Construction of thermally stable 3,6-disubstituted spiro-fluorene derivatives as host materials for blue phosphorescent organic light-emitting diodes

Dyes and Pigments 114 (2015) 222e230

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Construction of thermally stable 3,6-disubstituted spiro-fluorene derivatives as host materials for blue phosphorescent organic light-emitting diodes Lei Wang a, *, Biao Pan a, Liping Zhu b, Bo Wang a, Yixing Wang a, Yakun Liu a, Jiangjiang Jin a, Jiangshan Chen b, Dongge Ma b a

Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, PR China State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 June 2014 Received in revised form 13 November 2014 Accepted 15 November 2014 Available online 28 November 2014

Four novel thermally stable host materials, 9,9'-(9,9-diphenyl-9H-fluorene-3,6-diyl)bis(9H-carbazole) (BBDC), 3-(3,6-bis(diphenylphosphoryl)-9,9-diphenylfluorene (BBDP), 3,6-di(9H-carbazol-9-yl)-9,9'spirobi[fluorene] (BSBDC), 3,6-di(9H-carbazol-9-yl)-9,9'-spirobi[fluorene] (BSBDP) were designed and synthesized by linking phosphine oxide or carbazole moieties to the 3,6-position of a spirofluorene core. In comparison to the conventional 2,7-disubstituted homologues, 3,6-disubstituted spiro-fluorene can offer more versatile and advanced properties in terms of higher triplet energy (>2.8 eV) and higher glasstransition temperatures (ca. 215
C). Accordingly, doped blue-emitting devices were fabricated and the device with BSBDP as host exhibited a very low turn-on voltage of 2.8 V and maximum current efficiency of 34.2 cd A1, external quantum efficiency of 18.7% and power efficiency of 34.4 lm W1. These results demonstrate that 3,6-disubstituted spiro-fluorene derivatives are promising host materials for blue phosphorescent organic light-emitting devices (PhOLEDs). © 2014 Elsevier Ltd. All rights reserved.

Keywords: 3,6-Spirobifluorene High triplet energy High Tg Blue phosphorescent organic light-emitting device Host materials Low roll-off

1. Introduction PhOLEDs have currently attracted considerable scientific and industrial interest because of their potential applications in fullcolor flat-panel displays and solid-state lighting [1e11]. During the past decades, green and red phosphorescent electroluminescent devices with high efficiency, long lifetime, and appropriate color coordinates have been developed [12e15]. Nevertheless, blue PhOLEDs with an improved performance remain a challenge mainly due to the lack of appropriate host materials [16e20]. Generally speaking, two intrinsic criteria restrict the development of blue host materials: (i) their triplet energy (ET) level must be higher than blue dopant (2.7 eV) to prevent exothermic reverse energy transfer from guest to host and confine the triplet exciton within the emitting layer [21]; and (ii) in order to extend the

* Corresponding author. Tel./fax: þ86 27 87793032. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.dyepig.2014.11.011 0143-7208/© 2014 Elsevier Ltd. All rights reserved.

operational lifetime of the device, the blue host must possess good morphological and chemical stabilities to form stable amorphous films [22]. Therefore, in recent years, tremendous effort has been devoted to the pursuit of developing blue host materials with desired optoelectronic performance [23e25]. Fluorene-based polymers and oligomers are receiving much research attention because of their good thermal and chemical stability come along with high emission efficiency [26e28]. However, most fluorene derivatives have a relatively low-lying triplet exciton level and long-wavelength emission induced by the inherent photo- or electro-oxidized cleavage of C9-substituted pendant alkyl groups, which inhibit their application in blue PhOLEDs [29]. To surmount this constraint, a common strategy toward manipulating the electronic structure, emission spectrum and thermal morphological stability is tailoring the species of the substituents and their substitution patterns on C9 and biphenyl [30]. In 2006, by substituting fluorene with diphenylphosphine oxide (DPPO) moiety, Burrows and co-workers firstly created a fluorene derivative 2,7-bis(diphenylphosphineoxide)-9,9-

