Highly Efficient Color-Tunable OLED Based on Poly(9,9

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J. Phys. Chem. C 2009, 113, 2290–2295

Highly Efficient Color-Tunable OLED Based on Poly(9,9-dioctylfluorene) Doped with a Novel Europium Complex Umberto Giovanella,* Mariacecilia Pasini, Christelle Freund, Chiara Botta, William Porzio, and Silvia Destri Istituto per lo Studio delle Macromolecole, CNR Via Bassini 15, 20133 Milano, Italy, and Polo scientifico e tecnologico del CNR, Via Fantoli 16, 20138 Milano, Italy ReceiVed: October 14, 2008; ReVised Manuscript ReceiVed: December 9, 2008

Tunable dichromic electroluminescence is obtained from a blend of Eu3+ complex in polyfluorene by a single spin-coatable emitting layer device with external quantum efficiency as high as 1%, the highest reported in literature to our knowledge. The design of a complex whose ligand increases the site isolation of Eu3+ ion allows reduction of the back-transfer quenching effects that prevented up to now the multicolor electroluminescence of Eu3+ complex/polyfluorene blends. Energy transfer mechanisms involving singlet and triplet excitons are analyzed by focusing on the effects of site isolation and polarity of the new ligand that provide homogeneous dispersion of the complex in the conjugated polymer. 1. Introduction Light emitting diodes employing heavy-metal complexes are very attractive since the electroluminescence (EL) efficiency, which is limited by the radiative recombination of singlet excitons, can be increased by the addition of the phosphorescent emission from the triplet excitons of the complexes.1 In order to obtain efficient emission from the complexes, they are usually dispersed in a suitable host displaying the two main properties of (i) providing efficient energy/charge transfer to the phosphorescent dopants and (ii) avoiding concentration quenching effects of the complexes. The use of an electroluminescent polymer as host for the phosphorescent complexes has the further advantage that its own fluorescent component can be exploited for multicolor emission, provided that the energy transfer (ET) from polymer to phosphorescent guest is incomplete and that no direct charge trapping at the dopant site occurs.2,3 Thanks to their narrow red emission, Eu3+ β-diketonate complexes4 have attracted considerable interest in designing organic light emitting diodes (OLEDs) for full color displays. Multicolor and white polymer-based OLEDs have been obtained using iridium complexes,5 while no report with Eu3+ complexes has been published to our knowledge. In the last few years, many reports appeared where blends of Eu3+ β-diketonate in polyvinylcarbazole (PVK) have been used,6 but few examples are reported with emissive conjugated polymers. Bazan and McGehee, using poly(2-(6′-cyano-6′-methyl-heptyloxy)-1,4phenylene (CN-PPP) and Eu3+ tris(dibenzoylmethane)phenantholine (Eu(dbm)3Phen), obtained a multicolor emission in photoluminescence (PL), while only a red-emitting OLED could be fabricated.7 A more luminescent complex, Eu(TTA)3Phen (TTA, 2-thienoyltrifluoroacetone) was exploited by Morgado and co-workers8b dispersed in poly(9,9-dioctylfluorene) (PFO) and poly(9,9-dioctylfluorene)-alt-2,1,3-benzothiadiazole (F8BT). A multicolored, PL spectrum of the PFO blend was dominated by Eu3+ complex emission; conversely the typical green luminescence of F8BT was observed from the latter. These results indicate that Fo¨rster resonant energy transfer (FRET) takes place in PFO blend while the null overlap between F8BT * Corresponding author, [email protected].

