Highly efficient solution-processed pure yellow OLEDs based on dinuclear Pt(II) complexes†

Yuanhui Sun ^a, Bochen Liu ^b, Bo Jiao ^c, Yue Guo ^b, Xi Chen ^a, Guijiang Zhou *^a, Zhao Chen *^b and Xiaolong Yang *^a
aSchool of Chemistry, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Xi’an Key Laboratory of Sustainable Energy Material Chemistry, Xi'an Jiaotong University, Xi'an 710049, P. R. China. E-mail: zhougj@xjtu.edu.cn; xiaolongyang@xjtu.edu.cn
^bSchool of Applied Physics and Materials, Wuyi University, Jiangmen 529020, P. R. China. E-mail: chenzhao2006@163.com
^cSchool of Electronic and Information Engineering, Key Laboratory for Physical Electronics and Devices of the Ministry of Education, Shaanxi Key Lab of Information Photonic Technique, Xi'an Jiaotong University, Xi'an 710049, P. R. China

First published on 16th June 2021

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Introduction

Since the phosphorescent organometallic complexes were successfully applied to the fabrication of organic light-emitting diodes (OLEDs), electroluminescence (EL) efficiencies of OLEDs have been dramatically improved because of the full utilization of both singlet and triplet excitons.^1–4 To date, the great potential of applying OLEDs in advanced displays and solid-state lighting has been unmistakably demonstrated.^5–8 Foldable or even rollable OLEDs are being used in mobile phones, TVs, and other electronic products. In the process of pursuing high-quality wide-color gamut displays, much attention has been paid to the development of three primary color luminescent materials. Therefore, efficient red, green, and blue (RGB) OLEDs achieve remarkably high external quantum efficiencies (EQE) of over 30%.^9–14 Besides the three primary colors, yellow emission is also vitally important.¹⁵ Yellow emission is essential for RGBY-TVs, and can help to increase the color rendering indexes (CRI) of white OLEDs for high quality solid-state lighting.^16,17 In addition, the monochromatic yellow light is widely used in light-sensitive material preparation laboratories or factories and signal systems. However, due to the very narrow region of the yellow-light chromaticity in the Commission Internationale de L’Eclairage (CIE) diagram, it is not easy to obtain yellow emission with high purity. Thus, there is shortage of efficient phosphorescent complexes with pure yellow emissions.

Up to now, only a few yellow phosphorescent complexes showed high EL performance in vacuum-deposited OLEDs with EQEs exceeding 20%.^17–23 For example, Wang et al. synthesized a yellow Ir(III) complex which showed an impressively high EQE of 25.3% in the vacuum-deposited OLED.¹⁸ Che et al. demonstrated that vacuum-deposited pure yellow OLEDs with CIE coordinates of (0.43, 0.56) and an EQE of 26.3% could be realized with the Pt(II) complex bearing a rigid tetradentate ligand.¹⁹ Due to the advantages in the preparation of flexible large-area devices through inkjet printing or roll-to-roll coating with low-cost manufacturing equipment, solution-processed methods are getting more and more attention in fabricating OLEDs.^24,25 However, efficiencies of yellow phosphorescent OLEDs fabricated with solution-processed methods were relatively lower. To the best of our knowledge, solution-processed yellow OLEDs which could obtain EQEs over 20% were very rare. Jou et al. reported solution-processed OLEDs based on an Ir(III) complex which could show a high EQE of 22.7% and pure yellow CIE coordinates of (0.43, 0.56).¹⁶ Our group recently showed that a trinuclear Pt(II) complex could exhibit yellow emission in the solution-processed OLEDs with an EQE of 16.92% and CIE coordinates of (0.47, 0.52).²⁶ However, this trinuclear Pt(II) complex had relatively low solubility, which hindered the further improvement of device efficiency by optimizing the doping concentration.

Herein, with the aim to increase the EL performance of solution-processed yellow OLEDs, two efficient dinuclear Pt(II) complexes SO-DPt and AB-DPt (Fig. 1) were developed. The diphenylsulfone and arylboron groups were decorated to the dinuclear Pt(II) complex frame for two reasons. Firstly, these two functional groups were expected to increase the photoluminescence quantum yields (PLQYs) and tune the charge transport properties, which had been well demonstrated in other related phosphorescent complexes.^27–30 Secondly, the incorporation of bulky diphenylsulfone and arylboron groups would increase the solubility of the resultant complexes to facilitate the employment of solution methods to fabricate devices. Consequently, doped in host materials, complexes SO-DPt and AB-DPt showed pure yellow emissions with impressively high PLQYs close to 0.9. More importantly, solution-processed OLEDs based on SO-DPt and AB-DPt exhibited excellent EL performance with EQEs reaching up to 21.54% and 21.39%, respectively, which were not only the highest efficiencies reported for OLEDs based on multinuclear Pt(II) complexes, but also among the best performance of solution-processed yellow phosphorescent OLEDs.

