Sky-Blue-Emitting Dendritic Alkynylgold(III) Complexes for Solution

Jul 21, 2017 - The present carbazole-based dendritic alkynylgold(III) complexes exhibit intense sky-blue emission with peak maximum at ca. 490 nm and ...
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Sky-Blue-Emitting Dendritic Alkynylgold(III) Complexes for SolutionProcessable Organic Light-Emitting Devices Chin-Ho Lee, Man-Chung Tang, Yi-Chun Wong, Mei-Yee Chan,* and Vivian Wing-Wah Yam* Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China ABSTRACT: A new class of tridentate ligand-containing cyclometalated gold(III) complexes featuring dendritic alkynyl ligands with carbazole moieties as dendrons and peripheral groups has been synthesized up to the third generation. High-performance solutionprocessable organic light-emitting devices (OLEDs) with maximum current efficiency of up to 23.7 cd A−1 and external quantum efficiency of up to 6.9% have been realized by a simple spin-coating technique. With the incorporation of bulky carbazole moieties to form higher generation dendrimers, the undesirable excimeric emission could be effectively reduced, allowing the fine-tuning of the emission color toward the blue region. This represents the first successful demonstration of sky-blue-emitting alkynylgold(III) complexes and its application in solution-processable OLEDs.

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to the non-emissive distorted triplet metal-centered (3MC) state.15 As a result, the energy of the emissive states can be easily channeled to the 3MC state, where it will be lost as thermal energy through molecular vibrations and rotations.15 The incorporation of strong field ligands such as Nheterocyclic carbenes2a,3a,7a,b into the heavy metal centers is one of the possible solutions to raise the 3MC state as well as to avoid such energy dissipation.15 Forrest, Thompson, and co-workers recently reported a new class of N-heterocyclic carbene-based iridium(III) blue-emitting phosphors, tris(Nphenyl,N-methyl-pyridoimidazol-2-yl)iridium(III) [Ir(pmp)3].2a Both facial (fac) and meridional (mer) isomers were found to exhibit high PLQYs of over 70% in the solution state.2a PHOLEDs based on the fac and mer isomers gave Commission Internationale de L’Eclairage (CIE) coordinates of (0.16, 0.09) and (0.16, 0.15), together with high EQEs of 10.1% and 14.4%, respectively.2a Another common approach involves the attachment of electronwithdrawing groups, such as fluoro, trifluoromethyl, and cyano groups, to the aryl ring of the cyclometalating ligands to stabilize the highest occupied molecular orbital (HOMO), which is mainly localized on the aryl ring of the cyclometalating ligands, and thus widening the energy gap.16 Of particular interest is the iridium(III) bis(4,6-difluorophenyl)pyridinato-N,C2′)picolinate (FIrpic), which emits at ca. 470 nm in the solid and solution states with high PLQYs of up to 60%.2b High-performance devices based on sky-blue-emitting FIrpic have been demonstrated with a maximum EQE of 5.7% and a PE of 6.3 lm W−1.2c The EQE has further been boosted to 21.7% via strategic device engineering.2d While most of the research on blue phosphorescent emitters are mainly focused on the octahedral d6 iridium(III)

INTRODUCTION Organic light-emitting devices (OLEDs) represent one of the promising candidates for flat-panel displays and solid-state lighting applications.1 Owing to their high photoluminescence quantum yields (PLQYs) and their capability to harvest both singlet and triplet excitons for realizing 100% internal quantum efficiency, phosphorescent emitters such as those based on iridium(III),2 platinum(II),3 ruthenium(II),4 and osmium(II)5 systems have been extensively studied in past decades.6 More recently, there has been a growing interest in phosphorescent emitters of other metal centers, such as those of gold(III),7 palladium(II),8 copper(I),9 and tungsten(VI).10 Through the extensive studies to fine-tune the luminescence properties, efficient red-11 and green-emitting12 phosphorescent OLEDs (PHOLEDs) with high external quantum efficiencies (EQEs) have been demonstrated. For instance, Jou and co-workers demonstrated the use of novel device architecture with stepwise energy levels for iridium(III)based red-emitting PHOLEDs, in which a remarkably high power efficiency (PE) of 47.0 lm W−1 at luminance of 100 cd m−2 and EQE of 20.3% have been realized.11a Meanwhile, Kido and co-workers designed a tailor-made bipolar host for the preparation of iridium(III)-based green-emitting PHOLEDs with an extraordinary EQE of 26.9%.12a While the EQEs of the existing red- and green-emitting OLEDs have almost reached the theoretical limit, blue-emitting PHOLEDs are still generally considered to be lagging behind in their performance in terms of efficiencies, color purity, and device stability when compared to their red- and green-emitting counterparts.13 However, the development of blue-emitting PHOLEDs is indispensable for full-color displays and solid-state illumination.14 The large triplet energy of blue phosphorescent emitters has rendered the emissive triplet intraligand (3IL) and charge-transfer (3CT) excited states to be close in energy © 2017 American Chemical Society

