Highly efficient red electrophosphorescent device incorporating a bipolar
triphenylamine/bisphenylsulfonyl-substituted fluorene hybrid as the host
Fang-Ming Hsu,
aChen-Han Chien,
aYa-Jou Hsieh,
aChen-Hao Wu,
aChing-Fong Shu,*
aShun-Wei Liu
band Chin-Ti Chen*
bReceived 27th May 2009, Accepted 19th August 2009
First published as an Advance Article on the web 15th September 2009 DOI: 10.1039/b910292b
We have fabricated highly efficient red phosphorescent organic light-emitting diodes (PHOLEDs) incorporating a bipolar host material, 2,7-bis(phenylsulfonyl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (SAF), doped with 7 wt% tris(1-phenylisoquinolinolato-C2,N)iridium(III) [Ir(piq)
3].
Attaching the electron-donating (p-type) triphenylamine group onto the electron-accepting (n-type) 2,7-bis(phenylsulfonyl)fluorene segment (through the C9 position of the fluorene unit) imparts SAF with good morphological stability, high triplet energy gap (ET), bipolar transporting ability, and
matching energy levels with adjacent carrier-transporting layers. Consequently, the SAF-based red-PHOLED exhibited a very low turn-on voltage (2.4 V) and high electroluminescence efficiencies of 15.8% and 22.0 lm W1, superior to those of the corresponding device incorporating a conventional host material, 4,40-N,N0-dicarbazolbiphenyl (CBP; 3.2 V, 8.5%, and 8.4 lm W1, respectively). At a practical brightness of 1000 cd m2, the efficiencies of the SAF-based red-PHOLED remained high (13.1%, 14.4 lm W1).
Introduction
Phosphorescent organic light-emitting diodes (PHOLEDs) have attracted much attention because they can, in theory, approach 100% internal quantum efficiency by harvesting both singlet and triplet excitons.1–10In these PHOLEDs, the triplet emitters are usually doped into a host material to reduce the self-quenching associated with the relatively long excited state lifetimes of triplet emitters and triplet–triplet annihilation.11Thus, the development of effective hosts and phosphors is equally important for the fabrication of highly efficient PHOLEDs. Being one of the primary colors, the efficient emission of red light from OLEDs is critical for applications in full-color displays and solid state lighting.12–16 Realizing efficient red-PHOLEDs, however, remains a great challenge because of the lack of suitable host materials. Although the carbazole derivative 4,40-N,N0
-dicarba-zolbiphenyl (CBP) has been used widely as a host for red-PHOLEDs, they usually require high driving voltages because the wide energy gap of CBP leads to large energy barriers for charge injection from adjacent carrier-transporting layers. In addition, CBP is inherently a p-type material; its hole mobility is at least one order of magnitude higher than its electron mobility.17,18 Moreover, CBP exhibits a relatively low glass-transition temperature (Tg¼ 62C)19and is prone to
crystalli-zation.20–22 Its poor thermal and morphological stabilities may reduce CBP’s ability to bear joule heat during device operation.
In this study, we synthesized a host material, SAF, comprising electron-rich triphenylamine and electron-deficient 2,7-bis(phe-nylsulfonyl)fluorene units linked through the C9 position of the fluorene moiety. This molecular design endows SAF with several advantageous properties: (i) bipolarity, arising from the presence of the electron-accepting phenylsulfonyl groups appended to the triphenylamine/fluorene hybrid,23–25 thereby facilitating both electron and hole injection and transport; (ii) a high triplet energy gap (ET¼ 2.71 eV), resulting from the individual building blocks
being connected through a nonconjugated linkage (the sp3 -hybridized C9 atom of the fluorene unit), to effectively suppress reverse energy transfer from the guest emitter to the SAF host; and (iii) a sterically bulky and hindered structure imparting good thermal and morphological stabilities. As a result, we expected SAF to be an appropriate host for red-PHOLEDs. Indeed, an SAF-based red-PHOLED doped with 7 wt% tris(1-phenyl-isoquinolinolato-C2,N)iridium(III) [Ir(piq)
3] exhibited a very low
turn-on voltage (2.4 V) and high electroluminescence (EL) effi-ciencies of 15.8% and 22.0 lm W1, superior to those of the reference device based on the conventional material CBP (3.2 V, 8.5%, and 8.4 lm W1, respectively).
