Synthetic Metals 155 (2005) 206–210

A novel fluorene derivative containing four triphenylamine groups:Highly thermostable blue emitter with hole-transporting

ability for organic light-emitting diode (OLED)

Qiang Fanga,∗, Bing Xua, Biao Jianga,∗, Haitao Fua, Wenqing Zhub,Xueyin Jiangb, Zhilin Zhangb

a Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, PR Chinab College of Materials, Shanghai University, Shanghai 200436, PR China

Received 28 February 2005; received in revised form 12 August 2005; accepted 17 August 2005Available online 13 October 2005

Abstract

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Four triphenylamine groups-substituted fluorene with rather high glass transition temperature,Tg, of 165 C, was prepared and employed togewith TPD as a composite hole-transporting layer to give a device with very low turn-on voltage and high efficiency. When a hole blockfabricated, the new compound can be used as a light-emitting layer for OLED to give a device emitting blue color.© 2005 Elsevier B.V. All rights reserved.

Keywords: Organic light-emitting diode (OLED); Triphenylamine derivatives; Fluorene; Blue electroluminescent materials; Hole-transporting materials

1. Introduction

One of the important applications of triarylamines is ashole-transporting materials (HTMs) in organic light-emittingdiodes (OLEDs)[1]. To have an efficient and practical OLEDdevice, the requirements for triarylamines include low ion-ization potentials, highly thermal stability, ease of formingamorphous glass and high quantum yield of photoluminescence[1–4]. For majority of triarylamine derivatives used in OLED,their ionization ability is satisfactory[1] whereas there are stillneeds for improving the thermal stability, the ability for formingamorphous glass and quantum efficiency of luminescence[4]. For example, the most widely used HTMs,N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine(TPD) and 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl(NPB), have rather low glass transition temperatures,Tg,of 65 and 95◦C, respectively[2,5]. On the other hand, thewidely used electron-transporting material such as tris(8-hydroxyquinoline)aluminum (Alq), has aTg of 170◦C [6]. Thelow Tg of the HTMs implies that their thin films are easily to be

∗ Corresponding authors. Fax: +86 21 6416 6128.E-mail address: qiangfang@yahoo.com (Q. Fang).

physically transformed upon operation at high temperaturhigh power density. In addition, although films of TPD are amphous when deposited, they tend to crystallize in air at roomperature and at higher temperatures (60–80◦C) in the absencof air [7]. This tendency makes the device failure. Thus, mefforts[1,4,8–14]have been made to develop new amorphouarylamines with high thermal stability. Some examples incdendritic molecules with a triarylamine core[1], triarylamineswith spiro linkage[13], and isoindole derivatives[14]. Howeverthose endeavors are far from satisfactory on the whole. In mcases, some triarylamines have highTg, nevertheless they hadefinite degree of crystallization[14]; or some triarylamines caform stable amorphous glass, but their glass transition temtures are usually lower (in a range of 100–120◦C) [4]. In partic-ular, among the triarylamines mentioned above, a few emitrescence with high quantum yield. Further, many triarylamwere synthesized using complicated procedure and expereactants[1,4]. Undoubtedly, it is very desirable to exploand develop new triarylamines that meet the requiremfor OLED and can be synthesized using simple and low-processes.

It is known that fluorene derivatives have high quantum yof photoluminescence and high thermal stability[15]. Inspiredby this result, we design and synthesize the following new t

0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved.d

oi:10.1016/j.synthmet.2005.08.007

Q. Fang et al. / Synthetic Metals 155 (2005) 206–210 207

Scheme 1. The structure and the preparation procedure ofXB10. Regents and conditions: (i) NBS in DMF at room temperature (r.t.) for 24 h. (i) (a)n-BuLi in hexane(1.57 M), THF at−78◦C for 1 h; (b) B(OMe)3, stirred for 1 h at r.t.; (c) glycol/toluene (1:2, v/v), reflux for 10 h. (iii) CF3SO3H, at 140◦C for 8 h; (iv) aq. Na2CO3,Pd(PPh3)4, toluene/diether glycol, reflux for 36 h.

enylamine derivative containing a fluorene core,XB10, whosechemical structure and the synthetic procedure are shown inScheme 1. It is expected that a combination of rigid fluorine andmorphological triphenylamine could not only remain the ther-mal and morphological stability but also improve the quantumefficiency.

2. Experimental

2.1. Materials

All starting materials were purchased from Aldrich and Acroscompanies and used without further purification.

