Influencing the Spectral Stability and the Electroluminescence

Apr 10, 2008 - Dipartimento di Ingegneria delle Acque e di Chimica (DIAC) - Politecnico di Bari, Via Orabona, 4 I-70125. Bari, Italy, National Nanotec...
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J. Phys. Chem. C 2008, 112, 7005-7014

7005

Influencing the Spectral Stability and the Electroluminescence Behavior of New Blue-Emitting Bifluorene-Based Materials by the 7,7′-Functionalization of the Core Roberto Grisorio,† Claudia Piliego,‡,§ Paola Fini,| Pinalysa Cosma,⊥ Piero Mastrorilli,† Giuseppe Gigli,‡,# Gian Paolo Suranna,*,† and Cosimo Francesco Nobile† Dipartimento di Ingegneria delle Acque e di Chimica (DIAC) - Politecnico di Bari, Via Orabona, 4 I-70125 Bari, Italy, National Nanotechnology Laboratories (NNL) of CNR - INFM, Lecce, Italy, Istituto Superiore UniVersitario di Formazione Interdisciplinare (ISUFI)-sezione Nanoscienze, Lecce, Italy, IPCF - CNR c/o Dipartimento di Chimica, Bari, Italy, Dipartimento di Chimica dell’UniVersita` di Bari, Campus UniVersitario-Bari, Italy, Dipartimento di Ingegneria dell’InnoVazione, UniVersita` del Salento, Lecce, Italy ReceiVed: October 23, 2007; In Final Form: January 22, 2008

This study concerns the preparation of novel monodispersed organic materials based on a tetrahexyl-bifluorene core linked to end-groups by double or triple bonds. The molecules were designed aiming at tuning their HOMO-LUMO levels without remarkably changing their optical properties. The syntheses were carried out through a convergent-divergent approach by reaction of a 7,7′-diiodobifluorene building block with 2-fluoren7-yl-ethynyl (BF1), 2-carbazol-3-yl-ethynyl (BF2), 2-fluoren-7-yl-ethenyl (BF3), and 2-carbazol-3-yl-ethenyl (BF4) moieties. The analysis of the optical properties evidenced for BF1-4 an efficient blue emission in solution. Their emission profile is preserved also in the solid state, except for that of BF4, the PL spectrum of which showed the appearance of a green band deriving from aerobic degradation. The HOMO value of -5.00 eV, measured for BF4 by cyclic voltammetry, suggested a correlation of the spectral instability with its low ionization potential. This hypothesis was confirmed by the photoluminescence behavior of BF1-3 upon UV photoirradiation showing a correlation between spectral stability and ionization potential. OLED devices of ITO/PEDOT-PSS/BF1-4/Ca/Al configuration exhibited a blue electroluminescence. Fair performances were obtained for the carbazole-containing BF2 (1000 cd/m2 with a current efficiency of 0.039 cd/A at 10 V) and BF4 (1431 cd/m2 and 0.027 cd/A at 14 V) presumably due to a more balanced transport of charge carriers in the active layer. The results obtained in devices embodying 7.5 nm of a hole-blocking layer (BCP) between the calcium cathode and the emitting material showed a remarkable enhancement of current efficiencies for all devices and confirmed a more balanced transport of charge carriers in the carbazolecontaining molecules. The figures of merit obtained for the carbazole-containing BF2 (1088 cd/m2 and 0.070 cd/A at 10 V) represent one of the best results for blue-emitting materials embodying ethynyl groups.

Introduction Fluorene-based organic compounds have been representing one of the most promising classes of blue-emitting materials for organic light-emitting diodes (OLEDs).1 An important goal which can be easily achieved with fluorene derivatives is the straightforward introduction of alkyl chains at the C-9 position, which ensures a good solubility for the corresponding macromolecules and contributes to the preservation of their optical properties in the solid state by limiting molecular interactions.2 However, blue-emitting devices fabricated with fluorene-based polymers and copolymers show poor spectral stability, as their emission spectra are characterized by the appearance of a lowenergy band after short operation times, a behavior that has been attributed to processes involving the fluorene 9-position.3 The presence in the fluorene moiety of a highly reactive site brings * Corresponding author. Tel: +390805963603. Fax: +390805963611. E-mail: [email protected]. † Dipartimento di Ingegneria delle Acque e di Chimica (DIAC) Politecnico di Bari. ‡ National Nanotechnology Laboratories (NNL) of CNR - INFM. § Istituto Superiore Universitario di Formazione Interdisciplinare (ISUFI)sezione Nanoscienze. | IPCF - CNR c/o Dipartimento di Chimica. ⊥ Dipartimento di Chimica dell’Universita ` di Bari. # Dipartimento di Ingegneria dell’Innovazione, Universita ` del Salento.

as a drawback its marked tendency toward oxidation forming green-emitting fluorenones, leading to an additional emission band centered at 520-540 nm for most fluorene-based materials.4 The physical origin of this green emission remains so far a fervidly debated argument characterized by two opposite points of viewsassociating the spectral instability of polyfluorenes either to on-chain defect emission5 or to fluorenone-based excimer formation.6 Regarding the formation of the keto defects, a hypothesis has been accepted, suggesting that the preferential sites for oxidation processes are the monoalkyl fluorenes (9-H fluorenes), deriving from the incomplete alkylation of fluorene.7 These 9-H fluorenes are difficult to detect and remove from the monomer feed by common procedures. It is only recently, however, that other degradation pathways have been proposed, accounting for more experimental data than the simple 9-H fluorene oxidation hypothesis. In addition to 9-H fluorene decomposition, these pathways also consider the dialkylfluorene itself as unstable and envisage fluorenone formation by sequential chain elimination triggered by dioxygen attack.8 In order to avoid or at least to limit the formation of fluorenones in fluorene-based materials, it is crucial not only to completely eliminate 9-H fluorene defects at an early stage of the synthesis9 but also to understand the causes leading to fluorene oxidation.

