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Unsymmetrical Dendrimers as Highly Efficient Light-Emitting Materials: Synthesis, Photophysics, and Electroluminescence Hengjun Zhang,†,‡ Xinjun Xu,†,‡ Wenfeng Qiu,† Ting Qi,†,‡ Xike Gao,†,‡ Ying Liu,†,‡ Kun Lu,†,‡ Chunyan Du,†,‡ Gui Yu,† and Yunqi Liu*,† Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, and Graduate School, Chinese Academy of Science, Beijing 100080, P. R. China ReceiVed: February 24, 2008; ReVised Manuscript ReceiVed: June 16, 2008
Highly efficient light-emitting materials can be archived by a rational design that involves rigid dendron and π-conjugated chromophores. This strategy is exemplified by three unsymmetrical dendrimers 5-(2′,3′,4′,5′tetraphenyl)phenyl-5′-(9- anthracenyl)-2,2′-bithiophene (1), 5-(2′,3′,4′,5′-tetraphenyl)phenyl-5′-(1-pyrenyl)2,2′ -bithiophene (2), and 5-(2′,3′,4′,5′-tetraphenyl)phenyl-5′-(1-pyrenylethynyl)-2,2′- bithiophene (3). Thermogravimetric analysis and differential scanning calorimetry suggest that unsymmetrical dendrimers 1 and 2 have good thermal stability (>400 °C) and high glass transition temperature (>130 °C) which make them promising candidates for highly stable organic light-emitting devices. The comparative study on their UV-vis absorption and fluorescence spectra in solution and as thin films indicates that the unsymmetrical dendrimers can form intermolecular π-π stacking in the solid state. Electroluminescence (EL) devices using them as emitters were made, and it was found that they emit intense green light, and the maximum luminance was 8659 cd/m2 at 13.5 V with a maximum efficiency of 4.9 cd/A for 1. The further comparative study on the EL performance of 1-3, combined with their single crystal structures, demonstrated that with felicitously chosen π-conjugated chromophores and better control of the space between the chromophore and dendron highly efficient light-emitting materials will be realized. Introduction Organic fluorescent materials are very important materials, as they can be used as light-emitting materials for organic lightemitting diodes (OLEDs), which are reputed as the new star of next generation plate-plane displays and lighting.1,2 Since the creative work of Tang and VanSlyke in electroluminescence (EL) devices using small molecule organic fluorescent materials,3 the research activities on the design and synthesis of new highly efficient organic light-emitting materials for OLEDs have made significant progress during the past decades.4-18 Meanwhile, the researchers reached a consensus that intermolecular interactions play a crucial role in the performance of OLEDs,19,20 whereas the correlations between molecular packing structures and optical properties are still not clear enough to direct the design and synthesis of highly efficient light-emitting materials. So, it is necessary to pay more attention to explore and clarify the correlations. As we know, hydrogen bonding and π-π stacking are important interactions usually observed in organic and coordinative supramolecular systems. As for organic fluorescent materials, π-π stacking can make molecules aggregate and cause fluorescence quenching, showing little or no fluorescence in the solid state. Great efforts have been made to reduce this quenching effect through the incorporation of dendrimers around the fluorophore.21-26 On the other hand, high charge carrier mobility correlated with π-π stacking. In view of this, one of the great challenges for developing next generation highly efficient organic light-emitting materials is to search for a tradeoff between luminescence enhancement and * To whom correspondence should be addressed. E-mail: liuyq@mail. iccas.ac.cn. † Institute of Chemistry. ‡ Graduate School.