2.1.1. Triphenylamine boronic ester (1)A solution of butyllithium in hexane (1.57 M, 6.4 mL,

10 mmol) was added dropwise to a stirring solution of 4-bromotriphenylamine (3.24 g, 10 mmol) in dry THF (100 mL)over 1 h under argon at−78◦C. The mixture was stirred atthis temperature for 1 h and followed by dropwise addition ofB(OMe)3 (2.6 ml, 25 mmol). After addition, the solution waswarned to room temperature and stirred for 1 h. The reactionmixture was condensed, and the residue was dissolved in a mix-ture of glycol and toluene (100 mL, 1:2, v/v). The mixture wasallowed to react under reflux for 10 h and then cooled to roomt mLo rinea l-v singa elu-e 3 g,4

2d

ol),t1a sives dis-s shedw ver

anhydrous Na2CO3. After removing the solvent, the residue waspurified by chromatography using a mixture of petroleum ether(60–90◦C) and dichloromethane (2:1, v/v) as eluent at SiO2 togive 2 (0.633 g, yield, 78%).

2.1.3. 2,7-Bis(4-diphenylaminophenyl)-9,9-bis(4-diphenylaminophenyl)-fluorene (XB10)

To a 100-mL Schlenk tube, charged with1 (1.26 g, 4 mmol),2(0.81 g, 1 mmol) and Pd(PPh3)4 (115 mg, 0.1 mmol), was addeda degassed mixed solvent of toluene (10 mL) and diether glycol(10 mL). The mixture was stirred for 20 min at room temper-ature under argon, and then a degassed 2 M Na2CO3 aqueoussolution (4 mL, 8 mmol) was added to the Schlenk tube. Thereaction mixture was heated to reflux and stirred at this tem-perature for 36 h. After being cooled to room temperature, themixture was diluted with 200 mL of ethyl acetate. The organiclayer was washed with saturated brine and dried over anhy-drous Na2SO4. After evaporation of the solvent, the residuewas treated by chromatography using a mixture of hexane anddichloromethane (1:1, v/v) as the eluent.XB10 was obtained asa white powder (696 mg, 61% yield).1H NMR (CDC13): δ 8.72(s, 2H), 7.80 (d,J = 7.8 Hz, 2H), 7.57–7.64 (m, 8H), 7.19–7.30(m, 16H), 7.12–7.17 (m, 16H), 7.02–7.08 (m, 12H), 6.96–7.00(m, 4H), 6.93 (d,J = 9.0 Hz, 2H);13C NMR (CDCl3): δ 64.7,120.53, 120.55, 122.9, 123.07, 123.11, 123.9, 124.5, 126.3,127.9, 129.1, 129.3, 129.4, 135.3, 138.7, 139.7, 140.1, 146.3,18 4,7

2

RX4 wereo tachiF C andT eat-i ft utionc g( ntere r thec

emperature. The obtained solution was diluted with 200f toluene. The organic layer was washed with saturated bnd dried over anhydrous Na2SO4. After evaporation of the soent, the crude product was purified by chromatography umixture of hexane and dichloromethane (1:1, v/v) as

nt at SiO2. 1 was thus obtained as a white powder (1.59% yield).

.1.2. 9,9-Bis(4-diphenylaminophenyl)-2,7-ibromofluorene (2)

To a 100-mL flask, 2,7-dibromofluorene (338 mg, 1 mmriphenylamine (3.43 g, 14 mmol) and CF3SO3H (91 mg,mmol) were charged. The mixture was heated to 140◦C underrgon. After maintaining this temperature for 8 h under intentirring, the mixture was cooled to room temperature andolved into dichloromethane. The resulting solution was waith 10% Na2CO3 and water. The organic layer was dried o

,47.2, 147.68, 147.73, 152.5; MALDI-MS: 1138 (M+), 1061,94, 817, 740; FT-IR (KBr):υ (cm−1) 1590, 1492, 1274, 8151, 694.

.2. Instrumentation

1H (13C) NMR spectrum was recorded on a Bruker D00 spectrometer. UV–vis and photoluminescent spectrabtained with a Hitachi 2800 spectrophotometer and a Hi-4500 fluorescence spectrophotometer, respectively. DSGA were determined With a Du Pont 2100 analyzer at a h

ng rate of 10◦C min−1 in nitrogen. Cyclic voltammetry ohe compounds was performed in an dichloromethane solontaining [Bu4N]BF4 (0.10 M, Bu = butyl) under argon usin0.10 M AgNO3)/Ag and platinum wire as reference and coulectrodes, respectively. A CHI 600B analyzer was used foyclic voltammetry.