Recently, we reported an efficient bipolar host 2,7- bis(diphenylphosphoryl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (POAPF) for a blue-PHOLED.26However, the relatively weak electron-withdrawing nature of diphenylphos-phoryl segments in the POAPF molecule leads to an elevated lowest unoccupied molecular orbital (LUMO) level (2.40 eV), which may impede electron injection from the electron-trans-porting layer. In addition, the mismatch of the LUMO energy level of POAPF with that of Ir(piq)3 (3.10 eV) would cause
serious electron trapping at the dopant sites. Both of these influences will significantly increase the operating voltage of the red-PHOLED based on POAPF.
aDepartment of Applied Chemistry, National Chiao Tung University,
Hsinchu, 300 Taiwan. E-mail: [email protected]
bInstitute of Chemistry, Academia Sinica, Taipei, Taiwan 11529, Republic
of China
ª The Royal Society of Chemistry 2009
PAPER www.rsc.org/materials | Journal of Materials Chemistry
Results and discussion
Synthesis and characterizationScheme 1 illustrates the synthetic route followed for the prepa-ration of the bipolar host SAF. The starting material, 2,7-dibromofluorenone, was treated with thiophenol in the presence of potassium carbonate in N,N-dimethylformamide (DMF) at 130C to afford the disulfide 1, which was then reacted with the Grignard reagent of bromobenzene to give the corresponding alcohol 2. Subsequent oxidation with 30% aqueous hydrogen peroxide yielded the disulfonyl intermediate 3. The target compound SAF was obtained from the acid-promoted Friedel– Craft-type substitution of 3 with excess triphenylamine. We characterized the structure of SAF using 1H and 13C NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis.
Thermal properties
SAF exhibits very high thermal stability; thermogravimetric analysis (TGA) performed under a nitrogen atmosphere revealed that the decomposition temperature (Td, corresponding to 5%
weight loss) was 430C. The high value of T
dof SAF implies that
it is capable of enduring vacuum thermal sublimation, which meets one of the basic requirements of a host to be used in PHOLEDs. From differential scanning calorimetry (DSC) measurements, we observed a distinct glass transition tempera-ture (Tg) at 127C—much higher than that of the conventional
host material CBP (Tg ¼ 62 C)19—and no exothermic peak
resulting from crystallization at temperatures up to 280C. We
attributed these superior thermal properties to the nonplanar, rigid structure of SAF, featuring a bulky triphenylamine group and two phenylsulfonyl units appended at the 9- and 2,7-posi-tions of the fluorene ring, respectively. It is important for OLEDs to be constructed from materials having relatively high values of Tg if they are to avoid degradation through morphological
changes of the amorphous organic layer.27–29To further investi-gate the morphological stability of the emitting layer (EML) within the red-PHOLEDs, we used vacuum deposition to prepare SAF and CBP thin films (30 nm) doped with 7 wt% Ir(piq)3 on silicon wafers and then measured their surface
morphologies using atomic force microscopy (AFM) before and after annealing at 90C for 12 h under a nitrogen atmosphere. Fig. 1 reveals that the SAF-based thin film had a fairly stable surface morphology, which did not change upon annealing. The root mean square (RMS) roughness before and after annealing were 0.42 and 0.45 nm, respectively. On the other hand,
annealing the CBP-based film induced a degradation of the sur-face morphology, with large crystals appearing on the annealed film. The RMS roughness before and after the annealing of the CBP-based film were 0.57 and 18.6 nm, respectively. SAF maintained its morphological stability in the glassy state for long periods of heat exposure.
Photophysical properties
Fig. 2a displays the UV-Vis absorption and photoluminescence (PL) spectra of SAF measured under different conditions; Table 1 summarizes the key spectral data. The absorption profiles of SAF in cyclohexane, toluene, and THF are nearly identical—i.e., independent on solvent polarity—indicating that the Franck– Condon excited state is subject to a rather small dipolar change with respect to the ground state. In sharp contrast, the fluores-cent maxima of SAF revealed a salient bathochromic shift: from
Scheme 1 Synthetic route for SAF.
Fig. 1 AFM images of 7 wt% Ir(piq)3-doped films based on the hosts (a)
SAF without annealing, (b) SAF with annealing, (c) CBP without annealing, and (d) CBP with annealing.