charge carrier mobility tuning. Unsymmetrical dendrimers, which were regarded as the third class of light-emitting materials,21 are undoubtedly the better choice. In 1996, Moore et al. reported a series of rigid phenylacetylene-based dendrimers with a fluorescent perylene chromophore terminated, and investigated their intramolecular energy transfer.27 Peng et al. synthesized the first unsymmetrical conjugated dendrimer using a simple method and studied its optical properties 4 years later, and pointed out that it can be a potentially efficient light harvesting material at the same time.28 Subsequently, Kwok and Wong reported a series of multifunctional, light-emitting distyrylstillbenes with first-generation or second-generation propoxy surface functionalized poly(benzylether) end capped one-side dendrimers, and first investigated their EL behavior. In addition, a contrasting study suggested that OLEDs based on these dendrimers exhibit better device performance than those of the two-side dendritic wedges substituted dendrimers.29 Recently, Tian et al. reported a series of naphthalimide dendrimers using Frechet-type poly(aryl ether) as dendrons and carbazole or oxadiazole as peripheral groups in order to enhance core luminescence and tune carrier injection. Their preliminary EL behaviors were investigated by means of a spin-coating technique;30 however, the EL performances reported by Kwok and Wong29 and Tian et al.30 were not outstanding, possibly due to the flexibility of dendrons and lack of felicitous π-π stacking interactions. On the basis of their research, we propose a strategy on the design of highly efficient light-emitting unsymmetrical dendrimers, which consist of three main units: the rigid dendrons, the spacer, and the π-conjugated chromophore as shown in Figure 1. The rigid dendrons can control the intermolecular interactions, and the π-conjugated chromophore can easily form intermolecular π-conjugation, whereas the spacer can regulate the space between the dendrons and
10.1021/jp801613v CCC: $40.75 2008 American Chemical Society Published on Web 08/01/2008
Dendrimers as Efficient Light-Emitting Materials
Figure 1. Schematic representation the combination of unsymmetrical dendrimers.
SCHEME 1: Synthetic Routes and Conditions: (a) (1) nBuLi, THF, -78 °C, (2) Sn(C4H9)3Cl, rt, Overnight; (b) 9-Bromoanthracene, Pd(PPh3)2Cl2, Reflux 24 h; (c) NBS, CHCl3, HAc; (d) 1-Pyrenyl Boronic Acid, Pd(PPh3)4, Na2CO3, Toluene, Reflux 24 h; and (e) 1-Ethynylpyrene, Pd(PPh3)2Cl2, PPh3, CuI, n-Propylamine, 70 °C, 12 h
chromophore to form efficient intermolecular π-conjugation, so as to improve the charge mobility within the material. Herein, we report the synthesis, photophysics, and EL of three unsymmetrical dendrimers 1, 2, and 3 with different π-conjugated chromophores and spacers. Experimental Section General Information. All starting materials were obtained from commercial suppliers and used as received. Solvents, such as toluene and tetrahydrofuran, were freshly distilled over sodium, and dichloromethane was distilled over CaH2. All reactions were performed under an argon atmosphere. The synthon 5-(2′,3′,4′,5′tetraphenyl)phenyl-2,2′-bi-thiophene (DTTPP) was prepared in a revisory procedure with excellent yield according to the literature31 and bromized with N-bromosuccinimide (NBS) in chloroform under acid conditions to afford 5-(2′,3′,4′,5′-tetraphenyl)phenyl5′-bromo-2,2′-bi-thiophene (BDTTPP). 1-Ethynylpyrene was prepared according to the literature with a high yield.32 1H NMR spectra were obtained on a Bruker DMX 400 NMR Spectrometer. The signals have been designated as follows: s (singlet), d (doublet), and m (multiplet). 1H chemical shifts are reported in ppm downfield from the tetramethylsilane (TMS) reference using the residual protonated solvent resonance as an internal standard. The UV-vis and photoluminescence (PL) spectra were obtained on Hitachi U-3010 and Hitachi F-4500 spectrometers, respectively. Quantum yields in solution were determined using quinine sulfate as the standard (φ ) 0.58 in 0.1 M H2SO4).33 Measurements of the photoluminescence quantum yield, φPLQY, are carried out using an integrating sphere according to the method outlined by De Mello.34 Films were fabricated using high vacuum vapor deposition, and the thickness was about 50 nm. Solid-state fluorescence spectra