Results and discussion

The bromotriphenylamine-based tetradentate ligand was firstly prepared, and then the functional groups of diphenylsulfone and arylboron were incorporated through Suzuki–Miyaura coupling reactions to obtain the final ligands L-SO and L-AB. The designed dinuclear Pt(II) complexes were synthesized according to our previous study with high yields close to 20%.³¹ Synthetic details are provided in the ESI.† The structures of the target dinuclear Pt(II) complexes were fully characterized by NMR (Fig. S1 in the ESI†) and MS (Fig. S2 in the ESI†). According to the thermal gravimetric analysis, the decomposition temperatures (T_d) of SO-DPt and AB-DPt were 302 and 298 °C, respectively (Table 1 and Fig. S3 in the ESI†), indicating the good thermal stability of these dinuclear Pt(II) complexes.

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The steady-state UV-vis absorption properties of SO-DPt and AB-DPt were characterized in CH₂Cl₂ at a concentration of ca. 3 × 10⁻⁵ M at room temperature. As shown in Fig. 2, two main absorption bands were observed for these dinuclear Pt(II) complexes. The high-energy absorption bands peaking around 256 nm, which showed very large extinction coefficients (logε > 4.4), were generally assigned to the spin-allowed singlet ligand-centered ¹ππ* transitions.³² The low-energy absorptions around 430 nm also displayed quite high intensity, which was the typical characteristics of triphenylamine-based Pt(II) complexes,^26,27,33,34 and could be attributed to the spin-allowed singlet intra-ligand charge transfer (¹ILCT) mixed with the ligand-to-metal charge transfer (¹LMCT) transition. The spin-forbidden triplet transitions might be completely overshadowed by the strong absorptions in the range of 450–480 nm. Studies had reported that Pt(II) and Ir(III) complexes bearing the arylboron segment usually displayed pronounced redshifts in low-energy absorption compared with those consisting of the diphenylsulfone group.^35,36 However, in contrast with SO-DPt, it was surprising that AB-DPt displayed only a tiny redshift in the low-energy absorption. To gain a deep understanding of this abnormal phenomenon, we calculated the UV-vis absorption spectra of SO-DPt and AB-DPt based on density functional theory (DFT) and time-dependent DFT (TD-DFT), which showed good agreement with experimental results (Fig. S4 in the ESI†). The calculation result indicated that the lowest-energy absorption peak of SO-DPt was essentially induced by the transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) with a significantly large oscillator strength (f = 0.5546). As presented in Fig. 3 and Table 2, for SO-DPt, the HOMO was concentrated on the triphenylamine core, and the LUMO was mainly distributed on the two pyridinyl and chelated phenyl segments, while the Pt centers made a slightly higher contribution to LUMO (5.58%) than to HOMO (1.53%). Therefore, the lowest-energy absorption of SO-DPt was mainly assigned to ILCT and ππ* transitions mixed with minor LMCT. The Pt centers in AB-DPt made little and comparable contribution to both the HOMO (1.39%) and LUMO (1.26%). However, calculation results suggested that the lowest-energy absorption peak of AB-DPt decisively resulted from the HOMO → LUMO+1 transition (Fig. S4 in the ESI†). Therefore, although the arylboron group made significant contribution to the LUMO of AB-DPt, it showed almost no contribution to LUMO+1 (Fig. 3), thus the lowest-energy absorption of AB-DPt was also mainly assigned to ILCT and ππ* transitions. Considering the contribution of Pt centers to LUMO+1 was higher than to HOMO, a little LMCT was also responsible for the lowest-energy absorption of AB-DPt. Therefore, both diphenylsulfone and arylboron groups did not notably participate in the lowest excited states, and this is the reason why the two dinuclear Pt(II) complexes exhibited similar UV-vis absorption behaviors. However, as shown in Fig. S4 (ESI†), the HOMO → LUMO+1 transition of SO-DPt and the HOMO → LUMO transition of AB-DPt resulted in strong absorptions very close to the lowest energy absorptions, indicating that diphenylsulfone and arylboron moieties had significant influence on higher excited states.

