Photophysical properties of the bidentate, tridentate, and tetradentate platinum(II) complexes.
Abstract
As one of the most important phosphorescent emitters, tetradentate cyclometalated platinum(II) complexes have attracted much attention in recent years, because of the high luminescent efficiency, emission spectra, and color tuned easily, especially for the development of high-efficient deep-blue and “pure” blue emitters and single-doped white organic light-emitting diodes (OLEDs). Also, some platinum(II)-based OLEDs exhibited superior operational stability, indicating their potentials in full-color display and solid-state lighting applications. In this chapter, we will introduce the recent advances of the tetradentate cyclometalated platinum(II) complexes, including pyrazole, N-heterocyclic carbene, imidazole and pyridine-based complexes, molecular design, photophysical properties, and some of their device performances.
Keywords
- platinum complex
- tetradentate
- OLED
- blue emitter
- phosphorescence
- operational lifetime
1. Introduction
In the 1960s, the first organic electroluminescent spectrum was reported from the crystal of anthracene [1]. In 1987, Tang and VanSlyke from Eastman Kodak Company successfully demonstrated an efficient and practical organic light-emitting diode (OLED) employing tris(8-hydroxyquinolinato)aluminum (Alq3) as a fluorescent emitter [2]. After that, OLEDs began to attract more and more attention in both academic and industrial researches for their potential applications for full-color displays and solid-state lighting industry.
From the spin statistics, it is well known that the singlet and triplet in the electrogenerated excitons are 25 and 75%, respectively [3]. As a result, OLEDs using fluorescent emitters, which emit from the singlet excited state, can achieve a peak internal quantum efficiency (IQE) only 25%. However, if heavy metal ion is incorporated into the organic ligand, phosphorescent emitters can break the spin-forbidden reactions, and fast intersystem crossing (IC) from singlet to triplet state can occur owing to the strong electron spin-orbit coupling (SOC); thus, heavy metal complexes have the potential to harvest both the electrogenerated singlet and triplet excitons and achieve 100% IQE. In 1998, Forrest and Thompson et al. and Che et al. first reported the electrogenerated phosphorescent platinum(II) [4] and osmium(II) [5] complexes, respectively. Afterward, more heavy metal complexes were found to be used as efficient phosphorescent materials, like iridium(III), ruthenium(II), palladium (II), rhodium (III), gold(III), and so on, and some reviews about these complexes have been published [6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]. Among them, iridium(III) complexes have been most widely studied. Green and red phosphorescent iridium(III) emitters developed by Universal Display Corporation (UDC) have been successfully commercialized due to their superior efficiency and long operational lifetime. OLED display doped these emitters that have been adopted for several types of high-end personal electronics, such as Samsung Galaxy, LG OLED television, Apple smart watch, and iPhone X. Compared with the liquid crystal display (LCD), OLED display have many outstanding merits, such as low-cost fabrication methods, high color quality, and high-luminance efficiency and also many advantages of low power consumption, wide-viewing angle, wide temperature range, fast response, etc [19, 20]. Thus, OLED has been widely considered as the next generation of full-color display and solid-state lighting technologies.
The development of high efficient and stable phosphorescent emitters is of the most importance for the development of OLEDs and their application. Although thousands of phosphorescent heavy metal complexes have been reported, the emitters can meet the requirement of commercialized displays, which are extremely rare. Now, considerable challenges still remain, for example, the development of efficient green and red emitters with high color quality, especially for the efficient and stable blue and deep-blue phosphorescent emitters. Much of the previous research work and the commercialized phosphorescent emitters mainly focused on the iridium(III) complexes. However, in the past few years, many reports demonstrated that the photophysical properties and device performances of the platinum(II)-based emitters could compare with or even superior to the iridium(III) ones in many aspects [16]. Also, some unique properties were found for some of the platinum(II) complexes, like narrowband emissive spectra, efficient deep-blue emitting, and excimer formation for single-doped white OLEDs [16]. These properties enable the platinum(II) complexes to have potential to be utilized in commercialized displays.