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13259 were recorded with thin solid films of the compound cast from tetrahydrofuran solutions on a 1 cm × 4 cm quartz plate. MALDITOF MS spectra were determined on a Bruker BIFLEX III mass spectrometer, and HR-EI MS spectra were obtained on a GCTMS micromass UK instrument. Elemental analyses were carried out on a Carlo-Erba 1160 elemental analyzer. Cyclic voltammetric measurements were carried out in a conventional three-electrode cell using Pt button working electrodes of 2 mm diameter, a platinum wire counter electrode, and a Ag/AgCl reference electrode on a computer-controlled CHI 660C instrument at room temperature (rt). Thermogravimetric analysis (TGA) was carried out using a Perkin-Elmer thermogravimeter (model TGA7) under a dry N2 flow at a heating rate of 10 °C/min. Differential scanning calorimetry (DSC) analyses were performed on a TA DSC 2010 instrument under a dry N2 flow, heating from rt to about 250 °C at a heating rate of 10 °C/min. OLED Fabrication and Characterization. All OLEDs were fabricated on bare indium tin oxide (ITO) substrates that were cleaned with detergent, deionized water, acetone, and ethanol. Films of organic semiconductors and cathodes were formed by vacuum deposition at a pressure of 3 × 10-4 Pa. Their deposition rates were 1 and 5 Å/s, respectively. The electrontransporting material is 1,3,5-tris[N-phenylbenzimidazol-2-yl]benzene (TPBI), while the hole-transporting material is 4,4′bis[1-naphthylphenylamino]biphenyl(NPB)or(R)-2,2′-dimethoxyl3,3′-di(phenyl-4-yl-diphenyl-amine)-[1,1′]-binaphthalenyl (TPABN-TPA). The thickness of the thin films was monitored by using a quartz crystal oscillator placed near the substrates, and was calibrated ex situ by using an Ambios Technology XP-2 surface profilometer. A Newport 2835-C multifunction optical meter was used to measure luminescence output. Current-voltage characteristics were measured with a Hewlett-Packard 4140B semiconductor parameter analyzer. Commission Internationale de L’Eclairage (CIE) coordinates were measured with a Photo Research PR-650 spectrophotometer. X-ray Crystallography. Single crystals suited for X-ray structural analysis were obtained by slow diffusion of methanol into tetrahydrofuran solutions of 1 and 3. Diffraction data were collected on a Rigaku Saturn charge-coupled device (CCD) area detector with graphite monochromated Mo KR (γ ) 0.71070 Å) radiation at 113 K. The structure was solved with patterson methods and refined with a full matrix least-squares technique using SHELXL-97. Hydrogen atoms were assigned isotropic displacement coefficients. Synthesis of 5-(2′,3′,4′,5′-Tetraphenyl)phenyl-5′-(9-anthracenyl)-2,2′-bithiophene (1). To a solution of DTTPP (0.55 g, 1 mmol) in THF (20 mL) was added dropwise n-butyllithium (0.44 mL, 1.1 mmol, 2.5 M in hexanes) at -78 °C. The mixture was stirred at this temperature for 30 min, tributyltin chloride (0.32 mL, 1.1 mmol) was added dropwise, and the mixture was allowed to warm to room temperature and stirred overnight. And then the flask was charged with 9-bromoanthracene (0.26 g, 1mmol) and bis(triphenylphosphine)palladium(II) chloride (0.071 g, 0.1 mmol). The reaction mixture was refluxed for 24 h and then poured into saturated potassium fluoride. The aqueous layer was extracted with dichloromethane, and the organic extracts were washed with brine and dried over sodium sulfate. The filtrate was concentrated on a rotary evaporator, and the following residue was purified by column chromatography on silica gel with dichloromethane/petroleum ether (1:10) to afford 1 as a light green solid. Yield: 0.505 g (70%). Data for 1 are as follows. 1H NMR (CDCl3, 400 MHz): δ ) 8.567 (s, 1H, anthracene-H), 8.046 (d, J ) 7.6 Hz, 2H, anthracene-H), 7.968 (d, J ) 8 Hz, 2H, anthracene-H), 7.798 (s, 1H), 7.532-7.440
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TABLE 1: Physical Properties of the Three Materials 1-3 solution material
Td/Tg (°C)
1 2 3
455/138 440/172 168/-
abs
λmax
(nm)
353, 372, 391 384 284, 354, 410
film em
λmax
(nm)
504 488 472, 500
φf
a
0.03 0.31 0.44
opt
b
Eg (eV) 2.79 2.56 2.46
λmaxabs
(nm)
357, 377, 396 401 363, 420
energy levels (eV) λmax
em
(nm)
488 503 523, 555
HOMO
LUMO
-5.40 -5.38 -5.40
-2.61 -2.82 -2.94
a Measured in CH2Cl2 solution by using quinine sulfate as a standard at room temperature. b Band gap estimated from the onset wavelength of optical absorption in thin film.