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Consistent with the fact that these two dinuclear Pt(II) complexes had similar UV-vis absorption behaviors, they also displayed almost identical emission wavelengths. Doped in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) films, both SO-DPt and AB-DPt emitted bright yellow light with peaks around 550 nm (Fig. 2). The emissions showed lifetimes of ca. 4.8 μs, confirming their phosphorescent nature. The structured emission profiles with small shoulders located at ca. 580 nm suggested the ³ππ* character of the phosphorescence.³⁷ Theoretical calculations on the natural transition orbitals (NTO) of SO-DPt and AB-DPt were carried out to reveal the feature of their lowest triplet states. As depicted in Fig. 4, hole and particle orbitals of SO-DPt and AB-DPt exhibited almost the same distribution pattern. The hole → particle transition contributed ca. 97% to T₁ states of both SO-DPt and AB-DPt, which clearly implied the dominant ³ππ* character of the phosphorescence. Again, both diphenylsulfone and arylboron groups hardly contributed either to hole or particle orbitals, and thereby exerted tiny influence on the emission wavelengths. The Pt centers made a slightly higher contribution to particle orbitals (4.62% for SO-DPt and 4.61% for AB-DPt) than to hole orbitals (3.87% for SO-DPt and 3.52% for AB-DPt) (Table S1 in the ESI†), implying that the T₁ states of SO-DPt and AB-DPt had very little LMCT feature. This is because the triphenylamine group usually showed stronger electron-donating ability than metal centers in triphenylamine-based organometallic complexes.^33,35,38,39 Although the direct contribution from Pt centers to hole → particle transitions was tiny, strong spin–orbit coupling (SOC) induced by the two Pt centers was still active to facilitate the intersystem crossing from S₁ to T₁, leading to intense phosphorescence at room temperature.^33,40 Furthermore, as aforementioned, diphenylsulfone and arylboron moieties had significant influence on higher excited states. Those closely located higher lying states would contribute to the emitting triplet substates via SOC, leading to enhanced phosphorence.⁴¹ Therefore, the PLQYs of doped films were determined to be as high as 0.89 and 0.86 for SO-DPt and AB-DPt, respectively, which were among the highest efficiencies for multinuclear metal complexes.^42–44 This result demonstrates that the incorporation of diphenylsulfone and arylboron groups is good for improving phosphorescence efficiency.

Cyclic voltammetry (CV) was performed to investigate the electrochemical properties of SO-DPt and AB-DPt in CH₂Cl₂ solutions (Fig. S5 in the ESI†). The first oxidation onsets were recorded at 0.19 and 0.16 V for SO-DPt and AB-DPt, respectively. Considering the low potentials and the high contribution of the triphenylamine group to HOMOs, these irreversible oxidations should occur on the triphenylamine groups. Accordingly, the HOMO levels were deduced to be −4.99 and −4.96 eV for SO-DPt and AB-DPt, respectively. No notable oxidation processes were recorded for Pt centers at higher potentials, which was in line with the fact that Pt centers had little contribution to HOMOs. During cathodic scans, reduction behaviors were observed at similar potentials (−2.23 and −2.26 V for SO-DPt and AB-DPt, respectively), indicating that the reductions occurred on pyridinyl segments. Therefore, LUMO levels of SO-DPt and AB-DPt were estimated to be −2.57 and −2.54 eV, respectively.

To explore the EL properties of SO-DPt and AB-DPt, OLEDs were fabricated by the solution-processed method with a common device structure of ITO/PEDOT:PSS (30 nm)/PVK (25 nm)/x wt% Pt complex:CBP (20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) (Fig. 5). The compounds PEDOT:PSS [poly(ethylenedioxythiophene):poly(styrenesulfonate)]/PVK [poly(9-vinylcarbazole)], CBP, and TmPyPB [1,3,5-tri(m-pyridin-3-ylphenyl)benzene] were employed as the hole-injection/transport, host, and electron-transport materials, respectively. The EL performance was optimized by adjusting the doping concentrations at 1, 2, and 3 wt% for devices A1–A3 or B1–B3.

These devices could be turned on at relatively low voltages in the range from 3.7 to 4.1 V, and show EL spectra matching well with the corresponding PL spectrum of the doped film (Fig. 6 and Fig. S6, S7 in the ESI†). No emissions from adjacent hole/electron transport layers or CBP host were recorded, indicating the effective energy transfer and good exciton confinement within the emissive layers. The current density–voltage–luminance (J–V–L) characteristics and efficiencies versus luminance curves of these devices are provided in Fig. 6. Among all devices, the 2 wt% doped OLEDs exhibited the best EL performance (Table 3). The device A2 based on SO-DPt achieved the highest EQE, CE, and PE of 21.54%, 76.64 cd A⁻¹, and 52.33 lm W⁻¹, respectively. The device B2 based on AB-DPt displayed comparable efficiencies with EQE, CE, and PE of 21.39%, 74.35 cd A⁻¹, and 48.08 lm W⁻¹, respectively. Devices with other doping concentrations also showed impressively high EQEs around 20% (Table 3). To the best of our knowledge, these efficiencies are the highest EQEs reported for OLEDs employing dinuclear Pt(II) complexes as emitters (Fig. S8, ESI†).^40,45–52 The excellent device performance could be attributed to the high PLQYs and good charge transport ability of SO-DPt and AB-DPt. In addition, as illustrated in Fig. 6, the efficiency roll-offs of these devices were also relatively small. It is worth mentioning that these devices emitted pure yellow light. In particular, the emission of the AB-DPt-based device had CIE coordinates of (0.45, 0.54), which precisely located at the very narrow region of the yellow-light chromaticity in the CIE diagram (Fig. 6a). Therefore, compared with other solution-processed yellow phosphorescent devices, OLEDs based on SO-DPt and AB-DPt not only achieved top efficiencies, but also showed attractively high color purity (Table S2 in the ESI†).^{16,21,53–62} These results demonstrated that dinuclear Pt(II) complexes SO-DPt and AB-DPt are promising for practical application in high performance pure yellow OELDs.

