Novel Light-Emitting Ternary Eu3+ Complexes Based on

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Novel Light-Emitting Ternary Eu3+ Complexes Based on Multifunctional Bidentate Aryl Phosphine Oxide Derivatives: Tuning Photophysical and Electrochemical Properties toward Bright Electroluminescence Hui Xu,*,†,§ Kun Yin,§ and Wei Huang*,‡ Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, and School of Chemistry and Materials, Heilongjiang UniVersity, Harbin 150080, P. R. China, Institute of AdVanced Materials (IAM), Jiangsu Key Laboratory of Organic Electronics and Flat-Panel Displays, Nanjing UniVersity of Posts and Telecommunications (NJUPT), Nanjing 210003, P. R. China, and Institute of AdVanced Materials (IAM), Fudan UniVersity, Shanghai 200433, P. R. China ReceiVed: October 5, 2009; ReVised Manuscript ReceiVed: December 5, 2009

Three functional bidentate aryl phosphine oxide (APO) derivatives characterized by two diphenylphosphine oxide moieties [2-(diphenylphosphoryl)-N-(2-(diphenylphosphoryl)-4-methoxyphenyl)-4-methoxy-N-(4methoxyphenyl)aniline (TMOADPO), 3,6-bis(diphenylphosphoryl)-9-ethyl-9H-carbazole (EtCzDPO), and 3,6-bis(diphenylphosphoryl)-9-phenyl-9H-carbazole (PhCzDPO)] bridged with a hole-transporting arylamine, as well as their tertiary complexes [Eu(TTA)3(TMOADPO)2 1, Eu(TTA)3(EtCzDPO)2 2, and Eu(TTA)3(PhCzDPO)2 3 (TTA ) 2-thenoyltrifluoroacetonate)], were designed and synthesized. The strong absorption antennae effect of the functional APO ligands was proved. It is shown that their more rigid structure and chelate coordinate mode impart a decreased degree of freedom and form much more compact complex structures, which not only reduces the energy loss caused by the structure relaxation but also restrains the solvent quenching and facilitates the energy transfer from the APO ligands to Eu3+. Thermal analysis was also performed to demonstrate the improved thermal stability and phase stability of the complexes. CV analysis not only indicated excellent carrier-injection ability but also showed the feasibility to tune it by adjusting the kinds and number of functional groups and by designing alternative complex structures. All of the complexes exhibited excellent electroluminescent (EL) performance, such as maximum brightness around 1000 cd m-2, an external quantum efficiency (EQE) around 3%, and stable monochromic red emission at 614 nm. Our investigations demonstrate the potential application of bidentate APO ligands in high EL performance Eu3+ complexes. Introduction Luminescent lanthanide complexes have received much attention because of their applications in chemical sensing,1,2 biological imaging,3-7 and optical electronic devices.8-12 Among the complexes, the electroluminescent (EL) ternary lanthanide complexes such as the red-emitting europium(3+) complexes,13 show great potential as candidates for high color-purity displays with advantages of nearly monochromic characteristic emission, chemical and environmental stability, and theoretically approaching 100% internal device quantum efficiency. However, the weaker carrier injection/transporting ability and weaker stability during vacuum deposition of the lanthanide lightemitting materials led to diminished electroluminescent (EL) performance, short of expectations.13 Generally, because the triplet energy levels of anionic ligands, especially β-diketones, easily fit the excited levels of Eu3+ ion, these anionic ligands mainly serve as bridges between neutral ligands and Eu3+ in the intramolecular energy transfer.14,15 Many functional groups including hole-transporting moieties,16 long, conjugated aryls,17 and polyfluorinated alkyl moieties18 were used to modify EL * Corresponding authors. (W.H.) Fax: +86 25 83492333, e-mail: [email protected]. (H.X.) Tel: +86 451 81773032, fax: +86 451 86608042, e-mail: [email protected]. † Heilongjiang University. ‡ Nanjing University of Posts and Telecommunications. § Fudan University.

Eu3+ complexes to improve carrier injection/transporting and sublimation ability. Bright Eu3+-originated, pure red electroluminescence was observed. Although β-diketones account for the majority of organic moieties in these complexes, compared with the common β-diketones, such as 2-thenoyltrifluoroacetonate (TTA) and 1,3-diphenylpropane-1,3-dione (DBM), EL performance of the complexes based on the functionalized β-diketones did not exhibit outstanding improvement. One of the main reasons might be that the discontinuous conjugation of the β-diketone reduces the effect of the functional modification. Actually, TTA has the first singlet and triplet (S1 and T1) energy levels fitting to the 5D0 level of Eu3+, which affords highly efficient energy transfer.14 It was proved that the neutral ligands in these ternary systems have remarkable effects on the optical electric properties of their lanthanide complex, and it is believed that their functionalization is an effective method to improve the EL performance of their complexes.19-34 The most popular simple neutral ligands used in red light-emitting Eu3+ complexes, such as 1,10-phenanthroline (phen), 2,2′-bipyridine (bpy), and triphenylphosphine oxide (TPPO), are electrontransporting.13 In order to eliminate the unbalance of the hole and electron injection and transport, hole-transporting arylamine groups were introduced into phenanthroline derivatives to improve their hole-injection ability.22,23,26 Several Eu3+ complexes with these modified neutral ligands exhibited good EL properties with brightness more than 1000 cd m-2 and external

10.1021/jp909548t  2010 American Chemical Society Published on Web 12/30/2009

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CHART 1: Structure of Complexes 1-3