Taking into account the rapid development and unique properties of the platinum(II) complexes, in this chapter, we will mainly highlight their recent progress regarding their molecular design, photophysical properties, and device performances, especially for the tetradentate ones with cyclometalating ligands based on pyrazole,
2. Why employ tetradentate ligands?
Because of the relatively long luminescent lifetime and poor quantum efficiency (ϕ), platinum(II) complexes were historically not considered as ideal emitters. However, through judicious molecular design, bidentate platinum(II) complex can also emit strongly with lifetime in microsecond region, such as (ppy)Pt(acac) (Table 1) (1) [21]. Due to dsp2 hybrid orbitals that are adopted for the Pt(II) ion, the molecular configuration of the platinum(II) complexes is square planar. Consequently, bidentate platinum(II) complexes are usually very flexible, and the excited state energy can be consumed by many nonradiative decay pathways, like molecular distortion and bond vibration. This can be proven by the emission spectrum of (ppy)Pt(acac) (Figure 1), which exhibits a strong vibrational transition v0,1 at 518 nm, and, also, the nonradiative decay rate is 4.5 times faster than that of the radiative decay rate in CH2Cl2 solution at room temperature (RT).
The rigidity of the molecule would be enhanced if the tridentate ligand was employed, which could suppress the nonradiative decay pathway and favor to increase the ϕ. Therefore, Pt(dpyd)Cl (
Judicious tetradentate ligand design could provide rational coordination sites to the platinum(II) ions and maintain the square planar configuration, which are also of benefit to the material synthesis with high metallization yields. Most importantly, tetradentate platinum(II) complexes would have more rigid molecular configuration and improved photophysical and chemical properties. For example, the ϕ of the phenoxyl-pyridine (popy)-based complex PtOO3 [16, 23] could be up to over 80% in CH2Cl2 solution and be achieved to nearly unity in rigid PMMA matrix. If more rigid carbazolyl-pyridine was incorporated and served as ancillary ligand, the ϕ could be further improved to 100% yield even in CH2Cl2 solution for complex PtON3. Furthermore, tetradentate platinum(II) complexes could be easily modified to improve their photophysical and chemical properties through changing ligand’s conjugation degree, utilizing different coordination atoms, adopting various linking groups, or forming five- or six-membered chelates. Thanks to the continuous efforts of the scientific community, many efficient and stable platinum(II) complexes had been developed, making them serve as ideal phosphorescent emitters for OLED applications.
3. Pyrazole-based tetradentate platinum(II) complexes
Because of electron-donating character and relatively weak π-conjugation ability of the nitrogen atom at the 2-position of the pyrazole ring, 1-phenyl-pyrazole (ppy) and its derivatives are widely incorporated into the tetradentate platinum(II) complexes (Figure 2). These complexes usually have a high LUMO energy level, making them suitable for developing green to blue emitting materials. The photophysical properties and some of the device performances of the pyrazole-based complexes are summarized in Tables 2 and 3.
