Fast Synthesis of Green Iridium(III) Complexes Incorporating A Bis

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Fast Synthesis of Green Iridium(III) Complexes Incorporating A Bis(diphenylphorothioyl)amide Ligand for Efficient Pure OLEDs Xiao Liang, Feng Zhang, Zhi-Ping Yan, Zheng-Guang Wu, Youxuan Zheng, Gang Cheng, Yi Wang, Jing-Lin Zuo, Yi Pan, and Chi-Ming Che ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19318 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Applied Materials & Interfaces

Fast Synthesis of Green Iridium(III) Complexes Incorporating A Bis(diphenylphorothioyl)amide Ligand for Efficient Pure OLEDs Xiao Liang1#, Feng Zhang3#, Zhi-Ping Yan1, Zheng-Guang Wu1, You-Xuan Zheng1,3*, Gang Cheng2*, Yi Wang1,3*, Jing-Lin Zuo1,3, Yi Pan1,3, Chi-Ming Che2* 1State

Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic

Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China, e-mail: [email protected], [email protected] 2State

Key Laboratory of Synthetic Chemistry, Institute of Molecular Functional Materials, and

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, P. R. China, HKU Shenzhen Institute of Research and Innovation, Shenzhen 518053, P. R. China, e-mail: [email protected], [email protected] 3MaAnShan #Liang

High-Tech Research Institute of Nanjing University, MaAnShan, 238200, P. R. China

and Zhang have the same contributions to this paper.

Abstract: The bis(diphenylphorothioyl)amide (Stpip) containing phoshpor-sulfur (P=S) bonds was used as ancillary ligand for three pure green iridium(III) emitters Ir1, Ir2 and Ir3, which were synthesized in few minutes at room temperature with high reaction yields above 70%. All these complexes show good thermal stability and excellent sublimation yields of around 80~90%, which are considered beneficial for industry practical production and organic lightemitting diodes (OLEDs) fabrication. The emission profiles of these complexes meet the green standards of CIE1931 with coordinates of (0.33, 0.63), (0.33, 0.62) and (0.34, 0.62), respectively, and high photoluminescence quantum yields of up to 98% are achieved. Utilizing these complexes as emissive dopants, these OLEDs exhibited high current efficiency up to 91.94 cd A-1, external quantum efficiency (EQE) up to 26.52% and power efficiency up to 92.60 lm W-1 with very small efficiency roll-off, without adopting internal or external outcoupling methods. These results indicate that Stpip is a potentially suitable ligand scaffold for highly efficient phosphorescent Ir(III) emitters that endow corresponding OLEDs with high efficiency and small roll-off. Keywords: Green iridium(III) complex; Bis(diphenylphorothioyl)amide; Fast synthesis; Sublimation yield, Efficiency

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Introduction Phosphorescent OLEDs (PhOLEDs) have been extensively studied for decades as this technology has been shaping the landscape of display industry.1 OLEDs incorporating iridium(III) or platinum(II) complexes2 as emitters are capable of exhibiting superior efficiency, high contrast, low turn-on voltage and vibrant color gamut capability,3 which are the main pillars towards the construction of decent display technology. The origin of such advantages comes from the intrinsic properties of phosphorescent materials that can theoretically achieve 100% of internal quantum efficiency (IQE) endowed by strong spin-orbit coupling effect (SOC),4 additionally realizing relatively short luminescence decay lifetime in the order of only several microseconds or even less.5 Consequently, the radiative processes prevails over the non-radiative ones, inducing a much smaller efficiency roll-off.6 Even though PhOLEDs have been commercialized for several years, highly efficient and stable phosphorescent emitters that can both be easily processed and meet the requirements of National Television Standards Committee (NTSC) are still in high demand.1,3,7 The coordination pattern of iridium complexes has been thoroughly investigated throughout the years, however iridium complexes in direct coordination with sulfur atoms are seldomly reported and studied. Sulfur atom is electron-rich and capable of stabilizing metal ions in unusual oxidation states.8 The lone pair on sulfur atom can have additional two metal-sulfur (M-S) bonding interaction in co-planar structures.9 The M-S units are commonly reported in Ag, Au, Zn, Fe, Cu, Pd, Rh complexes which lead to unique structural and photophysical properties.10 However, Ir(III) complexes in direct coordination with sulfur atoms are rarely researched, especially in the context of OLEDs. In 1991, P. Biscarini et. al. reported a tri-cyclic neutral Ir(III) complex with S-Ir-S framework that is optically stable and has interesting circular dichroism properties.11 J. Browning et. al. investigated an Ir(III) complex containing tris(diphenylthiophosphinoyl)methanide ligand with a trigonal planar geometry and a free non-coordinated sulfur atom.12 V. H. Nguyen et. al. prepared several anionic heteroleptic Ir(III) complexes that exhibit different emission profiles with the coordinated S atoms in various oxidation states.8,9 A series of Ir(III) or Rh(II) complexes with dichalcogenoimidodiphosphinato ligands, which features a S-M-S pattern, were systematically studied based on their single crystal analysis10c. It is not until 2005 that the first OLED based on Ir(III) complex containing coordinated Ir-S bonds was reported, however, the device performance