polymer emission and TTA absorption does not allow any ET. Very different percentages of Eu3+ complexes in the blends were used by the two research groups for the active layer in the devices, but the electroluminescence was always red. In order to reach multicolor emission and enhance the emission efficiency of the blends of PFO with Eu3+-based complexes, much effort should be devoted to the design of the ligands. A proper understanding of the design criteria should take into account all the ET and charge transfer processes among the different singlet and triplet levels of the conducting polymer and the ligand of the Eu3+ complex. Moreover, only a proper monitoring of the morphological aspects in blending phosphorescent guests with conducting polymers allows control and balance these different processes, since ET between singlet states occurs at longer length scales with respect to ET between triplet states and charge trapping. In this contribution we design and prepare a new Eu3+ 2-thienoyltrifluoroacetone complex with ligands bringing a long alkyl chain on the 5 position of the thiophene ring. A phenylsubstituted ring ends two alkyl chains to increase the site isolation of the lanthanide ion, and a bromine atom at the end of the third chain will permit a further insertion of complex directly onto the PFO backbone. We show that the new complex, thereafter labeled Eu(Phen)(L1-Br)(L1)2, allows both PL and EL multicolor (blue and red) emissions to be obtained and insight on the quenching processes of the total PL of the Eu3+ complex/PFO blend. 2. Experimental Section 2.1. Materials and Synthesis. Poly(9,9-dioctylfluorene) end capped with N,N-bis(4-methylphenyl)-4-aniline (PFO) with a Mn of 118000 was purchased from American Dye Source, Inc., and used as received. Commercially available reagents were used without further purification unless otherwise stated. 1-(5-(11-Bromoundecyl)thiophen-2-yl)ethanone. (1s,5s)-9undecyl-9-borabicyclo[3.3.1]nonane was obtained by in situ hydroboration between 11-bromoundec-1-ene (3.6 mL, 16.4 mmol) in dry THF (12 mL) and 0.5 M 9-borabicyclo[3.3.1]nonane in THF (33 mL, 16.4mmol) added at 0 °C and then stirred for 5 h at room temperature. To the solution of alkyl borane were

10.1021/jp809088n CCC: $40.75  2009 American Chemical Society Published on Web 01/21/2009

Energy Transfer in Eu2+/Polyfluorene Blends added DMF (70 mL) PdCl2(dppf) (360 mg, 0.44 mmol), 2-acetyl-5-bromothiophene (3.06 g, 14.9 mmol), and K2CO3 (4.12 g, 29.8 mmol). The mixture was stirred at 50 °C for 18 h, then poured into water, and extracted with Et2O and CH2Cl2, and the solvents were removed to dryness. The crude product was chromatographed on silica gel (CH2Cl2). The product was obtained with 42% yield. MS (EI):m/z 358 [M+]. 1H NMR (CDCl3, 400 MHz, δ ppm, J in Hz): 7.53 (d, 1H, J ) 3.5), 6.81 (d, 1H, J ) 3.5), 3.40 (t, 2H, J ) 6.5), 2.83 (t, 2H, J ) 7.5), 2.51 (s, 3H), 1.85 (m, 4H), 1.43-1.29 (br, 14H). 1-(5-(11-(m-Tolyloxy)undecyl)thiophen-2-yl)ethanone. A 0.78 g (2.18 mmol) portion of 1-(5-(11-bromoundecyl)thiophen2-yl)ethanone, 0.33 g (3.05 mmol) of 3-methylphenol, 0.19 g (4.93 mmol) of sodium hydroxide, and 19 mg (0.07mmol) of TEBA (triethybenzylammonium chloride) were mixed in 1.3 mL of water in a round-bottom flask. The resulting red mixture was heated under a gentle reflux (80 °C) during 1.30 h under vigorous stirring. The light brown mixture was cooled to room temperature. After extraction with Et2O, the combined organic fractions were washed with 1 M NaOH neutralized with water, dried over MgSO4, filtrated, and evaporated; the resulting yellow powder was chromatographed (MeOH, reverse phase) 62% yield. MS (EI): m/z 386 [M+]. 1H NMR (CDCl3, 400 MHz, δppm, J in Hz): 7.53 (d, 1H, J ) 3.7), 7.15 (m, 1H), 6.81 (d, 1H, J ) 3.7), 6.76-6.69 (br, 3H), 3.93 (t, 2H, J ) 6.5), 2.83 (t, 2H, J ) 7.5), 2.51 (s, 3H), 2.33 (s, 3H), 1.74 (m, 4H), 1.41-1.26 (br, 14H). 4,4,4-Trifluoro-1-(5-(11-(m-tolyloxy)undecyl)thiophen-2yl)butane-1,3-dione (L1). The reaction was carried out under nitrogen. MeONa (1 mmol, 25 wt % in MeOH solution,) was poured via a syringe into dry Et2O (5 mL). To this solution ethyl trifluoroacetate (0.12 mL, 0.9 2mmol, 1.05 equiv) and 1-(5(11-(m-tolyloxy)undecyl)thiophen-2-yl)ethanone (340 mg, 0.9 mmol, 1 equiv) were successively added at room temperature. The resulting mixture, which went from initial yellow to light orange within 2 h, was stirred at room temperature overnight. Et2O (20 mL) and then 1 N HCl were added to the mixture. The aqueous phase was neutralized and extracted with Et2O several times. The organic phases were gathered, washed, and dried over MgSO4. The filtrate was evaporated to give a yellow powder purified by column chromatography on silica gel using heptane/acetone (3/1) as eluent, 65% yield. MS (EI):m/z 482 [M+]. 1H NMR (CDCl3, 400 MHz, δ ppm, J in Hz): 7.76 (d, 1H, J ) 3.6), 7.15 (m, 1H), 6.81 (d, 1H, J ) 3.6), 6.76-6.69 (br, 3H), 6.36 (s, 1H), 3.93 (t, 2H, J ) 6.5), 2.83 (t, 2H, J ) 7.5), 2.33 (s, 3H), 1.74 (m, 4H), 1.41-1.26 (br, 14H). IR (film CH2Cl2): ν 2927, 2854, 1601, 1597, 1489, 1448, 1388, 1251, 1185, 1136, 933, 786. 1-(5-(11-Bromoundecyl)thiophen-2-yl)-4,4,4-trifluorobutane1,3-dione (L1-Br). The same procedure of L1 was applied to obtain L1-Br starting from 1-(5-(11-bromoundecyl)thiophen2-yl)ethanone (300 mg, 0.83 mmol), yield 61%. MS (EI):m/z 454 [M+]. 1H NMR (CDCl3, 400 MHz, δ ppm, J in Hz): 7.76 (d, 1H, J ) 3.6), 6.81 (d, 1H, J ) 3.6), 6.36 (s, 1H), 3.40 (t, 2H, J ) 6.5), 2.86 (t, 2H, J ) 7.5), 1.85 (m, 2H), 1.7 (m, 2H), 1.4-1.25 (br 14H). IR (film CH2Cl2): ν 2927, 2854, 1601,1597, 1489, 1448, 1388, 1251, 1185, 1136, 933, 786. Eu(Phen)(L1-Br)(L1)2. L1-Br (50 mg, 1equiv, 0.11 mmol), L1 (106 mg, 2 equiv, 0.22 mmol), and phenanthroline (18 mg, 0.1 mmol) were dissolved in hot EtOH (7 mL). To this solution cooled to room temperature, a 1 M aqueous NaOH solution (0.3 mL, 0.3 mmol, 3 equiv) was added. After the mixture was stirred for 20 min, EuCl3 · 6H2O (36.6 mg, 0.1 mmol, 1 equiv) in water (7 mL) was added to the solution. The addition was

