Versatile Design Strategy for Highly Luminescent Vacuum-Evaporable

Article pubs.acs.org/JACS

Versatile Design Strategy for Highly Luminescent VacuumEvaporable and Solution-Processable Tridentate Gold(III) Complexes with Monoaryl Auxiliary Ligands and Their Applications for Phosphorescent Organic Light Emitting Devices Man-Chung Tang, Chin-Ho Lee, Shiu-Lun Lai, Maggie Ng, Mei-Yee Chan,* and Vivian Wing-Wah Yam* Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee, Hong Kong) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China S Supporting Information *

ABSTRACT: A new class of brightly blue-green-emitting arylgold(III) complexes has been synthesized, characterized, and applied as phosphorescent dopants in the fabrication of solution-processable and vacuum-deposited organic lightemitting devices (OLEDs). These arylgold(III) complexes can be readily synthesized by reacting the corresponding arylboronic acids with the gold(III) precursor complexes in a one-pot Suzuki−Miyaura coupling reaction. When compared to the structurally related alkynylgold(III) complex, arylgold(III) complexes 1 and 2 exhibit much higher photoluminescence quantum yields in solution state. High photoluminescence quantum yields are also observed in solid-state thin films. More importantly, the solid-state emission spectra show strong resemblance to those in solution, irrespective of the dopant concentration, leading to significant improvement in the color purity of the OLEDs by suppressing any excimer emission resulting from the π-stacking of the tridentate ligand. High performance solution-processable and vacuum-deposited blue-green-emitting OLEDs have also been realized, with maximum external quantum efficiencies of 7.3% and 14.7%, respectively, representing the first demonstration of efficient blue-greenemitting OLEDs based on cyclometalated arylgold(III) complexes.

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of the d−d ligand field states to improve the luminescence quantum yields of the aryl- and alkylgold(III) complexes.5 Subsequent reports on encouraging performances based on green-, yellow- and red-emitting alkynylgold(III) complexes with bidentate, tridentate and tetradentate ligands have been demonstrated;6 particularly, efficient vacuum-deposited and solution-processable OLEDs with EQEs of up to 11.5% and 11.1%, respectively, have been realized.6a,b Very recently, our research group has developed new classes of bipolar dendritic alkynylgold(III) complexes, in which highly efficient solutionprocessable OLEDs with EQEs of up to 10.0% and extremely small efficiency roll-offs of less than 1% at brightness of 1000 cd m−2 have been demonstrated.6c Most of these alkynylgold(III) complexes are prepared in good yields by reacting the corresponding alkynes with the chlorogold(III) precursor in the presence of a catalytic amount of copper(I) iodide.7 This synthetic method is useful for the incorporation of alkynyl ligands into the cyclometalated gold(III) center; however, this

INTRODUCTION Recent advances in the design and synthesis of phosphorescent metal complexes lead to a leap-forward development of organic light-emitting devices (OLEDs) that holds great potentials to replace other flat panel display technologies for mobile phones, televisions and solid-state lighting applications.1 Extensive efforts have been focused on the iridium(III)2 and platinum(II) systems,1a,3 where the strong spin−orbit coupling associated with the presence of the heavy metal centers can harvest both singlet and triplet excitons to yield an internal quantum efficiency of up to unity. Recently, platinum(II) and iridium(III) complexes with horizontally aligned emitters were reported to show the external quantum efficiencies (EQEs) of over 30%.4 On the other hand, the development of phosphorescent gold(III) emitters, which are isoelectronic to the platinum(II) emitters, is relatively less explored. A probable reason for the limited luminescence behavior in gold(III) species is the presence of low-energy d−d ligand field excited states that would quench the luminescence excited state through thermal equilibration or energy transfer. This limitation was first overcome by our research group via the incorporation of strong σ-donating ligands to raise the energy © 2017 American Chemical Society

Received: May 9, 2017 Published: July 3, 2017 9341

DOI: 10.1021/jacs.7b04788 J. Am. Chem. Soc. 2017, 139, 9341−9349

Article

Journal of the American Chemical Society Scheme 1. Chemical Structures of Cyclometalated Gold(III) Complexes 1−3

c h l o r o g o l d ( I I I ) p r e c u r s o r co m p l e x , [ A u - ( 3 , 5 - F− C∧NPh∧CtBu)Cl] with the corresponding boronic acids.9 The alkynylgold(III) complex 3 was prepared according to the literature procedure.7,10 The identities of the complexes have also been confirmed by 1H and 13C NMR spectroscopy, FAB mass spectrometry, IR spectroscopy, and elemental analyses. The IR spectrum of complex 3 features a weak band at 2154 cm−1, corresponding to the ν(CC) stretching frequency. All the complexes have been isolated as thermally stable pale yellow solids with high decomposition temperatures (Td) of >300 °C (see Figure S1). Photophysical Properties. The UV−vis absorption and emission spectra of 1−3 in dichloromethane at 298 K are shown in Figure 1a and Figure 1b, respectively. All the complexes show intense absorption bands at ca. 280−320 nm