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was unsatisfactory with a maximum current efficiency of merely 2.9 cd/A.13 Since 2011, our group has reported a panel of efficient Ir(III) complexes supported by diphenylphosphinic amide (tpip) ligand for OLED applications, revealing that diphenylphosphinic amide ligand can drastically enhance the device efficacy while maintaining a rather small efficiency roll-off even at high operating voltages.14 The strategy has been subsequently proven to be effective with coordination of this ligand, the corresponding Ir(III) or Pt(II) analogues often exhibit good electron mobility and decent device performances. For the practical application, the device operation lifetime is the top priority, which requires all the materials have good thermal stability. Furthermore, in order to speed up the process of OLEDs production, the cost reduction is also important. But for the production of most Ir(III) complexes, the last reaction process is always to reflux the [(C^N)2Ir(µ-Cl)]2 chloride-bridged dimmer with cyclometalated or ancillary ligands at high temperature for a long time, which would inevitably increase the manpower and cost. It is important, but difficult to find suitable ligands which can form phosphorescent complexes at room temperature efficiently to reduce the cost of OLEDs industrialization significantly. Herein, we incorporate S atoms into the original tpip framework bearing a novel SÎr^S coordination structure and obtain three Ir(III) complexes that are qualified as standard green emitters. The unique structural and electronic properties of S atoms could potentially alter the photophysical and electrochemical properties of the corresponding complexes.15 Since S atom is known to be able to bind strongly with transition metal, the unique coordination SÎr^S structure could also help to improve the stability of the corresponding complexes. In this work, three new Ir(III) emitters were prepared in 10 minutes at room temperature with high synthesis and vacuum purification yields. Meanwhile, the OLEDs with these emitters show excellent device performance with the external quantum efficiency (EQE) up to 26.52% and impressively small efficiency rolloff.

Experimental General synthesis of ligands and complexes Scheme 1 depicts the overall synthetic procedures of the ligands and three Ir(III) complexes Ir1, Ir2 and Ir3. All reactions were carried out under nitrogen atmosphere, solvents and reactants were purchased without further purification. Bis(diphenylphosphorothioyl)amide (HStpip) was



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synthesized by refluxing hexamethyldisilazane and chlorodiphenylphosphine in degassed toluene followed by addition of sulfur powder. Three Ir(III) complexes were prepared by reacting their corresponding Ir(III) chloro-birdge intermediates with KStpip even at room temperature (Figure S1).

P

H N

Si

S

a)

P

b)

NH

Si S

Cl

S

P

S

P

NK

P

KStpip

HStpip B(OH)2 Br N

Cl

d) c)

3

R

2

Ir

4 R 5

6

R

L1: R = 4-CF3 L2: R = 5-CF3 CF3

Cl

N

N1

B(OH)2

c)

N

Ir1: R = 4-CF3 Ir2: R = 5-CF3 CF3 Cl

F3C

Ir

CF3

CF3

Ir Cl

N

e)

2

C3

S P

F3C

N

Ir N

N

2

L3

S P 2

R

CF3

d)

F3C

2

C1: R = 4-CF3 C2: R = 5-CF3

N F3C

N R

N

Ir

N

2

CF3

Br

S P

e)

Ir

S P 2

Ir3

Scheme 1: Synthetic routes of iridium complexes Ir1, Ir2 and Ir3. Reagents and conditions: a) toluene, N2, reflux; S powder, THF, N2; b) KOH, Et2O; c) K2CO3, THF/H2O, N2, Pd(PPh3)4, reflux; d) IrCl3, EtOCH2CH2OH/H2O, N2, reflux; e) KStpip, EtOCH2CH2OH, N2, room temperature.

Synthesis of HStpip: Hexamethyldisilazane (1.46 g, 9.06 mmol) was added dropwise into the solution of chlorodiphenylphosphine (4.10 g, 18.58 mmol) in anhydrous toluene (30 mL). The mixture was refluxed for 3 h, then the byproduct Me3SiCl was distilled off. After the addition of S powder (0.66 g, 20.67 mmol) in tetrahydrofuran (THF) (30 mL), the reaction was refluxed overnight. On completion, the reaction mixture was evaporated and then filtered to obtain a white precipitate. This solid was washed with methanol and recrystallized from dichloromethane to give the desired product (3.05 g, 6.79 mmol, 75%). 1H NMR (400 MHz, CD2Cl2): δ 7.92 – 7.81 (m, 8H), 7.49 – 7.42 (m, 4H), 7.37 (t, J = 9.0 Hz, 8H), 4.57 (s, 1H) ppm. 31P NMR (400 MHz, CD2Cl2): δ 57.66 ppm. HRMS (ESI) m/z: calculated for C24H21NP2S2 [M+H]+450.0669; observed [M+H] 450.0672.