J. Phys. Chem. C, Vol. 113, No. 6, 2009 2291 accompanied by a large precipitation (pale yellow precipitate). The mixture was then heated for 3 h at 60 °C. After the mixture was allowed to cool to room temperature, the yellow precipitate was collected by filtration and dried under vacuum to afford 91% of the complex. MS (MALDI): m/z 1756 [M + H]+. 1H NMR (CD2Cl2, 400 MHz, δ ppm, J in Hz): 10.57 (s, 2H), 10.22 (d, 2H, JHH ) 7.6), 9.53 (s, 2H,), 8.55 (d, 2H, JHH ) 7.6), 7.14 (br, 2H), 6.72 (br, 6H), 6.12 (br, 3H), 5.78 (br, 3H), 3.96 (t, 4H, JHH ) 6.54), 3.46 (t, 2H, JHH ) 6.9), 3.32 (m, 6H,), 2.6 (br, 3H), 2.31 (s, 6H), 1.92-1.75 (br, 14H), 1.48-1.40 (br, 40H). IR (film CH2Cl2): ν 2927, 2854, 1624,1597, 1541, 1505, 1448, 1388, 1251, 1185, 1136, 933, 786. Anal. Calcd for C83H97BrF9N2O8S3Eu: C, 56.48; H, 5.54; N, 1.59; Br, 4.53. Found: C, 55.95; H, 5.65; N, 1.50; Br, 4.43. 2.2. Apparatus and Procedures. 1H NMR (400 MHz) spectra were measured with a Bruker Avance instrument. Gas chromatograms were obtained using an Agilent Network GC System 6890N instrument. FTIR spectra were recorded on a Bruker IFS 48 instrument using KBr pellets. Mass spectrometric measurements were performed on a Hewlett-Packard 5985 B GC-MS instrument equipped with a DB-5 MS (30 m) capillary column. Positive MALDI-TOF mass spectra were acquired by a Voyager DE-STR (PerSeptive Biosystem) using a simultaneous delay extraction procedure and detection in linear mode. The instrument was equipped with a nitrogen laser (emission at 337 nm for 3 ns) and a flash AD converter (time base 2 ns). trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenylidene]-malonitrile was used as a matrix. The elemental analyses were performed with a Vario EL elemental analyzer. Cyclic voltammetry experiments were performed at room temperature under nitrogen in a three-electrode cell using a 5 × 10-4 M acetonitrile solution with 0.1 M L-1 Bu4NClO4. The counter electrode was platinum; the reference electrode was silver/0.1 M silver perchlorate in acetonitrile (0.34 V vs SCE). The voltammetric apparatus (AMEL, Italy) included a 551 potentiostat modulated by a 568 programmable function generator and coupled to a 731 digital integrator. The working electrode for cyclic voltammetry was a platinum minidisk electrode (0.003 cm2). PL and EL were obtained with a monochromator equipped with a CCD detector, by exciting with a monochromated xenon lamp or by applying a constant bias, respectively. PL quantum yields (QY) were obtained by using quinine sulfate as a reference in solution and with a homemade integrating sphere for the films. External EL quantum efficiency (EQE) was measured in the forward direction by a calibrated photodiode OSD Centronic, and electrical characterization was performed by Keithley 2602 apparatus. Atomic force microscopy (AFM) investigations were performed using a NT-MDT NTEGRA apparatus in tapping mode. Fluorescence images were collected with a Nikon Eclipse TE2000-U inverted confocal microscope with a long working distance using a Plan Apo VC objective (magnification 60× or 100× N.A. 1.4). Excitation was obtained with a 100 W Hg lamp with 330-380 nm band-pass excitation filter. 2.3. Device Fabrication. Films were prepared by spincoating blend toluene solutions with a concentration of approximately 15 mg/mL. The total concentration of the solutions and deposition parameters were unchanged to give comparable thickness films. OLEDs were prepared by depositing the films over ITO (indium tin oxide) coated glass substrates with spincoated poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS, BaytronP Bayer). In optimized architecture, the PVK layer was then deposited on it from chloroform solution.

