Novel Deep Blue OLED Emitters with 1,3,5-Tri(anthracen-10-yl

Mar 2, 2011 - The compounds (T1−T3) possess high glass transition temperatures (Tg's) at 107, 109, and ... Chemistry of Materials 2015 27 (11), 3892...
0 downloads 0 Views 2MB Size
Download PDF
ARTICLE pubs.acs.org/JPCC

Novel Deep Blue OLED Emitters with 1,3,5-Tri(anthracen-10-yl)benzene-Centered Starburst Oligofluorenes Hong Huang,† Qiang Fu,‡ Shaoqing Zhuang,† Yakun Liu,† Lei Wang,†,* Jiangshan Chen,‡,* Dongge Ma,‡ and Chuluo Yang†,§,* †

Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, People's Republic of China § Department of Chemistry, Wuhan University, Wuhan 430072, People's Republic of China

bS Supporting Information ABSTRACT: A series of starburst materials (T1-T3) bearing a 1,3,5-tri(anthracen-10-yl)benze-ne core (T0) and three oligofluorenes arms have been synthesized and characterized. Singlecrystal diffraction analysis has shown that the core of these starburst materials possess a propeller twist topology, which made the starburst materials exhibit good film-forming capabilities and display deep blue emission both in solution and in the thin solid film. The compounds (T1-T3) possess high glass transition temperatures (Tg’s) at 107, 109, and 110 °C, and high decomposition temperatures (Td’s) at 438, 440, and 434 °C, respectively. In addition, the double-layered devices fabricated with the three materials as the emitter show a stable deep-blue emission and the device performance increases with arm length at some extent. The double-layered device based on T2 has a maximum brightness of over 3400 cd/m2 and a maximum current efficiency of 1.80 cd/A with CIE coordinates of (0.149, 0.098), which is among the best of the deep-blue starburst material devices reported so far in the current available literature.

’ INTRODUCTION π-Conjugated polymers have been extensively investigated and explored for use in organic electronics1 such as organic lightemitting diodes (OLED)2-4 and organic thin-film transistors (OTFT),5-7 which can be deposited via spin-coating or inkjet printing techniques. However, the purity problem of a polymer is its inherent bottleneck, which encouraged us to turn our interests into conjugated dendrimer or starburst oligomers. Starburst oligomers combine the advantages of both small molecules and polymers, such as, precise chemical structures and high purity of small molecules and good solution processability of the polymer. Additionally, their optoelectronic and thermal properties can be independently optimized via choice of different cores or branch arms. Among the reported starburst molecules, the monodisperse πconjugated oligofluorenes have attracted much attention as blue material for semiconducting devices, due to their well-defined and uniform molecular structures, good chemical purity, good thermal properties, and ease of manipulation of the conjugated length compared to the polyfluorenes.8-12 Among the polyfluorenes, a long wavelength band in the emission spectrum is always generated soon after thermal annealing or passing of current, turning the desired pure blue emission into a blue-green r 2011 American Chemical Society

or even yellow emission with a drastic loss in device efficiency. Recent research attributed it to physical (excimers or aggregate formation)13 or chemical (keto defects)14 degradation processes. Many chemical approaches have been developed to address this issue. For example, Rothberg15 et al. have synthesized monodispersed starburst oligofluorenes with spirofluorene as the core. Cao16 et al. synthesized six-arm triazatruxenes; its application shows a maximum luminance efficiency of 1.46 cd/A with CIE coordinates of (0.15, 0.09) and with luminance of 1343 cd/m2. Subsequently, Cao17 et al. reported starburst materials with a planar pyrene as the core, the Tg were enhanced to 90 °C, and the CIE of the applications is (0.19, 0.32) with a maximum current efficiency of 1.75 cd/A. Liu18 et al. reported a series of Si-based tetrahedral luminescent materials, the highest Tg of the oligofluorenes is 78 °C. Although the material color stability and Tg were improved with varying degrees of success, the device performance of pure blue still needs optimizing and the research of high-performance blue light-emitting materials continues. In the design of starburst conjugated materials, the core usually Received: November 8, 2010 Revised: December 29, 2010 Published: March 02, 2011 4872