Experimental Section

quantum efficiency (EQE) of about 2%.22,26-31 However, phanthroline derivatives showed several disadvantages including the relative weak coordination to lanthanide ions and a structure too rigid to accommodate bulky modifications. Therefore, it is still a big challenge to develop new neutral ligands to yield efficient Eu3+ complexes. Compared with N-heterocyclic ligands, the aryl phosphine oxide (APO) derivatives seem attractive as neutral ligands for light-emitting lanthanide complexes with much stronger coordination to rare earth ions, adaptability of functionalization, and tunable excited energy levels. In our former works, we first reported an efficient EL bidentate APO Eu3+ complex Eu(TTA)3DPEPO (TTA ) 2-thenoyltrifluoroacetonate, DPEPO ) bis(2-(diphenylphosphino)phenyl) ether oxide).32,33 The improved structural stability, intermediate excited energy levels, and efficient photoluminescence (PL) and EL were demonstrated. Nevertheless, EL brightness of Eu(TTA)3DPEPO was not large enough, which may be induced by the limited carrier injection ability of DPEPO. Reddy also reported another APO bidentate ligand-based Eu3+ complex with a high PL efficiency of 48%.35 Recently, we reported another series of arylaminemodified monodentate APO Eu3+ complexes.36-38 The double carrier injection ability of the complexes was improved much more, which further enhanced their EL performance. The fourlayer devices based on the complexes exhibited improved EL performance including a maximum brightness of more than 1000 cd m-2 and a maximum external quantum yield of about 3%. It seems natural and logical that the same method may also be equally effective on the bidentate APO ligands (Chart 1). As a continuation of our studies of bidentate APO ligands, herein we report a series of modified bidentate APO derivatives and their bright EL Eu3+ complexes as the following: 2-(diphenylphosphoryl)-N-(2-(diphenylphosphoryl)-4-methoxyphenyl)-4methoxy-N-(4-methoxyphenyl)aniline (TMOADPO), 3,6-bis(diphenylphosphoryl)-9-ethyl-9H-carbazole (EtCzDPO), and 3,6bis(diphenylphosphoryl)-9-phenyl-9H-carbazole (PhCzDPO), as well as their tertiary complexes Eu(TTA)3TMOADPO 1, Eu(TTA)3EtCzDPO 2, and Eu(TTA)3PhCzDPO 3, whose structures are depicted in Scheme 1. These bidentate APO derivatives were designed as ambipolar structures with an arylamine bridging two electron-injection diphenylphosphine oxide (DPPO) moieties, which may induce a relative balance of carrier injection and form the chelate coordinate mode. Compared with the basic Eu(TTA)3DPEPO, a highly improved EL performance of complexes 1-3 was achieved, which verified the validity of our molecular design.

Materials and Instruments. All the reagents and solvents used for the synthesis of 1-3 were purchased from Aldrich and Acros. Alq3, CBP, BCP, and NPB used for EL device fabrication were purchased from Lumthech Corporation. All the reagents were used without further purification. 1 H NMR spectra were recorded using a Varian Mercury plus 400NB spectrometer relative to tetramethylsilane (TMS) as internal standard. Molecular masses were determined with a Shimadzu laser desorption/ionization time-of-flight mass spectrometer (LDI-TOF-MASS) or a Shimadzu GCMS-QP2010 Plus equipped with a DB-5 ms column. Elemental analyses were performed on a Vario EL III elemental analyzer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on Shimadzu DTG-60A and DSC-60A thermal analyzers under nitrogen atmosphere at a heating rate of 10 °C min-1. Absorption and photoluminescence (PL) emission spectra of the target compounds were measured in dichloromethane using a Shimadzu UV-3150 spectrophotometer and a Shimadzu RF-5301PC spectrophotometer, respectively. Phosphorescence spectra and decay lifetimes were measured in dichloromethane using an Edinburgh FPLS 920 fluorescence spectrophotometer at 77 K cooled by liquid nitrogen. Cyclic voltammetric (CV) studies were conducted using an Eco Chemie B. V. Autolab potentiostat 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 acetonitrile with tetrabutylammonium hexafluorophosphate (0.1 M in acetonitrile) as the supporting electrolyte and a scanning rate of 0.1 V s-1. Bis(4-methoxyphenyl)amine (4). Copper(I) iodide (76.2 mg, 0.4 mmol), potassium carbonate (1.106 g, 8 mmol), 1-iodo-4methoxybenzene (0.936 g, 4 mmol), and 4-methoxyaniline (0.739 g, 6 mmol) were added under N2 to a 100 mL two-necked round-bottomed flask. The mixture was heated to 140 °C for 8 h, cooled to room temperature (R.T.), and extracted with dichloromethane and water. The organic layer was dried with MgSO4. Then the solvent was removed in vacuo, and the residue was purified by flash column chromatography. White crystals in a yield of 0.61 g (67%). 1H NMR (400 MHz, CDCl3, ppm): δ ) 7.840-7.911 (d, 4H, J ) 8.8), 6.963-7.034 (d, 4H, J ) 9.2), 5.710 (s, 1H), 3.881 (s, 6H); GC-MS (m/z): 229 (M+, 100%). Bis(2-bromo-4-methoxyphenyl)amine (5). 4 (229 mg, 1 mmol) was dissolved in CH2Cl2/CH3OH (3:1). The solution was cooled to 0 °C, and then benzyltrimethylammonium tribromide (751.8 mg, 2 mmol) was added. The mixture immediately turned purple. The reaction was quenched with water after 2 h and then extracted with dichloromethane. The organic layer was dried with MgSO4. Then the solvent was removed in vacuo, and the residue was purified by flash column chromatography. White crystals in a yield of 0.34 g (88%). 1H NMR (400 MHz, CDCl3, ppm): δ ) 7.119-7.183 (s, 2H), 6.992-7.039 (d, 2H, J ) 8.8), 6.738-6.838 (d, 2H, J ) 8.8), 5.815 (s, 1H), 3.776 (s, 6H); GC-MS (m/z): 385 (M+, 100%). 2-Bromo-N-(2-bromo-4-methoxyphenyl)-4-methoxy-N-(4methoxyphenyl)aniline (6). Copper(I) iodide (0.57 g, 3 mmol), potassium carbonate (8.295 g, 60 mmol), 1-iodo-4-methoxybenzene (7.020 g, 30 mmol), and 5 (2.322 g, 6 mmol) were added to a 100 mL two-necked round-bottomed flask. The mixture was heated to 200 °C for 8 h. The mixture was then cooled to R.T. and extracted with dichloromethane and water. The organic layer was dried with MgSO4. Then the solvent was

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SCHEME 1: Synthesis of the Ligands

