Article pubs.acs.org/cm
Highly Efficient Spiro[fluorene-9,9′-thioxanthene] Core Derived Blue Emitters and Fluorescent/Phosphorescent Hybrid White Organic Light-Emitting Diodes Yunchuan Li,† Zhiheng Wang,† Xianglong Li,† Gaozhan Xie,† Dongcheng Chen,† Yi-Fan Wang,‡ Chang-Cheng Lo,‡ A. Lien,§ Junbiao Peng,† Yong Cao,† and Shi-Jian Su*,† †
State Key Laboratory of Luminescent Materials and Devices and Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, 381 Wushan Road, Guangzhou 510640, China ‡ Shenzhen China Star Optoelectronics Technology Co., Ltd., Shenzhen 518132, China § TCL Corporate Research, Shenzhen 518052, China S Supporting Information *
ABSTRACT: A series of blue emitters incorporating spiro[fluorene-9,9′-thioxanthene] or spiro[fluorene-9,9′-thioxanthene-S,S-dioxide] as the core and phenylcarbazole or triphenylamine as the arms were designed and synthesized. Their spiro conformation is beneficial for their thermal stability and for reducing the trend of aggregation quenching. By tuning the valence of the sulfur atom, highly efficient local excited (LE) state deep blue emitters and charge-transfer (CT) state blue emitters are obtained. The devices based on the LE emitters TPA-S and CzB-S as the nondoped emissive layer exhibit high external quantum efficiency of 1.76% and 2.03% and Commission Internationale de l’Eclairage (CIE) coordinates of (0.158, 0.039) and (0.160, 0.054), respectively, and their CIEy values are among the smallest ever reported for the deep blue OLEDs and are readily very close to that of the inorganic light-emitting diode [CIE (0.16, 0.02)]. The nondoped device based on the CT emitter TPA-SO2 as the emissive layer also exhibits a high current efficiency of 5.46 cd A−1 and CIE coordinates of (0.154, 0.168). To fully utilize the 25% singlet and 75% triplet excitons, fluorescent/phosphorescent hybrid white organic light-emitting diodes in a single-emissive-layer architecture were also fabricated with TPA-SO2 as the blue emitter as well as the host of orange phosphorescent emitter to give forward-viewing power efficiency of 47.9 lm W−1, which is the highest value ever reported for the devices in a similar architecture without using any out-coupling technology.
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INTRODUCTION Full-color displays require primary RGB emission of relatively equal stability, efficiency, and color purity.1 As red and green light-emitting materials have acquired sufficient developments for commercial applying criteria of organic light-emitting diodes (OLEDs), blue light-emitting materials still play a decisive role for applying the OLED technology to commercial products including flat-panel displays and solid-state lighting sources.2−17 The device based on a nondoped light-emitting layer has advantages over the device based on an emitting layer (EML) consisting of a host doped with a fluorescent or phosphorescent emitter due to its relatively simple device structure and doping concentration insensitivity. In addition, a blue fluorescent emitter with an appropriate triplet energy may also function as the blue emitter as well as the host for green/red phosphorescent emitters to realize potential 100% exciton utilization in fluorescent/phosphorescent (F/P) hybrid white OLEDs (WOLEDs) in a simplified device architecture. As the intensity of the fluorescence from 25% of singlet excitons is inferior to that of the phosphorescence from 75% of triplet excitons, development of highly efficient blue fluorescent emitters is a prerequisite for high efficiency F/P hybrid © XXXX American Chemical Society
