Synthesis, photo- and electro-luminescence of novel red phosphorescent Ir(III) complexes with a silsesquioxane core

Journal of Organometallic Chemistry 830 (2017) 85e92

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Synthesis, photo- and electro-luminescence of novel red phosphorescent Ir(III) complexes with a silsesquioxane core Tianzhi Yu a, *, Yuying Niu a, Shupei Yu b, Zixuan Xu a, Yuling Zhao c, Hui Zhang a a

Key Laboratory of Opto-Electronic Technology and Intelligent Control (Ministry of Education), Lanzhou Jiaotong University, Lanzhou, 730070, China College of Materials Science and Engineer, Qingdao University of Science & Technology, Qingdao, 266042, China c School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, 730070, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 September 2016 Received in revised form 11 November 2016 Accepted 13 December 2016 Available online 18 December 2016

One new phosphorescent polyhedral oligomeric silsesquioxane (POSS) material consisting of one red emissive Ir(III) complex unit and seven carbazole moieties covalently attached to a POSS core was successfully synthesized and characterized. The phosphorescent POSS material had good thermal stability due to the significant effect of the POSS core, and its photoluminescence and electroluminescence performances were improved because the POSS core reduced interactions among the emissive units and diminished concentration quenching. Solution processed light-emitting device based on the POSS material showed a maximum current efficiency of 4.33 cd/A with Commission Internationale de LʹEclairage (CIE) coordinates of (0.67, 0.28). © 2016 Elsevier B.V. All rights reserved.

Keywords: Iridium complex Polyhedral oligomeric silsesquioxane (POSS) Red-emitting material Photoluminescence Electroluminescence

1. Introduction In contrast to fluorescent materials which have substantial losses due to injected charges combining to form nonemissive triplet states, phosphorescent materials caused considerable attentions because of their excellent luminescent properties such as harvesting singlet and triplet excitons through the strong spineorbit coupling of heavy metal ions (Os, Ir, Pt, Re) with d6 and d8 electronic structure, and enabling the internal quantum efficiencies approaching 100% [1e3]. In the phosphorescent metal complexes, Ir(III) complexes are particularly promising because of their favorable short excited-state lifetimes, thermally stability, high quantum efficiency, color diversity and environmental inertness. As a result of these characteristics, the neutral and cationic luminescent cyclometalated Ir(III) complexes have usually been applied in organic light-emitting diodes (OLEDs) [4e9]. The photophysical and photochemical properties (e.g., emission wavelength and quantum yield) of the Ir(III) complexes can be tuned by appropriate modification of the cyclometalated ligands or the ancillary ligands [10e12]. Generally, electron-withdrawing groups (fluorine or nitrile substituents) at the cyclometalating rings cause

* Corresponding author. E-mail address: [email protected] (T. Yu). http://dx.doi.org/10.1016/j.jorganchem.2016.12.015 0022-328X/© 2016 Elsevier B.V. All rights reserved.

the blue shifting trend of the emissions of Ir(III) complexes, whereas the cyclometalated ligands containing electron-pushing moieties (alkyl substituents or aromatic rings) will give rise to the red-shifted emissions. For heteroleptic Ir(III) complexes, changing the ancillary ligands would affect the HOMO (highest occupied molecular orbital) energy level of cyclometalated Ir(III) complexes but leave the LUMO (lowest unoccupied molecular orbital) energy level unchanged [13]. Thus, through careful selection of ancillary ligands, it is possible to tune the emission color. In the past decades, many efforts have been devoted to develop variable emissions from the blue light to the near-IR light with highly efficient Ir(III) complexes by modifying their coordination sphere [14e16]. As compared to green and blue Ir(III) complexes, red phosphorescent Ir(III) complexes show poor electroluminescent (EL) performances because their photoluminescence (PL) quantum yields tend to be intrinsically low. Additionally, many red phosphorescent Ir(III) complexes suffer from color purity due to their PL spectra with multiple and broad bands, which are not suitable for display applications. Thus, the development for high efficiency red-emitting Ir(III) complexes with single and narrow emission bands is still requisite. For small molecular phosphors, crystallization of their thin films may lead to the formation of excimers and exciplexes, which will decrease the device efficiency and impair the device stability.

