Pyrimidine-Based Mononuclear and Dinuclear Iridium(III) Complexes

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Pyrimidine-Based Mononuclear and Dinuclear Iridium(III) Complexes for High Performance Organic Light-Emitting Diodes Xiaolong Yang, Zhao Feng, Jiang Zhao, Jing-Shuang Dang, Boao Liu, Kaini Zhang, and Guijiang Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12446 • Publication Date (Web): 24 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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ACS Applied Materials & Interfaces

Pyrimidine-Based

Mononuclear

and

Dinuclear

Iridium(III)

Complexes for High Performance Organic Light-Emitting Diodes

Xiaolong Yang,† Zhao Feng,† Jiang Zhao,† Jing-Shuang Dang,† Boao Liu,† Kaini Zhang,† Guijiang Zhou,*, †

†

MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed

Matter, Institute of Chemistry for New Energy Materials, Department of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, P. R. China E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: Containing two nitrogen atoms, the electron-deficient pyrimidine ring have excellent coordinating capability with transition metal ions. However, compared with the widely used pyridine ring, applications of the pyrimidine ring in phosphorescent Ir(III) complexes are rare. In this research, two highly emissive pyrimidine-based mononuclear Ir(III) complexes and their corresponding dinuclear Ir(III) complexes were prepared with simple one-pot reaction. The incorporation of the second Ir(III) center can lead to dramatic differences of both photophysical and electrochemical properties between the mono- and dinuclear complexes. Besides, these properties can also be fine-tuned with different substituents. Theoretical calculations have also been performed to understand their photophysical behaviors. The electroluminescent investigations demonstrate that the pyrimidine-based monoand dinuclear Ir(III) complexes could show impressive device performance. The vacuum-deposited organic light-emitting diode (OLED) based on the mononuclear Ir(III) complex exhibited an external quantum efficiency (EQE) of 16.1% with almost no efficiency roll-off even at 10000 cd m−2. More encouragingly, the solution-processed OLED based on the dinuclear Ir(III) complex achieved the outstanding EQE, current efficiency (CE) and power efficiency (PE) of 17.9 %, 52.5 cd A−1 and 51.2 lm W−1, respectively, representing the highest efficiencies ever achieved by OLEDs based on dinuclear Ir(III) complexes.

KEYWORDS: pyrimidine; mononuclear iridium(III) complex; dinuclear iridium(III) complex; phosphorescence; organic light-emitting diode

2


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1. INTRODUCTION Since the pioneer phosphorescent organic light-emitting devices (OLEDs) reported by Ma et al. and Forrest et al.,1, 2 phosphorescent complexes based on transition metals, e.g., Os(II),3,

4

Pt(II),5,

6

Au(III)7,

8

and Ir(III),9,

10

have shown great potential in

fabricating high performance OLEDs because they can fully utilizing both the singlet and triplet excitons generated in the devices.11 The phosphorescence of these transition metal complexes usually involves the triplet metal-to-ligand charge transfer (3MLCT) processes.12 It means that both chelated organic ligands and the transition metal centers have affected the emission properties of phosphorescent complexes. Due to the limited number of the transition metal centers that can induce phosphorescent emission, more works have been done to synthesize various organic ligands in order to manipulate the emission properties as well as charge injection/transporting properties of phosphorescent complexes.13,

14

After nearly

twenty years of development, the prevailing strategy for developing phosphorescent complexes can be summarized as follows: one transition metal center coordinated to one or more nitrogen-containing heterocyclic ligands.1-14 In most cases, the nitrogen-containing heterocycles in organic ligands are the electron-deficient pyridine rings. Compared with pyridine ring, pyrimidine unit contains one more sp2 hybrid nitrogen atom. Besides, the pyrimidine ring also shows electron-deficient character which has been demonstrated by their successful application in the host and electron-transporting materials for high performance OLEDs,15-19 suggesting that the pyrimidine ring can be a good electron acceptor to facilitate the MLCT processes. Hence, it is reasonable to deduce that pyrimidine ring can also be used to synthesize stable phosphorescent complexes with properties suitable for fabricating high performance OLEDs. Actually, only a few of pyrimidine-based Ir(III) complexes have been

used

to

fabricate

OLEDs.

For

example,

the

complex

bis[5-(4-(tert-butyl)phenyl)-2-(4-(trifluoromethyl)phenyl)pyrimidine]iridium(III) acetylacetonate was employed to fabricate a green-yellow OLED showing a peak 3



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external quantum efficiency (EQE) of 8.5% and current efficiency (CE) of 30.0 cd A–1.20

The

green

emission

phosphorescent

bis(o-tolylpyrimidine)iridium(III)

acetylacetonate was reported to show electroluminescent (EL) performance of EQE of 8.0% and CE of 28.6 cd A−1.21 By switching substituted position of the methyl group on the phenyl ring from the ortho-position to the meta-position, the yellowish green phosphorescent emitter bis(m-tolylpyrimidine)iridium(III) acetylacetonate

was

synthesized and could exhibit even higher device efficiencies with EQE of 17.4% and CE of 60.4 cd A−1.22 Liao et al. reported two 4-phenylpyrimidine-based Ir(III) complexes showing high photoluminescence quantum yield (PLQY) up to 0.92 with good horizontal orientation property.23 Accordingly, the resultant orange OLED could achieve a peak EQE of 28.2%, which is among the highest efficiencies for orange phosphorescent OLEDs. In our previous work, we have also reported three pyrimidine-based Ir(III) complexes exhibiting very interesting photophysical properties and impressive EL efficiencies.24 One mononuclear and two dinuclear Ir(III) complexes (Scheme 1) were obtained at the same time by using 2-(4-(tert-butyl)phenyl)pyrimidine as the ligand. Compared with the mononuclear Ir(III) counterpart, the dinuclear Ir(III) complexes possess higher HOMO level and lower LUMO level, resulting in a dramatic emission red-shift of ca. 80 nm from the mononuclear Ir(III) complex to the dinuclear Ir(III) complexes. More importantly, the simple solution-processed orange-red OLEDs using the dinuclear Ir(III) complex as emitter can achieve the maximum EQE, CE and PE of 14.4 %, 27.2 cd A−1 and 19.5 lm W−1, which are among the highest efficiencies ever obtained by solution-processed orange-red emitting OLEDs.24 These results further indicate the great potential of using pyrimidine-based ligands to synthesize highly efficient phosphorescent Ir(III) complexes. Furthermore, compared with the tedious and complicated synthesis processes reported for some dinuclear Ir(III) complexes (Scheme