WOLEDs. In the past decade, numerous highly efficient donor−π-acceptor (DA) type blue light-emitting fluorescent compounds have been developed for blue OLEDs. Li et al. reported a twisting triphenylamine-imidazole DA molecule as a nondoped blue emitter to give a maximum current efficiency (CE) of 5.7 cd A−1.18 Sulfone is also a promising electronwithdrawing building block for constructing high efficiency DA type blue fluorescent emitters. Adachi et al. reported a series of blue emitters with a sulfone core, and an external quantum efficiency (EQE) of 9.9% was achieved at low current density due to the successful utilization of triplet energy via thermally activated delayed fluorescence (TADF). Unfortunately, it suffered a serious efficiency roll-off at high current density maybe due to the triplet−triplet annihilation although the blue emitters are doped into a suitable host.19 Bryce et al. systematically studied a series of fluorophores based on electron-accepting dibenzothiophene-S,S-dioxide (DBTO) and their photophysical properties. However, it is hard to Received: December 3, 2014 Revised: January 13, 2015
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DOI: 10.1021/cm504441v Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Scheme 1. Synthetic Routes of the Spiro[fluorene-9,9′-thioxanthene] Core Derived Compounds CzB-S, TPA-S, CzB-SO2, and TPA-SO2a
(i) n-BuLi (1 equiv), THF, −78 °C; (ii) AcOH, HCl, 80°C, under N2; (iii) DCM, AcOH, H2O2 (5.0 equiv); (iv) boronic acid ester (2.1 equiv), Pd(PPh3)4 (5 mol %), Na2CO3 (5.0 equiv, 2 M aq), toluene/EtOH (5/1), reflux. a
thioxanthene], where the sulfur atom may disturb the π conjugation and the planar structure of the fluorene unit. Electron-rich units like phenylcarbazole and triphenylamine (TPA) are combined with the thioxanthene unit at the para site of sulfur to give LE type deep blue emitters CzB-S and TPA-S (Scheme 1). To the best of our knowledge, this would be the first report on the LE emitters consisting of sulfur as a conjugation-disturbing linkage. The sulfur atom can be further oxidized to sulfone to give spiro[fluorene-9,9′-thioxantheneS,S-dioxide]-based compounds CzB-SO2 and TPA-SO2, which may function as an electron-accepting unit to induce ICT. Most recently, during the preparation of this Article, a similar molecule based on the spiro[fluorene-9,9′-thioxanthene-S,Sdioxide] core was developed as a hole-transporting layer in organic photovoltaics.24 Similar to the previously reported spirofluorene-based molecules, the TPA units are combined with the fluorene unit of the spiro[fluorene-9,9′-thioxantheneS,S-dioxide] core to give an elongated conjugation, and efficient ICT might not be expected due to the sp3 hybridized carbon. In the current structures, the medium electron-withdrawing property of sulfone may avoid an excess of red-shift caused by ICT and maintain a suitable energy band gap to give blue emission, and the sulfone group may offer the developed materials excellent electron injection/transport properties.25 With the combination of electron-donating and hole-transporting phenylcarbazole and TPA units, bipolar carriertransporting ability is also anticipated, and this is favorable for a nondoped emission layer and a host of phosphorescent emitter. Different from the previously reported sulfonecontaining blue emitters, the current spiro[fluorene-9,9′thioxanthene-S,S-dioxide]-based compounds contain a spiro conformation to reduce the intermolecular π−π stacking and thus the excimer formation26 and to improve the film thermal stability which could be hardly compatible for the previously reported DA type molecules. In addition, the similar molecular structure but quite different electronic structure of the compounds based on spiro[fluorene-9,9′-thioxanthene] and