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Moreover, at high doping concentration, the intermolecular interaction in thin film will cause the self-quenching of luminescence. Thus, most efficient phosphorescent OLEDs based on the Ir(III) complexes are usually fabricated through doping the phosphorescent emitters into a host matrix to inhibit the luminescence selfquenching or triplet-triplet (T-T) annihilation at high doping concentrations. Unfortunately, this method usually gives rise to the phase separation during the device manufacturing process by the vacuum deposition. To solve the these issues, many research groups have used three dimensional bulky structure and sterically hindered configuration in the cyclometalating ligands of the Ir(III) complexes to suppress the close packing among the molecules in the solid state and enhance the luminant performances of the devices [17e20]. Especially, the multifunctional star-shaped Ir(III) complexes have proved to be good materials for solutionprocessable OLEDs, in which an Ir(III) complex core is surrounded by carrier transporting groups (such as carbazole, triphenylamine, etc.) to prevent the intermolecular aggregation of the luminophores and phase segregation, and to improve charge recombination. Such purposeful strategy certainly enhances photo- and electroluminescence properties of phosphorescent Ir(III) complexes. Recently, polyhedral oligomeric silsesquioxane (POSS)-based phosphorescent emitters have received tremendous attention due to their unique nanoscale cage-shaped structures and good solution processability well suited for OLEDs [21e23]. Efficient monochromatic and white-emitting OLEDs based on phosphorescent POSS materials containing Ir(III) complexes were reported by Yang et al. [21], in which the monochromatic OLEDs exhibited maximum external quantum efficiencies (EQEs) in the range of 5e9% and maximum brightness of 1000 cd/m2, and the white-emitting OLEDs shown an EQE of 8.0% at 1000 cd/m2. Efficient light-emitting devices based on platinum complexes-anchored POSS core exhibited a peak external quantum efficiency of ca. 8% [22]. Very recently, we reported efficient green-emitting devices using Ir(III) complexesfunctionalized POSS materials [24,25], in which the devices based on these POSS materials shown maximum external quantum efficiency of 9.77%. As part of our continuous efforts, in this paper we used 2-(1-naphthyl)benzothiazole (nbt) as cyclometalated ligand and the carbazole-functionalized b-diketone Cz-acac-allyl (1-(9butyl-9H-carbazol-3-yl)hept-6-ene-1,3-dione) as the ancillary ligand to synthesize a red-emitting Ir(III) complex, [bis(1-naphthyl) 0 benzothiazolato-N,C2 )iridium(1-(9-butyl-9H-carbazol-3-yl)hept6-ene-1,3-dionato-O,O)] [Ir(nbt)2(Cz-acac-allyl)], and then the synthesis, characterization, photophysical properties and thermal stability of a new POSS material bearing seven carbazole units and one red-emitting Ir(III) complex moiety were reported. Furthermore, solution processed light-emitting devices based on the redemitting Ir(III) complex and the POSS material were fabricated to investigate their electroluminescence properties. 2. Results and discussion 2.1. Synthesis and characterization The method of synthesizing the red-emitting Ir(III) complex and the POSS material is outlined in Scheme 1. The red-emitting Ir(III) complex was prepared via reacting the chlorobridged dimer complex (nbt)2Ir(m-Cl)2Ir(nbt)2 with the ancillary ligand Cz-acac-allyl and anhydrous K2CO3 in refluxing dichloroethane under nitrogen atmosphere, which has a terminal alkene group in the ancillary ligand for later attachment to POSS core. The approach can obtain the Ir(III) complex in 85.5% yield. The Ir(III) complex was characterized by 1H NMR, mass spectra and elemental analysis. In the presence of platinum(0)-1,3-divinyl-1,1,3,3-