+

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Synthesis of KStpip: A 3% aqueous KOH solution (10 mL) was added quickly to the solution of HStpip (2.00 g, 4.45 mmol) in dichloromethane (50 mL) with stirring. After 3 h at room temperature, white precipitate was filtered and washed with H2O and dichloromethane sequentially. The washed product was dried under vacuum with 92% yield (2.00 g, 4.10 mmol). HR-MS (ESI) m/z: calculated for C24H20KNP2S2 [M+H] + 488.0228; observed [M+H] + 488.0269. General synthesis of ligands (L1, L2): A mixture of phenylboronic acid (3.24 g, 26.55 mmol), 2bromo-4-(trifluoromethyl)pyridine or 2-bromo-5-(trifluoromethyl)pyridine (5.00 g, 22.12 mmol), tetrakis(triphenylphosphine)palladium (0.56 g, 0.487 mmol) and K2CO3 (7.12 g, 53.63 mmol) was heated at 80 C in THF and water (2 : 1, v/v) for 24 h. After cooling to room temperature, the reaction mixture was added into water, extracted with ethyl acetate (60 mL × 3 times). After removal of solvents, the crude product was purified by silica gel column chromatography with PE: EA = 10:1 as eluent. L1. Yield 60% (2.96 g), a yellowish oil. 1H NMR (400 MHz, CDCl3): δ 8.87 (s, 1H), 8.03 (d, J = 8.3 Hz, 2H), 7.93 (s, 1H), 7.54 – 7.42 (m, 4H) ppm. HR-MS (ESI) m/z: calculated for C12H8F3N [M+H] + 224.0687; observed [M+H] + 224.0685. L2. Yield 87% (4.36 g), a white soild. 1H NMR (400 MHz, CDCl3): δ 8.95 (s, 1H), 8.03 (d, J = 9.2 Hz, 2H), 7.98 (d, J = 9.6 Hz, 1H), 7.84 (d, J = 8.6 Hz, 1H), 7.51 (d, J = 7.8 Hz, 3H) ppm. HRMS (ESI) m/z: calculated for C12H8F3N [M+H] + 224.0687; observed [M+H] + 224.0683. Synthesis of L3: A mixture of 2,4-bis(trifluoromethyl)phenylboronic acid (3.00 g, 11.63 mmol), 2-bromopyridine (1.53 g, 9.69 mmol), tetrakis(triphenylphosphine)palladium (0.25 g, 0.21 mmol) and K2CO3 (3.15 g, 22.79 mmol) in 60 mL of THF and water (2 : 1, v/v) was heated to 80 C for 24 h. After cooling to room temperature, the reaction mixture was added into water and extracted with ethyl acetate. After removal of solvents, the crude product was purified by silica gel column chromatography with petroleum ether (PE) / ethyl acetate (EA) = 10 : 1 as eluent to give L1 as white soild (2.51 g, 89% yield). 1H NMR (400 MHz, CDCl3): δ 8.72 (s, 1H), 8.04 (s, 1H), 7.89 (d, J = 7.9 Hz, 1H), 7.84 – 7.76 (m, 1H), 7.67 (d, J = 8.0 Hz, 1H), 7.45 (d, J = 7.8 Hz, 1H), 7.37 (t, J = 7.2 Hz, 1H) ppm. HR-MS (ESI) m/z: calculated for C13H7F6N [M+H] + 292.0561; observed [M+H] + 292.0559.

General synthesis of C1, C2 and C3: IrCl3.nH2O (Ir% = 54.5%, 1.21 g, 3.43 mmol) and two equivalent L1 (L2 or L3) were added into 40 mL of 2-ethoxyethanol (2-EtOCH2CH2OH) and H2O



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(3 : 1, v/v) and the solution was refluxed at 125 C for 24 h. After cooling down to room temperature, yellow precipitate was filtered and washed with H2O, EtOH and PE sequentially. The product was dried under vacuum. C1. Yield 79% (3.72 g), a yellow solid. 1H NMR (400 MHz, CDCl3): δ 9.31 (d, J = 5.4 Hz, 4H), 8.10 (s, 4H), 7.59 (d, J = 8.3 Hz, 4H), 6.95 – 6.91 (m, 4H), 6.91 – 6.78 (m, 4H), 6.76 – 6.60 (m, 4H), 5.92 (d, J = 7.3 Hz, 4H) ppm. HRMS (ESI) m/z: calculated for C48H28Cl2F12Ir2N4 [M+H] + 1343.0813; observed [(L1)2Ir] + 637.0646, [M-(L1)2Ir] + 706.0831. C2. Yield 82% (4.72 g), a yellow soild. 1H NMR (400 MHz, CDCl3) δ 9.64 (s, 4H), 7.98 (q, J = 8.8 Hz, 8H), 7.58 (d, J = 7.8 Hz, 4H), 6.83 (d, J = 8.4 Hz, 4H), 6.67 (s, 4H), 5.75 (d, J = 7.9 Hz, 4H) ppm. HRMS (ESI) m/z: calculated for C48H28Cl2F12Ir2N4 [M+H] + 1343.0813; observed [(L2)2Ir] + 637.0756, [M-(L2)2Ir] + 706.0880. C3: 86% yield (2.38 g, 1.47 mmol). 1H NMR (400 MHz, CDCl3) δ 9.25 – 9.18 (m, 4H), 8.45 (d, J = 8.5 Hz, 4H), 7.98 (t, J = 8.8 Hz, 4H), 7.46 (s, 4H), 7.05 – 6.99 (m, 4H), 5.99 (s, 4H) ppm. HRMS (ESI) m/z: calculated for C52H24Cl2F24Ir2N4 [M+Na]

