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Strategy for the Realization of Efficient Solution-Processable Phosphorescent Organic Light-Emitting Devices: Design and Synthesis of Bipolar Alkynylplatinum(II) Complexes Fred Ka-Wai Kong, Man-Chung Tang, Yi-Chun Wong, Maggie Ng, Mei-Yee Chan,* and Vivian Wing-Wah Yam* Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China S Supporting Information *

ABSTRACT: A new class of highly luminescent bipolar alkynylplatinum(II) complexes has been synthesized, characterized, and applied as phosphorescent dopants in the fabrication of solution-processable organic light-emitting devices (OLEDs). Through the incorporation of a delicate balance of electrondonating carbazole moieties and electron-accepting phenylbenzimidazole or oxadiazole moieties into the platinum(II) core, the platinum(II) complexes have been demonstrated to exhibit bipolar charge transport character with high photoluminescence quantum yields of up to 0.75 in thin films. The introduction of meta-linkages into the complexes further helps weaken the donor−acceptor interactions, facilitating better carrier-transporting abilities. More importantly, high-performance solution-processable green-emitting OLEDs with maximum current efficiencies of up to 57.4 cd A−1 and external quantum efficiencies of up to 16.0% have been realized. This is among the best performances for solution-processable phosphorescent OLEDs reported based on platinum(II) complexes as well as bipolar metal complexes.

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INTRODUCTION Phosphorescent organic light-emitting devices (PHOLEDs) are among the most promising candidates for flat panel display applications owing to their exceptionally high device efficiencies and advantages of wide viewing angle, rich color gamut, and so on.1 However, severe efficiency roll-off at high current densities arising from triplet−triplet (T−T) annihilation remains a great challenge for practical use as solid-state lighting that requires a high brightness for illumination.2 The T−T annihilation can be reduced by using bipolar host materials in which both donor and acceptor moieties are incorporated into one single compound to avoid the accumulation of triplet exciton density within the emissive layer and to broaden the recombination zone for light emission.3−7 Meanwhile, the bipolar host materials should possess sufficiently high triplet energy to ensure efficient energy transfer from the host to the dopant as well as to prevent undesirable backward energy transfer.3 The common strategy for achieving high triplet energy in bipolar host materials is to introduce spacers between the hole- and electron-transporting moieties to weaken the donor−acceptor interaction.4−7 Various approaches have been adopted, such as the incorporation of either a saturated sp3-carbon,4a,b silicon center,4c−f or spiro-core5 to block the electronic communication between donor−acceptor pairs, to introduce a twisted conformation such as through ortho-methylation for steric control,6 or to connect the donor and acceptor groups through meta- or ortho-linkage7 in order to interrupt the π-conjugation © XXXX American Chemical Society

between the two moieties. Yang, Ma, and co-workers recently reported a series of carbazole/oxadiazole (OXD) hybrid host materials with various linkage modes.7c Both ortho-substituted and meta-substituted hosts demonstrated less intramolecular charge transfer behavior and higher triplet energies when compared to the para-substituted counterpart due to the reduced π-conjugation imparted by the twisted conformation.7c Green-emitting PHOLEDs based on tris(2-phenylpyridine)iridium(III) [Ir(ppy)3] as emitter and ortho-substituted 2,5bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole as bipolar host demonstrated encouraging performance with a maximum current efficiency of 77.9 cd A−1 and a maximum power efficiency of 59.3 lm W−1, much higher than those of the devices using a para-substituted analogue as host material (i.e., 21.2 cd A−1 and 17.0 lm W−1).7c On the other hand, the design and synthesis of bipolar phosphorescent materials are rare in the literature.8 Usually, the metal center is coordinated to one or more cyclometalating ligand(s). The cyclometalating ligand(s) can effectively control the luminescence properties including the emission color and photoluminescence quantum yields (PLQYs) of the complexes. However, the occurrence of charge-transfer emission arising from the incorporation of donor−acceptor moieties at the cyclometalating ligand(s) would inevitably affect the color Received: January 16, 2017

A

DOI: 10.1021/jacs.7b00479 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society Scheme 1. Chemical Structures of Bipolar Alkynylplatinum(II) Complexes 1−4

Scheme 2. Synthetic Routes of Bipolar Alkynylplatinum(II) Complexes 1−4

purity of the devices.8g Therefore, it is important to design bipolar phosphorescent materials with rather weak donor− acceptor communication using the aforementioned strategies in order to achieve stable color output for the devices. In 2011, Wang and co-workers reported a new class of bipolar heteroleptic green-emitting iridium(III) dendrimers, in which hole-transporting carbazole- and electron-transporting oxadiazole-based dendrimers were functionalized on the cyclometalating phenylpyridine ligands and the auxiliary acetylacetonate ligand, respectively.8b Nondoped solution-processable PHOLEDs based on the bipolar dendrimers have been found to show improved performance compared to those based on unipolar dendrimers, with a 1.5-fold increase in maximum external quantum efficiency (EQE) from 5.0% to 7.4%.8b Yam and co-workers have also developed a new class of bipolar

gold(III) complexes based on triphenylamine/phenylbenzimidazole (PBI) hybrid with sterically demanding methyl groups as spacer to weaken the donor−acceptor communication and to rigidify the molecules.8i Bipolar character could be revealed by the outstanding solution-processable PHOLED performance with a high EQE of 10.0% and a very small EQE roll-off of less than 1% at a luminance of 1000 cd m −2 . 8i These demonstrations open up a new avenue to generate new classes of bipolar phosphorescent materials. Herein we have specifically designed and synthesized a novel bipolar cyclometalating carbazole/PBI hybrid pincer ligand and have incorporated different carbazole/PBI and carbazole/OXD hybrid alkynyl ligands with either para- or meta-linkage modes into the platinum(II) center to generate a new class of bipolar platinum(II) complexes. Taking the advantages of high electron B


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Journal of the American Chemical Society mobilities (∼10−5 cm2 V−1 s−1), PBI- and OXD-containing materials are commonly used as electron-transporting materials in OLEDs.9 Through systematic control of a balance of electron-transporting and hole-transporting moieties and their extents of electronic communication, the platinum(II) complexes have been demonstrated to exhibit high PLQYs of up to 75% in solid-state thin films. In addition, it is found that the meta-substitution can effectively suppress the donor−acceptor interaction and thus enhances the bipolar character of the metal complexes. Notably, efficient solution-processable green-emitting PHOLEDs with remarkable EQEs and current efficiencies of up to 16.0% and 57.4 cd A−1, respectively, can be realized. To the best of our knowledge, this EQE is one of the highest values among the reported solution-processable PHOLEDs based on platinum(II) complexes,10 as well as solutionprocessable devices based on bipolar iridium(III)8a−d,j−l and gold(III)8i complexes. More importantly, the present result is comparable to the state-of-the-art solution-processable OLEDs based on iridium(III) complexes11a−c and thermally activated delayed fluorescence (TADF) materials.11d−f This suggests that the incorporation of bipolar moieties into phosphorescent emitters shall provide a useful strategy in the design for highly efficient PHOLED applications.

