phosphine oxide hybrids as host materials for deep blue phosphorescent organic light-emitting diodes

Organic Electronics 14 (2013) 1924–1930

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meta-Linked spirobifluorene/phosphine oxide hybrids as host materials for deep blue phosphorescent organic light-emitting diodes Lin-Song Cui a, Shou-Cheng Dong a, Yuan Liu a, Mei-Feng Xu a, Qian Li a, Zuo-Quan Jiang a,⇑, Liang-Sheng Liao a,b,⇑ a Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, PR China b The State Key Laboratory on Integrated Optoelectronics, Jilin University, Changchun, Jilin 130012, PR China

a r t i c l e

i n f o

Article history: Received 30 March 2013 Received in revised form 17 April 2013 Accepted 17 April 2013 Available online 3 May 2013 Keywords: Organic light-emitting diodes Host materials Deep blue phosphorescence Spirobifluorene derivatives

a b s t r a c t This study investigated the use of a novel modification in molecular design to get two new electron-transport host materials, SF3PO and BSF3PO. By linking the phosphine oxide moieties at meta-position of spirobifluorene rings, higher triplet energies could be easily achieved for these two new materials. The steric spirobifluorene structures could guarantee their good thermal stabilities. According to these properties, their applications as host materials for deep blue phosphorescent organic light-emitting diodes (PHOLEDs) were explored. As expected, the deep blue emitting devices with Ir-complex FIr6 as phosphorescent dopants and SF3PO and BSF3PO as hosts had been fabricated and showed high efficiency of 28.5 and 22.0 cd/A, respectively, which were significantly higher than that of the para-linked analogue SPPO1. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Phosphorescent organic light-emitting diodes (PHOLEDs) have made a big breakthrough on OLED technology because of they could utilize both singlet and triplet excitons to achieve theoretical 100% internal quantum efficiency [1,2]. So far, tremendous attention has been devoted to this work; green and red PhOLEDs with 100% internal quantum efficiency have been achieved [3–6]. This is because of the fact that the green or red emitters possess relatively low triplet energy and thus the adjacent hole and electron transport layers were easy designed to suppress

⇑ Corresponding authors. Address: Jiangsu Key Laboratory for CarbonBased Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, PR China (L.-S. Liao). Tel.: +86 13506135475 (Z.-Q. Jiang). E-mail addresses: [email protected] (Z.-Q. Jiang), lsliao@suda. edu.cn (L.-S. Liao). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.04.037

triplet energy transfer from emitting material to the charge transport layers. For the most popular sky-blue emitter 0 iridium (III) bis(4,6-(difluorophenyl)pyridine-N,C2 ) picolinate (FIrpic), the same concept was also employed to blue PHOLEDs and high efficiency could also be improved by optimizing the device structure [7–10]. As we know, significant effort have been achieved with sky blue PHOLEDs based on FIrpic with a maximum external quantum efficiency over 29% and a maximum power efficiency over 47.7 lm/W [11]. However, the sky blue phosphor FIrpic does not satisfy the blue color standards recommended by the National Television Committee. To improve the color quality of blue PhOLEDs, Thompson et. al. reported a phosphor of iridium(III) bis(40 ,60 -difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6), and the corresponding devices show deep-blue emission with CIE coordinates of (0.16, 0.26) [12]. Since then, elegant materials and elaborate device structures have been developed to improve the performance of FIr6-based devices.