+

1637.0128; observed [M+Na]

+

1637.0127. General synthesis of iridium complexes Ir1, Ir 2 and Ir3: 1 mmol C1 (C2 or C3) and 2.5 equivalent KStpip (1.10 g, 2.26 mmol) were dissolved in 30 mL of 2-EtOCH2CH2OH, then the reaction solution was stirred at room temperature for 10 minutes and the solvent was removed under vaccum, the resulting yellow soild was then washed with water, EtOH and PE in order and recrystallized from dichloromethane / PE. The product was further purified by gradient sublimation Ir1. Yield 70% (2.25 g, 2.07 mmol), an orange solid. 1H NMR (400 MHz, DMSO-d6): δ 9.34 (s, 2H), 8.39 (s, 2H), 8.03 – 7.94 (m, 6H), 7.49 (d, J = 13.9 Hz, 10H), 7.29 (s, 2H), 7.10 (d, J = 8.5 Hz, 4H), 6.85 (s, 2H), 6.79 – 6.73 (m, 2H), 6.69 (s, 2H), 5.86 – 5.79 (m, 2H) ppm. 31P NMR (400 MHz, CDCl3): δ 29.10 ppm. HR-MS (ESI) m/z: calculated for C48H34F6IrN3P2S2 [M+H]

+

1086.1281;

observed [M+H] + 1086.1284. Ir2. Yield 73% (2.38 g, 2.19 mmol), an orange solid. 1H NMR (400 MHz, DMSO-d6): δ 9.85 (s, 2H), 8.29 (d, J = 8.7 Hz, 2H), 8.09 (d, J = 10.6 Hz, 2H), 7.93 – 7.86 (m, 2H), 7.78 – 7.67 (m, 4H), 7.41 (s, 10H), 7.29 (s, 2H), 7.09 (t, J = 9.4 Hz, 4H), 6.87 (s, 2H), 6.75 (q, J = 7.3, 6.8 Hz, 2H), 5.86 (t, J = 5.7 Hz, 2H) ppm. 31P NMR (400 MHz, CDCl3): δ 30.37 ppm. HR-MS (ESI) m/z: calculated for C48H34F6IrN3P2S2 [M+H]+ 1086.1281; observed [M+H]+ 1086.1288.


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Ir3: 75% yield (1.65 g, 1.35 mmol,). 1H NMR (400 MHz, CD2Cl2): δ 9.44 (s, 2H), 8.21 (d, J = 8.4 Hz, 2H), 7.96 (dd, J = 15.2, 5.7 Hz, 4H), 7.64 – 7.48 (m, 8H), 7.42 (s, 6H), 7.28 (d, J = 7.9 Hz, 2H), 7.09 (d, J = 7.9 Hz, 4H), 6.58 (s, 2H), 6.15 (s, 2H) ppm. 31P NMR (400 MHz, CDCl3): δ30.18 ppm. HR-MS (ESI) m/z: calculated for C50H32F12IrN3P2S2 [M+H] + 1222. 1029; observed [M+H] + 1222.1030.

Results and Discussion Synthesis and characterization As shown in Scheme 1, three Ir(III) complexes Ir1, Ir2 and Ir3 were obtained by reacting their corresponding Ir(III) chloro-birdge intermediates with KStpip at room temperature with high yields, which were characteriazed by high resolution mass and NMR spectroscopies. In most cases, the Ir(III) complexes were prepared via Ir(III) chloro-birdge intermediate with the ancillary or another cyclometalated ligand at 120 oC in 2-ethoxyethanol for two hours or more (Figure S2). But in this experimental, three complexes can be obtained with high reaction yields at room temperature, evidently as a result of introduction of S atoms. Furthermore, they were purified through gradient sublimation prior to device fabrication and the yields of sublimation were significantly high at around 80~90% with almost no residue left. Similar compounds with tpip as ancillary ligand were reported to give less than 60% yield via the sublimation. These results suggest that the Ir(III) complexes containg sulfur atoms might be suitable for industrial production.

Figure 1. ORTEP diagrams of Ir2 (CCDC No.1846760) shown at 30% probability level. The hydrogen atoms are omitted for clarity.

