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Blue emitting fluorophores of phenyleneethynylenes substituted by diphenylethenyl terminal groups for organic light-emitting diodes
Materials Chemistry and Physics 115 (2009) 378–384
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Blue emitting fluorophores of phenyleneethynylenes substituted by diphenylethenyl terminal groups for organic light-emitting diodes Guoliang Mao a , Akihiro Orita a , Larysa Fenenko b , Masayuki Yahiro b , Chihaya Adachi b,∗ , Junzo Otera a a b
Department of Applied Chemistry, Okayama University of Science, Ridai-cho, Okayama 700-0005, Japan Center for Future Chemistry, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
a r t i c l e
i n f o
Article history: Received 3 August 2008 Received in revised form 19 October 2008 Accepted 13 December 2008 Keywords: Electronic materials Electrical properties Optical properties
a b s t r a c t Phenyleneethynylene motifs substituted by diphenylethenyl groups at both ends were prepared successfully by use of double elimination protocol of -substituted sulfones for introducing phenyleneethynylene arrays followed by Wittig–Horner reaction for introducing diphenylethenyl moiety. The hybrid blue fluorophores exhibited strong emission even in the solid-state films (˚F 0.60) while, in CHCl3 solution, incorporation of substituents on the central phenylene unit significantly enhanced emission efficiency up to ˚F = 0.57. The OLED devices with use of these blue fluorophores as an emitting material provided maximum external quantum efficiency of ext = 2.4%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Acetylenes have attracted extensive attention in material sciences [1–5]. In particular, phenyleneethynylenes are interesting materials since they exhibit high quantum yields in solution and their emission peaks can be facilely tuned by attachment of electron-donating or withdrawing substituents on benzene rings [6–9]. Further, poly(phenyleneethynylene)s (PPE) were revealed to emit strong fluorescence even in the solid thin-films [10]. Based on these high photoluminescence (PL) characteristics, PPE was used as a thin-film sensor which can detect very small amount of trinitrotoluene and dinitrotoluene effectively [11,12]. Further, there are several reports on organic light-emitting diodes (OLEDs) using PPE as an emitter, although their external electroluminescence (EL) quantum efficiencies (ext ) are still very low level of ext ∼ 0.035% [13–16]. Thus, further improvement in material design for high EL efficiency is still in demand. Recently, we have been involved in synthesis of various types of phenyleneethynylenes by use of double elimination protocol of -substituted sulfone [17–25] (Scheme 1) and disclosed that bis(4-(phenylethynyl)phenyl)ethynes indicate rather high hole mobilities in the range of h = 10−3 to 10−5 cm2 V−1 s−1 in ptype organic field-effect transistors (OFET) [26]. In addition, we have found that attachment of phenyleneethynylene to a silole fluorophore enables emission enhancement giving rise to a high quantum yield (˚ = 0.50) [19]. Further, we were intrigued by
triarylethene/phenyleneethynylene-hybrid blue fluorophores 1, which might be promising for emission materials in OLEDs (Fig. 1). We postulated that triarylethene moiety serves as a blue fluorophore which emits intense luminescence in the nanoparticle and the solid state [27,28], while phenyleneethynylene moiety provides efficient carrier transport ability. Herein, we report two double elimination routes for 1, optical properties such as ultraviolet–visible (UV–vis) absorption and PL in solution and in solid state as well as OLED characteristics using 1 as an emitting material. 2. Experimental details 2.1. General: syntheses and characterization All reactions were carried out under an atmosphere of argon with freshly distilled solvents, unless otherwise noted. Tetrahydrofuran (THF) was distilled from sodium/benzophenone. Other solvents such as toluene, diisopropylamine and CH2 Cl2 were distilled from CaH2 . THF and ether solution of LiHMDS and Grignard reagents were purchased and used without titration. A hexane solution of BuLi was purchased and titrated by Gilman method prior to use. A pentane solution of t-BuLi was purchased and used without titration. Silica gel (Daiso gel IR-60) was used for column chromatography. Sulfones 2 [29], 3 [30,31], 6 [32], and 7 [29] were prepared according to the reported procedure. NMR spectra were recorded at 25 ◦ C on JEOL Lambda 300 and JEOL Lambda 500 instruments and calibrated with tetramethylsilane (TMS) as an internal reference. Elemental analyses were performed by the PerkinElmer PE 2400. UV–vis and photofluorescence were recorded by JASCO V-560 and JASCO FP-6500 at room temperature, respectively. Absolute quantum yields of fluorescence were recorded by an integration sphere system (Hamamatsu photonics C9920-02). 2.2. Synthetic procedure for 1 (bromide route)
∗ Corresponding author. Tel.: +81 92 802 3306; fax: +81 92 802 3306. E-mail address: [email protected] (C. Adachi). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.12.015
2.2.1. 1,4-Bis(4-bromophenylethynyl)-2,5-diethynylbenzene (4b) (representative) To a THF solution (10 mL) of 4-bromophenylmethyl phenyl sulfone (747 mg, 2.4 mmol), 1,4-diethyl-2,5-diformylbenzene (190 mg, 1.0 mmol) and diethyl
G. Mao et al. / Materials Chemistry and Physics 115 (2009) 378–384
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Scheme 1. Double elimination protocol for diphenylacetylenes.
chlorophosphate (0.345 mL, 2.4 mmol) was added lithium hexamethyldisilazide (8.0 mL of 1.0 M THF solution, 8.0 mmol) at 0 ◦ C, and the mixture was stirred at rt for 4 h. After usual workup with ethyl acetate and aqueous NH4 Cl, the organic layer was evaporated, the residue was subjected to column chromatography on silica gel (hexane) to give 1,4-bis(4-bromophenylethynyl)-2,5-diethynylbenzene (4b) in a pure form (409 mg, 83%). 1,4-Bis(4-bromophenylethynyl)-2,5-diethynylbenzene: m.p. 147–149 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 1.30 (t, J = 7.7 Hz, 6H), 2.83 (q, J = 7.7 Hz, 4H), 7.39 (s, 2H), 7.39 (d, J = 8.3 Hz, 4H), 7.50 (d, J = 8.3 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 14.7, 27.1, 89.2, 93.1, 122.2, 122.3, 122.5, 131.5, 131.6, 132.9, 143.5; HRMS (ESI+ ) exact mass calcd for C26 H20 Br2 [M+ ] requires m/z 489.9932, found m/z 489.9922. 2.2.2. 1,4-Bis(4-bromophenylethynyl)benzene (4a) 78% yield (hexane:CH2 Cl2 = 3:1 for column chromatography); m.p. > 300 ◦ C; 1 H NMR (300 MHz, CDCl3 ) ı 7.43 (d, J = 8.8 Hz, 4H), 7.50 (d, J = 8.8 Hz, 4H), 7.51 (s, 4H); 13 C NMR spectra could not be measured because of poor solubility; HRMS (ESI+ ) exact mass calcd for C22 H12 Br2 [M+ ] requires m/z 433.9306, found m/z 433.9310. 2.2.3. 1,4-Bis(4-bromophenylethynyl)-2,5-dimethoxybenzene (4c) 60% yield (hexane:CH2 Cl2 = 2:1 for column chromatography); m.p. 235–237 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 3.90 (s, 6H), 7.02 (s, 2H), 7.43 (d, J = 8.5 Hz, 4H), 7.49 (d, J = 8.5 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 56.4, 86.7, 94.0, 113.2, 115.5, 122.1, 122.7, 131.6, 133.1, 153.9; HRMS (ESI+ ) exact mass calcd for C24 H16 Br2 O2 [M+ ] requires m/z 493.9517, found m/z 493.9523. 2.2.4. 1,4-Bis(4-bromophenylethynyl)-2,5-(3,7-dimethyloctyloxy)benzene (4d) 75% yield (hexane:Et2 O) = 20:1 for column chromatography); m.p. 96–98 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 0.85 (d, J = 6.7 Hz, 12H), 0.97 (d, J = 6.7 Hz, 6H), 1.07–1.40 (m, 12H), 1.45–1.54 (m, 2H), 1.58–1.68 (m, 2H), 1.70–1.82 (m, 2H), 1.85–1.94 (m, 2H), 4.00–4.10 (m, 4H), 7.00 (s, 2H), 7.38 (d, J = 8.5 Hz, 4H), 7.48 (d, J = 8.5 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 19.7, 22.6, 22.7, 24.8, 28.0, 29.9, 36.3, 37.4, 39.2, 67.9, 87.1, 93.8, 113.8, 116.7, 122.4, 122.5, 131.6, 132.9, 153.6; HRMS (ESI+ ) exact mass calcd for C42 H52 Br2 O2 [M+ ] requires m/z 746.2334, found m/z 746.2343. 2.2.5. 1,4-Diethyl-2,5-bis(4-formylphenylethynyl)benzene (5b) (representative) To a THF solution (10 mL) of 1,4-bis(4-bromophenylethynyl)-2,5-diethylbenzene (492 mg, 1.0 mmol) was added t-BuLi (2.72 mL of 1.47 M pentane solution, 4.0 mmol) at −78 ◦ C, and the mixture was stirred at the same temperature. After 1 h, N,N-dimethyl formamide (0.192 mL, 2.5 mmol) was added at −78 ◦ C, and the mixture was stirred at rt for 4 h. After usual workup with ethyl acetate and aqueous NH4 Cl, the organic layer was evaporated, and the residue was subjected to column chromatography on silica gel (hexane:CH2 Cl2 = 2:1) to give 1,4-diethyl-2,5-bis(4-formylphenylethynyl)benzene (5b) in a pure form (312 mg, 80%). 1,4-Bis(4-formylphenylethynyl)-2,5-diethyl benzene: m.p. 175–177 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 1.33 (t, J = 7.7 Hz, 6H), 2.87 (q, J = 7.7 Hz, 4H), 7.43 (s, 2H), 7.68 (d, J = 8.3 Hz, 4H), 7.88 (d, J = 8.3 Hz, 4H), 10.03 (s, 2H); 13 C NMR (125 MHz, CDCl3 ) ı 14.7, 27.2, 92.1, 93.5, 122.4, 129.6, 129.6, 131.9, 132.0, 135.5, 143.9, 191.3; HRMS (ESI+ ) exact mass calcd for C28 H22 O2 [M+ ] requires m/z 390.1620, found m/z 390.1627.