L. Wang et al. / Dyes and Pigments 114 (2015) 222e230

dimethylfluorene (PO6) [31], which has a triplet energy level (ET ¼ 2.72 eV) that meets the requirement of blue PhOLEDs. Subsequently, 9,9'-spirobifluorene derivatives with high Tg, good thermal stability and no long-wavelength emission in comparison with oligo- and polyfluorene derivatives (PFs) have been considered to be a potential substitution strategy [32]. Lee and co-workers reported two spirobifluorene/phosphine oxide hybrid hosts ortho-(SPPO11) and para-(SPPO1), which acquired a satisfactory electroluminescent performance by using [bis(4,6-difluorophenyl) pyridinato- N,C2’]picolinate iridium (III) (FIrpic) as the blue dopant [33,34]. These results revealed that the utilization of the DPPO at the ortho position of the spirobifluorene has obvious advantages in terms of efficiency and morphological stability. Most recently, Liao and co-workers also reported that the meta-substituted spirofluorene derivatives possessed the highest triplet energy compared with ortho- and para-analogues [35]. This can be attributed to the steric hindrance which leads to a twisted geometry in these molecules and makes extension of conjugation between the spirobifluorene and the substituent moiety unlikely [36]. Considering that the Tg reported was as high as 119
C for the mono-substituted SF3PO, we envisaged that 3,6-disubstituted spirobifluorene derivatives will be endowed with even higher Tg since rigid molecules possessing symmetry matrixes usually have higher thermal stability than asymmetric homologues [37]. Above all, it is anticipated that the synergistic combination of high ET and high thermal stability in one molecule is feasible by adopting 3,6-disubstituted spirobifluorene. In fact, it is an appealing strategy which was widely adopted in the field of polymer instead of small molecular PhOLEDs [38e40]. In this contribution, we rationally designed and synthesized two 3,6-disubstituted spirobifluorene blue host materials by using spirobifluorenyl as the building blocks and carbazole/DPPO as periphery moieties through meta-linking topology. In order to have a comprehensive understanding of the structure-property relationship, the other two analogues with 9,9-diphenylfluorene were also prepared for comparison. Through the involvement of carbazole and DPPO moiety, the ET and molecular configurations were accurately tuned. All of the four designed molecules could retain high triplet energies and thermal stability because of the perpendicular spiro-conformation and meta-linking topology. We aim to elucidate the feasibility of constructing hosts with both high ET and Tg through the appropriate linkage mode. 2. Experimental section 2.1. Material and methods All the reagents and solvents used for the synthesis were purchased from Aldrich and were used without further purification. All reactions were performed under a dry nitrogen atmosphere. 1H NMR, 31P NMR and 13C NMR spectra were measured on a BrukerAF301 AT 400 MHz spectrometer. Elemental analyses of carbon, hydrogen, and nitrogen were performed on an Elementar (Vario Micro cube) analyzer. Mass spectra were carried out on an Agilent (1100 LC/MSD Trap) using APCI ionization. UVeVis absorption spectra were recorded on a Shimadzu UVeVISeNIR Spectrophotometer (UV-3600). PL spectra were recorded on Edinburgh instruments (FLSP920 spectrometers). Differential scanning calorimetry (DSC) was performed on a PE Instruments DSC 2920 unit at a heating rate of 10
C/min from 30 to 250
C under nitrogen. The glass transition temperature (Tg) was determined from the second heating scan. Thermogravimetric analysis (TGA) was undertaken with a PerkinElmer Instruments (Pyris1 TGA). The thermal stability of the samples under a nitrogen atmosphere was determined by measuring their weight loss while heating at a rate of 10
C/min