208 Q. Fang et al. / Synthetic Metals 155 (2005) 206–210

Fig. 1. UV–vis spectra ofXB10.

3. Results and discussion

3.1. Preparation, optical and thermal properties of XB10

As shown inScheme 1, the precursors1, boronic monoesterof triphenylamine, was synthesized by employing a facile pro-cedure starting from the reaction of 4-bromotriphenylaminewith butyllithium and following from a reaction of the resultingtriphenylamine–lithium salt with boric ester. For compound2,it is prepared by the reaction between a commercially available2,7-dibromofluorenone and triphenylamine in the presence ofCF3SO3H. In order to prevent the side reaction of the 4-positionof amine with fluorenone, a large excess of triphenylamine areemployed. Finally,XB10 was prepared by the condensation of1 with 2.

XB10 is an amorphous and white powder. It can easily forma transparent film by using spin-coating method. Both the THFsolution and the film ofXB10 are colorless and emit strong bluephotoluminescence (PL) under the irradiation with a general UVlamp (? = 365 nm).

Fig. 1 depicts the UV–vis spectra ofXB10 in THF and inthe solid state. The two absorption peaks in THF at 305 and373 nm, respectively, are attributed to the absorption of a triph-enylamine unit and a fluorene unit linked with two phenyl rings,respectively. In the film,XB10 shows the UV–vis peak at asomewhat longer wavelength (307 and 376 nm), suggesting thep ow-e thoseo -to-f

rei ngtho at4 yieldo int ith aq tedi estst solids ak at

Fig. 2. PL spectra ofXB10.

Fig. 3. DSC trace ofXB10.

453 nm; such a couple of PL peaks appears for various fluorenederivatives[15].

Thermal properties of the compound are characterized bydifferential scanning calorimetry (DSC) and TGA.Figs. 3 and 4show DSC and TGA curves ofXB10, respectively. As shownin Fig. 3, XB10 has rather high glass transition temperature,Tg, of 165◦C, which is higher than the popular HTMs such as

Fig. 4. TGA curve ofXB10.

resence of certain intermolecular electronic interaction. Hver, the degree of the red shift is considerably smaller thanbserved with the�-conjugated compounds that take face

ace stacking[15].PL spectra ofXB10 in toluene and in THF indicate that the

s an effect of the solvent polarity on the emission wavelef the compound. For example,XB10 shows a main peak34 nm and a shoulder peak at 450 nm with a quantumf 92% (a 0.5 M H2SO4 solution of quinine as a reference)

oluene, whereas it shows the emission peak at 450 nm wuantum yield of 87% in THF with strong polarity, as depic

n Fig. 2. Such solvatochromism observed for the PL sugghat the photoexcited state has a polar structure. In thetate,XB10 gives a main peak at 439 nm and a shoulder pe

Q. Fang et al. / Synthetic Metals 155 (2005) 206–210 209

NPB and TPD[1]. In addition, no obvious crystallization peak isobserved during the scanning from room temperature to 220◦C,implying thatXB10 shows limited tendency to crystallize. TGAcurve shows thatXB10 has high thermostability with 5-wt.%loss temperature of 505◦C.

3.2. Electroluminescence properties

3.2.1. As a hole-transporting layerThe HOMO energy ofXB10 is estimated[16–17] by

using cyclic voltammetry (CV). For comparison, the HOMOenergy of TPD is also measured under the same condi-tion. The results show that HOMO energies ofXB10 andTPD are 5.22 and 5.10 eV, respectively. These results ren-der XB10 quite possible to be used as a hole-transportinglayer in the fabrication of OLEDs. To evaluate the per-formance of XB10 as a HTM in OLED, four deviceswith configurations of ITO/XB10/Alq/LiF/Al (deviceXB10), ITO/TPD/XB10/Alq/LiF/Al (device TPD/XB10),ITO/XB10/TPD/Alq/LiF/Al (device XB10/TPD), andITO/TPD/Alq/LiF/Al (device TPD) were fabricated. Thethickness of the each layer is as below: TPD, 30 nm;XB10,30 nm; Alq, 30 nm; LiF, 0.5 nm; Al, 120 nm. All devices emitthe green color of Alq (530 nm), suggesting thatXB10 is onlyas a HTM in the devices.Fig. 5 shows theV–B (operationvoltage versus brightness) characteristics of four devices. Asc ef eda5 ndA erH Ino lassia erwa urd e,o e,or ce in

Fig. 6. Efficiency of devices TPD and TPD/XB10.