Fig. 1. Chemical structures of acetylenic fluorophores 1.
2.2.6. 1,4-Bis(4-formylphenylethynyl)benzene (5a) 65% yield (hexane:CH2 Cl2 = 1:4 for column chromatography); m.p. 242–244 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 7.57 (s, 4H), 7.69 (d, J = 8.3 Hz, 4H), 7.89 (d, J = 8.3 Hz, 4H), 10.04 (s, 2H); 13 C NMR spectra could not be measured because of poor solubility; HRMS (ESI+ ) exact mass calcd for C24 H14 O2 [M+ ] requires m/z 334.0994, found m/z 334.1002.
2.2.7. 1,4-Bis(4-formylphenylethynyl)-2,5-dimethoxybenzene (5c) 71% yield (hexane:CH2 Cl2 = 1:4 for column chromatography); m.p. 229–231 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 3.93 (s, 6H), 7.06 (s, 6H), 7.72 (d, J = 8.3 Hz, 4H), 7.88 (d, J = 8.3 Hz, 4H), 10.03 (s, 2H); 13 C NMR (125 MHz, CDCl3 ) ı 56.5, 89.6, 113.4, 115.7, 129.4, 129.6, 132.2, 135.5, 154.1, 191.4; HRMS (ESI+ ) exact mass calcd for C26 H18 O4 [M+ ] requires m/z 394.1205, found m/z 394.1211.
2.2.8. 1,4-Bis(4-formylphenylethynyl)-2,5-(3,7-dimethyloctyloxy)benzene (5d) 53% yield (hexane:EtOAc = 5:1 for column chromatography); m.p. 105–108 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 0.84 (d, J = 6.4 Hz, 12H), 0.99 (d, J = 6.4 Hz, 6H), 1.05–1.42 (m, 12H), 1.43–1.57 (m, 2H), 1.58–1.71 (m, 2H), 1.72–1.85 (m, 2H), 1.87–1.97 (m, 2H), 4.03–4.14 (m, 4H), 7.05 (s, 2H), 7.67 (d, J = 8.3 Hz, 4H), 7.87 (d, J = 8.3 Hz, 4H), 10.03 (s, 2H); 13 C NMR (125 MHz, CDCl3 ) ı 19.7, 22.6, 22.7, 24.8, 28.0, 29.9, 36.2, 37.3, 39.2, 67.8, 90.0, 94.2, 113.8, 116.6, 129.6, 132.0, 135.4, 153.8, 191.4; HRMS (ESI+ ) exact mass calcd for C44 H54 O4 [M+ ] requires m/z 646.4022, found m/z 646.4031.
2.2.9. 1,4-Diethyl-2,5-bis(4-(2,2-diphenylethenyl)phenylethynyl)benzene (1b) (representative) To a DMF solution (25 mL) of 1,4-diethyl-2,5-bis(4-formylphenyl ethynyl)benzene (781 mg, 2.0 mmol) were added dimethyl diphenylmethylphosphonate (1.326 g, 4.8 mmol) and potassium t-butoxide (898 mg, 8.0 mmol), and the mixture was stirred at rt for 12 h. After usual workup with ethyl acetate and aqueous NaCl, the organic layer was evaporated, and the residue was subjected to column chromatography on silica gel (hexane:CH2 Cl2 = 10:1) to give 1,4-diethyl-2,5-bis(4(2,2-diphenyl ethenyl)phenylethynyl)benzene (1b) as pale yellow powder (940 mg, 68%). 1,4-Diethyl-2,5-bis(4-(2,2-diphenylethenyl)phenylethynyl)benzene: m.p. 231–233 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 1.26 (t, J = 7.6 Hz, 6H), 2.80 (q, J = 7.6 Hz, 4H), 6.96 (s, 2H), 7.00 (d, J = 8.3 Hz, 4H), 7.19–7.23 (m, 4H), 7.28 (d, J = 8.3 Hz, 4H), 7.29–7.31 (m, 2H), 7.31–7.33 (m, 10H), 7.34 (d, J = 1.9 Hz, 4H), 7.35 (d, J = 1.8 Hz, 2H); 13 C NMR (125 MHz, CDCl3 ) ı 14.7, 27.1, 88.9, 94.3, 121.5, 122.4, 127.5, 127.6, 127.7, 128.2, 128.7, 129.5, 130.3, 131.1, 131.5, 137.5, 140.1, 143.2, 143.3, 143.6. Anal. calcd for C54 H42 : C, 93.87; H, 6.13. Found: C, 93.74; H, 6.19.
2.2.10. 1,4-Bis(4-(2,2-diphenylethenyl)phenylethynyl)benzene (1a) 57% yield (hexane:CH2 Cl2 = 1:2 for column chromatography); m.p. 269–272 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 6.95 (s, 2H), 7.00 (d, J = 8.6 Hz, 4H), 7.19–7.21 (m, 4H), 7.28 (d, J = 8.2 Hz, 4H), 7.30–7.37 (m, 16H), 7.44 (s, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 89.8, 91.5, 121.1, 123.1, 127.4, 127.6, 127.7, 127.8, 128.3, 128.7, 129.5, 130.4, 131.2, 131.5, 137.7, 140.1. Anal. calcd for C50 H34 : C, 94.60; H, 5.40. Found: C, 94.69; H, 5.58.
2.2.11. 1,4-Dimethoxy-2,5-bis(4-(2,2-diphenylethenyl)phenylethynyl)benzene (1c) 61% yield (hexane:CH2 Cl2 = 10:1 for column chromatography); m.p. 252–254 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 3.86 (s, 6H), 6.95 (s, 2H), 6.97 (s, 2H), 6.99 (d, J = 8.2 Hz, 4H), 7.18–7.22 (m, 4H), 7.28–7.37 (m, 20H); 13 C NMR (125 MHz, CDCl3 ) ı 56.5, 86.3, 95.3, 113.4, 115.6, 121.3, 127.5, 127.6, 127.7, 127.8, 128.3, 128.7, 129.4, 130.4, 131.3, 137.7, 140.1, 143.2, 143.7, 153.9. Anal. calcd for C52 H38 O2 : C, 89.88; H, 5.51. Found: C, 90.09; H, 5.83.
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G. Mao et al. / Materials Chemistry and Physics 115 (2009) 378–384
Scheme 2. Synthetic route 1 for 1 (bromide route).
2.2.12. 1,4-(3,7-Dimethyloctyloxy)-2,5-bis(4-(2,2diphenylethenyl)phenylethynyl)benzene (1d) 60% yield (hexane:EtOAc = 10:1 for column chromatography); m.p. 101–103 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 0.85 (d, J = 6.4 Hz, 12H), 0.95 (d, J = 6.4 Hz, 6H), 1.07–1.20 (m, 6H), 1.21–1.39 (m, 6H), 1.46–1.53 (m, 2H), 1.57–1.64 (m, 2H), 1.70–1.79 (m, 2H), 1.83–1.90 (m, 1H), 3.98–4.07 (m, 4H), 6.94 (s, 2H), 6.95 (s, 2H), 6.99 (d, J = 8.3 Hz, 4H), 7.17–7.22 (m, 4H), 7.28 (d, J = 8.3 Hz, 6H), 7.30–7.35 (m, 14H); 13 C NMR (125 MHz, CDCl3 ) ı 19.7, 22.6, 22.7, 24.7, 27.9, 29.9, 36.3, 37.3, 39.2, 67.9, 86.7, 95.1, 113.9, 116.8, 121.5, 127.5, 127.6, 127.6, 127.7, 128.2, 128.7, 129.4, 130.3, 131.1, 137.5, 140.1, 143.2, 143.5, 153.5. Anal. calcd for C70 H74 O2 : C, 88.75; H, 7.87. Found: C, 88.95; H, 7.87. 2.3. Synthetic procedure for 5 (acetal route) 2.3.1. 1,4-Bis(4-(1,3-dioxolan-2-yl)phenylethynyl)-2,5-diethylbenzene (8b) (representative) To a THF solution (10 mL) of 4-(1,3-dioxolan-2-yl)phenylmethyl phenyl sulfone (7) (730 mg, 2.4 mmol), 1,4-diethyl-2,5-diformylbenzene (190 mg, 1.0 mmol) and chlorotrimethylsilane (0.304 mL, 2.4 mmol) was added lithium hexamethyl disilazide (10.0 mL of 1.0 M THF solution, 10.0 mmol) at 0 ◦ C, and the mixture was stirred at rt for 4 h. After usual workup with ethyl acetate and aqueous NH4 Cl, the organic layer was evaporated, the residue was subjected to column chromatography on silica gel (hexane:EtOAc = 5:1) to give 1,4-bis(4-(1,3-dioxolan-2-yl)phenylethynyl)2,5-diethylbenzene (8b) in a pure form (383 mg, 80%). 1,4-Bis(4-(1,3-dioxolan-2-yl) phenylethynyl)-2,5-diethylbenzene: m.p. 146–148 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 1.30 (t, J = 7.7 Hz, 6H), 2.85 (q, J = 7.7 Hz, 4H), 4.10–4.18 (m, 8H), 5.83 (s, 2H), 7.39 (s, 2H), 7.47 (d, J = 8.3 Hz, 4H), 7.55 (d, J = 8.3 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 14.7, 27.1, 65.3, 88.7, 93.8, 103.2, 122.4, 124.2, 126.5, 131.4, 131.5, 137.9, 143.4; HRMS (ESI+ ) exact mass calcd for C32 H30 O4 [M+ ] requires m/z 478.2144, found m/z 478.2159. 2.3.2. 1,4-Bis(4-(1,3-dioxolane-2-yl)phenylethynyl)benzene (8a) 85% yield (CH2 Cl2 :EtOAc = 10:1 for column chromatography); m.p. 208–210 ◦ C; H NMR (500 MHz, CDCl3 ) ı 4.01–4.16 (m, 8H), 5.83 (s, 2H), 7.47 (d, J = 8.3 Hz, 4H), 7.51 (s, 4H), 7.55 (d, J = 8.3 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 65.3, 89.6, 91.0, 103.3, 123.1, 123.9, 126.5, 131.6, 138.2; HRMS (ESI+ ) exact mass calcd for C28 H22 O4 [M+ ] requires m/z 422.1518, found m/z 422.1526. 1
2.3.3. 1,4-Bis(4-(1,3-dioxolane)2-yl)phenylethynyl)2,5-dimethoxybenzene (8c) 67% yield (CH2 Cl2 :EtOAc = 6:1 for column chromatography); m.p. 237–239 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 3.90 (s, 6H), 4.00–4.17 (m, 8H), 5.82 (s, 2H), 7.03 (s, 2H), 7.47 (d, J = 8.3 Hz, 4H), 7.58 (d, J = 8.3 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 56.5, 65.3, 86.2, 94.8, 103.3, 113.4, 115.7, 124.0, 126.4, 131.7, 138.0, 153.9; HRMS (ESI+ ) exact mass calcd for C30 H26 O6 [M+ ] requires m/z 482.1729, found m/z 482.1723.
