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* E-mail: liudi@dlut.edu.cn; Fax: 0086-0411-84986233 Received September 20, 2011; revised October 19, 2011; accepted October 25, 2011. Project supported by the National Natural Science Foundation of China (Nos. 20704002, 20923006, 21072026), the Ministry of Education for the

New Century Excellent Talents in University (No. NCET-08-0074), and the Fundamental Research Funds for the Central Universities (No. DUT10LK16).

† Dedicated to Professor Weiyuan Huang on the occasion of his 90th birthday.

Chin. J. Chem. 2011, 29, 2655—2658 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2655

Synthesis and Characterization of New Thienopyrazine-cored Dendrimer for Non-Doped Organic Red Light-Emitting Diodes†

Li, Qinga,c(李青) Li, Jiuyanb(李久艳) Duan, Yonghengb(段永恒) Gao, Zhanxiana(高占先) Liu, Di*,a(刘迪)

a School of Chemistry, Dalian University of Technology, Dalian, Liaoning 116024, China b State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, Liaoning 116024, China c Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai, Shangdong 264003, China

A new thieno-[3,4-b]-pyrazine-cored dendrimer with polyphenyls dendrons (D2TP) was designed and synthe-sized for application as red emitter in non-doped organic light-emitting diodes (OLEDs). The bulky dendritic arms enable this molecule to possess significant molecular weight and good solubility in common organic solvents. As a result, D2TP is suitable for solution processing methods such as spin coating to make thin films and devices. In ad-dition, these large dendrons provide site-isolation effect on the planar emissive core to prevent fluorescence quenching or intermolecular interaction. The photophysical and electrochemical properties of D2TP were investi-gated in its dilute solutions. D2TP was used as non-doped emitter to fabricate OLEDs by spin coating method and saturated red electroluminescence was obtained.

Keywords thienopyrazine, dendrimer, light-emitting diodes, red electroluminescence

Introduction

Organic light-emitting diodes (OLEDs) have been drawing broad research attention because of their prac-tical applications in large-area flat-panel displays and solid-state lighting. Currently, the red light-emitting materials and devices still lag behind the green and blue components in both efficiency and stability and thus require further improvements to match the criteria of full-color displays and white-light lighting.1-3 Recently, developing red emitters for non-doped light-emitting diodes has become a prevalent strategy to gain highly efficient red electroluminescence (EL).3-7 It is well es-tablished that the solution processing is the most favor-able fabrication technique of OLEDs for practical ap-plications.8,9 The dendritic molecules have been em-ployed for solution processible OLEDs due to the in-herent topological features, in which the emissive core is surrounded by a branched shell to prevent self-ag- gregation and concentration-quenching in the solid state.10-12 Furthermore, the dendritic molecules have such a merit that they possess repeatable monodispersity and high level of purity, both of which are essential for ideal device performance.

In this article, we designed and synthesized a novel thieno-[3,4-b]-pyrazine-cored dendrimer (D2TP) with polyphenyls (Müllen type dendron) as dendritic arms

(shown in Scheme 1). Thieno[3,4-b]pyrazine was se-lected as the luminescent core because of its saturated red fluorescence, large Stokes shift and high fluorescent quantum yield.3 To prevent the undesired intermolecular aggregation and concentration quenching, the bulky dendrons were introduced around the thieno-[3,4-b]- pyrazine core to construct a nonplanar molecule.

Experimental

Materials and instruments

All the chemicals and reagents for the synthesis (including 2,5-dibromothiophene, trimethylsilylethyne, 1,2-bis(4-bromophenyl)-1,2-ethanedione) are of ana-lytical grade and purchased from Across and Aldrich. They were used as received without further purification. 1H NMR spectra were recorded on a Bruker AvanceII (400 MHz) and a Varian INOVA spectrometer (400 MHz). Mass spectra were recorded on a GC-Tof MS (Micromass, UK) mass spectrometer for TOF-MS-EI, a MALDI micro MX (Waters, USA) for MALDI-TOF- MS. The fluorescence and UV-vis absorption spectra measurements were performed on a Perkin-Elmer LS55 fluorescence spectrometer and a Perkin-Elmer Lambda 35 UV-Visible spectraphotometer, respectively. The fluorescence quantum yields were determined in di-

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Scheme 1 Synthetic route for D2TP

OO

Br Br

O

S

N N

D2TP

2

1

o-Xylenereflux, N2

1SBr Br

S

N N

chloromethane or toluene solutions against rhodamine B as the standard (ΦF＝0.97 in ethanol). Melting points were recorded on a WRS-1B digital melting point in-strument (Shanghai Precision and Scientific Instrument Co.).

