Efficient non-doped blue-light-emitting diodes incorporating an anthracene
derivative end-capped with fluorene groups
Chen-Hao Wu, Chen-Han Chien, Fang-Ming Hsu, Ping-I Shih and Ching-Fong Shu*
Received 29th September 2008, Accepted 4th December 2008First published as an Advance Article on the web 27th January 2009 DOI: 10.1039/b817031b
We have synthesized and characterized a novel blue-emitting material, 2-tert-butyl-9,10-bis[40
-(9-p-tolyl-fluoren-9-yl)biphenyl-4-yl]anthracene (BFAn), containing an anthracene core end-capped with 9-phenyl-9-fluorenyl groups. The presence of the sterically congested fluorene groups imparts BFAn
with a high thermal decomposition temperature (Td¼ 510C) and results in its forming a stable glass
(Tg¼ 227C). Atomic force microscopy measurements revealed that BFAn forms high-quality
amorphous films and possesses good morphological stability after annealing. Organic light-emitting diodes (OLEDs) featuring BFAn as the emitter exhibited an excellent external quantum efficiency of
5.1% (5.6 cd A1) with Commission Internationale de L’Eclairage coordinates of (0.15, 0.12) that are
very close to the National Television Standards Committee’s blue standard. The power efficiency of our
BFAn-based devices reached as high as 5.7 lm W1, making them superior to other reported non-doped
deep-blue OLEDs.
Introduction
Organic light-emitting diodes (OLEDs) have attracted much scientific and commercial attention because of their potential use in
high-resolution, full-color, flat-panel displays.1–5 To meet the
requirements for full-color displays, red-,6,7 green-,8,9 and
blue-emitting10–12 materials (i.e., the three primary colors) must all
feature high electroluminescence (EL) efficiencies, good thermal stabilities, and excellent charge-carrier injection/transport abilities. Although remarkable improvements in OLED performance have been achieved over the past decade, the performance of blue OLEDs is relatively poor in comparison with those of red and green OLEDs. Because of their intrinsically wide band-gap, the synthesis of highly efficient blue-light emitters exhibiting good color purity remains a great challenge for the development of new OLEDs.
Anthracene derivatives have been studied extensively and developed as blue-light-emitting materials in OLEDs because of their excellent photoluminescence (PL) and electroluminescence
(EL) properties.13–20Among these derivatives,
9,10-diphenylan-thracene (DPA) is an attractive material for its unity fluorescence quantum efficiency in dilute solution and high fluorescence in the solid state. Unfortunately, DPA tends to crystallize in the solid
state,21,22thereby limiting its OLED applications because crystal
formation destroys film homogeneity and raises the resistance of
the sample, ultimately leading to device failure.23 In general,
amorphous thin films (in OLEDs) having high glass transition temperatures (Tg) are less vulnerable to heat and, hence, their
devices perform more stably.24–27 Consequently, light-emitting
materials possessing high values of Tgare required to retain the
film morphology during the operation of the device.
In this study, we developed a highly efficient blue-emitting
material, 2-tert-butyl-9,10-bis[40
-(9-p-tolylfluoren-9-yl)biphenyl-4-yl]anthracene (BFAn), possessing high thermal stability and
good amorphous film-formation capability. This new blue emitter contains an anthracene core and two end-capping fluo-rene groups, the 3D cardo structures of which improve the molecular rigidity and mitigate the close-packing of molecules in the solid state, leading to an amorphous organic thin film
dis-playing pronounced morphological stability.28 We fabricated
non-doped blue-light OLEDs incorporating BFAn to evaluate its applicability. A device incorporating BFAn as the emitter and
2-tert-butyl-9,10-bis[40-(1-phenylbenzoimidazyl)biphenyl-4-yl]
anthracene (BIAn) as the electron-transporting layer exhibited
a high external quantum efficiency of 5.1% (5.6 cd A1) and an
excellent power efficiency (5.7 lm W1at 7.8 mA cm2), together
with satisfactory Commission Internationale de L’Eclairage (CIE) coordinates (0.15, 0.12). To the best of our knowledge, this power efficiency is among the best ever reported for fluorescent
blue-light-emitting OLEDs.13–20,29–33
Results and discussion
SynthesisScheme 1 illustrates the synthetic route we used to prepare the anthracene derivative BFAn presenting two fluorene terminal groups. The acid-promoted Friedel–Crafts-type substitution of toluene with 9-(4-bromophenyl)-fluoren-9-ol (1) afforded the bromide 2, which in turn underwent cross-coupling with the
pinacol ester of diboron to give the arylboronic ester 3.34We
prepared 2-tert-butyl-9,10-bis(4-bromophenyl)anthracene (6)
using a procedure reported previously: the reaction of mono-lithiated 1,4-dibromobenzene with 2-tert-butylanthraquinone and then reduction of the intermediate diol with potassium
iodide and sodium hypophosphite in acetic acid.15 Suzuki
coupling of the boronic ester 3 and the dibromide 6 yielded the
target compound BFAn. The1H and13C NMR spectra,
high-resolution mass spectrum, and elemental analysis (EA) were consistent with the proposed molecular structure of BFAn.