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from 30 to 700
C. Cyclic voltammetry (CV) measurements were carried out in a conventional three electrode cell using a glassy carbon working electrode of 2 mm in diameter, a platinum wire counter electrode, and an Ag/AgNO3 (0.1 M) reference electrode on a computer-controlled EG&G Potentiostat/Galvanostat model 283 at room temperature. Oxidations CVs of all compounds were performed in dichloromethane containing 0.1 M tetrabutylammoniumhexafluorophosphate (Bu4NPF6) as the supporting electrolyte. All solutions were purged with a nitrogen stream for 10 min before measurement. The experimental conditions and equipment used have been described in our previous work [41]. 2.2. Preparation of compounds 2.2.1. Synthesis of 3,6-dibromo-9, 9-diphenyl-9H-fluorene (2) A fraction of bromobenzene (10.9 mL, 104 mmol) in dry THF (50 mL) was added to the initially charged magnesium turnings (6 g, 250 mmol) to initiate the Grignard reaction. Then the reaction mixture was refluxed for 3 h to complete the reaction. Afterwards, a solution of 3,6-dibromo-fluorenone (18.0 g, 105 mmol) in THF (50 mL) was slowly added to the Grignard solution and the mixture was refluxed overnight (CAUTION: organomagnesium reagent is pyrophoric and should be handled with extreme care). The precipitated yellow magnesium complex was collected and washed with dry THF. The solid was poured into ice-cold saturated ammonium chloride solution stirred for 2 h, and the precipitate was collected and dried. The solid was then dissolved in dried benzene (50 mL) to which CF3SO3H (1.1 mL, 12 mmol) was added dropwise. The mixture was heated to 80
C to afford a brown solution and stirred for another 6 h and then was poured into an icecold saturated NaHCO3 solution. The organic layer was separated and washed with water and then brine. The extract was evaporated to dryness affording a white solid, which was further purified by column chromatography. Yield: 58%. 1H NMR: (CDCl3, 400 MHz): d(ppm) 7.86 (d, J ¼ 1.6 Hz, 2H), 7.40 (dd, J ¼ 6.4 Hz, 2H), 7.27e7.22 (m, 8H), 7.13e7.15 (m, 4H). MS (APCI): calcd for C25H16Br2:473.96, found, 475.7(Mþ2)þ. 2.2.2. Synthesis of 3,6-dibromo-9,9'-spirobi[fluorene] (3) In a dry three-necked flask, equipped with a sealed wire stirrer, a condenser protected by a drying tube, and a dropping funnel, were placed magnesium turnings (2.8 g, 117 mmol) and dry THF (50 mL). A solution of 2-bromo-1,1'-biphenyl (3.9 mL, 50 mmol) in about 25 ml of the THF solution was added and the reaction mixture was refluxed for 3 h after completion of the addition. Afterwards, a solution of 3,6-dibromo-fluorenone (9.9 g, 55 mmol) in THF was slowly added to the Grignard solution and the mixture was refluxed overnight. The suspension was cooled, collected and washed well with water. The crude residue was placed in another two-necked flask (200 mL) and dissolved in acetic acid (50 mL). A catalytic amount of aqueous HCl (5 mol%, 12 N) was then added and the whole solution was refluxed for 12 h. After cooling to ambient temperature, purification by silica gel chromatography using dichloromethane/n-hexane gave a white powder. Yield: 73%. 1H NMR: (CDCl3, 400 MHz): d(ppm) 7.97 (s, 2H), 7.8 (d, J ¼ 8.0 Hz, 2H), 7.41 (t, J ¼ 1.2 Hz, 2H), 7.27 (t, J ¼ 4.0 Hz, 2H), 7.15 (t, J ¼ 1.6 Hz, 2H), 6.73 (d, J ¼ 8.0 Hz, 2H), 6.31 (d, J ¼ 8.0 Hz, 2H). MS (APCI): calcd for C25H14Br2:474.19, found, 476.4 (Mþ2)þ. 2.2.3. Synthesis of 9,9'-(9,9-diphenyl-9H-fluorene-3,6-diyl)bis(9Hcarbazole) (BBDC) A mixture of 2 (800 mg, 1.68 mmol), carbazole (842 mg, 5.04 mmol), copper iodide (342 mg, 1.8 mmol), 18-crown-6 (89 mg, 3.4 mmol), K2CO3 (1.25 g, 9.07 mmol) dissolved in DMPU (tetrahydro-1,3-dimethyl-2(1H)pyrimidine, 2 mL) was heated at 180
C