2.3.4. 1,4-Bis(4-(1,3-dioxolane-2-yl)phenylethynyl)-2,5-bis(3,7dimethyloctyloxy)benzene (8d) 77% yield (hexane:EtOAc = 4:1 for column chromatography); m.p. 111–113 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 0.85 (d, J = 6.7 Hz, 2H), 0.97 (d, J = 6.7 Hz, 6H), 1.08–1.41 (m, 12H), 1.46–1.54 (m, 2H), 1.58–1.70 (m, 2H), 1.72–1.83 (m, 2H), 1.86–1.96 (m, 2H), 3.98–4.19 (m, 12H), 5.83 (s, 2H), 7.02 (s, 2H), 7.46 (d, J = 8.3 Hz, 4H), 7.55 (d, J = 8.3 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 19.6, 22.5, 22.6, 24.7, 27.8, 29.8, 36.2, 37.3, 39.1, 65.2, 67.7, 86.4, 94.5, 103.2, 113.7, 116.6, 124.2, 126.3, 131.4, 137.8, 153.5; HRMS (ESI+ ) exact mass calcd for C48 H62 O6 [M+ ] requires m/z 734.4546, found m/z 734.4552. 2.3.5. Transformation of 8 to 5 (representative) To a solution of 1,4-bis(4-(1,3-dioxolan-2-yl)phenylethynyl)-2,5-diethylbenzene (8b) (479 mg, 1.0 mmol) in acetone (5 mL) and water (5 mL) was added ptoluenesulphonic acid monohydrate (38 mg, 0.2 mmol) at rt, and the mixture was stirred at rt for 12 h. After usual workup with ethyl acetate and water, the organic layer was washed with aqueous NaHCO3 and brine, and dried over MgSO4 . After evaporation, the residue was subjected to column chromatography on silica gel (hexane:CH2 Cl2 = 2:1) to give 1,4-diethyl-2,5-bis(4-formylphenyl ethynyl)benzene (5b) in a pure form (379 mg, 97%). Other derivatives were prepared according to the same procedure. 2.4. UV–vis and PL characterization and fabrication of OLED device Neat films of 1a–1d were deposited in vacuum (10−4 Pa) on pre-cleaned glass, silicon and quartz substrates. The thickness of the deposited films was measured using a Stylus Profiler (DEKTAK 6M). The HOMO levels of 1a–1d were measured with ultraviolet photoelectron spectroscopy (AC-1, Riken Keiki). The absorption spectra were recorded using a UV–vis–NIR recording spectrophotometer (UV-3100, Shimadzu). The PL spectra were measured using a spectrofluorometer (FP-6500-A-51, Jasco). The absolute PL efficiency (˚F ) of the films was measured under argon flow using an integrating sphere with a xenon lamp as the excitation source and a multi-channel spectrometer (Hamamatsu, PMA-11) as an optical detector. To investigate EL properties, OLED devices were prepared using 4,4 -bis[N(1-naphtyl)-N-phenyl-amino]biphenyl (␣-NPD) as a hole-transport layer (HTL), and 4,7-diphenyl-1,10-phenanthroline (Bphen) as an electron-transport layer (ETL) and hole-blocking layer (HBL). A magnesium silver (Mg:Ag) alloy layer (10:1) capped with a silver layer was deposited on top of the organic layer. Current density–voltage–luminance (J–V–L) characteristics were measured using a semiconductor parameter analyzer (HP4155C, Agilent) with an optical power meter (Model 1835-C, Newport). The OLED devices were fabricated under conventional vacuum deposition procedure. An indium tin oxide (ITO) coated glass substrate was pre-cleaned by solvent degreasing and ultraviolet–ozone treatment prior to loading into the deposition system. Organic layers were deposited by high-vacuum (about 5 × 10−4 Pa) ther-
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Scheme 3. Synthetic route 2 for 1 (acetal route).
mal evaporation onto pre-cleaned substrates. First, a 50-nm thick (␣-NPD) HTL was deposited, followed by deposition of a 20-nm thick 1 light-emitting layer. Then, 4,7-diphenyl-1,10-phenanthroline (Bphen) was used to confine holes within 1 lightemitting layer and inject electrons. Finally, a shadow mask with 1-mm diameter openings was used to define the cathode consisting of a 100-nm thick Mg:Ag alloy (10:1) with a 20-nm thick silver cap layer. The active device area was 2 mm × 2 mm. To ensure the reproducibility of OLED devices, we measured six different devices and obtained reliable OLED characteristics.
3. Results and discussion For preparation of a phenyleneethynylene array, we planned to take advantage of double elimination protocol using -substituted sulfone [14–25]. Acetylenic fluorophores 1a–1d were successfully prepared by two alternative synthetic routes 1 and 2 which invoke the double elimination protocol as a key step for introducing acetylenic bonds (Scheme 2 for route 1 and Scheme 3 for route 2). 3.1. Synthesis of 1 by double elimination protocol Acetylenic fluorophores 1a–1d were prepared first by synthetic
of 5 were lower; for instance, 4b was transformed to 5b in 68% yield. Wittig–Horner olefination of 5a–5d with dimethyl diphenylmethylphosphonate 6 and t-BuOK afforded the desired acetylenic fluorophores 1a–1d in moderate yields (1a (57%), 1b (68%), 1c (61%), and 1d (60%)). Compounds 1 could be prepared alternatively according to synthetic route 2 (Scheme 3). Diacetal 8a was prepared in 85% yield by one-shot double elimination between acetal sulfone 7 and terephthalaldehyde (3a). In this reaction, use of trimethylsilyl chloride as a trapping agent afforded better results than the chlorophosphate (8b (80%), 8c (67%), 1d (77%)). When the chlorophosphate was used, the desired diacetal 8a was obtained only in 65% yield. Although employing ClP(O)(OEt)2 and a large excess of LiHMDS (16 equiv.) resulted in a high yield of 8a (85%), the same reactions for 8b–8d were somewhat sluggish under even in such reaction conditions giving rise to lower yields (8b (75%), 8c (61%), and 1d (72%)). This reaction proceeded as well in one-pot which involves aldol-type reaction with LiHMDS followed by double elimination with t-BuOK: the same product 8b was obtained in 70% yield (Eq. (1))
(1) route 1 (Scheme 2). Bis(4-bromophenylethynyl)benzenes 4 were prepared by one-shot double elimination [20] between 2 equiv. of 4-bromobenzyl sulfone 2 and terephthalaldehydes 3. When an excess amount of LiHMDS (lithium hexamethyldisilazide) in THF was added to a THF solution of 2, terephthalaldehyde and diethyl chlorophosphate at 0 ◦ C, and the mixture was stirred at rt for 4 h, the double elimination reaction proceeded smoothly to provide the desired 1,4-bis(4-bromophenylethynyl)benzene (4a) in 78% yield. According to the same procedure, diethyl (4b), dimethoxy (4c) and bis(3,7-dimethyloctyloxy) (4d) derivatives were prepared, respectively, in moderate to good yields (83%, 60% and 75%). When trimethylsilyl chloride (TMSCl) was used instead of ClP(O)(OEt)2 , the reaction for 4 was somewhat sluggish (4c (51%), 4d (73%)). Dibromide 4a was transformed easily to dialdehyde 5a in 65% yield by lithiation with a pentane solution of t-BuLi (2 equiv. for bromine) followed by formylation with dimethylformamide (DMF). The other formyl derivatives 5b–5d were provided successfully as well. When BuLi was used for lithiation instead of t-BuLi, the yields
Acetals 8 were transformed quantitatively to the corresponding dialdehydes 5 by acid-catalyzed hydrolysis in acetone and water, and the dialdehydes 5 were transformed to 1. All acetylenic fluorophores 1 derived from both routes were stable yellow powder and characterized by elemental analysis and spectroscopies. The third synthetic route was attempted through one-shot double elimination between 3 and 4-(2,2diphenylethenyl)phenylmethyl sulfone 10 (Scheme 4). However, the first step Wittig–Horner reaction did not proceed smoothly giving rise to formation of a complicated mixture. 3.2. Photoluminescence and electroluminescence characteristics UV–vis absorption and PL spectra of acetylenic fluorophores 1 in degassed CHCl3 were measured (Figs. 2 and 4). All compounds have large ε (>104 ), and acetylenic fluorophores 1a and 1b exhibited strong absorption bands at 350–450 nm. Since 1,4-bis(phenylethynyl)benzene [33] (11) and 1,4-(2,2-
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Fig. 2. UV–vis spectra of 1a–1d in CHCl3 at concentration of 8.0 × 10−5 M.