Department of Applied Chemistry, National Chiao Tung University, Hsin-Chu, Taiwan, 30050
PAPER www.rsc.org/materials | Journal of Materials Chemistry
Thermal properties
We investigated the thermal properties of BFAn using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Fig. 1a displays DSC curves of BFAn recorded over the
temperature range 50–400C. The sublimated sample melted at
391 C on the first heating only, and then it transformed into
a glassy state upon cooling from the melt. When we heated the amorphous glassy sample again, a glass transition (Tg) occurred
at 227C; we observed no exothermic peak due to crystallization
at temperatures up to 400C. This relatively high value of T
g, which suggests potentially enhanced device stability, is a very desirable feature for emissive materials in light-emitting appli-cations. To verify the role played by the fluorene terminal units in determining the thermal behavior of BFAn, we prepared the corresponding anthracene derivative lacking fluorene end-capping groups (DBPAn, Scheme 1) as a reference compound.
DBPAn exhibits its glass transition at 126 C, followed by
a broad crystallization feature at 176 C and a well-defined
melting point at 293 C (Fig. 1b). These observations indicate
that the enhanced thermal stability of the amorphous glass state of BFAn can be attributed to the presence of its rigid fluorene end-groups and its high molecular weight. In addition, BFAn possesses excellent thermochemical stability, as evidenced through TGA, with its 5% weight loss temperature under
nitrogen atmosphere being 510C. In contrast, DBPAn exhibits
inferior thermal stability; its 5% weight loss temperature is
343C.
To further investigate the morphological stabilities of BFAn and DBPAn, we used vapor deposition to prepare thin films of each of these materials on silicon wafer substrates and then used atomic force microscopy (AFM) to determine their surface morphologies. After vapor deposition, we performed an annealing process
involving heating the films at 120C for 10 h under a nitrogen
atmosphere and then cooling to room temperature. The topo-graphical images in Fig. 2 reveal that BFAn provided a uniform surface that underwent no morphological changes after annealing. The root-mean-square (RMS) roughnesses of the pristine and annealed film were 0.31 and 0.32 nm, respectively. In contrast, thermal annealing induced crystallite formation in the DBPAn film, as indicated by changes in the surface morphology; the RMS roughness of this annealed film was 40.1 nm, ca. 100-fold higher than that of the film prior to annealing (0.35 nm). These images provide further evidence that the thermal stability of BFAn is greater than that of DBPAn, presumably because the presence of the fluorene end-capping groups hinders the close packing of BFAn and suppresses its crystallizability, resulting in an amor-phous material exhibiting pronounced morphological stability. Photophysical properties
The photophysical data of BFAn and DBPAn are summarized in Table 1. The UV–Vis absorptions of BFAn and DBPAn showed
Scheme 1 Reagents: (i) CF3SO3H, toluene; (ii) bis(pinacolato)diboron,
Pd(dppf)Cl2, KOAc, DMF; (iii) 3, Pd(PPh3)4, K2CO3, toluene/H2O,
Aliquat 336; (iv) phenylboronic acid, Pd(PPh3)4, K2CO3, toluene/H2O,
Aliquat 336; (v) 5, Pd(PPh3)4, K2CO3, toluene/H2O, Aliquat 336. Fig. 1 DSC traces of (a) BFAn and (b) DBPAn (heating rate: 20C
min1). Insets: Expanded views of the second DCS heating scans of (a)
BFAn (100–350C) and (b) DBPAn (100–150C). Tc: crystallization
temperature; Tg: glass-transition temperature; Tm: melting-point
temperature.