L.-S. Cui et al. / Organic Electronics 14 (2013) 1924–1930

The research on host materials is of equal importance for PHOLEDs to afford high efficiency of phosphorescent emitters [13–15]. Generally speaking, the first criterion for suitable host materials is that the triplet energy of the host should be higher than that of the phosphorescent emitter to prevent exothermic reverse energy transfer and confine the triplet excitons within the emitting layer. Considering that the triplet energy of deep blue dopant is about 2.75 eV, this requirement means that the host material with triplet energy below 2.80 eV is not applicable in deep blue PHOLEDs [16–24]. In the early stage of seeking host materials, the carbazole-based derivatives, which usually possess high triplet energies and good hole transport (HT) ability, played the major role on this topic [25,26]. Accompanied with the development of materials research, the researcher gradually realize that the electron transport (ET) type host materials may be another alternative to achieve carrier balance for lowering electron injection barriers, blocking hole transporting to cathode, and increasing electron mobility [27,28]. The phosphine oxide (PO) derivatives, undoubtedly, should be highlighted in the progress of ET-type host materials. The PO functional group has attracted great attention for host materials because the PO groups meet all the requirements for host [29–31]. One success example of these materials is the fluorene based PO type host materials for their deep HOMO levels to block hole transporting. Most recently, Lee and co-workers reported two ortho- (SPPO11) and para- (SPPO1) linked spirobifluorene/phosphine-oxide hybrid molecules as hosts, which acquired a satisfactory electroluminescent performance by using FIrpic as the blue dopant [32,33]. We tried to repeat this work for deep blue PHOLEDs but regrettably we found the triplet energy of SPPO1 is just 2.77 eV, which is

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not suitable for the host of the phosphorescent emitter FIr6. Fortunately, the synthetic versatility of organic materials allows us to optimize and tailor the molecular physical properties including the energy level. Unlike the reported ortho- or para- analogues, the meta- position derivation will possess the highest triplet energy. In this work, we firstly utilize the meta-position of spirobifluorene to synthesis two new spirobifluorene-based materials SF3PO and BSF3PO. In addition to their higher triplet energies, SF3PO and BSF3PO also show good thermal stabilities resulted from the introducing of bulky spirobifluorene group. As expected, deep blue PHOLEDs containing SF3PO/ BSF3PO as hosts and FIr6 as a dopant exhibited good performance with the maximum current efficiency of 28.0 cd/ A and 22.0 cd/A respectively, which are superior to the results of SPPO1.

2. Results and discussions 2.1. Synthesis and characterization The syntheses of new hosts according are shown in Scheme 1. 2-amino-4-bromobenzophenone was prepared according to the literature methods [34]. The important intermediates 3-bromofluorenone was synthesized by the pschorr cyclization reaction [35]. 2-bromobiphenyl was reacted with n-BuLi by lithium–halogen exchange reaction at 78 °C, then reacted with 3-bromofluorenone, followed by an intramolecular ring closure via Friedel–Crafts reaction to afford the 3-bromo-9,90 -spirobifluorene. Finally, the target compounds SF3PO and BSF3PO were successfully achieved by the reaction of 3-bromo-9,90 -spirobifluorene

Scheme 1. Synthetic routes and chemical structures of the host materials.

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with chlorodiphenylphosphine or dichlorophenylphosphine through the lithiation and then oxidized by hydrogen peroxide. Detailed synthetic procedures are presented in the experimental section. Afterwards, all the target materials were further purified by repeated temperature gradient vacuum sublimation. The chemical structures of target compounds were fully characterized by 1H NMR and 13C NMR spectroscopies, mass spectrometry and elemental analysis.