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RESULTS AND DISCUSSION Synthesis and Characterization. In order to facilitate bipolar charge transport character, electron-transporting and hole-transporting moieties are introduced on both the cyclometalating pincer ligand and the alkynyl ligand of the platinum(II) complexes. The substitutents have been specifically incorporated at either the para- or the meta-position to control the π-communication between the donor and the acceptor units. Bipolar alkynylplatinum(II) complexes 1−4 (Scheme 1) were successfully synthesized as shown in Scheme 2 and fully characterized by 1H NMR spectroscopy (Figures S1−S4), high-resolution ESI-mass spectrometry (Figures S5− S8), and IR spectroscopy and gave satisfactory elemental analysis. They were all isolated as air-stable and thermal-stable yellow solids with decomposition temperatures (Td) above 300 °C (Figure 1). The IR spectra of the complexes feature a weak band at 2085 cm−1, corresponding to the ν(CC) stretching mode. Photophysical and Electrochemical Properties. In general, all complexes in dichloromethane show intense vibronic-structured absorption bands at 290−340 nm with a moderately intense vibronic-structured band at 365−410 nm (Figure 2). The absorption bands at λ ≤ 300 nm are attributed to the spin-allowed intraligand (IL) π → π* transitions of the carbazole and bzimb units, whereas the lower-energy absorption bands at ca. 305−390 nm could be assigned as the π → π* transitions from the electron-donating carbazole moiety to the electron-accepting PBI or OXD moiety, with mixing of the IL π → π* transitions of the cyclometalating (CbztBu2−Ph)2bzimb ligand as supported by previous literature.8i,10a−d By weakening the π-conjugation between the donor and acceptor moieties imparted by meta-substitution compared with para-substitution, a blue shift of the charge transfer band has been observed when comparing 1 (352 nm) with 2 (305 nm), as well as comparing 3 (363 nm) with 4 (307 nm), which clearly demonstrates the improved bipolar character of the meta-substituted complexes. In addition, the transition energies of these charge transfer bands are found to be dependent on the nature of the electron-accepting units. A

Figure 1. Thermogravimetric curves of (a) 1, (b) 2, (c) 3, and (d) 4.

Figure 2. Electronic absorption spectra of 1−4 in dichloromethane at 298 K.

red-shifted absorption band is observed from 1 (352 nm) to 3 (363 nm), which bear PBI and OXD units, respectively. The lower-energy absorption bands beyond 390 nm are attributed to IL π → π* transitions of the (CbztBu2−Ph)2bzimb ligand, with substantial mixing of metal-to-ligand charge transfer (MLCT) [dπ(Pt) → π*((CbztBu2−Ph)2bzimb)] and ligandto-ligand charge transfer (LLCT) [π(CC−R) → π*((CbztBu2−Ph)2bzimb)] character.10a−d C


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orbitals (LUMOs) of the complexes, the cyclic voltammetry of 1−4 in dichloromethane (0.1 mol dm−3 nBu4NPF6) has been investigated. In general, an irreversible reduction wave at around −1.90 V vs saturated calomel electrode (SCE) is found for 1−4. The reduction process is assigned as the (CbztBu2− Ph)2bzimb-centered reduction as supported by previous studies as well as the insensitivity toward different bipolar alkynyl ligands.10a−d Upon the anodic sweep of 1−4, an irreversible first oxidation wave at around +1.10 V vs SCE is observed with insignificant shift of the potentials upon varying the electron transport moieties and the linkage modes. This oxidation process is assigned as the alkynyl ligand-based oxidation with mixing of a metal-centered contribution, as supported by previous studies.10a−d One quasi-reversible oxidation couple at around +1.27 V vs SCE is observed for 1−4, in which the couple is also found in the oxidative scan for the chloroplatinum(II) precursor, [Pt{(CbztBu2−Ph)2bzimb}Cl] (+1.27 V vs SCE) (Figure S9), and the structurally related analogue, [Pt( n Bu 2 bzimb)(CC−C 6 H 4 −Cbz t Bu 2 )] (nBu2bzimb = 2,6-bis(N-n-butylbenzimidazol-2′-yl)benzene) (+1.22 V vs SCE).10a The oxidation process is therefore assigned as the carbazole-based oxidation for both the carbazole moieties on the (CbztBu2−Ph)2bzimb ligand and the bipolar alkynyl ligand.10a−d,12 The electrochemical data of 1−4 are summarized in Table 2, and the cyclic voltammograms of 1−4 are shown in Figure 5 and Figures S10−S12. Computational Studies. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been performed in order to gain further insights into the origins of the electronic transitions of complexes 1 and 2. Model complexes labeled as 1′ and 2′, in which all the tert-butyl groups are replaced by methyl groups, are used in the computational study. On the basis of the TDDFT calculations, the S0 → S1 transition computed at ca. 400 nm mainly corresponds to the HOMO → LUMO excitation for 1′ and 2′. The HOMO refers to the π orbital localized on the alkynyl phenyl carbazole moieties of the auxiliary ligand mixed with the dπ(Pt) orbital, while the LUMO corresponds to the π* orbital localized on the bzimb ligand (Figures S13 and S14). This indicates that the low-lying absorption band at ca. 400 nm is mainly contributed by the S0 → S1 transition, which is predominantly a LLCT[π(CC−R) → π*(bzimb)] transition, with mixing of a MLCT[dπ(Pt) → π*(bzimb)] character. The first 15 singlet−singlet transitions of 1′ and 2′ are listed in Table S1. The higher-lying absorption bands computed at ca. 300−350 nm are mainly attributed to the HOMO → LUMO+2 transition. The LUMO+2 for 1′ refers to the π* orbital localized on the carbazole and the PBI moieties of the auxiliary ligand, while the LUMO+2 for 2′ refers to the π* orbital localized on the PBI moieties (Figure 6). Therefore, this absorption band can be assigned as predominantly a charge transfer transition from the carbazole moiety to the PBI moiety of the auxiliary ligand for both 1′ and 2′, with mixing of an IL[π → π*(alkynyl phenyl carbazole)] transition in 1′. By changing from para- to meta-substitution, the πconjugation between the electron-donating carbazole moiety and the electron-accepting PBI moieties is weakened as reflected from the spatial plots of the HOMO and LUMO+2 of 1′ and 2′ obtained from PBE0/CPCM calculation, and the charge transfer band is computed to show a blue shift in energy from 1′ (ca. 340 nm) to 2′ (ca. 320 nm), which is in line with the trend observed in the electronic absorption studies (ca. 352 nm for 1 and ca. 305 nm for 2). The computed absorption

The emission spectra of 1−4 in degassed dichloromethane at 298 K are obtained with excitation at λ ≥ 350 nm. They all show a strong vibronic-structured emission band with emission maxima at around 511 nm and progressional spacings of about 1300 cm−1, corresponding to typical aromatic vibrational modes of the (CbztBu2−Ph)2bzimb ligand (Figure 3). The

Figure 3. Normalized emission spectra of 1−4 in degassed dichloromethane at 298 K.