the characteristic vibrational patterns of the isolated anthracene
core (lmax¼ 358, 378, and 398 nm for BFAn and lmax¼ 356, 376
and 398 nm for DBPAn).21,35 In addition, we assigned the
absorption bands of BFAn in the region from 290 to 320 nm to
the peripheral fluorene groups (lmax¼ 289, 299, and 311 nm).36
Upon excitation at 365 nm, the solutions of BFAn and DBPAn exhibited blue emissions having an emission maximum at 446 and 431 nm, respectively. Fig. 3 presents the absorption and photoluminescence (PL) spectra of BFAn in dilute toluene solution and as a solid film on a quartz plate. The absorption and emission spectra of the BFAn thin film were similar to those acquired in dilute solution, but with slight red-shifts (3 and 4 nm, respectively). These small spectral shifts in the solid state spectra without the appearance of the excimer emission imply that the intermolecular interactions were weak, i.e., they were restrained effectively by the bulky end-capping fluorene groups. We calculated the fluorescence quantum yield (Ff) of BFAn in the
dilute toluene solution to be 0.93, using DPA (Ff ¼ 0.90 in
cyclohexane) as a calibration standard. In addition, we used an integrating sphere apparatus to measure the solid state quantum
yield (Ff¼ 0.84) on a quartz plate. The high quantum yields of
BFAn make it an excellent candidate for use as an efficient blue-light-emitting material in OLEDs.
Electroluminescence properties of OLEDs
To evaluate the applicability of using BFAn as the emitting layer (EML), we fabricated two non-doped blue-emitting devices
(I and II) through sequential vapor deposition of the materials
onto ITO glass under vacuum (3 106torr). Device I had the
following configuration: indium tin oxide (ITO)/4,40
-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) (20 nm)/4,40,400
-tris(N-carbazolyl)triphenylamine (TCTA) (10 nm)/BFAn (40 nm)/ 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) (40 nm)/ LiF (1 nm)/Al (100 nm). Fig. 4 displays the device structure and relative HOMO/LUMO energy levels of the materials used in this study. We determined the HOMO energy level of BFAn (5.85 eV) using an AC-2 photoelectron spectrometer; we esti-mated the LUMO energy level (2.92 eV) by adding the optical energy gap to the obtained HOMO energy level. NPB and TCTA were employed as bilayer hole-transporting layers (HTLs) to provide cascade hole injection and transport; TCTA also func-tioned as an effective electron/exciton blocker; TPBI was used as both the electron-transporting layer (ETL) and a hole-blocker. Device I exhibited a maximum external quantum efficiency as
Fig. 2 AFM images (top and angled views) of (a) BFAn and (b) DBPAn layers, after annealing at 120C for 10 h under a nitrogen atmosphere.
Table 1 Physical properties of the anthracene derivatives
Compound UV lmax(nm)a PL lmax(nm)a HOMO (eV)b LUMO (eV)c Tg(C) Tm(C) Td(C)d
BFAn 289, 299, 311, 358, 378, 398 446 5.85 2.92 227 391 510
DBPAn 356, 376, 398 431 5.76 2.82 126 293 343
BIAn 316, 378, 398 438 6.02 3.07 192 355 502
aMeasured in dilute toluene solution.bDetermined using a photoelectron spectrometer (AC-2).cEstimated by adding the optical energy gap to the
obtained HOMO energy level.dTemperature at which a 5% weight loss was detected.
Fig. 3 UV–Vis absorption and PL spectra of BFAn in dilute toluene solution and in the solid state.
Fig. 4 Relative HOMO/LUMO energy levels of the materials used in devices I and II.