Fig. 4. Photoluminescence spectra of 1a–1d in CHCl3 (10−6 M).
Fig. 3. Chemical structures of compounds 11–13.
Scheme 4. Attempted synthetic route 3 for 1 (olefin route).
diphenylethenyl)benzene [27] (12) (Fig. 3) exhibited the highest emission peaks (max ) at 320 nm (CH2 Cl2 ) and 354 nm (CHCl3 ), respectively, it seems that -system in 1 is expanded effectively between phenyleneethynylene array and triarylethene moiety. In particular, incorporation of alkoxy groups on the central phenylene unit induced bathochromic shift of the –* transitions, and 1c and 1d showed the characteristic vibronic structure with the highest max at around 390 nm (Fig. 2). When a UV light was irradiated to the CHCl3 solution of 1a and 1b at the highest max of the absorption band, 1a indicated almost no emission, and diethyl derivative 1b showed only weak luminescence (Fig. 4). Since the fluorescence quantum efficiencies for 12 and 1,4-diethyl-2,6-bis(phenylethynyl)benzene (13) (Fig. 3) are ˚F = 0.059 (max = 461 nm)[27] and ˚F = 0.95 (max = 355 nm) [19], respectively, non-emissive characteristics of 1a and 1b is rationalized in terms of predominant effect of triarylethene moiety over
bis(phenylethynyl)benzene moiety. In sharp contrast to these nonemissive characteristics of 1a and 1b, the alkoxy derivatives 1c and 1d emitted strong luminescence with max at 437 nm and 441 nm, respectively (Fig. 4 and Table 1), and 1d with longer alkyloxy groups exhibited the strongest emission. Although we have already disclosed that incorporation of alkoxy group on the benzene ring of phenyleneethynylenes enhances PL [34,35], the same mechanism serves for the emission enhancement in 1c and 1d. Further noteworthy is the PL in the solid state. When a UV light was irradiated to the powders of 1a–1d, all these compounds showed strong blue emission giving high quantum yields (˚ 0.47) as shown in Table 1. Such strong emission can be explained in terms of steric congestion of the triarylethene moieties which reduces intermolecular vibronic interactions to circumvent fluorescence quenching. This result indicates a possibility that acetylenic triarylethenes 1 are applicable to blue-emission materials for OLEDs as we expected. Next, we fabricated vacuum deposited films of 1a–1d and measured the PL characteristics. The neat films of 1a–1d were successfully deposited by vacuum evaporation without decomposition, resulted in the transparent uniform morphology. When a UV light was irradiated, the deposited films of 1a–1d showed strong emission and the quantum yields are summarized in Table 1. All films emitted strong fluorescence (˚F > 0.60) in their deposited films, and 1b especially indicated the highest quantum yield of ˚F = 0.80. Further, we examined PL of co-deposited films of 6 wt% X:CBP (X, 1a–1d; CBP, 4,4 -bis(carbazol-9-yl)biphenyl), where the emitting molecules 1a–1d were distributed in a solid solution of a CBP host. The co-deposited films fabricated by 1a and 1b showed strong blue fluorescence which can be characterized as monomer emission with wavelength maxima at max = 448 nm (1a) and 434 nm and 452 nm (1b), respectively (Fig. 5). On the other hand, 1c and 1d emitted blue-greenish PL with f,max = 481 nm and 491 nm, respectively. The PL efficiencies of all co-deposited films
Table 1 Photoluminescence data (max : fluorescence peak, ˚F : absolute PL efficiency) of 1a–1d in CHCl3 solution (10−6 M), powder, vacuum deposited films and co-deposited films. Solution
1a 1b 1c 1d
Vacuum deposited neat films
Co-deposited films
max
˚F
max
Powder ˚F
max
˚F
max
˚F
446 423 437 441
0.02 0.04 0.43 0.57
456 456 471 502
0.50 0.47 0.54 0.64
453 455 492 490
0.60 0.80 0.69 0.63
448 434, 452 481 491
0.65 0.63 0.71 0.63
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Fig. 5. Photoluminescence spectra of co-deposited films of 6 wt% 1a–1d:CBP.
showed rather high values of ˚F = 0.65 (1a), 0.63 (1b), 0.71 (1c) and 0.63 (1d). These spectral characteristics in solution, neat films and doped films are summarized in Fig. 6. Although three PL spectra of 1a and 1b in solution, neat film and doped film have almost same max of around 450 nm, 1c and 1d showed significantly different characteristics between them, resulted in large redshift in both neat and co-deposited films. Also, very delicate spectral changes in these films are observed. For example, 1a showed the main peak at 450 nm and the sub-peaks at 430 nm, 470 nm and 500 nm. Although the solution peaks provide rather
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simplified one peak and two shoulders, the solid films provided both red- and blue-shift of the spectrum. These peak intensities change gradually depending on the three molecular environments, indicating that very delicate molecular interactions happen to occur in these molecules. Further, we note that ˚F and max characteristics are almost same in both neat and doped films (Table 1). Thus, these unique PL characteristics can be ascribed to their molecular interaction of 1c and 1d in the neat and even 6 wt% co-deposited films. Introduction of electron-donating substituents such as methoxy (MeO) and dimethyloctyloxy (DMOO) units would induce polarized electronic structure, leading to enhanced electrostatic interaction between the molecules. The PL study clarified that the neat deposited films demonstrate almost similar PL characteristics compared with those of the doped films. Thus, we fabricated OLED devices of ITO (110 nm)/␣-NPD (50 nm)/1a–1d (20 nm)/Bphen (30 nm)/Mg:Ag (100 nm)/Ag (20 nm) having the neat emitter layers (Fig. 7). The devices emitted blue-green EL with the maximum wavelength at max = 480–500 nm (Fig. 6 and Table 2). In their current density–voltage (J–V) characteristics, a significant increase of the operation voltages was observed in the device with compound 1d while the lowest driving voltage was observed in compound 1a, indicating that the presence of long alkyl chains disturbs the carrier transport process. In our future study, we would like to examine carrier mobilities of these materials and clarify the relationship between the driving voltage and the molecular structures. The maximum of EL = 2.4% [36] and maximum luminance of 70,650 cd m−2 at 2.7 A cm−2 were observed in the OLED with 1c. Since there is no straightforward relationship between EL and ˚F efficiencies, the balance of hole and electron injection and transport would control EL .
Fig. 6. Photoluminescence spectra of compounds 1a (a), 1b (b), 1c (c) and 1d (d) in CHCl3 (10−6 M), neat films, co-deposited films of 6 wt% 1a–1d:CBP and their EL spectra in OLED device structure of ITO/␣-NPD/1/Bphen/Mg:Ag/Ag.
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Table 2 OLED characteristics in device structure of ITO/␣-NPD (50 nm)/1a–1d (20 nm)/Bphen (30 nm)/Mg:Ag/Ag.
1a 1b 1c 1d
HOMO (eV)
LUMO (eV)
Driving voltage (V) at J = 100 mA cm−2
EL (%)
Maximum luminance (cd m−2 )
5.84 5.75 5.49 5.57
3.05 3.86 2.75 2.82
6.4 8.4 8.3 11.5
1.0 1.7 2.4 0.83
17,800 35,950 70,650 1,380
1c as an emission layer exhibited the maximum external quantum efficiency of EL = 2.4%, indicating that these fluorophores are a promising candidate for an emitter layer in OLEDs. Acknowledgments This work has been supported financially by Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) and Tokuyama foundation. This work was also supported by a Grant-in-Aid for the Global COE Program, “Science for Future Molecular Systems” from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References
Fig. 7. Current density–voltage (J–V) (a) and luminance–current density (b) characteristics of ITO/␣-NPD/1a (a), 1b (b), 1c (c) and 1d (d)/Bphen/Mg:Ag/Ag. External EL quantum efficiency–current density (EL –J) characteristics and device structure are provided in their insets, respectively.