Fig. 2 (a) Room-temperature absorption and PL spectra (excited at 335 nm) of SAF in cyclohexane (-), toluene (C), and THF (:) solu-tions and as a solid film (B). The absorption spectrum of the Ir(piq)3film
is also depicted (O). All films were formed on quartz substrates through thermal evaporation (30 nm). (b) Emission peak frequencies plotted as a function of the solvent polarities for SAF in various solutions. The solvent polarity parameter function Df is expressed using the formula Df¼ [(3 1)/(23 + 1)] [(n2 1)/(2n2+ 1)], where 3 is the static dielectric
constant and n is the optical refractivity index of the solvent.
ª The Royal Society of Chemistry 2009
460 nm in nonpolar cyclohexane to 560 nm in highly polar THF. Furthermore, the excitation spectra of SAF, within experimental error, were effectively identical to the absorption spectra, indi-cating that the emissions result from a common Franck–Condon excited state. In addition, when we varied the concentration of SAF in each solution over the range 104 to 106 M, the absorption and emission profiles all exhibited concentration-independence. Thus, we can eliminate the possibility of the emission being caused by aggregates and/or excimers. We tentatively attribute the distinct solvatochromism in PL spectra of SAF to a mechanism involving rapid photoinduced electron transfer between the electron donor (triphenylamine) and the electron acceptor [2,7-bis(phenylsulfonyl)fluorene], resulting in a large change in the dipole moment in the excited state; a subsequent solvent relaxation process leads to the solvent polarity-dependent emission. When we plotted the emission peak frequencies of SAF in various organic solvents (from cyclo-hexane to ethanol) as a function of the solvent polarity (Fig. 2b), we obtained a linear relationship together with a slope as steep as 14 240 cm1, consistent with our assignment of a charge-transfer emission. Similar results were reported and reviewed for other bipolar-type molecules.30–32
Moreover, there is an effective overlap between the PL spec-trum of SAF and the absorption specspec-trum of the red-emitting phosphor Ir(piq)3(Fig. 2a). Thus, we anticipated that efficient
F€orster energy transfer of singlet excitons would occur from SAF to Ir(piq)3. In the EL spectrum of the 7 wt% Ir(piq)3–doped device
(vide infra), we observe only the red emission from the triplet dopant, indicating the complete energy transfer from SAF to Ir(piq)3. The absorption spectra (not shown) of the blue-emitting
bis[(4,6-difluorophenyl)pyridinato-N,C20
](picolinato)iridium(III) (FIrpic) and green-emitting fac-tris(2-phenylpyridinato)-iridium(III) [Ir(ppy)3] overlap poorly with the emission spectrum
of SAF, thereby prohibiting efficient energy transfer between the host and these phosphors. In addition, the nonconjugated linkage between the n- and p-type groups endows SAF with a high value of ETof 2.71 eV, estimated from the highest-energy 0–0
phospho-rescent emission measured in frozen 2-methyltetrahydrofuran at 77 K. This value of ETis sufficiently high for SAF to undergo
efficient exothermic energy transfer of triplet excitons to the red triplet emitter Ir(piq)3(ET¼ 2.0 eV)33without reversing.
Electrochemical properties
Fig. 3 displays the cyclic voltammograms of SAF and CBP in solutions containing ferrocene/ferrocenium (Fc/Fc+) as the internal standard. Table 2 summarizes the redox onset potentials and calculated energy levels. During cathodic sweeping in DMF, both SAF and CBP exhibited reversible reduction waves, with
onset potentials at1.92 and 2.70 V, respectively. The end-capping of the two electron-withdrawing phenylsulfonyl units at the fluorene group provided SAF with a less negative reduction potential relative to that of CBP. When performing anodic scans in CH2Cl2, SAF and CBP underwent oxidation processes with
onset potentials of 0.49 and 0.72 V, originating from their elec-tron-rich triphenylamine and carbazole segments, respectively. The oxidation of SAF occurred at a lower potential, indicating that the oxidation process was facilitated upon replacing the carbazole building block with a triphenylamine unit.25On the basis of the onset potentials for oxidation and reduction, we estimated the highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO) levels of SAF and CBP with regard to the energy level of ferrocene (4.8 eV below vacuum). Fig. 4 displays the relative HOMO/LUMO energy levels of the mate-rials used to prepare the devices. SAF possesses lower LUMO (2.88 eV) and shallower HOMO (5.29 eV) levels than those of CBP (2.10 and 5.52 eV, respectively), revealing its superior ability to inject electrons and holes.