Figure 3. Cyclic voltammograms of 1-3 in dichloromethane/Bu4NPF6 at a scan rate of 100 mV/s.
Figure 2. Normalized optical absorption and PL emission spectra of 1 (a), 2 (b), and 3 (c) in dichloromethane solution and as thin films.
(m, 4H, anthracene-H), 7.287-7.213 (m, 6H), 7.116-7.051 (m, 6H), 6.984-6.828 (m, 11H), 6.548 (d, J ) 3.4 Hz, 1H, thiophene-H). MALDI-TOF MS (M+): 722.4. Elemental analysis (%) calcd for C52H34S2: C, 86.39; H, 4.74. Found: C, 86.15; H, 4.76. Synthesis of 5-(2′,3′,4′,5′-Tetraphenyl)phenyl-5′-(1-pyrenyl)-2,2′-bithiophene (2). BDTTPP (0.44 g, 0.7 mmol) and 1-pyrenyl boronic acid (0.21 g, 0.84 mmol) were dissolved in
toluene (20 mL) and 2.0 mol/L Na2CO3 aqueous solution (5 mL). After degassing by argon gas, Pd(PPh3)4 (0.025 g, 0.022 mmol) was added to the mixed solution. The reaction mixture was then refluxed under an argon atmosphere. After 24 h, the reaction mixture was diluted with diethyl ether (50 mL) and washed with water three times. The organic layer was dried over MgSO4 and concentrated under vacuum. The residue was purified by column chromatography on silica gel by eluting with CH2Cl2/petroleum ether (1:10) to get 2 as a yellow solid. Yield: 0.36 g (69%). Data for 2 are as follows. 1H NMR (CDCl3, 400 MHz): δ ) 8.541 (d, J ) 9.2 Hz, 1H, pyrene-H), 8.215-8.167 (m, 3H, pyrene-H), 8.111-8.006 (m, 5H, pyrene-H), 7.783 (s, 1H), 7.199 (s, 6H), 7.103-6.798 (m, 17H), 6.558 (d, J ) 3.4 Hz, 1H). HRMS (M+): calcd for C54H34S2, 746.2102; found, 746.2109. Elemental analysis (%) calcd for C54H34S2: C, 86.83; H, 4.59. Found: C, 86.29; H, 4.58. Synthesis of 5-(2′,3′,4′,5′-Tetraphenyl)phenyl-5′-(1-pyrenylethynyl)-2,2′-bi-thiophene (3). BDTTPP (0.32 g, 0.5 mmol), Pd(PPh3)2Cl2 (17 mg, 0.025 mmol), PPh3 (13 mg, 0.05 mmol), CuI (5 mg, 0.025 mmol), and a magnetic stirring bar were placed in a two-necked round-bottom flask fitted with a condenser. The whole setup was degassed and back-filled with argon. To the reaction flask was added previously degassed 10 mL of THF and n-propylamine (10 mmol) using syringes. 1-Ethynylpyrene (0.12 g, 0.5 mmol) was then dissolved in 5 mL of THF and added to the reaction mixture at about 70 °C. The reaction mixture was stirred at reflux for 12 h under the atmosphere of argon. The solvents were evaporated, and the crude product was extracted with ether/ethyl acetate. The combined organic layers were washed with water followed by brine before drying and evaporating. The residue was purified by column chromatography on silica gel by eluting with CH2Cl2/petroleum ether (1: 10) to get 3 as a yellow solid. Yield: 0.30 g (78%). Data for 3 are as follows. 1H NMR (CDCl3, 400 MHz): δ ) 8.571 (d, J ) 9.0 Hz, 1H, pyrene-H), 8.232-8.006 (m, 8H), 7.764 (s, 1H), 7.276 (d, 1H), 7.195 (s, 5H), 7.106-6.809 (m, 17H), 6.555 (d, J ) 3.4 Hz, 1H). HRMS (M+): calcd for C56H34S2, 770.2102;
Dendrimers as Efficient Light-Emitting Materials
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13261
Figure 4. ORTEP view of 1 (30% probability displacement ellipsoids).