According to the good thermal and amorphous stabilities of new compounds, they could form morphologically stable and uniform amorphous films by vacuum deposition for OLED fabrication. The UV–vis absorption and emission photoluminescence spectra of SF3PO and BSF3PO at room temperature in CH2Cl2 are shown in Fig. 2. SF3PO and BSF3PO exhibit very similar absorption and emission profiles. The emission peaks around 322 nm are assigned to the p–p transition of spirobifluorene between phenyl phosphine oxide moiety, which is blue-shifted by 24/15 nm as compared to that of SPPO11/SPPO1. Similarly, the absorption spectra as well as a hypsochromic shift could be attributed to meta-linkage for SF3PO and BSF3PO instead of paralinkage (SPPO1) and ortho-linkage (SPPO11). The optical energy bandgaps of SF3PO and BSF3PO are 3.84 and 3.81 eV, respectively, which are calculated from the threshold of the absorption spectra in CH2Cl2 solution. The phosphorescence spectra were measured in frozen 2-methyltetrahydrofuran matrix at 77 K, and the triplet energies (ET) calculated from the spectra followed the sequence SPPO11 (2.78 eV)  SPPO1 (2.77 eV) < SP3PO (2.87 eV)  BSP3PO (2.86 eV). Apparently, the introduction of meta-linkage in spirobifluorene benefit to generating higher triplet energy than para and ortho-linkage. These results demonstrate that both the two compounds can be used as potential hosts for deep blue phosphorescent emitters. The HOMO levels of SF3PO and BSF3PO were determined by Ultraviolet photoemission spectroscopy (UPS) as 6.39 and 6.23 eV (see Supplementary Information), and the LUMO levels were calculated from the HOMOs and optical energy gaps. The all pertinent data are summarized in Table 1.

2.2. Physical properties The thermal properties of SF3PO and BSF3PO were investigated through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in nitrogen atmosphere at a scanning rate of 10 °C/min. SF3PO and BSF3PO exhibit high thermal decomposition temperatures (Td, corresponding to 5% weight loss, see Supplementary Information) of 372 and 409 °C, respectively. Their glasstransition temperatures were observed at 119 and 189 °C, respectively. The Tg of SF3PO was improved from the 96 °C of SPPO1 to 119 °C, but slightly lower than 127 °C of SPPO11. These results accord well with the steric hindrance of the three isomers, and BSF3PO exhibits the highest Tg value because of its biggest molecular size (Fig. 1).

BSF F3PO O o

Heat Flow

9 C Tg = 189

SF3PO o

Tg = 119 C

2.3. Electroluminescent properties

100

150

200

250

300

To evaluate the potentials of SF3PO and BSF3PO as host materials for deep blue phosphorescent emitters, we fabricated PHOLEDs with typical sandwiched structures by sequential vapor deposition of the materials onto a glass substrate coated with ITO, the deep blue Ir-complex FIr6

350

o

Temperature ( C) Fig. 1. DSC traces recorded at a heating rate of 10 °C/min.

1.0

(a)

SPPO1 SPPO11 SF3PO BSF3P0

0.8 0.6

0.8 0.6 0.4

0.4

0.2

0.2

0.0

0.0 300

350

400

Wavelength (nm)

450

PL Intensity (arb. unit)

Absorption (arb. unit)

1.0

Phosphorescence Intensity (arb. unit)

50

4 3 2 1 0 350

(b) BSF3PO

SF3PO SPPO11 SPPO1

400

450

500

550

Wavelength (nm)

Fig. 2. (a) UV–Vis absorption and PL spectra of SPPO1, SPPO11, SF3PO and BSF3PO in dichloromethane solution at 105 M and (b) phosphorescence spectra of these compounds measured in a frozen 2-methyltetrahydrofuran matrix at 77 K.

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L.-S. Cui et al. / Organic Electronics 14 (2013) 1924–1930 Table 1 Physical properties of SF3PO and BSF3PO.

c d e f

Tga (°C)

Tda (°C)

kabsb (nm)

kemb (nm)

kphc (nm)

Egd (eV)

ETe (eV)

HOMO/LUMOf (eV)

SF3PO BSF3PO SPPO1 SPPO11

119 189 96 127

372 409 – –

294, 308,314 294, 308,314 308, 318 308,323

322 322 346 337

430 432 447 446

3.83 3.83 3.78 3.65

2.88 2.87 2.77 2.78

6.39/2.56 6.23/2.40 6.43/2.65 6.51/2.86

Tg: glass transition temperature. Measured in dichloromethane solution at room temperature. Measured in 2-MeTHF glass matrix at 77 K. Eg: The band gap energy is estimated from the optical absorption edges of UV–Vis absorption spectra. ET: The triplet energy is estimated from the onset peak of the phosphorescence spectra. HOMO levels were calculated from UPS data, LUMO levels were calculated from HOMO and Eg.