1),25-28

the

2-phenylpyrimidine-type

ligand

possessing

two

N^C-coordinating sites shows great advantage in synthesizing neutral dinuclear Ir(III) complexes through a quasi-one-pot reaction. Scheme 1. Examples of Some Reported Dinuclear Ir(III) Complexes 4


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4+ N

N

N Ir

N

N Ir

N

N N

N

N DIr1 2+

F

2+

F F

N N N

N Ir N

H

N

N

N

N

N

N

N Ir

F

Ir N

F F

N

F DIr3

DIr2

R R N

N

R

N Ir

Ir Cl

N

R

R

N R

N

N

O

O N N

Ir

R

DIr5, R = CHO

DIr4

N

R

N Cl

N Ir N

Ir

Ir

N

N

Ir

O

Ir

N N

N O

N

N DIr6

DIr7

O

N

O

N Ir

N N

N

N N

Ir N

O

N

N

H

N N

N

F

Ir

N

N Ir

N

Ir N

O

O

DIr8

N O

O

O

DIr9

In this paper, we have prepared two mononuclear Ir(III) complexes and their corresponding dinuclear Ir(III) complexes with 2-phenylpyrimidine-type ligands. The crystal structures of a mononuclear Ir(III) complex and its corresponding dinuclear 5



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Ir(III) complex had been confirmed by the X-ray diffraction results. These mono- and dinuclear Ir(III) complexes can show high PLQYs. In addition, through introducing different substituents to the ligands can further adjust the electrochemical and photophysical behaviors of these mono- and dinuclear Ir(III) complexes. The vacuum-deposited

OLEDs

based

on

the

mononuclear

Ir(III)

complex

bis[2-(4-(tert-butyl)phenyl)pyrimidine]iridium(III) acetylacetonate (M1) can exhibit decent EQE over 16.0%. Moreover, the solution-processed OLEDs based on the dinuclear Ir(III) complex FDIr1 can achieve maximum EQE, CE and PE of 17.9 %, 52.5 cd A−1 and 51.2 lm W−1, respectively, representing highest EL efficiencies ever reported for the dinuclear Ir(III) complexes (Scheme 1).24, 29, 30 This work shows that 2-phenylpyrimidine-type ligands are very promising for developing highly efficient mono- and dinuclear Ir(III) complexes.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization After the synthesis of 2-phenylpyrimidine-type ligands by Suzuki cross-coupling reactions between 2-chloropyrimidine and the corresponding boronic acids, the ligands were treated with IrCl3 to give the intermediate complexes. Subsequently, the intermediate complexes were treated with acetylacetone thallium(I) [Tl(acac)] in CH2Cl2 at room temperature to give the mono- and dinuclear Ir(III) complexes (Scheme 2). The detailed synthetic process and characterizations of NMR, MS for these complexes were described in the Supporting Information (Figure S1−S3), which are in good agreement with their structures. Scheme 2. Synthesis of the Mono- and Dinuclear Ir(III) Complexes

6


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R R

N N O

Ir

N

N

O R = H, HIr R = F, FIr R

B(OH)2

DME, Pd(Ph3)4, 90 °C

+ N R

N

N

N Cl

1) IrCl 3 H2O, THF/H2O

N

2) Tl(acac), CH2Cl2

CH3CH2OH, K2CO3 (2M)

R

N

O

Ir

O

O

N

Ir

N

N acac

O R = H, HDIr1 R = F, FDIr1

R

R

R

R R

N

N N

N Ir O

acac

N

O

N

Ir N O

O

acac

acac

R = H, HDIr2 R = F, FDIr2

The single crystals of the mononuclear Ir(III) complex FIr and its corresponding dinuclear Ir(III) complex FDIr1 were cultivated by slowly diffusing their tetrahydrofuran solutions into hexane. Single-crystal X-ray diffraction studies of FIr and FDIr1 clearly confirm their structures. Details of each structure are summarized in Table 1. As shown in Figure 1, two 2-(4-fluorophenyl)pyrimidine ligands and an acetylacetone ligand are coordinated to the central Ir(III) ion to form a slightly distorted octahedral configuration with the cis-C,C trans-N,N chelate disposition, which is similar to the typical structural pattern of the mononuclear Ir(III) complex bearing pyridine-type ligands.31, 32 The average bond lengths of N−Ir and C−Ir in FIr are 2.038(7) Å and 1.998(9) Å, respectively (Table S1 in Supporting Information), which are similar to the values reported for the pyridine-based Ir(III) complexes.31, 32 Compared with the reported O−Ir bond lengths of the pyridine-based Ir(III) complexes (1.154(5) and 1.175(4) Å),31, 32 the average bond length of O−Ir in FIr is slightly shortened to 1.138(6) Å. In the crystal packing of FIr, the π-π overlap with a ca. 3.45 Å face-face separation between the pyrimidine ring and the phenyl ring of the other complex indicates the existence of π-π interactions between the FIr molecules (Figure 2). Besides, the shortest distance of 2.7782(66) Å between the fluorine atom and hydrogen atom of the other complex discloses that the hydrogen bond is another 7



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kind of interaction between the FIr molecules (Figure 2).

Figure 1. Crystal structural drawings of FIr and FDIr1 with ellipsoids shown at the 30% probability level. The structure of FDIr2 shown here is the theoretical optimized structure. (Hydrogen atoms are omitted for clarity).

8


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Table 1. Crystal Data and Structure Refinements for FIr and FDIr1 FIr

FDIr1

1017101

1017103

C100H76F8Ir4N16O8

C40H31F3Ir2N6O4

M (g mol )

2550.57

1101.11

crystal system

Triclinic

Triclinic

space group

P−1

P−1

a (Å)

14.4570(16)

13.836(4)

b (Å)

17.8391(19)

13.926(4)

c (Å)

18.624(2)

14.038(4)

α (°)

113.594(2)

71.393(6)

β (°)

90.914(2)

65.594(5)

γ (°)

90.477(2)

83.900(5)

V (Å3)

4400.4(8)

2333.3(11)

2

2

Dcalcd (g cm )

1.925

1.567

−1

µ (mm )

6.117

5.750

F(000)

2464

1052

θ range (°)

1.19 to 27.61

1.54 to 26.37

reflections collected

51944

12924

independent

20155

9196

Rint

0.0822

0.0547

parameters

1233

496

R (I > 2σ(I))

R1 = 0.0568,

R1 = 0.0654,

wR2 = 0.0928

wR2 = 0.1739

R1 = 0.1104,

R1 = 0.1260,

wR2 = 0.1056

wR2 = 0.1973

1.035

0.845

CCDC formula −1

Z −3

reflections

R (all data)

S

9



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Figure 2. The π-π interactions and F−H hydrogen bonds between adjacent complexes in the crystal of FIr.