counterpoise the high efficiency and the deep blue emission suffered from the planar structure of the DBTO unit and the elongated π conjugation of the two benzene rings.20 Zhang et al. also reported a series of sulfone-containing highly efficient blue light-emitting materials.21 In contrast, light-emitting materials consisting of sulfur as a connecting unit have been rarely reported to date. The DA type emitters containing donor and acceptor units may offer the emitter bipolar charge-transporting property and thus improved carrier balance. However, the emission efficiency of charge-transfer (CT) state molecules is generally low due to the forbidden electronic transition rule arising from the largely separated HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital).22 In this regard, local excited (LE) state is a more efficient radiation state than CT state, arising from its larger transition moment which benefitted from a much larger orbital overlap.22 Besides, DA molecular systems induce obvious intramolecular chargetransfer (ICT), which brings about a large bathochromic shift and impairs the color purity of emission. Considering the color purity, the LE emitter which is not affected by ICT could be a more suitable choice. The extensive conflict between them could be solved by systematic study of these two types of emitters. Although LE and CT emitters are widely investigated, there are few reports that systematically investigate the LE and CT emission in a similar molecular structure. As for the design of blue emission molecules, the confinement of the conjugation length as well as the introduction of bulky structure is vital for realizing thermally stable blue light-emitting materials.23 Spirofluorene is a well-known unit used for the design of blue light-emitting materials, in which the perpendicular arrangement of the spiro conformation not only hinders close packing and intermolecular aggregation but also increases molecular rigidity to maintain the color stability and to improve their electroluminescence (EL) efficiency. In this work, a sulfur atom is incorporated into spirofluorene to give a novel spiro structure of spiro[fluorene-9,9′B
DOI: 10.1021/cm504441v Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
2-yl)-9H-thioxanthen-9-ol (2) was purified by column chromatography and used for the next step directly. A mixture of 2, 20 mL of AcOH, and 0.5 mL of hydrochloric acid was stirred under argon at 80 °C for 3 h. The reaction mixture was extracted with DCM and further purified by column chromatography to afford 3 (0.578 g, yield 54.3%) as a white solid. 1H NMR (600 MHz, CDCl3, δ, ppm): 7.78−7.82 (d, J = 7.6 Hz, 2H), 7.52−7.54 (d, J = 7.7 Hz, 2H), 7.28−7.30 (t, 2H), 7.24−7.28 (m, 6H), 6.60−6.62 (s, 2H). 2′,7′-Dibromospiro[fluorene-9,9′-thioxanthene-S,S-dioxide] (4). Compound 3 (1.2 g, 2.25 mmol), 30 mL of DCM, 15 mL of AcOH, and 2.5 mL of H2O2 (22.5 mmol, 10 equiv) were stirred under atmosphere at 80 °C for 24 h. The reaction mixture was extracted with DCM and further purified by column chromatography to afford 4 (1.07 g, yield 88%) as a white solid. 