tetramethyldisiloxane (Pt-dvs) as the catalyst, the star POSS material containing seven carbazole units and one red-emitting Ir(III) complex moiety was prepared in a stepwise mode by sequentially reacting the Ir(III) complex Ir(nbt)2(Cz-acac-allyl) with octakis(dimethylsiloxy)silsesquioxane (Q8MH 8 ) in a 1:1 ratio, and followed by reaction with excess 9-allyl-9H-carbazole (Cz-allyl). The reaction was monitored by measuring the decrease in intensity of the FT-IR spectral signal at 2147 cm1 for the SieH bonds until the end of the reaction. In this reaction, the star POSS material 7CzIr(nbt)2(Cz-acac)-POSS was found in low yield (17.4%) after isolating from the reaction mixture chromatographically, the most compound was octakis[3-(carbazol-9-yl)propyldimethylsiloxy]silsequioxane (POSS-8Cz) [26]. In addition, there are several very small amount of new products finding in the reaction system, which could be 6Cz-2Ir(nbt)2(Cz-acac)-POSS and/or 5Cz3Ir(nbt)2(Cz-acac)-POSS materials, but these materials were difficult to be isolated from the reaction mixture, and thus we did not separate them. The star POSS material 7Cz-Ir(nbt)2(Cz-acac)-POSS was characterized using 1H NMR, FT-IR and ESI-MS. Its molecular weight ([M þ H]þ ¼ 3528.97) was consistent with the structure of the molecule presented in the Experimental section. In the FT-IR spectrum of 7Cz-Ir(nbt)2(Cz-acac)-POSS, the strong absorption peak at 1089 cm1 represents the vibrations of the siloxane Sie OeSi groups, which indicates the general feature of POSS derivatives. In addition, the characteristic stretching vibration of the SieH groups (2147 cm1) of Q8MH 8 disappeared completely, indicating that the hydrosilylation occurred to completion in 7CzIr(nbt)2(Cz-acac)-POSS material. The characteristic stretching vibration bands of eCH3 and eCH2e are clearly observed at 2800e3000 cm1. In 1H NMR spectra of 7Cz-Ir(nbt)2(Cz-acac)POSS, the peaks for the vinyl groups of Cz-allyl (5.03, 5.15 and 5.99 ppm) and Ir(nbt)2(Cz-acac-allyl) (4.64, 4.72 and 5.51e5.44 ppm) and SieH protons of Q8MH 8 (4.7 ppm) disappeared, supporting the complete hydrosilylation reaction and the vinyl groups of Cz-allyl and Ir(nbt)2(Cz-acac-allyl) underwent hydrosilylation of the SieH bonds of Q8MH 8. The thermal behaviors of the red-emitting Ir(III) complex and the POSS material have been investigated by thermogravimetric analyses (TGA) under nitrogen atmosphere. The results of their TGA measurements are shown in Fig. 1. The TGA result of the redemitting Ir(III) complex shows that the Ir(III) complex is thermally stable up to 340  C (2% weight loss). At about 345 and 411  C, there are two sharp weight losses in its TGA curve, indicating that the Ir(III) complex undergoes two large-stage decomposition processes. From the TGA result of the POSS material, the POSS material exhibits good thermal stability up to 358  C (2% weight loss). With increasing temperature, the POSS material also undergoes two large-stage decomposition processes at 360 and 453  C accompanied by two sharp weight losses in its TGA curve. Compared with the Ir(III) complex, the thermal stability of the POSS material has been improved due to introducing the inorganic silsesquioxane (POSS) core. The good thermal property of the POSS material will benefit its application in the field of OLEDs. 2.2. Photophysical and electrochemical properties Fig. 2 displays the UVevisible absorption and photoluminescence (PL) spectra of the red-emitting Ir(III) complex and the POSS material dissolved in dichloromethane solutions. The absorption peaks for Ir(nbt)2(Cz-acac-allyl) appeared at 227, 274, 345 and 460 nm, respectively. The intense absorption bands ranging from 220 to 360 nm can be assigned to intraligand spinallowed p e p* transitions, and the broad weak absorption band at 460 nm can be attributed to both spin-orbit coupling enhanced

T. Yu et al. / Journal of Organometallic Chemistry 830 (2017) 85e92 O

87

O

N O

Cl Ir S

N

Ir

Ir N

Cl

S

K CO , ClCH CH Cl

2

S

O

N 2

2

N

Ir(nbt)2(Cz-acac-allyl)

S

H

N O

+

Ir S

N

O

Pt-catalyst

O

Ir

O Si O Si O Si O Si H O O Si O Si O H Si O O Si H

R

N 2

H Si O Si H O O Si O Si O O O Si Si O O O Si Si H H

Si O Si O O Si O Si O O O Si Si O O O Si Si R R

O

Toluene 2

N

N

R O Si O Si O Si O Si R O O Si O O O Si R Si O Si R

7Cz-Ir(nbt)2(Cz-acac)-POSS

R =

N

Scheme 1. Synthetic routes to the red-emitting Ir(III) complex and the POSS material.