10.1021/jp7102403 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/10/2008

7006 J. Phys. Chem. C, Vol. 112, No. 17, 2008 The strategies of synthesizing suitable fluorene copolymers has led to significant results in obtaining materials with improved blue electroluminescence.10 The behavior of polymers however, is complicated by their statistically defined structure and by their variable molecular weight. The complexity of interpreting results obtained from polymeric materials can be bypassed by investigating the optical behavior of well-defined oligomeric structures, due to the ease of their purification and characterization.11 To this purpose, recent studies have demonstrated a better stability toward oxidation under thermal treatment of blue-emitting monodispersed oligofluorenes and fluorene-based systems8a,12 with respect to polyfluorenes. The reasons for this behavior have been attributed to the higher chemical purity of the monodispersed systems with respect to polymers. However, the literature does not provide further examples of investigations on the spectral stability of functionalized oligofluorenes, notwithstanding for these systems much more precise structure-property relations could be drawn regarding not only their optical properties but also their ionization potential and electron affinity.10,13 These parameters can be dramatically changed by the introduction of suitable electron-donating or -withdrawing moieties as end-groups of an oligofluorene π-conjugated backbone. If one also considers the applications of these monodispersed organic materials as active layers in OLEDs, the presence of electron-donating or -withdrawing moieties can improve the balancing between hole and electron transport within the device active layer, thus avoiding complex multilayer device configurations aimed at OLED optimization.14 In this framework, we deemed it worthwhile to design and synthesize molecular materials constituted by tetrahexyl-bifluorene (the simplest oligofluorene core) containing fluoren-2yl and carbazol-3-yl groups linked to its 7,7′-positions through double or triple C-C bonds, aiming at tuning their HOMOLUMO levels and their transport properties without remarkably changing their optical properties. We report on the structureproperty relationships that we drew on the series of functionalized bifluorene-based materials, focusing in particular on the key issues of the spectral stability and electroluminescence behavior. Experimental Section All manipulations were carried out under an inert nitrogen atmosphere using standard Schlenk techniques. All solvents were carefully dried and freshly distilled prior to use according to common laboratory techniques. 2,7-Dibromofluorene (1) and all other reactants were purchased from Aldrich, Acros, Fluka, or Strem and used without further purification. C, H, and N elemental analyses were carried out with a Carlo Erba CHNS-O EA 1108 instrument. NMR spectra were recorded at 295 K on a Bruker Avance 400 MHz. UV-vis spectra were recorded on a Kontron Uvikon 942 instrument and fluorescence spectra were obtained on a Varian Cary Eclipse spectrofluorimeter; quantum yields were measured by the dilute solution method, using a 9,10-diphenylanthracene solution in cyclohexane (Φ ) 0.90) as standard.15 UV-photodecomposition experiments were carried out by illuminating the sample with a 150 W high-pressure Hglamp. FT-IR spectra were recorded on a Bruker Vector 22 spectrometer. Differential scanning calorimetry (DSC) analyses were carried out on a DSC Q200 TA instrument at a scanning rate of 10 °C/min under a nitrogen flow. GPC analyses were carried out on a Agilent Series 1100 instrument equipped with a Pl-gel 10 µm-100 Å column. THF solutions for the GPC analysis were eluted at 25 °C at a flow rate of 1 mL/min and

Grisorio et al. analyzed using a multiple wave detector. Cyclic voltammetry (CV) measurements were carried out under an inert nitrogen atmosphere with an Autolab potentiostat PGSTAT 10 using a three-electrode cell. The CV measurements were carried out in acetonitrile solutions of tetrabutylammonium tetrafluoroborate (0.10 M) at a scan rate of 100 mV/s. An indium tin oxide (ITO)coated glass was used as the working electrode, on which the film was deposited. A Pt wire and a Ag/Ag+ pseudoreference electrode were utilized as the counter electrode and reference electrode, respectively. Each measurement was calibrated against ferrocene, the ionization potential of which is +4.80 eV, and the evaluation of the HOMO level of the materials was carried out by measuring the onset of the oxidation potential in the anodic scan with respect to the Fc/Fc+ couple. The OLED devices were prepared by spin-coating poly(3,4-ethylenedioxythiophene) poly(styrenesulphonate) (PEDOT-PSS) onto an O2plasma-treated ITO-coated glass substrate. Subsequently, the active material was spin cast from a chloroform solution (2 mg in 0.3 mL of CHCl3) and finally a calcium (45 nm)/aluminum (150 nm) electrode was deposited by thermal evaporation (10-6 mbar). For the optimized devices, 2,9-dimethyl-4,7-diphenyl1,10-phenanthroline (BCP, 7.5 nm) was thermally evaporated before Ca/Al deposition. The characterization of the devices was performed at room temperature in air. 2,7-Dibromo-9,9-dihexylfluorene (2). A suspension of 1 (10.93 g, 33.73 mmol) and tetrabutylammonium bromide (3.62 g, 11.25 mmol) in a 50% w/v NaOH aqueous solution (75 mL) was stirred for 15 min at 60 °C. To the red suspension, n-hexyl bromide (11.69 g, 70.84 mmol) was added and the mixture was kept under stirring overnight. After the solution was cooled, the product was extracted with diethyl ether (3 × 75 mL) and the organic layer was dried over Na2SO4. After the solvent was removed, the crude product was purified by flash chromatography (SiO2, petroleum ether 40-60 °C) to afford 2 in 92% yield as a white solid. 1H NMR (400 MHz, CDCl3): δ 7.54 (d, J ) 8.9 Hz, 2H), 7.50-7.45 (m, 4H), 1.99-1.91 (m, 4H), 1.211.02 (m, 12H), 0.83 (t, J ) 7.0 Hz, 6H), 0.68-0.58 (m, 4H). 13C{1H} NMR (100 MHz, CDCl ): δ 152.6, 139.1, 130.2, 3 126.2, 121.5, 121.1, 55.7, 40.2, 31.5, 29.6, 23.7, 22.6, 14.0. 2-Bromo-7-trimethylsilyl-9,9-dihexylfluorene (3). To a solution of 2 (7.30 g, 14.83 mmol) in THF (90 mL) kept at -80 °C, n-BuLi (1.6 M in hexanes, 9.3 mL, 14.9 mmol) was added dropwise. The resulting mixture was allowed to react at -80 °C for 1 h before addition of Me3SiCl (1.93 g, 17.79 mmol). Then the flask was warmed at room temperature and stirred for 4 h. After solvent removal, diethyl ether (40 mL) was added and the resulting solution was washed with water (3 × 20 mL) and dried over Na2SO4. The crude product, obtained by solvent evaporation, was purified by flash chromatography (SiO2, petroleum ether 40-60 °C) affording 3 in 98% yield as a colorless oil. 1H NMR (400 MHz, CDCl3): δ 7.69 (d, J ) 7.6 Hz, 1H), 7.60 (d, J ) 7.9 Hz, 1H), 7.55-7.46 (m, 4H), 2.09-1.90 (m, 4H), 1.22-1.02 (m, 12H), 0.82 (t, J ) 7.0 Hz, 6H), 0.76-0.57 (m, 4H), 0.36 (s, 9H). 13C{1H} NMR (100 MHz, CDCl3): δ 153.2, 149.5, 140.6, 140.1, 139.7, 132.0, 129.9, 127.6, 126.2, 121.2, 121.1, 119.0, 55.3, 40.1, 31.4, 29.6, 23.6, 22.5, 14.1, -0.8. 7,7′-Bis(trimethylsilyl)-9,9,9′,9′-tetrahexyl-[2,2′]-bifluorene (4). A mixture of 3 (7.00 g, 14.42 mmol), Ni(COD)2 (4.76 g, 17.31 mmol), 2,2′-bipyridine (2.70 g, 17.31 mmol), 1,5cyclooctadiene (1.56 g, 14.42 mmol), and toluene (90 mL) was stirred overnight at 80 °C. After cooling to room temperature, the reaction solution was filtered on a celite plug, washed with water (3 × 50 mL), and dried over Na2SO4. After the solvent

