**4.** *N***-heterocyclic carbene-based tetradentate platinum(II) complexes**

Because of the strong δ-donating ability and relatively weak π-accepting property, the *N*-heterocyclic carbene (NHC) unit could shorten the metal-carbene bond length of the NHC-based platinum(II) complexes, shallow the LUMO energy level to widen the HOMO and LUMO gap, and raise the d-d level of the excited state to suppress the thermally activated nonradiative decay. These will be beneficial for the stability of the complexes and the enhancement of quantum efficiency [12, 17]. Therefore, the NHC-based platinum(II) complexes are appropriate to serve as blue and deep-blue phosphorescent OLEDs. However, due to synthetic challenges and shortage of stable host materials with high T1 state, the reported NHC-based tetradentate platinum(II) complexes are very rare. Especially, their operational lifetime remains unclear. The NHC-based platinum(II) complexes discussed in this chapter are illustrated in **Figure 4**, their photophysical properties are summarized in **Table 4**, and some of their device performances are illustrated in **Table 5**.

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 (**27**–**30**, **Figure 4**) [35]. All the four complexes exhibit intense blue phosphorescence ether in solutions (ϕ, 3–18%) or in PMMA films (ϕ, 24–29%). Incorporating electron-donating groups into the phenyl rings, like -Me and -tBu, can destabilize the HOMO, resulting in 3–4 nm redshift for the emission spectra of **28** and **29** compared with that of **27**. On the other hand, electron-withdrawing group -F can stabilize the HOMO and make a significant blueshift (**Table 4**). Moreover, blue device doped with **28** exhibited emission peak at about 460 nm with CIE coordinates of (0.16, 0.16), but the EQE was low and was not reported. In 2013, Che's group optimized the blue device doped with 4% complex **29**, which could achieve a high EQE of about 15% with CIE coordinates of (0.19, 0.21)

**Figure 4.** Molecular structures of NHC-based tetradentate platinum(II) complexes.

**Table 4.** Photophysical properties of NHC-based tetradentate platinum(II) complexes.

**32** [36] CH<sup>2</sup>

Pt7O7 (**33**) [37] CH<sup>2</sup>

PtOO7 (**34**) [26] CH<sup>2</sup>

PtON7 (**35**) [26] CH<sup>2</sup>

PtON7-tBu (**36**) [29] CH<sup>2</sup>

PtON7-dtb (**37**) [28] CH<sup>2</sup>

Data were collected in solid state.

**a**

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

 [35] THF-DMF 457 — 3 0.5 443 29 — [35] THF-DMF 460 — 7 1.8 448 24 — [35] THF-DMF 461 — 8 1.8 449 26 — [35] THF-DMF 443 — 18 3.5 434;451 26 — [36] THF-DMF 531;562 — 15 47.3 — — —

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

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

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

85

Cl<sup>2</sup> — — — — 480<sup>a</sup> — 0.1<sup>a</sup>

Cl<sup>2</sup> 471 20 71 3.2 — — —

Cl<sup>2</sup> 442 66 — — 442 58 2.5

Cl<sup>2</sup> 452 64 78 4.2 452 89 4.1

Cl<sup>2</sup> 446 20 83 6.6 — — —

Cl<sup>2</sup> 446 20 85 5.4 447 91 4.7

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes http://dx.doi.org/10.5772/intechopen.76346 85

**Figure 4.** Molecular structures of NHC-based tetradentate platinum(II) complexes.

device by adding an electron-blocking layer (EBL) could achieved peak EQE of 22.1% and still

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 phenyl-

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

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>

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.

to synthetic challenges and shortage of stable host materials with high T1

some of their device performances are illustrated in **Table 5**.