Received: June 7, 2017 Published: July 21, 2017 10539

DOI: 10.1021/jacs.7b05872 J. Am. Chem. Soc. 2017, 139, 10539−10550

Article

Journal of the American Chemical Society Scheme 1. Synthetic Routes to Alkynes

Scheme 2. Synthetic Route and Chemical Structures of Alkynylgold(III) Dendrimers 1−5

system,2 those based on square-planar d8 metal complexes, including platinum(II)3 and gold(III)7 systems, are relatively less studied, with blue emitters of the latter almost unexplored. In particular, the π-stacking of the cyclometalating ligands in most of the square-planar metal complexes usually leads to a significant spectral shift upon an increase in the dopant concentration that leads to dopant aggregation, yielding a broad red-shifted excimeric emission.7c,d This inevitably increases the difficulty to prepare blue emitters for the square-planar metal complex systems. Very recently, our research group has successfully demonstrated that the intermolecular interactions can be effectively suppressed by incorporating a dendritic structure into the square-planar metal complexes.2e,f,3b,7e High-performance solution-processable PHOLEDs with EQEs of up to 10.1% and 10.4% for gold(III)7e and platinum(II)3b systems, respectively, have been realized. Herein, we take advantage of the dendritic structure

to incorporate a bulky tridentate ligand as well as sterically demanding fluorene groups as the core unit and carbazolebased dendrons to feature a new class of solution-processable sky-blue-emitting alkynylgold(III) complexes. The present carbazole-based dendritic alkynylgold(III) complexes exhibit intense sky-blue emission with peak maximum at ca. 490 nm and high PLQYs of up to 40%. Notably, solution-processable PHOLEDs with high EQEs of up to 6.9% have been demonstrated, and this work represents the first example of dendritic alkynylgold(III) complexes emitting in the bluegreen region.

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RESULTS, AND DISCUSSION Synthesis and Characterization. The carbazole-based dendrons and dendrimers were mainly synthesized by Ullmann coupling.17 Triisopropylsilyl (TIPS)-protected alkynes and terminal alkynes were synthesized according to 10540

DOI: 10.1021/jacs.7b05872 J. Am. Chem. Soc. 2017, 139, 10539−10550

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Journal of the American Chemical Society modification of literature methods,7m,18 and the synthetic route is shown in Scheme 1. The dendrimer core unit (TIPS−CC−C6H4−N{C13H6(C6H13)2}2Br2) was synthesized by Buchwald−Hartwig amination 18 between 4((triisopropylsilyl)ethynyl)aniline and 2,7-bromo-9,9-dihexyl9H-fluorene. The carbazole dendrons were attached to the core unit through Buchwald−Hartwig amination.18 Deprotection of the triisopropylsilyl alkynes with tetra-n-butylammonium fluoride (TBAF) in THF afforded the target terminal alkynes. 2,6-Bis(4-(tert-butyl)phenyl)pyridine (tBuC^N^CtBu) and the chlorogold(III) precursor, [Au(tBuC^N^CtBu)Cl], were prepared by slight modifications of the literature procedures.7h Dendritic alkynylgold(III) complexes 1−5 were synthesized by modification of the literature procedure for bis-cyclometalated diarylpyridine alkynylgold(III) complexes previously reported by our group.7e,f The molecular structures of gold(III) dendrimers are shown in Scheme 2. The gold(III) dendrimers exhibit high stability and can be stored in the dark for prolonged periods without decomposition. These alkynylgold(III) complexes have been fully characterized by 1H nuclear magnetic resonance (NMR), 13 C{1H} NMR, fast-atom bombardment mass spectrometry (FAB-MS), or matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and gave satisfactory elemental analysis. The infrared spectra of the complexes feature a weak absorption at around 2147− 2149 cm−1, corresponding to the ν(CC) stretching frequency. In addition, these gold(III) complexes show good thermal stability with high decomposition temperatures of over 300 °C, where the decomposition temperature is defined as the temperature at which the thermogravimetric analysis (TGA) curve shows a 5% weight loss. The TGA traces of complexes 2−4 have been selected and are shown in Figure 1 for illustration purposes.