7,7′-Functionalized Bifluorene-Based Materials was removed, the crude product was purified by flash chromatography (SiO2, petroleum ether 40-60 °C) to afford 4 in 81% yield as a white solid. 1H NMR (400 MHz, CDCl3): δ 7.83 (d, J ) 7.9 Hz, 2H), 7.77 (d, J ) 7.9 Hz, 2H), 7.71-7.65 (m, 4H), 7.71-7.65 (m, 4H), 2.09 (t, J ) 7.9 Hz, 8H), 1.23-1.08 (m, 24H), 0.86-0.72 (m, 20H), 0.38 (s, 18H). 13C{1H} NMR (100 MHz, CDCl3): δ 151.8, 150.3, 141.6, 140.8, 140.3, 139.0, 132.0, 127.7, 126.1, 121.6, 120.1, 119.1, 55.2, 40.2, 31.4, 29.7, 23.8, 22.6, 14.1, -0.7. 7,7′-Diiodo-9,9,9′,9′-tetrahexyl-[2,2′]-bifluorene (5). To a solution of 4 (4.11 g, 5.06 mmol) in CH2Cl2 (30 mL), ICl (1.0 M in CH2Cl2, 10.8 mL, 10.8 mmol) was added dropwise at 0 °C. The resulting mixture was stirred at room temperature for 3 h, before portionwise addition of a 10 wt % Na2S2O3 aqueous solution until discoloration of the solution. The organic layer was separated, washed with water (3 × 30 mL), and dried over Na2SO4. After the solvent was removed, the crude product was purified by flash chromatography (SiO2, petroleum ether 40-60 °C) yielding 5 in 94% yield as a white solid. 1H NMR (400 MHz, CDCl3): δ 7.77 (d, J ) 7.9 Hz, 2H), 7.74-7.68 (m, 4H), 7.66 (d, J ) 7.9 Hz, 2H), 7.60 (s, 2H), 7.51 (d, J ) 7.6 Hz, 2H), 2.13-1.94 (m, 8H), 1.22-1.05 (m, 24 H), 0.81 (t, J ) 7.0 Hz, 12H), 0.77-0.67 (m, 8H). 13C{1H} NMR (100 MHz, CDCl3): δ 153.4, 150.9, 141.0, 140.4, 139.4, 135.9, 132.1, 126.3, 121.5, 121.4, 120.1, 92.6, 55.5, 40.2, 31.4, 29.6, 23.7, 22.6, 14.0. 7,7′-Bis(9,9-dihexylfluoren-2-yl-ethynyl)-9,9,9′,9′-tetrahexyl[2,2′]-bifluorene (BF1). A mixture of 5 (0.275 g, 0.30 mmol), 10 (0.226 g, 0.63 mmol), Pd(PPh3)4 (69.3 mg, 0.06 mmol), CuI (11.4 mg, 0.06 mmol), and diethylamine (5 mL) was refluxed for 4 h. After the solution was cooled to room temperature, diethyl ether (50 mL) was added and the resulting mixture was washed with water (3 × 30 mL) and dried over Na2SO4. After the solvent was removed, the crude was purified by flash chromatography (SiO2, petroleum ether 40-60 °C) to give BF1 in 88% yield as a yellow solid. Anal. calcd. for C104H130: C, 90.51%; H, 9.49%. Found: C, 90.16%; H, 9.49%. 1H NMR (400 MHz, CDCl3): δ 7.81 (d, J ) 7.9 Hz, 2H), 7.78-7.68 (m, 8H), 7.66-7.58 (m, 10H), 7.40-7.33 (m, 6H), 2.17-1.94 (m, 16H), 1.21-1.02 (m, 48H), 0.84-0.57 (m, 40H). 13C{1H} NMR (100 MHz, CDCl3): δ 151.9, 151.1, 151.0, 150.8, 141.4, 141.0, 140.8, 140.5, 139.8, 130.7, 130.6, 127.5, 126.9, 126.3, 126.0, 125.9, 122.9, 121.6, 121.5, 121.4, 120.3, 120.0, 119.8, 119.7, 90.7, 90.6, 55.3, 55.2, 40.5, 31.6, 31.5, 29.8, 29.7, 23.8, 23.7, 22.7, 22.6, 14.0. IR (KBr): v [cm-1] 3052, 2920, 2857, 1449, 1255, 811, 749. 7,7′-Bis(N-hexylcarbazol-3-yl-ethynyl)-9,9,9′,9′-tetrahexyl[2,2′]-bifluorene (BF2). A mixture of 5 (0.275 g, 0.30 mmol), 15 (0.173 g, 0.63 mmol), Pd(PPh3)4 (69.3 mg, 0.06 mmol), CuI (11.4 mg, 0.06 mmol), and diethylamine (5 mL) was refluxed for 6 h. After the solution was cooled to room temperature, diethyl ether (50 mL) was added and the resulting mixture was washed with water (3 × 30 mL) and dried over Na2SO4. After the solvent was removed, the crude product was purified by flash chromatography (SiO2, petroleum ether 40-60 °C/CH2Cl2 ) 9:1) to give BF2 in 93% yield as a yellow solid. Anal. calcd. for C90H104N2: C, 89.06%; H, 8.64%; N, 2.31%. Found: C, 88.63%; H, 8.61%; N, 2.29%. 1H NMR (400 MHz, CDCl3): δ 8.39 (s, 2H), 8.15 (d, J ) 7.9 Hz, 2H), 7.84-7.59 (m, 14H), 7.56-7.50 (m, 2H), 7.47-7.40 (m, 4H), 7.33-7.27 (m, 2H), 4.34 (t, J ) 7.1 Hz, 4H), 2.17-2.03 (m, 8H), 1.92 (quint, J ) 7.1 Hz, 4H), 1.48-1.27 (m, 12H), 1.22-1.07 (m, 24H), 0.91 (t, J ) 6.0 Hz, 6H), 0.85-0.71 (m, 20H). 13C{1H} NMR (100 MHz, CDCl3): δ 151.8, 151.1, 140.8, 140.7, 140.6, 140.1, 139.9,