Fig. 1. Thermogravimetric analyses (TGA) of Ir(nbt)2(Cz-acac-allyl) and the POSS material (7Cz-Ir(nbt)2(Cz-acac)-POSS).

(p / p*) and spin-forbidden 3MLCT transitions [27]. The absorption spectrum of 7Cz-Ir(nbt)2(Cz-acac)-POSS material also exhibits an intense absorption region ranging from 220 to 360 nm and a weak absorption region ranging from 415 to 550 nm. In intense absorption region, there are five visible absorption peaks at 242, 265, 295, 332 and 347 nm, which can be attributed to the p / p* transitions of the ligands. In weak absorption region, there is an absorption band at 460 nm, which can be assigned to both spinorbit coupling enhanced 3(p / p*) and spin-forbidden 3MLCT transitions. The photoluminescence spectra of the red-emitting Ir(III) complex and the POSS material are shown in Fig. 2. The photoluminescence spectra of Ir(nbt)2(Cz-acac-allyl) and the POSS material strongly resemble each other, they all exhibit a strong red emission with a maximum main peak at 613 nm and a weak 3

Fig. 2. UVevis absorption and photoluminescence spectra of Ir(nbt)2(Cz-acac-allyl) and the POSS material (7Cz-Ir(nbt)2(Cz-acac)-POSS) in dichloromethane solutions (C ¼ 1.0  105 mol L1, lex ¼ 460 nm).

shoulder peak at 665 nm. From their emission spectra, it is shown that the red-emitting Ir(III) complex and the POSS material nearly display pure red emissions, with a full width at half maximum (FWHM) of only 30e35 nm. It was reported that the enlarged conjugation rings in ligands could be effective in narrowing emission bands of their corresponding Ir(III) complexes [28]. The solid state photoluminescence quantum yields of the redemitting Ir(III) complex and the POSS material were measured by an absolute method using the Edinburgh Instruments (FLS920) integrating sphere excited at 380 nm with the Xe lamp. The photoluminescence quantum yields of the Ir(III) complex and the POSS material at room temperature were measured to be 3.18% and 5.79%, respectively. The result indicated that the POSSfunctionalized Ir(III) material showed higher photoluminescence efficiency than the corresponding pristine Ir(III) complex because

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the bulky POSS units strongly suppressed the intermolecular interaction and aggregation of the luminophores. Cyclic voltammetry (CV) was carried out to investigate the electrochemical behavior of the red-emitting Ir(III) complex and the POSS material (Fig. 3). Their HOMO energy levels were determined from the onset of the oxidation potentials of their cyclic voltammetry measurements, while their LUMO energy levels were deduced from their HOMO energy levels and optical band gaps determined from their extrapolated UVeVis absorption edges. The cyclic voltammograms were recorded in degassed dichloromethane solutions containing tetra-n-butylammonium tetrafluoroborate (0.1 mol/L) as the electrolyte. A platinum working electrode and a saturated Ag/AgCl reference electrode were used. Ferrocene was used for potential calibration. As shown in Fig. 3, the first oxidation potentials of the Ir(III) complex and the POSS material were at ca. 0.89 and 0.88 V, respectively. At the same condition, the oxidation and reductive potentials of ferrocene were observed at 0.55 and 0.40 V, respectively, then the E1/2 (Fc/Fcþ) is 0.475 V. Thus the HOMO energy levels of the Ir(III) complex and the POSS material were determined to be 5.22 and 5.21 eV regarding the energy level of ferrocene/ferrocenium as 4.80 eV. The optical band edges of the Ir(III) complex and the POSS material were estimated to be ca. 554 and 557 nm, which correspond to 2.24 and 2.23 eV. Then the LUMO energy levels of the Ir(III) complex and the POSS material are calculated to be 2.98 eV. Table 1 summarizes the photophysical, thermal and electrochemical properties of the Ir(III) complex and the POSS material. 2.3. OLEDs performances In order to evaluate the OLEDs performances of the Ir(III) complex and the POSS material, the solution-processed light-emitting devices have been fabricated with the configuration of ITO/ PEDOT:PSS (30 nm)/PVK: Dopant (x wt %, 50 nm)/TPBi (50 nm)/Liq (2 nm)/Al(150 nm), in which the Ir(III) complex and the POSS material were used as dopants, respectively (Fig. 4(a)). Where PEDOT:PSS is poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate), PVK is Poly(9-vinylcarbazole), TPBi is 1,3,5Tri(N-phenylbenzimidazol-2-yl)benzene and Liq is 8Hydroxyquinolinolato-lithium. The PEDOT:PSS layer was used as hole-injection layer. PVK was chosen as the host material for its good film-forming property and hole-transport characteristics. The emitting layers are consisted of host material PVK and dopants at different concentrations (x wt%), and prepared by solutionprocessable method. TPBi was used as the electron transport and