Synthesis of compounds

The important intermediates 1 and 2 were synthe-sized according to the literature methods.13-16

Compounds 1 (200 mg, 0.37 mmol) and 2 (1.88 mmol) in 10 mL of o-xylene were stirred and refluxed for 72 h under nitrogen. After cooling to room tempera-ture, the solvent was evaporated under reduced pressure. The crude product was purified by column chromatog-raphy on silica gel (CH2Cl2/petroleum ether) to give pure product D2TP as red powder (300 mg, 16%). m.p.＞350 ℃; 1H NMR (400 MHz, CD2Cl2) δ: 8.17 (d, J＝12 Hz, 8H, ArH), 8.09—8.05 (m, 32H, ArH), 7.79 (d, J＝12 Hz, 8H, ArH), 7.42 (d, J＝8 Hz, 8H, ArH), 7.39 (d, J＝8 Hz, 8H, ArH), 7.34—7.27 (m, 40H, ArH), 7.25—7.20 (m, 24H, ArH), 7.18—7.16 (m, 56H, ArH), 7.10—7.05 (m, 56H, ArH), 7.02 (d, J＝8 Hz, 12H,

ArH), 6.98 (d, J＝8 Hz, 8H, ArH); MS (MALDI-TOF) calcd for C390H260N2S, 5006.3200, found 5006.0112. Anal. calcd for C390H260N2S: C 93.57, H 5.23, N 0.56; found C 93.12, H 5.17, N 0.51.

Results and discussion

Design and synthesis

The dendrimer D2TP was synthesized through the procedure described in Scheme 1. The bulky polyphen- yls groups were introduced through a phenylene bridge into the 2,3,5,7-positions of thieno-[3,4-b]-pyrazine core as dendritic arms in order to protect the core via site-isolation effect, to provide significant molecular weight and viscosity so that the target molecules are suitable for solution processing, and to generate mor- phologically stable molecules due to the excellent sta-bility of this type of dendron. The molecule designed in this way is expected to have the advantageous features of high fluorescent quantum yield (ΦF), large Stokes shift, and good solubility for potential application in solution-processed and non-doped OLEDs.

Synthesis and Characterization of New Thienopyrazine-cored Dendrimer

Chin. J. Chem. 2011, 29, 2655—2658 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 2657

The thienopyrazine-containing core 113,14 bearing the terminal ethynyl groups as the reactive sites at the sur-face of the emissive core, and polyphenylene-substi- tuted cyclopentedieneone 215,16 are two most important intermediates and were prepared according to the litera-ture methods. The target dendrimer D2TP was synthe-sized through Diels-Alder cycloaddition of 2 to 1 in o-xylene at high temperature. D2TP is well soluble in normal organic solvents and was purified by flash col-umn chromatograph over silica gel with mixed petro-leum ether and dichloromethane (V∶V＝3∶2) as elu-ent. The observation of [M]＋ peak at ca. m/z 5006.0112 in matrix-assisted laser desorption/ionization time- of-light (MALDI-TOF) mass spectra, elemental analysis and 1H NMR spectral data (see supporting information) confirmed the chemical structure of the target dendrimer D2TP.

Photophysical properties and electrochemistry

In the UV-vis absorption spectrum in Figure 1, D2TP exhibits two major electronic absorption bands: π-π* transition at 330—360 nm and charge transfer (CT) transition at 490—530 nm.3,9 It should be noted that the absorption intensity of the long wavelength band is much lower than that of the short wavelength band. Upon photoexcitation at 480 nm, D2TP emits strong red fluorescence with emission peak at 622 nm in CH2Cl2 solution at room temperature. Meanwhile, the structure-less emission spectrum remains unaltered irrespective of the excitation wavelength, which is possibly due to an efficient relaxation to the lowest excited state.3 D2TP exhibits a high fluorescence quantum yield (ΦF) of 37.6% in toluene solutions (relative to rhodamine B in ethanol as the standard, ΦF＝0.97). However, ΦF in CH2Cl2 is only a half of that in toluene. This interesting phenomenon may be due to intramolecular charge transfer (ICT) emission, which varies with polarity of the solvent.