J. Phys. Chem. C, Vol. 112, No. 17, 2008 7007 130.6, 129.3, 126.3, 126.1, 125.9, 124.0, 122.9, 122.5, 122.1, 121.4, 120.5, 120.2, 119.7, 119.3, 113.3, 108.9, 108.8, 91.0, 88.7, 55.3, 43.3, 40.5, 31.6, 31.5, 29.8, 29.0, 27.0, 23.8, 22.7, 22.6, 14.1. IR (KBr): v [cm-1] 3052, 2943, 2927, 2849, 2196 (CtC), 1488, 1457, 1340, 1231, 811, 734. 7,7′-Bis(9,9-dihexylfluoren-2-yl-ethenyl)-9,9,9′,9′-tetrahexyl[2,2′]-bifluorene (BF3). A mixture of 5 (0.919 g, 1.00 mmol), 8 (0.757 g, 2.10 mmol), Pd(OAc)2 (44.9 mg, 0.20 mmol), tri(o-tolyl)phoshane (121.7 mg, 0.40 mmol), DMF (13 mL), and triethylamine (3 mL) was stirred at 90 °C overnight. After the solution was cooled to room temperature, diethyl ether (50 mL) was added. The resulting solution was washed with water (3 × 20 mL) and then dried over Na2SO4. After the solvent was removed, the crude product was purified by column chromatography (SiO2, petroleum ether 40-60 °C/CH2Cl2 ) 10:1) to afford BF3 in 14% yield as a yellow solid. Anal. calcd. for C104H134: C, 90.24%; H, 9.76%. Found: C, 89.87%; H, 9.79%. 1H NMR (400 MHz, CD Cl ): δ 7.85 (d, J ) 7.3 Hz, 2H), 2 2 7.80 (d, J ) 7.9 Hz, 2H), 7.78-7.70 (m, 8H), 7.66-7.57 (m, 8H), 7.43-7.32 (m, 10H), 2.21-2.00 (m, 16H) 1.25-1.02 (m, 48H), 0.86-0.58 (m, 40H). 13C{1H} NMR (100 MHz, CD2Cl2): δ 151.8, 151.6, 151.3, 151.0, 140.9, 140.9, 140.5, 140.4, 140.1, 136.6, 136.5, 128.5, 128.4, 126.8, 126.1, 126.0, 122.9, 121.4, 120.7, 120.6, 119.9, 119.9, 119.8, 119.6, 55.2, 55.0, 40.4, 31.6, 31.5, 29.7, 23.9, 23.8, 22.6, 13.8, 13.8. IR (KBr): ν [cm-1] 3030, 2957, 2924, 2855, 1461, 1265, 1090, 1021, 957, 801. 7,7′-Bis(N-hexylcarbazol-3-yl-ethenyl)-9,9,9′,9′-tetrahexyl[2,2′]-bifluorene (BF4). A mixture of 5 (0.534 g, 0.58 mmol), 13 (0.339 g, 1.22 mmol), Pd(OAc)2 (26.1 mg, 0.12 mmol), tri(o-tolyl)phosphane (70.8 mg, 0.23 mmol), DMF (8 mL), and triethylamine (1.5 mL) was stirred at 90 °C overnight. After the solution was cooled to room temperature, diethyl ether (50 mL) was added. The resulting solution was washed with water (3 × 20 mL) and then dried over Na2SO4. After the solvent was removed, the crude product was purified by column chromatography (SiO2, petroleum ether 40-60 °C/CH2Cl2 ) 8:1) to afford BF4 in 23% yield as a yellow solid. Anal. calcd. for C90H108N2: C, 88.76%; H, 8.94%; N, 2.30%. Found: C, 88.71%; H, 8.90%; N, 2.33%. 1H NMR (400 MHz, CD2Cl2): δ 8.34 (s, 2H), 8.19 (d, J ) 8.2 Hz, 2H), 7.85 (d, J ) 8.5 Hz, 2H) 7.82-7.76 (m, 4H), 7.75-7.70 (m, 4H), 7.65 (s, 2H), 7.62 (dd, J ) 8.2, 1.3 Hz, 2H), 7.55-7.43 (m, 8H), 7.33 (d, J ) 16.2 Hz, 2H), 7.30-7.27 (m, 2H), 4.37 (t, J ) 6.7 Hz, 4H), 2.20-2.12 (m, 8H), 1.93 (quint, J ) 6.7 Hz, 4H), 1.50-1.38 (m, 12H), 1.24-1.01 (m, 24H), 0.92 (t, J ) 7.0 Hz, 6H), 0.840.73 (m, 20H). 13C{1H} NMR (100 MHz, CD2Cl2): δ 151.8, 151.6, 140.9, 140.2, 140.1, 137.0, 128.9, 128.6, 126.4, 126.0, 125.8, 125.4, 124.3, 123.1, 122.7, 121.4, 120.4, 120.3, 119.9, 119.8, 118.9, 118.4, 109.1, 109.0, 55.2, 43.2, 40.5, 31.6, 29.7, 28.9, 26.9, 23.9, 22.6, 13.8. IR (KBr): ν [cm-1] 3029, 2958, 2924, 2855, 1461, 1349, 1325, 1265, 1091, 1021, 957, 801. Results and Discussion Synthesis and Characterization. The structure of the synthesized materials is reported in Figure 1. Compounds BF1-4 were prepared by a convergent-divergent approach. In order to avoid a potentially unselective direct halogenation of the bifluorene, a more convenient protocol, consisting of the Ni(0)-mediated coupling of bromofluorenes functionalized with trimethylsilyl groups (Scheme 1) was adopted.16 The commercially available 2,7-dibromofluorene (1) was converted into the corresponding 9,9-dihexylfluorene derivative (2) by reaction with n-hexylbromide, and it was subsequently functionalized in the C-7 position with a trimethylsilyl group (3). The homo-

7008 J. Phys. Chem. C, Vol. 112, No. 17, 2008

Figure 1. Bifluorene molecular materials BF1-4.