Figure 2. UV−visible absorption spectra of complexes 1−5 in CH2Cl2 at 298 K.

structured absorption band at 270−310 nm in 3 to 5 are found to increase with increasing dendrimer generations. This absorption band is assigned as IL π → π* transition of the carbazole moieties, similar to that found in other carbazolebased dendrimers.3b,7e With reference to the structurally related alkynylgold(III) complexes,7c,e−i the absorption band at ca. 320−400 nm could be assigned as metal-perturbed IL π → π* transition of the cyclometalated tBuC^N^CtBu ligand with charge transfer character from the aryl ring to the pyridine unit of the C^N^C ligand,7c,e−i with mixing of IL π → π* transitions of the fluorene units,19 while the low-energy absorption tail beyond 410 nm could be assigned as the ligand-to-ligand charge transfer (LLCT) [π(alkynylfluorene unit) → π*(tBuC^N^CtBu)] transition.19 Upon excitation at λ ≥ 400 nm in degassed CH2Cl2 solution at 298 K, all the complexes feature Gaussian-shape emission bands with emission maxima at ca. 620−650 nm (Figure 3). These

Figure 3. Normalized emission spectra of complexes 1−5 in degassed CH2Cl2 at 298 K.

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

emission bands have been tentatively assigned as derived from the 3LLCT [π(alkynylfluorene unit) → π*(tBuC^N^CtBu)] excited state.7c,e−i The emission energies are found to be slightly blue-shifted with increasing dendrimer generations [i.e., 2 (647 nm) < 3 (645 nm) < 4 (630 nm) < 5 (623 nm)]. This phenomenon is in good agreement with the previously reported gold(III) carbazole-containing dendrimers and can be ascribed to the negative inductive effect of the electronegative nitrogen atoms of the peripheral carbazole

Photophysical Properties. All the complexes show a moderately intense absorption band at ca. 320−400 nm with an absorption tail at ca. 410−450 nm with extinction coefficients (ε) on the order of 104 dm3 mol−1 cm−1. An additional vibronic-structured absorption band at ca. 270−310 nm is also observed for the carbazole dendrimers 3−5. The UV−visible spectra of complexes 1−5 in CH2Cl2 are shown in Figure 2. The molar extinction coefficients of the vibronic10541

DOI: 10.1021/jacs.7b05872 J. Am. Chem. Soc. 2017, 139, 10539−10550

Article

Journal of the American Chemical Society units.7e With increasing dendrimer generation from complexes 2 to 5, the stabilized HOMO level results in a blue shift in emission energy of 744 cm−1. On the other hand, the modification of alkyl chain length (CH3 in complex 1; C6H13 in complex 2) does not have a significant influence on both the absorption and the emission behaviors of the complexes in solution state (Figure 3). The emission properties of complexes 1−5 doped in N,N′-dicarbazolyl-3,5-benzene (MCP) thin films (5−50 wt%) have also been investigated, and Figure 4 shows the emission spectra of complexes 1−5

Figure 4. Normalized solid-state thin film emission spectra of complexes 1−5 doped at 5 wt% in MCP (asterisk denotes artifact).