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Conclusions

To conclude, two dinuclear Pt(II) complexes SO-DPt and AB-DPt were successfully developed with the triphenylamine-based tetradentate ligands. The diphenylsulfone and arylboron groups were used to adjust the properties of the resultant emitters. Theoretical calculation results revealed that the diphenylsulfone and arylboron groups were not directly involved in the lowest excited states, but showed significant contribution to closely located higher lying excited states, which could benefit the phosphorescence efficiency. The films doped with SO-DPt and AB-DPt exhibited pure yellow emissions with impressively high PLQYs of 0.89 and 0.86, respectively. Solution-processed OLEDs were fabricated and showed outstanding EL performance with EQEs over 21%, which were the highest efficiencies reported for OLEDs based on dinuclear Pt(II) complexes to date. In addition, the yellow emissions had CIE coordinates of (0.45, 0.54), which demonstrated the attractively high color purity. This work highlights the great potential of SO-DPt and AB-DPt for application in pure yellow OLEDs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge financial support from the National Natural Science Foundation of China (51803163, 21875179, 21602170, and 21572176), the Natural Science Foundation of Shaanxi Province (2021JM-023, 2019JZ-29, and 2019JQ-188), and the China Postdoctoral Science Foundation (2016M600778 and 2020M673369). The characterization assistance from the Instrument Analysis Center of Xi’an Jiaotong University was also acknowledged.