Table 2 Electroluminescent characteristics of the devices. a

b ext

b p

Device

Host/ guest

V (v)

g

A

SPPO1/ FIr6 SF3PO/ FIr6 BSF3PO/ FIr6

4.1

4.5, 4.0, 2.7

3.8

13.6, 13.4, 11.9 10.2, 10.1, 8.9

B C

4.1

(%)

doped into the hosts as emitting layer. MoO3 doped into N,N0 -dicarbazolyl-3,5-benzene (mCP) was utilized as the hole-injection layer (HIL) and the hole transport layer (HTL), mCP served as electron blocking layer (EBL). 1,3,5tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) was used as electron-transporting layer (ETL) as well as hole-blocking layer (HBL). Liq served as electron-injecting layer (EIL) and Al was utilized as cathode. FIr6 doped in SF3PO and BSF3PO were used as the emitting layer. These PHOLEDs have the configuration of ITO/MoO3: mCP (45 nm, 15 wt.%)/ mCP (10 nm)/ Host: FIr6 (25 nm, 8 wt.%)/TmPyPB (30 nm)/Liq (2 nm)/Al (120 nm) (Host = SF3PO: device B;

b

g (lm/W)

gc (cd/A)

8.8, 6.5, 3.3 23.7, 22.9, 14.4 18.5, 17.2, 10.0

10.5, 9.2, 6.2 28.5, 28.1, 23.1 22.0, 21.4, 16.9

a

Voltage (V) at 100 cd/m2. External quantum efficiency (gext), power efficiency (gp) or current efficiency (gc) in the order of maximum, at 100 cd/m2 and at 1000 cd/m2.

4

(a) SPPO1 SF3PO BSF3PO

40

16

10

3

10

30 20

2

10 10

Luminance (cd/m 2)

50

2

Current Density (mA/cm )

b

10 3

4

5

6

7

SPPO1 SF3PO BSF3PO

8

4

1

0

(b)

12

EQE (%)

a b

Compound

0

8

0

(c)

1.0 SPPO1 SF3PO BSF3PO

20

10

0

0

10

20

30

40 2

Current Density (mA/cm )

EL Intensity (a.u.)

Power Efficiency (lm/W)

30

10

20

30

40

Current Density (mA/cm2 )

Voltage (V)

(d) SPPO1 SF3PO BSF3PO

0.8 0.6 0.4

2

@5mA/cm

0.2 0.0 300

400

500 600 Wavelength (nm)

700

800

Fig. 3. (a) Current density–voltage–luminance characteristics; (b) external quantum efficiency versus current density curves; (c) power efficiency versus current density curves and (d) the EL spectrum of devices A, B and C at 5 mA/cm2.

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Normalized PL Intensity (arb. unit)

Host = BSF3PO: device C). For comparison, the control devices using SPPO1 (device A) as host were also fabricated. The key parameters summarized in Table 2. The current density–voltage–luminance (J–V–L) and external quantum efficiency and power efficiency versus current density for the devices are depicted in Fig. 3. The detail electroluminescence (EL) data are summarized in Table 2. As shown in Fig. 3, device B hosted by SF3PO exhibited a maximum power efficiency and external quantum efficiency of 23.7 lm/W and 13.7%, respectively, while device C hosted by BSF3PO exhibited slightly lower efficiencies of 18.5 lm/W and 10.2%, accordingly. In comparison, the control device A hosted by SPPO1 as the emitting layer showed relatively poorer performance (8.8 lm/W, 4.5%). This can be attributed to the higher triplet energy levels of SF3PO and BSF3PO compared with SPPO1, which could efficiently suppress triplet energy back transfer from the guest to the host in emission layer. All the devices exhibit the identical spectra with a peak at 461 nm and a shoulder at 488 nm, which resembles the PL of FIr6 phosphorescent emitter in dilute solution. Thus, the Commission International de I’Eclairage (CIE) coordinates of devices A, B and C have values of (0.16, 0.31), (0.15, 0.30) and (0.15, 0.30), respectively. Actually, we also fabricated the devices D–F doped by the sky-blue triplet emitter FIrpic, although the best device performance was also achieved by SF3PO as host with a maximum current efficiency of 30.5 cd/A, the efficiency difference between SPPO1 (27.0 cd/A) and SF3PO (30.5 cd/A) was not so significant as that in the devices with FIrpic as emitter (see Supplementary Information). These results could partly support the importance of tuning the energy level by appropriate molecular design to fit the demand of the emitter with higher triplet energy. To further verify the exciton confinement properties of the hosts and understand the difference of the device performance of A, B, and C. Transient photoluminescence decays of thin films (formed on quartz substrates with a