Crystals of Ir2 were obtained through gradient sublimation, the crystal structure is illustrated in the ORTEP diagram (Figure 1). The iridium center exhibits distorted octahedral coordination



geometry, and the P-S-Ir-S-P-N heterocycle shows a chair-like formation, with slight twist between P-N-P plane and two adjacent P-S-Ir planes, with the torsion angles between two planes being ±18o. The Ir-S bond lengths range from 2.455(2) to 2.473(2) Å, significantly longer than the corresponding Ir-O bond in the similar reported Ir-tpip structures; the Ir-C bond lengths are in the range of 1.999(10) to 2.028(10) Å, while the bond lengths of Ir-N are between 2.042(8) and 2.071(9) Å. Meanwhile, the dihedral angles between S-Ir-S atoms are within the range from 99.92o(9) to 103.24 o(8), and the torsion angle between P-S-N atoms are within the range from 117.8 o(3) to 120.8 o(4). The rest of C-C and C-N bond lengths are similar to that of other reported Ir(III) complexes.16 The thermal stability of Ir(III) complexes is of great importance for their OLEDs application, revealed by thermogravimetric analysis (TGA), the decomposition temperatures of these three Ir(III) complexes are 274.0 oC, 271.2 oC and 372.9 oC for Ir1, Ir2 and Ir3, respectively (see Table 1 and Figure S3). No obvious glass transition point for Ir1, Ir2 and Ir3 was observed, indicating of potentially better morphologic stability during the evaporation process at the film state.

Photophysical property

a)

b)

300

400

500

600

Ir1-77 K Ir2-77 K Ir3-77 K

Normalized Intensity (a.u.)

Ir1 Ir2 Ir3

Normalize Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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700

450

Wavelength (nm)

500

550

600

650

700

Wavelength (nm)

Figure 2. a) UV-Vis and PL spectra at room temperature of Ir1,Ir2 and Ir3 in dichloroform; b) PL spectra in dichloroform at 77 K.

The UV-Vis absorption spectra and emission spectra at room temperature and 77 K in dichloromethane are illustrated in Figure 2 and 3D correlation spectra of excitation and emission of three emitters are depicted in Figure S4. The absorption bands between 280 nm and 340 nm can be assigned to spin-allowed ligand-centered (LC) 1π-π* transitions in the ligands, and the


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absorption of Ir3 is slightly blue-shifted compared with Ir1 and Ir2 attributed to electron withdrawing property of two trifluoromethyl groups in the phenyl group. Spin-allowed metalligand charge transfer (1MLCT) bands can be observed at around 350 nm to 430 nm for the three complexes. Additionally, the weak absorption located between 400 nm to 450 nm could be attributed to 3MLCT. All complexes exhibit green emission peak with maximum at 522, 528 and 516 nm for Ir1, Ir2 and Ir3, respectively. However, the emission profile of Ir3 is quite different from the other two complexes. The shoulder peak is much more obvious for Ir3 at around 542 nm, while Ir1 and Ir2 exhibit almost no shoulder peak at room temperature. At 77 K, the molecular relaxation is restrained in the frozen media, the emission band shows better resolved vibronic structures. In doped films of Ir1, Ir2 and Ir3 in the TCTA (4,4',4''-tris(carbazol-9-yl)-triphenylamine), a widely used host material in OLED, all the emission spectra exhibit pure peaks from the emitters, with the emission maxima of 538 nm, 539 nm and 527 nm for Ir1, Ir2 and Ir3, similarly, a shoulder peak at 548 nm can be observed for Ir3. The emission profile of three complexes, despite slight difference in emission maxima due to the matrix environment, fits well with the corresponding emission in solutions. indicating of efficient energy transfer in the photoluminescence process (Figure S5). Table 1: Photophysical properties Ir(III) complexes Ir1, Ir2 and Ir3. Absorption

Emission

Emission

λaba/nm

λemb/nm

λemc/nm

Ir1

226,275,364

522

538

Ir2

226,296,365

528

Ir3

224,290,364

516,542

Complex

a)In

krg10-

knrh10-

5

6

/s-1

/s-1

1.87

5.21

0.89

1.59

3.38

2.22

τeme

τemf

/μs

/μs

70%(97%)

1.50

539

66%(95%)

527,548

98%(92%)

Ф

d em

HOMOi

LUMOj

/eV

/eV

1.40

-5.35

-3.15

5.99

0.31

-5.36

-3.15

4.15

0.36

-5.57

-3.28

dichloromethane; b)In dichloromethane at room temperature; c)Emission maxima; d)photoluminescence quantum

yields in dichloromethane and co-deposited films of Ir1, Ir2 and Ir3 in TCTA (in brackets); e)Photoluminescence decay lifetime measured in dichloromethane; f)Photoluminescence decay lifetime measured in co-deposited films; g)Radiative

analysis;

rate constants; h)Non-radiative rate constants; i)HOMO energy levels calculated from cyclic voltammetry

j)LUMO

energy levels calculated from cyclic voltammetry analysis and UV-vis onset.