large Stokes shifts and emission lifetimes in the microsecond regime are indicative of emissions of triplet parentage. The emission is assigned to be originated from the 3IL[π → π*((CbztBu2−Ph)2bzimb)]/MLCT[dπ(Pt) → π*((CbztBu2− Ph)2bzimb)] excited state.10a−d This is also supported by the insensitivity toward different bipolar alkynyl ligands. The thinfilm emission spectra of 20 wt % 1−4 doped in mixed host consisting of tris(4-carbazoyl-9-ylphenyl)amine (TCTA):2,7bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13) (1:1) are shown in Figure 4. The emission spectra are almost

Figure 4. Normalized emission spectra of 1−4 doped in TCTA:SPPO13 (1:1) at 20 wt % at 298 K.

identical to their corresponding emission spectra in solution without undesirable emission from the host materials, indicating the complete energy transfer from the host materials to the guest complexes. Table 1 summarizes the photophysical data for 1−4. Remarkably, all complexes show high PLQYs ranging from 62 to 75% in TCTA:SPPO13-doped thin films, suggesting that they are promising candidates for PHOLED applications. To estimate the energy levels of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular D


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Journal of the American Chemical Society Table 1. Photophysical Properties of Complexes 1−4 complex

absorption λmax/nm (ε/dm3 mol−1 cm−1)

medium (T/K)

emission λmax/nm (τo/μs)

1

297 (121970), 318 (134385), 342 (103920), 352sh (84960), 412 (13650)

CH2Cl2 (298) glass (77)c thin film (298)d 5% 10% 15% 20% CH2Cl2 (298) glass (77)c thin film (298)d 5% 10% 15% 20% CH2Cl2 (298) glass (77)c thin film (298)d 5% 10% 15% 20% CH2Cl2 (298) glass (77)c thin film (298)d 5% 10% 15% 20%

511, 550, 595 (3.3) 495, 534, 580, 634 (7.1)

297 (167070), 305sh (147940), 342 (50900), 365sh (31705), 412 (13650)

2

297 (133250), 316 (131570), 343 (103320), 363sh (85560), 412 (13650)

3

297 (187940), 307sh (162030), 342 (50900), 365sh (31705), 412 (13650)

4

517, 518, 519, 520, 511, 495,

556, 558, 559, 560, 550, 534,

605 605 606 607 595 (3.3) 580, 634 (7.1)

517, 518, 518, 519, 511, 495,

556, 558, 558, 560, 550, 534,

605 606 606 607 595 (3.6) 580, 634 (7.6)

517, 518, 519, 520, 511, 495,

557, 559, 560, 561, 550, 534,

605 606 607 607 595 (3.5) 580, 634 (7.4)

516, 517, 519, 519,

557, 558, 560, 560,

604 605 607 607

ΦPL (soln)a

ΦPL (film)b

0.49

0.47 0.62 0.61 0.58 0.59

0.62 0.62 0.66 0.60 0.64

0.63 0.59 0.66 0.61 0.59

0.65 0.61 0.73 0.75

a

The luminescence quantum yield, measured at room temperature using [Ru(bpy)3]Cl2 in degassed aqueous solution as the reference (excitation wavelength = 436 nm, Φlum = 0.042). bAbsolute ΦPL(film) of 1−4 doped into TCTA:SPPO13 (1:1) measured using 300 nm as the excitation wavelength. cMeasured in butyronitrile glass. dTCTA:SPPO13 (1:1) thin film.

Table 2. Electrochemical Data for 1−4a complex 1 2 3 4

oxidation E1/2/V vs SCEb (ΔEp/mV)c [Epa/V vs SCEd] [+1.10], [+1.09], [+1.08], [+1.10],

+1.27 +1.27 +1.28 +1.27

(80) (79) (71) (77)

reduction [Epc/V vs SCEe]

EHOMO/eVf

ELUMO/eVf

[−1.92] [−1.91] [−1.89] [−1.90]

−5.90 −5.89 −5.88 −5.90

−2.88 −2.89 −2.91 −2.90

In CH2Cl2 solution with 0.1 M nBu4NPF6 as supporting electrolyte at 298 K working electrode, glassy carbon; scan rate = 100 mV s−1. bE1/2 = (Epa + Epc)/2; Epa and Epc are the peak anodic and peak cathodic potentials, respectively. cΔEp = (Epa − Epc). dEpa refers to the anodic peak potential for the irreversible oxidation waves. eEpc refers to the cathodic peak potential for the irreversible reduction waves. fEHOMO and ELUMO levels were calculated from electrochemical potentials, i.e., EHOMO = −e (4.8 V + Epa); ELUMO = −e (4.8 V + Epc). a

spectra of 1′ and 2′ are shown in Figure S15, and in general, they are in good agreement with the experimental absorption spectra of 1 and 2, respectively. Electroluminescence (EL) Properties. Taking advantage of high PLQYs of the bipolar platinum(II) complexes, solutionprocessable PHOLEDs have been fabricated with the configuration of indium tin oxide (ITO)/poly(ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS; 70 nm)/ x % platinum(II) complex:TCTA:SPPO13 (1:1; 60 nm)/1,3bis(3,5-di(pyridine-3-yl)phenyl)benzene (BmPyPhB; 30 nm)/ LiF (0.8 nm)/Al (100 nm), in which PEDOT:PSS and BmPyPhB are used as hole-transporting and electron-transporting layers, respectively. The emissive layer is prepared by spin-coating a solution of platinum(II) complex:TCTA:SPPO13 blend at different concentrations in chloroform; specifically, a mixed host consisting of TCTA and SPPO13 is

used to improve the device performance. Figure 7 depicts the normalized EL spectra of the devices at a current density of 20 mA cm−2. All the devices exhibit vibronic-structured emission, and the EL spectra for all the devices are exactly identical to their emission spectra in solution. Notably, the full-width-athalf-maxima (fwhm) of all the devices remain unchanged upon increasing the dopant concentration from 5% to 20%. In addition, both x and y chromaticity coordinates of all the devices are only varied by less than 0.02. This concentrationindependent EL suggests the low tendency of the bipolar platinum(II) complexes to form aggregates through Pt···Pt and/or π−π stacking in the thin films, in excellent agreement with the photoluminescence studies. Device characteristics for 1−4 are shown in Figures S16−S19. While all the complexes give similar EL spectra, the choice and linkage mode of the substituents play crucial roles in E