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Figure 1. PL (exc. 380 nm) and EL (9 V) spectra of the device ITO/ PEDOT:PSS/Eu(TTA)3Phen:PFO:PMMA (0.1:1:5 by weight)/Ca/Al. In the inset, PL-QY of neat components, binary, and ternary blends are shown.

Calcium/aluminum (Ca/Al) or barium/aluminum (Ba/Al) cathodes were then grown on top of them at 10-6 mbar vacuum. 3. Results and Discussion 3.1. Blends with a Standard Eu3+ Complex. In order to obtain multicolor emission with Eu3+ complexes, first of all we analyze the interaction between the blue-emitting polymer (PFO) and the red-emitting Eu3+ complexes by using the Eu(TTA)3Phen complex, hereafter called standard Eu3+ complex. This complex was already studied in blends with PVK8a and PFObased polymer,8b but neither multicolor emission nor high efficiency have been obtained. In order to understand the origin of the quenching mechanism in the PFO blends of the standard complex, we study the ET processes between PFO and the complex by introducing a further inert host matrix, i.e., poly(methyl methacrylate) (PMMA), in the PFO/Eu(TTA)3Phen blend. In Figure 1 we show the results obtained for blends of PFO with 10 wt % of Eu(TTA)3Phen by adding PMMA. The PL and EL spectra perfectly overlap for the blend Eu(TTA)3Phen/ PFO/PMMA (0.1:1:5 by weight), suggesting that the same excited states are generated during photoexcitation and electrical excitation (charge trapping is negligible). Therefore the addition of 80 wt % of PMMA interrupts the “communication” at the short scale lengths between the conducting polymer and the complex, maintaining the overall PL-QY similar to that of the two neat components (see inset of Figure 1). A reduction of the amount of PMMA in the ternary blend (0.1:1:0.5 by weight) produces a severe decrease of the PL-QY of the total emission (see inset of Figure 1), with an increase of Eu3+ contribution in both PL and EL spectra with respect to PFO. Direct contact between the PFO and the Eu3+ complex therefore introduces a detrimental quenching of both the PL and EL of the blend. ET9 from PFO (excitation donor) to Eu3+ complex (acceptor) comes into question as depletion mechanism of the polymer excited state in the presence of the dopant. However, the very small overlap between PFO’s emission and Eu3+ complex absorption suggests an inefficient FRET from PFO to the complex,7,8 with a theoretical Fo¨rster radius9 of 1.5 nm. On the other side, if the ligand is directly excited (λexc ) 350 nm), a stronger contribution of Eu3+ emission to the overall PL is achieved, although PL-QY is reduced with respect to the neat film of diketonate Eu3+ complex. This fact suggests that another quenching process takes place. Diketonate ligands have