0

10

SPPO1 SF3PO BSF3PO

0

2

4

6

8

10

Time ( µsec) Fig. 4. Transient photoluminescence decay (excited at 330 nm) curves at room temperature at 461 nm for the FIr6 co-deposited with SPPO1, SF3PO and BSF3PO.

thickness of 40 nm) with 10 wt.% FIr6 dispersed in SPPO1, SF3PO and BSF3PO were measured. As shown in Fig. 4, the SPPO1: FIr6 film exhibits a complicated biexponential decay curve, indicating the possibility of triplet energy transferring from FIr6 to SPPO1 in emission layer. This could be attributed to the close triplet energy levels of SPPO1 (2.77 eV) and FIr6 (2.72 eV), thereby leading to excitons diffusion out of the emissive layer and thus lower the device performance. However, the SF3PO: FIr6 and BSF3PO: FIr6 films exhibited nearly mono-exponential decay curves, indicating that the triplet energy transfer from FIr6 to hosts is completely suppressed and the energy is well confined in emission layer, consequently resulting in efficient deep blue PHOLEDs for SF3PO and BSF3PO based devices. 3. Conclusions In summary, we have developed two new host materials by substituting spirobifluorene at meta-position with the phenylphosphine oxide unit. This design strategy endows these materials with high triplet energies and relatively high thermal stability. Deep blue PHOLEDs hosted by SF3PO and BSF3PO achieved maximum current efficiencies as high as 28.5 and 22.0 cd/A, respectively, which are much higher than that of SPPO1. This can be attributed to their high triplet energies to confine triplet exciton on the guest. This work reveals that the 3-position of the commonly used fluorene unit could solve the problem of insufficient triplet energy derived at the traditional 2-position of fluorene and thus could be widely utilized to get better performance in electrophosphorescence research. The tuning method is simple and the further study will be continued in future work. 4. Experimental section 4.1. Materials and measurements All chemicals i.e., 2-bromobiphenyl, chlorodiphenylphosphine and dichlorophenylphosphine were purchased from Aldrich and were used without further purification. THF was purified by PURE SOLV (Innovative Technology) purification system. Chromatographic separations were carried out by using silica gel (200–300 mesh). All other reagents were used as received from commercial sources unless otherwise stated. 1H NMR and 13C NMR spectra were recorded on a Varian Unity Inova 400 spectrometer at room temperature. Mass spectra were recorded on a Thermo ISQ mass spectrometer using a direct exposure probe. UV–Vis absorption spectra were recorded on a Perkin Elmer Lambda 750 spectrophotometer. PL spectra and phosphorescent spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer. Transient PL decays were measured by a single photon counting spectrometer from HORIBA JOBIN YVON with a Nano LED pulse lamp as the excitation source. Differential scanning calorimetry (DSC) was performed on a TA DSC 2010 unit at a heating rate of 10 °C/min under nitrogen. The glass transition temperatures (Tg) were determined from the second heating scan.