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mode. For instance, with the para-substituted electrontransporting group, the current efficiency and EQE are increased from 24.5 cd A−1 and 7.1% for a device made with 1 to 35.1 cd A−1 and 9.8% for a device doped with 3. The discrepancies on device efficiencies are believed to be due to a better electron-transporting property of OXD than PBI.13 More importantly, the device performances can be further boosted by the introduction of meta-substitution. Devices with metasubstituted PBI moieties (i.e., 2) are much more superior to those with para-substituted counterpart (i.e., 1), in which both current efficiency and EQE are increased by ∼64% via metalinkage. Similar EQE increment can be obtained for devices with OXD electron-transporting groups (i.e., 3 and 4). High current efficiency of 57.4 cd A−1 and EQE of 16.0% can be realized for devices made with 4. Such a high EQE value is one of the highest among solution-processable PHOLEDs based on platinum(II) complexes,10 as well as solution-processable PHOLEDs based on bipolar iridium(III)8a−d,j−l and gold(III)8i complexes (Table 4). It is also worth noting that this encouraging result is comparable to some of the bestperforming solution-processable devices based on iridium(III) complexes11a−c and TADF materials.11d−f This substantial improvement is believed to be due to the weakened donor− acceptor interaction via meta-substitution that enhances the bipolar character of the platinum(II) complexes and thus facilitates both hole- and electron-transporting properties. This could be reflected by the high power efficiencies of the devices based on bipolar platinum(II) complexes. Apparently, devices based on the meta-substituted bipolar complexes 2 and 4 outperform a device based on the structurally related derivative, [Pt(nBu2bzimb)(CC−C6H4−CbztBu2)],10a in which the power efficiencies of the devices are ∼2−3 times higher than that of the optimized control one (i.e., 9.1 lm W−1). In order to demonstrate the bipolar characters of the platinum(II) complexes, two platinum(II) complexes, i.e., 3 and

Figure 5. Cyclic voltammograms for the (a) oxidative and (b) reductive scans of 4 in dichloromethane (0.1 M nBu4NPF6).

determining the device performance. Apparently, the use of OXD as electron-transporting group can significantly improve the device performance (Figure 8 and Table 3). As expected, devices with OXD electron-transporting groups are much better than those with PBI moieties with the same linkage

Figure 6. Spatial plots (isovalue = 0.03) of selected frontier molecular orbitals of 1′ and 2′ obtained from PBE0/CPCM calculation. F


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Figure 7. Normalized EL spectra of devices made with (a) 1, (b) 2, (c) 3, and (d) 4.

Figure 8. EQEs of devices made with bipolar complexes (a) 1, (b) 2, (c) 3, and (d) 4.

[Pt(nBu2bzimb)(CC−C6H4−CbztBu2)] have also been prepared. Figure 9 depicts the current densities of hole- and electron-only devices. For the hole-only devices, the current densities of all the devices are comparable, i.e., within an order of magnitude under the same electric field. This implies that the presence of carbazole moieties is favorable for hole transport. On the other hand, the current densities of devices with bipolar

4, have been selected to test their hole- and electrontransporting properties. Hole-only devices with structure of ITO/PEDOT:PSS (70 nm)/10% 3 or 4:TCTA (60 nm)/ molybdenum trioxide (MoO3) (2 nm)/Al (100 nm) and electron-only devices with structure of ITO/LiF (2 nm)/10% 3 or 4:TCTA (60 nm)/LiF (2 nm)/Al (100 nm) have been fabricated. For comparison, the control devices based on G


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Journal of the American Chemical Society Table 3. Key Characteristics of Devices Doped with Complexes 1−4

complex

dopant concentration/ wt %

max. current efficiency/ cd A−1

max. power efficiency/ lm W−1

max. EQE/%

1 2 3 4

15 10 10 15

24.5 43.6 35.1 57.4

8.1 18.3 14.7 27.7

7.1 12.1 9.8 16.0

a

CIE (x, y)a 0.38, 0.36, 0.37, 0.37,

0.59 0.60 0.60 0.60

Data were collected at a current density of 10 mA cm−2.

platinum(II) complexes are much higher than that of the control device (i.e., 4 > 3 > [Pt(nBu2bzimb)(CC−C6H4− CbztBu2)]). In good agreement with the EL studies, this clearly indicates that the incorporation of OXD moieties can effectively improve the electron-transporting properties of the platinum(II) complexes, while the bipolar characters can further be enhanced by interrupting the donor−acceptor π-conjugation through meta-substitution.

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CONCLUSION A new class of highly luminescent bipolar alkynylplatinum(II) complexes containing hole-transporting carbazole and electrontransporting PBI or OXD moieties has been successfully designed and synthesized. All bipolar platinum(II) complexes exhibit intense green emission with high PLQYs of up to 75% in thin films. Notably, these bipolar platinum(II) complexes are capable to serve as phosphorescent dopants for solutionprocessable PHOLEDs, in which high current efficiencies of up to 57.4 cd A−1 and EQEs of up to 16.0% have been realized. These superior EQE values are believed to arise from the improved bipolar characters via the incorporation of electrondonating carbazole and electron-accepting PBI or OXD moieties, as well as the enhancement by meta-substitution.

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Figure 9. Current density−voltage curves of (a) hole- and (b) electron-only devices doped with 3, 4, and a reference [Pt(nBu2bzimb)(CC−C6H4−CbztBu2)] compound ([Pt]ref). (Aldrich, 98%) was recrystallized for no less than three times from hot absolute ethanol prior to use. Synthesis of Triisopropylsilyl (TIPS)-Protected Alkynes and Alkynes L1−L4. All reactions were carried out under anaerobic and anhydrous conditions using standard Schlenk techniques. {(CH3)2CH}3Si−CC−C6H4−Cbz−(PBI-4)2. This was synthesized according to a modification of a literature procedure for a Suzuki coupling reaction.8i A white solid of {(CH3)2CH}3Si−CC−C6H4− Cbz−(PBI-4)2 was obtained. Yield: 950 mg, 55%. 1H NMR (400 MHz, CDCl 3, 298 K, relative to Me4 Si): δ 1.17 (s, 21H, −Si{CH(CH3)2}3), 7.26−7.28 (m, 4H, benzimidazolyl protons), 7.35−7.42 (m, 6H, benzimidazolyl and phenyl protons), 7.45 (d, J = 8.6 Hz, 2H, carbazolyl protons), 7.51−7.60 (m, 8H, phenyl protons), 7.66−7.69 (m, 10H, phenyl and carbazolyl protons), 7.75 (d, J = 8.0 Hz, 2H, phenyl protons), 7.92 (d, J = 7.8 Hz, 2H, benzimidazolyl protons), 8.39 (d, J = 1.6 Hz, 2H, carbazolyl protons). HRMS (positive ESI) found 960.4452 [M + H]+; calcd for C67H58N5Si (m/z) 960.4456. {(CH3)2CH}3Si−CC−C6H4−Cbz−(PBI-3)2. This was synthesized according to a procedure similar to that of {(CH3)2CH}3Si−CC− C6H4−Cbz−(PBI-4)2 except that 1-phenyl-2-(3-(4,4,5,5-tetramethyl1,3,2-dioxaborolan-2-yl)phenyl)-1H-benzimidazole (1.50 g, 3.80 mmol) was used in place of 1-phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)phenyl)-1H-benzimidazole. A white solid of {(CH3)2CH}3Si−CC−C6H4−Cbz−(PBI-3)2 was obtained. Yield:

EXPERIMENTAL SECTION

Material and Reagents. 3,6-Dibromo-9-(4-((triisopropylsilyl)ethynyl)phenyl)carbazole,14 1-phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)phenyl)-1H-benzimidazole, 15a 1-phenyl-2-(3(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1H-benzimidazole,15b 2-(4-(tert-butyl)phenyl)-5-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1,3,4-oxadiazole,15c 2-(4-(tert-butyl)phenyl)-5(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1,3,4-oxadiazole,15d and 1,3-bis(N-(4-bromophenyl)-benzimidazol-2′-yl)benzene16 were synthesized according to reported procedures with slight modifications. All solvents were purified and distilled using standard procedures before use. All other reagents were of analytical grade and were used as received. Tetra-n-butylammonium hexafluorophosphate