To further elucidate the photophysical properties of three complexes, the photoluminescence quantum yield (PLQY) and decay lifetime of each complex were measured both in solution and films (Table 1, Figure S6). The PLQYs in degassed dichloromethane calculated using a relative to Ir(ppy)3 (Φ = 0.4) in degassed CH2Cl2 solution at room temperature for Ir1, Ir2, Ir3 are 70%, 66%



and 98%, respectively. In co-deposited films of Ir1, Ir2 and Ir3 in TCTA, however, the PLQY is much higher, with 97%, 95% and 92% for Ir1, Ir2 and Ir3, respectively. Such difference indicates of stronger interaction with solvents in the solution state for Ir1 and Ir2 than Ir3, since the PLQY of Ir1 and Ir2 in films are substantially higher than that of solutions. The PLQYs of films can better reflect the actual photophysical properties of the device and hence taken as the reference of comparing the corresponding devices’ performances. The radiative rate constants (kr) and nonradiative rate constants (knr) are determined from the emission decay lifetime in co-deposited films at room temperature combined with PLQY adopting the following equations: τrad = τem/Фem; kr = 1/τrad; τem = 1/(kr+knr), under the presumption that the intersystem crossing quantum yields are close to unity.

Electrochemical property and theoretical calculations The redox properties of these three complexes are examined using cyclic voltammetry (Figure S7) and the electrochemical potential data are used to calculate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. In the anodic scan, oxidation and reduction peaks between 0.8 V and 1.2 V can be observed for all complexes. The quasi-reversible oxidation peaks are 0.87 V, 0.91 V and 1.07 V for Ir1, Ir2 and Ir3, respectively, using ferrocene/ferrocenium (Fc/Fc+) couple as reference. Distinct reduction waves are observed for the three complexes. The HOMO energy levels for each complex are determined to be -5.35 eV, -5.36 eV and -5.57 eV, respectively, combining with the band gap derived from onset of UV-vis spectra, the LUMO energy levels are determined to be -3.15 eV, -3.15 eV and 3.28 eV for Ir1, Ir2 and Ir3, respectively. It is evident that the electrochemical properties of Ir1 and Ir2 are almost identical in spite of their structural difference. For Ir3, the HOMO and LUMO energies are simultaneously lowered because of the introduction of more trifluoromethyl units. Theoretical calculations were further undertaken to gain better insight into the electronic properties of three complexes. Frontier orbitals of each complex were calculated using time dependent density functional theory (TD-DFT), B3LYP (6-31G) method with Genecp basis set specifically for the calculation of iridium center. The HOMO and LUMO distribution patterns and compositions of each fragment are presented in Figure 3. The Stpip ligands evidently contribute much less in the overall HOMO and LUMO distributions than the cyclometalated ligands and the compositions of iridium vary very slightly in these complexes. Besides, the substitution position of


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single trifluoromethyl group in Ir1 and Ir2 on the pyridine rings appears to have negligible impact on the composition of frontier orbital levels; hence the corresponding HOMO and LUMO levels of Ir1 and Ir2 are almost identical. However, the composition of cyclometalated ligands is significantly smaller for Ir3 compared with Ir1 and Ir2. The major components in ancillary ligands for the distributions of HOMO and LUMO are the nitrogen atoms connecting the two phosphine sulfur bonds. The HOMO and LUMO energy levels all shift deeper simultaneously for Ir3, evidently brought by two trifluoromethyl groups on phenyl ring, which is consistent with the cyclic voltammetry measurement. Accordingly, the energy gaps between HOMO and LUMO of these three complexes are very close, with 3.60 eV, 3.59 eV and 3.68 eV for Ir1, Ir2 and Ir3, respectively, leading to much resembled emission profiles.

Figure 3. HOMO and LUMO distribution and composition of each fragment.

OLEDs Performance To assess the electroluminescence (EL) properties of these complexes, devices based on Ir1 (D1),

Ir2 (D2) and Ir3 (D3) as emitters with the device configuration of : ITO/ HAT-CN

(dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile, 5 nm)/ TAPC (di-[4-(N,Nditolyl-amino)-phenyl]cyclohexane, 50 nm)/ TCTA: x wt% Ir complex (10 nm)/ TmPyPb (1,3,5tri(m-pyrid-3-yl-phenyl)benzene, 50 nm)/ LiF (1.2 nm)/ Al (100 nm) were fabricated. The energy diagram of the device structure is illustrated in Figure 4. In this device configuration, TAPC was employed as hole transporting layer (HTL), TCTA acted as host for Ir(III) complexes while TmPyPb was introduced as electron transporting layer (ETL). Devices with various doping



Page 12 of 19

concentrations of the iridium complexes were fabricated. As a result, the optimal doping concentrations for Ir1, Ir2 and Ir3 are 8 wt%, 12 wt% and 12 wt%, respectively. The overall electroluminescence performances are shown in Figure 5 and Table 2, and additional device performance details can also be found in Figure S8, S9, S10 and Table S2. All these devices display bright green emission with CIE coordinates of (0.33, 0.63), (0.34, 0.62) and (0.33, 0.62) for D1, D2 and D3, respectively, qualify themselves as suitable candidates for standard green emission. The EL spectra are very similar to the emission profiles of the complexes in dichloromethane solutions for iridium emitters, suggesting that the devices’ EL emissions originate from the triplet excited states of the phosphors. No obvious emission from the host has been observed in all spectra, indicating of complete and effective energy transfer from host to guests during the electroluminescence process, which is in good agreement with the PL spectra measured in codeposited films. CN NC

N

NC

N

N

N

CN

N

N

N N

N

CN HATCN

CN

N TCTA

N N

N

N

TmPyPb N

TAPC

Figure 4. Device energy diagram and molecule structures of each layer.