1H NMR (600 MHz, CDCl3, δ, ppm): 8.06−8.08 (d, J = 8.5 Hz, 2H), 7.86−7.88 (d, J = 7.7 Hz, 2H), 7.60−7.62 (d, J = 8.5 Hz, 2H), 7.46−7.49 (m, 2H), 7.26−7.34 (m, 4H), 6.64−6.66 (s, 2H). 2′,7′-Bis(4-(9H-carbazol-9-yl)phenyl)spiro[fluorene-9,9′-thioxanthene] (CzB-S). Toluene (60 mL), ethanol (20 mL), and 2 M aqueous Na2CO3 (15 mL) were added to a mixture of 3 (0.798 g, 1.5 mmol), 5 (1.156 g, 3.15 mmol), and Pd(PPh3)4 (52 mg, 3 mol %). The suspension was stirred at 90 °C for 24 h under a nitrogen atmosphere. When cooled to room temperature, the mixture was extracted with CH2Cl2 and dried over Na2SO4. After the removal of solvent, the residue was purified by column chromatography on silica gel to afford CzB-S (0.97 g, yield 78%) as a white solid. 1H NMR (600 MHz, CDCl3, δ, ppm): 8.13−8.14 (d, J = 7.8 Hz, 4H), 7.86−7.88(d, J = 7.7 Hz, 2H), 7.75−7.77 (d, J = 7.7 Hz, 2H), 7.58−7.60 (d, J = 8.1 Hz, 2H), 7.46−7.54 (m, 12H), 7.28−7.38 (m, 14H), 6.92−6.93 (s, 2H). 13 C NMR (150 MHz, CDCl3, δ, ppm): 153.96, 140.75, 139.72, 139.20, 138.16, 137.83, 136.78, 128.39, 127.21, 128.56, 125.63, 130.52, 126.76, 123.39, 119.96, 125.95, 120.46, 127.94, 109.78, 60.58. MS (MALDITOF): m/z calcd for C61H38N2S 830.28; found 830.063. Anal. Calcd for C61H38N2S: C, 88.16; H, 4.61; N, 3.37; S, 3.86. Found: C, 88.08; H, 4.49; N, 3.25; S, 3.76. 2′,7′-Bis(4-(9H-carbazol-9-yl)phenyl)spiro[fluorene-9,9′-thioxanthene-S,S-dioxide] (CzB-SO2). CzB-SO2 (0.95 g, yield 76%) was synthesized as a white solid in a similar manner of CzB-S with 4 instead of 3. 1H NMR (600 MHz, CDCl3, δ, ppm): 8.38−8.48 (d, J = 8.2 Hz, 2H), 8.14−8.19 (d, J = 7.7 Hz, 4H), 7.92−7.94 (d, J = 7.7 Hz, 2H), 7.78−7.82 (d, J = 8.3 Hz, 2H), 7.47−7.58 (m, 8H), 7.40−7.43 (d, J = 8.4 Hz, 4H), 7.32−7.39 (m, 10H), 7.25−7.28 (m, 4H), 6.88− 6.90 (s, 2H). 13C NMR (150 MHz, CDCl3, δ, ppm): 129.24, 128.58, 144.61, 125.96, 140.59, 120.74, 137.80, 120.19, 152.22, 135.77, 123.52, 109.68, 138.05, 120.36, 140.91, 124.22, 127.29, 128.91, 58.5. MS (MALDI-TOF): m/z calcd for C61H38N2O2S 862.27; found 862.208. Anal. Calcd for C61H38N2O2S: C, 84.89; H, 4.44; N, 3.25; O, 3.71; S, 3.72. Found: C, 84.72; H, 4.38; N, 3.20; S, 3.59. 4,4′-(Spiro[fluorene-9,9′-thioxanthene]-2′,7′-diyl)bis(N,N-diphenylaniline) (TPA-S). TPA-S (0.93 g, yield 74%) was synthesized as a white solid in a similar manner of CzB-S with 6 instead of 5. 1H NMR (600 MHz, CDCl3, δ, ppm): 7.50−7.54 (d, J = 7.6 Hz, 2H), 7.64−7.66 (d, J = 7.6 Hz, 2H), 7.46−7.48 (d, J = 7.6 Hz, 2H), 7.36−7.38 (m, 8H), 7.17−7.27 (m, 10H), 6.92−7.14 (m, 20H), 6.75−6.76 (s, 2H). 13 C NMR (150 MHz, CDCl3, δ, ppm): 128.50, 126.50, 125.58, 139.65, 124.43, 154.04, 137.60, 122.93, 147.07, 133.92, 120.22, 138.32, 123.60, 147.55, 125.37, 126.16, 127.13, 128.33, 60.63. MS (MALDI-TOF): m/ z calcd for C61H42N2S 834.31; found 834.068. Anal. Calcd for C61H42N2S: C, 87.74; H, 5.07; N, 3.35; S, 3.84. Found: C, 87.66; H, 4.89; N, 3.31; S, 3.77. 4,4′-(Spiro[fluorene-9,9′-thioxanthene-S,S-dioxide]-2′,7′-diyl)bis(N,N-diphenylaniline) (TPA-SO2). TPA-SO2 (1.01 g, yield 78%) was synthesized as a white solid in a similar manner to TPA-S with 6 instead of 5. 1H NMR (600 MHz, CDCl3, δ, ppm): 8.25−8.28 (d, J = 8.3 Hz, 2H), 7.82−7.84 (d, J = 7.6 Hz, 2H), 7.63−7.65 (d, J = 8.3 Hz, 2H), 7.38−7.42 (m, 4H), 7.18−7.26 (m, 10H), 7.02−7.24 (m, 16H), 6.94−6.96 (d, J = 8.7 Hz, 4H), 6.72−6.75 (s, 2H). 13C NMR (150 MHz, CDCl3, δ, ppm): 129.90, 127.72, 125.86, 148.32, 123.87, 147.22, 134.78, 122.80, 144.77, 131.97, 120.47, 152.36, 140.58, 123.47, 124.87, 126.45, 128.61, 129.35, 58.4. MS (MALDI-TOF): m/z calcd for