dx.doi.org/10.1021/jp110652y | J. Phys. Chem. C 2011, 115, 4872–4878

The Journal of Physical Chemistry C plays an important role in the molecular shape and the emission color. According to the molecule structure design principle, the nonplanar propeller-topology core can efficiently prevent the close-packing of the molecules in solid state and thus enable formation of smooth and pinhole-free thin film, but the high luminous propeller skeleton core is rare. In this context, we have designed a series of starburst oligofluorenes based on 1,3,5-tri(anthracen-10-yl)benzene core, in which the core possesses a propeller topology and keeps the molecule in a good amorphous state and depresses aggregation and recrystallization. As we expected, these blue light-emitting materials with enhanced functional properties in terms of morphology, thermal stability, and luminescence properties were achieved. Importantly, solution-processed, double-layer OLED devices fabricated with these new emitters exhibited pure-blue EL with low driving voltages, high brightness, high efficiency, and excellent color stability, displaying rather fascinating EL performance compared to the linear oligofluorenes and polyfluorene counterparts. Additionally, we also investigated the influence of the oligofluorene's arm length and its subsequent affect on the EL performance of the material. The results show that the 1,3,5tri(anthracen-10-yl)benzene-centered starburst oligofluorenes could be a promising class of solution-processable deep-blue emitters in OLEDs.

’ EXPERIMENTAL SECTION General synthesis and Characterization. All reagents and solvents were used as purchased from Aldrich and were used without further purification. 1H NMR spectra were recorded at Bruker-AF301 AT 400 MHz. High resolution mass spectrometric measurements were carried out using a Bruker autoflex MALDI-TOF mass spectrometer. Fluorescence spectra were obtained on a Perkin-Elmer LS55 Luminescence spectrometer. The differential scanning calorimetry (DSC) analysis was performed under a nitrogen atmosphere on a TA Instruments DSC 2920. To measure the PL quantum yields (Φf), degassed solutions of the compounds in CH2Cl2 were prepared. The concentration was adjusted so that the absorbance of the solution would be lower than 0.1. The excitation was performed at 360 nm, and 2-methyl-9, 10-di(naphthalen-1-yl)anthracene (MADN) in toluene (Φ = 0.54 in toluene) was used as a standard.19 Cyclic voltammetric measurements were carried out in a conventional three-electrode cell using a Pt button working electrode of 2 mm in diameter, a platinum wire counter electrode, and a Ag/Agþ (0.1 M) reference electrode on a computer-controlled EG&G Potentiostat/Galvanostat model 283 at room temperature. Reduction CV of all compounds were performed in dichloromethane containing tetrabutylammonium hexafluorophosphate (TBAPF6) (0.1 M) as the supporting electrolyte. Oligofluorene boronic acids of different chain lengths (4a-4c) were prepared by the procedure reported in the literature.16,20 9-Anthraceneboronic acid was synthesized according to the literature.21 The Suzuki coupling reaction was conducted under a nitrogen atmosphere and avoiding light exposure. Device Fabrication. Prior to the deposition of organic materials, the indium-tin-oxide (ITO)/glass was cleaned with a routine cleaning procedure and pretreated with UV-ozone. Devices were fabricated under a base vacuum of about 10-5 Pa in a thin-film evaporation coater. We fabricated OLEDs using oligofluorenes T1-T3 as the emissive layer with a structure of