synthetic methodology is rather limited for the synthesis of other classes of organogold(III) complexes. Early work by us has utilized trans-metalation reaction using Grignard reagent for the preparation of the luminescent aryland alkylgold(III) diimine complexes in moderate to good yields in one steps.5 In 2012, Bochmann and co-workers reported the synthesis of the cyclometalated hydroxide complex by reacting the gold(III) hydroxide with arylboronic acids.8 This approach provides a versatile synthon for producing a range of perfluoroarylgold(III) complexes with emission spanning from yellow to blue. However, the synthetic route involves at least two synthetic steps to obtain the target complexes.8 Apart from the use of Grignard reagents reported by Yam and co-workers,5 Gray and co-workers recently reported the Suzuki−Miyaura coupling of arylboronic acids to bidentate ligand-containing chlorogold(III) precursor.9 Both monoarylated intermediates and diarylated complexes can be obtained by these one-step reactions. In the search for versatile synthetic routes, herein we report the design and synthesis of a new class of tridentate ligand-containing arylgold(III) complexes 1 and 2 by directly reacting the chlorogold(III) precursor, [Au(3,5-F−C∧NPh∧CtBu)Cl], with the corresponding 3,5-difluorophenyl boronic and 4-tert-butylphenylboronic acid under the Suzuki−Miyaura coupling condition. Complex 3, an alkynylgold(III) analogue of 2 but with the presence of the CC bond, has also been synthesized for comparison study (see Scheme 1). Unlike the alkynylgold(III) analogue 3 that shows lower emission quantum yield in solution on the order of 10−3, it is interesting to note that both complexes 1 and 2 are capable of emitting intense blue-green emission in both solution or solid-state thin films. Solution-processed and vacuum-deposited OLEDs based on 2 have been demonstrated with EQEs of 5.9% and 14.7%, respectively. Notably, this represents the first demonstration of efficient blue-greenemitting OLEDs based on cyclometalated gold(III) complexes. Taking advantages of the diverse library of boronic acids that is commercially available, this work may open up a simple method for the design and synthesis of new classes of arylgold(III) complexes with emission colors spanning the entire visible spectrum.

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RESULTS AND DISCUSSION Synthesis of Gold(III) Complexes. The synthetic route and molecular structures of complexes 1−3 are shown in Scheme S1 and Scheme 1, respectively. Complexes 1 and 2 were synthesized using Suzuki coupling reaction of the

Figure 1. (a) UV−vis absorption spectra in CH2Cl2 and (b) emission spectra in degassed CH2Cl2 of complexes 1−3 at 298 K. 9342


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Journal of the American Chemical Society Table 1. Photophysical Properties of Complexes 1−3 complex

medium (T/K)

absorption λmax/nm (εmax/dm3 mol−1cm−1)

emission λmax/nm (τo/μs)

1

CH2Cl2 (298) Glass (77)c Thin film (298) 5% in MCP 10% in MCP 15% in MCP 20% in MCP CH2Cl2 (298) Glass (77)c Thin film (298) 5% in MCP 10% in MCP 15% in MCP 20% in MCP CH2Cl2 (298) Glass (77)c Thin film (298) 5% in MCP 10% in MCP 15% in MCP 20% in MCP

292 (51365), 360 (2980), 380 (4710), 400 (4330)

476, 508, 548 (16.3) 470, 504, 542 (225)

2

3

291 (43780), 330 (8140), 380 (3635), 400 (2535)

294 (44170), 370 (5955), 390 (4620), 410 (3185)

484, 484, 484, 484, 476, 470,

516, 516, 516, 516, 508, 504,

556 (108) 556 556 556 548 (45.8) 542 (187)

482, 482, 482, 482, 480, 470,

514, 514, 514, 514, 512, 510,

552 (86) 552 552 552 550 (0.5) 550 (67)

490, 490, 490, 490,

522, 522, 522, 522,

556 (68) 556 556 556

Φsola

Φfilmb

0.08

0.30 0.31 0.31 0.25 0.07

0.41 0.38 0.35 0.31 4 × 10−3

0.38 0.27 0.29 0.24

a The luminescence quantum yield, measured at room temperature using quinine sulfate in 0.5 M H2SO4 as the reference (excitation wavelength = 365 nm, Φlum = 0.546). bΦfilm of gold(III) compound doped into MCP excited at wavelength of 320 nm. cMeasured in EtOH−MeOH−CH2Cl2 (40:10:1, v/v).

C∧NPh∧CtBu ligand with charge transfer character from the aryl ring to the pyridine unit of the C∧N∧C ligand, mixed with some LLCT transition from the aryl ligand to the pyridine unit of the C∧N∧C ligand. Upon excitation at λ ≥ 350 nm in dichloromethane solution at 298 K, the emission energies are slightly blue-shifted from 480 nm in 3 to 476 nm in 1 and 2, in good agreement with the UV−vis studies. On the basis of the UV−vis studies and previous spectroscopic work,7,10 the emission bands of 1 and 2 have been assigned as metalperturbed 3IL [π → π*(3,5-F−C∧NPh∧CtBu)] state, with some aryl-to-pyridine 3ILCT character,7,10 while the vibronicstructured band of 3 has been assigned to a mixture of metalperturbed 3IL [π → π*(3,5-F−C∧NPh∧CtBu)] state with some aryl-to-pyridine 3 ILCT character and 3 LLCT [π(C CC6H4tBu-p) → π*(3,5-F−C∧NPh∧CtBu)] state. The change in the emission origin can also be revealed by the longer emission lifetimes of 1 and 2, that is, from the relative long-lived 3IL state of 1 (16.3 μs) and 2 (45.8 μs) to that of 3 IL/3LLCT state of 3 (0.5 μs). In addition, the luminescence quantum yields of complexes 1 (Φsol = 0.08) and 2 (Φsol = 0.07) in solution are much higher than that of 3 (Φsol = 0.004). This could be attributed to a closer proximity between the aryl auxiliary ligand and the 3,5-F−C∧NPh∧CtBu moiety in the arylgold(III) complexes that locks the aryl ligand in a twisted conformation, reducing its rotational degree of freedom and slowing down the nonradiative decay pathway. Such twisting of the aryl auxiliary ligands would also disfavor the aryl-to-pyridine (C∧N∧C) LLCT contribution to the transition and the excited states in 1 and 2. Interestingly, the luminescence quantum yields of all the complexes in the doped N,N′-dicarbazolyl-3,5benzene (MCP) thin films are similar (Table 1). This further supports that the enhancement of the luminescence quantum yields of the arylgold(III) complexes in the solution state is mainly due to the reduced rotational degree of freedom arising from this steric effect. It is worth noting that all the complexes