high as 5.0% (5.5 cd A1, 5.0 lm W1) at 2.0 mA cm2 with
a brightness of 110 cd m2. Even at a much higher brightness of
1000 cd m2, the external quantum efficiency and power
effi-ciency remained high at 4.6% and 3.6 lm W1, respectively. These
values reveal that BFAn exhibits potential for practical use in display applications. For the purpose of comparison, we also fabricated a reference device using a conventional blue-emitting material, 9,10-di-(2-naphthyl)anthracene (ADN), as the EML. The control ADN-based device showed inferior EL performance
with maximum efficiencies of 2.9% and 1.9 lm W1, which were
similar to the reported values of other ADN-based devices.37–39
In the case of device II, we replaced TPBI with a novel ETL
material, 2-tert-butyl-9,10-bis[40
-(1-phenylbenzoimidazyl)biphenyl-4-yl]anthracene (BIAn, Scheme 1), which has a lower LUMO (3.07 eV, Table 1) than that of TPBI (2.7 eV), to facilitate electron injection from the cathode to the organic layer. Fig. 5 displays typical current density–voltage–luminance (I–V–L) characteristics of devices I and II. At the same driving voltage, device II exhibits a higher current density and enhanced bright-ness relative to those of device I. Fig. 6 reveals that device II displays improved EL performance in comparison with that of device I; its maximum external quantum efficiency reached
5.1% (5.6 cd A1, 5.0 lm W1) at 7.8 mA cm2with a brightness of
436 cd m2. The efficiency of device II decreased only slightly to
4.7% (5.2 cd A1, 4.1 lm W1) at a higher luminance of 1000 cd
m2. Table 2 summarizes the EL performance of these two
BFAn-based devices.
The turn-on voltage (corresponding to 1 cd m2) decreased
from 3.0 V for device I to 2.5 V for device II, leading to the higher
power efficiency of device II (5.7 lm W1; Fig. 6). We attribute
this enhancement to the better electron injection from the LiF/Al electrode to BIAn than to TPBI. To verify this hypothesis, we constructed electron-only devices to investigate the injection of electrons from the cathode to the organic layer. These electron-only devices had the configuration ITO/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (30 nm)/TPBI or BIAn (60 nm)/LiF (1 nm)/Al (100 nm). Because of the lower HOMO energy level of BCP (6.5 eV), no hole injection occurred from the anode to the organic layers. Because only electrons could be injected from the cathode to the organic layers, electrons domi-nated the measured current density–voltage (I–V) characteristics. As indicated in Fig. 7, the electron-only device incorporating BIAn had a much higher injection current relative to that of the TPBI-based device, suggesting that the energy barrier for elec-tron injection from the cathode to the BIAn layer was consid-erably lower than that to the TPBI layer. Consequently, the use of BIAn facilitated effective electron injection, resulting in the higher power efficiency of device II.
Fig. 5 I–V–L characteristics of devices I and II.
Fig. 6 External quantum efficiency and power efficiency of devices I and II plotted as functions of the current density.
Table 2 EL performance of devices I–II
I II Voltage (V)a 3.0 2.5 Brightness (cd m2)b 530 (4183) 543 (4382) E.Q.E. (%)b 4.8 (3.8) 5.0 (4.0) L.E. (cd A1)b 5.4 (4.2) 5.5 (4.4) P.E. (lm W1)b 4.2 (2.3) 4.8 (2.5) Max. E.Q.E. (%) 5.0 5.1 Max. L.E. (cd A1) 5.5 5.6 Max. P.E. (lm W1) 5.0 5.7 EL lmax(nm)c 450 450 CIE (x, y)c (0.15, 0.12) (0.15, 0.12) a Recorded at 1 cd m2.b
At 10 mA cm2, the data in the parentheses
were taken at 100 mA cm2.cAt 7 V.
Fig. 7 I–V curves of the electron-only devices.
The EL spectra of devices I and II display deep-blue emissions with maxima centered at 450 nm (Fig. 8). These EL spectra are quite similar to the PL spectrum of BFAn in the solid state (Fig. 3), indicating that all of the emissions originate exclusively from the BFAn layer. The inset to Fig. 8 provides the corre-sponding CIE coordinates of devices I and II; the CIE chroma-ticity coordinates of (0.15, 0.12) are very close to the standard blue emission recommended by the National Television Stan-dards Committee (NTSC). Furthermore, the emission color of devices I and II remained almost constant under the different bias conditions. The corresponding CIE coordinates changed
only slightly, from (0.15, 0.11) at 5.0 V (43 mA/cm2, 2042 cd/m2)
to (0.15, 0.12) at 9.0 V (562 mA/cm2, 15314 cd/m2) for device I
and from (0.15, 0.11) at 5.0 V (65 mA/cm2, 3000 cd/m2) to (0.15,
0.12) at 9.0 V (560 mA/cm2, 18191 cd/m2) for device II. This result
demonstrated that BFAn was a promising blue OLEDs material with good color stability.