There is little overlap between the absorption and emission spectra of D2TP since the long-wavelength absorption band is relatively weak. Calculated from the positions of the long-wavelength absorption maximum and the fluorescence maximum, the Stokes shift for F-DTP is determined as large as 106 nm. Such a large Stokes shift is especially valuable for light-emitting materials used in non-doped OLEDs since each emis- sive molecule is densely surrounded in the neat film and the absence of self-absorption will definitely facilitate efficient light output from the device.

The redox behavior of D2TP was investigated by cyclic voltammetry. The onset potentials of the first

Figure 1 UV-vis absorption and fluorescence spectra of D2TP in dilute CH2Cl2 solutions.

oxidation ( onsetoxE ) and reduction ( onset

redE ) were 0.94 and －1.01 V vs. saturated calomel electrode (SCE) elec-trode, respectively. From the values of onset

oxE and onsetredE , the highest occupied molecular orbital (HOMO)

and the lowest unoccupied molecular orbital (LUMO) as well as the electrochemical band gaps ( CV

gE ) of D2TP were calculated as －5.34, －3.39 and 1.95 eV (HOMO＝－e( onset

oxE ＋4.4) eV, LUMO＝－e( onsetredE ＋

4.4) eV, CVgE ＝e( onset

oxE －onsetredE ) eV).17-19 All the

photophysical and electrochemical data of D2TP are summarized in Table 1.

Electroluminescence

In order to evaluate the electroluminescence (EL) properties, D2TP was used as the neat emitting layer to fabricate non-doped OLEDs. Based on the excellent solubility of D2TP in common organic solvents, high quality neat films without pinholes can be obtained by spin coating their solutions in chlorobenzene to form the EML (emitting layer). The OLEDs have a configuration of ITO/PEDOT:PSS (40 nm)/D2TP (25 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm), where PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrene sulfo- nate)) acts as hole injecting layer, TPBI (2,2',2"-(1,3,5- benzenetriyl)tris(1-phenyl-1H-benzimidazole)) as elec-tron transporting layer, ITO (indium tin oxide) and LiF/Al as anode and cathode, respectively.

D2TP based OLEDs transmited saturated red EL with peak at 636 nm, as shown by the EL spectra in Figure 2. In comparison with the photoluminescence of D2TP solution, the EL exhibited a red-shift of 14 nm. This is a frequently observed phenomena that is typi-cally caused by the additional effect of the electrical field on the excited states of emitting molecules in

Table 1 Photophysical and electrochemical data of D2PT

Toluene/CH2Cl2

λabs/nm λem/nm Φ a/% Stock’s shift/nm HOMO (LUMO)b/eV Eg

c/eV

343, 509/346, 516 614/622 37.6/19.3 105/106 －5.34 (－3.39) 1.95 a Related to Rhodamine B as the standard (Φ＝0.97 in ethanol). b HOMO and LUMO energies were calculated with reference to saturated calomel electrode (SCE). c The electrochemical band gap was calculated by subtracting LUMO energy from HOMO.

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OLEDs. The CIE coordinates of these spectra are con-stant at (0.62, 0.32). These coordinates are quite close to (0.64, 0.33), which are the coordinates of the standard red color of the National Television System Committee (NTSC). Moreover, the EL spectra and CIE coordinates almost kept unchanged with increasing driving voltage, which offers better device operation compared to red OLEDs with dopants in which the color usually changes with voltage.20 The voltage-current density-brightness characteristics of D2TP device are shown in Figure 3. The D2TP device reached a maximum brightness of 210 cd•m－2 at 12 V and a peak luminous efficiency of 0.23 cd•A－1 at a current density of 16 mA•cm－2.

Figure 2 Electroluminescence spectra of D2TP based OLEDs at different voltages.

Figure 3 Voltage-current density-brightness characteristics of D2TP device. Inset: the current efficiency curve as a function of current density.

Conclusions

We designed and synthesized a novel thieno-[3,4-b]- pyrazine-cored dendrimer (D2TP) containing polyphen-yls dendrons. In addition to providing the site-isolation effect on the planar emissive core, the bulky dendrons

enable this molecule to be solution processible to form good quality thin films. The solution processed OLEDs using D2TP as non-doped emitting layer exhibited pure saturated red electroluminescence. This report provides a practical strategy to decorate the highly efficient but planar luminophors to be suitable for application in so-lution-processible and non-doped OLEDs.

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(CJOC201100370 Li, L.)