with a moderately intense vibronic-structured band at ca. 380− 410 nm with extinction coefficients (ε) on the order of 104 dm3 mol−1 cm−1. Table 1 summarizes the photophysical data of 1− 3. Upon replacing the 4-tert-butylphenyl auxiliary ligand of 2 by 4-tert-butylphenylalkynyl in 3, a slight red shift in the low-lying absorption bands at ca. 380 and 400 nm in 2 to ca. 390 and 410 nm in 3 is observed. This could probably be rationalized by the good π-donating ability of the alkynyl ligand, which gives rise to high-lying filled π(CCAr) orbitals as well as causes the destabilization of the filled dπ(Au) orbitals through effective pπ−dπ overlap between the tert-butylphenylalkynyl ligand and the metal center. Mixing of the π-donor character of the alkynyl ligand also causes an extension of the π-conjugation length of the diphenylpyridine ligand through the gold center. Such effects would result in the destabilization of the filled π orbitals on the C∧N∧C ligand and the stabilization of the π*(C∧N∧C) orbitals. These would lead to the narrowing of the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) energy gap, given the mixed π(CCAr)/dπ(Au)/π(C∧N∧C) nature of the HOMO and the π*(C∧N∧C) character of the LUMO, leading to the redshifted absorption band of π(CCAr) → π*(C∧N∧C) ligandto-ligand charge transfer (LLCT)/π→ π*(C∧N∧C) intraligand (IL) transition with aryl-to-pyridine intraligand charge transfer (ILCT) character. Such assignment is found to be consistent with the results from the density functional theory (DFT), time-dependent DFT (TDDFT) and emission and electrochemical studies (vide infra). Interestingly, upon replacing the 4-tert-butylphenyl group in 2 by fluoro substituents at the 3,5positions of the phenyl ring in 1 has led to negligible effect on the absorption energies. Considering the vibrational progressional spacings in the range of 1300−1400 cm−1, typical of skeletal vibrational modes of the diarylpyridine ligands, the lowenergy absorption bands have been tentatively assigned as metal-perturbed IL π → π* transition of the 3,5-F− 9343


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Journal of the American Chemical Society

oxidation. On the contrary, the first oxidation wave of 3 (i.e., +1.50 V vs SCE) is assigned as an alkynyl ligand-centered oxidation, similar to that of the previously reported structurally related alkynylgold(III) complexes.6e,7 Computational Studies. DFT and TDDFT calculations have been performed in order to provide further insights into the electronic structures and the origins of the electronic transitions of 1−3. The optimized ground-state (S0) geometries of 1−3 with selected structural parameters are shown in Figure 3. Interestingly, it is observed that the aryl ring in the auxiliary ligand in 3 is coplanar with the C∧N∧C ligand while those in 1 and 2 are orthogonal to the plane of the C∧N∧C ligand. This is probably due to the steric interactions between the hydrogen atoms on the aryl ring and the fluorine and hydrogen atoms on the C∧N∧C ligand in 1 and 2. The presence of alkynyl group in 3 not only reduces this steric repulsion, but also provides the π−donating ability to enhance the pπ−dπ overlap between the phenylalkynyl unit and the metal center, with extended πconjugation with the C∧N∧C ligand, rendering the phenyl ring in the auxiliary ligand to be able to lie on the same plane as the C∧N∧C ligand, further supporting the observation of the red shift in the absorption and emission maxima from 1 and 2 to 3. The first ten singlet−singlet transitions of 1−3, computed by the TDDFT/CPCM method, are listed in Table S2 and the frontier molecular orbitals involved in these transitions are shown in Figures S3−S5. The S0 → S1 transition is computed to show a red shift in energy from 1 ≈ 2 to 3, which is in good agreement with the trend observed in the low-lying absorption band. The transitions with significant oscillator strengths computed in the region of the low-energy absorption bands (ca. 380 nm) are mainly contributed by the HOMO → LUMO and HOMO−1 → LUMO excitations in 2 and 3 and by the HOMO → LUMO excitation only in 1. The HOMO−1 in 2 and 3 is predominantly the π orbital of the C∧N∧C ligand mixed with a metal dπ orbital, and their HOMO is predominantly the π orbital of the auxiliary ligand mixed with a metal dπ orbital, whereas the HOMO in 1 is predominantly the π orbital of the C∧N∧C ligand mixed with a metal dπ orbital. The LUMO of 1−3 is the π* orbital on the pyridine moiety of the C∧N∧C ligand and therefore the low-lying absorption band of 1−3 can be assigned as metal-perturbed IL[π → π*(C∧N∧C)] transition with ILCT from the aryl-topyridine of C∧N∧C, with some mixing of LLCT[π(auxiliary) → π*(C∧N∧C)] transition in 2 and 3, supporting the spectral assignments of the low-lying absorption bands. The orbital energy diagrams of the frontier molecular orbitals of 1−3 are shown in Figure 4. The trend of the orbital energies is in line with that observed in both the UV−vis absorption trend as well as in the cyclic voltammetric studies, in which 1 is the most difficult to be oxidized and 3 is the easiest to be oxidized, whereas 2 is the most difficult to be reduced and 3 is the easiest to be reduced. The HOMO is stabilized by −0.26 eV with increasing electron-withdrawing ability of the aryl ligand upon going from 2 to 1, and with the introduction of the alkynyl group in the auxiliary ligand, the LUMO is stabilized by −0.17 eV in 3, when compared to that in 2. In order to gain more insights into the nature of the emissive states of complexes 1−3 and the structural changes from their corresponding ground states, geometry optimizations of the lowest-lying triplet excited states (T1) have been performed with the unrestricted UPBE0 method. Selected changes in the structural parameters of the T1 states of 2 and 3 relative to their corresponding ground states are shown in Figure S6. The major