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Thermo gravimetric analysis (TGA) was performed on a TA SDT 2960 instrument at a heating rate of 10 °C/min under nitrogen. Temperature at 5% weight loss was used as the decomposition temperature (Td). 4.2. Computational methodology The geometrical and electronic properties of SF3PO and BSF3PO were performed with the Gaussian 09 program package. The calculation was optimized by means of the wb97xd (Becke three parameters hybrid functional with Lee-Yang-Perdew correlation functionals) with the 6-31G(d) atomic basis set. Molecular orbitals were visualized using Gaussview. 4.3. Fabrication of OLEDs The OLEDs were fabricated through vacuum deposition of the materials at ca. 2  106 Torr onto ITO-coated glass substrates having a sheet resistance of ca. 30 X per square. The ITO surface was cleaned ultrasonically – sequentially with acetone, ethanol, and deionized water, then dried in an oven, and finally exposed to UV–ozone for about 30 min. Organic layers were deposited at a rate of 2–3 Å/ s, subsequently, Liq was deposited at 0.2 4 Å/s and then capped with Al (ca. 4 Å/s) through a shadow mask without breaking the vacuum. For all the OLEDs, the emitting areas were determined by the overlap of two electrodes as 0.09 cm2. The EL spectra, CIE coordinates and J–V–L curves of the devices were measured with a PHOTO RESEARCH SpectraScan PR 655 photometer and a KEITHLEY 2400 SourceMeter constant current source at room temperature. The EQE values were calculated according to the previously reported methods [36]. 4.4. 3-Bromofluorenone (3-BrF) To 10 g (36.21 mmol) of 2-amino-4-bromobenzophenone was stirred into 50 ml of 80% sulfuric acid at 60 °C over half hour. The solution was cooled to 5 °C and diazotized with 2.4 g (36.21 mmol) of sodium nitrite in 5 ml water. After 2 h at 50 °C the mixture was cooled and filtered, washed with water. The solid was recrystallization from ethyl acetate and give a light yellow solid (7.5 g, 80% yield). 1H NMR (400 MHz, CDCl3) d (ppm): 7.62 (d, J = 8.0 Hz, 1H) 7.59 (s, 1H) 7.52–7.35(m, 4H) 7.33–7.24 (m, 1H) 13C NMR (100 MHz, CDCl3) 192.6, 146.1, 143.1, 134.2, 132.0, 129.7, 125.5, 124.5, 123.8, 120.6. MS (EI): m/z 259.9 [(M + 1)+]. Anal. calcd for C13H7BrO (%): C 60.02, H 2.69; found: C 60.23, H 2.72. 3-Bromo-9,90 -spirobifluorene (3-BrSF). A solution of 2-bromobiphenyl (4.6 g, 19.73 mmol) in THF (100 ml) was treated with n-Butyl lithium (2.4 M in hexane, 8.63 ml) under nitrogen at 78 °C. After 30 min, a solution of 3-bromo-fluorenone (5.1 g, 19.73 mmol) in THF (50 ml) was added drop wise slowly. The resulting mixture was stirred for 30 min at 78 °C, and allowed to warm to room temperature. After 12 h, the mixture quenching with 30 ml of water and the organic layer was extracted with ether acetate, then washed with water and brine, dried over