Comp. | In solution at RT | In PMMA at RT | ||||||
---|---|---|---|---|---|---|---|---|
Solvent | λmax/nm | FWHM/nm | ϕ/% | τ/μs | λmax/nm | ϕ/% | τ/μs | |
2-MeTHF | 484;512 | — | 56 | 4.9 | — | — | — | |
2-MeTHF | 474 | — | 37 | 3.4 | — | — | — | |
2-MeTHF | 486;516 | — | 63 | 5.7 | — | — | — | |
CH2Cl2 | 555 | — | 17 | 4.4 | — | — | — | |
CH2Cl2 | 430;456 | — | 39 | 3.0 | — | 83 | 7.5 | |
CH2Cl2 | 454;478 | 85 | 71 | 3.3 | 449 | 85 | 4.5 | |
CH2Cl2 | 442 | 15 | 80 | 13.5 | 440 | 88 | 11.3 | |
CH2Cl2 | 444 | 20 | 89 | 10.0 | 445 | 84 | 7.6 | |
CH2Cl2 | 444 | 20 | 95 | 8.9 | 445 | 88 | 8.8 | |
CH2Cl2 | 496 | 84 | 53 | 1.8 | 480 | 64 | 2.0 | |
CH2Cl2 | 450;476 | 79 | 82 | 3.5 | 445 | 84 | 4.3 | |
CH2Cl2 | 450;478 | 121 | 45 | 3.1 | 449 | 78 | 4.8 | |
CH2Cl2 | 546 | 95 | 19 | 0.8 | 503 | 88 | 2.2 | |
CH2Cl2 | 568 | 104 | 1.1 | 0.6 | 544 | 29 | 0.9 | |
CH2Cl2 | 448 | 19 | — | — | 447 | 81 | 7.4 | |
CH2Cl2 | 491 | 18 | 81 | 12.9 | — | 90 | — | |
CH2Cl2 | 573 | 26 | 40 | 3.4 | — | — | — | |
CH2Cl2 | 536 | 111 | — | — | 476 | 18.0 | ||
CH2Cl2 | 486 | 46 | — | — | 476 | 68 | — | |
CH2Cl2 | 443;471 | — | 70 | — | — | — | — | |
CH2Cl2 | 449;477 | — | 24 | — | — | — | — | |
CH2Cl2 | 444;474 | — | 34 | — | — | — | — |
Dopant | λmax/nm | FWHM/nm | CIE | ηEQE | |
---|---|---|---|---|---|
Peak (%) | 100 cd/m2 (%) | ||||
6% PtON1( |
454 | 46 | (0.15, 0.13) | 25.2 | 23.3 |
2% PtOO1-tBu ( |
448 | 24 | (0.151, 0.098) | 5.3 | 2.7 |
2% PtON6-tBu ( |
452 | 30 | (0.147, 0.093) | 10.9 | 6.6 |
7% PtN1N ( |
498 | 20 | (0.15, 0.56) | 26.1 | 25.8 |
2% PtN8ppy ( |
576 | 28 | (0.53, 0.47) | 19.3 | 16.0 |
6% PtN’1 N-tBu ( |
490 | 34 | (0.157, 0.491) | 17.8 | 17.3 |
30% |
540 | — | (0.33, 0.57) | 16.4 | — |
20% |
456 | — | (0.18, 0.30) | 7.7 | — |
20% |
541 | — | (0.35, 0.55) | 15.7 | — |
In 2010, Huo et al. reported a series of symmetric tetradentate platinum(II) complexes (
To develop blue or deep-blue emitters, the arylamino liker should be replaced with less-conjugated ones, like oxygen or functionalized carbon groups. Based on this design, PtOO1 (
In 2013, Li′s group developed a new type of tetradentate platinum(II) complex PtON1 (
To afford deep or “pure” blue emitters, the CIEy ≤ 0.1 is needed. To achieve this goal, narrowband emission is required to eliminate the color contamination from the green region. Through a systemic research work, it was found that the emission from the PyCz ligand could be suppressed by introducing electron-donating group, like -Me, -tBu, and -NMe2, to the 4-position of the pyridine ring to increase the energy level of the metal-to-ligand charge-transfer (MLCT) states of the PyCz moiety. Therefore, a series of deep-blue emitters, PtON1-NMe2, PtON1-Me, and PtON1-tBu (
In addition to modifying the cyclometalating ligand, the 1-phenyl-pyrazole ligand can be replaced with low-energy ligand, like pyrazolyl-carbazole, and green emitter PtN1N (
The development of efficient and stable blue emitters still maintains a challenge. In order to achieve this goal, chemically and thermally stable ligands must be adopted. Based on the above work, the carbazole in PtN1N was replaced with 9,10-dihydroacridine to break conjugation and increase the T1 state energy without changing the linking nitrogen atom; therefore, two new tetradentate platinum(II) complexes PtN’1 N (