In this paper, our preliminary examination demonstrated that 1a–1d have high potential for OLED’s emitters. We would expect further enhancement of device performance by optimizing device parameters such as thickness, doping concentration, and carrier transport materials which would balance hole and electron injection and transport. 4. Summary In conclusion, acetylenic fluorophores 1 were prepared successfully by use of double elimination protocol for construction of phenyleneethynylene array. All molecules showed high PL efficiency over ˚F = 0.6 in their solid-state films. An OLED device using
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Blue emitting fluorophores of phenyleneethynylenes substituted by diphenylethenyl terminal groups for organic light-emitting diodes Guoliang Mao a , Akihiro Orita a , Larysa Fenenko b , Masayuki Yahiro b , Chihaya Adachi b,∗ , Junzo Otera a a b
Department of Applied Chemistry, Okayama University of Science, Ridai-cho, Okayama 700-0005, Japan Center for Future Chemistry, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
a r t i c l e
i n f o
Article history: Received 3 August 2008 Received in revised form 19 October 2008 Accepted 13 December 2008 Keywords: Electronic materials Electrical properties Optical properties
a b s t r a c t Phenyleneethynylene motifs substituted by diphenylethenyl groups at both ends were prepared successfully by use of double elimination protocol of -substituted sulfones for introducing phenyleneethynylene arrays followed by Wittig–Horner reaction for introducing diphenylethenyl moiety. The hybrid blue fluorophores exhibited strong emission even in the solid-state films (˚F 0.60) while, in CHCl3 solution, incorporation of substituents on the central phenylene unit significantly enhanced emission efficiency up to ˚F = 0.57. The OLED devices with use of these blue fluorophores as an emitting material provided maximum external quantum efficiency of ext = 2.4%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Acetylenes have attracted extensive attention in material sciences [1–5]. In particular, phenyleneethynylenes are interesting materials since they exhibit high quantum yields in solution and their emission peaks can be facilely tuned by attachment of electron-donating or withdrawing substituents on benzene rings [6–9]. Further, poly(phenyleneethynylene)s (PPE) were revealed to emit strong fluorescence even in the solid thin-films [10]. Based on these high photoluminescence (PL) characteristics, PPE was used as a thin-film sensor which can detect very small amount of trinitrotoluene and dinitrotoluene effectively [11,12]. Further, there are several reports on organic light-emitting diodes (OLEDs) using PPE as an emitter, although their external electroluminescence (EL) quantum efficiencies (ext ) are still very low level of ext ∼ 0.035% [13–16]. Thus, further improvement in material design for high EL efficiency is still in demand. Recently, we have been involved in synthesis of various types of phenyleneethynylenes by use of double elimination protocol of -substituted sulfone [17–25] (Scheme 1) and disclosed that bis(4-(phenylethynyl)phenyl)ethynes indicate rather high hole mobilities in the range of h = 10−3 to 10−5 cm2 V−1 s−1 in ptype organic field-effect transistors (OFET) [26]. In addition, we have found that attachment of phenyleneethynylene to a silole fluorophore enables emission enhancement giving rise to a high quantum yield (˚ = 0.50) [19]. Further, we were intrigued by
triarylethene/phenyleneethynylene-hybrid blue fluorophores 1, which might be promising for emission materials in OLEDs (Fig. 1). We postulated that triarylethene moiety serves as a blue fluorophore which emits intense luminescence in the nanoparticle and the solid state [27,28], while phenyleneethynylene moiety provides efficient carrier transport ability. Herein, we report two double elimination routes for 1, optical properties such as ultraviolet–visible (UV–vis) absorption and PL in solution and in solid state as well as OLED characteristics using 1 as an emitting material. 2. Experimental details 2.1. General: syntheses and characterization All reactions were carried out under an atmosphere of argon with freshly distilled solvents, unless otherwise noted. Tetrahydrofuran (THF) was distilled from sodium/benzophenone. Other solvents such as toluene, diisopropylamine and CH2 Cl2 were distilled from CaH2 . THF and ether solution of LiHMDS and Grignard reagents were purchased and used without titration. A hexane solution of BuLi was purchased and titrated by Gilman method prior to use. A pentane solution of t-BuLi was purchased and used without titration. Silica gel (Daiso gel IR-60) was used for column chromatography. Sulfones 2 [29], 3 [30,31], 6 [32], and 7 [29] were prepared according to the reported procedure. NMR spectra were recorded at 25 ◦ C on JEOL Lambda 300 and JEOL Lambda 500 instruments and calibrated with tetramethylsilane (TMS) as an internal reference. Elemental analyses were performed by the PerkinElmer PE 2400. UV–vis and photofluorescence were recorded by JASCO V-560 and JASCO FP-6500 at room temperature, respectively. Absolute quantum yields of fluorescence were recorded by an integration sphere system (Hamamatsu photonics C9920-02). 2.2. Synthetic procedure for 1 (bromide route)
∗ Corresponding author. Tel.: +81 92 802 3306; fax: +81 92 802 3306. E-mail address: [email protected] (C. Adachi). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.12.015
2.2.1. 1,4-Bis(4-bromophenylethynyl)-2,5-diethynylbenzene (4b) (representative) To a THF solution (10 mL) of 4-bromophenylmethyl phenyl sulfone (747 mg, 2.4 mmol), 1,4-diethyl-2,5-diformylbenzene (190 mg, 1.0 mmol) and diethyl
G. Mao et al. / Materials Chemistry and Physics 115 (2009) 378–384
379
Scheme 1. Double elimination protocol for diphenylacetylenes.
chlorophosphate (0.345 mL, 2.4 mmol) was added lithium hexamethyldisilazide (8.0 mL of 1.0 M THF solution, 8.0 mmol) at 0 ◦ C, and the mixture was stirred at rt for 4 h. After usual workup with ethyl acetate and aqueous NH4 Cl, the organic layer was evaporated, the residue was subjected to column chromatography on silica gel (hexane) to give 1,4-bis(4-bromophenylethynyl)-2,5-diethynylbenzene (4b) in a pure form (409 mg, 83%). 1,4-Bis(4-bromophenylethynyl)-2,5-diethynylbenzene: m.p. 147–149 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 1.30 (t, J = 7.7 Hz, 6H), 2.83 (q, J = 7.7 Hz, 4H), 7.39 (s, 2H), 7.39 (d, J = 8.3 Hz, 4H), 7.50 (d, J = 8.3 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 14.7, 27.1, 89.2, 93.1, 122.2, 122.3, 122.5, 131.5, 131.6, 132.9, 143.5; HRMS (ESI+ ) exact mass calcd for C26 H20 Br2 [M+ ] requires m/z 489.9932, found m/z 489.9922. 2.2.2. 1,4-Bis(4-bromophenylethynyl)benzene (4a) 78% yield (hexane:CH2 Cl2 = 3:1 for column chromatography); m.p. > 300 ◦ C; 1 H NMR (300 MHz, CDCl3 ) ı 7.43 (d, J = 8.8 Hz, 4H), 7.50 (d, J = 8.8 Hz, 4H), 7.51 (s, 4H); 13 C NMR spectra could not be measured because of poor solubility; HRMS (ESI+ ) exact mass calcd for C22 H12 Br2 [M+ ] requires m/z 433.9306, found m/z 433.9310. 2.2.3. 1,4-Bis(4-bromophenylethynyl)-2,5-dimethoxybenzene (4c) 60% yield (hexane:CH2 Cl2 = 2:1 for column chromatography); m.p. 235–237 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 3.90 (s, 6H), 7.02 (s, 2H), 7.43 (d, J = 8.5 Hz, 4H), 7.49 (d, J = 8.5 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 56.4, 86.7, 94.0, 113.2, 115.5, 122.1, 122.7, 131.6, 133.1, 153.9; HRMS (ESI+ ) exact mass calcd for C24 H16 Br2 O2 [M+ ] requires m/z 493.9517, found m/z 493.9523. 2.2.4. 1,4-Bis(4-bromophenylethynyl)-2,5-(3,7-dimethyloctyloxy)benzene (4d) 75% yield (hexane:Et2 O) = 20:1 for column chromatography); m.p. 96–98 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 0.85 (d, J = 6.7 Hz, 12H), 0.97 (d, J = 6.7 Hz, 6H), 1.07–1.40 (m, 12H), 1.45–1.54 (m, 2H), 1.58–1.68 (m, 2H), 1.70–1.82 (m, 2H), 1.85–1.94 (m, 2H), 4.00–4.10 (m, 4H), 7.00 (s, 2H), 7.38 (d, J = 8.5 Hz, 4H), 7.48 (d, J = 8.5 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 19.7, 22.6, 22.7, 24.8, 28.0, 29.9, 36.3, 37.4, 39.2, 67.9, 87.1, 93.8, 113.8, 116.7, 122.4, 122.5, 131.6, 132.9, 153.6; HRMS (ESI+ ) exact mass calcd for C42 H52 Br2 O2 [M+ ] requires m/z 746.2334, found m/z 746.2343. 2.2.5. 1,4-Diethyl-2,5-bis(4-formylphenylethynyl)benzene (5b) (representative) To a THF solution (10 mL) of 1,4-bis(4-bromophenylethynyl)-2,5-diethylbenzene (492 mg, 1.0 mmol) was added t-BuLi (2.72 mL of 1.47 M pentane solution, 4.0 mmol) at −78 ◦ C, and the mixture was stirred at the same temperature. After 1 h, N,N-dimethyl formamide (0.192 mL, 2.5 mmol) was added at −78 ◦ C, and the mixture was stirred at rt for 4 h. After usual workup with ethyl acetate and aqueous NH4 Cl, the organic layer was evaporated, and the residue was subjected to column chromatography on silica gel (hexane:CH2 Cl2 = 2:1) to give 1,4-diethyl-2,5-bis(4-formylphenylethynyl)benzene (5b) in a pure form (312 mg, 80%). 1,4-Bis(4-formylphenylethynyl)-2,5-diethyl benzene: m.p. 175–177 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 1.33 (t, J = 7.7 Hz, 6H), 2.87 (q, J = 7.7 Hz, 4H), 7.43 (s, 2H), 7.68 (d, J = 8.3 Hz, 4H), 7.88 (d, J = 8.3 Hz, 4H), 10.03 (s, 2H); 13 C NMR (125 MHz, CDCl3 ) ı 14.7, 27.2, 92.1, 93.5, 122.4, 129.6, 129.6, 131.9, 132.0, 135.5, 143.9, 191.3; HRMS (ESI+ ) exact mass calcd for C28 H22 O2 [M+ ] requires m/z 390.1620, found m/z 390.1627.