All devices exhibit excellent electroluminescence performances (Figure 5 and Table 2). These three devices can all be turned on (Von) at relatively low voltages ranging from 2.8 V to 3.1 V and reach 1000 cd/m2 at only 3.7 V, 3.5 V and 4.1V for D1, D2 and D3 devices, respectively. Since all three complexes have similar molecular structures and the variation of cyclometalated ligands has little impact on their emission and electrochemical properties, their corresponding EL performances are majorly attributed to their photoluminescence efficiencies. For device D1 with Ir1 (PL = 70% in solution and 97% in film), the turn-on voltage is 2.9 V with a maximum current efficiency (CEmax) of 88.42 cd A-1, a maximum external quantum efficiency (EQEmax) of 24.13% and a maximum power efficiency (PEmax) of 92.60 lm W-1, respectively. Because Ir2 has slightly


Page 13 of 19

lower PL efficiency (PL = 66% in solution and 95% in film), device D2 also exhibited the lowest efficiency with a CEmax of 79.21 cd A-1, an EQEmax of 22.50% and a PEmax of 86.48 lm W-1, respectively. Despite the PLQY of corresponding film of Ir3 is not the best among three iridium complexes (PL = 98% in solution and 92% in film), the D3 device still showed the highest device performances with a CEmax of 91.94 cd A-1 and an EQEmax of 26.52%, probably due to a much better electron-transporting capability introduced by two trifluoromethyl groups, which are normally considered to be beneficial to a more balanced carrier balance. However, perhaps due to the lowest HOMO level of Ir3, the turn-on voltage of D3 is also the highest (3.1 V) among three

a)

b)

400

500

600

Wavelength/nm

700

100 90 80 70 60

Current Efficiency (cd A-1)

Normalized Intensity (a.u.)

8 wt% Ir1 12 wt% Ir2 12 wt% Ir3

c)

50 40

40

30

30 20

D1 D2 D3 10

800

100 90 80 70 60 50

1

10

100

Luminance (cd m-2)

1000

20

Power Efficiency (lm W-1)

devices, which consequently results in the lowest power efficiency of 85.02 lm W-1.

10

d)

60

103

40

102

D1 D2 D3

20

0

3

4

5

6

Voltage (V)

7

8

10

EQE (%)

80

104

Luminance (cd m-2)

Current Density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


D1 D2 D3

101

9

100

1

1

10

100

Luminance (cd m-2)

1000

Figure 5. Electroluminescence performance of OLEDs based on Ir1, Ir2, Ir3: a) electroluminescence spectrum; b) current efficiency-luminance curve (left axis) and power efficiency-luminance curve (right axis); c) current densityvoltage curve (left axis) and luminance-voltage curve (right axis); d) external quantum efficiency-luminance curve.

Notably, all devices exhibited impressively low efficiency roll-offs. The EQE of D1, D2 and D3 could still be kept at 23.05%, 21.00% and 24.06%, respectively, even at the practicable brightness of 1 000 cd m-2, which decreased by 1.08%, 1.50% and 2.46% for D1, D2 and D3 compared with



Page 14 of 19

the maximum values. Combined with standard green emission profiles and decent device performances of these complexes, the bis(diphenylphorothioyl)amide could be an excellent ligand for efficient Ir(III) emitters and OLEDs.

Table 2: Summary of device performance CE [cd A-1]b)

PE [lm W-1]c)

EQE [%]d)

CIE

Device

Von

(wt%)

[V]a)

Max

@1000 cd m-2

Max

@1000 cd m-2

Max

@1000 cd m-2

[(x, y)]e)

D1 (8)

2.9

88.42

84.12

92.60

72.85

24.13

23.05

(0.33, 0.63)

D2 (12)

2.8

79.21

76.44

86.48

68.35

22.50

21.00

(0.34, 0.62)

D3 (12)

3.1

91.94

83.30

85.02

63.87

26.52

24.06

(0.33, 0.62)

a)Maximum e)external

luminance; b)turn-on voltage, the driving voltage at 1 cd m-2; c)current efficiency; d)power efficiency;

quantum efficiency calculated within visible spectrum region; f)CIE coordinates at 1000 cd m-2.