Recently, Fan and Liao et al. designed and synthesized a series of platinum(II) complexes (
4. N -heterocyclic carbene-based tetradentate platinum(II) complexes
Because of the strong δ-donating ability and relatively weak π-accepting property, the
Comp. | In solution at RT | In PMMA at RT | ||||||
---|---|---|---|---|---|---|---|---|
Solvent | λmax/nm | FWHM/nm | ϕ/% | τ/μs | λmax/nm | ϕ/% | τ/μs | |
THF-DMF | 457 | — | 3 | 0.5 | 443 | 29 | — | |
THF-DMF | 460 | — | 7 | 1.8 | 448 | 24 | — | |
THF-DMF | 461 | — | 8 | 1.8 | 449 | 26 | — | |
THF-DMF | 443 | — | 18 | 3.5 | 434;451 | 26 | — | |
THF-DMF | 531;562 | — | 15 | 47.3 | — | — | — | |
CH2Cl2 | — | — | — | — | 480a | — | 0.1a | |
Pt7O7 ( |
CH2Cl2 | 471 | 20 | 71 | 3.2 | — | — | — |
PtOO7 ( |
CH2Cl2 | 442 | 66 | — | — | 442 | 58 | 2.5 |
PtON7 ( |
CH2Cl2 | 452 | 64 | 78 | 4.2 | 452 | 89 | 4.1 |
PtON7-tBu ( |
CH2Cl2 | 446 | 20 | 83 | 6.6 | — | — | — |
PtON7-dtb ( |
CH2Cl2 | 446 | 20 | 85 | 5.4 | 447 | 91 | 4.7 |
Dopant | λmax/nm | FWHM/nm | CIE | ηEQE | |
---|---|---|---|---|---|
Peak (%) | 100 cd/m2 (%) | ||||
3% |
~460b | ~70b | (0.16, 0.16) | — | — |
4% |
460 | ~70b | (0.19, 0.21) | ~15 | |
2% Pt7O7( |
472 | 20 | (0.12, 0.24) | 26.3 | 20.5 |
14% Pt7O7( |
— | — | (0.37, 0.42) | 25.7 | 21.5 |
2% PtOO7 ( |
446 | 50 | (0.15, 0.10) | 7.0 | 4.1 |
6% PtON7 ( |
458 | 54 | (0.15, 0.14) | 23.7 | 20.4 |
6% PtON7-tBu ( |
450 | 28 | (0.14, 0.09) | 17.6 | 10.7 |
2% PtON7-dtb ( |
451 | 23 | (0.146, 0.088) | 17.2 | 12.4 |
6% PtON7-dtb ( |
452 | 25 | (0.146, 0.091) | 19.8 | 14.7 |
10% PtON7-dtb ( |
452 | 39 | (0.155, 0.130) | 19.6 | 14.9 |
14% PtON7-dtb ( |
454 | 47 | (0.161, 0.169) | 19.0 | 15.5 |
6% PtON7-dtb ( |
451 | 29 | (0.148, 0.079) | 24.8 | 22.7 |
Early in 2011, Che’s group had developed a series of symmetric bis-NHC-based platinum(II) complexes by employing O^C*C^O ligands (
In 2014, Li′s group reported a symmetric bis-NHC-based platinum(II) complex Pt7O7 (
In 2013–2015, Li′s group successively developed a series of blue and deep-blue OLEDs by employing rigid NHC-based platinum(II) complexes, like PtOO7 (
On the other hand, all the PtON7 series of complexes (
Further modifications are needed for the development of deep-blue OLEDs. Fortunately, incorporating-tBu group into the 4-position of the pyridine ring can elevate the T1 energy of the pyridinyl-carbazole moiety and suppress its emission, just like the discussion of PtON1 and PtON1-tBu above [27]. Thus, very narrow emission spectra can be obtained for the PtON7-tBu and PtON7-dtb, which have FWHM of only 20 nm, making them suitable for deep-blue emitters (Figure 5) [29]. Importantly, the introduction of the other-tBu group to the phenyl ring can significantly enhance the thermal stability of PtON7-dtb and benefit to the high-quality device fabrication. As expected, PtON7-tBu-based device exhibited a deep-blue color and CIE coordinates of (0.14, 0.09) owing to its narrow emission spectrum and also had a peak EQE of 17.6% [29]. What’s more is that PtON7-dtb-based devices demonstrated excellent performances. Increasing the concentration of the PtON7-dtb would broaden the emission spectra; however, no signs of excimer or aggregation formation were observed. Through optimizing the device structure by employing a co-host of hole- and electron-transporting materials, the peak