feature vibronic-structured emission bands with peak maxima at ca. 480−490 nm in MCP thin films (Figure S2). Unlike the other structurally related gold(III) complexes,6a,d,e the emission energies are found to be independent of the dopant concentration in MCP thin films, without the presence of the undesirable excimer emission arising from the π−π stacking of the C∧N∧C ligand. In view of these, the present alkynyl- and arylgold(III) complexes are anticipated to be promising phosphorescent dopants for blue-green-emitting OLEDs with good color purity. Electrochemistry. The cyclic voltammetry of 1−3 in dichloromethane (0.1 mol dm−3 nBu4NPF6) has been investigated. Estimation of their HOMO and LUMO energy levels has also been made. In general, a quasi-reversible reduction couple at −1.36 V to −1.51 V vs saturated calomel electrode (SCE), and an irreversible oxidation wave at +1.50 V to +1.98 V vs. SCE are found for 1−3, respectively. The electrochemical data of 1−3 are summarized in Table S1, and their cyclic voltammograms are shown in Figure 2. With reference to our

Figure 2. Cyclic voltammograms for the (a) oxidation and (b) reduction scans of 1−3 in dichloromethane (0.1 M nBu4NPF6).

previous studies,6e,7 the first reduction is attributed to the ligand-centered reduction of the cyclometalated ligand. In parallel to the photophysical studies, the use of 4-tertbutylphenylalkynyl as the auxiliary ligand could cause a stabilization of the π* orbital of the 3,5-F−C∧NPh∧CtBu, allowing 3 to be reduced at a less negative reduction potential than those of 1 and 2, [i.e., 1 (−1.47 V vs SCE), 2 (−1.51 V vs SCE), 3 (−1.36 V vs SCE)], while the first oxidation waves for 1 and 2 are found to be sensitive to the nature of the aryl ligands. The weaker electron-donating ability of the −C6H3F2 moiety in 1 than that of the −C6H4−tBu-p moiety in 2 has led to a more positive potential of the oxidation wave of 1 [i.e., 1 (+1.98 V vs SCE) and 2 (+1.74 V vs SCE)]. This irreversible anodic wave has been assigned as an aryl ligand-centered 9344


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Figure 4. Orbital energy diagram of the frontier molecular orbitals (H = HOMO and L = LUMO) of 1−3.

Figure S7 are the plots of the spin density of their emissive states. The spin density in 3 is mainly localized on both the C∧N∧C ligand and the auxiliary ligand, as well as the metal center, supporting an admixture of 3IL and 3LLCT character with a slight perturbation from the metal center in the T1 state, i.e., metal-purtubed 3IL and 3LLCT state. For 1 and 2, the spin density is mainly localized on the C∧N∧C ligand and the metal center and this supports that their T1 states have a 3IL character with some perturbation from the metal center. Listed in Table 2 are the calculated emission energies of complexes 1−3, which Table 2. Relative Energies of the Optimized Triplet Excited States of 1−3 complex

ΔE(T1 − S0)/cm−1 (λ/nm)a

1 2 3

21683 (461) 21734 (460) 21404 (467)

a Energy difference between the triplet excited state and the ground state at the corresponding optimized geometry in dichloromethane solution.

are approximated from the energy difference between the S0 and T1 states at their corresponding optimized geometries in dichloromethane solution. The emission wavelength of the metal-perturbed 3IL/3LLCT state in 3 (467 nm) is computed to be slightly red-shifted relative to the metal-perturbed 3IL states in 1 (461 nm) and 2 (460 nm), and this is in line with the trend observed in the emission spectra. OLED Device Fabrication and Characterization. Taking advantages of high solubilities of the gold(III) complexes, solution-processable devices with the configuration of indium tin oxide (ITO)/poly(ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS; 70 nm)/x % Au(III):MCP (60 nm)/tris(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB; 5 nm)/1,3,5-tri[(3-pyridyl)phen-3-yl]benzene (TmPyPB; 30 nm)/LiF (0.8 nm)/Al (100 nm) have been fabricated. 3TPYMB and TmPyPB are used as the holeblocking and electron-transporting layers, respectively. The emissive layer was prepared by spin-coating a solution of Au(III):MCP blend at different concentrations in dichloromethane. Figure 5 depicts the normalized electroluminescence (EL) spectra of the solution-processable OLEDs based on 1−3. All devices exhibit vibronic-structured emission, and the EL spectra for all the devices are almost identical to their emission