coupling of 3, promoted by Ni(COD)2, afforded the bifluorene derivative 4, which was then reacted with ICl to give the building block 5. The end-groups 8 and 13 were obtained starting from the suitable bromo derivatives 7 and 12, respectively. The latter were prepared by the alkylation of 6 and 11 with n-hexylbromide. The synthesis of the vinyl derivative 8 was achieved by reaction of 2-bromo-9,9-dihexylfluorene (7) with potassium vinyltrifluoroborate, using PdCl2(dppf) as catalyst and triethylamine as base, following a literature protocol.17 The 3-vinylcarbazole 13 was obtained analogously starting from 3-bromoN-hexylcarbazole (12). The introduction of the triple bond to give 9 (14) was achieved by a Sonogashira coupling between 7 (12) and trimethylsilylacetylene using PdCl2(PPh3)2 as catalyst and CuI as co-catalyst. The deprotection of the triple bond was performed by reaction with KF, affording the final terminal acetylene building blocks 10 and 15. The synthesis of the bifluorene systems BF1-2 was carried out in high yield by a Sonogashira coupling between the diiododerivative 5 and two equivalents of the suitable alkynes 10 and 15. Compounds BF3-4 were prepared by a Heck reaction of the bifluorene derivative 5 with two equivalents of 8 and 13, respectively, using Pd(OAc)2/tri-(o-tolyl)phosphane as catalytic system and triethylamine as base. These compounds were obtained with relatively low yields (14%, 23% for BF3 and BF4, respectively) due to the formation of byproducts difficult to remove. It is worth remarking that, notwithstanding GC-MS and NMR confirming their purity, the building blocks 2 and 7 have been subjected to repeated basic treatment with potassium tert-butylate (a treatment aimed at eliminating traces of 9-H fluorene defects) according to a literature procedure.9 A color modification of the THF solution of the fluorene compounds upon tert-butylate addition is a qualitative test for the presence of 9-H sites. Notwithstanding this test proved negative for the building blocks 2 and 7, the same test carried out on BF1-4 resulted in a slightly colored solution indicating that 9-H defects could not completely be removed. The higher sensitivity of the test for the oligomeric compounds can be ascribed to a hyperchromic effect deriving from the π-conjugation extension. Compounds BF1-4 were characterized by 1H NMR, 13C{1H} NMR, IR, elemental analysis, and GPC (see Supporting

Grisorio et al. Information). In the case of BF4, the double-bond configuration was deduced by the analysis of the 1H NMR spectrum showing a doublet with J ) 16.2 Hz attributable to one of the two vinyl protons and confirmed by the correlation of these signals in the 1H1H-COSY spectrum. The IR analysis revealed the presence of the CH out-of-plane bending of a trans-substituted double bond at 957 cm-1; this feature was helpful for assigning the configuration of the vinyl bonds in BF3 because the superimposition of the vinyl protons resonance (assigned to the unresolved signal at δ 7.34) with other aromatic protons complicated the assignment of the double-bond configuration from the mere analysis of the 1H NMR spectrum (see Supporting Information). The thermal characterization of BF1-4 was carried out by differential scanning calorimetry (DSC). The samples were subjected to a heating scan to 300 °C, a cooling scan, and a reheating scan (all carried out at 10 °C/min under inert atmosphere). All samples show a glass transition at temperatures (Tg) between 39 and 65 °C (Table 1). These glass transitions were accompanied by an endothermic peak, due to enthalpic relaxation, which was not observed during the cooling and reheating scans. During the first heating scan, BF1 and BF3 samples also showed endothermic events at 162.4 and 170.3 °C, respectively, which could be attributed to melting. During both cooling and reheating scans the only observed events were glass transitions, except for BF3 which showed, during the reheating scan, an exothermic event at 83.9 °C followed by an endothermic one at 169.7 °C. This behavior can be explained by assuming that, at temperatures above the Tg, the increase in molecular mobility can facilitate crystallization (exothermic peak) followed by melting (endothermic peak). No thermal event attributable to decomposition could be identified in the explored temperature range (see Supporting Information). Electrochemical Properties. We carried out a cyclic voltammetry (CV) comparative study aimed at investigating the influence of the substituents on the HOMO-LUMO energy levels of BF1-4. The HOMO levels were measured by the onset of the oxidation peak, following an empirical correlation.18 The onset of UV-vis absorption curves in solution (see below), used for the calculation of the energy gaps, permitted an estimate of the LUMO energy levels (Table 1). Quasi-reversible oxidation waves and no reduction events could be observed for BF1-4. In terms of HOMO energy levels, a lowering of ionization potentials of carbazole-containing structures BF2 and BF4 (-5.33 and -5.00 eV) was observed with respect to BF1 and BF3 (-5.40 and -5.23 eV). This observation is ascribable to the presence of the electron-donating nitrogen atoms. The incorporation of double bonds, on the other hand, sensibly lowers the ionization potentials with respect to the triple-bond moieties as can be deduced by a comparison of the HOMO levels of BF1 with that of BF3 and that of BF2 with that of BF4. Optical Behavior and Spectral Stability. The optical properties of the materials were studied both in chloroform solution (Figure 2A,B) and in the solid state (Figure 3A,B). Concerning the behavior in solution, BF1-2 show absorption maxima at 380 and 378 nm, respectively, the latter with a band at 286 nm due to the π-π* transition of the carbazole unit excluded from the conjugation. Photoluminescence (PL) spectra of BF1 showed a maximum at 413 nm followed by a vibronic replica at 437 nm. The PL spectrum of BF2 is very similar, showing maxima at 414 and 438 nm. As to BF3, the absorbance spectrum exhibits a maximum at 390 nm with a shoulder at 407 nm. The emission profile of BF3 presents a fine structure with maxima at 431 nm and at 458 nm. The absorption spectrum

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SCHEME 1: Synthetic Procedures for BF1-4

TABLE 1: Electrochemical Data and Thermal Properties of BF1-4 Eonset a HOMOb energy LUMO first DSC second DSC (V) (eV) gapc (eV) (eV) scan (°C) scan (°C) BF1 BF2 BF3

0.80 0.94 0.58

-5.40 -5.33 -5.23

3.00 3.00 2.87

-2.40 -2.33 -2.36

BF4

0.41

-5.00

2.86

-2.14

162.4 170.3

55.2 (Tg) 50.1 (Tg) 64.5 (Tg)/ 83.9/169.7 39.3 (Tg)