ARTICLE

ITO/PEDOT-PSS/T1-T3/TPBI/LiF/Al, where the polyethylene dioxythiophene-polystyrene sulfonate (PEDOT-PSS) and 2,2 0 ,2 00 -(1,3,5-benzenetriyl)tris[1-phenyl-1H-benimidazole] (TPBI) were used as hole injection and hole-blocking/electrontransporting layer, respectively. The active layer was spin-coated from toluene solution, and the TPBI layer was deposited by means of conventional vacuum deposition onto the ITO-coated glass substrates. The J-V-L characteristics of the EL devices were measured using a Keithley 2400 Source meter and a Keithley 2000 Source multimeter equipped with a calibrated silicon photodiode. The EL spectra were measured using a JY SPEX CCD3000 spectrometer. All of the measurements were carried out at room temperature under ambient conditions. X-ray Crystallography. A single crystal of T0 3 CH2Cl2 of suitable dimensions (0.30  0.20  0.20 mm) was mounted onto a thin glass fiber. All of the intensity data were collected on a Bruker SMART CCD diffractometer (Mo-K radiation = 0.71073 Å) in Φ and scan modes. The collected frames were processed with SAINTþ22 software and an absorption correction (SADABS)23 was applied to the collected reflections. The structure was solved by direct methods (SHELXTL)24 in conjunction with standard difference Fourier techniques and subsequently refined by full-matrix least-squares analyses on F2. All other non-hydrogen atoms were refined with anisotropic thermal parameters, and all hydrogen atoms were placed in calculated positions and refined isotropically using a riding model. Crystallographic data for T0 3 CH2Cl2: C49H32Cl2, Mr = 691.65, Monoclinic, space group P2(1)/c, Z = 4, a = 11.849(3) Å, b = 17.350(5) Å, c = 21.055(4) Å, β = 121.146(11)°, U = 3704.5(16) Å3, F(000) = 1440. A total of 23 424 reflections were measured in the range 2.01° e θ e 22.50° (hkl range indices: -12 e h e 12, -18 e k e 18, -22 e l e 22), 4842 unique reflections. The structure was refined on F2 to R1 = 0.1121, wR2 = 0.2491 (reflections with I > 2σ (I), GOOF = 1.159 on F2 for 460 refined parameters). Synthesis of 1,3,5-Tri(anthracen-10-yl) Benzene (T0). To a mixture of compound 1 (2.7 g, 12.0 mmol) and 1,3,5-tribromobenzene (0.63 g, 2.0 mmol) in toluene (60 mL), K2CO3 (60 mL, 120 mmol), and ethanol (30 mL) were added. Then the mixture was degassed for 5 min and tetrakis(triphenylphosphine)palladium (0.35 g, 0.30 mmol) was added in one portion under an atmosphere of N2. The solution was then heated under reflux for 24 h. After the reaction mixture was cooled, the product was filtered and washed with the methanol. Yield: 90% (1.1 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.50 (s, 3H), 8.19-8.17 (d, J = 8.4 Hz, 6H), 8.06-8.04 (d, J = 8.4 Hz, 6H), 7.76 (s, 3H), 7.56-7.47 (m, 12H). MS (FAB): 606.5 (m/z). Synthesis of 1,3,5-Tri(10-bromoanthracen-9-yl) Benzene (3). N-Bromosuccinimide (NBS) (534 mg, 3.0 mmol) was added into a solution of 2 (304 mg, 0.5 mmol) in dry DMF (10.0 mL). The reaction mixture was stirred at room temperature under an atmosphere of nitrogen for 4 h. The product was filtered to afford a light yellow solid 3. Yield: 75% (0.32 g). 1H NMR: (400 MHz, CDCl3): δ (ppm) 8.64-8.62 (d, J = 8.0 Hz, 6H), 8.15-8.13 (d, J = 8.0 Hz, 6H), 7.73 (s, 3H), 7.66-7.57 (m, 12H). MS (FAB):843.9 (m/z). Synthesis of T1. To a solution of compound 3 (442 mg, 0.5 mmol) and compound 4a (1.13 g, 3.0 mmol) in 30 mL of toluene, 2 M K2CO3 (15.0 mL, 30.0 mmol) and ethanol (15.0 mL) were added. The mixture was stirred for 10 min and tetrakis(triphenylphosphine) palladium (0.1 g, 0.025 mmol) was added in one portion under an atmosphere of N2. The solution 4873

dx.doi.org/10.1021/jp110652y |J. Phys. Chem. C 2011, 115, 4872–4878

The Journal of Physical Chemistry C

ARTICLE

Scheme 1. Molecular Structures and Syntheses of T0-T3a

a Reagents and conditions: (a) n-BuLi, tri(isopropyl)borate, 2 M HCl. -78 °C; (b) Toluene, K2CO3, ethanol, 1,3,5-tribromobenzene, Pd(PPh3)4, reflux; (c) NBS, DMF, 30 °C; and (d) Toluene, K2CO3, ethanol, Pd(PPh3)4, reflux.