Giovanella et al. a very low singlet emission PL-QY due to the nature of the π*-n transition,10 but triplets are formed very efficiently via ISC due to the presence of Eu3+ heavy ion. Since triplet state energy of the ligand matches typically with 5D1-level energy of the Eu3+ ion, Dexter-type (exchange mechanism) ET leading to the red-sharp emission of Eu3+ ion takes place. However, in the PFO host matrix, this Dexter-type ET process T1Ligand f 5D1 is competitive with another ET process, supposed to be mainly Dexter-type, from the ligand to the host triplets T1Ligand f T1PFO (backflow of excitation or backtransfer)11 that is active since the triplet state of PFO2 (2.15 eV) is lower than the ligand triplet. This process strongly reduces the emission efficiency of the system. In fact triplet excitons transferred to PFO are lost through nonradiative decays as T1PFO f S0PFO is spin forbidden. Since Dextertype ET depends strongly on the overlap between PFO and the orbitals of the ligand, by adding PMMA in the blend a reduction of this unwanted backflow process is attained, as the average distance between the conjugated ligand of Eu3+ complex and PFO backbone is increased. This analysis therefore suggests that the introduction of a “shielding” of the conjugated ligand through L1 can confine triplet excitons on the complex and favor the ET toward the red-emitting Eu3+ ion with respect to the back-transfer toward the host. In the following, we intend to substitute the role of PMMA as insulating system by a phenyl-capped undecyl chain on β-diketone ligand that increases the mutual distance between complex and polymer chain up to more than 1 nm, while does not affect the electronic level of the β-diketone ligand. 3.2. Blends with the New Eu3+ Complex. Synthesis. New 2-thenoyltrifluoroacetone derivatives used as ligands in the Eu3+ complex depicted in Scheme 1 were conveniently prepared by a three-step process. First the alkylation at the 5 position of 2-acetyl-5-bromothiophene via B-alkyl Suzuki-Miyaura cross-coupling reaction in that a reaction occurs between an alkyl borane and an aryl, i.e., involving an sp3 carbon. The reaction occurs in the presence of a base and a Pd0 catalyst.12 The alkyl borane component of the B-alkyl Suzuki reaction can be derived from the hydroboration with (1s,5s)-9-undecyl-9-borabicyclo[3.3.1]nonane of the corresponding olefin, 11-bromoundec-1-ene,13 to obtain 1-(5-(11-bromoundecyl)thiophen-2-yl)ethanone (1). Starting from 1 we obtain 1-(5-(11-(m-tolyloxy)undecyl)thiophen-2-yl)ethanone (2) trough a standard Williamson etherification.14 Claisen condensation on both (1) and (2) gives the desired diketone ligands 1-(5-(11-bromoundecyl)thiophen-2-yl)-4,4,4trifluorobutane-1,3-dione (L1-Br) and 4,4,4-trifluoro-1-(5-(11(m-tolyloxy)undecyl)thiophen-2-yl)butane-1,3-dione (L1), respectively.15 The correct structures were confirmed by NMR spectroscopy, IR spectroscopy, and mass spectrometry. We choose to perform the etherification before the Claisen condensation, because the R-protons of the diketone are more acidic than those of the ketone; they may react more easily with the base used during the Williamson reaction, if the diketone is formed before the etherification. Eu(Phen)(L1-Br)(L1)2 is synthesized from the hydrated trichloride complex of europium and a mix of the diketonate L1 and L1-Br (in molar ratio 2:1) in EtOH/water with phenanthroline (phen) as ancillary ligand as described by Melby et al.16 The reaction is quantitative, and the complex is obtained as fine pale yellow powder. The complex is soluble in most of the organic solvents and is characterized by NMR, IR film, UV-vis absorption, and mass spectrometry.