Figure 3. Selected structural parameters of the ground-state geometries of 1−3 optimized at the PBE0 level of theory.

geometrical changes are found to mainly occur in 3, in which the phenyl ring in the auxiliary ligand becomes orthogonal, instead of being coplanar in the ground state, to the plane of the C∧N∧C ligand. However, 1 and 2 retain the orthogonality of the planes between the phenyl ring and the C∧N∧C ligand in both their S0 and T1 states due to the restricted rotation of the auxiliary ligand. This is also in line with the much higher emission quantum yields of 1 and 2 than 3 in the solution state when 3 shows a much larger excited state structural distortion. 9345


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Figure 5. EL spectra of solution-processable devices based on gold(III) complexes 1−3.

spectra in solution without any undesirable excimeric emission. Remarkably, the Commission International de L’Éclairage (CIE) coordinates of all the devices are almost constant over a wide range of dopant concentrations. Particularly, there are only small changes of ±0.01 in the CIE x and y values for devices doped with 1 and 2 even when the dopant concentrations have been increased to 20 wt %. Similarly, the CIE coordinates of devices made with 3 slightly changes from (0.21, 0.50) to (0.23, 0.51) upon increasing the dopant concentration from 5 wt % to 20 wt %. This is not the case for most of the other related square-planar cyclometalated Au(III) complexes, in which the EL spectra are found to show significant spectral shifts upon dopant aggregation to give a broad red-shifted excimer emission.6c,f These concentrationindependent EL properties are extremely valuable for the precise control of the emission energies of the gold(III) complexes via the modification of the C∧N∧C ligands as well as the color purity of the OLEDs. In addition to the excellent color purity, all the solutionprocessable devices have been found to show promising performance (see Figure 6). Table 3 summarizes the key parameters of devices made with 1−3. Interestingly, both devices with and without CC bond (i.e., 2 and 3) exhibit similar performance in terms of current efficiencies and EQEs. Maximum current efficiencies and EQEs of the optimized devices derived from 2 and 3 both are ∼15.5 cd A−1 and ∼5.9%, respectively. These efficiency values are in good agreement with their photoluminescence quantum yields (PLQYs), both of which are within 30−40% in solid-state thin films. Surprisingly, devices made with 1 are higher than those of 2 and 3, despite all the complexes having similar PLQYs in MCP thin films. Specifically, maximum current efficiency and EQE of the optimized device with 1 are 19.5 cd A−1 and 7.3%, respectively.

Figure 6. EQEs of solution-processable devices based on gold(III) complexes 1−3.

Table 3. Key Parameters of Solution-Processable Devices Made with 1−3 complex 1

2

3

a

dopant concentration (wt %)

max. current efficiency (cd A−1)

max. power efficiency (lm W−1)

max. EQE (%)

5 10 15 20 5 10 15 20 5 10 15 20

11.2 17.1 19.5 14.6 6.4 15.5 9.0 11.0 10.8 15.5 10.9 8.0

4.7 9.8 12.2 11.5 3.1 8.1 5.7 7.7 6.0 8.1 6.8 5.0

4.3 6.5 7.3 5.4 2.5 5.9 3.4 4.1 4.1 5.9 4.0 2.9

CIE (x, y)a 0.21, 0.22, 0.22, 0.22, 0.22, 0.22, 0.23, 0.23, 0.21, 0.21, 0.21, 0.23,

0.48 0.48 0.49 0.49 0.47 0.47 0.48 0.48 0.50 0.50 0.51 0.51

CIE coordinates are taken at a current density of 10 mA cm−2.

The performance improvement may be ascribed to the incorporation of more fluorine atoms to improve the electron-transporting property of complex 1, resulting in a better balance in the hole and electron currents at the emissive interface and thus higher device efficiencies.11 It should be highlighted that this new class of arylgold(III) complexes is capable to serve as phosphorescent dopants in the fabrication of vacuum-deposited OLEDs. Particularly, vacuum-deposited devices based on 2 with the configuration of ITO/molybdenum 9346


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phosphorescent dopants for both solution-processable and vacuum-deposited OLEDs.

oxide (MoO3; 2 nm)/4,4′-cyclohexylidenebis[N,N-bis(4methylphenyl)aniline] (TAPC; 40 nm)/x % 2:MCP (25 nm)/diphenyl-4-triphenylsilylphenyl-phosphine oxide (TSPO1; 5 nm)/1,3,5-tris(6(3-(pyridin-3-yl)phenyl)pyridine2-yl)benzene (Tm3PyP26PyB; 35 nm)/LiF (1 nm)/Al (100 nm) have been prepared by thermal evaporation, in which MoO3, TAPC, TSPO1, Tm3PyP26PyB are used as holeinjection, hole-transporting, hole-blocking, electron-transporting layers, respectively. Notably, efficient OLEDs with high current and power efficiencies of up to 38.1 cd A−1 and 39.9 lm W−1, respectively, can be obtained. These correspond to a maximum EQE of 12.8% (Figure 7a). Device performance can

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CONCLUSION A new class of highly luminescent arylgold(III) complexes has been successfully designed and synthesized. These arylgold(III) complexes can be simply synthesized by reacting the corresponding arylboronic acids with the gold precursor complexes via one-step reaction. As compared to the alkynylgold(III) analogues, the arylgold(III) complexes exhibit high emission quantum yields in both solution and solid-state thin films. Unlike most of the square-planar metal complexes, both photoluminescence and EL spectra are found to be concentration-independent and no excimer emission can be observed even at high dopant concentrations of up to 20 wt %. In addition, high-performance solution-processable and vacuum-deposited OLEDs can be realized; particularly, vacuumdeposited OLEDs made with 2 demonstrate superior EL performance with high EQE of 14.7%. Taking advantages of the diverse library of boronic acids that is commercially available, this work opens up a simple method for the design and synthesis of a new class of arylgold(III) complexes with emission colors spanning the entire visible spectrum.