hole/exciton-blocking material. Liq was used as the electroninjection layer. The doped concentrations of the Ir(III) complex and the POSS material were chosen as 10 wt%, 20 wt%, 30 wt% and 40 wt%, respectively. Fig. 4(b) displays the relative HOMO and LUMO energy levels of the materials investigated in this work. The energy level diagram shows that HOMO and LUMO energy levels of the Ir(III) complex and the POSS material lie above and below those of the host PVK, respectively. Consequently, it is possible that the Ir(III) complex and the POSS material would be able to trap both electrons and holes in the emitting layer. Furthermore, the LUMO energy level of TPBi is close to the LUMO energy levels of the Ir(III) complex and the POSS material, it could be expected that the electron transport and hole/ exciton-blocking TPBi layer was introduced for effective electron injection/facilitate and charge carrier balance within the emitting layer. Figs. 5 and 6 show the luminance vs. the driving voltage and the current efficiency vs. the current density curves of the devices with various doping concentrations of the Ir(III) complex and the POSS material, respectively. Table 2 summarizes the performances of the devices. As shown in Fig. 5, the maximum brightness of 618, 1140, 728 and 605 cd/m2 were observed in the devices with 10 wt%, 20 wt %, 30 wt% and 40 wt% doping concentrations of the Ir(III) complex, respectively, and the maximum luminous efficiencies of 2.39, 3.23, 2.66 and 2.23 cd/A were obtained. It is clearly seen that the devices based on the Ir(III) complex with 20 wt% doping concentration showed the best EL performance. The maximum current efficiency of 3.23 cd/A at 3.43 mA/cm2 and maximum brightness of 1140 cd/ m2 at 26.1 V have been observed in the device. From Fig. 6, the device with 20 wt% doping concentration of the POSS material exhibited the best EL performance, and the device had a maximum brightness of 1233 cd/m2 at 23.2 V and a maximum current efficiency of 4.33 cd/A at 3.78 mA/cm2. The electroluminescence (EL) spectra of the devices based on the Ir(III) complex and the POSS material were independent of the different dopant concentrations and the different driving voltages. Fig. 7 shows only the EL spectra of the devices based on the Ir(III) complex and the POSS material with 20 wt% concentration and at 19 V. The EL spectra exhibited red emissions with a maximum main peak at 613 nm and a shoulder peak at 667 nm. The Commission Internationale de LʹEclairage (CIE) coordinates of the devices are (0.66, 0.29) for the Ir(III) complex and (0.67, 0.28). As shown in Table 2, the EL performances of the devices fabricated from POSS material had shown significant enhancements in contrast with that of the devices based on the Ir(III) complex. This indicates that the Ir(III) complex anchored to POSS core provides good encapsulation for emissive unit and effectively suppresses the intermolecular aggregation-induced emission quenching. 3. Conclusion

Fig. 3. Cyclic voltammograms of ferrocene, Ir(nbt)2(Cz-acac-allyl) and the POSS material (7Cz-Ir(nbt)2(Cz-acac)-POSS) (scan rate: 10 mV s1, solvent: dichloromethane).