Table 4. Key Parameters of Solution-Processable PHOLEDs Based on Platinum(II) Complexes and Bipolar Iridium(III) and Gold(III) Complexes metal core

max. current efficiency/cd A−1

max. power efficiency/lm W−1

max. EQE/%

λmax/nm

CIE (x, y)

ref

Pt(II) Pt(II) Pt(II) Ir(III) Ir(III) Ir(III) Ir(III) Au(III) Pt(II)

37.6 53.6 22.1 23.1 25.5 21.6 26.9 33.6 57.4

11.4 26.0 12.3 14.0 --21.2 8.7 27.7

10.4 15.5 11.7 10.8 7.4 7.2 11.5 10.0 16.0

510 515 608 467 520 522 490 532 520

0.33, 0.62 0.31, 0.61 0.56, 0.42 0.16, 0.32 0.34, 0.60 0.35, 0.60 0.18, 0.37 0.34, 0.58 0.37, 0.60

10a 10e 10g 8a 8b 8d 8k 8i this work

H


Article


1H, −CCH), 7.55 (d, J = 8.6 Hz, 2H, carbazolyl protons), 7.58 (d, J = 8.4 Hz, 4H, phenyl protons), 7.63 (d, J = 8.4 Hz, 2H, phenyl protons), 7.77−7.80 (m, 4H, carbazolyl and phenyl protons), 7.91 (d, J = 8.4 Hz, 4H, phenyl protons), 8.11 (d, J = 8.4 Hz, 4H, phenyl protons), 8.26 (d, J = 8.4 Hz, 4H, phenyl protons), 8.51 (d, J = 1.6 Hz, 2H, carbazolyl protons). HRMS (positive ESI) found 820.3688 [M + H]+; calcd for C56H46N5O2 (m/z): 820.3652. H−CC−C6H4−Cbz−(OXD-3)2 (L4). This was synthesized according to a procedure similar to that of L1 except that {(CH3)2CH}3Si− CC−C6H4−Cbz−(OXD-3)2 (303 mg, 0.31 mmol) was used in place of {(CH3)2CH}3Si−CC−C6H4−Cbz−(PBI-4)2. A white solid of L4 was obtained. Yield: 209 mg, 82%.1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): δ 1.37 (s, 18H, tert-butyl protons), 3.22 (s, 1H, −CCH), 7.55−7.57 (m, 6H, carbazolyl and phenyl protons), 7.62−7.69 (m, 4H, phenyl protons), 7.84−7.80 (m, 4H, carbazolyl and phenyl protons), 7.94 (d, J = 7.8 Hz, 2H, phenyl protons), 8.01−8.13 (m, 6H, phenyl protons), 8.52 (t, J = 1.8 Hz, 2H, phenyl protons), 8.54 (d, J = 1.6 Hz, 2H, carbazolyl protons). HRMS (positive ESI) found 820.3680 [M + H]+; calcd for C56H46N5O2 (m/z): 820.3652. Synthesis of Bipolar Pincer Ligand, Chloroplatinum(II) Precursor, and Bipolar Alkynylplatinum(II) Complexes. All reactions were carried out under anaerobic and anhydrous conditions using standard Schlenk techniques. Synthesis of Bipolar Pincer Ligand. (CbztBu2−Ph)2bzimb. This was synthesized according to modification of a literature procedure for an Ullmann coupling reaction.18 A white solid of (CbztBu2− Ph)2bzimb was obtained. Yield: 566 mg, 32%. 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): δ 1.43 (s, 36H, tert-butyl protons), 7.12−7.20 (m, 8H, carbazolyl protons), 7.35−7.45 (m, 10H, benzimidazolyl and phenyl protons), 7.52 (t, J = 8.0 Hz, 1H, phenyl proton), 7.62 (d, J = 8.0 Hz, 4H, phenyl protons), 7.72 (s, 1H, phenyl proton), 7.92−7.96 (m, 4H, benzimidazolyl and phenyl protons), 8.10 (d, J = 1.6 Hz, 4H, carbazolyl protons). Positive FAB-MS: m/z 1017 [M]+. Elemental analyses. Found (%): C, 83.47; H, 6.64; N, 8.02. Calcd for C72H68N6·H2O: C, 83.52; H, 6.81; N, 8.12. Synthesis of Chloroplatinum(II) Precursor. [Pt{(CbztBu2− Ph)2bzimb}Cl]. This was synthesized according to modification of a procedure for the synthesis of the related chloroplatinum(II) bzimb complexes.10b A yellow solid of [Pt{(CbztBu2−Ph)2bzimb}Cl] was obtained. Yield: 308 mg, 45%. 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): δ 1.49 (s, 36H, tert-butyl protons), 6.61 (d, J = 8.0 Hz, 2H, phenyl protons), 6.85 (t, J = 8.0 Hz, 1H, phenyl proton), 7.29 (d, J = 7.8 Hz, 2H, benzimidazolyl protons), 7.41 (t, J = 7.8 Hz, 2H, benzimidazolyl protons), 7.52−7.55 (m, 10H, benzimidazolyl and carbazolyl protons), 7.81 (d, J = 8.6 Hz, 4H, phenyl protons), 7.94 (d, J = 8.6 Hz, 4H, phenyl protons), 8.19 (d, J = 1.6 Hz, 4H, carbazolyl protons), 9.22 (d, J = 7.8 Hz, 2H, benzimidazolyl protons). Positive FAB-MS: m/z 1210 [M − Cl]+. Elemental analyses. Found (%): C, 69.25; H, 5.72; N, 6.48. Calcd for C72H67N6PtCl: C, 69.36; H, 5.42; N, 6.74. Synthesis of Bipolar Alkynylplatinum(II) Complexes. [Pt{(CbztBu2−Ph)2bzimb}{CC−C6H4−Cbz−(PBI-4)2}] (1). This was synthesized according to modification of a procedure for the synthesis of the related alkynylplatinum(II) bzimb complexes.10a−d A yellow solid of 1 was obtained. Yield: 196 mg, 75%. 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): δ 1.49 (s, 36H, tert-butyl protons), 6.73 (d, J = 8.0 Hz, 2H, phenyl protons of (CbztBu2−Ph)2bzimb), 6.87 (t, J = 8.0 Hz, 1H, phenyl proton of (CbztBu2−Ph)2bzimb, 7.31−7.39 (m, 6H, phenyl and benzimidazolyl protons of −CC−C6H4−Cbz− (PBI-4)2), 7.41−7.45 (m, 6H, carbazolyl and benzimidazolyl protons of (CbztBu2−Ph)2bzimb), 7.52−7.58 (m, 18H, phenyl and benzimidazolyl protons of −CC−C6H4−Cbz−(PBI-4)2, carbazolyl protons of (CbztBu2−Ph)2bzimb), 7.60−7.63 (m, 4H, carbazolyl protons of −CC−C6H4−Cbz−(PBI-4)2 and benzimidazolyl protons of (CbztBu2−Ph)2bzimb), 7.72 (s, 8H, phenyl protons of (CbztBu2− Ph)2bzimb), 7.74 (dd, J = 8.2 and 1.6 Hz, 2H, carbazolyl protons of −CC−C6H4−Cbz−(PBI-4)2), 7.85 (d, J = 8.6 Hz, 4H, phenyl protons of −CC−C6H4−Cbz−(PBI-4)2), 7.92−7.99 (m, 8H, phenyl and benzimidazolyl protons of −CC−C6H4−Cbz−(PBI4)2, benzimidazolyl protons of (CbztBu2−Ph)2bzimb), 8.19 (d, J = 1.6