was then heated at reflux for 24 h under N2. After the reaction mixture cooled to room temperature and was removed from the solvent, the residue was purified by the column chromatography. Yield: 54% (450 mg). 1H NMR: (400 MHz, CDCl3): δ (ppm) 8.34-8.30 (m, 6H), 8.17-7.35 (m, 42H), 2.01-1.99 (m, 12H), 1.14-1.08 (m, 36H), 0.79-0.74 (m, 30H); 13C NMR (100 MHz, CDCl3) δ 150.97, 150.87, 140.95, 140.58, 138.16, 137.54, 136.24, 134.12, 130.17, 130.02, 129.86, 127.29, 127.20, 126.87, 126.81, 126.09, 125.51, 125.03, 122.89, 119.78, 119.63, 55.24, 40.42, 31.53, 29.63, 23.86, 22.50, 14.03; MS (MALDI-TOF): calcd for C123H126: 1603.9888, found, 1603.9846 (m/z); Anal. Calcd. (%) for C123H126: C 92.08, H 7.92; Found: C 92.20, H 7.80. Synthesis of T2. The procedure is similar to compound T1: Yield: 55%. 1H NMR: (400 MHz, CDCl3):δ (ppm) 8.37-8.33 (m, 6H), 7.98-7.32 (m, 60H), 2.06-2.04 (m, 24H), 1.26-1.10 (m, 72H), 0.88-0.76 (m, 60H); 13C NMR (100 MHz, CDCl3) δ 151.50, 151.21, 151.00, 140.79, 140.72, 140.43, 140.36, 140.31, 140.15, 139.31, 138.16, 137.57, 136.28, 134.12, 133.95, 130.20, 130.05, 127.33, 127.00, 126.79, 126.20, 126.06, 125.55, 125.08, 122.93, 121.43, 121.39, 120.04, 119.90, 119.72, 55.42, 55.19, 40.40, 31.52, 31.48, 29.70, 29.62, 23.93, 23.78, 22.57, 22.50, 14.05, 14.02; MS (MALDI-TOF): calcd for C198H222: 2601.7434, found 2601.7513 (m/z); Anal. Calcd. (%) for C198H222: C 91.40, H 8.60; Found: C 91.24, H 8.76. Synthesis of T3. The procedure is similar to compound T1: Yield: 58%. 1H NMR: (400 MHz, CDCl3):δ (ppm) 8.52-8.50 (m, 6H), 8.27-7.32 (m, 78H), 2.10-2.05 (m, 36H), 1.25-1.24 (m, 106H), 0.88-0.71(m, 92H); 13C NMR (100 MHz, CDCl3) δ 151.82, 151.77, 151.46, 151.17, 151.00, 140.78, 140.67, 140.54, 140.48, 140.43, 140.32, 140.17, 140.05, 139.97, 139.24, 138.90, 138.09, 137.57, 136.21, 136.19, 133.82, 133.67, 131.44, 131.41, 130.26, 130.14, 130.03, 129.96, 128.54, 127.27, 126.97, 126.97, 126.78, 126.59, 126.17, 126.02, 125.79, 125.75, 125.49, 125.12, 125.03, 122.92, 121.42, 119.97, 119.88, 119.71, 55.40, 55.34, 55.16,

40.37, 31.51, 31.47, 31.45, 29.69, 29.66, 29.61, 23.92, 23.83, 23.77, 22.56, 22.49, 14.02; MS (MALDI-TOF): calcd for C273H318: 3598.4951, found 3598.5210 (m/z); Anal. Calcd. (%) for C273H318: C 91.10, H 8.90; Found: C 91.14, H 8.86.

’ RESULTS AND DISCUSSION The structures and synthetic routes of the three well-defined oligofluorenes are shown in Scheme 1. Oligofluorenes boronic acids with different chain length (4a-c) were prepared by the procedure reported in the literature.16,18 9-Anthracene boronic acid was synthesized according to the literature by lithiumbromide exchange at -78 °C.21 Oligofluorenes (T1-T3) were synthesized by the Pd-catalyzed Suzuki cross-coupling of the 1,3,5-tri (10-bromoanthracen-9-yl) benzene and the corresponding oligofluorenes boronic acids with high yield. All of the newly synthesized oligofluorenes were fully characterized with 1H NMR, 13C NMR, MALDI-TOF, Elemental analysis, and found to be in good agreement with their structure. Normally, with respect to the 13C NMR spectrum, the chemical-shift of the C9 in the fluorene is in the range of 50-60 ppm. With comparison of the three starburst compounds, there are one (55.24 ppm), two (55.42, 55.19 ppm), and three apparent C9 peaks (55.40, 55.34, 55.16 ppm), respectively. Single crystal X-ray diffraction analysis revealed that the T0 3 CH2Cl2 crystallizes in the monoclinic space group of P2(1)/c, and the asymmetric unit is composed of one T0 molecule and one CH2Cl2 solvate molecule. As shown in Figure 1a, for the host tripodal framework of T0 molecule, the three anthracene rings link the central phenyl core at the m-position with the C-C single bonds, showing a propeller appearance in a pseudo-C3 symmetry. It is worth noting that the dihedral angle between each of three rigid anthracene rings and the central core is 75.5(2)°, 97.4(2)°, or 114.2(2)°, with a 4874

dx.doi.org/10.1021/jp110652y |J. Phys. Chem. C 2011, 115, 4872–4878

The Journal of Physical Chemistry C

ARTICLE

Figure 1. (a) Perspective drawing of compound T0 3 CH2Cl2, H atoms and solvate molecules are omitted for clarity; and (b) view of the stacking from intermolecular C-H hydrogen bond interactions.