In this paper, we have reported on the synthesis, photophysics, electrochemical characterization and thermal stabilities of a redemitting Ir(III) complex and an inorganic/organic hybrid material consisting of a red-emitting Ir(III) complex unit and seven carbazole moieties covalently attached to a polyhedral oligomeric silsesquioxane (POSS) core. Solution processed EL devices using the Ir(III) complex and the POSS material as the dopants were also fabricated, respectively. At the doping concentration of 20 wt%, the devices fabricated from POSS material exhibited a red emission with a maximum brightness of 1233 cd/m2 and a maximum current efficiency of 4.33 cd/A, while the devices based on the Ir(III) complex showed a maximum brightness of 1140 cd/m2 and a maximum current efficiency of 3.23 cd/A. Compared the device performances of the Ir(III) complex, it is found that the EL performances of the

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Table 1 Photophysical, thermal and electrochemical properties of the Ir(III) complex and the POSS material. Material

UVevis (nm)

PL (nm)

Td ( C)

Ff (%)

EOX (V)

HOMO (eV)

LUMO (eV)

Ir(III) complexa POSS materialb

227, 274, 345, 460 242, 265, 295, 332, 347, 460

613, 665 613, 665

345 360

3.18 5.79

0.89 0.88

5.22 5.21

2.98 2.98

a b

Ir(nbt)2(Cz-acac-allyl). 7Cz-Ir(nbt)2(Cz-acac)-POSS.

Fig. 4. The schematic diagram of solution-processed OLEDs (a) and the relative HOMO/LUMO energy levels of the materials (b) investigated in this work.

Fig. 5. The luminance vs. voltage (a) and current efficiency vs. current density (b) curves of the doped devices with different concentrations of Ir(nbt)2(Cz-acac-allyl).

Fig. 6. The luminance vs. voltage (a) and current efficiency vs. current density (b) curves of the doped devices with different concentrations of the POSS material (7CzIr(nbt)2(Cz-acac)-POSS).

devices fabricated from POSS material had shown significant enhancements, which can be attributed to reduced interactions

among the Ir(III) complex units and diminished concentration quenching.

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Table 2 EL performances of the Ir(III) complex and the POSS material. Materials

Concentration

Lmax (cd/m2)

LEmax (cd/A)

CIE (x, y)

Ir(III) complexa

10 20 30 40 10 20 30 40

[email protected] V [email protected] V [email protected] V [email protected] V [email protected] V [email protected] V [email protected] V [email protected] V

[email protected] mA/cm2 [email protected] mA/cm2 [email protected] mA/cm2 [email protected] mA/cm2 [email protected] mA/cm2 [email protected] mA/cm2 [email protected] mA/cm2 [email protected] mA/cm2

(0.66, (0.67, (0.65, (0.66, (0.67, (0.66, (0.67, (0.68,

POSS materialb

a b

wt% wt% wt% wt% wt% wt% wt% wt%

0.29) 0.28) 0.29) 0.30) 0.27) 0.29) 0.30) 0.28)

Ir(nbt)2(Cz-acac-allyl). 7Cz-Ir(nbt)2(Cz-acac)-POSS.

(TGA) was performed on a Perkin-Elmer Pyris system. UVevis absorption and photoluminescent spectra were recorded on a Shimadzu UV-2550 spectrometer and on a Perkin-Elmer LS-55 spectrometer, respectively. 4.2. Synthesis and characterization of Ir(nbt)2(Cz-acac-allyl) and the POSS materials

Fig. 7. EL spectra of Ir(nbt)2(Cz-acac-allyl) and the POSS material (7Cz-Ir(nbt)2(Czacac)-POSS) with 20 wt% concentration and at 19 V.

4. Experimental 4.1. Materials and methods 2-Aminothiophenol, 1-naphthaldehyde, carbazole, potassium tert-butoxide and allyl bromide were bought from Alfa Aesar. 4Pentenoic acid was obtained from Energy Chemical (China). IrCl3nH2O (iridium content > 60.0%) was bought from Shanxi Kaida Chemical Co. Ltd. (China) and used without further purification. Platinum complex (platinum-1,3-divinyl-1,1,3,3tetramethyldisiloxane, Pt-dvs, 2 wt% Pt in xylene) was purchased from Aldrich, USA. Octakis(dimethylsiloxy)silsesquioxane (Q8MH 8) containing eight hydro-silane groups was purchased from the Hybrid Plastics Co., USA. 8-Hydroxyquinolinolato-lithium (Liq), Poly(9-vinylcarbazole) (PVK) and 1,3,5-Tri(N-phenylbenzimidazol2-yl)benzene (TPBi) were purchased from Electro-Light Technology Corp., Beijing. Toluene was dried by distillation before use in the hydrosilylation reaction. All other chemicals were analytical grade reagent. The ancillary ligand 1-(9-butyl-9H-carbazol-3-yl)hept-6-ene1,3-dione (Cz-acac-allyl) was synthesized as previously described [25]. The cyclometalated ligand 2-(1-naphthyl)benzothiazole (nbt) and the cyclometalated Ir(III) m-chlorobridged dimmer ((nbt)2Ir(mCl)2Ir(nbt)2) were prepared as previously described [29]. 9-allyl9H-carbazole (Cz-allyl) was synthesized as previously described [24]. 1 H NMR spectra were obtained on Unity Varian-500 MHz. IR spectra (400e4000 cm1) were measured on a Shimadzu IRPrestige-21 FT-IR spectrophotometer. C, H, and N analyses were obtained using an Elemental Vario-EL automatic elemental analysis instrument. Mass spectrum was obtained from a Thermo Scientific Orbitrap Elite mass spectrometer. Thermogravimetric analysis