Figure 5. Stacking diagram of 1 showing π-π interactions and C-H · · · S hydrogen bonds.
Figure 6. ORTEP view of 3 (30% probability displacement ellipsoids).
Figure 7. Crystal packing diagram of 3 along the c axis.
found, 770.2110. Elemental analysis (%) calcd for C56H34S2: C. 87.24; H, 4.44. Found: C, 87.29; H, 4.55.
Figure 8. Current density-voltage-luminance characteristics of the OLED device of 1. The inset shows the normalized EL spectra of the three materials 1-3.
Results and Discussion Compound 1 was prepared in 70% yield via the one-pot Stille coupling reaction starting from steric bulk building block DTTPP as a light green solid, and the yellow solid compound 2 was synthesized through the Suzuki coupling reaction using
BDTTPP, which can be easily bromized by DTTPP with NBS, and 1-pyrenyl boronic acid in 69% yield, while compound 3 was prepared by the Sonogashira coupling reaction of BDTTPP with 1-ethynylpyrene in 78% yield as outlined in Scheme 1.
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TABLE 2: Device Performances of the Three Materials 1-3 material
EL λmax (nm)
VT (V)
Lmax (cd/m2)
ηcurr (cd/A)
ηex (%)
CIE (x, y)
1 2 3
516 503 524
4.5 7.2 6.5
8659 5970 2898
4.90 3.75 1.68
1.85 1.3 0.59
0.27, 0.51 0.25, 0.57 0.33, 0.56
The three novel compounds were characterized by 1H NMR, mass spectrometry (MS), and elemental analysis. Their thermal properties were investigated by TGA and DSC. The onset decomposition temperatures (Td) were 455, 440, and 168 °C for 1, 2, and 3, respectively. The glass-transition temperature (Tg) for 1 and 2 were observed at 138 and 172 °C, respectively. It is worth noting that the Tg of 2 compares favorably with that of the famous EL material Alq3. All the thermal data are summarized in Table 1, and the results indicate that compounds 1 and 2 possess good thermal stability and high glass-transition temperatures, which are necessary features of functional materials for applications in thin film molecular devices; however, compound 3 has poor thermal stability, and the presence of acetylenyl groups may be the reason. Photophysical Properties. The photophysical properties of compounds 1, 2, and 3 were examined by UV-vis and fluorescence spectroscopy in dichloromethane solutions and as thin films (Figure 2). All the data including absorption maxima (λmaxabs), PL emission maxima (λmaxem), optical band gaps (Egopt), and fluorescence quantum yields (φf) are summarized in Table 1. For 1, the absorption spectrum in solution displays three vibronic resolved bands at 353, 372, and 391 nm, which attribute to the anthracene S0-S1 transition, whereas 2 exhibits only one absorption peak centered at 384 nm. As for 3, two main absorption peaks centered at 284 and 410 nm were observed accompanied by a shoulder peak at 354 nm, and the longest wavelength absorption maximum bathochromically shifted compared with that of 2. The effect of this extra bathochromic shift might be due to π-π interaction between the pyrene ring and the acetylenic π-bond. As a thin film, 1 also has three absorption peaks with maxima at 357, 377, and 396 nm, which is slightly bathochromic shifted compared with its solution state. As for 2 and 3, the thin films have an absorption maximum at 401 and 420 nm, respectively, which represents a large bathochromic shift relative to their solution state, indicating a greater π-conjugation in their solid state. This means that the extent of electron delocalization in the unsymmetrical dendrimer with pyrene chromophores is greater than that with the anthracene chromophore. The Egopt of the three compounds 1-3, calculated from their absorption band edge, is 2.79, 2.56, and 2.46 eV, respectively. This also confirms that the dendrimer with pyrene chromophores has a greater π-conjugation than that with anthracene chromophores. Differing from the absorption spectrum, the fluorescence spectrum of 1 in solution has a single broad emission peak centered at 504 nm and a large full width at half-maximum (fwhm) value of 131 nm as shown in Figure 2a. On the contrary, the fluorescence spectrum of the thin film has a narrow emission peak centered at 488 nm and a moderate fwhm value of 78 nm. As for 2, the fluorescence spectrum in solution has a single emission peak centered at 488 nm and a fwhm value of 72 nm, whereas for 3 the fluorescence spectrum in solution has double emission peaks located at 472 and 500 nm and a fwhm value of 65 nm. Compared to their solution spectra, compounds 2 and 3 show bathochromicshifted solid-state emission apparently, indicating strong π-π interactions in their solid state. In addition, compound 1 has a very low fluorescence quantum yield (φf ) 0.03) in dichloromethane solution and a high absolute photoluminescence quantum yield (φf ) 0.40) in solid state at room temperature. This behavior had been