Conclusion
We have fabricated highly efficient non-doped deep-blue OLEDs incorporating the anthracene derivative BFAn, a blue-emitting material featuring phenyl-substituted fluorene end-capping groups that enhance its thermal and morphological stabilities while maintaining the excellent fluorescence quantum yield of the anthracene core. The BFAn-based non-doped devices exhibit satisfactory blue emissions having CIE coordinates located at (0.15, 0.12), very close to the NTSC’s standard blue CIE coor-dinates. Moreover, the BFAn-based devices exhibit outstanding efficiencies, with maximum values of external quantum efficiency
reaching as high as 5.1% (5.6 cd A1) at 7.8 mA cm2. Notably,
the power efficiency of device II (5.7 lm W1) is superior to those
of previously reported blue OLEDs.41–43
Experimental
General information9-(4-Bromophenyl)-fluoren-9-ol (1)36and
2-(4-bromophenyl)-1-phenylbenzoimidazole (4)44were synthesized as reported
previ-ously. The solvents were dried using standard procedures. All reagents were used as received from commercial sources, unless
otherwise stated. The1H and13C NMR spectra were recorded on
Bruker-DRX 300 MHz spectrometers. Mass spectra were obtained using a JEOL JMS-HX 110 mass spectrometer. DSC was performed using a SEIKO EXSTAR 6000DSC unit at
heating and cooling rates of 20 and 50C min1, respectively.
Samples were scanned from 50 to 400C, cooled to 0C, and
then scanned again from 50 to 400C. The values of T
gwere determined from the second heating scan. TGA was performed using a DuPont TGA 2950 instrument. The thermal stability of the samples under a nitrogen atmosphere was determined by
measuring their weight loss while heating at a rate of 20C min1.
UV–Vis spectra were measured using an HP 8453 diode-array spectrophotometer. PL spectra were obtained using a Hitachi F-4500 luminescence spectrometer. The HOMO energy levels of organic thin films were measured using a Riken-Keiki AC-2 photoelectron spectrometer; the LUMO energy levels of mate-rials were estimated by subtracting the optical energy gap from the measured HOMO energy level. AFM images were recorded under ambient conditions using a Digital Nanoscope IIIa instrument operated in the tapping mode.
9-(4-Bromophenyl)-9-p-tolylfluorene (2)
A mixture of 1 (2.00 g, 5.95 mmol), toluene (20 mL), and
CF3SO3H (5 mL) was stirred for 4 h at 25C under nitrogen. The
reaction mixture was then treated with saturated NaHCO3(aq)
and extracted with EtOAc. The organic extracts were dried (MgSO4) and the solvent evaporated under vacuum. The crude
product was purified through column chromatography
(n-hexane) to give 2 (1.95 g, 80%).1H NMR (300 MHz, CDCl3): d7.76 (d, J¼ 8.1 Hz, 2H), 7.37–7.26 (m, 8H), 7.07 (d, J ¼ 8.7 Hz, 2H), 7.06 (d, J¼ 8.1 Hz, 2H), 7.05 (d, J ¼ 6.3 Hz, 2H), 2.29 (s, 3H).13C NMR (75 MHz, CDCl3): d 151.4, 145.8, 142.8, 140.6, 136,9, 131.7, 130.4, 129.6, 128.7, 128.4, 128.4, 128.1, 126.5, 121.2, 120.8, 65.2, 21.5. MS (EI) m/z 410. 9-[4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-9-p-tolylfluorene (3)
A solution of 2 (1.95 g, 4.76 mmol), 4,4,40,40,5,5,50,50
-octamethyl-2,20-bis(1,3,2-dioxaborolane) (1.33 g, 5.23 mmol), KOAc (1.39 g,
14.2 mmol), and [1,10
-bis(diphenylphosphino)ferrocene]di-chloropalladium(II) [Pd(dppf)Cl2, 15 mg] in anhydrous DMF
(25 mL) was deoxygenated by purging with N2and then it was
heated at 85C under N
2. After 12 h, the reaction mixture was
cooled to room temperature, mixed with water (50 mL), and
extracted with CH2Cl2 (3 50 mL). The combined organic
phases were dried (MgSO4) and concentrated under reduced pressure. The crude product was purified through column chromatography (n-hexane/EtOAc, 12:1) to give 3 (1.67 g, 76%).
