**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 dsp<sup>2</sup> 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 CH<sup>2</sup> Cl<sup>2</sup> 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 (**2**) has a weaker vibrational transition v0,1 at 523 nm than that of (ppy)Pt(acac), and the ϕ is increased to 60% [22]. However, the other monodentate ligand was needed to ensure the neutrality of the molecule. Furthermore, the chloride ion is a weak coordination ligand. All these would disfavor the molecular thermal and electrochemical stabilities. Therefore, more rigid and stable ligands are needed for further development of efficient and stable platinum(II)-based phosphorescent emitters.

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 CH<sup>2</sup> Cl<sup>2</sup> 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 CH<sup>2</sup> Cl<sup>2</sup> 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.


b ϕ and τ were measured in a solution of 2-MeTHF.

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 [6–18]. 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-

78 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

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, *N*-heterocyclic carbene, imidazole, and

Because of the relatively long luminescent lifetime and poor quantum efficiency (ϕ), platinum(II) complexes were historically not considered as ideal emitters. However, through

state lighting technologies.

pyridine derivatives.

**2. Why employ tetradentate ligands?**

**Table 1.** Photophysical properties of the bidentate, tridentate, and tetradentate platinum(II) complexes.

blue emitting materials. The photophysical properties and some of the device performances

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes

http://dx.doi.org/10.5772/intechopen.76346

81

In 2010, Huo et al. reported a series of symmetric tetradentate platinum(II) complexes (**5**–**7**) containing 1-phenyl-pyrazole moieties [24]. All these complexes emit strongly with ϕ in the range of 37–63%; however, due to the π-conjugation character of the arylamino linking group, their emission energies are relatively low with maximum emission wavelength (λmax) at 474–486 nm in sky-blue and green region. Moreover, excimer emissions were observed for complexes **5** and **7** in solid state peaking at 512 and 516 nm, respectively, because of the strong intermolecular interaction. In 2013, Huo et al. synthesized a 1-(2-pyridinyl)-pyrazole-coordinated complex **8**, which exhibit an even lower emission energy with λmax at 555 nm due to the localization of the

**Comp. In solution at RT In PMMA at RT**

**5** [24] 2-MeTHF 484;512 — 56 4.9 — — — **6** [24] 2-MeTHF 474 — 37 3.4 — — — **7** [24] 2-MeTHF 486;516 — 63 5.7 — — —

) mainly on the biphenyl moiety and the platinum(II) ion [25].

**Solvent λmax/nm FWHM/nm ϕ/% τ/μs λmax/nm ϕ/% τ/μs**

Cl<sup>2</sup> 555 — 17 4.4 — — —

Cl<sup>2</sup> 430;456 — 39 3.0 — 83 7.5

Cl<sup>2</sup> 454;478 85 71 3.3 449 85 4.5

Cl<sup>2</sup> 442 15 80 13.5 440 88 11.3

Cl<sup>2</sup> 444 20 89 10.0 445 84 7.6

Cl<sup>2</sup> 444 20 95 8.9 445 88 8.8

Cl<sup>2</sup> 496 84 53 1.8 480 64 2.0

Cl<sup>2</sup> 450;476 79 82 3.5 445 84 4.3

Cl<sup>2</sup> 450;478 121 45 3.1 449 78 4.8

Cl<sup>2</sup> 546 95 19 0.8 503 88 2.2

Cl<sup>2</sup> 568 104 1.1 0.6 544 29 0.9

Cl<sup>2</sup> 448 19 — — 447 81 7.4

Cl<sup>2</sup> 491 18 81 12.9 — 90 —

Cl<sup>2</sup> 573 26 40 3.4 — — —

Cl<sup>2</sup> 536 111 — — 476 18.0

Cl<sup>2</sup> 486 46 — — 476 68 —

Cl<sup>2</sup> 443;471 — 70 — — — —

Cl<sup>2</sup> 449;477 — 24 — — — —

Cl<sup>2</sup> 444;474 — 34 — — — —

**Table 2.** Photophysical properties of pyrazole-based tetradentate platinum(II) complexes.

of the pyrazole-based complexes are summarized in **Tables 2** and **3**.