**4.** *N***-heterocyclic carbene-based tetradentate platinum(II) complexes**

Because of the strong δ-donating ability and relatively weak π-accepting property, the *N*-heterocyclic carbene (NHC) unit could shorten the metal-carbene bond length of the NHC-based platinum(II) complexes, shallow the LUMO energy level to widen the HOMO and LUMO gap, and raise the d-d level of the excited state to suppress the thermally activated nonradiative decay. These will be beneficial for the stability of the complexes and the enhancement of quantum efficiency [12, 17]. Therefore, the NHC-based platinum(II) complexes are appropriate to serve as blue and deep-blue phosphorescent OLEDs. However, due

NHC-based tetradentate platinum(II) complexes are very rare. Especially, their operational lifetime remains unclear. The NHC-based platinum(II) complexes discussed in this chapter are illustrated in **Figure 4**, their photophysical properties are summarized in **Table 4**, and

pyridine (ppy) gives an orange emitter PtN8ppy (**21**) because of the localization of the T1

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

with operational lifetime LT70 of nearly 1200 and 60,000 h at 1000

mainly

[30].

state, the reported

, respectively [29]. This device performance is among the highest-efficient green

state energy without changing the linking nitrogen atom; therefore, two

remained 20.3% at 1000 cd/m<sup>2</sup>

and 100 cd/m<sup>2</sup>

and increase the T1


**Table 4.** Photophysical properties of NHC-based tetradentate platinum(II) complexes.

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 (**27**–**30**, **Figure 4**) [35]. All the four complexes exhibit intense blue phosphorescence ether in solutions (ϕ, 3–18%) or in PMMA films (ϕ, 24–29%). Incorporating electron-donating groups into the phenyl rings, like -Me and -tBu, can destabilize the HOMO, resulting in 3–4 nm redshift for the emission spectra of **28** and **29** compared with that of **27**. On the other hand, electron-withdrawing group -F can stabilize the HOMO and make a significant blueshift (**Table 4**). Moreover, blue device doped with **28** exhibited emission peak at about 460 nm with CIE coordinates of (0.16, 0.16), but the EQE was low and was not reported. In 2013, Che's group optimized the blue device doped with 4% complex **29**, which could achieve a high EQE of about 15% with CIE coordinates of (0.19, 0.21)


ligands containing phenoxyl-pyridine or pyridinyl-carbazole moieties. All of them exhibit distorted molecular geometry that suppresses the excimer and aggregation formation. PtOO7

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

Cl<sup>2</sup>

2.5 μs in PMMA film at RT. As expected, PtOO7-based device exhibited a deep-blue emission with a CIE coordinates of (0.15, 0.10); however, the peak EQE was only 7%, due to its high T1 state level (2.87 eV) and incompatibility with the host material or improper energy-level align-

On the other hand, all the PtON7 series of complexes (**35**–**37**) have high ϕ of 78–91% and τ of 4.1–6.6 μs in solution and PMMA film at RT. Additionally, they have a relatively low T1 state level (2.81–2.82 eV), allowing them to be compatible with the known and efficient host materials. Encouragingly, PtON7-based device demonstrated a blue color with a CIE coordi-

due to the broad device emission spectrum (FWHM = 54 nm) and significant green emission contamination, the CIE coordinates of (0.15, 0.14) still fail to reach the standard of the "pure"

Further modifications are needed for the development of deep-blue OLEDs. Fortunately,

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

with a highly desirable CIE coordinates of (0.148, 0.079), very close to the "pure"

Cl<sup>2</sup>

(solid lines) and 77 K in 2-MeTHF (dash-

blue coordinates of (0.14, 0.08) [28]. This device performance is the best for the deep-blue

dotted lines) with molecular structures and CIE coordinates (RT) of each emitter inset (adapted with permission) [27].

**Figure 5.** PL spectra of (a) PtON7, (b) PtON7-tBu, and PtON7-dtb at RT in CH<sup>2</sup>

nates of (0.15, 0.14) and peak EQE of 23.7% still remained 20.4% at 100 cd/m<sup>2</sup>

incorporating-tBu group into the 4-position of the pyridine ring can elevate the T1

solution and has a ϕ of 58% and a short τ of

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

[26]. However,

87

energy of the

shows a broad emission peak at 442 nm in CH<sup>2</sup>

ment inside the emissive layer [16].

blue coordinates of (0.14, 0.08) [38].