References

1. M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R. Forrest, Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices, Nature, 1998, 395, 151–154 Search PubMed .
2. Y. Ma, H. Zhang, J. Shen and C. Che, Electroluminescence from Triplet Metal-Ligand Charge-Transfer Excited State of Transition Metal complexes, Synth. Met., 1998, 94, 245–248 Search PubMed .
3. Y. Chi and P.-T. Chou, Transition-metal phosphors with cyclometalating ligands: fundamentals and applications, Chem. Soc. Rev., 2010, 39, 638–655 Search PubMed .
4. P. Tao, Y. Miao, H. Wang, B. Xu and Q. Zhao, High-Performance Organic Electroluminescence: Design from Organic Light-Emitting Materials to Devices, Chem. Rec., 2019, 19, 1531–1561 Search PubMed .
5. W.-J. Joo, J. Kyoung, M. Esfandyarpour, S.-H. Lee, H. Koo, S. Song, Y.-N. Kwon, S. H. Song, J. C. Bae, A. Jo, M.-J. Kwon, S. H. Han, S.-H. Kim, S. Hwang and M. L. Brongersma, Metasurface-driven OLED displays beyond 10,000 pixels per inch, Science, 2020, 370, 459–463 Search PubMed .
6. D. Zhang, T. Huang and L. Duan, Emerging Self-Emissive Technologies for Flexible Displays, Adv. Mater., 2020, 32, 1902391 Search PubMed .
7. Z. Chen, C.-L. Ho, L. Wang and W.-Y. Wong, Single-Molecular White-Light Emitters and Their Potential WOLED Applications, Adv. Mater., 2020, 32, 1903269 Search PubMed .
8. Y. Yin, M. U. Ali, W. Xie, H. Yang and H. Meng, Evolution of white organic light-emitting devices: from academic research to lighting and display applications, Mater. Chem. Front., 2019, 3, 970–1031 Search PubMed .
9. Y.-K. Wang, S.-H. Li, S.-F. Wu, C.-C. Huang, S. Kumar, Z.-Q. Jiang, M.-K. Fung and L.-S. Liao, Tilted Spiro-Type Thermally Activated Delayed Fluorescence Host for ≈100% Exciton Harvesting in Red Phosphorescent Electronics with Ultralow Doping Ratio, Adv. Funct. Mater., 2018, 28, 1706228 Search PubMed .
10. W.-C. Chen, C. Sukpattanacharoen, W.-H. Chan, C.-C. Huang, H.-F. Hsu, D. Shen, W.-Y. Hung, N. Kungwan, D. Escudero, C.-S. Lee and Y. Chi, Modulation of Solid-State Aggregation of Square-Planar Pt(II) Based Emitters: Enabling Highly Efficient Deep-Red/Near Infrared Electroluminescence, Adv. Funct. Mater., 2020, 30, 2002494 Search PubMed .
11. X.-F. Ma, J.-C. Xia, Z.-P. Yan, X.-F. Luo, Z.-G. Wu, Y.-X. Zheng and W.-W. Zhang, Highly efficient green and red electroluminescence with an extremely low efficiency roll-off based on iridium(III) complexes containing a bis(diphenylphorothioyl)amide ancillary ligand, J. Mater. Chem. C, 2019, 7, 2570–2576 Search PubMed .
12. Z. Yan, Y. Wang, J. Ding, Y. Wang and L. Wang, Highly Efficient Phosphorescent Furo[3,2-c]pyridine Based Iridium Complexes with Tunable Emission Colors over the Whole Visible Range, ACS Appl. Mater. Interfaces, 2018, 10, 1888–1896 Search PubMed .
13. H. Shin, Y. H. Ha, H.-G. Kim, R. Kim, S.-K. Kwon, Y.-H. Kim and J.-J. Kim, Controlling Horizontal Dipole Orientation and Emission Spectrum of Ir Complexes by Chemical Design of Ancillary Ligands for Efficient Deep-Blue Organic Light-Emitting Diodes, Adv. Mater., 2019, 31, 1808102 Search PubMed .
14. R. Zaen, K.-M. Park, K. H. Lee, J. Y. Lee and Y. Kang, Blue Phosphorescent Ir(III) Complexes Achieved with Over 30% External Quantum Efficiency, Adv. Opt. Mater., 2019, 7, 1901387 Search PubMed .
15. C. Fan and C. Yang, Yellow/orange emissive heavy-metal complexes as phosphors in monochromatic and white organic light-emitting devices, Chem. Soc. Rev., 2014, 43, 6439–6469 Search PubMed .
16. J.-H. Jou, S.-C. Fu, C.-C. An, J.-J. Shyue, C.-L. Chin and Z.-K. He, High efficiency yellow organic light-emitting diodes with a solution-process feasible iridium based emitter, J. Mater. Chem. C, 2017, 5, 5478–5486 Search PubMed .
17. P. Tao, W.-L. Li, J. Zhang, S. Guo, Q. Zhao, H. Wang, B. Wei, S.-J. Liu, X.-H. Zhou, Q. Yu, B.-S. Xu and W. Huang, Facile Synthesis of Highly Efficient Lepidine-Based Phosphorescent Iridium(III) Complexes for Yellow and White Organic Light-Emitting Diodes, Adv. Funct. Mater., 2016, 26, 881–894 Search PubMed .