removed in vacuo, and the residue was purified by flash column chromatography. White crystals in a yield of 2.21 g (75%). 1H NMR (400 MHz, CDCl3, ppm): δ ) 7.104-7.186 (s, 2H), 6.994-7.065 (d, 2H, J ) 8.8), 6.699-6.862 (m, 4H), 6.550-6.677 (d, 2H, J ) 9.2), 3.789 (s, 6H), 3.762 (s, 3H); GC-MS (m/z): 491 (M+, 100%). 3,6-Dibromo-9-ethylcarbazole (7). 3,6-Dibromocarbazole (3.25 g, 10 mmol) and tetrabutylammonium chloride (322 mg) were dissolved in DMSO (50 mL), and then aqueous NaOH solution (50%, 2.55 mL) and ethyl bromide (1.12 mL, 15 mmol) were added. The mixture turned to orange. The mixture was heated to 60 °C for 8 h, quenched by water, and extracted with dichloromethane. The organic layer was dried with MgSO4. Then the solvent was removed in vacuo, and the residue was purified by flash column chromatography. White crystals in a yield of 2.8 g (80%). 1H NMR (400 MHz, CDCl3, ppm): δ ) 8.148 (s, 2H), 7.533-7.593 (d, 2H, J ) 8.8), 7.262-7.311 (d, 2H, J ) 8.4), 4.266-4.374 (q, 2H, J1 ) 7.2, J2 ) 14.4), 1.353-1.465 (tr, 3H, J ) 7.2); GC-MS (m/z): 351 (M+, 100%). 3,6-Dibromo-9-phenylcarbazole (8). Copper(I) iodide (0.57 g, 3 mmol), potassium carbonate (8.295 g, 60 mmol), iodobenzene (3 mL, 27 mmol), and 3,6-dibromocarbazole (3.25 g, 10 mmol) were mixed and heated to 200 °C for 8 h. The mixture was then cooled to R.T. and extracted with dichloromethane and water. The organic layer was dried with MgSO4. Then the solvent was removed in vacuo, and the residue was purified by flash column chromatography. White crystals in a yield of 2.40 g (60%). 1H NMR (400 MHz, CDCl3, ppm): δ ) 8.199 (s, 2H), 7.582-7.655 (tr, 2H, J ) 7.6), 7.470-7.536 (m, 5H), 7.236-7.274 (d, 2H, J ) 8.8); GC-MS (m/z): 399 (M+, 100%). General Procedure for the Preparation of the Phosphine Oxide Compounds. The dibromides (1 mmol) was dissolved in 15 mL of ether under N2 and then cooled to 0 °C. Butyllithium (2 mL, 1.6 M in hexane) was added dropwise in 1 h. Then chlorodiphenylphosphine (3.5 mmol, 0.62 mL) in ether (5 mL) was added dropwise over 30 min. The mixture was warmed to R.T. and stirred for 8 h. Then hydrogen peroxide (0.1 mL, 30%)

was added. A precipitate was formed immediately. The precipitate was filtered after stirring for 2 h and then washed with ether (3 × 10 mL) and purified by flash column chromatography. 2-(Diphenylphosphoryl)-N-(2-(diphenylphosphoryl)-4-methoxyphenyl)-4-methoxy-N-(4-methoxyphenyl)aniline (TMOADPO). Yellow powder in a yield of 418 mg (60%). 1H NMR (400 MHz, CDCl3, ppm): δ ) 7.101-7.255 (m, 22H), 6.799-6.903 (d, 2H, J ) 8.8), 6.660-6.584 (d, 2H, J ) 8.4), 6.387-6.459 (d, 2H, J ) 8.8), 6.151-6.223 (d, 2H, J ) 9.2), 3.563 (s, 6H), 3.550 (s, 3H); LDI-TOP (m/z): 735 (M+, 100%); elemental analysis calcd (%) for C45H39NO5P2: C 73.46, H 5.34, N 1.90, O 10.87. Found: C 73.65, H 5.21, N 2.08, O 10.66. 3,6-Bis(diphenylphosphoryl)-9-ethyl-9H-carbazole (EtCzDPO). White powder in a yield of 370 mg (62%). 1H NMR (400 MHz, CDCl3, ppm): δ ) 8.311-8.441 (d, 2H, J ) 12.0), 7.638-7.734 (m, 8H), 7.420-7.573 (m, 16H), 4.335-4.500 (q, 2H, J1 ) 7.2, J2 ) 14.4), 1.425-1.503 (tr, 3H, J ) 7.2); LDI-TOP (m/z): 595 (M+, 100%); elemental analysis calcd (%) for C38H31NO2P2: C 76.63, H 5.25, N 2.35, O 5.37. Found: C 76.81, H 5.11, N 2.49, O 5.29. 3,6-Bis(diphenylphosphoryl)-9-phenyl-9H-carbazole (PhCzDPO). White powder in a yield of 367 mg (57%). 1H NMR (400 MHz, CDCl3, ppm): δ ) 8.413-8.492 (d, 2H, J ) 12.4), 7.592-7.720 (m, 12H), 7.420-7.559 (m, 17H); LDI-TOP (m/z): 643 (M+, 100%); elemental analysis calcd (%) for C42H31NO2P2: C 78.37, H 4.85, N 2.18, O 4.97. Found: C 78.63, H 4.83, N 2.29, O 4.60. General Procedure for the Preparation of the Eu3+ Complexes. The complexes were prepared according to wellestablished protocols (Scheme 2).39 2-Thenoyltrifluoroacetone (HTTA; 672.7 mg, 3 mmol) was dissolved in ethanol, and NaOH (120 mg, 3 mmol) in aqueous solution (2 M) was added to remove the H+ in the TTA molecule. Then EuCl3 · 6H2O (370.1 mg, 1 mmol) in aqueous solution was added dropwise, the mixture was stirred at 60 °C for 30 min, the PO ligand (2 mmol) in ethanol was added dropwise, and the mixture was stirred at 60 °C for 4 h to afford the title complexes 1-3.