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Figure 7. EQEs of vacuum-deposited devices made with 2.

be further boosted up to 41.5 cd A−1, 43.5 lm W−1 and 14.7% by using 2,6-di(9H-carbazol-9-yl)pyridine (PYD-2Cz) as host (Figure 7b and Table 4). The efficiency enhancement is believed to be due to the higher triplet energy of PYD-2Cz that provides a high triplet exciton confinement effect for the blue emitter. More importantly, the CIE x and y values are kept almost constant upon increasing dopant concentration from 2 wt % to 11 wt %. These findings clearly demonstrate that the present arylgold(III) complexes are potential candidates as Table 4. Key Parameters of Vacuum-Deposited Devices Made with 2 host MCP

PYD2Cz

a

dopant concentration (wt %)

max. current efficiency (cd A−1)

max. power efficiency (lm W−1)

max. EQE (%)

2 5 8 11 2

19.3 21.7 25.4 38.1 23.6

17.4 19.5 22.8 39.9 21.2

7.5 8.5 9.3 12.8 9.0

0.20, 0.20, 0.21, 0.21, 0.21,

5 8 11

24.5 36.3 41.5

22.0 38.0 43.5

8.9 12.6 14.7

0.21, 0.50 0.21, 0.50 0.22, 0.51

EXPERIMENTAL SECTION

Material and Reagents. The tridentate ligand, 3,5-F− C∧NPh∧CtBu, and the gold(III) precursor complex, [Au(3,5-F− C∧NPh∧CtBu)Cl], were prepared according to a modification of a procedure reported in the literature.6e,12 Benzaldehyde, 3,5-difluoroacetophenone, 4-tert-butylacetophenone, 4-tert-butylphenylacetylene, 4-tert-butylboronic acid and 3,5-difluorophenyl boronic acid were purchased from Sigma-Aldrich or Matrix Scientific. All solvents were purified and distilled using standard procedures before use. All other reagents were of analytical grade and were used as received. Triethylamine was distilled over calcium hydride before use. Tetra-nbutylammonium hexafluorophosphate (Aldrich, 98%) was recrystallized for no less than three times from hot absolute ethanol prior to use. All reactions were performed under anaerobic and anhydrous conditions using standard Schlenk techniques under an inert atmosphere of nitrogen. Physical Measurements and Instrumentation. The UV−vis absorption spectra were recorded on a Cary 60 UV−vis (Agilent Technology) spectrophotometer equipped with a Xenon flash lamp. 1 H and 13C NMR spectra were recorded on a Bruker Avance 600 (600 MHz for 1H or 150 MHz for 13C{1H} nuclei) Fourier-transform NMR spectrometer with chemical shifts reported relative to tetramethylsilane (δ = 0 ppm) in chloroform. 19F{1H} NMR spectra were recorded on a Bruker Avance 400 (376 MHz for 19F nucleus) Fourier-transform NMR spectrometer with chemical shifts reported relative to trifluoroacetic acid (δ = −76.55 ppm) in chloroform. Positive FAB mass spectra were recorded on a Thermo Scientific DFS High Resolution Magnetic Sector Mass Spectrometer. IR spectra were recorded as KBr disk on a Bio-Rad FTS-7 FTIR spectrometer (4000− 400 cm−1). Elemental analyses were performed on the Carlo Erba 1106 elemental analyzer at the Institute of Chemistry, Chinese Academy of Sciences in Beijing. Steady-state excitation and emission spectra were recorded on a Horiba Scientific FluoroMax-4 fluorescence spectrofluorometer equipped with a R928P PMT detector. Liquid nitrogen was placed into the quartz-walled optical Dewar flask for low temperature (77 K) photophysical measurements. Solid-state photophysical measurements were performed with solid sample loaded into a quartz tube inside a quartz-walled Dewar flask. Low temperature (77 K) photophysical measurements were done by placing liquid nitrogen into the optical Dewar flask. Excited-state lifetimes of solution and glass samples were measured with a conventional laser system. The excitation source used was the 355 nm output (third harmonic, 8 ns) of a Spectra-Physics Quanta-Ray Q-