18. T. Peng, G. Li, K. Ye, C. Wang, S. Zhao, Y. Liu, Z. Hou and Y. Wang, Highly efficient phosphorescent OLEDs with host-independent and concentration-insensitive properties based on a bipolar iridium complex, J. Mater. Chem. C, 2013, 1, 2920–2926 Search PubMed .
19. G. Cheng, S. C. F. Kui, W.-H. Ang, M.-Y. Ko, P.-K. Chow, C.-L. Kwong, C.-C. Kwok, C. Ma, X. Guan, K.-H. Low, S.-J. Su and C.-M. Che, Structurally robust phosphorescent [Pt(O^N^C^N)] emitters for high performance organic light-emitting devices with power efficiency up to 126 lm W⁻¹ and external quantum efficiency over 20%, Chem. Sci., 2014, 5, 4819–4830 Search PubMed .
20. H.-B. Han, X.-F. Ma, Z.-G. Wu and Y.-X. Zheng, Highly efficient yellow electroluminescence of iridium complexes with good electron mobility, Mater. Chem. Front., 2018, 2, 1284–1290 Search PubMed .
21. J.-H. Jou, Y.-X. Lin, S.-H. Peng, C.-J. Li, Y.-M. Yang, C.-L. Chin, J.-J. Shyue, S.-S. Sun, M. Lee, C.-T. Chen, M.-C. Liu, C.-C. Chen, G.-Y. Chen, J.-H. Wu, C.-H. Li, C.-F. Sung, M.-J. Lee and J.-P. Hu, Highly Efficient Yellow Organic Light Emitting Diode with a Novel Wet- and Dry-Process Feasible Iridium Complex Emitter, Adv. Funct. Mater., 2014, 24, 555–562 Search PubMed .
22. C.-W. Hsu, K. T. Ly, W.-K. Lee, C.-C. Wu, L.-C. Wu, J.-J. Lee, T.-C. Lin, S.-H. Liu, P.-T. Chou, G.-H. Lee and Y. Chi, Triboluminescence and Metal Phosphor for Organic Light-Emitting Diodes: Functional Pt(II) Complexes with Both 2-Pyridylimidazol-2-ylidene and Bipyrazolate Chelates, ACS Appl. Mater. Interfaces, 2016, 8, 33888–33898 Search PubMed .
23. H. T. Kidanu and C.-T. Chen, Achieving pure yellow, high-efficiency (EQE > 20%) electroluminescence from ultrathin emitting layer (0.6–2.0 nm) OLEDs having a rare aggregation-free heteroleptic platinum complex, J. Mater. Chem. C, 2021, 9, 1410–1418 Search PubMed .
24. J.-H. Jou, S. Sahoo, D. K. Dubey, R. A. K. Yadav, S. S. Swayamprabha and S. D. Chavhan, Molecule-based monochromatic and polychromatic OLEDs with wet-process feasibility, J. Mater. Chem. C, 2018, 6, 11492–11518 Search PubMed .
25. S. Wang, H. Zhang, B. Zhang, Z. Xie and W.-Y. Wong, Towards high-power-efficiency solution-processed OLEDs: Material and device perspectives, Mater. Sci. Eng., R, 2020, 140, 100547 Search PubMed .
26. Y. Sun, B. Liu, Y. Guo, Z. Feng, G. Zhou, Z. Chen and X. Yang, Triphenylamine-based trinuclear Pt(II) complexes for solution-processed OLEDs displaying efficient pure yellow and red emissions, Org. Electron., 2021, 91, 106101 Search PubMed .
27. Z. M. Hudson, C. Sun, M. G. Helander, H. Amarne, Z.-H. Lu and S. Wang, Enhancing Phosphorescence and Electrophosphorescence Efficiency of Cyclometalated Pt(II) Compounds with Triarylboron, Adv. Funct. Mater., 2010, 20, 3426–3439 Search PubMed .
28. X. Yang, H. Guo, B. Liu, J. Zhao, G. Zhou, Z. Wu and W.-Y. Wong, Diarylboron-Based Asymmetric Red-Emitting Ir(III) Complex for Solution-Processed Phosphorescent Organic Light-Emitting Diode with External Quantum Efficiency above 28%, Adv. Sci., 2018, 5, 1701067 Search PubMed .
29. H. Benjamin, Y. Zheng, A. S. Batsanov, M. A. Fox, H. A. Al-Attar, A. P. Monkman and M. R. Bryce, Sulfonyl-Substituted Heteroleptic Cyclometalated Iridium(III) Complexes as Blue Emitters for Solution-Processable Phosphorescent Organic Light-Emitting Diodes, Inorg. Chem., 2016, 55, 8612–8627 Search PubMed .
30. J. Zhao, Y. Yu, X. Yang, X. Yan, H. Zhang, X. Xu, G. Zhou, Z. Wu, Y. Ren and W.-Y. Wong, Phosphorescent Iridium(III) Complexes Bearing Fluorinated Aromatic Sulfonyl Group with Nearly Unity Phosphorescent Quantum Yields and Outstanding Electroluminescent Properties, ACS Appl. Mater. Interfaces, 2015, 7, 24703–24714 Search PubMed .
31. Y. Sun, C. Chen, B. Liu, Y. Guo, Z. Feng, G. Zhou, Z. Chen and X. Yang, Efficient dinuclear Pt(II) complexes based on the triphenylphosphine oxide scaffold for high performance solution-processed OLEDs, J. Mater. Chem. C, 2021, 9, 5373–5378 Search PubMed .
32. S.-Y. Chang, J.-L. Chen, Y. Chi, Y.-M. Cheng, G.-H. Lee, C.-M. Jiang and P.-T. Chou, Blue-emitting platinum (II) complexes bearing both pyridylpyrazolate chelate and bridging pyrazolate ligands: synthesis, structures, and photophysical properties, Inorg. Chem., 2007, 46, 11202–11212 Search PubMed .