Table 1. Optical, Photophysical and Thermal Properties of T0-T3 PL (nm)

Abs (nm)

HOMO

LUMO

ΦF

Eg(eV)

solution

T0

-5.50

-2.45

0.65

3.10

397, 417

T1

-5.53

-2.58

0.65

2.95

430

435

361, 380, 400

266, 308, 358, 385, 399

107/438

T2 T3

-5.55 -5.57

-2.61 -2.64

0.66 0.65

2.94 2.93

433 434

433 434

361, 380, 401 352

262, 341, 397 260, 360

109/440 110/434

film

solution

film

Tg/Td(°C)

350, 370, 390

Figure 2. AFM topographic images of oligofluorenes T1, T2, and T3 in thin solid films (40 nm thick, size: 5 μm  5 μm).

distinctive increase in a clockwise direction, which should be attributed to the steric effect of the anthracene rings in 3D room. Moreover, There are only weak intermolecular C-H hydrogen bond interactions (3,375 (8) Å and 139° or 3.596(10) Å and 143°), as shown in Figure 1b, although there are several aromatic planes present, the shortest centroid-to-centroid distances are 3.987(5) Å for the compound, which shows the stereoscopic conformation of the host T0 molecule, is helpful for the molecules with the tendency to form smooth and amorphous thin films in solid state. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (see Supporting Information Figure S1) were employed to investigate the thermal properties of T1-T3, the related data are listed in Table 1. TGA measurements showed that all compounds exhibit high decomposition temperatures (Td, 5% weight loss) at 438, 440, and 434 °C, respectively. In the DSC experiment, the three compounds also revealed high glass transition temperature (Tg) at 107, 109, and 110 °C, respectively, which indicates (T1-T3) are stable materials. The high Tg may be attributed to the incorporation of the more bulky arms and rigid 1,3,5-Tri (anthracen-9-yl) benzene core.

As we know, the film-forming properties of organic materials are crucial for OLED performances, therefore, the surface morphologies of thin films of oligofluorenes (T1-T3) were investigated. Figure 2 shows the AFM images of T1-T3 on ITO/PEDOT substrate. The thin film was prepared by spincoating and then annealed under N2 gas condition at 100 °C for 2 h. The annealed film had a fairly smooth surface morphology with a root-mean-square (rms) roughness of 0.423, 0.411, and 0.410 nm for T1, T2, and T3, respectively. The excellent filmforming properties were attributed to the propeller twist topology of the core and the flexible arms. Figure 3 shows the UV-vis and fluorescence spectra of the three oligofluorenes in toluene solution and in thin film. The photophysical properties of these oligofluorenes are listed in Table 1. In solution, all of the oligofluorenes exhibited characteristic vibronic patterns of the isolated anthracene units except compound T3. As shown in Figure 3a, T0 has three distinctive absorption peaks located at around 350, 370, and 390 nm in toluene solution, compounds T1 and T2 also have a similar spectral shape, but there is a small hypsochromic shift (∼10 nm) compared to T1-T2. The absorption band from 350 to 400 nm belongs to the π-π* transition of the central anthracene moiety. 4875

dx.doi.org/10.1021/jp110652y |J. Phys. Chem. C 2011, 115, 4872–4878

The Journal of Physical Chemistry C

ARTICLE

Figure 3. (a) Absorption (left) and photoluminescence (PL) spectra (right) of T0-T3 in toluene, (b) Absorption and PL spectrum of the compounds T1-T3 in thin film (40 nm).

Figure 4. (a) J-V-B curves of device1, device 2, and device 3; (b) dependence of the current efficiency on the drive current density for device 1, device 2, and device 3; and (c) EL spectra of the three devices at 20 mA/cm2.