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under nitrogen protection for 2 days. After the mixture was cooled to room temperature, the product was extracted with CH2Cl2 and washed by water. The combined organic layer was washed with brine and dried over MgSO4. The subjection of the crude mixture to silica gel chromatography (n-hexane/toluene ¼ 10/1) afforded BBDC. Yield: 82%. Sublimation Temperature: 350
C Purity: 99.35% 1 H NMR: (CDCl3, 400 MHz):d (ppm) 8.14 (d, J ¼ 7.6 Hz, 4H), 7.94 (d, J ¼ 1.6 Hz, 2H), 7.71 (d, J ¼ 8.0 Hz, 2H), 7.55 (dd, J ¼ 10.4 Hz, 6H), 7.41e7.29 (m, 18H) 13C NMR (100 MHz, CDCl3) d 150.56, 145.29, 141.21, 140.90, 137.53, 128.57, 128.22, 127.66, 127.17, 127.04, 125.97, 123.43, 120.33, 120.03, 119.00, 109.83, 64.95. MS (APCI): calcd for C49H32N2, 648.79; found, 649.2. (Mþ1)þ. Anal. calcd for C49H32N2 (%): C 90.33, H 5.46, N 4.21; found: C 90.45, H 5.38, N 4.17. 2.2.4. Synthesis of 3-(3,6- bis (diphenylphosphoryl)-9,9diphenylfluorene (BBDP) To a mixture of NiCl2$6H2O (0.07 g, 0.3 mmol), zinc (0.39 g, 6.0 mmol), 2, 20 -bipyridine (bpy) (0.09 g, 0.6 mmol) and compound 3 (0.48 g, 1.0 mmol) in DMAc (20.0 ml) solution, biphenylphosphine oxide (0.4 g, 2.0 mmol) was added. The reaction mixture was stirred at 110
C for 48 h. After it, the mixture was cooled to room temperature and washed with CH2Cl2 and water. The organic layer was isolated and dried with anhydrous MgSO4. After it, the crude product was purified by silica gel column chromatography using dichloromethane-methanol as the eluent to afford the corresponding product. Yield: 55%. Sublimation Temperature: 370
C Purity: 98.51% 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.01 (s, 2H), 7.74e7.69 (m, 8H), 7.61e7.47 (m, 16H), 7.27e7.25 (m, 6H), 7.18e7.15 (m, 4H). 13C NMR (100 MHz, CDCl3): d 155.00, 144.30, 139.66, 139.53, 132.83, 132.70, 132.22, 132.13, 132.03, 131.80, 131.66, 128.71, 128.59, 128.49, 128.11, 128.59, 128.49, 128.11, 127.26, 126.49, 126.36, 124.36, 124.26, 65.84. MS (APCI): calcd for C49H36O2P2, 719.226; found, 720.223, (Mþ1)þ. 2.2.5. Synthesis of 3,6- di (9H-carbazol-9-yl)-9,9'- spirobi [fluorene](BSBDC) In the manner described for the compound BBDC, the final product BSBDC was obtained as the white power. Yield: 76%. Sublimation Temperature: 350
C Purity: 99.65% 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.14 (d, J ¼ 7.6 Hz, 2H), 8.01 (d, J ¼ 1.6 Hz, 4H), 7.93 (d, J ¼ 7.6 Hz, 2H), 7.52e7.41 (m, 12H), 7.26 (m, 6H), 6.98 (dd, J ¼ 12.4 Hz, 4H). 13C NMR (100 MHz, CDCl3): d 150.47, 145.29, 143.73, 142.11, 140.67, 134.55, 130.76, 127.52, 126.22, 124.66, 122.17, 120.02, 119.89, 119.00, 109.83, 69.34. MS (APCI): calcd for C49H30N2, 646.78; found, 647.4 (Mþ1)þ. Anal. calcd for C49H30N2 (%): C 90.99, H 4.68, N 4.33; found: C 90.57, H 4.38, N 5.05. 2.2.6. Synthesis of 3,6-di (9H-carbazol-9-yl)-9,9'-spirobi [fluorene](BSBDP) In the manner described for the compound BBDP, the final product BSBDP was obtained as the white power. Yield: 80% Sublimation Temperature: 370
C Purity: 98.50% 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.03e8.05 (d, J ¼ 7.6 Hz, 2H), 7.81 (d, J ¼ 7.6 Hz, 2H), 7.66 (m, 4H), 7.35e7.56 (m, 18H), 7.04e7.07 (t, J ¼ 7.6 Hz, 4H), 6.66 (t, J ¼ 1.6 Hz, 2H), 6.51 (m, 2H). 13C NMR (100 MHz, CDCl3): d 153.18, 147.08, 142.01, 141.34, 141.21, 133.02, 132.78, 132.53, 132.37, 132.27, 132.18, 132.01, 131.75, 129.02, 128.90, 128.78, 128.50, 128.23, 124.34 124.33, 124.26, 120.51, 65.64. MS (APCI): calcd for C49H34O2P2, 716.2; found, 717.5 (Mþ1)þ. 2.3. Computational details The geometrical and electronic properties were performed with the Amsterdam Density Functional (ADF) 2009.01 program package. The calculation was optimized by means of the B3LYP (Becke