Energy Transfer in Eu2+/Polyfluorene Blends

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SCHEME 1: (a) PFO, (b) Eu(Phen)(L1-Br)(L1)2

Figure 2. Cyclic voltammogram of Eu(Phen)(L1-Br)(L1)2.

The designed ligand offers the possibility to obtain site isolation between the polymer and the complex and eventually to reduce back-transfer. Moreover, the addition of alkyl chains modifies the polarity of the complex making it compatible with nonpolar PFO. The three long chains having C-C linkages in trans and/or gauche conformations can produce a sort of shell around the lanthanide ion because the presence of 2-methylbenzoxy groups prevents interdigitation with fluorene alkyl chains. In addition, the overall size of the chain, larger than 1 nm, should reduce the probability of the Dextertype T1Ligand f T1PFO ET process requiring an efficient molecular overlap between the involved molecular orbitals.17 Electrochemistry. The cyclic voltammogram of Eu(Phen)(L1Br)(L1)2 film, reported in Figure 2, shows one irreversible oneelectron reduction process at -1.69 V and a reversible oxidation at 1.16 V as expected for an alkyl-substituted thiophene ring. Optical Properties. The introduction of the alkyl chain does not significantly modify the optical properties of the ligand L1 with respect to TTA. The absorption and PL spectra of Eu(Phen)(L1-Br)(L1)2 in solution and in solid state are reported in Figure 3. Solution PL of Eu(Phen)(L1-Br)(L1)2 shows the five-peak emission of Eu ion being the 5D0 f 7F2 at 614 nm the most intense one. Solid-state PL-QY of the complex is 41%. To measure the triplet energy level of the complex, Gd(L1)3Phen complex was synthesized, following the procedure reported elsewhere.18 Because the lowest excited state 6P7/2 of Gd3+ is too high to get energy from ligands, the data obtained from the phosphorescence spectra actually reveal the triplet energy level

Figure 3. Normalized absorption (solid line) and PL spectra (dotted line) of solution (bottom) and film (top) of Eu(Phen)(L1-Br)(L1)2; the phosphorescence spectrum (dashed line) of a frozen solution of Gd(L1)3Phen is also reported.

Figure 4. Abs and PL spectra of Eu(Phen)(L1-Br)(L1)2:PFO films with different doping ratios. In the inset, overall PL-QY (by exciting at 380 nm) and EQE%.

of the ligand in lanthanide complexes. From its phosphorescence spectrum in frozen solution (see dashed line spectrum in Figure 3), the triplet energy level of the complex is determined at 2.43 eV (511 nm) while its singlet state energy level is estimated (by its absorbance onset), at 2.98 eV (416 nm), in agreement with the value derived from cyclic voltammetry experiments. Figure 4 shows the absorption and PL spectra, under excitation of 380 nm monochromatic light (PFO excitation), of

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Figure 5. AFM tapping mode height image of Eu(Phen)(L1-Br)(L1)2: PFO 20 wt % film. The white bar is 1 µm.