1H NMR (300 MHz, CDCl 3): d 7.78 (d, J¼ 7.2 Hz, 2H), 7.69 (d, J¼ 8.1 Hz, 2H), 7.40–7.37 (m, 5H), 7.34–7.23 (m, 4H), 7.10–7.06 (m, 4H), 2.31 (s, 3H), 1.33 (s, 12H).13C NMR (75 MHz, CDCl3): d 151.5, 149.7, 143.2, 140.6, 136.7, 135.1, 129.3, 128.4, 128.2, 127.8, 120.5, 84.1, 65.8, 25.2, 21.4. MS (EI) m/z 458. 1-Phenyl-2-[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]benzoimidazole (5)
According to the procedure described for 3, the reaction of 2-(4-bromophenyl)-1-phenylbenzoimidazole (4, 1.50 g, 4.31 mmol),
Fig. 8 EL spectra of devices I and II operated at 7 V. Inset: CIE coor-dinates of devices I (diamond) and II (cross).
4,4,40,40,5,5,50,50-octamethyl-2,20-bis(1,3,2-dioxaborolane) (1.20
g, 4.73 mmol), KOAc (1.26 g, 12.9 mmol), Pd(dppf)Cl2(15 mg),
and DMF (30 mL) gave 5 as a white solid (1.15 g, 67.6%). 1H
NMR (300 MHz, CDCl3): d 7.83 (d, J¼ 7.8 Hz, 1H), 7.66 (d, J ¼ 8.3 Hz, 2H), 7.49 (d, J¼ 8.3 Hz, 2H), 7.41–7.37 (m, 3H), 7.27– 7.15 (m, 5H), 1.25 (s, 12H).13C NMR (75 MHz, CDCl 3): d 152.6, 143.3, 137.3, 135.0, 130.3, 129.0, 128.9, 127.8, 123.9, 120.3, 110.9, 84.4, 25.3. MS (FAB) m/z 397 [M + 1]. 2-tert-Butyl-9,10-bis[40 -(9-p-tolylfluoren-9-yl)biphenyl-4-yl]anthracene (BFAn)
Aqueous K2CO3(2.0 M, 10 mL) was added to a solution of 3
(1.67 g, 3.64 mmol), 9,10-bis(4-bromophenyl)-2-tert-butylan-thracene (6, 1.00 g, 1.84 mmol), and Aliquat 336 (ca. 220 mg) in toluene (60 mL). The reaction mixture was degassed and then
tetrakis(triphenylphosphine)palladium [Pd(PPh3)4, ca. 25 mg]
was added under a flow of nitrogen. The reaction mixture was
then heated at 85C while stirring under nitrogen. After 10 h, the
reaction mixture was cooled to room temperature and then poured into aqueous 70% MeOH (80 mL). The precipitate was collected by filtration, washed with MeOH, and dried under vacuum. The crude product was purified through column chro-matography (n-hexane/EtOAc, 12:1) to afford BFAn (1.23 g,
65%) as a yellow powder.1H NMR (300 MHz, CDCl3): d 7.85– 7.79 (m, 12H), 7.74–7.62 (m, 5H), 7.58–7.49 (m, 8H), 7.42–7.32 (m, 14H), 7.21 (dd, J¼ 8.2 Hz, 1.5 Hz, 4H), 7.11 (d, J ¼ 6.6 Hz, 4H), 2.36 (s, 6H), 1.31 (s, 9H). 13C NMR (75 MHz, CDCl 3): d 151.3, 147.3, 145.4, 145.3, 142.9, 142.8 140.2, 139.7, 139.4, 139.0, 138.1, 136.5, 136.3, 131.7, 130.9, 130.1, 129.9, 129.6, 129.0, 128.7, 128.6, 128.5, 128.1, 128.0, 127.8, 127.7, 127.4, 126.9, 126.8, 126.2, 124.8, 124.6, 121.1, 65.0, 34.9, 31.2, 20.9. MS (FAB) m/z
1047 [M + 1]. HRMS (FAB) calcd. for C82H63 [M + H]+m/z
1047.4852; found 1047.4932. Anal. Calcd for C82H63: C, 94.03;
H, 5.97. Found: C, 93.77; H, 6.09%.