To make it clear that the Stpip is an universal ligand for Ir(III) complexes, the sky-blue complexes Ir(dfppy)2Stpip (dfppy: 2-(2,4-difluorophenyl)pyridine) and Ir(dfppm)2Stpip (dfppm: 2(2,4-difluorophenyl)pyrimidine) show the emission with peaks at 466 and 474 nm (Figure S11), respectively. Furthermore, if the 4-(4-(trifluoromethyl)phenyl)quinazoline (tfpqz) is used as the cyclometalated ligand, the complex Ir(tfpqz)2Stpip shows red emission peak at 624 nm (Figure S11). All complexes can be rapid-synthesized with excellent stability, and their device performances are under investigation.

Conclusion In summary, the bis(diphenylphorothioyl)amide containing phoshpor-sulfur (P=S) bonds was used as ancillary ligand for efficient pure green Ir(III) complexes bearing a novel SÎr^S coordination. All complexes can be synthesized in a rapid fashion within several minutes and also show good thermal stability and high submission yields, owing to introduction of sulfur atoms, which lead to reduced material loss in sublimation and device fabrication. Utilizing these complexes as emitters, the OLEDs exhibited superier performances with low turn-on voltages down to 2.8 V, high current efficiency up to 91.94 cd A-1, power efficiency up to 92.60 lm W-1 and external quantum efficiency up to 26.52%, respectively. Furthermore, all devices exhibited low efficiency roll-off at high driving voltages, which are among the best green OLEDs without employing out-coupling methods. Therefore, the introduction of bis(diphenylphorothioyl)amide as


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ancillary ligand may be a feasible strategdy towards more diversified Ir(III) complexes with excellent OLED performances and potentially suitable for pragmatic applications.

Supporting Information General information for the ligands and complexes measurement; OLED fabrication and measurement; 1H NMR, 13C NMR and 31P NMR spectra of ligands, intermediates and complexes; TGA analysis, 3D excitation-emission correlation spectra and Cyclic Voltammetry spectra of Ir1, Ir2, Ir3; PL spectra of three iridium emitters doped in TATC films; The crystallographic data of Ir2; Summary of device performance based on various configurations; Device performances of D1, D2 and D3.

Acknowledgements This work was supported by the National Natural Science Foundation of China (51773088), the Fundamental Research Funds for the Central Universities (020514380148) and Basic Research Program

of

Shenzhen

(JCYJ20160229123546997,

JCYJ20160530184056496

and

JCYJ20170818141858021).

Author information Corresponding Authors [email protected] [email protected] [email protected] [email protected]

Notes The authors declare no competing financial interest. References (1) Xiao, L.; Chen, Z.; Qu, B.; Luo, J.; Kong, S.; Gong, Q.; Kido, J., Recent Progresses on Materials for Electrophosphorescent Organic Light‐Emitting Devices, Adv. Mater. 2011, 23, 926-952. (2) (a) Du, M.; Feng, Y.; Zhu, D.; Peng, T.; Liu, Y.; Wang, Y.; Bryce, M. R., Novel Emitting System Based on a Multifunctional Bipolar Phosphor: An Effective Approach for Highly Efficient Warm-White Light-Emitting Devices with High Color-Rendering Index at High Luminance, Adv. Mater. 2016, 28, 5963-5968; (b) Kim, K.-H.; Liao, J.-L.; Lee, S. W.; Sim, B.; Moon, C.-K.; Lee, G.-H.; Kim, H. J.; Chi, Y.; Kim, J.-J., Crystal Organic Light-



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Indolocarbazole Derivative as a Universal Host Material for RGB and White Phosphorescent OLEDs, Adv. Funct. Mater. 2015, 25, 5548-5556; (c) Kim, J.; Lee, K. H.; Lee, S. J.; Lee, H. W.; Kim, Y. K.; Kim, Y. S.; Yoon, S. S., Red Phosphorescent Bis‐Cyclometalated Iridium Complexes with Fluorine‐, Phenyl‐, and Fluorophenyl‐Substituted 2‐Arylquinoline Ligands, Chem-Eur. J. 2016, 22, 4036-4045; (d) Soellner, J.; Tenne, M.; Wagenblast, G.; Strassner, T., Phosphorescent Platinum(II) Complexes with Mesoionic 1H‐1,2,3‐Triazolylidene Ligands, Chem-Eur. J. 2016,


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22, 9914-9918; (e) Wu, T. L.; Yeh, C. H.; Hsiao, W. T.; Huang, P. Y.; Huang, M. J.; Chiang, Y. H.; Cheng, C. H.; Liu, R. S.; Chiu, P. W., High-Performance Organic Light-Emitting Diode with Substitutionally Boron-Doped Graphene Anode, ACS Appl. Mater. Interfaces 2017, 9, 14998-15004. (8)

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Graphic for manuscript

Three

pure

green

iridium(III)

complexes

coordinated

with

a

unique

bis(diphenylphorothioyl)amide ligand were prepared at room temperature in 10 minutes with satisfacotry synthesis, sublimission yields and high photoluminescence quantum efficiencies up to 98%. Highly efficient devices utilizing these emitters exhibit EQE up to 24.06% with slightly efficiency roll-off, rendering them suitable candidates for pragmatic OLEDs application.


Fast Synthesis of Green Iridium(III) Complexes Incorporating A Bis

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