first lowest triplet state (T1

**8** [25] CH<sup>2</sup>

**9** [23] CH<sup>2</sup>

**10** [26] CH<sup>2</sup>

**11** [27] CH<sup>2</sup>

**12** [27] CH<sup>2</sup>

**13** [27] CH<sup>2</sup>

**14** [27] CH<sup>2</sup>

**15** [27] CH<sup>2</sup>

**16** [27] CH<sup>2</sup>

**17** [27] CH<sup>2</sup>

**18** [27] CH<sup>2</sup>

**19** [27, 28] CH<sup>2</sup>

**20** [29] CH<sup>2</sup>

**21** [29] CH<sup>2</sup>

**22** [30] CH<sup>2</sup>

**23** [30] CH<sup>2</sup>

**24** [31] CH<sup>2</sup>

**25** [31] CH<sup>2</sup>

**26** [31] CH<sup>2</sup>

**Figure 1.** Molecular structures of (ppy)Pt(acac), Pt(dpyd)Cl, PtOO3, PtON3, and their PL spectra in CH<sup>2</sup> Cl<sup>2</sup> solution (adapted with permission) [23].

#### **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

**Figure 2.** Molecular structures of pyrazole-based tetradentate platinum(II) complexes and operational lifetime of related OLEDs.

blue emitting materials. The photophysical properties and some of the device performances of the pyrazole-based complexes are summarized in **Tables 2** and **3**.

In 2010, Huo et al. reported a series of symmetric tetradentate platinum(II) complexes (**5**–**7**) containing 1-phenyl-pyrazole moieties [24]. All these complexes emit strongly with ϕ in the range of 37–63%; however, due to the π-conjugation character of the arylamino linking group, their emission energies are relatively low with maximum emission wavelength (λmax) at 474–486 nm in sky-blue and green region. Moreover, excimer emissions were observed for complexes **5** and **7** in solid state peaking at 512 and 516 nm, respectively, because of the strong intermolecular interaction. In 2013, Huo et al. synthesized a 1-(2-pyridinyl)-pyrazole-coordinated complex **8**, which exhibit an even lower emission energy with λmax at 555 nm due to the localization of the first lowest triplet state (T1 ) mainly on the biphenyl moiety and the platinum(II) ion [25].


**Table 2.** Photophysical properties of pyrazole-based tetradentate platinum(II) complexes.

**3. Pyrazole-based tetradentate platinum(II) complexes**

(adapted with permission) [23].

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

Cl<sup>2</sup>

solution

**Figure 1.** Molecular structures of (ppy)Pt(acac), Pt(dpyd)Cl, PtOO3, PtON3, and their PL spectra in CH<sup>2</sup>

80 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

**Figure 2.** Molecular structures of pyrazole-based tetradentate platinum(II) complexes and operational lifetime of related OLEDs.


by introducing electron-donating group, like -Me, -tBu, and -NMe<sup>2</sup>

80% in CH<sup>2</sup>

Cl<sup>2</sup>

are attributed to the high T1

trum peaking at 491 nm in CH<sup>2</sup>

25.8% at a luminance of 100 cd/m<sup>2</sup>

PyCz moiety. Therefore, a series of deep-blue emitters, PtON1-NMe<sup>2</sup>

and all the PtON1 series showed intensive emitting except PtON1-CF<sup>3</sup>

narrowband emission spectrum peaking at 448 nm in CH<sup>2</sup>

Cl<sup>2</sup>

lifetime at 70% initial luminance, LT70, of 1436 h at 1000 cd/m<sup>2</sup>

**Figure 3.** PL spectra comparison of the PtON1 series at RT in CH<sup>2</sup>

(adapted with permission) [27].

was estimated nearly 72,000 h at a practical luminance of 100 cd/m<sup>2</sup>

important for the development of deep-blue OLEDs.

dine ring to increase the energy level of the metal-to-ligand charge-transfer (MLCT) states of the