100 cd/m<sup>2</sup>

a Device structure: ITO/2-TNATA/NPB/dopant: DP4/TPBi/LiF/Al.

b Estimated from the EL spectrum in the reported literature.

c Device structure: ITO/TAPC/TCTA/CzSi/dopant: CzSi/TmPyPB/LiF/Al.

dDevice structure: ITO/HATCN/NPD/TAPC/dopant: mCBP/DPPS/BmPyPB/LiF/Al.

e Device structure: PEDOT:PSS/NPD/TAPC/dopant: 26mCPy/PO15/LiF/Al.

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

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

<sup>h</sup>Device structure: ITO/HATCN/NPD/TAPC/dopant: TAPC:PO15/PO15/BmPyPB/LiF/Al.

**Table 5.** Summary of blue OLED performances of the NHC-based tetradentate platinum(II) complexes.

(**Table 5**) [36]. What's more is that extended π-conjugation (**31**) or prolonged linking group (**32**) would result in redshift for the emission spectra [36].

In 2014, Li′s group reported a symmetric bis-NHC-based platinum(II) complex Pt7O7 (**33**) by employing C^C\*C^C ligands (**Figure 4**) [37]. Pt7O7 exhibits a very narrow emission spectrum peaking at 471 nm with FWHM of only 20 nm in diluted CH<sup>2</sup> Cl<sup>2</sup> solution at RT. Two percent of Pt7O7-doped blue device demonstrated a peak EQE of 26.3% and still remained 20.5% at 100 cd/m<sup>2</sup> with broadening the electroluminescent (EL) spectrum (**Table 4**), paving a new way for the development of efficient blue phosphorescent emitters. Importantly, due to the square planar configuration, excimer would form in elevated concentration, and Pt7O7 could serve as single-doped white OLEDs. The device with the best emitting color could be achieved using a concentration of 14% Pt7O7 with a CRI of 70 and CIE coordinates of (0.37, 0.42), which also exhibited a peak EQE of 25.7%. This was the first reported emitter with both efficient monomer and excimer emissions.

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 (**34**), [26] PtON7 (**35**) [26], PtON7-tBu (**36**) [27, 29], and PtON7-dtb (**37**) [27, 28], which adopted asymmetric tetradentate ligands containing phenoxyl-pyridine or pyridinyl-carbazole moieties. All of them exhibit distorted molecular geometry that suppresses the excimer and aggregation formation. PtOO7 shows a broad emission peak at 442 nm in CH<sup>2</sup> Cl<sup>2</sup> solution and has a ϕ of 58% and a short τ of 2.5 μs in PMMA film at RT. As expected, PtOO7-based device exhibited a deep-blue emission with a CIE coordinates of (0.15, 0.10); however, the peak EQE was only 7%, due to its high T1 state level (2.87 eV) and incompatibility with the host material or improper energy-level alignment inside the emissive layer [16].

**Dopant λmax/nm FWHM/nm CIE ηEQE**

 [36] 460 ~70b (0.19, 0.21) ~15 2% Pt7O7(**33**)d [37] 472 20 (0.12, 0.24) 26.3 20.5

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

[35] ~460b ~70b (0.16, 0.16) — —

<sup>d</sup> [37] — — (0.37, 0.42) 25.7 21.5

[26] 446 50 (0.15, 0.10) 7.0 4.1

[26] 458 54 (0.15, 0.14) 23.7 20.4

[29] 450 28 (0.14, 0.09) 17.6 10.7

[28] 451 23 (0.146, 0.088) 17.2 12.4

[28] 452 25 (0.146, 0.091) 19.8 14.7

[28] 452 39 (0.155, 0.130) 19.6 14.9

[28] 454 47 (0.161, 0.169) 19.0 15.5

h [28] 451 29 (0.148, 0.079) 24.8 22.7

(**Table 5**) [36]. What's more is that extended π-conjugation (**31**) or prolonged linking group