As depicted in Figure 1, in the crystal of FDIr1, the bridging 2-(4-fluorophenyl)pyrimidine is coordinated to two Ir(III) centers through its two N^C-coordinating

sites, resulting in a

plane consisting of the

bridging

2-(4-fluorophenyl)pyrimidine ligand and two Ir(III) centers. The two acetylacetone ligands (acac) are located on the opposite side of this plane (Scheme 2 and Figure 1). This coordination geometry is almost identical to the structure of another pyrimidine-based dinuclear Ir(III) complex reported by us.24 The average bond lengths of N−Ir, C−Ir and O−Ir in FDIr1 are 2.036(13) Å, 2.012(15) Å and 2.148(10) Å, respectively, which are similar to those of FIr. The distance between the two Ir(III) centers in the crystal of FDIr1 is 5.4989(15) Å, which is large enough to ignore the metal-metal interactions due to the fact that typical Ir−Ir bond length is in the range from ca. 2.6 to ca. 3.1 Å in some multimetallic clusters.33-35 As shown in Figure 3, there is no π-π interaction between the FDIr1 molecules, while several hydrogen bonds can be found due to the short distances between the fluorine atoms and hydrogen atoms on the adjacent complex, indicating that FDIr1 has strong intermolecular interactions in the solid state. 10


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Figure 3. The F−H hydrogen bonds between adjacent complexes in the crystal of FDIr1.

According to our previous study, one 2-phenylpyrimidine-type bridging ligand can coordinate to two Ir(III) ions to form two isomers of dinuclear Ir(III) complexes, this is the inevitable result of the cis-C, C trans-N, N chelate pattern in both monoand dinuclear Ir(III) complexes.24, 31, 32 Therefore, when synthesizing the dinuclear Ir(III) complex FDIr1, we also obtained another isomer of dinuclear Ir(III) complex FDIr2 at the same time. The 1H-1H COSY NMR spectra clearly showed the different proton coupling pattern between FDIr1 and FDIr2 (see Figure S2). Although we did not successfully get the crystal of FDIr2, we still can reasonably predict the structure of FDIr2 based on our previous study.24 Considering the fact that the ligands used in this paper and in our previous study both are 2-phenylpyrimidine-type ligands, and the isomer FDIr1 has almost the identical structural configuration as one isomer of the dinuclear Ir(III) complexes does in our previous study,24 we believe that the isomer FDIr2 also has almost the same structure configuration as another isomer of dinuclear Ir(III) complexes does, i.e., the two acac ligands are located on the same side of the bridging ligand. Based on this, the structure of FDIr2 can be theoretical optimized as it shown in Figure 1.

11



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2.2. Thermal and Photophysical Properties The thermal stabilities of these mono- and dinuclear Ir(III) complexes were evaluated by thermogravimetric analysis (TGA) under a nitrogen atmosphere with a heating rate of 20 K/min. Both the mono- and dinuclear Ir(III) complexes show high degradation temperatures (5% loss of weight) from 287 to 336 °C (Figure S4 in Supporting Information and Table 2), indicating that these Ir(III) complexes are thermal stable enough for fabricating OLEDs. The UV-Vis absorption spectra of the mono- and dinuclear Ir(III) complexes in CH2Cl2 solutions are shown in Figure 4a, and the detailed optical data are summarized in Table 2. The distinct absorption bands in the range from 225 to 310 nm with peaks centered at ca. 260 nm can be attributed to the spin-allowed ligand-centered

1

π−π* intraligand charge transfer (1ILCT) transitions involving

2-phenylpyrimidine-type ligands. The strong absorption bands between 310 and 420 nm can be assigned to spin-allowed metal-to-ligand charge transfer (1MLCT) from the Ir(III) centers to the 2-phenylpyrimidine-type ligands. The weak absorption bands extended to the visible region can be attributed to 3MLCT along with ligand-centered 3

π−π* transitions. On the whole, the isomers of dinuclear Ir(III) complexes with the

same 2-phenylpyrimidine-type ligand show nearly identical absorption behaviors; the dinuclear Ir(III) complexes exhibit significant red-shifted 3MLCT absorption bands compared with their corresponding mononuclear Ir(III) complexes. Compared with that of HIr, the 3MLCT absorption band of FIr is obviously blue shifted due to the introduction of fluorine atoms into the 2-phenylpyrimidine ligands. This phenomenal also occurs in the same way in the cases of dinuclear Ir(III) complexes. The fluorinated dinuclear Ir(III) complexes FDIr1 and FDIr2 display pronounced blue-shifted 3MLCT absorption bands compared with the non-fluorinated dinuclear Ir(III) complexes HDIr1 and HDIr2.

12


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Absorbance (a.u.)

1.0 (a)

FIr FDIr1 FDIr2 HIr HDIr1 HDIr2

0.8 0.6 0.4 0.2 0.0 250

300

350

400

450

500

550

Wavelength (nm) 1.0

PL 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


(b)

FIr FDIr1 FDIr2 HIr HDIr1 HDIr2

0.8 0.6 0.4 0.2 0.0 500

550

600

650

700

750

Wavelength (nm)

Figure 4. (a) The UV-Vis absorption spectra and (b) the PL spectra of the mono- and dinuclear Ir(III) complexes in CH2Cl2 solutions at 298 K.