For compound T3, there is only one main broad absorption peak at 352 nm, which is attributed to the π-π* transition of the oligofluorenes. Oligofluorenes play an important role in absorbance when the fluorene units are increased to three. In solution, the emission peak is red-shifted with the increasing arm length, which could be due to the intermolecular energy transfer between the core and oligofluorene. The difference between the emission peaks for T1 and T2 is 3 nm, and for T2 and T3 is 1 nm, which indicates that the effective conjugation quickly saturates. Interestingly, the absorption peak at 341 nm is stronger than the peak at 397 nm in thin film (40 nm) for T2, which indicates that fluorene may play a more important role than 1,3,5Tri (anthracen-9-yl) benzene core in solid state. More interestingly, the photoluminescence spectral peaks in thin films are nearly similar to the solution state spectra except for a small redshift for T1 in emission. This indicates that the propeller core can efficiently prevent the intermolecular interaction, which is usually

observed for small molecules and polymer materials. Compared to the PL of T1-T3 in thin film, increasing the length of the oligofluorene arm from one to three fluorene units resulted in a slight blue-shift (1-2 nm), it is possible that the steric-hindrance plays a more important role than conjugation. Anyway, the emission of these starburst oligomers were mainly determined by the core, the arms only play an accessory role and adjust the interaction between the molecules. The electrochemical behaviors of T1-T3 were examined by cyclic voltammetry using a standard three-electrode electrochemical cell in an electrolyte solution of 0.1 M TBAPF6 dissolved in dichloromethane. The working electrode was glassy carbon, the counter electrode was a platinum wire, and the reference electrode was Ag/Agþ (0.1 M). The onset oxidation peaks (Eox) for T1-T3 were 1.23, 1.25, and 1.27 eV, respectively. Thus, the HOMO levels were determined to be -5.53, -5.55, and -5.57 eV by the method—(Eox þ 4.31) reported by Li et al25 (Table 1). 4876

dx.doi.org/10.1021/jp110652y |J. Phys. Chem. C 2011, 115, 4872–4878

The Journal of Physical Chemistry C

ARTICLE

Table 2. Electroluminescence Characteristics of the Devicesa device

emitter

Lmaxb(cd/m2), voltages(V)

ηc.maxc (cd/A)

ηpmaxd (lm/W)

CIE(x, y)e

Von (V)

1

T1

2300, 9.6

1.13

0.74

(0.151, 0.121)

4.17

2

T2

3400, 8.5

1.80

1.35

(0.149, 0.098)

3.75

3

T3

3000, 9.7

1.39

0.93

(0.149, 0.101)

4.72

Devices configuration: ITO/PEDOT:PSS (40 nm)/ T1-T3 (40 nm) /TPBI (40 nm) /LiF (1 nm) /Al (100 nm). b Maximum luminance. c Maximum current efficiency. d Maximum powder efficiency. e Commission International de I’Eclairage coordinates. a

This indicates that the 1,3,5-tri(anthracen-10-yl)benzene core could enhance the HOMO level, and thereby enhance the holeaffinity and hole-injection ability of the conjugated fluorene derivatives (compared to the polyfluorenes, the HOMO level is -5.8 eV).26 The optical band gaps (Eg) were calculated to be 3.10, 2.95, 2.94, and 2.93 eV for T0-T3, respectively. The excellent solubility, deep blue emission, and good filmforming ability enable the novel starburst oligofluorenes to be used as the bulky deep blue emitters in electroluminescent devices. To investigate the EL properties of the three oligofluorenes, double-layer devices were fabricated with the configuration of ITO/PEDOT: PSS (40 nm)/T1-T3 (40 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm) (emitter: T1, device 1; T2, device 2; T3, device 3) by spin-coating from a toluene solution of the oligofluorenes (7 mg 3 ml-1). Their EL spectra and current efficiency-current density characteristics are presented in Figure 4. The data of device performance are summarized in Table 2. The current efficiency varies with the arm length, the device based on the T2 has a highest maximum current efficiency (1.80 cd/A) with a maximum brightness of 3400 cd/m2. The device shows deep blue emission similar to that of the solid state photoluminescence, and no additional green emissions were found. The electroluminescence remained quite stable under different voltages. For all devices, when the voltage increased from 6 to 8 V, the emission is identical and the CIE coordinate is also the same value of the deep blue emission (see Supporting Information Figure S2). For device 1, the maximum luminance is 2300 cd/m2, the maximum current efficiency is 1.13 cd/A when the voltage reached 9.6 V; For device 3, the maximum luminance is 3000 cd/m2 when the voltage reached 9.7 V, the maximum current efficiency is 1.39 cd/A. However, the EL emission of device 1 shows light red shift with CIE coordinates of (0.151, 0.121) compared with (0.149, 0.098) of device 2, and the CIE(0.149, 0.101) of device 3 has no obvious change. It indicated that the application’s efficiency and luminance would be reach a maximum value when the length of the oligofluorene arm increased to an appropriate length. Meanwhile, we found that the threshold voltage (1 cd/m2) for device 1, device 2, and device 3 are 4.17, 3.75, 4.72 V, respectively (see Table 2). Above all, the arm length plays an important role in obtaining good EL performances for its balance between conductivity and hindrance. The excellent EL stability and the interesting color purity render these materials as promising candidates for display applications. We believe that optimization of the device structure and full exploration of the potential of these starburst-type materials could further improve the device performance.