Electroluminescence properties
To evaluate the bipolar characteristics of SAF, we prepared hole-only devices having the architecture indium tin oxide (ITO)/N,N0-diphenyl-N,N0-bis(3-methylphenyl)-(1,10 -biphenyl)-4,40-diamine (TPD, 30 nm)/host (30 nm)/TPD (30 nm)/Al
(100 nm) and electron-only devices having the configuration
Table 1 Optical and thermal properties of SAF
Host
labsa[nm] lem, max[nm]
ET[eV] Tg/Td[C]
cHex Tol THF Film cHex Tol THF Film
SAF 299, 321 299, 323 299, 323 301, 325 460 506 560 528 2.71 127/430
aAbsorption wavelength, measured in cyclohexane (cHex), toluene (Tol), and tetrahydrofuran (THF) solutions and in the thin film.
Fig. 3 Cyclic voltammograms of SAF and CBP.
Table 2 Electrochemical data for the hosts Host Eonset
ox [V] Eonsetred [V] HOMO [eV] LUMO [eV]
SAF 0.49 1.92 5.29 2.88
CBP 0.72 2.70 5.52 2.10
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ITO/4,7-diphenyl-1,10-phenanthroline (BPhen, 30 nm)/host (30 nm)/BPhen (30 nm)/LiF (15 A˚ )/Al (100 nm). Here, the host materials were SAF and the conventional host CBP. In the case of the hole-only devices, because of the large energy injection barrier between TPD (LUMO¼ 2.20 eV) and Al (4.30 eV), most electrons were restrained from injection into the organic layer. Only holes could be injected from the anode to the organic layers; therefore, the measured current density–voltage (I–V) characteristics were dominated by holes. On the other hand, in the electron-only devices, the BPhen layer possessed a large HOMO energy level (–6.40 eV) and, therefore, it behaved as a hole-blocker to inhibit the injection of holes from the ITO anode (4.80 eV) to the organic layer. Thus, the measured I–V curves mostly reflect the injected electrons. Fig. 5 reveals that both the SAF-based hole- and electron-only devices exhibited lower turn-on voltages and higher current densities than did the corresponding CBP-based devices under the same bias. We attribute the reduced driving voltages in the SAF-based devices to the decreased barrier heights for hole and electron injection from the charge transport layers to SAF. From these studies of carrier-only devices, we anticipated that the use of SAF as a host in PHOLEDs would provide suitable energy levels to facilitate carrier injection.
Furthermore, we measured the carrier mobility in the SAF film using time-of-flight (TOF) transient photocurrent techniques. The bipolar charge-transporting capability of SAF was evidenced by the similar values for the electron (3.4 105cm2V1s1) and hole mobilities (3.5 105cm2 V1 s1) under an electric
field of 4.1 105 V cm1. In contrast, the electron and hole mobilities of CBP were on the orders of 104and 103cm2V1s1, respectively,17,18indicating relatively unbalanced charge fluxes in the CBP-based EML. Although the electron and hole mobilities of SAF were more than one order of magnitude lower than those of CBP, the measured current densities of the SAF-based carrier-only devices (Fig. 5) were higher than those of its CBP counter-parts at the same driving voltages. This behavior may suggest that the current fluxes in CBP carrier-only devices are significantly limited by the potential barriers presented at the carrier-transporting layer–CBP interfaces, with the device currents being dictated primarily by the efficiencies of carrier injection (i.e., energy barriers), rather than the carrier mobility of the host material.34,35
Subsequently, we doped the bipolar host material SAF with a commercially available triplet phosphor, Ir(piq)3, to realize
a highly efficient red-PHOLED. The device was fabricated in the configuration ITO/TPD (30 nm)/EML (30 nm)/BPhen (30 nm)/ LiF (15A˚ )/Al (100 nm). Here, ‘‘EML’’ refers to a layer of SAF doped with 7 wt% Ir(piq)3; TPD and BPhen functioned not only
as hole- and electron-transporting layers but also as the carrier-blockers for effective emission.36,37 For comparison, we also fabricated the reference device incorporating the conventional host material CBP. Fig. 6 displays the current density–voltage– luminance (I–V–L) characteristics of these red-PHOLEDs. The SAF-based device exhibited significantly lower operating volt-ages than its CBP counterpart under the same current density, presumably because of the smaller energy barriers when using SAF as the host material, as demonstrated in the carrier-only devices (vide supra). As a result, the SAF device had a relatively low turn-on voltage of 2.4 V (corresponding to 1 cd m2). In addition, the SAF-based device exhibited a much better peak external quantum efficiency (EQE, 15.8% at 21.9 cd m2) than its CBP counterpart (8.5% at 93.0 cd m2). We attribute this behavior to the relatively balanced bipolar transport of SAF (vide supra), thereby increasing the opportunity for carrier recombination, and the higher value of ET of SAF (2.71 eV)
relative to that of CBP (2.56 eV), thereby effectively suppressing reverse energy transfer from the triplet dopant to the host.20The combination of a low operating voltage and a high EQE afforded our SAF-based device with an extremely high power efficiency (PE) of 22.0 lm W1. Even at a practical brightness of 1000 cd m2, the operating voltage was merely 3.6 V and the
Fig. 4 Relative HOMO/LUMO energy levels of the materials used for the EL devices.