three parameters hybrid functional with Lee-Yang-Perdew correlation functionals) [42,43]with the 6-31 G(d) atomic basis set. Then the electronic structures were calculated at t-HCTHhyb/6-311þþG (d, p) level [44]. Molecular orbitals were visualized using ADFview. The experimental conditions and equipment in this work have been described in our previous work [45]. 2.4. Device fabrication and measurement The hole-injection material MoO3, hole-transporting material 1, 4-bis [(1-naphthyl-phenyl)amino]biphenyl (NPB), electron/ exciton-blocking material N,N0 -Dicarbazolyl-3,5-benzene (mCP), electron-transporting material 3,3'-(5'-(3-(pyridine-3-yl)phenyl)[1,1':30 ,100 -terphenyl]-3,300 -diyl)dipyridine (TmPyPB) and dopant FIrpic were commercially available. Commercial ITO (purchased from CSG Holding Co. Ltd) coated glass with sheet resistance of 20 U per square was used as the substrates. Before device fabrication, the ITO glass substrates were pre-cleaned carefully and treated by oxygen plasma for 2 min MoO3 was firstly deposited to ITO substrate, followed by NPB, mCP, emissive layer, and TmPyPB. Finally, a cathode composed of lithium fluoride and aluminum were sequentially deposited onto the substrate in the vacuum of 106 Torr. The density-voltage-luminance (J-V-L) of the devices was measured with a Keithley 2400 Source meter equipped with a calibrated silicon photodiode. The EL spectra were measured by PR655 spectrometer. The EQE values were calculated according to previously reported methods [45]. 3. Results and discussion 3.1. Synthesis The synthetic routes to these new hosts are shown in Scheme 1. The intermediate 3,6-dibromo-9H-fluoren-9-one (1) was prepared by the bromination and oxidation of 9,10-phenanthrenequinone in a yield of 35% according to the reported literature method [46]. Subsequently, 3,6-dibromo-9,9-diphenyl-9H-fluorene (2) and 3,6dibromo-9,9'-spirobi[fluorene] (3) were synthesized via conventional Grignard Reaction. Finally, BBDC and BSBDC were achieved by the Ullmann coupling reaction of carbazole with 2 and 3, respectively. BBDP and BSBDP were achieved by one-pot Ni(II)/Zn catalyzed cross-coupling of DPPO with 2 and 3 at the temperature of 110
C in good yields, respectively. Repeated temperature gradient vacuum sublimation is required for further purification of the bipolar host materials. The final compounds were characterized by 1H NMR and 13C NMR spectroscopes, mass spectrometry and elemental analysis. All of the results are in agreement with the proposed structures. 3.2. Thermal properties The thermal properties of BBDC, BBDP, BSBDC and BSBDP were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), as shown in Fig. 1 and Table 1. The four compounds exhibit high thermal decomposition temperatures (Td, corresponding to 5% weight loss) in the range of 400e432
C. Intriguingly, there is obvious regularity for Td of the four compounds: The Td of carbazole derivatives is substantially higher than those of DPPO counterpart, that is, BSBDC > BSBDP and BBDC > BBDP. On the other hand, spirobifluorene core has an apparent advantage over broken spirobifluorene core for the same substituent group. The former can be attributed to the greater stability of the carbazole compared with the DPPO unit and the latter could be explained by the more twisted stereostructure of unbroken spirobifluorene core.

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Scheme 1. The Synthesis routes of the compounds BBDC, BBDP, BSBDC and BSBDP. Reagents and condition: i) a:Mg, bromobenzene, THF; b: benzene, CF3SO3H; ii) a:Mg, 2-bromo1,1'-biphenyl, THF; b: CH3COOH, HCl; iii) carbazole, CuI, 18-Crown-6, K2CO3, 180
C, 48 h; iv) Zn, NiCl2$6H2O, 2,2'-bipyridine, DMAc, diphenylphosphine oxide, 110
C, 24 h.

High glass transition temperatures(Tg) of 215
C were determined for BBDC and BSBDC, but as to BBDP and BSBDP, we have failed to detect their Tg. The almost identical values of BBDC and BSBDC are attributed to the similar non-planar molecular structures of the two compounds and are much higher than those of the analogous 2,7-disubstituted compounds SBFC-G1(174
C). This observation suggests that the molecular configuration of 3,6disubstituted spiro-fluorene is more twisted than 2, 7-

Fig. 1. (a) DSC and (b) TGA curves of BBDC, BBDP, BSBDC and BSBDP.