EQE could be further increased to 24.8% and remained 22.7% at practical luminance of 100 cd/m2 with a highly desirable CIE coordinates of (0.148, 0.079), very close to the “pure” blue coordinates of (0.14, 0.08) [28]. This device performance is the best for the deep-blue phosphorescent OLEDs reported to date [17], and this molecular design by employing asymmetric tetradentate NHC ligands is one of the most successful strategies for the development of deep-blue OLEDs with high color purity. There has also been much progress on the further understanding of the relationship between the molecular modifications and the narrowing of emission band, and research work had been carried out based on the study of the time-dependent density functional theory (TD-DFT), UV, IR, and transient Raman spectra [27, 39, 40].
5. Imidazole-based tetradentate platinum(II) complexes
Compared with the 1-phenyl-pyrazole and phenyl-NHC moieties discussed above, 2-phenyl-imidazole has a greater degree of π-conjugation and a relatively low T1 state level. Thus, imidazole-based phosphorescent metal complexes often exhibit redshift and serve as sky-blue emitters. Importantly, they are easily compatible with the known stable host and charger-transporting materials, making them suitable for the development of stable blue OLEDs. Imidazole-based iridium(III) blue emitters have been widely studied and demonstrated high quantum efficiency and impressive operational lifetime, although they are still far from meeting the strict requirements of commercialization [17, 34, 41, 42, 43, 44]. However, the reported tetradentate imidazole-based platinum(II) complexes are still rare, which are illustrated in Figure 6, their photophysical properties are summarized in Table 6, and the device performances are illustrated in Table 7.
Dopant | CIE | CRI | ηEQE | Device LT80 1000 cd/m2 (h) |
|
---|---|---|---|---|---|
Peak (%) | 1000 cd/m2 (%) | ||||
8% PtOO2 ( |
(0.16, 0.34) | — | 23.1 | 15.7 | — |
8% PtON2 ( |
(0.16, 0.32) | — | 22.9 | 17.5 | — |
2% Pt2O2 ( |
(0.23, 0.57) | — | 25.4 | 18.2 | — |
16% Pt2O2 ( |
(0.48, 0.48) | 72 | 24.6 | 21.0 | — |
16% Pt2O2 ( |
(0.46, 0.47) | 80 | 12.5 | — | 207 |
2% Pt1O2 ( |
(0.22, 0.44) | — | 24.1 | 16.9 | — |
16% Pt1O2 ( |
(0.49, 0.48) | 57 | 22.6 | 19.3 | — |
2% Pt1O2me2 ( |
(0.23, 0.44) | — | 26.5 | 17.6 | — |
16% Pt1O2me2 ( |
(0.42, 0.53) | 42 | 24.2 | 20.6 | — |
12% Pt1O2me2 ( |
(0.43, 0.50) | — | 12.3 | — | >400 |
Interestingly, although adopting different ancillary ligands, PtOO2 (
Compared with the nonplanar molecular PtOO2 and PtON2, all planar complexes Pt2O2 (
The operational lifetime of the devices is one of important parameters for their potential commercialization. Using the stable device structure III, white OLED doped with 16% Pt2O2 demonstrated an operational lifetime LT80 of over 200 h at an initial luminance of 1000 cd/m2 with a color rendering index (CRI) of up to 80 and peak EQE of 12.5% [46]. Due to the strong emission of the excimer, 12% Pt1O2me2-doped device exhibited a yellow emission; however, the operational lifetime LT80 could achieve over 400 h at an initial luminance of 1000 cd/m2, which was twice as long as that of Pt2O2 in the same device setting, and this could be attributed to the lack of high-energy blue emitters in the Pt1O2me2-based device.