spiro[fluorene-9,9′-thioxanthene-S,S-dioxide] make them a good platform to study photophysical properties and device performances of the LE and CT emitters. For the nondoped blue OLEDs, the LE emitter TPA-S based on TPA and spiro[fluorene-9,9′-thioxanthene] exhibits an external quantum efficiency (EQE) of 1.76% and Commission Internationale de l’Eclairage (CIE) coordinates of (0.158, 0.039). There are quite few deep blue emitters that exhibit CIEy values below 0.04 as well as such high EQE.21,27 The current CIEy value is readily very close to that of the inorganic lightemitting diode (LED) [CIE (0.16, 0.02)].28 In comparison, the CT emitter TPA-SO2 consisting of TPA and spiro[fluorene9,9′-thioxanthene-S,S-dioxide] exhibits the best efficiencies of 5.46 cd A−1 and 6.02 lm W−1 and slightly shifted CIE coordinates of (0.154, 0.168) among the four materials. In addition, F/P hybrid WOLEDs in a simplified single-emissivelayer single-dopant architecture were also fabricated by utilizing TPA-SO2 as the blue emitter and the host of orange phosphorescent emitter iridium(III) bis(4-phenylthieno[3,2c]pyridinato-N,C2′)acetylacetonate (PO-01), giving maximum forward-viewing efficiencies of 39.6 cd A−1 and 47.9 lm W−1, which are also hitherto one of the highest values for the ever reported F/P hybrid WOLEDs in a single-emissive-layer architecture.
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EXPERIMENTAL SECTION
General. 1H and 13C NMR spectra were recorded on a Bruker NMR spectrometer operating at 600 and 150 MHz, respectively, in deuterated chloroform (CDCl3) solution at room temperature. Differential scanning calorimetry (DSC) measurements were performed on a Netzsch DSC 209 under a N2 flow at a heating and cooling rate of 10 °C min−1. Thermogravimetric analyses (TGA) were performed on a Netzsch TG 209 under a N2 flow at a heating rate of 10 °C min−1. UV−vis absorption spectra were recorded on a HP 8453 spectrophotometer. Photoluminescence (PL) spectra were measured using a Jobin-Yvon spectrofluorometer. Cyclic voltammetry (CV) was performed on a CHI600D electrochemical workstation with a Pt working electrode and a Pt wire counter electrode at a scanning rate of 100 mV s−1 against a Ag/Ag+ (0.1 M of AgNO3 in acetonitrile) reference electrode with a nitrogen-saturated anhydrous acetonitrile and dichloromethane (DCM) solution of 0.1 mol L−1 tetrabutylammoniumhexafluorophosphate. PL quantum yields (PLQYs) of the solution and film were measured by using an integrating sphere on a HAMAMATSU absolute PL quantum yield spectrometer C11347. Transient PL was measured with an Edinburgh FL920 fluorescence spectrophotometer. The thin solid films used for absorption and PL spectral measurement were vacuum vapor deposited on quartz substrates. MALDI-TOF (matrix-assisted laser-desorption/ionization time-of-flight) mass spectra were performed on a Bruker BIFLEXIII TOF mass spectrometer. Materials. All solvents and materials were used as received from commercial suppliers without further purification. 9-(4-(4,4,5,5Tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole (5),29a N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (6),29b and 2,7-dibromo-9H-thioxanthen-9-one (1)29c were synthesized according to literature procedures. Synthetic routes of the spiro[fluorene-9,9′-thioxanthene] core derived compounds are outlined in Scheme 1. All the developed materials were purified by repeated temperature gradient vacuum sublimation. 