In 2014, Li′s group reported a symmetric bis-NHC-based platinum(II) complex Pt7O7 (**33**) by employing C^C\*C^C ligands (**Figure 4**) [37]. Pt7O7 exhibits a very narrow emission spectrum

of Pt7O7-doped blue device demonstrated a peak EQE of 26.3% and still remained 20.5% at

for the development of efficient blue phosphorescent emitters. Importantly, due to the square planar configuration, excimer would form in elevated concentration, and Pt7O7 could serve as single-doped white OLEDs. The device with the best emitting color could be achieved using a concentration of 14% Pt7O7 with a CRI of 70 and CIE coordinates of (0.37, 0.42), which also exhibited a peak EQE of 25.7%. This was the first reported emitter with both efficient

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 (**34**), [26] PtON7 (**35**) [26], PtON7-tBu (**36**) [27, 29], and PtON7-dtb (**37**) [27, 28], which adopted asymmetric tetradentate

with broadening the electroluminescent (EL) spectrum (**Table 4**), paving a new way

Cl<sup>2</sup>

solution at RT. Two percent

3% **28**<sup>a</sup>

4% **29**<sup>c</sup>

14% Pt7O7(**33**)

2% PtOO7 (**34**)

6% PtON7 (**35**)

6% PtON7-tBu (**36**)

2% PtON7-dtb (**37**)

6% PtON7-dtb (**37**)

10% PtON7-dtb (**37**)

14% PtON7-dtb (**37**)

6% PtON7-dtb (**37**)

a

b

c

e

f

g

100 cd/m<sup>2</sup>

monomer and excimer emissions.

e

f

g

g

g

g

g

Device structure: ITO/2-TNATA/NPB/dopant: DP4/TPBi/LiF/Al.

Device structure: ITO/TAPC/TCTA/CzSi/dopant: CzSi/TmPyPB/LiF/Al.

Device structure: PEDOT:PSS/NPD/TAPC/dopant: 26mCPy/PO15/LiF/Al.

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

(**32**) would result in redshift for the emission spectra [36].

peaking at 471 nm with FWHM of only 20 nm in diluted CH<sup>2</sup>

dDevice structure: ITO/HATCN/NPD/TAPC/dopant: mCBP/DPPS/BmPyPB/LiF/Al.

Device structure: ITO/HATCN/NPD/TAPC/dopant: 26mCPy/DPPS/BmPyPB/LiF/Al. <sup>h</sup>Device structure: ITO/HATCN/NPD/TAPC/dopant: TAPC:PO15/PO15/BmPyPB/LiF/Al.

**Table 5.** Summary of blue OLED performances of the NHC-based tetradentate platinum(II) complexes.

Estimated from the EL spectrum in the reported literature.

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

 **(%)**

On the other hand, all the PtON7 series of complexes (**35**–**37**) have high ϕ of 78–91% and τ of 4.1–6.6 μs in solution and PMMA film at RT. Additionally, they have a relatively low T1 state level (2.81–2.82 eV), allowing them to be compatible with the known and efficient host materials. Encouragingly, PtON7-based device demonstrated a blue color with a CIE coordinates of (0.15, 0.14) and peak EQE of 23.7% still remained 20.4% at 100 cd/m<sup>2</sup> [26]. However, due to the broad device emission spectrum (FWHM = 54 nm) and significant green emission contamination, the CIE coordinates of (0.15, 0.14) still fail to reach the standard of the "pure" blue coordinates of (0.14, 0.08) [38].

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/m<sup>2</sup> 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

**Figure 5.** PL spectra of (a) PtON7, (b) PtON7-tBu, and PtON7-dtb at RT in CH<sup>2</sup> Cl<sup>2</sup> (solid lines) and 77 K in 2-MeTHF (dashdotted lines) with molecular structures and CIE coordinates (RT) of each emitter inset (adapted with permission) [27].

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].