Figure 4b depicts the photoluminescent (PL) spectra of the mono- and dinuclear Ir(III) complexes in CH2Cl2 solutions. The structureless and broad emission profiles of both mono- and dinuclear IrIII complexes imply that the emissions mainly result from the 3MLCT transitions.12 The dinuclear complexes HDIr1 and HDIr2, FDIr1 and FDIr2 show similar emissions. This result may be explained from two aspects. On one hand, from the view of UV-Vis experiment results, HDIr1 and HDIr2, FDIr1 and FDIr2 display almost identical absorption curves in profile as well as intensity, indicating the similar excited states, and thereby resulting in similar emissions. On the other hand, from the view of theoretical calculation results (vide infra), the HOMO and LUMO distributions of HDIr1 and HDIr2, FDIr1 and FDIr2 are also quite similar, which means the similar charge transporting behaviors, leading to the similar 13



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Page 14 of 38

emissions. For the same reasons, HDIr1 and HDIr2, FDIr1 and FDIr2 possess similar PLQYs (ΦP) as well as lifetimes (τp) (Table 2). The dinuclear Ir(III) complexes show dramatically red-shifted phosphorescent emission peaks compared with their corresponding mononuclear Ir(III) complexes, which agrees well with the red-shifted 3MLCT absorption bands mentioned above. It has been well-established that incorporating electron withdrawing group such as fluorine atom to the phenyl ring

on

the

2-phenylpyridine

ligand

can

blue

shift

the

emissions

of

2-phenylpyridine-type Ir(III) complexes.36 This rule still works well for Ir(III) complexes bearing 2-phenylpymiridine-type ligands, because the fluorinated mononuclear Ir(III) complex FIr displays a green emission with the peak at 502 nm, which is 24 nm blue-shifted compared with the emission peak of the non-fluorinated mononuclear Ir(III) complex HIr. Similar to the cases in FIr and HIr, a blue shift of 23 nm is also observed between the dinuclear Ir(III) complexes HDIr1 and HDIr2, FDIr1 and FDIr2. Both FIr and HIr display high PLQYs (ΦP) close to unity in CH2Cl2 solutions (Table 2), indicating that it is very promising to using 2-phenylpymiridine-type ligands to develop efficient Ir(III) complex emitters. Compared with HDIr1 and HDIr2, the fluorinated dinuclear Ir(III) complexes FDIr1 and FDIr2 show much higher PLQYs of 0.68, which are among the highest PLQYs for dinuclear IrIII complexes.25, 27-29 The observed lifetimes (τp) for mononuclear complex HIr is 1.30 µs, which is comparable to that for the pyridine-based Ir(III) complex bis(2-phenylpyridine)iridium(III) acetylacetonate [Ir(ppy)2acac] (τp = 1.6 µs).31 However, when one more Ir(III) ion is chelated to the pyrimidine-type ligand, the resultant dinuclear Ir(III) complexes HDIr1 and HDIr2 show significant shortened τP about 0.2 µs. The fluorinated dinuclear Ir(III) complexes FDIr1 and FDIr2 also show much shorter lifetimes compared with its corresponding mononuclear Ir(III) complex FIr. The phenomenal of significant decrease of lifetimes for dinuclear Ir(III) complexes compared with their corresponding mononuclear Ir(III) complexes reflected the pronounced influence from the extra Ir(III) center on the photophysical properties of Ir(III) complexes. 14


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Table 2. Photophysical and Thermal Data for Ir(III) Complexes

Complex HIr HDIr1 HDIr2 FIr FDIr1 FDIr2 a

λem a (nm)

ФP b

261 (4.73), 338 (4.09), 378 (3.83), 411 (3.69), 460 (3.56) 261 (4.79), 338 (4.27), 410 (3.98), 475 (3.88), 515 (3.50) 260 (4.80), 342 (4.27), 407 (4.00), 476 (3.89), 515 (3.54) 258 (4.62), 333 (4.02), 367 (3.77), 397 (3.65), 441 (3.40) 259 (4.85), 328 (4.35), 400 (4.09), 447 (3.98), 504 (3.24) 259 (4.81), 329 (4.32), 400 (4.05), 447 (3.95), 504 (3.26)

526

0.99

1.30

(°C) 327

603

0.36

0.20

336

603

0.43

0.19

306

502

0.99

0.91

326

580

0.68

0.48

322

580

0.68

0.41

287

Measured in CH2Cl2 at a concentration of 2×10–5 M.

b

τP c (µs)

∆T5

λabs (logε) a (nm)

%

Measured in degassed CH2Cl2 solution relative to

fac-[Ir(ppy)3] (ΦP = 0.97). c Recorded in degassed CH2Cl2 solution with a 369 nm light source.

2.3. Theoretical Calculations To provide deeper insight into the optoelectronic properties of these Ir(III) complexes, theoretical calculations based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were undertaken. As summarized in Table S1, the calculated bond lengths are very close to those determined by crystal structures, showing the good agreement between the experimental and calculated structures, which confirms the validity of the theoretical calculations. Theoretical calculated energy levels, HOMO and LUMO patterns for the Ir(III) complexes are given in Figure 5, and the percentages of orbital contributions from each designated group in the Ir(III) complexes are given in TableS2 in the Supporting Information. The HOMOs of the mononuclear Ir(III) complexes HIr and FIr contain significant contributions from the dπ orbital of Ir(III) center (ca. 52.0%) and the π orbitals of the two chelated phenyl fragments. Each of the Ir(III) center in the dinuclear Ir(III) complexes also makes over 25% contribution to the HOMOs of HDIr1, HDIr2, FDIr1 and FDIr2. As depicted in Figure 5, different from the situation in HIr and FIr that the two chelated phenyl fragments make equal 15



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contribution to HOMOs, the phenyl fragment of the bridging ligand makes much more contribution to the HOMOs of the dinuclear Ir(III) complexes than the phenyl fragments of the peripheral ligands. This result may be interpreted from that the high coplanarity between the phenyl fragment of the bridging ligand and the two Ir(III) centers will ensure a good conjugation among the dπ orbitals of the two electron-rich Ir(III) centers and the π orbital of the phenyl fragment of the bridging ligand. The electron-donating conjugation effect from the two electron-rich Ir(III) centers will increase the electron-density of phenyl fragment of the bridging ligand and hence enhance its contribution to the HOMOs of the dinuclear Ir(III) complexes. As a result, the uneven HOMO distribution on the phenyl fragments from the bridging and peripheral ligands can be seen in the dinuclear Ir(III) complexes (Figure 5). Consequently, the HOMO levels of the dinuclear Ir(III) complexes are elevated compared with those of the corresponding mononuclear Ir(III) complexes due to the high electron-density on the phenyl fragments of the bridging ligand. Similar to the status of the pyridine ring giving large contribution to the LUMO of (ppy)2Ir(acac),37 the electron-deficient pyrimidine rings also make major contribution to the LUMOs of these pyrimidine-based Ir(III) complexes (Figure 5). Interestingly, different from the even distribution of LUMOs on the two pyrimidine rings in mononuclear complexes, the LUMOs of dinuclear Ir(III) complexes contain dominated contributions from the pyrimidine ring (68.8−87.9%) of the bridging ligand and nearly negligible contribution from the two pyrimidine rings (less than 5% for each) of the peripheral ligands. Because the pyrimidine ring of the bridging ligand donates two lone-electron pairs to two Ir(III) ions to form N–Ir bonds. It will decrease the electron density on the pyrimidine ring of the bridging ligand. Thereby, it has lower electron density than the two pyrimidine rings of the peripheral ligands, which will make the pyrimidine ring of the bridging ligand more electron-deficient, in turn, to stabilize the LUMOs of dinuclear Ir(III) complexes. As a result, the LUMO levels of dinuclear Ir(III) complexes are lower than those of the corresponding mononuclear Ir(III) complexes. On the whole, the dinuclear Ir(III) complexes possess higher HOMO levels and lower LUMO levels than the corresponding mononuclear Ir(III) 16