6. Pyridine-based tetradentate platinum(II) complexes
2-Phenylpyridine has been widely used as ligand for the iridium(III)- and platinum(II)-based phosphorescent complexes, like Ir(ppy)3, due to its high stability and easy preparation. However, owing to the low T1 state level, the emitting colors are usually from green to red. So far, various types of pyridine-based tetradentate platinum(II) complexes have been reported; importantly, most of them are highly efficient, and some complexes are so stable that they can achieve the early stage of commercial applications. The pyridine-based tetradentate platinum(II) complexes are illustrated in Figure 7, their photophysical properties are summarized in Table 8, and some of the device performances based on these complexes are showed in Table 9.
Comp. | In solution at RT | In PMMA at RT | |||||
---|---|---|---|---|---|---|---|
Solvent | λmax/nm | ϕ/% | τ/μs | λmax/nm | ϕ/% | τ/μs | |
2-MeTHF | 512, 548 | 74 | 7.6 | 514, 551, 595a | — | — | |
2-MeTHF | 488, 523 | 75 | 11.4 | 541, 583a | — | — | |
2-MeTHF | 613 | 14 | 7.6 | 741, 782a | — | — | |
— | — | — | — | 621b | 58 | — | |
— | — | — | — | 620b,c | — | — | |
CH2Cl2 | 512 | 63 | 2.0 | — | 97 | 4.5 | |
CH2Cl2 | 614 | — | 3.6 | — | 40 | — | |
CH2Cl2 | 502 | 34 | — | — | — | — | |
CH2Cl2 | 582 | 63 | 7.3 | — | — | — | |
CH2Cl2 | 508 | 31 | 2.6 | 474 | 83 | 3.8 | |
CH2Cl2 | 595 | 12 | 1.9 | — | — | — | |
CH2Cl2 | 479, 510, 624 | 60 | 5.8 | — | — | — | |
CH2Cl2 | 480, 510, 616 | 66 | 5.4 | — | — | — | |
CH2Cl2 | 482, 512, 624 | 75 | 17.7 | — | — | — | |
CH2Cl2 | 503 | 76 | 4.1 | — | — | — | |
CH2Cl2 | 551 | 90 | 4.3 | — | 74 | — | |
CH2Cl2 | 517 | 80 | 5.1 | — | 91 | — | |
CH2Cl2 | 553, 587 | 86 | 6.6 | — | — | — | |
CH2Cl2 | 526 | 47 | 5.9 | — | — | — | |
CH2Cl2 | 527 | 49 | 8.8 | — | — | — |
Dopant | λmax/nm | CIE | CRI | ηEQE | Device LT 1000 cd/m2 (h) |
|
---|---|---|---|---|---|---|
Peak (%) | 1000 cd/m2 (%) | |||||
4% |
512 | (0.32, 0.62) | — | 14.7 | — | — |
6% |
— | (0.662, 0.337) | — | 18.5 | 14.4 | LT80: 1920 |
6% |
— | (0.657, 0342) | — | 18.2 | 14.5 | LT80: 3330 |
8% |
500 | — | — | 22.3 | 17.6 | — |
6% |
— | (0.60, 0.36) | — | 4.7 | 4.6 | LT97: 62 |
10% |
— | (0.63, 0.37) | — | 10.8 | 7.8 | LT97: 638 |
2% |
— | (0.58, 0.42) | — | 21.5 | 13.5 | LT97: 25 |
10–6% |
— | (0.55, 0.45) | — | 16.9 | 15.3 | LT97: 2057 |
6% |
— | (0.17, 0.32) | — | 10.7 | 9.1 | LT70: 624 |
10% |
— | (0.41, 0.44) | 75 | 11.6 | 5.5 | — |
16% |
— | (0.41, 0.45) | 74 | 17.0 | 12.4 | — |
20% |
— | (0.41, 0.45) | 76 | 9.6 | 8.4 | — |
4% |
— | (0.29, 0.63) | — | 9.7 | 9.5 | — |
10% |
555 | (0.44, 0.55) | — | 26.0 | 23.1 | — |
10% |
— | (0.31, 0.64) | — | 27.6 | 25.6 | — |
In 2010, Huo’s group reported three pyridine-based platinum complexes (
In 2012, Fukagawa et al. developed two modified complexes TLEC-025 (
Pyridine-based tetradentate platinum(II) complex PtOO3 (