Fig. 1. Chemical structures of acetylenic fluorophores 1.
2.2.6. 1,4-Bis(4-formylphenylethynyl)benzene (5a) 65% yield (hexane:CH2 Cl2 = 1:4 for column chromatography); m.p. 242–244 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 7.57 (s, 4H), 7.69 (d, J = 8.3 Hz, 4H), 7.89 (d, J = 8.3 Hz, 4H), 10.04 (s, 2H); 13 C NMR spectra could not be measured because of poor solubility; HRMS (ESI+ ) exact mass calcd for C24 H14 O2 [M+ ] requires m/z 334.0994, found m/z 334.1002.
2.2.7. 1,4-Bis(4-formylphenylethynyl)-2,5-dimethoxybenzene (5c) 71% yield (hexane:CH2 Cl2 = 1:4 for column chromatography); m.p. 229–231 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 3.93 (s, 6H), 7.06 (s, 6H), 7.72 (d, J = 8.3 Hz, 4H), 7.88 (d, J = 8.3 Hz, 4H), 10.03 (s, 2H); 13 C NMR (125 MHz, CDCl3 ) ı 56.5, 89.6, 113.4, 115.7, 129.4, 129.6, 132.2, 135.5, 154.1, 191.4; HRMS (ESI+ ) exact mass calcd for C26 H18 O4 [M+ ] requires m/z 394.1205, found m/z 394.1211.
2.2.8. 1,4-Bis(4-formylphenylethynyl)-2,5-(3,7-dimethyloctyloxy)benzene (5d) 53% yield (hexane:EtOAc = 5:1 for column chromatography); m.p. 105–108 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 0.84 (d, J = 6.4 Hz, 12H), 0.99 (d, J = 6.4 Hz, 6H), 1.05–1.42 (m, 12H), 1.43–1.57 (m, 2H), 1.58–1.71 (m, 2H), 1.72–1.85 (m, 2H), 1.87–1.97 (m, 2H), 4.03–4.14 (m, 4H), 7.05 (s, 2H), 7.67 (d, J = 8.3 Hz, 4H), 7.87 (d, J = 8.3 Hz, 4H), 10.03 (s, 2H); 13 C NMR (125 MHz, CDCl3 ) ı 19.7, 22.6, 22.7, 24.8, 28.0, 29.9, 36.2, 37.3, 39.2, 67.8, 90.0, 94.2, 113.8, 116.6, 129.6, 132.0, 135.4, 153.8, 191.4; HRMS (ESI+ ) exact mass calcd for C44 H54 O4 [M+ ] requires m/z 646.4022, found m/z 646.4031.
2.2.9. 1,4-Diethyl-2,5-bis(4-(2,2-diphenylethenyl)phenylethynyl)benzene (1b) (representative) To a DMF solution (25 mL) of 1,4-diethyl-2,5-bis(4-formylphenyl ethynyl)benzene (781 mg, 2.0 mmol) were added dimethyl diphenylmethylphosphonate (1.326 g, 4.8 mmol) and potassium t-butoxide (898 mg, 8.0 mmol), and the mixture was stirred at rt for 12 h. After usual workup with ethyl acetate and aqueous NaCl, the organic layer was evaporated, and the residue was subjected to column chromatography on silica gel (hexane:CH2 Cl2 = 10:1) to give 1,4-diethyl-2,5-bis(4(2,2-diphenyl ethenyl)phenylethynyl)benzene (1b) as pale yellow powder (940 mg, 68%). 1,4-Diethyl-2,5-bis(4-(2,2-diphenylethenyl)phenylethynyl)benzene: m.p. 231–233 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 1.26 (t, J = 7.6 Hz, 6H), 2.80 (q, J = 7.6 Hz, 4H), 6.96 (s, 2H), 7.00 (d, J = 8.3 Hz, 4H), 7.19–7.23 (m, 4H), 7.28 (d, J = 8.3 Hz, 4H), 7.29–7.31 (m, 2H), 7.31–7.33 (m, 10H), 7.34 (d, J = 1.9 Hz, 4H), 7.35 (d, J = 1.8 Hz, 2H); 13 C NMR (125 MHz, CDCl3 ) ı 14.7, 27.1, 88.9, 94.3, 121.5, 122.4, 127.5, 127.6, 127.7, 128.2, 128.7, 129.5, 130.3, 131.1, 131.5, 137.5, 140.1, 143.2, 143.3, 143.6. Anal. calcd for C54 H42 : C, 93.87; H, 6.13. Found: C, 93.74; H, 6.19.
2.2.10. 1,4-Bis(4-(2,2-diphenylethenyl)phenylethynyl)benzene (1a) 57% yield (hexane:CH2 Cl2 = 1:2 for column chromatography); m.p. 269–272 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 6.95 (s, 2H), 7.00 (d, J = 8.6 Hz, 4H), 7.19–7.21 (m, 4H), 7.28 (d, J = 8.2 Hz, 4H), 7.30–7.37 (m, 16H), 7.44 (s, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 89.8, 91.5, 121.1, 123.1, 127.4, 127.6, 127.7, 127.8, 128.3, 128.7, 129.5, 130.4, 131.2, 131.5, 137.7, 140.1. Anal. calcd for C50 H34 : C, 94.60; H, 5.40. Found: C, 94.69; H, 5.58.
2.2.11. 1,4-Dimethoxy-2,5-bis(4-(2,2-diphenylethenyl)phenylethynyl)benzene (1c) 61% yield (hexane:CH2 Cl2 = 10:1 for column chromatography); m.p. 252–254 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 3.86 (s, 6H), 6.95 (s, 2H), 6.97 (s, 2H), 6.99 (d, J = 8.2 Hz, 4H), 7.18–7.22 (m, 4H), 7.28–7.37 (m, 20H); 13 C NMR (125 MHz, CDCl3 ) ı 56.5, 86.3, 95.3, 113.4, 115.6, 121.3, 127.5, 127.6, 127.7, 127.8, 128.3, 128.7, 129.4, 130.4, 131.3, 137.7, 140.1, 143.2, 143.7, 153.9. Anal. calcd for C52 H38 O2 : C, 89.88; H, 5.51. Found: C, 90.09; H, 5.83.
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Scheme 2. Synthetic route 1 for 1 (bromide route).
2.2.12. 1,4-(3,7-Dimethyloctyloxy)-2,5-bis(4-(2,2diphenylethenyl)phenylethynyl)benzene (1d) 60% yield (hexane:EtOAc = 10:1 for column chromatography); m.p. 101–103 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 0.85 (d, J = 6.4 Hz, 12H), 0.95 (d, J = 6.4 Hz, 6H), 1.07–1.20 (m, 6H), 1.21–1.39 (m, 6H), 1.46–1.53 (m, 2H), 1.57–1.64 (m, 2H), 1.70–1.79 (m, 2H), 1.83–1.90 (m, 1H), 3.98–4.07 (m, 4H), 6.94 (s, 2H), 6.95 (s, 2H), 6.99 (d, J = 8.3 Hz, 4H), 7.17–7.22 (m, 4H), 7.28 (d, J = 8.3 Hz, 6H), 7.30–7.35 (m, 14H); 13 C NMR (125 MHz, CDCl3 ) ı 19.7, 22.6, 22.7, 24.7, 27.9, 29.9, 36.3, 37.3, 39.2, 67.9, 86.7, 95.1, 113.9, 116.8, 121.5, 127.5, 127.6, 127.6, 127.7, 128.2, 128.7, 129.4, 130.3, 131.1, 137.5, 140.1, 143.2, 143.5, 153.5. Anal. calcd for C70 H74 O2 : C, 88.75; H, 7.87. Found: C, 88.95; H, 7.87. 2.3. Synthetic procedure for 5 (acetal route) 2.3.1. 1,4-Bis(4-(1,3-dioxolan-2-yl)phenylethynyl)-2,5-diethylbenzene (8b) (representative) To a THF solution (10 mL) of 4-(1,3-dioxolan-2-yl)phenylmethyl phenyl sulfone (7) (730 mg, 2.4 mmol), 1,4-diethyl-2,5-diformylbenzene (190 mg, 1.0 mmol) and chlorotrimethylsilane (0.304 mL, 2.4 mmol) was added lithium hexamethyl disilazide (10.0 mL of 1.0 M THF solution, 10.0 mmol) at 0 ◦ C, and the mixture was stirred at rt for 4 h. After usual workup with ethyl acetate and aqueous NH4 Cl, the organic layer was evaporated, the residue was subjected to column chromatography on silica gel (hexane:EtOAc = 5:1) to give 1,4-bis(4-(1,3-dioxolan-2-yl)phenylethynyl)2,5-diethylbenzene (8b) in a pure form (383 mg, 80%). 1,4-Bis(4-(1,3-dioxolan-2-yl) phenylethynyl)-2,5-diethylbenzene: m.p. 146–148 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 1.30 (t, J = 7.7 Hz, 6H), 2.85 (q, J = 7.7 Hz, 4H), 4.10–4.18 (m, 8H), 5.83 (s, 2H), 7.39 (s, 2H), 7.47 (d, J = 8.3 Hz, 4H), 7.55 (d, J = 8.3 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 14.7, 27.1, 65.3, 88.7, 93.8, 103.2, 122.4, 124.2, 126.5, 131.4, 131.5, 137.9, 143.4; HRMS (ESI+ ) exact mass calcd for C32 H30 O4 [M+ ] requires m/z 478.2144, found m/z 478.2159. 2.3.2. 1,4-Bis(4-(1,3-dioxolane-2-yl)phenylethynyl)benzene (8a) 85% yield (CH2 Cl2 :EtOAc = 10:1 for column chromatography); m.p. 208–210 ◦ C; H NMR (500 MHz, CDCl3 ) ı 4.01–4.16 (m, 8H), 5.83 (s, 2H), 7.47 (d, J = 8.3 Hz, 4H), 7.51 (s, 4H), 7.55 (d, J = 8.3 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 65.3, 89.6, 91.0, 103.3, 123.1, 123.9, 126.5, 131.6, 138.2; HRMS (ESI+ ) exact mass calcd for C28 H22 O4 [M+ ] requires m/z 422.1518, found m/z 422.1526. 1
2.3.3. 1,4-Bis(4-(1,3-dioxolane)2-yl)phenylethynyl)2,5-dimethoxybenzene (8c) 67% yield (CH2 Cl2 :EtOAc = 6:1 for column chromatography); m.p. 237–239 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 3.90 (s, 6H), 4.00–4.17 (m, 8H), 5.82 (s, 2H), 7.03 (s, 2H), 7.47 (d, J = 8.3 Hz, 4H), 7.58 (d, J = 8.3 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 56.5, 65.3, 86.2, 94.8, 103.3, 113.4, 115.7, 124.0, 126.4, 131.7, 138.0, 153.9; HRMS (ESI+ ) exact mass calcd for C30 H26 O6 [M+ ] requires m/z 482.1729, found m/z 482.1723.