disubstituted ones [47]. The twisted molecular configuration and the rigid structure of carbazole units cause entanglement in the amorphous state and hinder recrystallization, which is highly desirable for improving the efficiency and lifetime of OLEDs, especially for high-temperature applications of devices. 3.3. Photophysical properties Fig. 2(a) shows the UVevis absorption and photoluminescence (PL) spectra of BBDC, BBDP, BSBDC and BSBDP in toluene at room temperature and all the results are summarized in Table 1. The UVevis absorption spectra of BBDC and BSBDC are quite similar with peaks 285 nm, and shoulder peaks of 327 and 340 nm. The 285 nm peak can be attributed to carbazole-centered n-p* transition, and the 327 and 340 nm peaks can be attributed to the pep* transitions of carbazole chromophore. As to BBDP and BSBDP, the 280 nm peak can be attributed to PO-centered n-p* transition. There are two new absorption shoulders around 305 and 315 nm, which can be assigned to the pep* transitions of fluorene or spirofluorene. The energy gaps evaluated from the onset of the optical absorption of solution state are 3.48 eV for BBDC and BSBDC, while this value is enlarged to 3.88 eV for BBDP and 3.78 eV for BSBDP. This could be explained by that the electron-withdrawing group DPPO which will increase the band gap. The PL spectra of BBDC and BSBDC are almost coincident with each other with an emission peak around 358 nm. And for BBDP and BSBDP, the main emission peak lies at around 322 nm while there is a shoulder peak at 330 nm, which could be ascribed to the emission of the meta-linked DPPO. ET of BBDC, BBDP, BSBDC and BSBDP are determined by the highest-energy vibronic sub-band of the phosphorescence spectra in the solution state at 77 K (Fig. 2b) following the sequence of BBDP (2.87 eV) ¼ BSBDP (2.87 eV) > BBDC (2.81 eV) ¼ BSBDC (2.81 eV), which is accordant with the p-conjugation degree between carbazole and DPPO moieties. Notably, all of these values are higher than the triplet energy level of the 2,7-carbazole-disubstituted derivatives SBFC-G1 (2.66 eV) [48] and 2,7-DPPO-disubstituted derivatives SPPO13 (2.73 eV) [49]. In general, higher triplet energy level of the host material would facilitate energy transfer from host to dopant in the host-guest system. Thus, it is expected that 3, 6-disubstituted spiro-fluorene hosts will provide more

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Table 1 Physical properties of BBDC, BBDP, BSBDC and BSBDP. Host

lmax,abs/lmax,PL (nm)a

Eoxonset (V)

Eg (eV)b

HOMO/LUMO (eV)c

HOMO/LUMO (eV)d

ET (eV)e

Tg/Tdf (
C)

BBDC BBDP BSBDC BSBDP

288/358 282/322 285/352 280/322

1.12 1.55 1.08 1.48

3.48 3.88 3.48 3.78

5.8 6.23 5.76 6.16

5.37 6.03 5.38 5.89

2.81 2.87 2.81 2.87

215/426 NA/400 215/432 NA/411

a b c d e f

2.32 2.35 2.28 2.38

2.74 2.59 2.72 2.55

Measured in toluene at room temperature. Calculated from the onset of the absorption spectra in toluene solution. Estimated from the CV, HOMO ¼ Eoxonset (vs Ag/Agþ) eEAg/Agþ, the energy level of Ag/Agþ is 4.68 eV vs vacuum level. Calculated through the density function theory (DFT). Measured in 2-MeTHF glass matrix at 77 K. Tg: glass transition temperatures, Td: decomposition temperatures.

effective confinement of the triplet excitons on the guest and can prevent reverse energy transfer from guest to host. 3.4. Electrochemical properties The oxidative electrochemical behaviour of the four compounds was studied in CH2Cl2 solution through cyclic voltammetry (CV) measurements. Their cyclic voltammograms were shown in Fig. 3 and the respective electrochemical data were summarized in Table 1. The HOMO energy levels of the four compounds were calculated from the onset potentials of oxidation compared with the internal ferrocenium/ferrocene (Feþ/Fe) standard (4.68 eV) value and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated from the HOMO and their energy gaps (Eg). HOMOs of BBDC, BBDP, BSBDC and BSBDP are 5.80, 6.23, 5.76 and 6.16 eV, while their LUMOs are 2.32, 2.35, 2.28

Fig. 2. (a) Absorption and PL spectra of BBDC, BBDP, BSBDC and BSBDP in dilute toluene solution at room temperature. (b) The phosphorescence spectra of BBDC, BBDP, BSBDC and BSBDP in 2-methyltetrahydrogenfuran glass matrix at 77 K.

and 2.38 eV, respectively. CV analyses indicated that BBDC and BSBDC displayed reversible oxidation waves within the electrochemical window of CH2Cl2 and an additional peak at around 0.9 V, which most likely resulting from the electrochemical property of the C3 and C6 of the carbazole [50]. Conversely, BBDP and BSBDP displayed high irreversible oxidation peaks for the incorporation of DPPO groups, and finally led to low HOMO/LUMO. Apparently, the incorporation of the electron-withdrawing moiety DPPO can remarkably improve the electron injection ability compared with their carbazole counterparts. This trend is in accord with the results of DFT calculation. 3.5. Theoretical calculations To understand the structure-property relationship of the compounds at the molecular level, we obtained the electron density distributions of the HOMO and LUMO energy levels of the four compounds from the quantum calculations as shown in Fig. 4. According to DFT calculations, the HOMO of BBDC and BSBDC are mainly located on the electron-donating carbazole moiety, whereas the LUMO orbitals are mainly distributed on the fluorene moiety connecting to carbazole. The calculated HOMO/LUMO values are in the range of 5.37/-2.74 eV and 5.38/-2.72 eV, which correlates well with the experimental data. For the compound BBDP and BSBDP, their HOMO and LUMO are all localized on the fluorene, which can be explained by the electron-withdrawing nature of DPPO. But for compound BSBDP, spiro-configured biphenyl group is also contributed to the HOMO orbital, which can be attributed to the “spiroconjuagtion” of spiro-fluorene [51]. Since the separation between HOMO and LUMO levels is preferable for efficient holeand electron transporting properties and the prevention of reverse energy transfer, BBDC and BSBDC could be more suitable host