A great progress has been made for the development of stable and efficient platinum(II)-based red OLEDs in the past several years. In 2014, Li′s group demonstrated a stable red OLEDs with an estimated operational lifetime LT97 of 62 h at 1000 cd/m2 with CIE coordinates of (0.60, 0.36) by employing PtON11Me (
For the development of iridium(III)-based blue emitters containing phenylpyridine moiety, generally, electron-withdrawing groups, like fluorine, are needed to be introduced into the phenyl group to stabilize the HOMO level [21]. However, this would result in electrochemical stability problems to accelerate the device degradation, which was unfavorable to the development of the stable blue OLEDs. In 2016, Li′s group developed a new rout for stable and efficient blue OLEDs through breaking the conjugation of the phenylpyridine with a six-membered chelating rings (
In fact, a synthetic challenge still remains for the gram-scale preparation of the PtNON and the PtON1 series complexes. Recently, our group developed an efficient approach for the CuCl-catalyzed C-N band cross coupling of carbazoles and 2-bromopyridine derivatives to synthesize 2-bromo-
Early in 2013, Che’s group had developed a series of symmetric (
7. Other types of tetradentate platinum(II) complexes
Besides the four series of tetradentate platinum(II) complexes discussed above, there were also some other new types. In 2015, a series of sky-blue emitters based on 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole or 3-(trifluoromethyl)-5-(2-pyridyl)-1,2,4-triazole containing spiro-arranged tetradentate ligands were developed. The peak EQE of one blue OLED could reach 15.3% and CIE values of (0.190, 0.342) [58]. In 2017, Liao, Fan, and co-workers developed three 1-isopropyl-2-phenyl-benzo[
Very recently, Fukagawa and co-workers reported great progress in ultrapure green OLEDs based on a NHC emitter PtN7N [60], which was developed by Li′s group before 2014 [61]. The optimized OLED showed CIE coordinates of (0.18, 0.74) using a top-emitting OLED with a microcavity structure and also using a boron-based host material [60]. Fukagawa’s work demonstrated that the narrowband emitter PtN7N was superior to the iridium(II)-based emitter Ir(mppy)3 for the development of ultrapure green emitter to satisfy the BT.2020 for ultrahigh-definition displays [60], owing to the very small vibrational structures of PtN7N that could be well suppressed by microcavity technology. Similar phenomenon was also observed in the previous report of narrowband green emitter PtN1N vs. PtOO3 [62]. Moreover, Fukagawa’s work also demonstrated that the operational stability of PtN7N-based OLEDs could be comparable to that of the Ir(mppy)3-based ones, indicating the promise for the practical application of PtN7N by employing suitable host and charge-transporting materials [60, 63].