4.2.1. Ir(nbt)2(Cz-acac-allyl) The cyclometalated Ir(III) m-chlorobridged dimmer ((nbt)2Ir(mCl)2Ir(nbt)2) (1.00 g, 0.67 mmol), Cz-acac-allyl (1.16 g, 3.34 mmol) and anhydrous K2CO3 (0.93 g, 6.73 mmol) were refluxed in dichloroethane under nitrogen atmosphere for 24 h. After cooling, a small quantity of water was added. The mixture was extracted with dichloromethane (100 mL  3). The organic phase was washed with water (2  100 mL) and dried over anhydrous MgSO4. After filtering, the filtrate was evaporated to dryness under reduced pressure. The crude was purified by chromatography on silica gel using petroleum ether/acetone (15:1, v/v) as the eluent to give red powdery Ir(nbt)2(Cz-acac-allyl) in 85.5% yield (1.21 g). 1H NMR(CDCl3, d, ppm): 8.66 (d, 1H, J ¼ 8.0 Hz, Aryl-H), 8.60 (d, 1H, J ¼ 7.6 Hz, Aryl-H), 8.38 (s, 1H, Aryl-H), 8.18e8.13 (m, 3H, Aryl-H), 7.94 (d, 3H, J ¼ 7.2 Hz, Aryl-H), 7.81 (d, 1H, J ¼ 8.4 Hz, Aryl-H), 7.72e7.60 (m, 4H, Aryl-H), 7.37 (m, 6H, Aryl-H), 7.17e7.04 (m, 5H, Aryl-H), 6.74 (d, 1H, J ¼ 8.0 Hz, Aryl-H), 6.68 (d, 1H, J ¼ 7.2 Hz, ArylH), 5.94 (s, 1H), 5.51e5.44 (m, 1H, eCH]CH2), 4.72 (d, 1H, J ¼ 17.6 Hz, eCH]CH2), 4.64 (d, 1H, J ¼ 9.6 Hz, eCH]CH2), 4.20 (t, 2H, J ¼ 6.7 Hz, -NeCH2e), 2.23 (t, 2H, J ¼ 5.6 Hz), 2.10e2.06 (m, 2H), 1.76 (m, 2H), 1.31e1.27 (m, 2H), 0.87 (t, J ¼ 6.8 Hz, 3H). Anal. Calc. for C57H44IrN3O2S2 (%): C, 64.63; H, 4.19; N, 3.97. Found: C, 64.83; H, 4.08; N, 4.07. ESI-MS: Calcd for [C57H44IrN3O2S2 þ H]þ, 1060.33; Found, 1060.37. 4.2.2. 7Cz-Ir(nbt)2(Cz-acac)-POSS A round bottom flask (100 mL) was charged with octakis(dimethylsiloxy)silsesquioxane (0.92 g, 0.90 mmol), Ir(nbt)2(Cz-acacallyl) (0.96 g, 0.90 mmol) and anhydrous toluene (30 mL). The solution was degassed with argon for 5 min and then 5 drops of a solution of platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane (Ptdvs) (2 wt% Pt in xylene) were added, and the reaction mixture was stirred at room temperature under positive argon pressure for 4 h. Then Cz-allyl (1.87 g, 9.02 mmol) in 5 mL anhydrous toluene was introduced via syringe, and the reaction mixture was allowed to stir at room temperature overnight. The toluene was evaporated in vacuo and the crude product was isolated by chromatography on silica gel using ethyl acetate/petroleum ether (1:20, v/v) as the eluent to obtain red powdery 7Cz-Ir(nbt)2(Cz-acac)-POSS in 17.4% yield (0.55 g). 1H NMR (400 MHz, CDCl3, d, ppm): 8.66 (d, 1H, J ¼ 8.4 Hz, Aryl-H), 8.52 (d, 1H, J ¼ 8.4 Hz, Aryl-H), 8.38 (s, 1H, ArylH), 8.22 (d, 1H, J ¼ 7.2 Hz, Aryl-H), 8.16 (d, 1H, J ¼ 8.4 Hz, Aryl-H), 8.05e8.02 (m, 14H, Aryl-H), 7.90 (d, 1H, J ¼ 7.6 Hz, Aryl-H),