Onset of the oxidation potential referred to Ag/Ag+. b Estimated after calibration of the voltammograms by Fc/Fc+ internal reference. c Estimated from the onset of the absorption spectrum in chloroform. a

of BF4 showed no vibronic bands and exhibits a maximum at 392 nm together with another maximum at 299 nm due to the carbazol-3-yl moiety, excluded from the conjugation. In the emission spectrum, the molecule showed a maximum at 434 nm followed by a vibronic replica at 461 nm. The fluorescence quantum yields (Φ) of the compounds range from 0.74 to 0.84. The onset of UV-vis absorption curves in solution was used for the calculation of the HOMO-LUMO energy gap values. The results, shown in Table 1, indicate a lowering of the HOMO-LUMO energy gap (of 0.13-0.14 eV) for BF3-4 with respect to their ethynyl functionalized analogues BF1-2. In order to evaluate the optical behavior of BF1-4 in the solid state, a film of the corresponding material was spin-coated on a quartz substrate. The relevant spectra are reported in Figure 3A,B. The solid-state absorption spectra of BF1-4 show practically no difference in terms of λabs with respect to those recorded in solution (BF1: λabs ) 376 nm; BF2: λabs ) 379 nm; BF3: λabs ) 393 nm with a shoulder at λabs ) 414 nm; BF4: λabs ) 397 nm).

With respect to the spectra recorded in solution, the emission spectra of BF1-2 are characterized by the loss of the vibronic structure and by a red-shift of the emission (BF1: λem ) 449 nm; BF2: λem ) 464 nm). A similar behavior is observed in the emission spectrum of BF3, showing a λem at 465 nm. A peculiar aspect was observed in the emission spectrum of BF4: the λem at 470 nm is flanked by another band of lower intensity, peaked at 538 nm. On the basis of the resemblance of this green emission to the well-known low-energy band appearing in the PL spectra of fluorenone-containing oligofluorene systems, it is reasonable to identify the fluorescence at 538 nm as a consequence of the oxidation of fluorene units to fluorenones.4a,19 This feature may be due to a photoinduced decomposition process in air during the PL measurement (λex ) 400 nm). To verify this hypothesis, sequential PL measures were carried out on the same sample of BF4. It was observed that the intensity of the green emission progressively increased with the number of the measurements (Figure 4A). It is thus reasonable to suppose that the amount of keto defects increases as a function of the irradiation time at 400 nm, causing an oxidative degradation with consequent appearance of the green band. To gain further insight into the photodecomposition process, we analyzed a chloroform solution of BF4 kept under ambient illumination for 1 month and found that a thin film obtained by this solution exhibited only the green band in PL measurements (Figure 4B), indicating that the compound underwent similar oxidative degradation. It is noteworthy that, when the emission of this aged solution was recorded in CHCl3, no traces of the longer-wavelength band could be detected, indicating that the efficiency of the energy transfer toward fluorenone-based acceptors is dramatically dependent upon

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Figure 2. UV-vis and PL spectra in CHCl3 of BF1-2 (A) and BF3-4 (B). Figure 3. Solid-state UV-vis and PL spectra of BF1-2 (A) and BF3-4 (B).

intermolecular interactions between the host/guest systems and therefore extremely low in solution. The amount of keto defects formed is too low to allow its detection by IR spectroscopy; however, in order to qualitatively evaluate the effects of the photodegradation on BF4, a straightforward thin layer chromatography (TLC) analysis on silica was carried out on the 1-month aged solution using a hexane/CH2Cl2 mixture as eluent. This simple technique allowed the separation of the products formed after degradation from BF4, and a qualitative estimation of their fluorescence wavelength by inspecting the TLC plate under a 366 nm UV lamp. In this case, several products more polar than BF4 were observed (products that reasonably contain fluorenones and/or oxidized side chains)8b,c formed after the aging process and, even though only qualitatively, it was confirmed that green-emitting compounds originated after an oxidative degradation of a fluorenebased material. On these bases, it is reasonable to assume that these compounds act as acceptors of an efficient energy transfer process from the blue-emitting materials, generating the pure green emission when deposited as a thin film on quartz. The much lower stability exhibited by BF4 with respect to BF1-3 could be confirmed also by the unaltered blue photoluminescence profile of thin films obtained from aged solutions of BF1-3 kept under air and ambient illumination for 1 month. The high photosensitivity of BF4 in air motivated our interest in the possibility of studying the spectral modifications of BF1-3 in the solid state after exposure to an intense UV source. For the purpose, a 150 W high-pressure UV lamp was used. In the case of BF1-3, as reported in Figure 5A-C, after 15 min of UV-light exposure, the photoluminescence profile changed and a longer-wavelength broad emission appeared. In the case

of the more stable BF1, only a tail at a longer wavelength could be observed, while for BF2 and BF3 the whole photoluminescence profile is shifted to the green region. Also in these cases, a TLC analysis on the annealed film dissolved in chloroform was carried out, clearly revealing the formation of green-emitting degradation byproducts. Unfortunately, as also testified by recent studies on monodispersed fluorene-based molecular materials,20 the quantification of these defects is extremely difficult,8b and only a qualitative correlation between their presence and the relative intensity of the low-energy band is possible. On the basis of these results, we tried to find an explanation for the remarkably different behavior of our compounds toward the development of the green band. A rationalization of the data concerning the spectral stability of BF1-4 can be made by considering the pivotal role of intermolecular interactions in the efficiency of the energy transfer process and the amount of the oxidizable sites in the compounds. In fact, the presence of orthogonally placed alkyl chains also in the fluorene end-groups of BF1 and BF3 can better isolate these molecules in the solid state, reducing the efficiency of the energy transfer toward fluorenone-based acceptors. Taking these effects into account, it seems appropriate to draw a comparison of the optical behavior upon aerobic photodegradation of BF1 and BF3 on one hand and of the carbazole-containing BF2 and BF4 on the other. Only by the above-mentioned comparison can the longer wavelength emission intensity be regarded as an indication of the amount of fluorenones formed by oxidative degradation. From the analysis of the PL spectra of the annealed film, it can be concluded that the ethynyl-containing molecules BF1-2

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Figure 4. (A) Photoluminescence spectrum of a thin film of BF4 taken at a 2 min interval. (B) Photoluminescence spectrum of a solution of BF4 kept for 1 month under air and ambient illumination (filled circles); photoluminescence spectrum of a film of BF4 obtained from the abovementioned solution (open circles).