SCHEME 2: Synthesis of the Complexes

Purification was accomplished by precipitation from concentrated ethanol and water solution. 1: yellow powder in a yield of 71%. 1H NMR (400 MHz, CDCl3, ppm): δ ) 8.449-7.759 (m, 10H), 7.759-7.287 (m, 23H), 6.815-6.481 (tr, 3H, J ) 4.8 Hz), 6.717-6.535 (d, 3H, J ) 2.8 Hz), 4.126 (s, 3H), 3.495 (s, 6H), 3.333 (s, 3H); FT-IR (KBr pellet, cm-1): 1607 (CdO stretching in TTA), 1537 (CdC stretching in TTA), 1501, 1484, 1438 (C-P stretching), 1413, 1356, 1303, 1244, 1229, 1181 (PdO stretching), 1142, 1061, 1032, 933, 858, 782, 750, 722, 694, 641, 579, 539; ESI-MS (m/z): 1554 (M+, 100); elemental analysis calcd (%) for C69H51EuF9NO11P2S3: C 53.42, H 3.31, N 0.90, O 11.35, S 6.20. Found: C 53.70, H 3.34, N 0.91, O 11.26, S 6.17. 2: yellowish powder in a yield of 70%. 1H NMR (400 MHz, CDCl3, ppm): δ ) 7.929-7.391 (m, 26H), 6.949-6.673 (d, 3H, J ) 4.8 Hz), 6.496-6.295 (tr, 3H, J ) 4.4 Hz), 6.195-6.006 (d, 3H, J ) 2.4 Hz), 4.223 (s, 3H), 2.825-2.383 (br, 2H), 1.794-1.602 (br, 3H); FT-IR (KBr pellet, cm-1): 1612 (CdO stretching in TTA), 1536 (CdC stretching in TTA), 1516, 1498, 1476, 1438 (C-P stretching), 1414, 1355, 1298, 1238, 1229, 1181 (PdO stretching), 1138, 1111, 1060, 1029, 933, 859, 781, 741, 716, 693, 641, 605, 590, 578, 560, 532, 519; ESI-MS (m/z): 1443 (M+ + CH3OH, 100), 722 (M2+ + CH3OH, 60); elemental analysis calcd (%) for C62H43EuF9NO8P2S3: C 52.77, H 3.07, N 0.99, O 9.07, S 6.82. Found: C 53.09, H 3.05, N 1.33, O 9.23, S 6.79. 3: yellowish powder in a yield of 64%. 1H NMR (400 MHz, CDCl3, ppm): δ ) 8.621-8.364 (m, 19H), 8.364-8.140 (m, 12H), 8.108-8.014 (d, 3H, J ) 4.8 Hz), 7.594-7.440 (tr, 3H, J ) 4.8 Hz), 7.156-6.992 (d, 3H, J ) 2.8 Hz), 6.717-6.535 (s, 3H); FT-IR (KBr pellet, cm-1): 1611 (CdO stretching in TTA), 1536 (CdC stretching in TTA), 1501, 1473, 1438 (C-P stretching), 1414, 1356, 1303, 1242, 1229, 1179 (PdO stretching), 1142, 1061, 1029, 933, 858, 782, 721, 694, 641, 590, 579, 567, 521; ESI-MS (m/z): 1492 (M+ + CH3OH, 100), 1460 (M+, 50); elemental analysis calcd (%) for C66H43EuF9NO8P2S3: C 54.33, H 2.97, Eu 10.41, N 0.96, O 8.77, S 6.59. Found: C 54.67, H 2.71, N 1.02, O 8.98, S 6.70.

General Procedure of Preparation of Gadolinium Complexes. For triplet energy level measurement, gadolinium complexes comprising TMOADPO, EtCzDPO, or PhCzDPO without TTAs were also synthesized.32 Phosphine oxide ligand (1 mmol) was dissolved in ethanol (15 mL). Gd(NO3)3(H2O)6 (0.5 mmol) in water (0.1 mL) was added in solution dropwise under stirring, and the solution was heated to refluxing for 2 h. The gadolinium complexes were recrystallized from concentrated methanol-water mixed solvent. Gd(NO3)3(TMOADPO)2. White powder. Elemental analysis calcd (%) for C45H39GdN4O14P2: C 50.09, H 3.64, N 5.19, O 20.76; found: C 50.38, H 3.51, N 4.87, O 21.00. Gd(NO3)3(EtCzDPO)2. White powder. Elemental analysis calcd (%) for C38H31GdN4O11P2: C 48.61, H 3.33, N 5.97, O 18.75; found: C 48.77, H 3.08, N 5.85, O 18.91. Gd(NO3)3(PhCzDPO)2. White powder. Elemental analysis calcd (%) for C42H31GdN4O11P2: C 51.11, H 3.17, N 5.68, O 17.838; found: C 51.24, H 3.13, N 5.37, O 18.09. Fabrication and Testing of OLEDs. Four-layer OLEDs were fabricated by vacuum deposition with a configuration of ITO/ NPB (30 nm)/EuIII complex:CBP (10%, 40 nm) /BCP (30 nm)/ Alq3 (30 nm)/Mg0.9Ag0.1 (200 nm)/Ag (80 nm), wherein NPB is N,N-bis(R-naphthylphenyl)-4,4′-biphenyldiamine as the holetransporting layer, CBP is 4,4′-bis(carbazole-9-yl)biphenyl as host material, BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline as the electron transporting/hole blocking layer, Alq3 was used as the electron transporting layer, and ITO and MgAg were used as the anode and cathode, respectively. Before being loaded 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 25 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 MgAg with 200 nm thickness was deposited at a rate of 0.4 nm s-1 to improve electron injection. Finally, an 80-nm-thick layer of Ag was deposited at a rate of 0.6 nm s-1 as the cathode. The emission area of the devices was 0.12 cm2 as determined by

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the overlap area of the anode and the cathode. The EL spectra and current-voltage-luminance characteristics were measured with a Spectrascan PR 650 photometer and a computercontrolled DC power supply under ambient conditions. Results and Discussion Design and Synthesis. In our former work on monodentate functional APO ligands,34 by taking the advantage of the modification inertia of the phosphine oxide ligands the direct introduction of the hole-transporting groups as chromophore groups endows the ligands with a compact structure, excellent double carrier transport ability, and intermediate S1 and T1 energy levels. However, it was noticed that the relatively weak coordination ability of the monodentate ligands induced distinct emissions of the ligands in the PL spectra of the complexes in solution, which originate from structure relaxation of the complexes and solvent quenching. This implied that the intramolecular energy transfer was inefficient. In our work on the bidentate APO ligand DPEPO,32 it was proved that the chelate coordinate mode could not only increase rigidity of the complex but also improve the stability of the complex. However, the electron-transporting DPEPO induces unbalanced carrier injection/transporting in its complex, which might be the main reason for the limited maximum brightness of only 600 cd m-2. In general, it is clear that the bidentate APO ligands are superior to the monodentate APO ligands in the rigidity and stability of the complex structures. In addition, the successful modification of monodentate APO ligands suggests that the introduction of functional groups can be a feasible approach to improve EL performance of Eu3+ complexes based on the bidentate APO ligands. Furthermore, the study of the relationship between the structure and the performance is very important to provide a reference for the design of high-efficiency EL Eu3+ complexes based on functional APO ligands. It is well-known that the intramolecular energy transfer process in Eu3+ complexes follows the Dexter mechanism. Thus, a compact complex structure should be required for the efficient intramolecular energy transfer. By taking advantage of the modification inertia of APO ligands, herein the functional bidentate APO ligands were also designed as the direct combinations of hole-transporting amine moieties and two electron-transporting DPPO moieties. Two DPPO moieties were also used as the coordinate sites to form the chelate coordinate mode. It is obvious that since DPPO moieties serve as both coordinate sites and electron-transporting groups, the ratio of electron-transporting moieties to the hole-transporting moieties in bidentate APO ligands is 2:1. However, the ratio in momodentate APO ligands is 1:1. Therefore, by judging from the structures of the hole-transporting moieties in modified monodentate and bidentate APO ligands, the latter has the advantage in structure stability and improvement of intramolecular energy transfer, and the former is superior in stronger hole injection and transfer and endowing their complexes with an amorphous state. These bidentate APO ligands can be conveniently synthesized from the corresponding dibromides through phosphorization and oxidation in moderate isolated yields of 50%. The complexes were prepared according to the well-established protocols37 in yields more than 60%. Optical Properties. UV-vis absorption spectra and PL spectra of 1-3 in solution (10-6 mol L-1 in CH2Cl2 were measured (Figure 1). The absorption spectrum of 1 consists of three main absorption peaks at 227, 274, and 335 nm. 2 and 3 have nearly the same absorption characteristics with peaks at 250, 276, and 343 nm. TTA makes contributions to the