HOMO values between TPD andXB10. For device TPD/XB10,higher HOMO energy ofXB10 is helpful for the transfer ofthe holes from ITO glass through TPD into Alq, e.g.,XB10is as a “bridge” between TPD and Alq. In contrast, for deviceXB10/TPD, on one hand, the bigger energy barrier betweenXB10 and ITO glass makes the transfer of holes difficult; on theother hand, the mobile direction of holes in OLED is from lowHOMO level to high HOMO level, implying that the transferof holes from ITO throughXB10 to TPD followed enteringAlq will be confronted with a big energy barrier. Thus, deviceXB10/TPD needs high turn-on voltage.

The efficiency of device TPD/XB10 is investigated andshown inFig. 6. For comparison, the efficiency of device TPDis also given. As shown inFig. 6, the efficiency of deviceTPD/XB10 is very near to that of device TPD, indicating thatusing TPD/XB10 as a composite HTM endows both low turn-onvoltage and good efficiency to the device.

3.2.2. As a light-emitting layerHigh PL quantum yield ofXB10 motivates us to use

the compound as a light-emitting layer for OLED.Fig. 7shows the EL spectra of a device fabricated with a configu-ration of ITO/TPD (30 nm)/XB10 (30 nm)/PBD (30 nm)/Alq(30 nm)/LiF (0.5 nm)/Al (120 nm) (here, PBD is 2-(4-biphenyl)-

an be seen fromFig. 5, deviceXB10, a double layer devicabricated byXB10 and Alq exhibits a turn-on voltage (defins the voltage need to deliver a brightness of 1 cd/m2) ofV, which is higher than that of the device built by TPD alq (4 V). This is possibly attributed to that TPD has lowOMO energy than that ofXB10, as described above.ther words, the energy barrier between TPD and ITO g

s smaller, compared with the energy barrier betweenXB10nd ITO. Interestingly, whenXB10 was employed togethith TPD as a composite HTM, the two devices (XB10/TPDnd TPD/XB10) give much different results. Among the foevices device TPD/XB10 shows the lowest turn-on voltagf 3 V; whereas deviceXB10/TPD gives the highest voltagf 6 V, which is even higher than that of deviceXB10. Theseesults are considered to be also caused by the differen

Fig. 5. V–B curves of the four devices.

Fig. 7. EL spectra of the blue emission device.

210 Q. Fang et al. / Synthetic Metals 155 (2005) 206–210

Fig. 8. B–V curve of a blue emission device.

Fig. 9. Efficiency of a blue emission device.

5-(4-tertbutyl-phenyl)-1,3,4-oxadiazole). Because of stronghole transferring ability ofXB10, a hole block layer, PBD isrequired. As seen fromFig. 7, the device shows maximumemission peak at about 456 nm at 1 and 200 mA/cm2, respec-tively, agreed with the PL spectrum ofXB10 in the solid state,suggesting that the hole from ITO was restricted within the light-emitting layer and the blue emission derived fromXB10 layer.

The B–V curve and efficiency of the device are shown inFigs. 8 and 9, respectively. Under forward bias (a positive volt-age on the ITO electrode), the device begins to emit bluelight aabout 5 V, and the brightness increases with increasing applievoltage after turn-on voltage, indicating typical rectifying char-acteristics. The current efficiency is higher than 1.7 cd A−1 when

the current density changes from 0.1 to 40 mA/cm2, implyingthat the device has good properties.

4. Summary

A novel fluorene derivative containing four triphenylaminegroups was prepared. The compound has rather high glass transi-tion temperature, and when it is employed together with TPD asa composite hole-transporting layer, the device shows very lowturn-on voltage and high efficiency. When a hole block layer wasused, the compound emitted blue light with good efficiency.

Acknowledgement

Financial supports from Ministry of Science and Technologyof China (863 Project: number: 2001AA313070) and a pro-gram from the Science & Technology Commission of ShanghaiMunicipality (number: 011461057) are acknowledged. QF isthankful of the financial support from the Chinese Academy ofSciences (“Bai Ren” Project).

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