are more stable toward the formation of the green-emitting fluorenone-based compounds than the ethenyl-containing BF3-4 molecules, because the intensity of the longer-wavelength emission is higher in BF3 with respect to BF1 (upon analogous UV treatment) and BF4 undergoes degradation under milder conditions with respect to BF2. As the bifluorene core and the fluorene end-groups of BF1-4 derive from the same batch of 5 and 7, the molecules BF1 and BF3 contain the same (extremely low) amount of 9-H fluorene defects, and the same can be stated for BF2 and BF4. Consequently, the reason for the different tendency toward the development of the green band of BF1-4 must be correlated to parameters other than their chemical purity (intended as the amount of 9-H defects). Although mechanisms fully explaining the degradation pathways of fluorene-based materials have been recently put forward, these clarify the different evolution of the green band only as a function of the C-9 substitution of fluorene.8 A parameter that could explain the photodecomposition behavior of the materials is their tendency toward oxidation that can be estimated by the measure of the HOMO energy level. From the spectral stability data of BF1-4 (Figure 5) in the light of the electrochemically determined ionization potentials, the lower stability of BF3 (with respect to BF1) and that of BF4 (with respect to BF2) can be rationalized by their higher HOMO values (-5.23 and -5.00 eV for BF3 and BF4, respectively). On this basis, we propose a reaction pathway for the initiation step of the photo-oxidation reaction that takes into account the

Figure 5. Photoluminescence spectra of a thin film of BF1-3 before and after UV-photoirradiation (15 min) in air.

SCHEME 2: Proposed Reaction Pathway for the Formation of a C-9 Benzylic Radical Leading to Fluorenone

dependence on the ionization potential of the luminophor (Scheme 2). The π-conjugated segment undergoes electron abstraction, which is favored by a lower ionization potential,

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TABLE 2: Electroluminescence Properties of BF1-4 for OLED of Configuration ITO/PEDOT-PSS/BF1-4/Ca/Al (type A) and ITO/PEDOT-PSS/BF1-4/BCP/Ca/Al (type B)

BF1 (type A) BF1 (type B) BF2 (type A) BF2 (type B) BF3 (type A) BF3 (type B) BF4 (type A) BF4 (type B) a

max. lum. (cd/m2)

max. curr. eff. (cd/A)

ext. eff. a (%)

CIE coord.a (x,y)

345 420 1103 1150 714 553 1431 534

0.014 0.065 0.039 0.070 0.013 0.046 0.027 0.035

0.006 0.033 0.014 0.038 0.007 0.030 0.012 0.021

0.19, 0.21 0.19, 0.24 0.19, 0.22 0.19, 0.24 0.17, 0.15 0.17, 0.18 0.17, 0.18 0.19, 0.21

Data obtained from spectra at 12 V.

forming a radical cation. If the molecule contains a 9-H defect, proton elimination can occur leading to a C-9 radical species that, in turn, is either involved in further oxidation to fluorenone or in subsequent oxidation of dialkylfluorene units, following mechanisms previously proposed.8 Electroluminescence Properties. In order to carry out a survey of the electroluminescence properties of the obtained materials, we fabricated and measured diodes using the configuration ITO/PEDOT-PSS/BF1-4/Ca/Al. The results are reported in Table 2. For all devices, blue emission and turn-on voltages ranging from 3 to 4.5 V were observed. Compound BF1 showed a maximum luminance of 345 cd/m2 at 12 V with a maximum current efficiency of 0.014 cd/A at 8 V. The substitution of the terminal fluorenes with the electron-donating carbazol-3-yl groups (in BF2) exerts a profound effect on the device performances due to the favorable injection and transport of holes in the active layer (1103 cd/m2 at 12 V, 0.039 cd/A at 10 V). For BF3 a maximum luminance of 714 cd/m2 could be obtained at 16 V, with a maximum current efficiency of 0.013 cd/A at 14 V. Comparison of the behavior of BF4 (1431 cd/m2 and 0.027 cd/A at 14 V) with that of BF3 confirms the beneficial effect of an electron-donating group incorporated in the molecule. Notwithstanding the use of a simple OLED configuration, the performances of the devices were quite interesting, in particular for BF2 and BF4. This result can be rationalized by admitting, for these carbazole-containing molecular materials, a more balanced transport of electrons and holes within the active layer. This observation, in conjunction with the good current efficiencies obtained, in particular for BF2 (Figure 6C), encouraged us to pursue an optimization of the devices. As it is reasonable to suppose that most of the current density of the devices is due to the hole transport, the diode configuration was improved by introducing a hole-blocking layer (2,9-dimethyl4,7-diphenyl-1,10-phenanthroline, BCP) between the organic emitters and the cathode. The use of a hole-blocking material can also give indirect information about the transport ability of electrons and holes in the active layer of the device. The results in terms of luminance, current density, and current efficiencies are reported in Figure 7 and summarized in Table 2. The turn-on voltages of these optimized devices were not substantially modified and ranged from 4 to 5 V. This result suggests that the introduction of 7.5 nm BCP does not influence the electron injection and transport in our devices. As expected, an overall reduction of current density for all optimized OLEDs was observed, due to the higher resistance of hole mobility with respect to single-layer devices. However, the entity of the current suppression cannot be correlated to the energy barrier between the HOMO of BF1-4 and the HOMO of BCP (-6.7 eV).21 In fact, this effect is more evident for BF1 and BF3 notwithstanding their lower HOMO level. In the case of BF2 and BF4, the

Figure 6. Luminance (A), current density (B), and current efficiency (C) plot for diodes of ITO/PEDOT-PSS/BF1-4/Ca/Al configuration.

more contained drop in current density can be explained by assuming a more balanced conduction between electrons and holes within the active layer. The more ambipolar character of the carbazole-containing BF2 and BF4 is not too surprising and is corroborated by recently published results on the transport properties of fluorene-based22 or nitrogen-containing molecular materials.23 Notwithstanding the contained effect on the luminance, the introduction of BCP turned out to be beneficial in terms of current efficiency for all compounds, due to the favored recombination of holes and electrons within the active layer. Remarkably, in the case of BF1, a considerable (∼5-fold) increase of the current efficiency to 0.065 cd/A was obtained. This observation can be explained as a consequence of the confinement of a higher number of holes, favoring a more efficient recombination within the emitting material. A similar effect was obtained for BF3, the maximum current efficiencies of which reached 0.046 cd/A. In the case of BF2 and BF4, the

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Figure 8. EL spectra at 12 V for diodes of ITO/PEDOT-PSS/BF14/BCP/Ca/Al configuration.

spectra as a function of the bias variation, and the CIE coordinates at 12 V fall in the blue region for all devices (Table 2 and Figure 8). Conclusions