Figure 1. Absorption and PL spectra of complexes 1-3 in CH2Cl2 (10-6 mol L-1).

absorption bands around 340 nm in the spectra, while for 1 its strongest absorption peak at 335 nm is mainly due to the electron-donating methoxyl-modified TMOADPO. For 2 and 3, EtCzDPO and PhCzDPO mainly give a contribution to the maximum absorption peaks of the complexes at 250 and 276 nm. It indicates that the bidentate APO ligands are strongly absorbing “antennae” for light harvesting in the complexes. The PL spectra of 1-3 consist of four main peaks at 579, 593, 611, and 653 nm as characteristic emissions from Eu3+ corresponding to 5D0 to 7Fj (j ) 0-3) transitions. The peaks at 612 nm have the largest intensity as the main emission. Significantly, although the functional bidentate APO ligands are also strongly blue emitting, only the characteristic emissions originated from Eu3+ ion are observed in PL spectra of 1-3 in solution. However, in our former work the emissions attributed to monodentate APO ligands were distinct in PL spectra from their Eu3+ complexes. The relaxation of the complex structure would allow the solvent to enter into the complex structure more easily, which might diminish the solvent-quenching effect.29 It is obvious that the complexes based on monodentate APO ligands are inferior in structure rigidity and stability. Contrarily, the more rigid structure and chelate coordinate mode convey much less degree of freedom for EtCzDPO and PhCzDPO and facilitate the formation of more compact complex structures, which not only reduces the energy loss induced by the structure relaxation but also restrains the solvent quenching. For TMOADPO, although it is not as rigid as EtCzDPO and PhCzDPO, its three methoxy moieties would have strong steric effect on preventing the solvent molecules from entering into the complex structures. Furthermore, the chelate coordinate mode would create a shorter distance between the ligands and Eu3+. It is known that the intramolecular energy transfer in Eu3+ complexes follows the Dexter mechanism, which requires the overlap of electron clouds of host and guest. Thus, the energy transfer between bidentate APO ligands and Eu3+ can benefit from the compact complex structures. This also decreases the possibility of ligand emission. The relative PL quantum efficiencies (PL QE) of 1 and 2 were measured by using Ru(bpy)3Cl2 as a standard and listed in Table 1. 1-3 have moderate PL QE of more than 30%. Intermolecular Energy Transfer. The absorption and PL spectra of the free ligands (TMOADPO, EtCzDPO, PhCzDPO, and Gd(TTA)3 · 2H2O) (10-6 mol L-1 in CH2Cl2) were measured (Figure 2). The absorption spectra of EtCzDPO and PhCzDPO are similar with two strong absorption peaks at 249 and 275 nm. Their PL emissions are nearly the same as that of carbazole, which implies that the carbazole moieties in EtCzDPO and PhCzDPO are the main absorption antennae. It is shown that there are wide overlaps from 325 to 425 nm between the



TABLE 1: PL Properties and Frontier Energy Levels of Complex 1-3 complex 1 2 3

λexa (nm) 227, 273, 335 243, 276, 342 252, 275, 342

λem,maxa (nm) 612 612 612

lifetime (µs) 89.65 (55%), 432.50 (45%) 397.72 388.08

PL QE (%) b

c

33.4 /25.7 36.3b/24.1c 35.9b/23.8c

S1/T1 of APO ligandsd (eV)

HOMO/LUMOe (eV)

2.89/2.55 3.45/3.08 3.45/3.08

-5.14/-2.82 -5.56/-3.21 -5.61/-3.27

a 10-6 mol L-1 in CH2Cl2. b Using Ru(bpy)3Cl2 as standard. c By an integrating sphere. d S1 levels were estimated according to the absorption edges of PO ligands, and T1 levels were estimated according to the phosphorescent peaks of the corresponding Gd3+ complexes at 77 K. e Determined by the cyclic voltammetry (CV) at room temperature in acetonitrile measured against an Ag/Ag+ (0.1 M in acetonitrile) electrode (0.34 V versus saturated calomel electrode (SCE)).

SCHEME 3: Schematic Energy Level Diagram and the Energy Transfer Processa

Figure 2. Absorption and PL spectra of the chelate functional APO ligands and Gd(TTA)3 · 2H2O in CH2Cl2 solution with a concentration of 10-6 mol L-1.

Figure 3. Phosphorescent spectra of the gadolinium complexes (10-4 mol L-1 in CH2Cl2).

emission peaks of EtCzDPO and PhCzDPO and the absorption peak of Gd(TTA)3 · 2H2O, which shows the possibility of the energy transfer from them to TTA. Because of the strong electron-donating effect of its three methoxyl groups, the absorption and PL emission peaks of TMOADPO are remarkably bathochromic. There is an overlap between the emission peak of Gd(TTA)3 · 2H2O and the absorption peak of TMOADPO, but there is no overlap between the emission peak of TMOADPO and the absorption peak of Gd(TTA)3 · 2H2O. Therefore, the reverse singlet energy transfer from TTA anion to TMOADPO can occur. Nevertheless, considering the much stronger light harvesting ability of TMOADPO than that of TTA anion, this reverse singlet energy transfer from TTA to TMOADPO can be neglected. T1 levels of TMOADPO, EtCzDPO, and PhCzDPO were obtained by measuring the phosphorescence spectra of their gadolinium complexes corresponding to their peak emission wavelengths, which are 2.55 (486 nm), 3.08 (402 nm), and 3.08 eV (402 nm)) (Figure 3). The first singlet excited energy levels (S1) of TAPO, NaDAPO, and CPPO are estimated by referencing their absorbance edges, which are 2.89 (429 nm), 3.45 (359 nm), and 3.45 eV (359 nm). For comparison, the singlet and

a

S1 ) first singlet excited states; T1 ) first triplet excited states.