Page 16 of 38

Page 17 of 38

counterparts, leading to the much narrower energy gaps. Considering the predominated contribution (75.4−95.0%) from HOMO → LUMO transitions to the T1 states (Table S2), the narrowed energy gaps of the dinuclear Ir(III) complexes will surely show the dramatically red-shifted MLCT absorption bands and phosphorescent emission. It should be noted that the theoretical results also reveal that the electron-withdrawing fluorine atoms can stabilize both the HOMOs and LUMOs, especially the HOMOs, due to their direct bonding to the phenyl rings which give substantial contribution to the HOMOs (Figure 5b). Hence, the fluorinated Ir(III) complexes possess lower HOMO and LUMO levels as well as wider energy gaps, resulting in blue-shifted phosphorescent emission. This result is in good agreement with the experimental outcomes.

(a) 0 -1 -2 E (eV)

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


LUMO −1.67

−1.79

−1.80

−4.80

−4.79

HDIr1

HDIr2

-3 -4 -5

−5.01

HOMO

-6 -7 HIr

17



(b) 0 -1 -2 E / eV

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

Page 18 of 38

LUMO −1.86

−2.02

−2.01

−5.35

−5.17

−5.15

FDIr1

FDIr2

-3 -4 -5 HOMO -6 -7 FIr

Figure 5. Theoretical calculated energy levels and plots of HOMO and LUMO for (a) HIr, HDIr1 and HDIr2, (b) FIr, FDIr1 and FDIr2.

2.4. Electrochemical Properties The electrochemical properties of these Ir(III) complexes were investigated by cyclic voltammetry (CV) using ferrocene as the internal reference (see Figure S5), and the results are listed in Table 3. During the anodic scan in CH3CN, the mononuclear Ir(III) complexes FIr and HIr show reversible metal-centered oxidation peaks with potential (Eox1/2) at ca. 0.70 V and 0.79 V, respectively. However, each of the dinuclear Ir(III) complexes show two discernibly reversible oxidation waves with the first Eox1/2 in the region of 0.51−0.75 V and the second Eox1/2 in the region of 0.84−1.00 V. The successive redox processes can be assigned to the oxidation of the two Ir(III) centers.27, 38, 39 Due to the effect of delocalization of the positive charge over the two Ir(III) cations, the first Eox1/2 of the dinuclear Ir(III) complexes locate at less positive potentials compared with those of their corresponding mononuclear Ir(III) complexes. Once one Ir(III) core is oxidized, other Ir(III) core will be reluctant to be oxidized, switching the second Eox1/2 of dinuclear Ir(III) complexes to higher potentials. Besides, the oxidation potentials of both mono- and dinuclear Ir(III) 18


Page 19 of 38

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


complexes are raised by the incorporation of the fluorine atom due to its electron-withdrawing property,40 leading to lower HOMO levels of fluorinated complexes compared with those of corresponding non-fluorinated complexes (Table 3).

Table 3. Electrochemical Data for Ir(III) Complexes E ox 1/2

Epc

HOMO/ LUMO

(V)

(V)

(eV)

0.70

–2.22, –2.50

–5.50/–2.58

HDIr1

0.51, 0.85

–2.17, –2.62

–5.31/–2.63

HDIr2

0.53, 0.84

–2.16,

–5.33/–2.64

0.79

–2.25, –2.54

–5.59/–2.55

FDIr1

0.65, 0.95

–2.11, –2.68

–5.45/–2.69

FDIr2

0.75, 1.00

–1.99, –2.56

–5.55/–2.81

Complex

HIr

FIr

During the anodic cathodic scan, all Ir(III) complexes display the first irreversible reduction peak potential (Epc) between −1.99 V and −2.25 V. More specifically, as indicated by their less negative Epc, the dinuclear Ir(III) complexes are easier to be reduced, which may be caused by the fact that the electron density on the pyrimidine fragment in the bridging ligand will be notably lowered through coordinating to two Ir(III) centers. However, compared with their dramatic effects on oxidation potentials, the fluorine atoms show much less influence on the reduction potentials. For example, the first oxidation potential is notably raised from 0.70 V for HIr to 0.79 V for FIr, while the first reduction potential is only changed from –2.22 V for HIr to –2.25 V for FIr. Accordingly, the incorporation of the fluorine atoms can stabilize the HOMO more than the LUMO level as listed in Table 3, thus increasing the energy gap and blue-shifting the emission of fluorinated FIr. For the same reason, the fluorinated dinuclear Ir(III) complexes FDIr1 and FDIr2 show orange emissions (λ = 580 nm) while the non-fluorinated dinuclear Ir(III) complexes HDIr1 and HDIr2 19



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Page 20 of 38

show red emission (λ = 603 nm). These experimental results are in well agreement with the theoretical calculation predictions. In addition, the higher HOMO levels and lower LUMO levels imply that the dinuclear Ir(III) complexes have some degree of bipolar character, which is a highly desired property for developing high performance OLEDs. 2.5. Electroluminescence Properties To evaluate the transporting character of these mononuclear Ir(III) emitters, we fabricated hole- and electron-only devices based on these complexes with vacuum-deposited method. The famous pyridine-based Ir(ppy)2acac was used as the reference complex. The hole-only devices were fabricated with the structure of (3

ITO/MoO3

nm)/NPB

[

N,N'-bis(1-naphthalenyl)-N,N'-bisphenyl-(1,1'-biphenyl)-4,4'-diamine] (1 nm)/ Ir(III) complex (40 nm)/NPB (1 nm)/MoO3 (3 nm)/Al (100 nm). The electron-only devices

were

fabricated

with

the

structure

(1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene)

of (1

ITO/LiF nm)/Ir(III)