Fig. 3. The CV measurement of BBDC, BBDP, BSBDC and BSBDP.

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Fig. 4. Calculated spatial distributions of the HOMO and LUMO energy densities of BBDC, BBDP, BSBDC and BSBDP.

materials for FIrpic as compared with BBDP and BSBDP at high luminance. 3.6. Carrier transporting characteristics The charge-carrier properties of BBDC, BBDP, BSBDC and BSBDP were investigated in hole-only and electron-only devices. The holeonly device with the following structure: ITO/MoO3(1 nm)/ hosts(140 nm)/MoO3(10 nm)/Al and the electro-only device with the structure of ITO/TmPyPB (10 nm)/hosts(140 nm)/LiF(1)/Al. MoO3 and TmPyPB layers were used to prevent electron and hole injection from the cathode and anode, respectively. sFig. 1 shows

the current density versus voltage curves of the hole-only and electron-only devices for the compounds BBDC, BBDP, BSBDC and BSBDP. In both hole-only and electron-only devices, the current density of device based on spirobifluorene derivatives were higher than that of 9,9-diphenylfluorene derivatives, indicating better carrier-transport and injection properties of the spirobifluorenebased materials. 3.7. Electroluminescence To further verify the conjugation effects on device performance, four PhOLEDs devices (A, B, C, D) were fabricated with the

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L. Wang et al. / Dyes and Pigments 114 (2015) 222e230 Table 2 EL data of the devices AeD. Device Host A B C D a b c d e f

BBDC BBDP BSBDC BSBDP

Von Lmax [cd/m2] hcc (V)a (V at Lmax, V)b [cd/A]

hpd [lm/W]

[%]

3.5 3.3 3.2 2.8

28.1/7.2 28.8/12.5 34.1/31.5 34.4/30.3

13.8 13.4 16 18.7

26969 8642 20480 9304

(16.3) (12.9) (11.7) (9.4)

33.1/20.5 32.2/28.1 34.1/31.7 34.2/30.1

hEQEe CIE [x, y]f (0.15, (0.16, (0.14, (0.15,

0.39) 0.42) 0.36) 0.30)

Von: the voltage at 1 cd/m2. V: the voltage at the maximum brightness. The maximum hc and hc at 1000 cd/m2. The maximum hp and hp at 1000 cd/m2. Maximum external quantum efficiency. Measured at 8.0 V.

Scheme 2. Energy levels of the materials used in device AeD.

configuration of: ITO/MoO3 (10 nm)/NPB (70 nm)/mCP (5 nm)/ BBDC, BBDP, BSBDC and BSBDP-FIrpic (7%) (40 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (150 nm), respectively. NPB is the holetransporting layer and mCP is the exciton blocking layer. FIrpic doped host is used as the emitting layer, and the best electroluminescence (EL) performance was achieved with 7wt % FIrpic for all the hosts. TmPyPB served as both the electron-transporting layer and hole-blocking layer. MoO3 and LiF (lithium fluoride) served as the hole- and electron-injecting layers, respectively. The schematic energy level diagrams of the devices are shown in Scheme 2. The current densityvoltageluminance (JVL) characteristics, efficiency versus luminance curves and EL spectra for the devices are shown in Fig. 5 and the key EL data are summarized in Table 2. As can be seen from Fig. 5, all the phosphorescent devices display low turn-on voltages from 2.8 to 3.5 V. The blue devices AD attain a maximum current efficiency (hc.max) of 33.1, 32.2, 34.1, and 34.2 cd A1 and a maximum power efficiency (hp.max) of 28.1, 28.8, 34.1, and 34.4 lm/W, respectively. The hext.max reached 16% and