Fig. 5 I–V characteristics of (a) hole- and (b) electron-only devices. Fig. 6 I–V–L characteristics of Ir(piq)3–doped devices. ª The Royal Society of Chemistry 2009
efficiencies remained high (13.1%, 14.4 lm W1). Fig. 7 presents plots of the EQE and PE with respect to the current density for these red-electrophosphorescent devices; Table 3 lists their key characteristics. Moreover, the EL spectra of the SAF-based
device exhibited almost the same profiles, with the maximum signal intensity at 622 nm, over the entire range of operating voltages (Fig. 8), with Commission Internationale de L’Eclairage (CIE) coordinates at the standard red (0.67, 0.33) demanded by the NTSC. The spectra were assigned to the emission from Ir(piq)3, without any other emission originating from the
adja-cent organic layer, indicating the excellent confinement of carriers within the EML.
Conclusion
We have synthesized a novel bipolar host (SAF) for the fabri-cation of highly efficient Ir(piq)3-based red-PHOLEDs. SAF was
constructed by linking an electron-rich triphenylamine group to an electron-deficient bis(phenylsulfonyl)-substituted fluorene through the C9 atom of its fluorene unit. This geometry afforded SAF with a rather bulky and nonplanar structure and, hence, high thermal and morphological stabilities. In addition, the introduction of a nonconjugated linkage resulted in a high value of ET (2.71 eV), which effectively prevented reverse energy
transfer from the triplet emitter to the SAF host. More impor-tantly, the favorable bipolar characteristics of SAF facilitated both hole and electron injection from adjacent carrier-trans-porting layers and balanced charge fluxes within the EML. Consequently, the 7 wt% Ir(piq)3-doped SAF device exhibited
significantly improved EL efficiencies (15.8% and 22.0 lm W1) relative to those of the reference device incorporating the conventional host CBP (8.5%, 8.4 lm W1).
Experimental section
Characterization1H and13C NMR spectra were recorded using Varian UNITY INOVA 500 MHz and Varian UNITY 300 MHz spectrometers. Mass spectra and high-resolution mass spectra (HRMS) were obtained using JEOL JMS-HX 110 and Finnigan Thermo Quest MAT 95XL mass spectrometers. Differential scanning calorim-etry (DSC) was performed using a SEIKO EXSTAR 6000DSC unit operated at heating and cooling rates of 10 and 50C min1, respectively. The glass transition temperature (Tg) was
deter-mined from the second heating scan. Thermogravimetric analysis (TGA) was undertaken using a DuPont TGA 2950 instrument. The thermal stability of the samples under a nitrogen atmosphere was determined by measuring their weight losses while heating at a rate of 20 C min1. Atomic force microscopy (AFM) measurements were performed in the tapping mode under ambient conditions using a Digital Nanoscope IIIa instrument. UV-Vis spectra were measured using an HP 8453 diode array spectrophotometer. PL spectra were obtained using a Hitachi F4500 luminescence spectrometer. The low-temperature phos-phorescence spectra were obtained using a composite spec-trometer containing a monochromator (Jobin Yvon, Triax 190) coupled with a liquid nitrogen-cooled charge-coupled device (CCD) detector (Jobin Yvon, CCD-1024 256-open-1LS). Cyclic voltammetry (CV) measurements were performed using a BAS 100 B/W electrochemical analyzer operated at a scan rate of 100 mV s1. The potentials were measured against an Ag/Ag+ (0.01 M AgNO3) reference electrode using ferrocene/ferrocenium
(Fc/Fc+) as the internal standard.