(3

nm)/TPBI

complex

(40

nm)/TPBI (1 nm)/LiF (3 nm)/Al (100 nm). As shown in Figure 6a, compared with those of Ir(ppy)2acac, the hole transporting abilities of these mononuclear Ir(III) complexes were slightly decreased, but the electron transporting abilities of these mononuclear Ir(III) complexes were improved. This result indicates that the devices based on these mononuclear Ir(III) complexes could show good charge carrier balance property. We also compared the transporting characters between the mononuclear complex FIr and dinuclear complex FDIr1 by fabricating solution-processed holeand electron-only devices with structures of ITO/MoO3 (3 nm)/PEDOT: PSS (20 nm)/Ir(III) complex (40 nm)/PEDOT: PSS (20 nm)/MoO3 (3 nm)/Al (100 nm) and ITO/LiF (3 nm)/ Ir(III) complex (40 nm)/LiF (3 nm)/Al (100 nm), respectively. As Figure 6b depicted, both the hole and electron transporting abilities of the dinuclear complex FDIr1 were improved compared with those of the mononuclear complex FIr. Especially, the improvement of electron transporting ability is more obvious than that of hole transporting ability. These results imply that the dinuclear complex could show stronger bipolar character and thereby better charge carrier balance in the 20


Page 21 of 38

resultant device.

(a) 200 −2

Current density (mA cm )

Ir(ppy)2acac HIr FIr M1

150

Electron-only

Hole-only

100

50

0 8

10

12

14

16

18

20

22

Voltage (V)

(b) 200 −2

Current density (mA cm )

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


FIr FDIr1 150

Hole-only

Electron-only

100

50

0 8

10

12

14

16

18

20

22

Voltage (V) Figure 6. The Current Density-Voltage (J-V) curves for single carrier devices based on a) HIr, FIr, M1 and Ir(ppy)2acac using vacuum-deposition method and b) FIr and FDIr1 by spin-coating.

Mononuclear

Ir(III)

complexes

HIr

bis[2-(4-(tert-butyl)phenyl)pyrimidine]iridium(III)

and

FIr

acetylacetonate

21


as

well

(M1)24

as were


selected as dopants to fabricate vacuum-deposited OLEDs because of their higher PLQYs and good vacuum-sublimable properties. Device configuration was adopted with the structure of ITO/MoO3 (4 nm)/CBP (35 nm)/8.0 wt% M1 (Device 1) or HIr (Device 2) or FIr (Device 3): CBP (15 nm)/TPBI (65 nm)/LiF (1 nm)/Al (100 nm) (Figure 7a). The MoO3 layer was used to help inject the holes to the hole-transporting CBP (4,4′-N,N′-dicarbazolebiphenyl) layer. A 15 nm thick CBP layer doped with the mononuclear Ir(III) complex at 8.0 wt% level acted as the emission layer. TPBI served as the electron-transporting material. Figure 7 shows the energy levels41, 42 and the molecular structures of the materials used in the fabricated OLEDs. The EL spectra, current density(J)–voltage(V)–luminance(L) curves and efficiencies versus luminance curves of the OLEDs based on the mononuclear Ir(III) complexes are depicted in Figure 8 and Figure S6. The detailed EL data are summarized in Table 4.

-5

ITO −4.8

−6.0

-6 −6.7

M1

HIr

FIr

−5.45−5.50 −5.59

−3.4

-4 ITO

-5

−4.8

−2.9 LiF/Al

TPBI (45 nm)

-4

−2.69 −2.7

-3

E (eV)

CBP (35 nm)

-3

−2.4

DFIr1:TCTA

−2.9 −2.51 −2.59−2.55 −2.7 −2.9 LiF/Al

(b)

PEDOT:PSS (45 nm)

-2

(a)

TPBI (65 nm)

-2

E (eV)

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

Page 22 of 38

−5.45 −5.2

−6.2

−5.7 −6.2

-6 8 wt% Ir : CBP (15 nm)

x wt% Ir : TCTA (30 nm)

MoO3 (4 nm)

Figure 7. (a) The vacuum-deposited and (b) the solution-processed device structures, and energy level diagrams of HOMOs and LUMOs and molecular structures of the materials used in the fabricated OLEDs.

22


Page 23 of 38

Device 1 Device 2 Device 3

EL intensity (a.u.)

1.0 (a) 0.8 0.6 0.4 0.2 0.0

400 (b)

500 550 600 650 Wavelength (nm)

700

750

Device 1 Device 2 Device 3

400

105 104

320 103 240 102 160

101

80 0

−

Current density (mA cm−2)

480

450

Luminance (cd m−2)

100 4

6

8

10

12

Voltage (V)

(c) −1

CE/PE (cd A /lm W )

1

10

0

−1

10 Device 1

10

-1

10

EQE CE PE

EQE(%)

-2

10

-3

-1

0

10

1

10

2

3

4

10

5

10 10 10 10 −2 Luminance (cd m )

10

1

10

−1

−1

CE/PE (cd A /lm W )

(d)

0

10

10

EQE (%)

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


-1

10

Device 2

EQE

1

-2

10

CE PE

-3

10 0

10

1

10

2

3

4

10 10 10 −2 Luminance (cd m )

5

10

Figure 8. (a) EL spectra of Devices 1–3. (b) The J-V-L relationships of Devices 1–3. (c) Efficiencies vs luminance curve of Device 1. (d) Efficiencies vs luminance curve of Device 2. 23