CIE (x, y)a 0.49 0.49 0.50 0.50 0.50

CIE coordinates are taken at a luminance of 100 cd m−2. 9347


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Journal of the American Chemical Society switched GCR-150 pulsed Nd:YAG laser (10 Hz). Luminescence decay signals were recorded by a Hamamatsu R928 photomultiplier tube, recorded on a Tektronix model TDS-620A (500 MHz, 2 GSs−1) digital oscilloscope, and analyzed with a program for exponential fits. Relative photoluminescence quantum yields in solution were measured by the optical dilute method reported by Demas and Crosby.13 An aqueous solution of quinine sulfate in 1.0 N H2SO4 has been used as the reference (Φlum = 0.546, excitation wavelength at 365 nm),13 whereas absolute photoluminescence quantum yields in thin flims were measured on a Hamamatsu C9920−03 absolute PLQY measurement system. Excited-state lifetimes of thin films were measured on a Quantaurus-Tau C11367−34 fluorescence lifetime spectrometer. Cyclic voltammetry was performed with a CH Instruments Model CHI620E (CH Instruments, Inc.). All solutions for electrochemical measurements were purged with prepurified argon gas prior to measurement. Thermal analyses were performed on a TGA Q50 (TA Instruments), in which Td is defined as the temperature at which the sample shows a 5% weight loss. DFT and TDDFT Computational Calculations. All calculations were carried out with the Gaussian 09 program suite.14 The groundstate (S0) geometries of complexes 1−3 were fully optimized in dichloromethane by DFT method with the hybrid Perdew, Burke, and Ernzerhof (PBE0) functional,15 in conjunction with the conductor-like polarizable continuum model (CPCM).16 On the basis of the optimized S0 geometries, TDDFT method17 at the same level associated with CPCM (CH2Cl2) was employed to compute the singlet−singlet transitions. To gain more insight into the emissive states, unrestricted UPBE0/CPCM (CH2Cl2) method was employed to optimize the geometries of the lowest-lying triplet excited states (T1). Vibrational frequency calculations were performed on all stationary points to verify that each was a minimum (NIMAG = 0) on the potential energy surface. For all the calculations, the Stuttgart effective core potentials (ECPs) and the associated basis set were utilized to describe Au18 with f-type polarization functions (ζ = 1.050),19 while the 6-31G(d,p) basis set20 was applied for all other atoms. All DFT and TDDFT calculations were performed with a pruned (99,590) grid for numerical integration. OLED Fabrication and Characterization. Solution-processable OLEDs were fabricated on patterned ITO coated glass substrates with a sheet resistance of 30 Ω per square. The substrates were cleaned with Decon 90, rinsed with deionized water, dried in an oven, and finally treated in an ultraviolet-ozone chamber. A 70 nm thick PEDOT:PSS layer was spin-coated onto the ITO coated glass substrates as holetransporting layer. After that, the emissive layer was formed by mixing the gold(III) complex with MCP to prepare a 10 mg cm−3 solution in chloroform and spin-coated onto the PEDOT:PSS layer to give uniform thin films of 60 nm thickness. Onto this, a 5 nm thick 3TPyMB and a 30 nm thick TmPyPB were evaporated as a holeblocking layer and an electron-transporting layer, respectively; while LiF/Al was used as the metal cathode. For vacuum-deposited OLEDs, sequential thermal evaporation of MoO3, TAPC, emissive layer, TSPO1, Tm3P26PyB, LiF and Al were made onto the ITO substrate. All films were sequentially deposited at a rate of 0.1−0.2 nm s−1 without vacuum break. A shadow mask was used to define the cathode and to make four 0.1 cm2 devices on each substrate. Current density− voltage−luminance characteristics and EL spectra were measured simultaneously with a programmable Keithley model 2420 power source and a Photoresearch PR-655 spectrometer. Synthesis and Characterization of Gold(III) Complexes. Complexes 1 and 2 were synthesized by the reaction of the [Au(3,5-F−C∧NPh∧CtBu)Cl] with the corresponding arylboronic acids in the presence of a catalytic amount of palladium(II) catalyst in aqueous base and organic solvent. To a two-necked flask containing [Au(3,5-F−C∧NPh∧CtBu)Cl] (0.2 g, 0.32 mmol), K2CO3 (0.55 g, 0.43 mmol), Pd(OAc)2 (7.2 mg, 0.032 mmol), [HP(tBu)3]BF4 18.5 mg, 0.064 mmol) and the respective boronic acid (0.64 mmol) were added degassed toluene and H2O (4:1, v/v) and the mixture was stirred at 80 °C for 12 h under a nitrogen atmosphere. After removing the solvent, the crude product was purified by column chromatography on silica gel (70−230 mesh) using hexane-dichloromethane (6:1, v/v) as the