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes

tBu (**11**–**13**), were developed peaking at 442–444 nm with FWHM of 15–20 nm and ϕ not less than

could be easily tuned through changing the substitutions or their positions on the pyridine ring,

with ϕ of 29–88% (**Figure 3**) [27]. Furthermore, PtON6-tBu, employing the 4-phenylpyrazole in place of 3,5-dimethylpyrazole in PtON1, was also developed as a deep emitter, which exhibit

emission energy does not decrease significantly owing that the 4-phenyl group and the pyrazole are not coplanar in PtON6-tBu. What is more is that deep-blue OLEDs doped 2% PtON1-tBu or PtON6-tBu could reach peak EQEs of 5.3 or 10.9% with CIEy < 0.1 [28]. The unsatisfied EQEs

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 (**20**) was designed and synthesized in Li′s group [29]. PtN1N also gives a very narrow emission spec-

(SM) of 0.3 for the vibrational transition v0,1 peak at 525 nm can be achieved. Moreover, one 7% PtN1N-doped device demonstrated a peak EQE of 26.1% and only decreased slightly to

linking atom can significantly enhance the chemical and device stability. Therefore, using a known stable device structure, 10% PtN1N-doped green OLED could achieved an operational

Cl<sup>2</sup>

known state-of-the-art host materials. Thus, stable host materials with a high T1

solution at RT (**Figure 3**) [27]. Moreover, it was also found that the emission color

Cl<sup>2</sup>

energy of the deep-blue emitters, making them incompatible with

solution at RT; the FWHM of 18 nm and Huang-Rhys factor

. Importantly, employing the nitrogen of the carbazole as

, to the 4-position of the pyri-

http://dx.doi.org/10.5772/intechopen.76346

, PtON1-Me, and PtON1-

83

, especially in PMMA films

level are still

and FWHM of 20 nm [28]. The

with peak EQE of 14.3%, which

with molecular structures of each emitter inset

. Furthermore, improved

a Device structure: ITO/HATCN/NPD/TAPC/dopant: 26mCPy/DPPS/LiF/Al.

b Device structure: ITO/HATCN/NPD/TAPC/dopant: 26mCPy/DPPS/BmPyPB/LiF/Al.

c Device structure: PEDOT:PSS/NPD/TAPC/dopant: 26mCPy/DPPS/BmPyPB/LiF/Al.

dDevice structure: ITO/HATCN/NPD/TrisPCz/dopant:mCBP)/mCBT/BPyTP/Liq/Al.

e Device structure: ITO/HATCN/TAPC/TCTA/dopant: 26mCPy(or CBP)/TmPyPB/Liq/Al.

**Table 3.** Summary of OLED performances of the pyrazole-based 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 (**9**) was synthesized by employing 1-phenyl-3,5-dimethylpyrazole and phenoxyl-pyridine (popy) like oxygen in Li′s group in 2013 [23]. The dominant emission peaks of PtOO1 are at 420 nm at 77 K and 430 nm at room temperature (RT). The ϕ is relatively low in solution but can be up to 83% with τ of 7.5 μs in PMMA film. However, excimer emission could not be observed; this could be attributed to the boat-like conformation of the two six-membered rings containing the oxygen liker [23] to prevent intermolecular Pt-Pt bond formation.

In 2013, Li′s group developed a new type of tetradentate platinum(II) complex PtON1 (**10**) using 3,5-dimethyl-1-phenylpyrazole and thermally and electrochemically stable pyridinyl-carbazole (PyCz) as ligands linked by an oxygen atom [26]. PtON1 exhibits a peak emission at 440 nm with a full width at half maximum (FWHM) of 6 nm at 77 K. However, at RT, the emission spectrum is dramatically broadened, and the FWHM is up to 85 nm with two emission peaks at 454 and 478 nm, respectively, which attributed to dual emission from both the phenyl-pyrazole and PyCz moieties. The ϕ of PtON1 is much higher than that of PtOO1 in CH<sup>2</sup> Cl<sup>2</sup> solution, due to more rigid PyCz moiety. Importantly, PtON1-based blue OLED can achieve a peak external quantum efficiency (EQE) of 25.2% and Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.13) but still short of the "pure" blue CIE coordinates of (0.14, 0.08) designated by the National Television System Committee (NTSC) of the USA in 1931.