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

As shown in Figure 8a, both Device 1 and Device 2 could display bright green electroluminescence (EL) with the peaks (λELmax) at 528 nm and CIE (Commission Internationale de l'Eclairage) coordinates of (0.34, 0.63). The EL bands show good resemblance of the PL spectra of M1 and HIr in dilute CH2Cl2 solution (see Figure S7), indicating that the EL emission mainly originates from the radiative decay of M1 and HIr. The Device 1 based on M1 exhibits the best device performance with the maximum luminance (L), EQE, CE and PE of 144324 cd m−2, 16.1%, 61.9 cd A−1 and 37.5 lm W−1, respectively. Generally, with the luminance increased, the detrimental EL efficiency roll-off will take place due to the triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ).43 However, the Device 1 shows almost no EQE and CE efficiency roll-offs even at a high luminance of 10000 cd m−2. Although the Device 2 based on HIr shows slightly lower efficiencies (maximum EQE, CE and PE of 13.2%, 50.8 cd A−1 and 26.8 lm W−1, respectively), its EQE and CE efficiency roll-offs are also negligible at 10000 cd m−2. Unexpectedly, Device 3 based on FIr displays much poorer device performance with the maximum EQE, CE and PE of 5.3%, 17.0 cd A−1 and 8.3 lm W−1, respectively (see Figure S6). This may result from the relative strong intermolecular interaction between the FIr molecules indicated by both experimental and theoretical results. As shown in Figure 2, strong π−π interaction and F−H interactions between the neighbor molecules can be observed in the FIr crystal, indicating its stronger molecular interaction. In addition, emitters possessing larger permanent dipole moments have higher probability to form emitter aggregates in doped film, leading to stronger quenching effects in devices.44, 45 As the theoretical calculation results revealed, the permanent dipole moment of FIr is 6.88 D (debye), which is over 1.5 times larger than those of M1 (3.79 D) and HIr (4.43 D). Therefore, it is reasonable that FIr exhibits strong molecular aggregation behavior. At the doping level of 8 wt%, the strong intermolecular interactions of FIr will be suppressed to some extent, but it cannot be totally eliminated. As depicted in Figure S7, the EL profile and the PL profile are almost identical for M1 and HIr, while the EL spectrum of FIr shows a broader FWHM (full width at half maximum) due to the 24


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Page 25 of 38

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


appearing of a shoulder at ca. 530 nm. We further investigated the intermolecular interactions or molecular aggregation behaviors in CBP films at the doping level of 8 wt% by using atomic force microscopy (AFM). As shown in Figure 9, the films doped with M1 and HIr show quite similar graphic pattern with the root-mean-square (RMS) roughness around 6.3 nm. However, the FIr doped film displays obvious molecular clusters with a larger RMS of 10.21 nm. These results demonstrate that the intermolecular interactions or molecular aggregation behavior in FIr doped CBP film is stronger. Likewise, stronger molecular aggregation of the emitter will occur in Device 3, leading to detrimental effects of triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ).43 Therefore, the Device 3 exhibits poorer EL performance with notable EQE and CE efficiency roll-offs (above 10%) at the luminance of 10000 cd m−2 (Table 4). In brief, M1 and HIr with small dipole moments show very high EL efficiencies and extremely low efficiency roll-offs, indicating that the pyrimidine-based mononuclear Ir(III) complexes M1 and HIr are promising phosphorescent emitters for fabricating high-performance OLEDs. More importantly, these results have clearly indicated the crucial role of small dipole moments for the phosphorescent emitters in reducing the possibility of forming molecular aggregation to improve the EL performance.

Table 4. Electroluminescent Data for Vacuum-Deposited OLEDs Based on Mononuclear Ir(III) Complexes Device

dopant

λELmax (nm)

V (V) a

L (cd m−2) b

1

M1

528

4.3/5.8/7.3

2

HIr

528

3

FIr

500

a

EQE (%) c

CE (cd A−1) c

PE (lm W−1) c

144324 (12.7 V)

16.1/15.6/16.1

61.9/59.1/61.8

37.5/31.7/26.6

4.3/5.8/7.3

97692 (12.5 V)

13.2/12.7/13.1

50.8/48.6/50.5

26.8/26.0/21.5

4.5/6.5/8.8

26805 (12.3 V)

5.3/5.1/4.6

17.0/16.5/15.2

8.3/7.9/5.5

−2 b

Driving voltages (V) in the order of at 1, 1000 and 10000 cd m . The maximum value of L and the voltages at

which they were obtained. c EQE, CE and PE in the order of maximum value and at 1000 and 10000 cd m−2.

25



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

(a) M1:CBP

RMS：：6.42 nm

(b) HIr:CBP

RMS：：6.24 nm

(c) FIr:CBP

RMS：：10.21 nm Figure 9. AFM topographic images of the 8 wt % doped CBP with these three mononuclear Ir(III) complexes. (a) M1:CBP; (b) HIr:CBP; (c) FIr:CBP. Considering its higher thermal stability, better solubility and higher PLQY 26


Page 26 of 38

Page 27 of 38

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


among these dinuclear complexes, FDIr1 is more suitable for fabricating solution-processed devices. Firstly, we spin-coated FDIr1 with CBP and TCTA (4,4′,4″-tris(N-carbazolyl)triphenylamine) at the doping level of 8 wt% on glass substrates respectively to investigate their emission and morphology properties. Excited with UV light, the emission from the CBP is much stronger than that from FDIr1 (Figure 10a and 10b). However, FDIr1 can show very strong emission doped in TCTA (Figure 10a and 10b). It means that the energy transfer process is more efficient from TCTA to FDIr1 than from CBP to FDIr1. In addition, the PLQY of the doped CBP film is ca. 0.29, while that for the doped TCTA film is enhanced to ca. 0.85. The morphological investigation of spin-coated films doped with FDIr1 was performed using AFM. As shown in Figure 10c, 10d and Figure S8, large and deep (ca. 100 nm) pinholes can be clearly detected in the doped CBP film, leading to a very large root-mean-square (RMS) roughness of 29.54 nm. The poor quality of the doped CBP film blocks the energy transfer process.46 In contrast, the doped TCTA film is much smoother with the RMS roughness of 2.12 nm, which benefits the performance of the solution-processed devices.

27



1.0 (a) PL 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

FDIr1:CBP FDIr1:TCTA

0.8 0.6 0.4 0.2 0.0 400

450

500 550 600 Wavelength (nm)

650

(b)

(c) FDIr1:CBP

RMS：：29.54 nm

(d) FDIr1:TCTA

RMS：：2.12 nm

28


Page 28 of 38

Page 29 of 38

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


Figure 10. (a) The PL spectral of FDIr1 doped films at the 8 wt% doping level. (b) Pictures of FDIr1 doped films irradiated at 365 nm. (c) and (d) AFM topographic images of FDIr1 doped films.