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anhydrous Na2SO4, filtered and evaporated under reduced pressure. The crude residue dissolved in acetic acid (60 ml) and concentrated aqueous HCl (8 ml). After refluxing for 2 h, the mixture was cooled to ambient temperature and filtered. The crude product was purified by recrystallization from toluene and afforded a white solid (5.2 g, 67% yield). 1H NMR (400 MHz, CDCl3) d (ppm): 7.86–7.76 (m, 3H) 7.41–7.32 (t, 3H) 7.22(d, J = 8.0 Hz, 1H) 7.16–7.06 (m, 3H) 6.75–6.68 (t, 3H) 6.62–6.57 (d, J = 8.0 Hz, 1H) 13C NMR (100 MHz, CDCl3) 149.0, 148.0, 147.6, 143.9, 141.7, 140.4, 130.5, 128.5, 127.9, 125.5, 124.1, 123.9, 123.2, 121.7, 120.2, 120.1, 65.6. MS (EI): m/z 396.4 [(M + 1)+]. Anal. calcd for C13H7BrO (%): C 75.77, H 3.71; found: C 75.96, H 3.82. 3-Diphenylphosphine oxide-9,90 -spirobifluorene (SF3PO). 3-Bromo-9,90 -spirobifluorene (2.0 g, 5.05 mmol) was dissolved in anhydrous THF (30 ml) under nitrogen and was cooled to 78 °C. n-Butyl lithium (2.4 M in hexane, 2.2 ml) was then added drop wise slowly. After stirring for 30 min at 78 °C, dichlorophenylphosphine (1.2 g, 5.30 mmol) in THF (10 ml) of was added slowly. The resulting mixture was stirred for 1 h at 78 °C, and allowed to warm to room temperature. The mixture quenching with 20 ml of water and the organic layer was extracted with ether acetate, then washed with water and brine, dried over anhydrous Na2SO4, filtered and evaporated under reduced pressure. The crude material was purified by column chromatography with 1:3 (v/v) dichloromethane/petroleum ether as the eluent. The white powdery product obtained was dissolved in dichloromethane (30 ml) and to the solution was added 30% aqueous H2O2 (10 ml). The mixed solution was stirred overnight at ambient temperature. The organic layer was separated and washed with dichloromethane and water, then dried over anhydrous Na2SO4, filtered and evaporated under reduced pressure. The crude material was purified by column chromatography with 1:3 (v/v) dichloromethane/petroleum ether as the eluent to give a white solid (1.4 g, 54% yield). 1H NMR (400 MHz, CDCl3) d (ppm): 8.30 (d, J = 8.0 Hz, 1H) 7.87–7.76 (m, 3H) 7.77– 7.66(m, 4H) 7.59–7.51 (m, 2H) 7.38–7.33 (m, 2H) 7.50–7.43(m, 4H) 7.41–7.30(m, 3H) 7.25–7.17(m, 1H) 7.16–7.06(m, 3H) 6.69–6.67(m, 4H) 13C NMR (100 MHz, CDCl3) 153.2, 148.6, 147.8, 142.2, 132.0, 131.8, 131.4, 128.8, 127.8, 123.9, 120.7, 120.3, 120.0, 66.0. MS (EI): m/z 516.85 [M+]. Anal. calcd for C37H25OP (%): C 85.81, H 4.70; found: C 86.03, H 4.88. Bis(9,9-spirobifluorene-3-yl)-phenylphosphaneoxide (BSF3PO). BSF3PO was synthesized according to the same procedure as for SF3PO by using 3-bromo-9,90 -spirobifluorene (2 g, 5.05 mmol), dichlorophenylphosphine (0.45 g, 2.53 mmol) and 30% aqueous H2O2 (10 ml). BSF3PO was afforded as a white solid (0.8 g, 42% yield). 1H NMR (400 MHz, CDCl3) d (ppm):8.34 (d, J = 8.0 Hz, 2H) 7.86– 7.72 (m, 8H) 7.60–7.52(m, 1H) 7.52–7.44 (m, 2H) 7.40– 7.30 (m, 6H) 7.30–7.19(m, 2H) 7.18–7.05(m, 8H) 13C NMR (100 MHz, CDCl3) 153.7, 148.4, 147.9, 142.3, 141.5, 140.7, 132.2, 131.6, 128.7, 127.6, 123.9, 120.3, 120.7 119.6, 66.0. MS (EI): m/z 754.85 [M+]. Anal. calcd for C56H35OP (%): C 88.87, H 4.76; found: C 89.10, H 4.67.

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