described previously34 and can be explained by an intramolecular torsional motion in solution between the two molecular subunits anthracene and bithiophene, while the torsional motion will be restricted in the solid state. Meanwhile, additional nonradiative relaxation processes brought by intramolecular torsional motion can also decrease the overall quantum yield in solution. As for compounds 2 and 3, the intramolecular torsional motion in solution between pyrene and bithiophene is very weak, so they have high fluorescence quantum yield in dichloromethane solution with φf values of 0.31 and 0.44, respectively. Electrochemical Properties. All three compounds were characterized by cyclic voltammograms as shown in Figure 3, and of the data are listed in Table 1. An irreversible oxidation potential was observed in CH2Cl2 for 1 and 3, and two reversible oxidation potentials were observed for 2 as shown in Figure 3. In contrast, all of their reduction potentials were not detected. The onset oxidation potentials for 1-3 were located at +1.0, +0.98, and +1.0 V, respectively. According to the equation HOMO ) -([Eonset]ox + 4.4) eV,36 their respective HOMO energy levels were estimated to be 5.40, 5.38, and 5.40 eV. These values are lower than that of NPB (5.46 eV),4 which is one of the most widely used hole-transport materials. Thus, they would be beneficial for hole injection and transportation. Subsequently, their LUMO energy levels were estimated to be 2.61, 2.82, and 2.94 eV, respectively, by combining the HOMO energy levels together with the optical band gaps obtained from the edge absorption of the solid samples. Single Crystal X-ray Characterization and Molecular Packing Properties. Single crystals of 1 and 3 suitable for X-ray crystallography were obtained by layer-diffusion of methanol to their tetrahydrofuran solution, and single crystal X-ray determination provides insight into the structure-property relationship and the effect of a spacer on their photophysics and charge transport properties. Unfortunately, we were unable to obtain single crystals with 2 despite numerous attempts. All crystallographic data for these two compounds are listed in Table S1 in the Supporting Information. As shown in Figure 4, crystal 1 possesses an unsymmetric noplanar geometry structure. First, the anthracene ring is almost perpendicular to the thienyl ring (S1, C1, C2, C3, C4) with a dihedral angle of 103.47°, and the spacer bithiophene is also a nonplanar framework with a dihedral angle of 27.46° between the two thienyl rings. Moreover, another thienyl ring (S2, C5, C6, C7, C8) and phenyl ring (C23, C24, C25, C26, C27, C28) are noplanar with a dihedral angle of 43.62°. Finally, the central phenyl ring (C23, C24, C25, C26, C27, C28) and the four phenyl rings attached to it are not coplanar with dihedral angles of 49.6°, 60.1°, 75.3°, and 68.7°, respectively. However, from the packing structure shown in Figure 5, we observed that 1 has cofacial π-π stacking between neighboring anthracene rings with a intermolecular distance of 3.451 Å and double C-H · · · S hydrogen bonding with a distance of 3.885 Å, which result in the formation of onedimensional ladderlike molecular columns and facilitate the charge carrier transport consequently. Crystal 3 also has an unsymmetric noplanar geometry structure as shown in Figure 6. Although the dihedral angle for the thienyl ring (S2, C5, C6, C7, C8) and central phenyl ring (C27, C28, C29, C30, C31, C32) is 47.17°. It is noteworthy that the pyrene ring, thienyl ring (S1, C1, C2, C3, C4), and thienyl ring (S2, C5, C6, C7, C8) are almost coplanar with dihedral angles of only 5.11° and 1.15°, respectively. Additionally, from the packing diagram shown in Figure 7, we observed that crystal 3 has double π-π stacking between the pyrene ring and thienyl ring with an intermolecular distance of 3.454 Å, and two types of edge-to-face packing of the pendant phenyl rings to neighboring planar units, pyrene and ethynyl groups, which are