T. Yu et al. / Journal of Organometallic Chemistry 830 (2017) 85e92

7.83e7.65 (m, 6H, Aryl-H), 7.57 (d, 2H, J ¼ 7.6 Hz, Aryl-H), 7.50 (t, 2H, J ¼ 7.6 Hz, Aryl-H), 7.39e7.33 (m, 18H, Aryl-H), 7.26e7.23 (m, 14H, Aryl-H), 7.18e7.13 (m, 16H, Aryl-H), 7.07e7.03 (m, 2H, Aryl-H), 6.97 (d, 1H, J ¼ 8.4 Hz, Aryl-H), 6.68 (t, 2H, J ¼ 8.0 Hz, Aryl-H), 5.93 (s, 1H), 4.15 (t, 2H, J ¼ 6.4 Hz, -NeCH2e), 4.11e4.04 (m, 14H,-NeCH2e), 2.11 (t, 2H, J ¼ 7.2 Hz, eCH2e), 1.78e1.75 (m, 16H, eCH2e), 1.40e1.28 (m, 6H, eCH2e), 1.12e1.08 (m, 3H, eCH3), 0.56e0.40 (m, 16H, eCH2eSi), 0.05-(-0.04) (m, 48H, SieCH3). IR (KBr pellet, cm1): 3056, 2957, 2926, 2870, 1613, 1488, 1457, 1405, 1344, 1328, 1255, 1229, 1157, 1089, 842, 748, 722, 562, 489. ESI-MS: Calcd for [C178H191IrN10O22S2Si16 þ H]þ, 3529.2; Found, 3528.97. Anal. Calc. for C178H192IrN10O22S2Si16 (%): C, 60.58; H, 5.48; N, 3.97. Found: C, 60.67; H, 5.45; N, 4.02.

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4.3. OLEDs fabrication and characterization The multilayer OLEDs with a device architecture of ITO/ PEDOT:PSS (30 nm)/PVK: Dopant (x wt %, 50 nm)/TPBi (50 nm)/Liq (2 nm)/Al(150 nm) were fabricated, in which the red-emitting Ir(III) complex and the POSS material were used as dopants, respectively. Poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) was spin-coated onto precleaned and O2-plasma-treated indium tin oxide (ITO) substrates, yielding layers ca. 30 nm thick. PEDOT:PSS layers were heated at 180  C for 10 min to remove residual water. Blends of dopants þ poly(9vinylcarbazole) (PVK) in chlorobenzene solution was spin-coated on top of the PEDOT:PSS layers, yielding films ca. 50 nm thick. The samples were then dried at 80  C for 30 min. A TPBi hole/ exciton-blocking layer was deposited via thermal evaporation at a rate of ~2 Å s1. A cathode consisting of an ultrathin Liq interfacial layer with a nominal thickness of 2 nm and an Al layer ca. 150 nm thick was deposited by thermal evaporation. The deposition rates for Liq and Al were ~1 and 10 Å s1, respectively. The active area of the devices was 12 mm2. The EL spectra and Commission Internationale de L'Eclairage (CIE) coordinates were measured on a Hitachi MPF-4 fluorescence spectrometer. The characterization of brightness-current-voltage (BeIeV) were measured with a 3645 DC power supply combined with a 1980A spot photometer and were recorded simultaneously. All measurements were done in the air at room temperature without any encapsulation.

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