triplet state energy levels are summarized in Table 1 and illustrated in Scheme 3. Because of the different positions of S1 and T1 levels of TMOADPO, EtCzDPO, and PhCzDPO, two different intramolecular energy transfer mechanisms are suggested. For EtCzDPO and PhCzDPO, their S1 levels are higher than that of TTA, and their T1 levels are between the S1 and T1 levels of TTA. Therefore, their intramolecular energy transfer follows a successive process. The energy absorbed by EtCzDPO and PhCzDPO can be transferred to the S1 level of TTA because all S1 levels of these APO ligands are higher than that of TTA (Scheme 3). Different from the unfunctionalized DPEPO, the introduction of longer-conjugated planar carbazole moieties as chromophore groups remarkably reduces their S1 levels. Compared with DPEPO, whose S1 level is 3.94 eV, S1 levels of EtCzDPO and PhCzDPO are modified to fit that of TTA with S1 energy gaps of 0.33 eV, which is much less than 0.82 eV between DPEPO and TTA. However, T1 levels of EtCzDPO and PhCzDPO are higher than that of DPEPO (2.99 eV). Compared with DPEPO, T1 energy gaps between EtCzDPO, PhCzDPO, and TTA are slightly increased from 0.64 to 0.73 eV. It is believed that the energy transfer between two different levels can be facilitated if the energy gap between them is around 0.4 eV. Thus, this implies that compared with DPEPO, EtCzDPO and PhCzDPO can support more efficient singlet energy transfer and similar triplet energy transfer. For the intramolecular energy transfer process in 2 and 3, it is suggested that EtCzDPO and PhCzDPO are first excited to their S1 states, and then the energy transfer occurs to their T1 levels or S1 level of TTA. Consequently, all of the energy converges on the T1 level of TTA and finally transfers to 5D0 of Eu3+. Different from EtCzDPO and PhCzDPO, TMOADPO has a S1 level lower than that of TTA. A reverse energy transfer from TTA to TMOADPO might occur. As the energy-absorbing antenna, the majority of S1 excited energy is harvested through TMOADPO so the positive energy transfer can be neglected. However, the lower S1 level of TMOADPO induces the energy to transfer to the T1

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Figure 5. DSC curves of 1-3. Figure 4. TGA curves of 1-3.

level of TMOADPO. Moreover, the energy gap between T1 levels of TMOADPO and TTA is only 0.2 eV, which is too small for efficient positive energy transfer. Therefore, it is possible that the energy is transfered to 5D0 of Eu3+ from both T1 levels of TMOADPO and TTA. Nevertheless, the very compact complex structure created by the chelate coordinate mode facilitates the overlap of electron wave function between TMOADPO and Eu3+, which is required by the Dexter energy transfer mechanism. The equivalent PL QE of 1 with 2 and 3 further proves that the efficient direct energy transfer from TMOADPO to Eu3+ is also efficient. The investigation of the PL decay lifetime of the complexes also supports the different intramolecular energy transfer process in 1-3 (Table 1). Both 2 and 3 have single exponential decay characteristics, corresponding to long lifetimes of about 390 µs. However, the decay curve of 1 is double exponential, corresponding to two different lifetimes of 89 and 432 µs. Obviously, the long lifetime of 1 is attributed to the intramolecular energy transfer process involving the excited energy levels of TTA anion, which is similar for 2 and 3. The short lifetime of 1 undoubtedly corresponds to the direct energy transfer between TMOADPO and Eu3+. Thermal Properties. The thermal stability of the complexes is very important because decomposition leads to diminished EL performance of the Eu3+ complexes. Thermogravimetric analysis (TGA) of 1-3 was performed. It is shown that the temperatures of the thermal decomposition (Td) of 1-3 are higher than 300 °C (340 °C for 1, 315 °C for 2, and 323 °C for 3) (Figure 4), which make device fabrication by vacuum evaporation more feasible. In the first step of decomposition of 1 a weight loss of 42% corresponds to the loss of three TTA moieties. However, for 2 and 3 the first step with a weight loss of 27% corresponds to the decomposition of their APO ligands. The stronger thermal stability of TMOADPO might originate from its relatively flexible structure, which facilitates the structure adjustment rather than decomposition during heating and maintains stability. Circle differential scanning calorimetry (DSC) analysis of 1-3 was also performed to investigate their phase variation. For 1 no melting point (Tm) was observed, which means that 1 hardly crystallizes (Figure 5). The formation of the amorphous state might be attributed to the three methoxyl moieties in TMOADPO, which enhance the steric effect of the complex and weaken the intermolecular interaction. 1 also has a high temperature of glass transition (Tg) of 137 °C. The more rigid structures of 2 and 3 make them easy to crystallize because of strong intermolecular interaction and regular arrangement. The Tm of 2 is 272 °C with two distinct Tg of 145 and 220 °C. The most rigid 3 has the highest Tm of 306 °C and Tg of 216 °C. It

Figure 6. CV curves of 1-3.

is found that 2 and 3 have a similar Tg of about 220 °C. Thus, the first Tg of 2 must originate from the ethyl of EtCzDPO. Both 2 and 3 exhibit very excellent phase stability with very high Tg. It is known that the Joule heat during the device operation might induce the phase variation, which would make the thin films deform, shrink, or aggregate and consequently results in defects and pinholes. Therefore, the enhanced phase stability of 1-3 is significant for the improvement of device lifetime. Electrochemical Properties. The redox behavior of 1-3 deserves attention, as it can indicate the frontier orbital levels, which would relate to the carrier-injection ability of the complexes. Circle voltammetry analysis of 1-3 was performed (Figure 6). 1 has one reversible oxidation peak at 0.53 V and one irreversible oxidation peak at 1.16 V. Its irreversible reduction peak is at -2.56 V. 2 has two irreversible oxidation peaks at 1.05 and 1.40 V, respectively, and three irreversible reduction peaks at -1.71, -1.99, and -2.25 V. The redox behavior of 3 is similar to that of 2. There are two irreversible oxidation peaks at 1.06 and 1.47 V and three irreversible reduction peaks at -1.68, -1.91, and -2.22 V in the CV curve of 3. According to the onset voltages of their redox peaks and the equation reported by de Leeuw et al.,40 the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the complexes are calculated and listed in Table 1. 1 has the highest HOMO level, nearly -5.1 eV, which is even higher than that of many common hole-transport layer materials (HOMONPB ) -5.2 eV), but its LUMO level at -2.82 eV was also the highest. Both 2 and 3 have appropriate HOMO levels around -5.6 eV and much lower LUMO levels of about -3.2 eV (Figure 6 and Table 1). It is clear that the hole-injection ability of the complex can be tuned conveniently by introducing strong electron-donating groups. If the electron-donating effect