Figure 7. Luminance (A), current density (B), and current efficiency (C) plot for diodes of ITO/PEDOT-PSS/BF1-4/BCP/Ca/Al configuration.

maximum current efficiency enhancement is more contained (0.070 cd/A at 10 V for BF2, 0.035 cd/A for BF4 at 12 V). However, for BF2 the maximum current efficiency is accompanied by a luminance of 1088 cd/m2 at 10 V. This result, representing one of the best performances obtained for the electroluminescence of blue-emitting ethynyl-containing luminophors can be rationalized in light of the previously discussed balanced conduction within the emitting material. The devices could not be operated above 14 V. The low Tg values observed for BF1-4 (vide supra) seem the most probable cause of the OLED performance degradation at higher biases; however, for all devices, the electroluminescence profile obtained proved to be very stable (see Supporting Information)sno emission band attributable to fluorene oxidation can be observed in the EL

The preparation of novel monodispersed organic materials (BF1-4) based on a functionalized bifluorene core linked to fluoren-2-yl or carbazol-3-yl end-groups by triple or double bonds was aimed at studying their spectral stability and the electroluminescence behavior. The compounds, prepared using fluorene building blocks subjected to basic purification in order to remove most of the 9-H fluorene defects, exhibited pure blue emission in solution. On the other hand, differently from the other compounds, the solid-state photoluminescence of BF4 (containing carbazole end-groups linked to the core through ethenyl spacers) showed the appearance of a photoinduced green band that was indicative of a degradation process leading to the formation of green-emitting fluorenone-containing compounds during the measure. The particular sensitivity of BF4 with respect to BF1-3 toward oxidation, even under mild conditions, could be justified by invoking its lowest ionization potential, calculated by cyclic voltammetry. Further insights into the correlation between the spectral stability of fluorene-based compounds and their ionization potential were carried out analyzing the optical behavior during UV-irradiation of BF13. The results indicate that BF1 showed a higher stability with respect to BF3, and the same conclusions can be drawn for BF2 with respect to BF4, in agreement with the fact that ionization potentials of BF2 and BF4 are lower than those of BF1 and BF3, respectively. All molecules exhibited a blue electroluminescence obtained from devices of configuration ITO/PEDOT-PSS/BF1-4/Ca/Al. The introduction of carbazole moieties proved to be beneficial in terms of maximum luminance both for BF2 (1000 cd/m2 with a current efficiency of 0.039 cd/A at 10 V) and for BF4 (1431 cd/m2 and 0.027 cd/A at 14 V). The introduction of a hole-blocking layer (BCP) between the calcium cathode and the organic material resulted in blueemitting devices endowed with considerably improved current efficiencies that reached a maximum for BF2 (0.070 cd/A at 10 V with a luminance of 1088 cd/m2). The OLED characteristics obtained for BF2 and BF4 and, in particular, the more contained drop in current density of OLED containing these molecules as a consequence of the insertion of the BCP layer, is compatible with a more balanced

7014 J. Phys. Chem. C, Vol. 112, No. 17, 2008 carrier conduction within their films with respect to BF1 and BF3, due to improved electron transport. In particular, the results obtained for BF2 are among the best so far obtained for blueemitting materials embodying ethynyl groups, and they pave the way for further modifications of the core structure aimed at optimizing the device performances. Acknowledgment. The FIRB project MICROPOLYS (Italian MIUR) is acknowledged for funding. Dr. Paolo Pesce and Dr. Marcella Marasciulo (University of Bari) are gratefully acknowledged for skillful assistance in the carrying out of synthesis and cyclic voltammetry experiments. Supporting Information Available: The data relevant to the preparation and characterization of compounds 7-15, the 1H and 13C{1H} NMR spectra, the cyclic voltammetry scans, DSC scans, and electroluminescence spectra at different bias voltages of BF1-4, the 1H,1H COSY spectrum for BF4, IR spectrum of BF3, GPC traces of BF1-4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Grimsdale, A. C.; Mu¨llen, K. AdV. Polym. Sci. 2006, 199, 1. (b) Bernius, M. T.; Inbasekaran, M.; O’Brien, J.; Wu, W. AdV. Mater. 2000, 12, 1737. (c) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402. (2) (a) Scherf, U.; List, E. J. W. AdV. Mater. 2002, 14, 477. (b) Neher, D. Macromol. Rapid Commun. 2001, 22, 1365. (3) (a) Montilla, F.; Mallavia, R. AdV. Funct. Mater. 2007, 17, 71. (b) Gamerith, S.; Gadermaier, C.; Scherf, U.; List, E. J. W. Phys. Status Solidi A 2004, 201, 1132. (c) Zhao, W.; Cao, T.; White, J. M. AdV. Funct. Mater. 2004, 14, 783. (4) (a) Chi, C.; Im, C.; Enkelmann, V.; Ziegler, A.; Lieser, G.; Wegner, G. Chem. Eur. J. 2005, 11, 6833. (b) Zojer, E.; Pogantsch, A.; Hennebicq, E.; Beljonne, D.; Bre´das, J.-L.; Scandiucci de Freitas, P.; Scherf, U.; List, E. J. W. J. Chem. Phys. 2002, 117, 6794. (5) (a) Becker, K.; Lupton, J. M.; Feldmann, J.; Nehls, B. S.; Galbrecht, F.; Gao, D.; Scherf, U. AdV. Funct. Mater. 2006, 16, 364. (b) Kulkarni, A. P.; Kong, X.; Jenekhe, S. A. J. Phys. Chem. B 2004, 108, 8689. (c) Romaner, L.; Pogantsch, A.; Scandiucci de Freitas, P.; Scherf, U.; Gaal, M.; Zojer, E.; List, E. J. W. AdV. Funct. Mater. 2003, 13, 597. (6) (a) Jaramillo-Isaza, F.; Turner, M. L. J. Mater. Chem. 2006, 16, 83. (b) Sims, M.; Bradley, D. D. C.; Ariu, M.; Koeberg, M.; Asimakis, A.; Grell, M.; Lidzey, D. G. AdV. Funct. Mater. 2004, 14, 765. (7) List, E. J. W.; Guenter, R.; Scandiucci de Freitas, P.; Scherf, U. AdV. Mater. 2002, 14, 374. (8) (a) Grisorio, R.; Suranna, G. P.; Mastrorilli, P.; Nobile, C. F. AdV. Funct. Mater. 2007, 17, 538. (b) Liu, L.; Tang, S.; Liu, M.; Xie, Z.; Zhang,

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