’ CONCLUSIONS Three new propeller topology starburst materials T1-T3 have been synthesized and characterized. All of the starburst materials show good film-forming ability, excellent thermal stability, and good solubility. Likewise, the oligofluorenes exhibit deep blue PL

emissions. A double-layered device fabricated from the spincoating of T2 shows a maximum brightness of 3400 cd/m2 and a maximum current efficiency of 1.80 cd/A with CIE coordinates of (0.149, 0.098). The preliminary findings demonstrate that these starburst materials are promising blue light emitting materials. Further study on device engineering and quantum chemistry analysis is being undertaken and the results will be reported in due course.

’ ASSOCIATED CONTENT

bS

Supporting Information. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Figure S1); EL emission spectra of voltage increases from 6 to 8 V (Figure S2); cyclic voltammetric curve (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT This work is supported by the central allocation grant from Donghu New & High Technology Development Zone of Wuhan City and Wuhan Science and Technology Bureau (NO: 01010621227). ’ REFERENCES (1) (a) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1530. (b) Lai, W. Y.; He, Q. Y.; Zhu, R.; Chen, Q. Q.; Huang, W. Adv. Funct. Mater. 2008, 18, 265. (c) Tang, Z. M.; Lei, T.; Wang, J. L.; Ma, Y. G.; Pei, J. J. Org. Chem. 2010, 75, 3644. (d) Sax,:: S.; Penkalla, N. R.; Neuhold, A.; Schuh, S.; Zojer, E.; List, E. J. W.; MUllen, K. Adv. Mater. 2010, 22, 2087. (e) Zhang, B. H.; Qin, C. J.; Ding, J. Q.; Chen, L.; Xie, Z. Y.; Cheng, Y. X.; Wang, L. X. Adv. Funct. Mater. 2010, 20, 2951. (2) (a) Kong, Q. G.; Zhu, D.; Quan, Y. W.; Chen, Q. M.; Ding, J. F.; Lu, J. P.; Tao, Y. Chem. Mater. 2007, 19, 3309. (b) Li, Z. A.; Ye, S. H.; Liu, Y. Q.; Yu, G.; Wu, W. B.; Qin, J. G.; Li, Z. J. Phys. Chem. B. 2010, 114 (28), 9101. (c) Nagai, A.; Kobayashi, S.; Nagata, Y.; Kokado, K.; Taka, H.; Kita, H.; Suzuri, Y.; Chujo, Y. J. Mater. Chem. 2010, 20, 5196. (3) (a) Laughlin, B. J.; Smith, R. C. Macromolecules. 2010, 43 (8), 3744. (b) Allard, N.; Aïch, R. B.; Gendron, D.; Boudreault, P. L. T.; Tessier, C.; Alem, S.; Tse, S. C.; Tao, Y.; Leclerc, M. Macromolecules. 2010, 43 (5), 2328. (c) Park, M. J.; Kwak, J. H.; Lee, J. H.; Jung, I. H.; Kong, H.; Lee, C. H.; Hwang, D. H.; Shim, H. K. Macromolecules 2010, 43 (3), 1379. (d) Chen, H. Y.; Chen, C. T.; Chen, C. T. Macromolecules. 2010, 43 (8), 3613. (4) (a) Kamtekar, K. T.; Vaughan, H. L.; Lyons, B. P.; Monkman, A. P.; Pandya, S. U.; Bryce, M. R. Macromolecules. 2010, 43 (10), 4481. (b) Lee, J. K.; Fong, H. H.; Zakhidov, A. A.; McCluskey, G. E.; Taylor, P. G.; Berrios, M. S.; Abruna, H. D.; Holmes, A. B.; Malliaras, G. G.; Ober, C. K. Macromolecules. 2010, 43 (3), 1195. 4877