Accordingly, we chose TCTA as host to fabricate solution-processed OLEDs with the structure of ITO/PEDOT:PSS (45 nm)/x wt% FDIr1:TCTA (30 nm)/TPBI (45 nm)/LiF (1 nm)/Al (100 nm) (Figure 7b). We also optimized the doping levels to enhance the device performance. Devices 4–6 have the doping levels of 6, 8 and 10 wt%, respectively. It turns out that Device 5 exhibited the best EL performance (Table 5). Figure 11a shows the EL spectrum of Device 5, in which almost no emission from TCTA can be observed, indicating very efficient energy transfer from the host to FDIr1. Moreover, Device 5 shows a low turn-on voltage of 3.2 V with impressive maximum EQE, CE and PE of 17.9%, 52.5 cd A−1 and 51.2 lm W−1, respectively. Even at a high luminance of 1000 cd m−2, the EQE, CE and PE of Device 5 remain as high as 15.5%, 45.5 cd A−1 and 18.7 lm W−1, respectively, indicating the low efficiency roll-off (Figure 11). To the best of our knowledge, the performance of Device 5 should represent the best EL performance ever reported for devices based on dinuclear Ir(III) complexes.24, 29, 30, 39, 47, 48 Although Device 4 and Device 6 show inferior performance (see Figure S9), their high EL efficiencies and low efficiency roll-offs still render them among the best reported devices based on dinuclear Ir(III) complexes. The attractive EL devices achieved by Devices 4–6 can be attributed to several factors. Compared with FDIr1 possessing the HOMO level of –5.45 eV and LUMO level of –2.69 eV, TCTA has a lower HOMO level of –5.7 eV and higher LUMO level of –2.4 eV with triplet energy of 2.76 eV,49 which can effectively confine the electrical generated excitons within the emissive layer and promote the energy transfer process from TCTA to FDIr1. In addition, TCTA shows high glass transition temperature of 151 °C, which can improve the film quality (Figure 10c) upon annealing at 60 °C to avoid side effects of inhibiting energy transfer processes induced by emitter aggregation.46 Furthermore, the 8 wt% FDIr1 doped TCTA film shows a high PLQY of 0.85 and short τp of 0.54 µs. The better charge carrier balance in the 29



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

Page 30 of 38

device due to the bipolar character of FDIr1 (Figure 6) also contributes to the impressive device performance. In brief, the outstanding device performance of these solution-processed OLEDs can be attributed to the favorable characters of the FDIr1 doped TCTA emissive layers such as good film morphology, efficient energy transfer process, good exciton confinement, better charge carrier balance, high PLQY and short τp.

Table 5. EL Data for Solution-Processed OLEDs Based on the Dinuclear Ir(III) Complex FDIr1

a

Device

λELmax (nm)

V (V) a

L (cd m−2) b

EQE (%) c

CE (cd A−1) c

PE (lm W−1) c

4

564

3.4/8.3/10.6

43510 (13.9 V)

16.4/15.5/14.3

48.2/45.6/42.0

42.8/22.2/15.9

5

564

3.2/7.6/10.1

46206 (14.2 V)

17.9/16.8/15.5

52.5/49.6/45.5

51.2/26.1/18.7

6

564

3.4/7.5/10.4

42937 (12.5 V)

13.8/13.5/12.4

40.6/39.7/36.8

33.3/22.3/15.5

Driving voltages (V) in the order of at 1, 1000 and 10000 cd m−2. b The maximum value of L and the voltages at

which they were obtained. c EQE, CE and PE in the order of maximum value and at 100 and 1000 cd m−2.

30


Page 31 of 38

(a)

Device 5

EL intensity (a.u.)

1.0 0.8 0.6 0.4 0.2 0.0

400 450 500 550 600 650 700 750 Wavelength (nm) Devcie 4 Device 5 Device 6

104 103 102

80

101

−

Current density (mA cm− 2)

(b) 160

Luminance (cd m−2)

100

0 4

6

8

10

12

14

Voltage (V)

(c) −1

CE/PE (cd A /lm W )

1

10

0

−1

10 Device 5

10

EQE (%)

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-1

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2

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10 10 −2 Luminance (cd m )

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Figure 11. (a) The EL spectrum of Device 5. (b) The J-V-L relationships of Devices 4–6. (c) Efficiencies vs luminance curve of Device 5.

3. CONCLUSIONS In summary, using 2-phenylpyrimidine-type ligands, we synthesized two mononuclear Ir(III) complexes HIr and FIr accompanied by two corresponding kinds of dinuclear Ir(III) complexes HDIr1 and HDIr2, FDIr1 and FDIr2. The mononuclear Ir(III) complexes HIr and FIr show green emissions (λ = 502 and 526 nm, respectively) with high PLQYs of 0.99. The dinuclear Ir(III) complexes HDIr1 and HDIr2, FDIr1 and FDIr2 show red or orange emissions (λ = 603 and 580 nm, 31



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respectively) with PLYQs up to 0.68 in solutions, indicating the dramatic different photophysical properties between the mononuclear Ir(III) complexes and their corresponding dinuclear Ir(III) complexes. Besides, similar to situations in conventional mononuclear Ir(III) complexes, the photophysical properties of these dinuclear Ir(III) complexes can also be fine-tuned with substituents possessing different electron-withdrawing or -donating characters.

A vacuum-deposited

conventional OLED based on a mononuclear Ir(III) complex with small dipole moment shows the peak luminance, EQE, CE and PE of 144324 cd m−2, 16.1%, 61.9 cd A−1 and 37.5 lm W−1, respectively, with almost no efficiency roll-off even at a high luminance of 10000 cd m−2. Furthermore, the solution-processed device based on the dinuclear Ir(III) complex FDIr1 exhibits an outstanding performance with the peak luminance, EQE, CE and PE of 46206 cd m−2, 17.9%, 52.5 cd A−1 and 51.2 lm W−1, respectively, which is the best performance ever achieved for OLEDs based on dinuclear Ir(III) complexes reported so far. This research work shows that the electron-deficient pyrimidine ring is a good replacer of the pyridine ring for synthesizing transition metal complexes. Especially, 2-phenylpyrimidine-type ligands have great potential in development of both mono- and dinuclear Ir(III) complexes for high performance OLEDs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed synthesis of the pyrimidine-based mono- and dinuclear Ir(III) complexes. NMR, MS, TGA, CV characterization, details of the computational studies, EL performance of Device 3, Device 6 and Device 7. X-ray crystallographic file in CIF format for determination of the structure of FIr and DFIr1.

AUTHOR INFORMATION 32


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Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21572176 and 20902072), the Natural Science Foundation of Shaanxi Province (No. 2016JQ2011), the China Postdoctoral Science Foundation (Nos. 2015M580831 and 20130201110034), the Fundamental Research Funds for the Central Universities (Nos. cxtd2015003 and 01091191320074) and Creative Scientific Research Team in Yulin City.

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