eluent. Subsequent recrystallization by diffusion of diethyl ether vapor into a concentrated dichloromethane solution gave the product as a pale yellow solid. Complex 1: Yield: 68 mg, 40%. 1H NMR (600 MHz, CDCl3, 298 K, relative to Me4Si, δ/ppm) δ 7.71−7.68 (m, 3H, −py and −C6H5 of C∧N∧C), 7.60−7.54 (m, 5H, −C6H5 and −C6H3− of C∧N∧C), 7.27 (dd, J = 8.0 and 2.0 Hz, 1H, −C6H3− of C∧N∧C), 7.22 (dd, J = 8.8 and 2.0 Hz, 1H, −C6F2H2− of C∧N∧C), 7.16 (dd, J = 7.2 and 1.8 Hz, 2H, −C6F2H3), 7.09 (d, J = 1.8 Hz, 1H, −C6H3− of C∧N∧C), 6.73−6.70 (m, 1H, −C6F2H2− of C∧N∧C), 6.69−6.63 (m, 1H, −C6F2H3), 1.21 (s, 9H, −tBu). 13C{1H} NMR (150 MHz, THFd8, δ/ppm) δ 168.76, 168.68, 167.15, 167.07, 164.71, 164.68, 164.48, 164.40, 164.21, 164.19, 163.09, 163.07, 162.99, 162.86, 162.78, 156.22, 155.56, 153.58, 253.52, 153.46, 153.40, 148.70, 148.68, 148.40, 148.38, 147.67, 147.62, 132.22, 131.37, 130.12, 130.07, 128.58, 128.56, 128.51, 128.48, 128.47, 126.38, 124.66, 117.09, 117.07, 116.97, 116.94, 116.55, 110.25, 110.23, 110.10, 110.08, 106.82, 106.66, 106.59, 106.44, 99.92, 99.75, 35.86, 31.41. 19F{1H} NMR (376 MHz, CDCl3, 298 K, relative to CF3COOH, δ/ppm) δ −113.23, −112.37, −85.56. Positive FABMS m/z 708.2 [M]+. Elemental analyses: Found (%): C 56.22, H 3.37, N 2.05. Calcd for AuC33H24NF4: C 56.02, H 3.41, N 1.98. Complex 2: Yield: 73 mg, 34%. 1H NMR (600 MHz, CDCl3, 298 K, relative to Me4Si, δ/ppm) δ 7.80−7.78 (m, 3H, −py and −C6H5 of C∧N∧C), 7.73 (d, J = 2.0 Hz, 1H, −py), 7.63−7.54 (m, 6H, −C6H4−tBu-p, −C6H5 and −C6H3− of C∧N∧C), 7.34−7.27 (m, 4H, −C6H4−tBu-p, −C6H3− and −C6F2H2− of C∧N∧C), 7.02 (d, J = 2.0 Hz, 1H, −C6H3− of C∧N∧C), 6.79−6.74 (m, 1H, −C6F2H2− of C∧N∧C), 1.37 (s, 9H, −tBu), 1.17 (s, 9H, −tBu of C∧N∧C). 13C{1H}NMR (150 MHz, CDCl3, relative to Me4Si, δ/ppm) δ 168.28, 168.19, 166.35, 166.26, 163.99, 163.97, 163.27, 163.20, 161.65, 161.63, 161.35, 161.26, 155.01, 152.06, 151.99, 151.91, 151.84, 148.33, 148.30, 147.96, 147.93, 146.91, 141.66, 140.14, 140.11, 132.92, 132.13, 125.16, 124.84, 123.63, 117.14, 116.61, 108.73, 108.82, 108.80, 108.65, 108.62, 106.51, 106.32, 106.24, 106.06, 35.22, 34.25, 31.51, 30.89. 19F{1H} NMR (376 MHz, CDCl3, 298 K, relative to CF3COOH, δ/ppm) δ −113.16, −85.64. Positive FAB-MS m/z 727.9 [M]+. Elemental analyses: Found (%): C 61.31, H 4.92, N 2.00. Calcd for AuC37H34NF2: C 61.07, H 4.70, N 1.92. [Au(3,5-F−C∧NPh∧CtBu)(CCC6H4tBu-p)] (3). This was synthesized according to modification of a literature procedure for bis(cyclometalated) diarylpyridine alkynylgold(III) complexes reported previously.6e,12 Yield: 110 mg (36%). 1NMR (600 MHz, CDCl3, 298 K, relative to Me4Si, δ/ppm) δ 8.17 (d, J = 1.6 Hz, 1H, −C6H3− of C∧N∧C), 7.71 (d, J = 8.0 Hz, 2H, −C6H5 of C∧N∧C), 7.50−7.43 (m, 8H, −C6H4−tBu-p, −py, −C6H5 and −C6H3− of C∧N∧C), 7.33 (d, J = 8.2 Hz, 2H, −C6H4−tBu-p), 7.28−7.25 (m, 1H, −C6H3− of C∧N∧C), 7.08 (d, J = 8.8 Hz, 1H, −C6F2H2− of C∧N∧C), 6.72−6.69 (d, J = 8.8 Hz, 1H, −C6F2H2− of C∧N∧C), 1.35 (s, 18H, −tBu). 13C{1H} NMR (150 MHz, CDCl3, δ/ppm) δ 166.91, 166.83, 165.29, 165.21, 164.56, 163.02, 162.99, 162.84, 162.82, 161.58, 161.57, 161.44, 161.37, 155.36, 155.19, 151.18, 151.12, 151.06, 151.00, 149.76, 147.47, 147.45, 147.16, 145.44, 137.19, 132.30, 131.64, 130.40, 129.29, 127.46, 127.40, 125.15, 124.98, 124.92, 123.96, 123.68, 116.23, 115.65, 115.49, 108.92, 108.90, 108.77, 106.65, 106.50, 106.44, 106.28, 101.98, 86.02, 35.44, 34.66, 31.30, 31.14. 19F{1H} NMR (376 MHz, CDCl3, 298 K, relative to CF3COOH, δ/ppm) δ −112.27, −85.26. Positive FAB-MS m/z 751.6 [M]+. IR (KBr) 2154 cm−1 ν(CC). Elemental analyses: Found (%): C 62.38, H 4.52, N 1.87. Calcd for AuC39H34NF2: C 62.31, H 4.55, N 1.86.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b04788. Synthetic route for cyclometalated gold(III) complexes; thermogravimetric analysis; photophysical and electrochemical data; spatial plots of selected frontier MOs; change in bond lengths for 2 and 3; plots of spin density of the emissive states; TDDFT/CPCM orbital and 9348


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excitation energies; estimation of vapor pressure of complex 2 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Vivian Wing-Wah Yam: 0000-0001-8349-4429 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.W.-W.Y. acknowledges support from The University of Hong Kong and the URC Strategic Research Theme on New Materials. The work described in this paper was fully supported by a grant from the University Grants Committee Areas of Excellence (AoE) Scheme from the Hong Kong Special Administrative Region, China (Project No. AoE/P-03/08). C.-H.L. acknowledges the receipt of postgraduate studentships from The University of Hong Kong. This research is conducted in part using the HKU Information Technology Services computing resources and facilities.

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REFERENCES

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