Fig. 7 (a) EQE and (b) PE of the Ir(piq)3-doped devices plotted with
respect to the current density.
Table 3 Electroluminescence data for the devices
Host SAF CBP Va[V] 2.4/3.6 3.2/4.9 EQEb[%] 15.8/13.1 8.5/7.3 PEb[lm W1] 22.0/14.4 8.4/5.7 LEb[cd A1] 19.6/16.2 10.6/8.8 CIEc[x, y] 0.67,0.33 0.67,0.33
aMeasured operating voltages, presented in the order of the values at 1
and 1000 cd m2. bMeasured values, presented in the order of the
maximum value and the value at 1000 cd m2.cMeasured at 7 V.
Fig. 8 EL spectra of the SAF-based device recorded under various operating voltages.
ª The Royal Society of Chemistry 2009
Carrier mobility
The carrier mobility was measured using time-of-flight transient photocurrent techniques at ambient temperature. The sample was fabricated in the configuration glass/ITO (100 nm)/SAF (1 mm)/Ag (20 nm). The SAF layer was deposited at a constant rate of 0.15 nm s1. During the film deposition of the devices, the pressure of the chamber was maintained below 2 106torr to minimize defects and the evaporation rates of the organic materials were controlled using respective quartz-crystal moni-tors. The thickness of the organic layer was measured using a surface profiler (Dektak 150; Veeco). The frequency-doubled output of a 5 ns-pulsed dye laser at 295 nm was chosen to maximize the optical absorption of the organic layer. The laser beam was directed through the samples from the Ag side to create photoexcited carriers. The excitation power density was set at ca. 1.6 W cm2to avoid space-charge effects. The instru-mental response time was maintained much shorter than the transit time. The transient photocurrent was recorded by measuring the voltage across a 50 U resistor using a 1 GHz digital oscilloscope (LC574 AM, LeCroy). The transit time, tT, was
taken from the intersection of two clear straight lines in the double-logarithmic plot of the transient photocurrent profile. The carrier mobility m was given by m¼ D/tTE, where D is the
thickness of the organic layer and E is the applied electric field. The detection of the transient photocurrent signal was performed using a process similar to one described previously.38
Fabrication of PHOLEDs
The EL devices were fabricated through vacuum deposition of the materials at 106torr onto ITO glass having a sheet resistance of 25 U square1. All of the organic layers were deposited at a rate of 1.0 A˚ s1. The cathode was completed through thermal deposition of LiF at a deposition rate of 0.1 A˚ s1; it was then capped with Al metal through thermal evaporation at a rate of 4.0 A˚ s1. The current density–voltage and brightness–voltage relationships of the devices were measured using a Keithley 2400 source meter and a Newport 1835C optical meter equipped with an 818ST silicon photodiode. The EL spectrum was obtained using a Hitachi F4500 luminescence spectrometer.
2,7-Bis(phenylthio)fluorenone (1)
2,7-Dibromofluorenone (3.06 g, 9.05 mmol) was added to a mixture of K2CO3 (2.76 g, 20.0 mmol) and
N,N-dime-thylformamide (150 mL). Thiophenol (1.95 mL, 19.0 mmol) was added with stirring and then the solution was heated at 130C
for 6 h. The reaction mixture was cooled to room temperature and poured into water (600 mL). The orange precipitate was collected by filtration, washed with water, and dried under vacuum. The solid was recrystallized (ethyl acetate/n-hexane) to afford orange needles (2.94 g, 82%). 1H NMR (300 MHz, CDCl3, d): 7.51 (s, 2H), 7.43–7.28 (m, 14H);13C NMR (75 MHz,
CDCl3, d): 192.83, 142.51, 138.85, 136.19, 135.19, 134.32, 132.44,
129.85, 129.31, 126.20, 121.12; MS (EI, m/z): 396 [M+].