33. M. Velusamy, C.-H. Chen, Y. S. Wen, J. T. Lin, C.-C. Lin, C.-H. Lai and P.-T. Chou, Cyclometalated Platinum(II) Complexes of Lepidine-Based Ligands as Highly Efficient Electrophosphors, Organometallics, 2010, 29, 3912–3921 Search PubMed .
34. Z. Hu, Y. Wang, D. Shi, H. Tan, X. Li, L. Wang, W. Zhu and Y. Cao, Highly-efficiency red-emitting platinum(II) complexes containing 4′-diarylamino-1-phenylisoquinoline ligands in polymer light-emitting diodes: Synthesis, structure, photoelectron and electroluminescence, Dyes Pigm., 2010, 86, 166–173 Search PubMed .
35. G. Zhou, Q. Wang, X. Wang, C.-L. Ho, W.-Y. Wong, D. Ma, L. Wang and Z. Lin, Metallophosphors of Platinum with Distinct Main-Group Elements: a Versatile Approach Towards Color Tuning and White-Light Emission with Superior Efficiency/Color Quality/Brightness Trade-offs, J. Mater. Chem., 2010, 20, 7472–7484 Search PubMed .
36. G. J. Zhou, C. L. Ho, W. Y. Wong, Q. Wang, D. G. Ma, L. X. Wang, Z. Y. Lin, T. B. Marder and A. Beeby, Manipulating charge-transfer character with electron-withdrawing main-group moieties for the color tuning of iridium electrophosphors, Adv. Funct. Mater., 2008, 18, 499–511 Search PubMed .
37. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest and M. E. Thompson, Highly phosphorescent bis-cyclometalated iridium complexes: Synthesis, photophysical characterization, and use in organic light emitting diodes, J. Am. Chem. Soc., 2001, 123, 4304–4312 Search PubMed .
38. X. Yang, Y. Zhao, X. Zhang, R. Li, J. Dang, Y. Li, G. Zhou, Z. Wu, D. Ma and W.-Y. Wong, Thiazole-based metallophosphors of iridium with balanced carrier injection/transporting features and their two-colour WOLEDs fabricated by both vacuum deposition and solution processing-vacuum deposition hybrid strategy, J. Mater. Chem., 2012, 22, 7136–7148 Search PubMed .
39. M. Zhu, Y. Li, C. G. Li, C. Zhong, C. Yang, H. Wu, J. Qin and Y. Cao, Tuning the energy levels and photophysical properties of triphenylamine-featured iridium(III) complexes: application in high performance polymer light-emitting diodes, J. Mater. Chem., 2012, 22, 11128–11133 Search PubMed .
40. Z. Hao, K. Zhang, K. Chen, P. Wang, Z. Lu, W. Zhu and Y. Liu, More efficient spin–orbit coupling: adjusting the ligand field strength to the second metal ion in asymmetric binuclear platinum(II) configurations, Dalton Trans., 2020, 49, 8722–8733 Search PubMed .
41. H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck and T. Fischer, The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs, Coord. Chem. Rev., 2011, 255, 2622–2652 Search PubMed .
42. G. Li, D. Zhu, X. Wang, Z. Su and M. R. Bryce, Dinuclear metal complexes: multifunctional properties and applications, Chem. Soc. Rev., 2020, 49, 765–838 Search PubMed .
43. M. Chaaban, C. Zhou, H. Lin, B. Chyi and B. Ma, Platinum(II) binuclear complexes: molecular structures, photophysical properties, and applications, J. Mater. Chem. C, 2019, 7, 5910–5924 Search PubMed .
44. E. V. Puttock, M. T. Walden and J. A. G. Williams, The luminescence properties of multinuclear platinum complexes, Coord. Chem. Rev., 2018, 367, 127–162 Search PubMed .
45. A. Tronnier and T. Strassner, (C^C*) Cyclometalated binuclear N-heterocyclic biscarbene platinum(II) complexes-highly emissive phosphorescent emitters, Dalton Trans., 2013, 42, 9847–9851 Search PubMed .
46. B. Ma, P. I. Djurovich, S. Garon, B. Alleyne and M. E. Thompson, Platinum Binuclear Complexes as Phosphorescent Dopants for Monochromatic and White Organic Light-Emitting Diodes, Adv. Funct. Mater., 2006, 16, 2438–2446 Search PubMed .
47. X. Yang, B. Jiao, J.-S. Dang, Y. Sun, Y. Wu, G. Zhou and W.-Y. Wong, Achieving High-Performance Solution-Processed Orange OLEDs with the Phosphorescent Cyclometalated Trinuclear Pt(II) Complex, ACS Appl. Mater. Interfaces, 2018, 10, 10227–10235 Search PubMed .
48. M. Z. Shafikov, R. Daniels, P. Pander, F. B. Dias, J. A. G. Williams and V. N. Kozhevnikov, Dinuclear Design of a Pt(II) Complex Affording Highly Efficient Red Emission: Photophysical Properties and Application in Solution-Processible OLEDs, ACS Appl. Mater. Interfaces, 2019, 11, 8182–8193 Search PubMed .
