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Dibenzothiophene/Oxide and Quinoxaline/Pyrazine Derivatives Serving As Electron-Transport Materials**

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Dibenzothiophene/Oxide and Quinoxaline/Pyrazine Derivatives Serving As Electron-Transport Materials** FULL PAPER DOI: 10.1002/adfm.200500823 Dibenzothiophene/Oxide and Quinoxaline/Pyrazine Derivatives Serving as Electron-Transport Materials** By Tai-Hsiang Huang, Wha-Tzong Whang,* Jiun Yi Shen, Yuh-Sheng Wen, Jiann T. Lin,* Tung-Huei Ke, Li-Yin Chen, and Chung-Chih Wu* A series of 2,8-disubstituted dibenzothiophene and 2,8-disubstituted dibenzothiophene-S,S-dioxide derivatives containing quinoxaline and pyrazine moieties are synthesized via three key steps: i) palladium-catalyzed Sonogashira coupling reaction to form dialkynes; ii) conversion of the dialkynes to diones; and iii) condensation of the diones with diamines. Single-crystal characterization of 2,8-di(6,7-dimethyl-3-phenyl-2-quinoxalinyl)-5H-5k6-dibenzo[b,d]thiophene-5,5-dione indicates a triclinic crystal structure with space group P1 and a non-coplanar structure. These new materials are amorphous, with glass-transition temperatures ranging from 132 to 194 °C. The compounds (Cpd) exhibit high electron mobilities and serve as effective elec- tron-transport materials for organic light-emitting devices. Double-layer devices are fabricated with the structure indium tin oxide (ITO)/Qn/Cpd/LiF/Al, where yellow-emitting 2,3-bis[4-(N-phenyl-9-ethyl-3-carbazolylamino)phenyl]quinoxaline (Qn) serves as the emitting layer. An external quantum efficiency of 1.41 %, a power efficiency of 4.94 lm W–1, and a current efficien- cy of 1.62 cd A–1 are achieved at a current density of 100 mA cm–2. 1. Introduction efficiency, brightness, and durability.[4] In contrast, reports of the use of small molecules as electron-transporting materials [5] Organic and polymer light-emitting diodes (OLEDs and are still rare in the literature. Tris-(8-hydroxyquinoline) alu- PLEDs) based on small molecules and conjugated polymers minum (Alq3) is the most widely used electron-transport mate- –6 2 –1 –1 [6] are considered to be promising candidates for the next genera- rial, yet its electron mobility is very low (ca. 10 cm V s ). tion of flat-panel displays.[1] After Tang and Van Slyke’s[2] dis- 2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) covery in 1987, and that of Burroughes et al.[3] in 1990, rapid is also a widely used electron-injection/transport or hole-block- [7] [8] progress has been made on device performances. To date, ing material. The high electron affinity (EA= 2.16 eV) of many small molecules containing electron-rich aromatic the oxidazole ring in the molecule is believed to play an impor- amines have been widely used as hole-transport and/or emit- tant role in this aspect. However, PBD thin films (glass-transi- ting materials in electroluminescent devices, exhibiting good tion temperature, Tg = 60 °C) formed by vacuum deposition tend to crystallize over time, which is detrimental to device sta- bility.[9] Thermally stable quinoxaline compounds are another – useful n-type material with high electron affinity. The quinoxa- [*] Dr. W.-T. Whang, T.-H. Huang line moiety has been introduced into small molecules and poly- [10] Department of Materials Science and Engineering mers, and successfully applied in OLEDs and PLEDs. National Chiao Tung University In this paper, we report the synthesis and characterization 1001 Ta Hsueh Rd, Hsin Chu 300 (Taiwan) of thermally stable dibenzothiophene and dibenzothiophene- E-mail: [email protected] Dr. J. T. Lin, J. Y. Shen, Y.-S. Wen S,S-dioxide-based derivatives incorporating peripheral qui- Institute of Chemistry, Academia Sinica noxaline/pyrazine moieties. The physical properties of these Taipei 115 (Taiwan) materials, carrier mobilities, and OLEDs fabricated using these E-mail: [email protected] compounds are investigated. Dr. J. T. Lin, J. Y. Shen, Y.-S. Wen Department of Chemistry National Central University Chungli 320 (Taiwan) 2. Results and Discussion Dr. C.-C. Wu, T.-H. Ke, L.-Y. Chen Department of Electrical Engineering Graduate Institute of Electro-optical Engineering and Graduate 2.1. Synthesis of the Materials Institute of Electronics Engineering National Taiwan University, Taipei 106 (Taiwan) Scheme 1 illustrates the synthesis of the dibenzothiophene E-mail: [email protected] or dibenzothiophene-S,S-dioxide incorporating quinoxaline [**] We thank the Academia Sinica, National Taiwan University, National and pyrazine segments. The synthetic procedures consist of Chiao Tung University and the National Science Council for support- ing this work. Supporting Information is available from Wiley Inter- three key steps: i) synthesis of 2,8-diethynyl derivatives (1 and Science or from the authors. 