Dendrimers as Efficient Light-Emitting Materials separated by a distance of 3.613 and 3.650 Å, respectively. The presence of an ethynyl group in the spacer may be the reason for the absence of cofacial π-π stacking between neighboring pyrene rings. As we mentioned earlier, the differences between the two kinds of packing styles of 1 and 3 would be expected to result in a difference of charge mobility within the materials. Electroluminescent Devices. To investigate their EL properties, three multilayer devices with structures of ITO/NPB (30 nm)/1 or 3 (30 nm)/TPBI (30 nm)/LiF (1 nm)/Al and ITO/TPA-BN-TPA (30 nm)/2 (30 nm)/TPBI (30 nm)/LiF (1 nm)/Al were fabricated by thermal deposition in a vacuum chamber under a reduced pressure of 3 × 10-4 Pa. The current density-voltage-luminance and luminous efficiency-current density characteristics of 1, which has the best performance among the three compounds, are shown in Figure 8, and the normalized EL emission spectra of the three compounds are shown in the inset. First, the turn-on voltage (at a brightness of 1.0 cd/m2) of 1 is 4.5 V, and the device reaches a maximum luminance of 8659 cd/m2 at 13.5 V with a maximum efficiency of 4.9 cd/A (1121 cd/m2 at 23 mA/m2) and an external quantum efficiency of 1.85%. The device exhibited green-light emission with a peak centered at 516 nm and CIE 1931 chromaticity coordinates of (x ) 0.27, y ) 0.51). The emission peak has an apparent red-shift compared with that of the solid film, which is caused by the formation of an electroplex between NPB and the 1 interface under high electric field. The device of 2 also shows a green maximum emission peak at about 503 nm with CIE 1931 chromaticity coordinates of (x ) 0.25, y ) 0.57) and has a turnon voltage of 7.2 V, a maximum luminance of 5970 cd/m2 at 15.5 V with a maximum efficiency of 3.75 cd/A (381 cd/m2 at 10.4 mA/m2), and an external quantum efficiency of 1.3%. As a comparison, compound 3 has poor performance with brightness only up to 2898 cd/m2 (at 14 V) and efficiencies of 0.59% EQE and 1.68 cd/A at 1227 cd/m2. The presence of double π-π interactions in the solid state of 3 maybe the reason. All the data summarized in Table 2 suggest that 1 has the best EL performance among the three compounds even though 2 uses TPA-BN-TPA, which has a more superior hole transporting ability than NPB,37 as a hole transporting layer. Conclusions In summary, three unsymmetrical dendrimers containing a rigid dendron and different π-conjugated chromophores were designed and synthesized via simple coupling reactions. The comparative study on their crystal structure shows that π-π stacking interactions can be altered by introducing a suitable spacer and π-conjugated chromophore. Although the quantum yield of 1 in solution is lower by 1 order of magnitude than that of 2 and 3, the performance of its diode is the best among the three compounds, and the performance combined with their single crystal X-ray diffraction analysis indicates that the better the balance between luminescence enhancement and π-π stacking interactions is controlled, the better the EL performance will be. In addition, dendrimers 1 and 2 show good thermal stability and high glass transition temperature (>130 °C), making them promising candidates for highly stable OLED devices. At the least, the present results demonstrate the effectiveness and feasibility of our molecular design strategy for synthesizing organic fluorescent materials with highly efficient EL performance. Acknowledgment. This work was supported by the National Natural Science Foundation of China (60736004, 20721061, 60671047, 50673093), the Major State Basic Research Development Program (2006CB806200, 2006CB932100), and the Chinese Academy of Sciences. We also thank Professor Yong Qiu and Dr. Lian Duan for their help in measuring the quantum yield of compound 1 in the solid state.
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