49. J. Yu, X. Wu, H. Tan, Y. Liu, Y. Wang, M. Zhu and W. Zhu, High-efficiency saturated red emission from binuclear cyclo-metalated platinum complex containing 5-(4-octyloxyphenyl)-1,3,4-oxadiazole-2-thiol ancillary ligand in PLEDs, Sci. China: Chem., 2013, 56, 1137 Search PubMed .
50. J. Zhang, L. Wang, A. Zhong, G. Huang, F. Wu, D. Li, M. Teng, J. Wang and D. Han, Deep red PhOLED from dimeric salophen Platinum(II) complexes, Dyes Pigm., 2019, 162, 590–598 Search PubMed .
51. W. Xiong, F. Meng, C. You, P. Wang, J. Yu, X. Wu, Y. Pei, W. Zhu, Y. Wang and S. Su, Molecular isomeric engineering of naphthyl-quinoline-containing dinuclear platinum complexes to tune emission from deep red to near infrared, J. Mater. Chem. C, 2019, 7, 630–638 Search PubMed .
52. Y. Sun, C. Chen, B. Liu, Y. Guo, Z. Feng, G. Zhou, Z. Chen and X. Yang, Efficient dinuclear Pt(II) complexes based on the triphenylphosphine oxide scaffold for high performance solution-processed OLEDs, J. Mater. Chem. C, 2021, 9, 5373–5378 Search PubMed .
53. A. K.-W. Chan, M. Ng, Y.-C. Wong, M.-Y. Chan, W.-T. Wong and V. W.-W. Yam, Synthesis and characterization of luminescent cyclometalated platinum(II) complexes with tunable emissive colors and studies of their application in organic memories and organic light-emitting devices, J. Am. Chem. Soc., 2017, 139, 10750–10761 Search PubMed .
54. A. Liang, G. Huang, S. Dong, X. Zheng, J. Zhu, Z. Wang, W. Wu, J. Zhang and F. Huang, Novel iridium complexes as yellow phosphorescent emitters for single-layer yellow and white polymer light-emitting diodes, J. Mater. Chem. C, 2016, 4, 6626–6633 Search PubMed .
55. B.-Y. Ren, R.-D. Guo, D.-K. Zhong, C.-J. Ou, G. Xiong, X.-H. Zhao, Y.-G. Sun, M. Jurow, J. Kang, Y. Zhao, S.-B. Li, L.-X. You, L.-W. Wang, Y. Liu and W. Huang, A Yellow-Emitting Homoleptic Iridium(III) Complex Constructed from a Multifunctional Spiro Ligand for Highly Efficient Phosphorescent Organic Light-Emitting Diodes, Inorg. Chem., 2017, 56, 8397–8407 Search PubMed .
56. X. Yang, Z. Feng, J. Dang, Y. Sun, G. Zhou and W.-Y. Wong, High performance solution-processed organic yellow light-emitting devices and fluoride ion sensors based on a versatile phosphorescent Ir(III) complex, Mater. Chem. Front., 2019, 3, 376–384 Search PubMed .
57. X. Ouyang, D. Chen, S. Zeng, X. Zhang, S. Su and Z. Ge, Highly efficient and solution-processed iridium complex for single-layer yellow electrophosphorescent diodes, J. Mater. Chem., 2012, 22, 23005–23011 Search PubMed .
58. G. Sarada, J. Yoon, W. Cho, M. Cho, D. W. Cho, S. O. Kang, Y. Nam, J. Y. Lee and S.-H. Jin, Substituent position engineering of diphenylquinoline-based Ir(III) complexes for efficient orange and white PhOLEDs with high color stability/low efficiency roll-off using a solution-processed emission layer, J. Mater. Chem. C, 2016, 4, 113–120 Search PubMed .
59. T. Giridhar, W. Cho, Y.-H. Kim, T.-H. Han, T.-W. Lee and S.-H. Jin, A systematic identification of efficiency enrichment between thiazole and benzothiazole based yellow iridium(III) complexes, J. Mater. Chem. C, 2014, 2, 9398–9405 Search PubMed .
60. L. Fu, M. Pan, Y. H. Li, H. B. Wu, H. P. Wang, C. Yan, K. Li, S. C. Wei, Z. Wang and C. Y. Su, A butterfly-like yellow luminescent Ir(III) complex and its application in highly efficient polymer light-emitting devices, J. Mater. Chem., 2012, 22, 22496–22500 Search PubMed .
61. J. H. Jou, H. H. Yu, Y. X. Lin, J. R. Tseng, S. H. Peng, Y. C. Jou, C. H. Lin, S. M. Shen, C. Y. Hsieh, M. K. Wei, D. H. Lin, C. C. Wang, C. C. Chen, F. C. Tung, S. H. Chen and Y. S. Wang, High efficiency yellow organic light emitting diodes with a balanced carrier injection co-host structure, J. Mater. Chem. C, 2013, 1, 5110–5115 Search PubMed .
62. T. Ye, S. Shao, J. Chen, L. Wang and D. Ma, Efficient Phosphorescent Polymer Yellow-Light-Emitting Diodes Based on Solution-Processed Small Molecular Electron Transporting Layer, ACS Appl. Mater. Interfaces, 2011, 3, 410–416 Search PubMed .

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00507c

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