9,10-Bis(biphenyl-4-yl)-2-tert-butylanthracene (DBPAn)
Aqueous K2CO3 (2.0 M, 10 mL) was added to a solution of
phenylboronic acid (0.44 g, 3.61 mmol), 9,10-bis(4-bromo-phenyl)-2-tert-butylanthracene (6, 1.00 g, 1.84 mmol), and Ali-quat 336 (ca. 220 mg) in toluene (40 mL). The reaction mixture
was degassed and then Pd(PPh3)4(ca. 15 mg) was added under
a flow of nitrogen. The reaction mixture was heated at 85 C
while stirring under nitrogen. After 10 h, the mixture was cooled to room temperature and then poured into aqueous 50% MeOH (80 mL); the yellow precipitate was filtered, washed with MeOH and dried under vacuum. The crude product was purified through column chromatography (n-hexane/EtOAc, 12:1) to
afford DBPAn (0.68 g, 68.7%) as a yellowish powder.1H NMR
(300 MHz, CDCl3): d 7.91–7.72 (m, 12H), 7.62–7.52 (m, 9H),
7.49–7.42 (m, 2H), 7.39–7.35 (m, 2H), 1.31 (s, 9H).13C NMR (75
MHz, CDCl3): d 147.8, 141.3 140.6, 140.4, 138.7, 138.6, 136.9,
136.7, 132.2, 132.1, 130.5, 130.3, 129.3, 127.6, 127.5, 127.3, 125.1, 125.0, 121.6, 35.4, 31.2. MS (FAB) m/z 539 [M + 1]. HRMS
(FAB) calcd. for C42H35[M + H]+m/z 539.2661; found 539.2741.
Anal. Calcd for C42H34: C, 93.64; H, 6.36. Found: C, 93.67; H,
6.36%.
2-tert-Butyl-9,10-bis[40
-(1-phenylbenzoimidazyl)biphenyl-4-yl]anthracene (BIAn)
Aqueous K2CO3(2.0 M, 10 mL) was added to a solution of 5
(0.70 g, 1.76 mmol), 9,10-bis(4-bromophenyl)-2-tert-butylan-thracene (6, 0.48 g, 0.89 mmol), and Aliquat 336 (ca. 220 mg) in toluene (40 mL). The reaction mixture was degassed and then
Pd(PPh3)4(ca. 15 mg) was added under a flow of nitrogen. The
reaction mixture was heated at 85 C while stirring under
nitrogen. After 10 h, the mixture was cooled to room tempera-ture and then poured into aqueous 50% MeOH methanol (80 mL); the yellow precipitate was filtered, washed with MeOH, and dried under vacuum to provide a crude product that was purified through column chromatography (n-hexane/EtOAc, 8:1) to give
BIAn (0.54 g, 65.8%) as a yellowish powder.1H NMR (300 MHz,
CDCl3): d 7.94 (d, J¼ 7.8 Hz, 2H), 7.87–7.77 (m, 9H), 7.72–7.69 (m, 12H), 7.66–7.59 (m, 5H) 7.51–7.49 (m, 8H), 7.45–7.31 (m, 5H), 1.28 (s, 9H).13C NMR (75 MHz, CDCl 3): d 152.5, 147.9 143.4, 142.1, 139.4, 139.3, 137.7, 137.5, 136.8, 136.6, 132.3, 130.0, 129.9, 129.7, 129.3, 129.1, 128.9, 127.9, 127.5, 127.4, 127.3, 127.0, 125.9, 125.4, 123.8, 123.5, 120.2, 110.9, 35.4, 31.2. MS (FAB) m/z
923 [M + 1]. HRMS (FAB) calcd. for C68H51N4[M + H]+m/z
923.4035; found 923.4115. Anal. Calcd for C68H50N4: C, 88.47;
H, 5.46; N, 6.07. Found: C, 88.09; H, 5.46; N, 6.06%. Fabrication of OLEDs
The organic materials used for fabricating the devices were generally purified through high-vacuum, gradient temperature sublimation. The EL devices were fabricated through vacuum
deposition of the materials at 106torr onto glass precoated with
a layer of ITO having a sheet resistance of 25 U square1. A LiF/
Al cathode was deposited through sequential evaporation of LiF and Al metal. The electron-transporting material was TPBI or BIAn; the bilayer hole-transporting layers comprised NPB and
TCTA. The effective area of the emitting diode was 3.1 mm2. The
current, voltage, and light intensity were measured simulta-neously using a Keithley 2400 source meter and a Newport 1835-C optical meter equipped with a Newport 818-ST silicon photodiode. Electroluminescence spectra were measured using a Hitachi F-4500 fluorescence spectrophotometer.
Acknowledgements
We thank the National Science Council for funding and Professor C.-H. Cheng for his support during the preparation and characterization of the OLEDs.
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