2.3.4. 1,4-Bis(4-(1,3-dioxolane-2-yl)phenylethynyl)-2,5-bis(3,7dimethyloctyloxy)benzene (8d) 77% yield (hexane:EtOAc = 4:1 for column chromatography); m.p. 111–113 ◦ C; 1 H NMR (500 MHz, CDCl3 ) ı 0.85 (d, J = 6.7 Hz, 2H), 0.97 (d, J = 6.7 Hz, 6H), 1.08–1.41 (m, 12H), 1.46–1.54 (m, 2H), 1.58–1.70 (m, 2H), 1.72–1.83 (m, 2H), 1.86–1.96 (m, 2H), 3.98–4.19 (m, 12H), 5.83 (s, 2H), 7.02 (s, 2H), 7.46 (d, J = 8.3 Hz, 4H), 7.55 (d, J = 8.3 Hz, 4H); 13 C NMR (125 MHz, CDCl3 ) ı 19.6, 22.5, 22.6, 24.7, 27.8, 29.8, 36.2, 37.3, 39.1, 65.2, 67.7, 86.4, 94.5, 103.2, 113.7, 116.6, 124.2, 126.3, 131.4, 137.8, 153.5; HRMS (ESI+ ) exact mass calcd for C48 H62 O6 [M+ ] requires m/z 734.4546, found m/z 734.4552. 2.3.5. Transformation of 8 to 5 (representative) To a solution of 1,4-bis(4-(1,3-dioxolan-2-yl)phenylethynyl)-2,5-diethylbenzene (8b) (479 mg, 1.0 mmol) in acetone (5 mL) and water (5 mL) was added ptoluenesulphonic acid monohydrate (38 mg, 0.2 mmol) at rt, and the mixture was stirred at rt for 12 h. After usual workup with ethyl acetate and water, the organic layer was washed with aqueous NaHCO3 and brine, and dried over MgSO4 . After evaporation, the residue was subjected to column chromatography on silica gel (hexane:CH2 Cl2 = 2:1) to give 1,4-diethyl-2,5-bis(4-formylphenyl ethynyl)benzene (5b) in a pure form (379 mg, 97%). Other derivatives were prepared according to the same procedure. 2.4. UV–vis and PL characterization and fabrication of OLED device Neat films of 1a–1d were deposited in vacuum (10−4 Pa) on pre-cleaned glass, silicon and quartz substrates. The thickness of the deposited films was measured using a Stylus Profiler (DEKTAK 6M). The HOMO levels of 1a–1d were measured with ultraviolet photoelectron spectroscopy (AC-1, Riken Keiki). The absorption spectra were recorded using a UV–vis–NIR recording spectrophotometer (UV-3100, Shimadzu). The PL spectra were measured using a spectrofluorometer (FP-6500-A-51, Jasco). The absolute PL efficiency (˚F ) of the films was measured under argon flow using an integrating sphere with a xenon lamp as the excitation source and a multi-channel spectrometer (Hamamatsu, PMA-11) as an optical detector. To investigate EL properties, OLED devices were prepared using 4,4 -bis[N(1-naphtyl)-N-phenyl-amino]biphenyl (␣-NPD) as a hole-transport layer (HTL), and 4,7-diphenyl-1,10-phenanthroline (Bphen) as an electron-transport layer (ETL) and hole-blocking layer (HBL). A magnesium silver (Mg:Ag) alloy layer (10:1) capped with a silver layer was deposited on top of the organic layer. Current density–voltage–luminance (J–V–L) characteristics were measured using a semiconductor parameter analyzer (HP4155C, Agilent) with an optical power meter (Model 1835-C, Newport). The OLED devices were fabricated under conventional vacuum deposition procedure. An indium tin oxide (ITO) coated glass substrate was pre-cleaned by solvent degreasing and ultraviolet–ozone treatment prior to loading into the deposition system. Organic layers were deposited by high-vacuum (about 5 × 10−4 Pa) ther-
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Scheme 3. Synthetic route 2 for 1 (acetal route).
mal evaporation onto pre-cleaned substrates. First, a 50-nm thick (␣-NPD) HTL was deposited, followed by deposition of a 20-nm thick 1 light-emitting layer. Then, 4,7-diphenyl-1,10-phenanthroline (Bphen) was used to confine holes within 1 lightemitting layer and inject electrons. Finally, a shadow mask with 1-mm diameter openings was used to define the cathode consisting of a 100-nm thick Mg:Ag alloy (10:1) with a 20-nm thick silver cap layer. The active device area was 2 mm × 2 mm. To ensure the reproducibility of OLED devices, we measured six different devices and obtained reliable OLED characteristics.
3. Results and discussion For preparation of a phenyleneethynylene array, we planned to take advantage of double elimination protocol using -substituted sulfone [14–25]. Acetylenic fluorophores 1a–1d were successfully prepared by two alternative synthetic routes 1 and 2 which invoke the double elimination protocol as a key step for introducing acetylenic bonds (Scheme 2 for route 1 and Scheme 3 for route 2). 3.1. Synthesis of 1 by double elimination protocol Acetylenic fluorophores 1a–1d were prepared first by synthetic
of 5 were lower; for instance, 4b was transformed to 5b in 68% yield. Wittig–Horner olefination of 5a–5d with dimethyl diphenylmethylphosphonate 6 and t-BuOK afforded the desired acetylenic fluorophores 1a–1d in moderate yields (1a (57%), 1b (68%), 1c (61%), and 1d (60%)). Compounds 1 could be prepared alternatively according to synthetic route 2 (Scheme 3). Diacetal 8a was prepared in 85% yield by one-shot double elimination between acetal sulfone 7 and terephthalaldehyde (3a). In this reaction, use of trimethylsilyl chloride as a trapping agent afforded better results than the chlorophosphate (8b (80%), 8c (67%), 1d (77%)). When the chlorophosphate was used, the desired diacetal 8a was obtained only in 65% yield. Although employing ClP(O)(OEt)2 and a large excess of LiHMDS (16 equiv.) resulted in a high yield of 8a (85%), the same reactions for 8b–8d were somewhat sluggish under even in such reaction conditions giving rise to lower yields (8b (75%), 8c (61%), and 1d (72%)). This reaction proceeded as well in one-pot which involves aldol-type reaction with LiHMDS followed by double elimination with t-BuOK: the same product 8b was obtained in 70% yield (Eq. (1))
(1) route 1 (Scheme 2). Bis(4-bromophenylethynyl)benzenes 4 were prepared by one-shot double elimination [20] between 2 equiv. of 4-bromobenzyl sulfone 2 and terephthalaldehydes 3. When an excess amount of LiHMDS (lithium hexamethyldisilazide) in THF was added to a THF solution of 2, terephthalaldehyde and diethyl chlorophosphate at 0 ◦ C, and the mixture was stirred at rt for 4 h, the double elimination reaction proceeded smoothly to provide the desired 1,4-bis(4-bromophenylethynyl)benzene (4a) in 78% yield. According to the same procedure, diethyl (4b), dimethoxy (4c) and bis(3,7-dimethyloctyloxy) (4d) derivatives were prepared, respectively, in moderate to good yields (83%, 60% and 75%). When trimethylsilyl chloride (TMSCl) was used instead of ClP(O)(OEt)2 , the reaction for 4 was somewhat sluggish (4c (51%), 4d (73%)). Dibromide 4a was transformed easily to dialdehyde 5a in 65% yield by lithiation with a pentane solution of t-BuLi (2 equiv. for bromine) followed by formylation with dimethylformamide (DMF). The other formyl derivatives 5b–5d were provided successfully as well. When BuLi was used for lithiation instead of t-BuLi, the yields
Acetals 8 were transformed quantitatively to the corresponding dialdehydes 5 by acid-catalyzed hydrolysis in acetone and water, and the dialdehydes 5 were transformed to 1. All acetylenic fluorophores 1 derived from both routes were stable yellow powder and characterized by elemental analysis and spectroscopies. The third synthetic route was attempted through one-shot double elimination between 3 and 4-(2,2diphenylethenyl)phenylmethyl sulfone 10 (Scheme 4). However, the first step Wittig–Horner reaction did not proceed smoothly giving rise to formation of a complicated mixture. 3.2. Photoluminescence and electroluminescence characteristics UV–vis absorption and PL spectra of acetylenic fluorophores 1 in degassed CHCl3 were measured (Figs. 2 and 4). All compounds have large ε (>104 ), and acetylenic fluorophores 1a and 1b exhibited strong absorption bands at 350–450 nm. Since 1,4-bis(phenylethynyl)benzene [33] (11) and 1,4-(2,2-
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Fig. 2. UV–vis spectra of 1a–1d in CHCl3 at concentration of 8.0 × 10−5 M.