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 -NMe<sup>2</sup> , 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-NMe<sup>2</sup> , PtON1-Me, and PtON1 tBu (**11**–**13**), were developed peaking at 442–444 nm with FWHM of 15–20 nm and ϕ not less than 80% in CH<sup>2</sup> Cl<sup>2</sup> solution at RT (**Figure 3**) [27]. Moreover, it was also found that the emission color could be easily tuned through changing the substitutions or their positions on the pyridine ring, and all the PtON1 series showed intensive emitting except PtON1-CF<sup>3</sup> , especially in PMMA films with ϕ of 29–88% (**Figure 3**) [27]. Furthermore, PtON6-tBu, employing the 4-phenylpyrazole in place of 3,5-dimethylpyrazole in PtON1, was also developed as a deep emitter, which exhibit narrowband emission spectrum peaking at 448 nm in CH<sup>2</sup> Cl<sup>2</sup> and FWHM of 20 nm [28]. The emission energy does not decrease significantly owing that the 4-phenyl group and the pyrazole are not coplanar in PtON6-tBu. What is more is that deep-blue OLEDs doped 2% PtON1-tBu or PtON6-tBu could reach peak EQEs of 5.3 or 10.9% with CIEy < 0.1 [28]. The unsatisfied EQEs are attributed to the high T1 energy of the deep-blue emitters, making them incompatible with known state-of-the-art host materials. Thus, stable host materials with a high T1 level are still important for the development of deep-blue OLEDs.

**Dopant λmax/nm FWHM/nm CIE ηEQE**

82 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

[26] 454 46 (0.15, 0.13) 25.2 23.3

[28] 448 24 (0.151, 0.098) 5.3 2.7

[28] 452 30 (0.147, 0.093) 10.9 6.6

[29] 498 20 (0.15, 0.56) 26.1 25.8

[31] 540 — (0.33, 0.57) 16.4 —

[31] 456 — (0.18, 0.30) 7.7 —

[31] 541 — (0.35, 0.55) 15.7 —

Device structure: ITO/HATCN/NPD/TAPC/dopant: 26mCPy/DPPS/LiF/Al.

Device structure: ITO/HATCN/NPD/TAPC/dopant: 26mCPy/DPPS/BmPyPB/LiF/Al.

Device structure: PEDOT:PSS/NPD/TAPC/dopant: 26mCPy/DPPS/BmPyPB/LiF/Al. dDevice structure: ITO/HATCN/NPD/TrisPCz/dopant:mCBP)/mCBT/BPyTP/Liq/Al.

Device structure: ITO/HATCN/TAPC/TCTA/dopant: 26mCPy(or CBP)/TmPyPB/Liq/Al.

the oxygen liker [23] to prevent intermolecular Pt-Pt bond formation.

and PyCz moieties. The ϕ of PtON1 is much higher than that of PtOO1 in CH<sup>2</sup>

by the National Television System Committee (NTSC) of the USA in 1931.

**Table 3.** Summary of OLED performances of the pyrazole-based 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 (**9**) was synthesized by employing 1-phenyl-3,5-dimethylpyrazole and phenoxyl-pyridine (popy) like oxygen in Li′s group in 2013 [23]. The dominant emission peaks of PtOO1 are at 420 nm at 77 K and 430 nm at room temperature (RT). The ϕ is relatively low in solution but can be up to 83% with τ of 7.5 μs in PMMA film. However, excimer emission could not be observed; this could be attributed to the boat-like conformation of the two six-membered rings containing

In 2013, Li′s group developed a new type of tetradentate platinum(II) complex PtON1 (**10**) using 3,5-dimethyl-1-phenylpyrazole and thermally and electrochemically stable pyridinyl-carbazole (PyCz) as ligands linked by an oxygen atom [26]. PtON1 exhibits a peak emission at 440 nm with a full width at half maximum (FWHM) of 6 nm at 77 K. However, at RT, the emission spectrum is dramatically broadened, and the FWHM is up to 85 nm with two emission peaks at 454 and 478 nm, respectively, which attributed to dual emission from both the phenyl-pyrazole

to more rigid PyCz moiety. Importantly, PtON1-based blue OLED can achieve a peak external quantum efficiency (EQE) of 25.2% and Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.13) but still short of the "pure" blue CIE coordinates of (0.14, 0.08) designated