2,7-Bis(phenylthio)-9-phenyl-9-fluorenol (2)
The Grignard reagent was prepared from Mg powder (0.37 g, 15.2 mmol) and bromobenzene (1.71 mL, 16.2 mmol) in ether (150 mL). 1 (5.08 g, 12.8 mmol) was added into the cooled Grignard solution. The reaction mixture was stirred for 8 h at room temperature and then it was quenched with saturated NH4Cl aqueous solution (150 mL) and extracted with ethyl
acetate (3 100 mL). The collected organic phase was dried (MgSO4) and concentrated under reduced pressure. The residue
was washed with hot n-hexane (150 mL) to give a pale-orange solid (5.50 g, 90%).1H NMR (300 MHz, CDCl 3, d): 7.58 (d, J¼ 7.8 Hz, 2H), 7.38–7.24 (m, 19H), 2.50 (s, 1H); 13C NMR (75 MHz, CDCl3, d): 151.62, 142.64, 138.28, 136.71, 135.84, 132.17, 131.30, 129.58, 128.69, 127.87, 127.76, 127.53, 125.62, 121.10, 83.74; MS (EI, m/z): 474 [M+]. 2,7-Bis(phenylsulfonyl)-9-phenyl-9-fluorenol (3)
30% Aqueous hydrogen peroxide (30 mL) was added to a solu-tion of 2 (2.58 g, 5.43 mmol) in acetic acid (70 mL). The reacsolu-tion mixture was stirred at 80C for 24 h and then it was cooled to
room temperature, saturated with NaCl (ca. 15 g), and extracted with CH2Cl2(4 70 mL). The combined organic phases were
dried (MgSO4) and the solvents evaporated under reduced
pressure to provide a pale-yellow residue, which was recrystal-lized from a mixture of CHCl3and n-hexane to give a white solid
(2.76 g, 95%).1H NMR (300 MHz, CDCl 3, d): 7.94–7.90 (m, 5H), 7.87–7.84 (m, 5H), 7.77–7.73 (m, 3H), 7.56–7.42 (m, 8H);13C NMR (75 MHz, CDCl3, d): 152.56, 142.98, 142.55, 141.38, 140.72, 133.61, 129.61, 128.97, 128.47, 127.86, 125.36, 124.64, 122.06, 83.60; MS (EI, m/z): 538 [M+]. 2,7-Bis(phenylsulfonyl)-9-(4-N,N-diphenylamino)phenyl-9-phenylfluorene (SAF)
CF3SO3H (1.10 mL, 12.5 mmol) was added dropwise to a
solu-tion of triphenylamine (4.85 g, 19.8 mmol) and 3 (5.26 g, 9.78 mmol) in CH2Cl2 (200 mL). The mixture was stirred for
30 min at room temperature and then quenched with saturated aqueous NaHCO3 solution (100 mL). The organic phase was
washed twice with water (100 mL) and then it was dried (MgSO4). After evaporation of the volatiles, the residue was
purified through column chromatography (SiO2; CH2Cl2/
hexane, 1:5) to yield SAF as a yellow solid (6.14 g, 82%).1H NMR (300 MHz, [D8]THF, d): 8.11 (s, 2H), 8.07 (d, J¼ 8.1 Hz, 2H), 7.98 (dd, J¼ 8.1, 1.5 Hz, 2H), 7.89 (d, J ¼ 6.9 Hz, 4H), 7.55–7.43 (m, 6H), 7.25–7.14 (m, 9H), 7.04–6.98 (m, 8H), 6.88 (d, J ¼ 8.7 Hz, 2H); 13C NMR (75 MHz, [D 8]THF, d): 153.48, 147.79, 147.33, 144.12, 143.05, 142.99, 142.69, 137.26, 133.08, 129.34, 128.72, 128.69, 127.89, 127.86, 127.69, 127.39, 125.89, 124.62, 123.18, 123.11, 122.35, 65.67; HRMS (FAB, m/z): calcd for C49H36NO4S2 [M + H]+, 766.2086; found, 766.2073; Anal.
Calcd for C49H35NO4S2: C 76.84, H 4.61, N, 1.83; found:
C 76.46, H 4.70, N 1.72.
Acknowledgements
We thank the National Science Council for funding and Professor C.-H. Cheng for his support during the preparation and characterization of the PHOLEDs.
ª The Royal Society of Chemistry 2009
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