8. Conclusion
In summary, after over 10 years of development, the emission spectra of the tetradentate platinum(II)-based OLEDs can cover the whole visible spectrum, they also exhibit high efficiency, and some show high color purity and long operational lifetime, demonstrating their potential applications for the next-generation full-color display and solid-state lighting. Owing to the square planar and rigid molecular configurations, platinum(II) complexes have many unique and exciting photophysical properties. On the one hand, easy molecular modification enables tunable emission spectra, and the FWHM of the pyrazole- or NHC-carbene-based complexes can achieve no more than 20 nm and can be as narrow as 15 nm. This facilitates them to serve as efficient deep-blue emitters, and device-doped NHC-carbene-based complex successfully realized “pure” blue emitting with CIE coordinates of (0.148, 0.079) and peak EQE of 24.8%. On the other hand, some planar d8 platinum(II) complexes can form intermolecular Pt-Pt bond to achieve 18e structure in their excited state, making them serve as single-doped white OLEDs with high CRI values. Besides, tetradentate platinum(II)-based green, especially for the red OLEDs, demonstrated superlong operational lifetime and satisfied the requirements of the initial commercialization. What’s more is that sky-blue OLEDs also achieved encouraging performances, indicating their bright future for the development of the efficient and stable blue OLEDs.
Despite great progress that has been made for the tetradentate platinum(II) complexes, a challenge remains for the development of the stable deep-blue OLEDs, and more work still be needed. To overcome this challenge, it is important to develop stable host materials with a high enough T1 state level and highly balanced charge carrier ability. However, through continued efforts of the academia and industry, we believe that these critical issues can be solved and the platinum(II)-based OLEDs will be one candidate for display and lighting applications.
Acknowledgments
The authors thank the National Natural Science Foundation of China (21602198, 21776259, 21476270), the “Qianjiang Talents Plan” (QJD1602017), and AAC Technologies for their financial support. The authors also thank Dr. Tyler Fleetham from the University of Southern California for the measurements of the quantum efficiency and luminescent lifetime of the PtON3.
Abbreviations
Alq3 | tris(8-hydroxyquinolinato)aluminium |
BAlq | bis(2-methyl-8-quinolinolato) (biphenyl-4-olato)aluminum |
Bebq2 | bis(benzo[h]quinolin-10-olato-κN,κO)beryllium(II) |
BmPyPB | 1,3-bis[3,5-di(pyridin-3-yl)phenyl]benzene |
BPyTP | 2,7-di(2,2′-bipyridin-5-yl)triphenylene |
CBP | 4,4′-bis(N-carbazolyl) biphenyl |
CzSi | 9-(4-(tert-butyl)phenyl)-3,6-bis(triphenylsilyl)-9H-carbazole |
DPPS | diphenyl-bis[4-(pyridin-3-yl)phenyl]-silane |
HATCN | 1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile |
mCBT | 9,9′-(2,8-dibenzothiophenediyl)bis-9H-carbazole |
26mCPy | 2,6-bis(N-carbazolyl) pyridine |
mppy | 3-methyl-2-phenylpyridine |
2-MeTHF | 2-methyltetrahydrofuran |
NPD | N,N′-diphyenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine |
OXD-7 | 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene |
PEDOT:PSS | poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) |
PO15 | 2,8-bis(diphenylphosphoryl)-dibenzothiophene |
ppy | 2-phenylpyridine |
PVK | polyvinylcarbazole |
TAPC | di-[4-(N,N-ditolylamino)-phenyl]cyclohexane |
TCTA | 4,4′,4′′-tris(N-carbazolyl)triphenlyamine |
2-TNATA | tri(4-(naphthalen-2-yl(phenyl)amino)phenyl)amine |
TmPyPB | 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene |
Tm3PyBPZ | 2,4,6-tris(3-(3-(pyridin-3-yl)phenyl)phenyl)-1,3,5-triazine |
TPBi | 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) |
TrisPCz | 9,9′,9″-triphenyl-9H,9′H,9″H-3,3′:6′3″-tercarbazole |
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