Giovanella et al. nated by the emission of PFO, that is unchanged in position and in vibronic progression with peaks at 423, 447, and 478 nm; a rather weak sharp red emission from Eu3+ ion at 614 nm is observed only for doping levels higher than 1 wt %. Morphological InWestigations. AFM tapping mode investigation together with fluorescence microscopy were performed. Films with the contemporary emission from both components (10-20 wt %) are homogeneous and flat, showing root-meansquare roughness of 0.33 and 0.35 nm, for 10 and 20 wt %, respectively, comparable to neat PFO film. The fluorescence microscopy image of 20 wt % film shows pink homogeneous emission with no evidence of Eu3+ complex aggregates2 in agreement with AFM result (Figure 5) and then confirming the improved miscibility of the novel Eu3+ complex up to 20 wt % blend. Electroluminescent Properties. Single-layer OLED devices with the architecture ITO/PEDOT:PSS/Eu(Phen)(L1-Br)(L1)2: PFO/Ca/Al for different weight ratios are fabricated to measure the optoelectronic properties. The active layers of 80-100 nm are deposited by a spin-coating technique from toluene solutions. The EL spectra of Eu(Phen)(L1-Br)(L1)2-doped PFO devices reported in Figure 6 show that simultaneous emission from the polymer and the complex is obtained for concentrations of 0.1-1 wt % of complex. More pronounced Eu3+ ion emission is observed if compared with PL spectra at the same doping concentration. The difference between PL and EL spectra is attributed to charge-trapping effect on Eu(Phen)(L1-Br)(L1)219 and is further supported by electrochemical properties of Eu3+ complex. In fact, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the complex, determined by cyclic voltammetry, are -5.9 and -3.05 eV, respectively. These values are quite similar to those we obtained for Eu(TTA)3Phen. The energy band diagram of Eu(Phen)(L1-Br)(L1)2-doped PFO devices is shown in the inset of Figure 6 and indicates that the complex can more efficiently trap carriers with respect to PFO.

Figure 6. EL spectra of of Eu(Phen)(L1-Br)(L1)2:PFO films with different doping ratios. Spectra are shifted for clarity. In the inset, energy band diagram of the compounds.

the Eu3+ complex doped PFO films at doping concentration between 0.1 and 20 wt %. Absorption spectra are almost unchanged with respect to neat PFO due to both weak oscillator strength of L1 and 1,10phenantroline ligands and their amount. PL spectra are domi-

When PFO is doped with different amounts of Eu(Phen)(L1Br)(L1)2, a tunable electroluminescence from blue to pink to pure red is achieved, as shown by the CIE color coordinates, calculated according to 1931 standards, given in Table 1. The EQE of the nonoptimized devices is reported in Table 1. By optimization of the device architecture,20 EQE as high as 1% is reached with 0.1 wt % blend in the structure ITO/PEDOT:PSS/ PVK/Eu(Phen)(L1-Br)(L1)2:PFO/Ba/Al without any change in the PL and EL spectral shape. This is, to our knowledge, the highest EQE reported for a color-tunable spin-coatable OLED based on Eu3+ complex doped PF matrix.

TABLE 1: CIE Color Coordinates, EQE, and CIE Diagram of the ITO/PEDOT:PSS/Eu(Phen)(L1-Br)(L1)2:PFO/Ca/Al Devices

Energy Transfer in Eu2+/Polyfluorene Blends 4. Conclusions In summary, we obtained, for the first time, tunable multicolor EL emission from homogeneous dispersion of a Eu3+ complex in the electroluminescent polymer PFO. The result has been reached by designing a complex whose ligand, with long alkyl chains, increases site isolation of Eu3+ ion and reduces the strong quenching effect due to back-transfer typical of Eu3+ complex: PFO blends. Spin-coated devices showed blue and red contemporary emission, with a maximum EQE as high as 1%. We are improving the effectiveness of the shielding by introducing lateral groups (triphenylamine)21 on PFO chains with steric hindrance, hole transport properties, and antioxidant action. The analysis of the overall ET mechanisms involving singlet and triplet excitons by focusing on the effect of site isolation and polarity of the new ligand adds new insights on the design of novel metal complexes for multicolored emission. Acknowledgment. This work was partially supported by Italian project MIUR-FIRB RBNE03S7XZ Sinergy, FIRBRBIP0642YL LUCI, and PRIN 2007PBWN44. Thanks are due to Dr. G. Zotti and Dr. B. Vercelli of IENI-CNR for cyclic voltammetry analysis. References and Notes (1) (a) Bunzli, J.-C. G.; Piguet, C. Chem. Soc. ReV. 2005, 34, 1048. (b) Kido, J.; Okamoto, Y. Chem. ReV. 2002, 102, 2357. (2) Chen, F. C.; Chang, S. C.; He, G.; Pyo, S.; Yang, Y.; Kurotaki, M.; Kido, J. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2681. (3) Sudhakar, M.; Djurovich, P. I.; Hogen-Esch, T. E.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7796. (4) Canzler, T. W.; Kido, J. Org. Electron. 2006, 7, 29.

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