870 mg, 50%. 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): δ 1.19 (s, 21H, −Si{CH(CH3)2}3), 7.29−7.38 (m, 6H, benzimidazolyl protons), 7.42−7.49 (m, 8H, carbazolyl and phenyl protons), 7.55 (d, J = 8.6 Hz, 2H, phenyl protons), 7.61−7.65 (m, 8H, phenyl protons), 7.74−7.77 (m, 6H, phenyl protons), 7.92 (d, J = 7.8 Hz, 2H, benzimidazolyl protons), 7.96 (t, J = 1.8 Hz, 2H, phenyl protons), 8.15 (d, J = 1.6 Hz, 2H, carbazolyl protons). HRMS (positive ESI) found 960.4440 [M + H]+; calcd for C67H58N5Si (m/z) 960.4456. {(CH3)2CH}3Si−CC−C6H4−Cbz−(OXD-4)2. This was synthesized according to a procedure similar to that for {(CH3)2CH}3Si−CC− C6H4−Cbz−(PBI-4)2 except that 2-(4-(tert-butyl)phenyl)-5-(4(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1,3,4-oxadiazole (1.53 g, 3.80 mmol) was used in place of 1-phenyl-2-(4-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1H-benzimidazole. A white solid of {(CH3)2CH}3Si−CC−C6H4−Cbz−(OXD-4)2 was obtained. Yield: 1.05 g, 60%. 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): δ 1.20 (s, 21H, −Si{CH(CH3)2}3), 1.40 (s, 18H, tert-butyl protons), 7.51−7.61 (m, 8H, carbazolyl and phenyl protons), 7.75−7.80 (m, 4H, carbazolyl and phenyl protons), 7.91 (d, J = 8.6 Hz, 4H, phenyl protons), 8.11 (d, J = 8.6 Hz, 4H, phenyl protons), 8.26 (d, J = 8.6 Hz, 4H, phenyl protons), 8.50 (d, J = 1.6 Hz, 2H, carbazolyl protons). HRMS (positive ESI) found 976.4907 [M + H]+; calcd for C65H66N5O2Si (m/z) 976.4980. {(CH3)2CH}3Si−CC−C6H4−Cbz−(OXD-3)2. This was synthesized according to a procedure similar to that of {(CH3)2CH}3Si−CC− C6H4−Cbz−(PBI-4)2 except that 2-(4-(tert-butyl)phenyl)-5-(3(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1,3,4-oxadiazole (1.53 g, 3.80 mmol) was used in place of 1-phenyl-2-(4-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1H-benzimidazole. A white solid of {(CH3)2CH}3Si−CC−C6H4−Cbz−(OXD-3)2 was obtained. Yield: 1.00 g, 57%. 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): δ 1.19 (s, 21H, −Si{CH(CH3)2}3), 1.37 (s, 18H, tert-butyl protons), 7.53−7.57 (m, 6H, carbazolyl and phenyl protons), 7.60 (d, J = 7.7 Hz, 2H, phenyl protons), 7.66 (d, J = 7.7 Hz, 2H, phenyl protons), 7.77−7.79 (m, 4H, carbazolyl and phenyl protons), 7.94 (d, J = 7.7 Hz, 2H, phenyl protons), 8.09−8.12 (m, 6H, phenyl protons), 8.52 (t, J = 1.6 Hz, 2H, phenyl protons), 8.54 (d, J = 1.6 Hz, 2H, carbazolyl protons). HRMS (positive ESI) found 976.4918 [M + H]+; calcd for C65H66N5O2Si (m/z) 976.4980. H−CC−C6H4−Cbz−(PBI-4)2 (L1). This was synthesized according to a modification of a literature procedure using tetra-n-butylammonium fluoride for the deprotection of TIPS-protected alkynes.17 A white solid of L1 was obtained. Yield: 216 mg, 86%. 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): δ 3.28 (s, 1H, −CCH), 7.26−7.28 (m, 4H, benzimidazolyl protons), 7.33−7.42 (m, 6H, benzimidazolyl and phenyl protons), 7.47 (d, J = 8.6 Hz, 2H, carbazolyl protons), 7.51−7.59 (m, 8H, phenyl protons), 7.66−7.69 (m, 10H, phenyl and carbazolyl protons), 7.76 (d, J = 8.0 Hz, 2H, phenyl protons), 7.92 (d, J = 7.8 Hz, 2H, benzimidazolyl protons), 8.39 (d, J = 1.6 Hz, 2H, carbazolyl protons). HRMS (positive ESI) found 804.3087 [M + H]+; calcd for C58H38N5 (m/z) 804.3122. H−CC−C6H4−Cbz−(PBI-3)2 (L2). This was synthesized according to a procedure similar to that of L1 except that {(CH3)2CH}3Si−C C−C6H4−Cbz−(PBI-3)2 (300 mg, 0.31 mmol) was used in place of {(CH3)2CH}3Si−CC−C6H4−Cbz−(PBI-4)2. A white solid of L2 was obtained. Yield: 208 mg, 83%. 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): δ 3.21 (s, 1H, −CCH), 7.29−7.40 (m, 6H, benzimidazolyl protons), 7.43−7.49 (m, 8H, carbazolyl and phenyl protons), 7.58 (d, J = 8.6 Hz, 2H, phenyl protons), 7.61−7.65 (m, 8H, phenyl protons), 7.74−7.78 (m, 6H, phenyl protons), 7.92 (d, J = 7.8 Hz, 2H, benzimidazolyl protons), 7.98 (t, J = 1.6 Hz, 2H, phenyl protons), 8.15 (d, J = 1.6 Hz, 2H, carbazolyl protons). HRMS (positive ESI) found 804.3083 [M + H]+; calcd for C58H38N5 (m/z): 804.3122. H−CC−C6H4−Cbz−(OXD-4)2 (L3). This was synthesized according to a procedure similar to that of L1 except that {(CH3)2CH}3Si− CC−C6H4−Cbz−(OXD-4)2 (303 mg, 0.31 mmol) was used in place of {(CH3)2CH}3Si−CC−C6H4−Cbz−(PBI-4)2. A white solid of L3 was obtained. Yield: 222 mg, 87%.1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): δ 1.39 (s, 18H, tert-butyl protons), 3.23 (s, I