TABLE 2: EL Properties of the Four-Layer Devices Based on 1-3 EQE (%) complex 1 2 3 Eu(TTA)3 DPEPO32

max. brightness (cd m-2)

turn-on voltage (V)

max.

at 100 cd m-2

945 1276 1163 634

6.0 7.6 7.2 7

2.96 3.54 3.20 2.89

0.67 0.95 0.94 1.25

of the hole-injection group is too strong, the electron-injection ability of the complex would be reduced. Significantly, compared with monodentate 9-[4-(diphenylphosphinoyl)phenyl]-9Hcarbazole (CPPO),36,37 whose structure is the combination of N-phenylcarbazole and DPPO, EtCzDPO and PhCzDPO support their complexes with similar HOMO levels, but they endow their complexes with 0.2 eV lower LUMO levels. Obviously, when the hole-injection moieties in the PO ligands (both monodentate and bidentate) are the same, the hole-injection abilities of the corresponding complexes are similar. More importantly, more DPPO in the bidentate ligands induces an increase in the electron-injection ability of their complexes. It proves that the complex’s carrier-injection ability can be tuned conveniently by changing the kind and number of functional groups in the APO ligands and designing different coordination modes. EL Performance of OLEDs. The four-layer devices with a configuration of ITO/NPB (30 nm)/Eu3+ complex:CBP (40 nm, 10%)/BCP (30 nm)/Alq3 (30 nm)/Mg0.9Ag0.1 (200 nm)/Ag (80 nm) were fabricated. Devices A, B, and C were based on 1, 2, and 3, respectively. All of the complexes exhibited excellent EL performance, such as maximum brightness around 1000 cd m-2, EQE around 3%, and stable monochromic red emission at 614 nm (Table 2). The brightness-current density-voltage (B-J-V) curves of the four-layer devices were measured and are shown in Figure 7. The turn-on voltage of device A was 6.0 V at 1 cd m-2, and a maximum brightness of 945 cd m-2 was achieved at 19.2 V with a current density of 386 mA cm-2. Device B had a turn-on voltage at 7.6 V. Its maximum brightness was 1276 cd m-2 at 21.2 V with a current density of 324 mA cm-2. The turn-on voltage of device C was 7.2 V. Its maximum brightness was 1176 cd m-2 at 19.6 V with a current density of 326 mA cm-2. It is obvious that the turn-on voltage is relative to the carrier injection and transport ability of each layer. Although the complexes were only used as dopants, the high doping concentration of 10% makes the carrier injection/transporting

Figure 7. Brightness-current density-voltage (B-J-V) curves of the four-layer devices of 1-3.

Figure 8. Brightness-current density (B-J) curves of A-C.

Figure 9. EQE-J curves of devices A-C.

ability important for the carrier injection of the entire device. The much lower turn-on voltage of device A than those of B and C shows a much easier carrier injection and transport for 1. Obviously, the strongest hole-injection ability of 1 is the main reason for the lowest turn-on voltage of A. Furthermore, A had the largest maximum current density among the devices, which also implied the strongest carrier transporting ability of 1. However, the high current densities of A did not correspond to high brightness. Moreover, brightness-current density (B-J) curve of A was the most gradual among these three devices (Figure 8). The weakest electron-injection and strongest holeinjection ability of 1 among the complexes induced an unbalanced carrier injection. At high voltages, the hole as the major carrier in A partially flows to the anode rather than forming excitons. Thus, the maximum brightness of A was the lowest among these three devices. Inspiringly, the maximum luminance of 2 was almost twice of that of Eu(TTA)3DPEPO,32 and 2 and 3 have similar EL performance. It is still noticed that at the same voltages the current densities and brightness of 3 were much larger than those of 2. However, at the same current densities, especially when the current density was more than 170 mA cm-2, the brightness of 2 was larger than that of 3. The stronger intermolecular interaction induced by the rigid phenyl-substituted PhCzDPO benefits the carrier-injection and transport but also worsens the triplet-triplet (T-T) quenching. Contrarily, the ethyl in EtCzDPO has no effect on the facilitation of carrier-injection and transport but can mitigate the T-T quenching effect by steric hindrance. Thus, at the same voltages, the current densities of B are much less than those of C. Because at the same voltages the higher current density in C could form more excitons, the brightness of C is much larger than that of B. However, at the same current densities, for 3, the stronger T-T interaction-induced exciton annihilation remarkably reduces

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Xu et al. efficiencies. Because when the injected carriers were limited, the excitons were thoroughly formed on the host CBP. Then the energy was transferred to Eu3+ complexes either through exciton migration or Fo¨rster energy transfer. PL properties of 1-3 indicate that the efficiencies of host-guest energy transfer from CBP to the complexes were very similar. However, at high current densities, the unbalanced carrierinjection ability of 1 leads to an efficiency of A that is lowest among these three devices. It is believed that at high current densities the carrier trapping by dopants becomes effective.41 Thus, the complexes with relatively balanced carrier injection/ transporting ability are superior in the formation of excitons, decreasing invalid carriers and improving the device efficiencies. Notably, the efficiency of 3 at a current density less than 10 mA cm-2 was much higher than that of 2, but at high current density, 2 became more efficient. It implied that at low current density the efficient carrier injection and transport in the emitting layer was the key factor for the device efficiency. However, at high current density the T-T quenching effect became key factor. Therefore, the weaker T-T quenching in 2 causes the efficiency of B to gradually exceed that of C during the increase in current density. EL spectra of all of the complexes mainly consist of the characteristic Eu3+ emissions with the main peak at 614 nm as a pure red emission. It is shown that along with the voltage increase, the spectra of A-C remains stable (Figure 10), which demonstrates that charge trapping is the dominant luminescence process. The excellent EL spectra stability of A-C indicated that even at high voltages the efficient energy transfer from the host CBP to the complexes is still efficient. Moreover, the absence of the emission from hole-transporting NPB implies that the recombination zones are close to the interfaces between the emission layers and hole-blocking BCP. Conclusion

Figure 10. EL spectra of devices A-C at different voltages and the energy level schemes of the devices.

