stability. The low turn-on voltage, the high brightness, and the stable efficiency of the device make Zn(Lc)2 competitive with other blue-emitting ZnII complexes based on Schiff bases and benzo-heterocyclic compounds. The PL spectrum of the film of Zn(Lc)2 prepared by vacuum evaporation was measured, which is as the same as its PL spectrum in the solid state. It indicated that Zn(Lc)2 did not decompose during the device fabrication. The EL spectra of device B only consisted of the emission at 452 nm from Zn(Lc)2
Deep Blue-Emitting ZnII Complex as pure blue emission (CIE 1931 coordinate: x 0.17, y 0.13) (Figure 12). The fwhm of the emission was just 72 nm, which indicated the high EL color purity of Zn(Lc)2. No emission from NPB and Alq3 was discovered. It means that the hole injection and transporting ability of Zn(Lc)2 is strong enough to avoid any redundant electrons entering into the layer of NPB, and through the hole-blocking BCP the recombination zone was limited in the layer of the ZnII complex efficiently. Notably, the EL spectrum at 12 V was just the same as that at 4 V. An excellent spectral stability of Zn(Lc)2 was demonstrated. The pure blue emission, high color purity, and excellent spectral stability make Zn(Lc)2 favorable among the blue-emitting materials. Conclusion The hole-transporting carbazole moiety was introduced into the deep blue-emitting ZnII complex (Zn(Lc)2) by a modified method. The chelates exhibited a remarkable blue-shift of their emissions compared with the ligands. Significantly, the unexpected amplification effect of the substituents on this hypsochromic phenomenon was demonstrated by the larger blue shift of the emission of the complex in the solid state from the larger substituent. Zn(Lc)2 has the shortest peak wavelength at 422 nm in the solid state, which is 23 nm shorter than that of Zn(La)2. This effect facilitates the functional modification of the complexes without inducing bathochromic emission. Thermal analysis indicated that the introduction of carbazole moieties in Zn(Lc)2 can remarkably improve its thermal stability. The higher HOMO energy level of Zn(Lc)2 proved that by introducing the selected functional groups through flexible chains, the carrier injection ability of the corresponding complexes can be conveniently tuned. The pure blue EL emission from Zn(Lc)2 at 452 nm was demonstrated in its four-layer devices. Zn(Lc)2 seems favorable among the blue-emitting ZnII complexes with a brightness more than 2000 cd m-2, a high efficiency stability, and an excellent EL spectral stability. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China under Grants 60325412, 90406021, and 20572018, The Education Bureau of Heilongjiang Province under Grants 11521206 and 11521216, the Doctoral Initial Fund of Heilongjiang University, and The Key Laboratory of Universities in Heilongjiang Province. Authors want to thank Dr. Tian Xia and Dr. Qiang Li for their assistance in the thermal and optical analysises. H. Xu wants to show appreciation to his doctor tutor Prof. Dr. Wei Huang in Nanjing Univeristy of Posts and Telecommunications. References and Notes (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51 (12), 913– 915. (2) Chen, C. H.; Shi, J. Coord. Chem. ReV. 1998, 171 (1), 161–174. (3) Evans, R. C.; Douglas, P.; Winscom, C. J. Coord. Chem. ReV. 2006, 250, 2093–2126. (4) Matsumura, M.; Akai, T. Jpn. J. Appl. Phys 1996, 35, 5357–5360. (5) Hopkins, T. A.; Meerholz, K.; Shaheen, S.; Anderson, M. L.; Schmidt, A.; Kippelen, B.; Padias, A. B.; Hall, H. K.; Peyghambarian, N.; Armstrong, N. R. Chem. Mater. 1996, 8 (2), 344–351. (6) Sano, T.; Nishio, Y.; Hamada, Y.; Takahashi, H.; Usuki, T.; Shibata, K. J. Mater. Chem. 2000, 10 (1), 157–161. (7) Sapochak, L. S.; Benincasa, F. E.; Schofield, R. S.; Baker, J. L.; Riccio, K. K. C.; Fogarty, D.; Kohlmann, H.; Ferris, K. F.; Burrows, P. E. J. Am. Chem. Soc. 2002, 124 (21), 6119–6125.
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