2′,7′-Dibromospiro(fluorene-9,9′-thioxanthene) (3). 2-Bromobiphenyl 4.66 g (20 mmol, 10 equiv) in dry tetrahydrofuran (THF) (60 mL) was stirred under argon and cooled to −78 °C. n-Butyllithium (8.0 mL, 2.5 mol/L) was slowly added with vigorous stirring. Stirring was continued for 1 h at the same temperature. Compound 1 (2 mmol, 0.74 g) in dry THF (160 mL) was added to the reaction solution in one portion. After the reaction mixture stirred overnight, THF was removed by distillation under reduced pressure. 9-(BiphenylC
DOI: 10.1021/cm504441v Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
Table 1. Summary of the Thermal, Photophysical, and Electrochemical Properties of CzB-S, TPA-S, CzB-SO2, and TPA-SO2 compd
Tga (°C)
Tdb (°C)
CzB-S TPA-S CzB-SO2 TPA-SO2
135.4 132.7 132.7 133.2
452 477 485 467
λabsc (nm) 293, 299, 293, 298,
310, 310, 310, 309,
330, 341 356 329, 341 363
λabsd (nm) 314, 300, 312, 300,
λemc (nm)
λemd (nm)
IP/EAe (eV)
ΦPLf
ΦPLg
Egh (eV)
Egoptj (eV)
386 410 432 465
423 425 426 452
−5.25/−2.49 −5.31/−2.21 −5.31/−2.44 −5.28/−2.38
0.54 0.66 0.86 0.89
0.33 0.27 0.50 0.74
2.76 3.10 2.87 2.90
3.22 3.05 3.14 2.89
344 312, 359 343 310, 367
a
Glass transition temperature (Tg) obtained from DSC measurements. bDecomposition temperature (Td) obtained from TGA measurements. UV−vis absorption and PL spectra measured in DCM solution. dUV−vis absorption and PL spectra measured in thin solid film. eIonization potential (IP) and electron affinity (EA) estimated from the onset of the oxidation and reduction potentials [vs ferrocene/ferrocenium (Fc/Fc+)]. f PL quantum yields of the DCM solutions measured using an integrating sphere. gPL quantum yields of the thin solid films (50 nm) deposited on the quartz substrates measured using an integrating sphere. hEnergy band gaps estimated from CV. jOptical energy band gaps estimated from the film absorption edge. c
C61H42N2O2S 866.30; found 866.274. Anal. Calcd for C61H42N2O2S: C, 84.50; H, 4.88; N, 3.23; O, 3.69; S, 3.70. Found: C, 84.37; H, 4.82; N, 3.18; S, 3.51. Theoretical Calculation. Density functional theory (DFT) calculations were performed on the Gaussian suite of programs (Gaussian 09_B01 package). The geometry of the ground state of the four molecules in the gas phase was optimized, without imposing any symmetry constraints, using B3LYP/6-31G(d). The minima were confirmed with all real frequencies.32 The M06-2X functional, which is well-known for intermediate description of charge-transfer systems, was utilized to gain insight into the character of the excited state. For better understanding of LE and CT transitions, natural transition orbitals (NTOs) were performed on the basis of the optimized ground state structures. Additionally, the excited singlet energy (ES) and triplet energy (ET) were acquired to investigate the split energy between ES and ET, which is generally called ΔEST. Device Fabrication and Characterization. Glass substrates precoated with a 95-nm-thin layer of indium tin oxide (ITO) with a sheet resistance of 10 Ω per square were thoroughly cleaned in ultrasonic bath of acetone, isopropyl alcohol, detergent, deionized water, and isopropyl alcohol and treated with O2 plasma for 20 min in sequence. Organic layers were deposited onto the ITO-coated glass substrates by thermal evaporation under high vacuum (