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

[29] 576 28 (0.53, 0.47) 19.3 16.0

<sup>d</sup> [30] 490 34 (0.157, 0.491) 17.8 17.3

6% PtON1(**10**)<sup>a</sup>

7% PtN1N (**20**)

30% **24**<sup>e</sup>

20% **25**<sup>e</sup>

20% **26**<sup>e</sup>

a

b

c

e

2% PtN8ppy (**21**)

6% PtN'1 N-tBu (**23**)

2% PtOO1-tBu (**13**)

2% PtON6-tBu (**19**)

b

b

c

b

**Peak (%) 100 cd/m2**

Cl<sup>2</sup>

solution, due

 **(%)**

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 (**20**) was designed and synthesized in Li′s group [29]. PtN1N also gives a very narrow emission spectrum peaking at 491 nm in CH<sup>2</sup> Cl<sup>2</sup> solution at RT; the FWHM of 18 nm and Huang-Rhys factor (SM) of 0.3 for the vibrational transition v0,1 peak at 525 nm can be achieved. Moreover, one 7% PtN1N-doped device demonstrated a peak EQE of 26.1% and only decreased slightly to 25.8% at a luminance of 100 cd/m<sup>2</sup> . Importantly, employing the nitrogen of the carbazole as linking atom can significantly enhance the chemical and device stability. Therefore, using a known stable device structure, 10% PtN1N-doped green OLED could achieved an operational lifetime at 70% initial luminance, LT70, of 1436 h at 1000 cd/m<sup>2</sup> with peak EQE of 14.3%, which was estimated nearly 72,000 h at a practical luminance of 100 cd/m<sup>2</sup> . Furthermore, improved

**Figure 3.** PL spectra comparison of the PtON1 series at RT in CH<sup>2</sup> Cl<sup>2</sup> with molecular structures of each emitter inset (adapted with permission) [27].

device by adding an electron-blocking layer (EBL) could achieved peak EQE of 22.1% and still remained 20.3% at 1000 cd/m<sup>2</sup> with operational lifetime LT70 of nearly 1200 and 60,000 h at 1000 and 100 cd/m<sup>2</sup> , respectively [29]. This device performance is among the highest-efficient green OLEDs reported in literatures. PtN1N also could be employed as an efficient green emitter for the development of white OLED [32]. However, replacing the PyCz for PtN1N with phenylpyridine (ppy) gives an orange emitter PtN8ppy (**21**) because of the localization of the T1 mainly on the ppy moiety. The peak EQE of a PtN8ppy-based device could also reach close to 20% [29].

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 (**22**) and PtN'1 N-tBu (**23**) were designed and reported by Li′s group recently [30]. Both PtN'1 N and PtN'1 N-tBu show dominant peaks at 476 nm, which blueshifts by 8 nm compared to that of PtN1N in 2-MeTHF at 77 K. Optimized device by employing 10% PtN'1 N-tBu as dopant without EBL could achieve peak EQE of 15.9% and an estimated operational lifetime LT70 of 635 h at an initial luminance of 1000 cd/m<sup>2</sup> [30]. This device performance is comparable or superior to the best platinum(II)-[33] and iridium(III) [34]-based blue OLEDs reported in literatures [30]. It was believed that the device performance could be further improved if using state-of-the-art host, electron, and hole-blocking materials.

Recently, Fan and Liao et al. designed and synthesized a series of platinum(II) complexes (**24**– **26**) based on pyrazole[1,5-*f*]phenanthridine-containing ligands [31]. All of them showed high thermal stabilities and strong emission from blue to yellow-green spectral region with ϕ of 24–70%. The dominate emission peaks of all the three complexes are not much difference, but the emission spectra are more and more broad. Interestingly, the emission from PyCz moiety can be observed clearly for complex **26**, which is much like the PtON1 discussed above [27]. Complex **26** demonstrated the best device performance to achieve peak EQE of 16.4%, but unfortunately the operational lifetime of the device was not reported.