dx.doi.org/10.1021/jp110652y |J. Phys. Chem. C 2011, 115, 4872–4878

The Journal of Physical Chemistry C

ARTICLE

(5) (a) Murphy, A. R.; Frechet, J. M. J. Chem. Rev. 2007, 107, 1066. (b) Osaka, I.; Abe, T.; Shinamura, S. J.; Miyazaki, E.; Takimiya, K. J. Am. Chem. Soc. 2010, 132 (14), 5000. (6) (a) Yasuda, T.; Fujita, K.; Tsutsui, T.; Geng, Y. H.; Culligan, S. W.; Chen, S. H. Chem. Mater. 2005, 17, 264. (b) Jung, B. J.; Lee, K.; Sun, J.; Andreou, A. G.; Katz, H. E. Adv. Funct. Mater. 2010, 20, 2930. (7) (a) Rivnay, J.; Toney, M. F.; Zheng, Y.; Kauvar, I. V.; Chen, Z. H.; Wagner, V.; Facchetti, A.; Salleo, A. Adv. Mater. 2010, Doi: 10.1002/ adma.201001202. (b) Yuan, Q.; Mannsfeld, S. C. B.; Tang, M. L.; Roberts, M.; Toney, M. F.; DeLongchamp, D. M.; Bao, Z. A. Chem. Mater. 2008, 20, 2763. (8) Liu, Q. D.; Lu, J. P.; Ding, J. F.; Day, M.; Tao, Y.; Barrios, P.; Chan, J. S. K.; Li, J. J.; Chi, Y. Adv. Funct. Mater. 2007, 17, 1028. (9) Kong, Q. G.; Zhu, D.; Quan, Y. W.; Chen, Q. M.; Ding, J. F.; Lu, J. P.; Tao, Y. Chem. Mater. 2007, 19, 3309. (10) Kreger, K.; B€ate, M.; Neuber, C.; Schmidt, H. W.; Strohriegl, P. Adv. Funct. Mater. 2007, 17, 3456. (11) Zhang, X. M.; Quan, Y. W.; Cui, Z.; Chen, Q. M.; Ding, J. F.; Lu, J. P. Eur. J. Org. Chem. 2010, 12, 2295. (12) Wu, P. L.; Feng, X. J.; Tam, H. L.; Wong, M. S.; Cheah, K. W. J. Am. Chem. Soc. 2009, 131, 886. (13) Montilla, F.; Mallavia, R. Adv. Funct. Mater. 2007, 17, 71. (14) Sims, M.; Bradley, D. D. C.; Ariusimakis, A.; Grell, M.; Lidzey, D. G. Adv. Funct. Mater. 2004, 14, 765. (15) Katsis, D.; Geng, Y. H.; Ou, J. J.; Culligan, S. W.; Trajkovska, A.; Chen, S. H.; Rothberg, L. J. Chem. Mater. 2002, 14, 1332. (16) Lai, W. Y.; Zhu, R.; Fan, Q. L.; Hou, L. T.; Cao, Y.; Huang, W. Macromolecules 2006, 39, 3707. (17) Liu, F.; Lai, W. Y.; Tang, C.; Wu, H. B.; Chen, Q. Q.; Peng, B.; Wei, W.; Huang, W.; Cao., Y. Macromol. Rapid Commun. 2008, 29, 659. (18) Liu, X. M.; Xu, J.; Lu, X.; He, C. Org. Lett. 2005, 7, 2829. (19) Seo, J. H.; Lee, K. H.; Seo, B. M.; Koo, J. R.; Moon, S. J.; Park, J. K.; Yoon, S. S.; Kim, Y. K. Org. Electron. 2010, 11, 1605. (20) Li, B.; Li, J.; Fu, Y.; Bo, Z. J. Am. Chem. Soc. 2004, 126, 3430. (21) (a) Liu, X. M.; Xu, J.; Lu, X.; He, C. Org. Lett. 2005, 7, 2829. (b) Wang, L.; Wong, W. Y.; Lin, M. F.; Wong, W. K.; Cheah, K. W.; Tam, H. L.; Chen., C. H. J. Mater. Chem. 2008, 18, 1. (22) SAINTþ, ver. 6.02a, Bruker, Analytical X-ray System Inc.: Madison, WI, 1998. (23) Sheldrick, G. M. SADABS, Empirical Absorption Correction Program, University of G€ottingen, Germany, 1997. (24) Sheldrick, G. M. SHELXTL, Reference Manual, ver. 5.1; Bruker AXS Inc.: Madison, WI, 1997. (25) Sun, Q.; Wang, H.; Yang, C.; Li, Y. F. J. Mater. Chem. 2003, 13, 800. (26) Janietz, S.; Bradley, D. D. C.; Grell, M.; Giebeler, C.; Inbasekaran, M.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 2453.

4878

dx.doi.org/10.1021/jp110652y |J. Phys. Chem. C 2011, 115, 4872–4878