Article

Journal of the American Chemical Society Hz, 4H, carbazolyl protons of (CbztBu2−Ph)2bzimb), 8.44 (d, J = 1.6 Hz, 2H, carbazolyl protons of −CC−C6H4−Cbz−(PBI-4)2), 9.28 (d, J = 8.0 Hz, 2H, benzimidazolyl protons of (CbztBu2−Ph)2bzimb). HRMS (positive ESI) found 2014.8127 [M + H]+; calcd for C130H104N11Pt (m/z) 2014.8147. IR (KBr disk): 2085 cm−1 ν(C C). Elemental analyses. Found (%): C, 70.53; H, 4.84; N, 6.94. Calcd for C130H103N11Pt·2CHCl3: C, 70.37; H, 4.70; N, 6.84. [Pt{(CbztBu2−Ph)2bzimb}{CC−C6H4−Cbz−(PBI-3)2}] (2). This was synthesized according to a procedure similar to that of complex 1 except that L2 (100 mg, 0.12 mmol) was used in place of L1. A yellow solid of 2 was obtained. Yield: 208 mg, 83%. 1H NMR (400 MHz, CD2Cl2, 298 K, relative to Me4Si): δ 1.50 (s, 36H, tert-butyl protons), 6.85 (d, J = 8.0 Hz, 2H, phenyl protons of (CbztBu2− Ph)2bzimb), 6.96 (t, J = 8.0 Hz, 1H, phenyl proton of (CbztBu2− Ph)2bzimb), 7.31−7.39 (m, 6H, phenyl and benzimidazolyl protons of −CC−C6H4−Cbz−(PBI-3)2), 7.41−7.45 (m, 6H, carbazolyl and benzimidazolyl protons of (CbztBu2−Ph)2bzimb), 7.52−7.58 (m, 18H, phenyl and benzimidazolyl protons of −CC−C6H4−Cbz−(PBI-3)2, carbazolyl protons of (CbztBu2−Ph)2bzimb), 7.60−7.63 (m, 4H, carbazolyl protons of −CC−C6H4−Cbz−(PBI-3)2 and benzimidazolyl protons of (CbztBu2−Ph)2bzimb), 7.72 (s, 8H, phenyl protons of (CbztBu2−Ph)2bzimb), 7.74 (dd, J = 8.2 and 1.6 Hz, 2H, carbazolyl protons of −CC−C6H4−Cbz−(PBI-3)2), 7.85 (d, J = 8.6 Hz, 4H, phenyl protons of −CC−C6H4−Cbz−(PBI-3)2), 7.92−7.99 (m, 8H, phenyl and benzimidazolyl protons of −CC−C6H4−Cbz− (PBI-3)2, benzimidazolyl protons of (CbztBu2−Ph)2bzimb), 8.19 (d, J = 1.6 Hz, 4H, carbazolyl protons of (CbztBu2−Ph)2bzimb), 8.44 (d, J = 1.6 Hz, 2H, carbazolyl protons of −CC−C6H4−Cbz−(PBI-3)2), 9.28 (d, J = 8.0 Hz, 2H, benzimidazolyl protons of (CbztBu2− Ph)2bzimb). HRMS (positive ESI) found 2014.8021 [M + H]+; calcd for C130H104N11Pt (m/z) 2014.8147. IR (KBr disk): 2085 cm−1 ν(C C). Elemental analyses. Found (%): C, 75.09; H, 5.12; N, 7.52. Calcd for C130H103N11Pt·0.5CHCl3: C, 75.57; H, 5.03; N, 7.43. [Pt{(CbztBu2−Ph)2bzimb}{CC−C6H4−Cbz−(OXD-4)2}] (3). This was synthesized according to a procedure similar to that of complex 1 except that L3 (102 mg, 0.12 mmol) was used in place of L1. A yellow solid of 3 was obtained. Yield: 202 mg, 80%. 1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): δ 1.39 (s, 18H, tert-butyl protons of −CC−C6H4−Cbz−(OXD-4)2), 1.49 (s, 36H, tert-butyl protons of (CbztBu2−Ph)2bzimb), 6.73 (d, J = 8.0 Hz, 2H, phenyl protons of (CbztBu2−Ph)2bzimb), 6.88 (t, J = 8.0 Hz, 1H, phenyl proton of (CbztBu2−Ph)2bzimb), 7.33 (d, J = 8.0 Hz, 2H, benzimidazolyl protons of (CbztBu2−Ph)2bzimb), 7.45 (t, J = 8.0 Hz, 2H, benzimidazolyl protons of (CbztBu2−Ph)2bzimb), 7.56−7.66 (m, 16H, phenyl and carbazolyl protons of −CC−C6H4−Cbz−(OXD4)2, phenyl, carbazolyl, and benzimidazolyl protons of (CbztBu2− Ph)2bzimb), 7.70 (d, J = 8.2 Hz, 2H, carbazolyl protons of −CC− C6H4−Cbz−(OXD-4)2), 7.82−7.87 (m, 6H, phenyl protons of (CbztBu2−Ph)2bzimb and carbazolyl protons of −CC−C6H4− Cbz−(OXD-4)2), 7.95−7.98 (m, 8H, phenyl protons of (CbztBu2− Ph)2bzimb and −CC−C6H4−Cbz−(OXD-4)2), 8.02 (d, J = 8.0 Hz, 2H, phenyl protons of −CC−C6H4−Cbz−(OXD-4)2), 8.12 (d, J = 8.0 Hz, 4H, phenyl protons of −CC−C6H4−Cbz−(OXD-4)2), 8.20 (d, J = 1.6 Hz, 4H, carbazolyl protons of (CbztBu2−Ph)2bzimb), 8.28 (d, J = 8.0 Hz, 4H, phenyl protons of −CC−C6H4−Cbz−(OXD4)2), 8.56 (d, J = 1.6 Hz, 2H, carbazolyl protons of −CC−C6H4− Cbz−(OXD-4)2), 9.29 (d, J = 8.0 Hz, 2H, benzimidazolyl protons of (CbztBu2−Ph)2bzimb). HRMS (positive ESI) found 2030.8529 [M + H]+; calcd for C128H112N11O2Pt (m/z) 2030.8671. IR (KBr disk): 2085 cm−1 ν(CC). Elemental analyses. Found (%): C, 71.82; H, 5.31; N, 7.25. Calcd for C128H111N11O2Pt·CHCl3: C, 72.07; H, 5.25; N, 7.17. [Pt{(CbztBu2−Ph)2bzimb}{CC−C6H4−Cbz−(OXD-3)2}] (4). This was synthesized according to a procedure similar to that of complex 1 except that L4 (102 mg, 0.12 mmol) was used in place of L1. A yellow solid of 4 was obtained. Yield: 212 mg, 84%. 1H NMR (400 MHz, CD2Cl2, 298 K, relative to Me4Si): δ 1.39 (s, 18H, tert-butyl protons of −CC−C6H4−Cbz−(OXD-3)2), 1.50 (s, 36H, tert-butyl protons of (CbztBu2−Ph)2bzimb), 6.86 (d, J = 8.0 Hz, 2H, phenyl protons of (CbztBu2−Ph)2bzimb), 6.88 (t, J = 8.0 Hz, 1H, phenyl proton of