2) via a Sonogashira coupling reaction[11] catalyzed by Adv. Funct. Mater. 2006, 16, 1449–1456 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1449 T.-H. Huang et al./Dibenzothiophene/Oxide and Quinoxaline/Pyrazine Electron-Transport Materials X X X phenylacetylene I2/DMSO O O PdCl2(PPh3)2/CuI/PPh3 Br Br i-Pr2NH O O FULL PAPER X = S X = S or SO 1: X = S 3: 2 X = SO 2: X = SO2 4: 2 NH NC NH 2 2 3 or 4 or 5-8 EtOH/CHCl NH2 NC NH2 3 Scheme 1. Synthesis of compounds 1–8. DMSO: dimethyl sofoxide. PdCl2(PPh3)2 and CuI; ii) oxidiation of the internal alkynes using iodine in dimethyl sulfoxide (3 and 4) to form the dione derivatives, in a process developed by Yusybov and Filimo- nov;[12] (it is worth nothing that this method is more convenient than the similar oxidation of an alkyne to dione via the use of highly toxic selenium dioxide[13]) iii) condensation of a bis- dione compound with an appropriate diamine to afford the tar- geted diquinoxaline or dipyrazine (5–8, Scheme 2). 2.2. X-Ray Crystal Structure The ORTEP representation of 7·CH2Cl2,C44H32N4O2S· CH2Cl2 is shown in Figure 1 (CH2Cl2 molecules have been omitted for clarity). There are no hydrogen bonds between the CH2Cl2 solvent and 7, but there are very weak van der Waals’ intermolecular interactions. Crystallographic data are listed in Table S1 (see Supporting Information), and important bond Figure 1. ORTEP drawing of 7. angles, bond distances, and parameters are collected in Tables S2–S4 (see Supporting Information). There is significant deviation of the two neighboring aromatic segments from tends to disfavor intermolecular stacking. The intermolecular planarity. The dihedral angles are 46.57(8)°, 27.48(9)°, and spacing (4.3 Å) is greater than that observed for a typical p–p 54.21(8)°, respectively, for the terminal phenyl plane (P2)/azine interaction distance (3.8 Å).[14] The crystal structure of 7 (see plane (P3), P3/dibenzothiophene-S,S-dioxide plane (P1), and below) indicates the lack of planarity of the molecule, and P1/P2 pairs. The twist of these aromatic rings from coplanarity therefore there are no efficient intermolecular p–p interactions, although it should be noted that the struc- ture of a crystal grown from a solvent may S S not exactly reflect the real molecular N packing of the deposited film. N N N N N N N N NC CN N CN NC 2.3. Photophysical Properties 6 5 N Qn N The optical absorption and photolumi- O O O O nescence (PL) spectra of compounds 5–8 S S N in dilute dichloromethane solution are shown in Figures 2 and 3. All of the com- N N N N N N N N N pounds displayed two absorption peaks: NC CN CN NC around 257–281 nm and 335–373 nm. The 7 8 higher-energy bands are assigned to the absorption of dibenzothiophene, diben- Scheme 2. Structures of Qn and compounds 5–8. zothiophene-S,S-dioxide, and the terminal 1450 www.afm-journal.de © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2006, 16, 1449–1456 T.-H. Huang et al./Dibenzothiophene/Oxide and Quinoxaline/Pyrazine Electron-Transport Materials FULL PAPER 0.9 phenyl ring. The low-energy absorption bands are attributed to the p–p* transition of the quinoxaline[13] or pyrazine[15] moi- 0.8 eties. These compounds are fluorescent, and the emission col- 0.7 5 ors range from purplish-blue to blue-green, both in solution 6 and the solid state. The maximum wavelength of the fluores- 0.6 7 8 cence of the film is only slightly red-shifted compared to that 0.5 of the solution (Dk ca. 3–6 nm). This outcome can be attribut- 0.4 ed to the non-coplanarity of the molecules (see below), which Abs. [a.u] Abs. prevents “disklike” intermolecular stacking. Though it is 0.3 known that quinoxaline derivatives may form an excimer upon [10b,13,16] 0.2 excitation, there is no evidence of excimer formation in –3 our compounds, even at concentrations of up to 1.2 × 10 M, or 0.1 in the films. 0.0 250 300 350 400 450 Wavelength [nm] 2.4. Thermal Properties –5 –1 Figure 2. Electronic absorption spectra of 5–8 (5.0 × 10 mol L in dichlo- The thermal properties of these materials were investigated romethane). by differential scanning calorimetry (DSC), and their thermal stability was examined by thermogravimetric analysis (TGA). 1.3 The detailed data are collected in Table 1. The experiments 1.2 show that the quinoxaline/pyrazine compounds are able to 1.1 5 form a stable glass. All the compounds exhibited isotherms 1.0 6 with melting points (Tm) ranging from 309 to 348 °C during the 0.9 7 first DSC heating, and readily formed a glass upon cooling 0.8 8 (30 °C min–1). In further heating cycles, the glass transition was 0.7 readily detected, and Tg ranged from 132 to 194 °C. These com- 0.6 pounds exhibited high thermal decomposition temperatures PL [a.u] PL 0.5 (Td > 380 °C).
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