18.7% for devices C and D. More importantly, spirobifluorene-based devices demonstrated remarkable stability with a low efficiency roll-off at a brightness of 1000 cd/m2. It is found that even at a practical brightness of 1000 cd/m2, the device C and device D still have a hp as 31.5 and 30.3 lm/W with a roll-off of only 7.6% and 11.4%, respectively. The preferable efficiency of devices C and D should be attributed to the unbroken core, which led to more twisted geometry and better carrier transporting characteristics than 9,9-diphenylfluorene-based counterparts as can be seen from sFig. 1. On the other hand, there are also regular differences between carbazole-substituted and DPPO-substituted derivatives. The turnon voltage (Von) of device B (3.3 V) and D (2.8 V) were lower than device A (3.5 V) and C (3.2 V), respectively. It can be attributed to the superior electron-transporting property of DPPO, which lead to better electron-injection and low driving voltage. However, a maximum luminance (Lmax) of 26969 cd/m2 and 20480 cd/m2 was reached for device A and device C, which are obviously superior to device B (8642 cd/m2) and device D (9304 cd/m2). This can be induced by the instable nature of DPPO come along with the increase of luminance. Thus, DPPO-substituted derivatives achieved

Fig. 5. (a) current efficiency-luminance characteristics; (b) current efficiency-current density characteristics; (c) J-V-L curve of the three devices; (d) EL spectra for the devices.

L. Wang et al. / Dyes and Pigments 114 (2015) 222e230

the best performance but could not keep stability at high luminance as carbazole-substituted host materials. This contradiction may be solved by optimizing ratio of PO and carbazole and reduce the gap between singlet and triplet according to our latest reference [52]. In Fig. 5d, all the devices show identical spectra with a peak at 476 nm and a shoulder at 500 nm, with the Commission Internationale de l’Eclairage (CIE) coordinates ranging from (0.16, 0.42) to (0.15, 0.30) at 8 V, which is arising from the typical emission of the phosphor FIrpic. It is indicated that the energy transfer from hosts to FIrpic was efficient, and no emissions were observed other than the FIrpic emission. For the four devices, the CIE coordinate changed obviously, which should be induced by the fluctuation of the thickness of the emitting layer and exciton formation area (see sFig2). In a word, keeping the integrity of the spiro structure and choosing better substituent groups with higher ET are two synergistic factors to elevate the blue device performance based on 3, 6disubstituted spirobifluorene derivatives. 4. Conclusion In conclusion, four host materials characterized by 3, 6substituted spirobifluorene were synthesized and their basic photophysical properties were investigated. The host materials with meta-substitution modification showed higher triplet energy and Tg compared with ortho- or para-analogues. These promising characters render them feasible to realize highly efficient and low rolloff blue PhOLEDs adopting a common device configuration. Our results clearly indicated that tailor-made host materials concurrently possessing high triplet energies and thermal stability for blue PhOLEDs can be reasonably achieved. Acknowledgements This research work was supported by the NSFC/China (21161160442, 51203056), the National Basic Research Program of China (973 Program 2013CB922104). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2014.11.011. References [1] Tao Y, Yang C, Qin J. Organic host materials for phosphorescent organic lightemitting diodes. Chem Soc Rev 2011;40:2943e70. [2] Yook KS, Lee JY. Organic materials for deep blue phosphorescent organic lightemitting diodes. Adv Mater 2012;24:3169e90. [3] Murawski C, Leo K, Gather MC. Efficiency roll-off in organic light-emitting diodes. Adv Mater 2013;25:6801e27. [4] Sasabe H, Kido J. Development of high performance OLEDs for general lighting. J Mater Chem C 2013;1:1699e707. [5] Xu H, Chen R, Sun Q, Lai W, Su Q, Huang W, et al. Recent progress in metalorganic complexes for optoelectronic applications. Chem Soc Rev 2014;43: 3259e302. [6] Wong W-Y, Ho C-L. Heavy metal organometallic electrophosphors derived from multi-component chromophores. Coord Chem Rev 2009;253:1709e58. [7] Wong W-Y, Ho C-L. Functional metallophosphors for effective charge carrier injection/transport: new robust OLED materials with emerging applications. J Mater Chem 2009;19:4457e82. [8] Zhou G, Wong W-Y, Suo S. Recent progress and current challenges in phosphorescent white organic light-emitting diodes (WOLEDs). J Photoch Photobio C 2010;11:133e56. [9] Zhou G, Wong W-Y, Yang X. New design tactics in OLEDs using functionalized 2-phenylpyridine-type cyclometalates of iridium(III) and platinum(II). ChemAsian J 2011;6:1706e27. [10] Yang X, Zhou G, Wong W-Y. Recent design tactics for high performance white polymer light-emitting diodes. J Mater Chem C 2014;2:1760e78.

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