(CbztBu2−Ph)2bzimb), 7.41 (d, J = 8.0 Hz, 2H, benzimidazolyl protons of (CbztBu2−Ph)2bzimb), 7.50 (t, J = 8.0 Hz, 2H, benzimidazolyl protons of (CbztBu2−Ph)2bzimb), 7.57−7.63 (m, 12H, phenyl protons of −CC−C6H4−Cbz−(OXD-3)2, carbazolyl protons of (CbztBu2−Ph)2bzimb), 7.66 (t, J = 8.0 Hz, 2H, benzimidazolyl protons of (CbztBu2−Ph)2bzimb), 7.71−7.76 (m, 6H, carbazolyl and phenyl protons of −CC−C6H4−Cbz−(OXD3)2), 7.91 (m, 6H, phenyl protons of (CbztBu2−Ph)2bzimb and carbazolyl protons of −CC−C6H4−Cbz−(OXD-3)2), 7.99−8.05 (m, 8H, phenyl protons of (CbztBu2−Ph)2bzimb and −CC−C6H4− Cbz−(OXD-3)2), 8.12−8.17 (m, 6H, phenyl protons of −CC− C6H4−Cbz−(OXD-3)2), 8.23 (d, J = 1.6 Hz, 4H, carbazolyl protons of (CbztBu2−Ph)2bzimb), 8.61 (d, J = 1.6 Hz, 2H, phenyl protons of −CC−C6H4−Cbz−(OXD-3)2), 8.67 (d, J = 1.6 Hz, 2H, carbazolyl protons of −CC−C6H4−Cbz−(OXD-3)2), 9.24 (d, J = 8.0 Hz, 2H, benzimidazolyl protons of (CbztBu2−Ph)2bzimb). HRMS (positive ESI) found 2030.8569 [M + H]+; calcd for C128H112N11O2Pt (m/z) 2030.8671. IR (KBr disk): 2085 cm−1 ν(CC). Elemental analyses. Found (%): C, 73.81; H, 5.40; N, 7.33. Calcd for C128H111N11O2Pt· 0.5CHCl3: C, 73.84; H, 5.38; N, 7.37. Physical Measurements and Instrumentation. The UV−vis absorption spectra were recorded on a Cary 50 (Varian) spectrophotometer equipped with a Xenon flash lamp. 1H NMR spectra were recorded on a Bruker DPX-400 (400 MHz) Fouriertransform NMR spectrometer with chemical shifts reported relative to tetramethylsilane. Positive FAB mass spectra were recorded on a Thermo Scientific DFS High Resolution Magnetic Sector Mass Spectrometer. High-resolution ESI mass spectra were recorded on a Bruker maXis II High Resolution LC-QTOF 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. Excited-state lifetimes of solution and glass samples were measured using a conventional laser system. The excitation source used was the 355 nm output (third harmonic, 8 ns) of a Spectra-Physics Quanta-Ray Q-switched GCR-150 pulsed Nd:YAG laser (10 Hz). Luminescence decay signals were detected by a Hamamatsu R928 photomultiplier tube, recorded on a Tektronix model TDS-620A (500 MHz, 2 GS s−1) digital oscilloscope, and analyzed by using a program for exponential fits. Luminescence quantum yields were measured by the optical dilute method reported by Demas and Crosby.19a A degassed solution of [Ru(bpy)3]Cl2 in aqueous state (Φlum = 0.042, excitation wavelength at 436 nm) was used as the reference,19b while those of the thin films were measured on a Hamamatsu C9920-03 Absolute PL Quantum Yield Measurement System. Cyclic voltammetric measurements were performed by using a CH Instruments, Inc. model CHI 600A electrochemical analyzer. All solutions for electrochemical studies were deaerated with prepurified argon gas just before measurements. Thermal analyses were performed with the TA Instruments TGA Q50 thermogravimetric analyzer, in which Td is defined as the temperature at which the material showed a 5% weight loss. For PHOLED fabrication, devices with the structure of ITO/PEDOT:PSS (70 nm)/emissive layer (60 nm)/BmPyPhB (30 nm)/LiF (0.8 nm)/Al (100 nm) were fabricated, in which the emissive layer was formed by mixing the complexes with TCTA:SPPO13 (1:1) to prepare a 10 mg cm−3 solution in chloroform via spin-coating technique. Current density−voltage−luminance characteristics of devices were simultaneously measured by a programmable Keithley 2420 source meter and a PR-655 colorimeter. All devices were measured in ambient conditions without encapsulation. Computational Details. All calculations were performed using the Gaussian 09 software package.20 The ground-state geometries of the model complexes of 1 and 2, in which all the tert-butyl groups were replaced by methyl groups (labeled as 1′ and 2′), were fully optimized J


Article

Journal of the American Chemical Society in dichloromethane with density functional theory (DFT) at the PBE0 level21 in conjunction with the conductor-like polarizable continuum model (CPCM) using dichloromethane as the solvent.22 The Cartesian coordinates of the optimized structures of 1′ and 2′ are given in Tables S3 and S4. Vibrational frequencies were then calculated for all stationary points to verify that each was a minimum (NIMAG = 0) on the potential energy surface. On the basis of the ground-state optimized geometries, time-dependent density functional theory (TDDFT) method23 at the same level of theory associated with CPCM (dichloromethane) was employed to compute the singlet− singlet transitions in the electronic absorption spectra of 1′ and 2′. For all the calculations, the Stuttgart effective core potentials (ECPs) and the associated basis set were utilized to describe Pt24 with f-type polarization functions (ζ = 0.993),25 whereas the 6-31G(d) basis set was employed to describe all other atoms.26 The DFT and TDDFT calculations were performed with a pruned (99 590) grid for numerical integration.

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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.7b00479. 1 H NMR spectra and HR-ESI mass spectra of 1−4 (Figures S1−S8); cyclic voltammograms of [Pt{(CbztBu2−Ph)2bzimb}Cl], 1−3 (Figures S9−S12); spatial plots of selected frontier molecular orbitals and computed absorption spectra of 1′ and 2′ (Figures S13− S15); device characteristics for 1−4 (Figures S16−S19); table for first 15 singlet excited states of 1′ and 2′ (Table S1); table for key parameters of devices made with 1−4 (Table S2); and Cartesian coordinates of the optimized structures for 1′ and 2′ (Tables S3 and S4) (PDF)

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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 This paper is dedicated to Professor Chi-Ming Che on the occasion of his 60th birthday. V.W.-W.Y. acknowledges UGC funding administered by The University of Hong Kong for supporting the Electrospray Ionization Quadrupole Time-ofFlight Mass Spectrometry Facilities under the Support for Interdisciplinary Research in Chemical Science, and the support from the URC Strategic Research Theme on New Materials of The University of Hong Kong. The work described in this paper was fully supported by a grant from the Theme-Based Research Scheme of the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23713/11). F.K.-W.K. and Y.-C.W. acknowledge the receipt of postgraduate studentships from The University of Hong Kong. F.K.-W.K. acknowledges the receipt of a University Postgraduate Fellowship from The University of Hong Kong.

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