doped in MCP thin films. Apparently, the emission maximum in the solid-state thin films only shifts by ∼4 nm (∼163 cm−1), i.e., from 497 nm (2) to 493 nm (5). However, the vibronic-structured emission bands can be well-resolved with increasing dendrimer generation. More importantly, a significant red shift in the emission energies of complex 2 is observed upon increasing the dopant concentration (Figure 5a), whereas complex 5 bearing the highest generation carbazole dendrimer features a vibronic-structured emission band, even when the dopant concentration increases up to 50 wt% in MCP thin film (Figure 5b). Such concentrationdependent behavior in complex 2 is likely to arise from the excimeric emission caused by the π−π stacking interactions of the C^N^C ligand, which is one of the biggest challenges for the design and synthesis of sky-blue emitters of square-planar metal complexes of d8 electronic configuration. This clearly illustrates that the introduction of bulky dendrons to the alkynyl ligands can effectively suppress the intermolecular interactions. It should be noted that the thin film emission spectrum of complex 1 has been red-shifted when compared to that of complex 2, different from those in the solution state. This suggests that long alkyl chains on the fluorene moieties play an important role in suppressing the intermolecular interaction in the solid state. A summary of key photophysical data of complexes 1−5 has been collected in Table 1. Electrochemistry. The electrochemical data of complexes 1−5 and the corresponding alkynyl ligands in dichloromethane (0.1 M nBu4NPF6) have been studied by cyclic voltammetry. Estimation of their HOMO and LUMO energy levels has also been made, as depicted in Table 2. The cyclic voltammogram of complex 2 is shown in Figure 6. In general, all the complexes feature a quasi-reversible reduction couple at

Figure 5. Normalized solid-state thin film emission spectra of (a) 2 and (b) 5 doped at 5, 10, 20, and 50 wt% in MCP (asterisk denotes artifact).

around −1.60 V versus standard calomel electrode (SCE). With reference to previously reported cyclometalated gold(III) complexes7c,e−i and the insensitive potential value of the reduction couple to the nature of the alkynyl ligands, the reduction couple could be assigned as the ligand-centered reduction of the cyclometalated tBuC^N^CtBu ligand. In the oxidative scan, all the complexes show a quasi-reversible oxidation couple at around +0.74 to +0.81 V vs SCE, which is attributed to alkynyl ligand-centered oxidation.7c,e−i Additional second and third oxidation couples are observed at around +1.02 to +1.16 V and +1.18 to +1.25 V vs SCE, respectively, for higher generation dendrimeric complexes 3−5. These multiple oxidation processes at higher potentials could be assigned as the carbazole-based oxidation.3b,7e,20 It is interesting to mention that the first oxidation couple becomes more positive with increasing dendrimer generation. On the contrary, the second and third oxidation couples become less positive with increasing number of generations. This is in agreement with the photophysical data where the core unit becomes more electron-deficient as more electron-withdrawing carbazole units are attached and will be more difficult to be oxidized, whereas the peripheral carbazole moieties are more electron-rich and hence are much easier to be oxidized. This results in an enlargement of the HOMO−LUMO gap with increasing dendrimer generation. Similar redox behaviors have also been observed in their corresponding alkynyl ligands, further confirming the origin of the oxidations of the complexes being from the alkynyl ligands. 10542

DOI: 10.1021/jacs.7b05872 J. Am. Chem. Soc. 2017, 139, 10539−10550

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Journal of the American Chemical Society Table 1. Photophysical Properties of Complexes 1−5 complex 1

2

3

4

5

absorption λmax/nm (εmax/dm3 mol−1 cm−1) 359 (54390), 412 sh (16910)

363 (57990), 410 sh (19605)

298 (64790), 352 sh (65245), 383 (76100)

285 (155300), 296 (155990), 350 (80380), 387 (86620)

285 (332180), 298 (338190), 350 (111225), 393 sh (85060)

medium

emission λmax/nm (τo/μs)

Φluma

CH2Cl2 (298) glass (77)b,c solid (298) solid (77) thin film (298) 5% in MCP 10% in MCP 20% in MCP 50% in MCP

653 (