the conversion efficiency, which is the main reason for the larger brightness of B than that of C at higher current densities. Figure 9 shows the external quantum efficiency-current density (EQE-J) curves of A-C. The maximum current efficiency and power efficiency of A achieved at 6.0 V and 0.0252 mA cm-2 was 4.69 cd A-1, corresponding to a EQE (ηext) of 2.96%. When the brightness rose to 100 cd m-2 at 11.8 V with a current density of 9.527 mA cm-2, its current efficiency and power efficiency decreased to 1.06 cd A-1 and 0.28 lm W-1, respectively, corresponding to a ηext of 0.67%. At 7.8 V with a current density of 0.0191 mA cm-2, the current efficiency and power efficiency of B reached a maximum of 5.60 cd A-1 and 2.26 lm W-1 corresponding to a ηext of 3.54%. At 100 cd m-2 with a voltage of 14.0 V and a current density of 6.773 mA cm-2, its current efficiency and power efficiency were still 1.51 cd A-1 and 0.34 lm W-1, respectively, corresponding to a ηext of 0.95%. The maximum current efficiency of C was 5.08 cd A-1 achieved at 7.0 V with a current density of 0.0208 mA cm-2, corresponding to a power efficiency of 2.28 lm W-1 and a ηext of 3.20%. At 12.4 V with a current density of 6.558 mA cm-2 and a brightness of 100 cd m-2, its current efficiency and power efficiency were 1.48 cd A-1 and 0.38 lm W-1, corresponding to a ηext of 0.94%. The EQE curves of the devices also showed the same phenomenon. At lower voltages with very small current densities, these three devices exhibited similar

A series of bidentate APO ligands with carrier-injection moieties and strong-coordinate PO moieties, as well as their Eu3+ complexes, were designed and synthesized. The investigations show that the bidentate APO ligands are strongly absorbing “antennae” for light harvesting in the complexes. The more rigid structure of the bidentate APO ligands and their chelate coordinate mode reduce their degree of freedom and form much more compact complex structures, which not only reduce the excited energy loss of the complexes induced by the structure relaxation but also restrain the solvent quenching. All of the complexes exhibited improved thermal and phase stability. Their excellent carrier-injection ability was proved by CV analysis. Significantly, more DPPO in the bidentate APO ligands induce the stronger electroninjection ability of their complexes. It is feasible that the carrier-injection ability of the complexes can be tuned by choosing different kinds of moieties, changing the proportion between the functional groups, and designing different complex structures. All of the complexes exhibited excellent EL performance, such as maximum brightness around 1000 cd m-2, EQE around 3%, and stable monochromic red emission at 614 nm. Our investigations demonstrate the potential application of functional bidentate APO ligands in high performance EL Eu3+ complexes, and further modifications for the improvement of PL properties and aggregation of the complexes are ongoing in our laboratory. Acknowledgment. This project was supported by NSFC (90406021, 50903028, and 20972043), Science and Technology

Light-Emitting Ternary Eu3+ Complexes Bureau of Heilongjiang Province (QC08C10), Supporting Program of Universities’ Science and Technology Innovation Team of Education Bureau of Heilongjiang Province, Open/ Innovation Fund of KLFIMC, and Heilongjiang University Funds for Distinguished Young Scientists. Helpful discussions with Prof. Chun-Hui Huang, Prof. Lian-Hui Wang, and Prof. Fu-You Li (Fudan University) were highly appreciated. References and Notes (1) Alexander, V. Chem. ReV. 1995, 95, 273–342. (2) De Paoli, G.; Dzo˘lic, Z.; Rizzo, F.; De Cola, L.; Vo¨gtle, F.; Mu¨ller, W. M.; Richardt, G.; Zı˘nic, M. AdV. Funct. Mater. 2007, 17, 821–828. (3) Parker, D.; Dickins, R. S.; Puschmann, H.; Crossland, C.; Howard, J. A. K. Chem. ReV. 2002, 102, 1977–201. (4) Tremblay, M. S.; Sames, D. Chem. Commun. 2006, 4116–4118. (5) Bunzli, J.-C. G. Acc. Chem. Res. 2006, 39, 53–61. (6) Bunzli, J.-C. G.; Piguet, C. Chem. Soc. ReV. 2005, 34, 1048–1077. (7) Deiters, E.; Song, B.; Chauvin, A.-S.; Vandevyver, C. D. B.; Gumy, F.; Buzli, J.-C. G. Chem.sEur. J. 2009, 15, 885–900. (8) Kuriki, K.; Koike, Y. Chem. ReV. 2002, 102, 2347–2356. (9) Ling, Q. D.; Song, Y.; Ding, S. J.; Zhu, C. X.; Chan, D. S. H.; Kwong, D.-L.; Kang, E.-T.; Neoh, K.-G. AdV. Mater. 2005, 17, 455–459. (10) Ling, Q. D.; Liaw, D.-J.; Zhu, C. X.; Chan, D. S. H.; Kwong, D.L.; Kang, E.-T.; Neoh, K.-G. Prog. Polym. Sci. 2008, 33, 917–978. (11) Mcgehee, M. D.; Bergstedt, T.; Zhang, C.; Saab, A. P.; O’Regan, M. B.; Bazan, G. C.; Srdanov, V. I.; Heeger, A. J. AdV. Mater. 1999, 11, 1349–1354. (12) Mancino, C.; Ferguson, A. J.; Beeby, A.; Long, N. J.; Jones, T. S. J. Am. Chem. Soc. 2005, 127, 524–525. (13) Kido, J.; Okamoto, Y. Chem. ReV. 2002, 102, 2357–2368. (14) Xin, H.; Shi, M.; Gao, X. C.; Huang, Y. Y.; Gong, Z. L.; Nie, D. B.; Cao, H.; Bian, Z. Q.; Li, F. Y.; Huang, C. H. J. Phys. Chem. B 2004, 108, 10796–10800. (15) Shi, M.; Li, F. Y.; Yi, T.; Zhang, D. Q.; Hu, H. M.; Huang, C. H. Inorg. Chem. 2005, 44, 8929–8936. (16) (a) Robinson, M. R.; O’Regan, M. B.; Bazan, G. C. Chem. Commun. 2000, 1645–1646. (b) Li, S. F.; Zhong, G. Y.; Zhu, W. H.; Li, F. Y.; Pan, J. F.; Huang, W.; Tian, H. J. Mater. Chem. 2005, 15, 3221–3228. (c) Zheng, Y. X.; Cardinali, F.; Armaroli, N.; Accorsi, G. Eur. J. Inorg. Chem. 2008, 12, 2075–2080. (17) De Silva, C. R.; Li, F. Y.; Huang, C. H.; Zheng, Z. P. Thin Solid Films 2008, 517, 957–962. (18) Yu, J. B.; Zhou, L.; Zhang, H. J.; Zheng, Y. X.; Li, H. R.; Deng, R. P.; Peng, Z. P.; Li, Z. F. Inorg. Chem. 2005, 44, 1611–1618.

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