Fig. 4. Photoluminescence spectra of 1a–1d in CHCl3 (10−6 M).
Fig. 3. Chemical structures of compounds 11–13.
Scheme 4. Attempted synthetic route 3 for 1 (olefin route).
diphenylethenyl)benzene [27] (12) (Fig. 3) exhibited the highest emission peaks (max ) at 320 nm (CH2 Cl2 ) and 354 nm (CHCl3 ), respectively, it seems that -system in 1 is expanded effectively between phenyleneethynylene array and triarylethene moiety. In particular, incorporation of alkoxy groups on the central phenylene unit induced bathochromic shift of the –* transitions, and 1c and 1d showed the characteristic vibronic structure with the highest max at around 390 nm (Fig. 2). When a UV light was irradiated to the CHCl3 solution of 1a and 1b at the highest max of the absorption band, 1a indicated almost no emission, and diethyl derivative 1b showed only weak luminescence (Fig. 4). Since the fluorescence quantum efficiencies for 12 and 1,4-diethyl-2,6-bis(phenylethynyl)benzene (13) (Fig. 3) are ˚F = 0.059 (max = 461 nm)[27] and ˚F = 0.95 (max = 355 nm) [19], respectively, non-emissive characteristics of 1a and 1b is rationalized in terms of predominant effect of triarylethene moiety over
bis(phenylethynyl)benzene moiety. In sharp contrast to these nonemissive characteristics of 1a and 1b, the alkoxy derivatives 1c and 1d emitted strong luminescence with max at 437 nm and 441 nm, respectively (Fig. 4 and Table 1), and 1d with longer alkyloxy groups exhibited the strongest emission. Although we have already disclosed that incorporation of alkoxy group on the benzene ring of phenyleneethynylenes enhances PL [34,35], the same mechanism serves for the emission enhancement in 1c and 1d. Further noteworthy is the PL in the solid state. When a UV light was irradiated to the powders of 1a–1d, all these compounds showed strong blue emission giving high quantum yields (˚ 0.47) as shown in Table 1. Such strong emission can be explained in terms of steric congestion of the triarylethene moieties which reduces intermolecular vibronic interactions to circumvent fluorescence quenching. This result indicates a possibility that acetylenic triarylethenes 1 are applicable to blue-emission materials for OLEDs as we expected. Next, we fabricated vacuum deposited films of 1a–1d and measured the PL characteristics. The neat films of 1a–1d were successfully deposited by vacuum evaporation without decomposition, resulted in the transparent uniform morphology. When a UV light was irradiated, the deposited films of 1a–1d showed strong emission and the quantum yields are summarized in Table 1. All films emitted strong fluorescence (˚F > 0.60) in their deposited films, and 1b especially indicated the highest quantum yield of ˚F = 0.80. Further, we examined PL of co-deposited films of 6 wt% X:CBP (X, 1a–1d; CBP, 4,4 -bis(carbazol-9-yl)biphenyl), where the emitting molecules 1a–1d were distributed in a solid solution of a CBP host. The co-deposited films fabricated by 1a and 1b showed strong blue fluorescence which can be characterized as monomer emission with wavelength maxima at max = 448 nm (1a) and 434 nm and 452 nm (1b), respectively (Fig. 5). On the other hand, 1c and 1d emitted blue-greenish PL with f,max = 481 nm and 491 nm, respectively. The PL efficiencies of all co-deposited films
Table 1 Photoluminescence data (max : fluorescence peak, ˚F : absolute PL efficiency) of 1a–1d in CHCl3 solution (10−6 M), powder, vacuum deposited films and co-deposited films. Solution
1a 1b 1c 1d
Vacuum deposited neat films
Co-deposited films
max
˚F
max
Powder ˚F
max
˚F
max
˚F
446 423 437 441
0.02 0.04 0.43 0.57
456 456 471 502
0.50 0.47 0.54 0.64
453 455 492 490
0.60 0.80 0.69 0.63
448 434, 452 481 491
0.65 0.63 0.71 0.63
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Fig. 5. Photoluminescence spectra of co-deposited films of 6 wt% 1a–1d:CBP.
showed rather high values of ˚F = 0.65 (1a), 0.63 (1b), 0.71 (1c) and 0.63 (1d). These spectral characteristics in solution, neat films and doped films are summarized in Fig. 6. Although three PL spectra of 1a and 1b in solution, neat film and doped film have almost same max of around 450 nm, 1c and 1d showed significantly different characteristics between them, resulted in large redshift in both neat and co-deposited films. Also, very delicate spectral changes in these films are observed. For example, 1a showed the main peak at 450 nm and the sub-peaks at 430 nm, 470 nm and 500 nm. Although the solution peaks provide rather
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simplified one peak and two shoulders, the solid films provided both red- and blue-shift of the spectrum. These peak intensities change gradually depending on the three molecular environments, indicating that very delicate molecular interactions happen to occur in these molecules. Further, we note that ˚F and max characteristics are almost same in both neat and doped films (Table 1). Thus, these unique PL characteristics can be ascribed to their molecular interaction of 1c and 1d in the neat and even 6 wt% co-deposited films. Introduction of electron-donating substituents such as methoxy (MeO) and dimethyloctyloxy (DMOO) units would induce polarized electronic structure, leading to enhanced electrostatic interaction between the molecules. The PL study clarified that the neat deposited films demonstrate almost similar PL characteristics compared with those of the doped films. Thus, we fabricated OLED devices of ITO (110 nm)/␣-NPD (50 nm)/1a–1d (20 nm)/Bphen (30 nm)/Mg:Ag (100 nm)/Ag (20 nm) having the neat emitter layers (Fig. 7). The devices emitted blue-green EL with the maximum wavelength at max = 480–500 nm (Fig. 6 and Table 2). In their current density–voltage (J–V) characteristics, a significant increase of the operation voltages was observed in the device with compound 1d while the lowest driving voltage was observed in compound 1a, indicating that the presence of long alkyl chains disturbs the carrier transport process. In our future study, we would like to examine carrier mobilities of these materials and clarify the relationship between the driving voltage and the molecular structures. The maximum of EL = 2.4% [36] and maximum luminance of 70,650 cd m−2 at 2.7 A cm−2 were observed in the OLED with 1c. Since there is no straightforward relationship between EL and ˚F efficiencies, the balance of hole and electron injection and transport would control EL .
Fig. 6. Photoluminescence spectra of compounds 1a (a), 1b (b), 1c (c) and 1d (d) in CHCl3 (10−6 M), neat films, co-deposited films of 6 wt% 1a–1d:CBP and their EL spectra in OLED device structure of ITO/␣-NPD/1/Bphen/Mg:Ag/Ag.
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Table 2 OLED characteristics in device structure of ITO/␣-NPD (50 nm)/1a–1d (20 nm)/Bphen (30 nm)/Mg:Ag/Ag.
1a 1b 1c 1d
HOMO (eV)
LUMO (eV)
Driving voltage (V) at J = 100 mA cm−2
EL (%)
Maximum luminance (cd m−2 )
5.84 5.75 5.49 5.57
3.05 3.86 2.75 2.82
6.4 8.4 8.3 11.5
1.0 1.7 2.4 0.83
17,800 35,950 70,650 1,380
1c as an emission layer exhibited the maximum external quantum efficiency of EL = 2.4%, indicating that these fluorophores are a promising candidate for an emitter layer in OLEDs. Acknowledgments This work has been supported financially by Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) and Tokuyama foundation. This work was also supported by a Grant-in-Aid for the Global COE Program, “Science for Future Molecular Systems” from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References
Fig. 7. Current density–voltage (J–V) (a) and luminance–current density (b) characteristics of ITO/␣-NPD/1a (a), 1b (b), 1c (c) and 1d (d)/Bphen/Mg:Ag/Ag. External EL quantum efficiency–current density (EL –J) characteristics and device structure are provided in their insets, respectively.
In this paper, our preliminary examination demonstrated that 1a–1d have high potential for OLED’s emitters. We would expect further enhancement of device performance by optimizing device parameters such as thickness, doping concentration, and carrier transport materials which would balance hole and electron injection and transport. 4. Summary In conclusion, acetylenic fluorophores 1 were prepared successfully by use of double elimination protocol for construction of phenyleneethynylene array. All molecules showed high PL efficiency over ˚F = 0.6 in their solid-state films. An OLED device using
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