**5. Imidazole-based tetradentate platinum(II) complexes**

Compared with the 1-phenyl-pyrazole and phenyl-NHC moieties discussed above, 2-phenylimidazole 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, 41–44]. 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**.

Interestingly, although adopting different ancillary ligands, PtOO2 (**38**) and PtON2 (**39**) nearly have the same T1 state level, corresponding to their dominant peaks at 462 and 460 nm

in 2-MeTHF at 77 K, respectively [45]. Both of them emit strongly in diluted CH<sup>2</sup>

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

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

**Table 7.** Device performances of pyrazole-based tetradentate platinum(II) complexes.

**Dopant CIE CRI ηEQE Device LT80**

[45] (0.16, 0.34) — 23.1 15.7 —

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

[45] (0.16, 0.32) — 22.9 17.5 —

[46] (0.23, 0.57) — 25.4 18.2 —

[46] (0.48, 0.48) 72 24.6 21.0 —

[46] (0.46, 0.47) 80 12.5 — 207

[46] (0.22, 0.44) — 24.1 16.9 —

[46] (0.49, 0.48) 57 22.6 19.3 —

[46] (0.23, 0.44) — 26.5 17.6 —

[46] (0.42, 0.53) 42 24.2 20.6 —

[46] (0.43, 0.50) — 12.3 — >400

22.9% and still could remain 15.7 and 17.5% at 1000 cd/m<sup>2</sup>

Device structure III: ITO/HATCN/NPD/dopant: CBP/BAlq/Alq/LiF/Al.

Cl<sup>2</sup>

was not run in the literature.

8% PtOO2 (**38**)<sup>a</sup>

8% PtON2 (**39**)

2% Pt2O2 (**40**)

16% Pt2O2 (**40**)

16% Pt2O2 (**40**)

2% Pt1O2 (**41**)

16% Pt1O2 (**41**)

2% Pt1O2me<sup>2</sup>

16% Pt1O2me<sup>2</sup>

12% Pt1O2me<sup>2</sup>

**a**

b

c

a

b

b

c

b

(**42**)b

(**42**)b

(**42**)<sup>c</sup>

b

Pt1O2 (**41**), and Pt1O2me<sup>2</sup>

at 490 nm in diluted CH<sup>2</sup>

m2

at RT and exhibit λmax at 468 and 466 nm, respectively (**Table 6**). PtOO2- and PtON2-based devices emitted in the blue-green region and demonstrated high peaking EQEs of 23.1 and

Due to the P=O, double bond can be irreversibly reduced by electrons in the device to result in the poor electrochemical stability of the hole-blocking material PO15, and the device lifetime

Compared with the nonplanar molecular PtOO2 and PtON2, all planar complexes Pt2O2 (**40**),

configuration, which results in strong intermolecular Pt-Pt interaction to form efficient excimers, enabling them suitably for serving as single-doped white OLEDs for lighting application [46]. All the devices doped with either low or high concentrations exhibited very high peak EQEs from 22.6 to 26.5% using the device structure II and could achieve 16.9–21.0% even at 1000 cd/

 (**Table 7**); this device performance indicated that both the monomer and the excimer were highly efficient in the device settings, which were superior to that of the literature reporting bidentate and tridentate platinum(II) complexes, like FPt, Pt-4, and Pt-16 [46]. What's more is that the triplet-triplet annihilation (TTA) processes at high dopant concentrations, which were often observed in the iridium(III)-based devices, were also not significant for these complexes. 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/m<sup>2</sup>

(**42**) show redshift, especially for the symmetric Pt2O2, which peaks

at RT. Importantly, all the three planar complexes have more rigid

Cl<sup>2</sup>

in the device structure I (**Table 7**).

**1000 cd/m2 Peak (%) 1000 cd/m (h) <sup>2</sup> (%)**

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

89

solution

with

**Figure 6.** Molecular structures of imidazole-based tetradentate platinum(II) complexes.


**Table 6.** Photophysical properties of imidazole-based tetradentate platinum(II) complexes.

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes http://dx.doi.org/10.5772/intechopen.76346 89


**a** Device structure I: ITO/PEDOT: PSS/NPD/TAPC/dopant:26mCPy/PO15/BmPyPB/LiF/Al.

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

c Device structure III: ITO/HATCN/NPD/dopant: CBP/BAlq/Alq/LiF/Al.

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].

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

Compared with the 1-phenyl-pyrazole and phenyl-NHC moieties discussed above, 2-phenyl-

ole-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, 41–44]. 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**.

Interestingly, although adopting different ancillary ligands, PtOO2 (**38**) and PtON2 (**39**)

state level, corresponding to their dominant peaks at 462 and 460 nm

 **at RT In PMMA at RT λmax/nm ϕ/% τ/μs λmax/nm ϕ/% τ/μs**

state level. Thus, imidaz-

**5. Imidazole-based tetradentate platinum(II) complexes**

imidazole has a greater degree of π-conjugation and a relatively low T1

**Figure 6.** Molecular structures of imidazole-based tetradentate platinum(II) complexes.

**Table 6.** Photophysical properties of imidazole-based tetradentate platinum(II) complexes.

**Cl2**

PtOO2 (**38**) [45] 468 64 9.0 — — — PtON2 (**39** [45] 466 61 6.5 — — — Pt2O2 (**40**) [46] 490 — — — 84 — Pt1O2 (**41**) [46] 474 — — — — —

(**42**) [46] 470 — — — — —

**Comp. In CH2**

nearly have the same T1

Pt1O2me<sup>2</sup>

**Table 7.** Device performances of pyrazole-based tetradentate platinum(II) complexes.

in 2-MeTHF at 77 K, respectively [45]. Both of them emit strongly in diluted CH<sup>2</sup> Cl<sup>2</sup> solution at RT and exhibit λmax at 468 and 466 nm, respectively (**Table 6**). PtOO2- and PtON2-based devices emitted in the blue-green region and demonstrated high peaking EQEs of 23.1 and 22.9% and still could remain 15.7 and 17.5% at 1000 cd/m<sup>2</sup> in the device structure I (**Table 7**). Due to the P=O, double bond can be irreversibly reduced by electrons in the device to result in the poor electrochemical stability of the hole-blocking material PO15, and the device lifetime was not run in the literature.

Compared with the nonplanar molecular PtOO2 and PtON2, all planar complexes Pt2O2 (**40**), Pt1O2 (**41**), and Pt1O2me<sup>2</sup> (**42**) show redshift, especially for the symmetric Pt2O2, which peaks at 490 nm in diluted CH<sup>2</sup> Cl<sup>2</sup> at RT. Importantly, all the three planar complexes have more rigid configuration, which results in strong intermolecular Pt-Pt interaction to form efficient excimers, enabling them suitably for serving as single-doped white OLEDs for lighting application [46]. All the devices doped with either low or high concentrations exhibited very high peak EQEs from 22.6 to 26.5% using the device structure II and could achieve 16.9–21.0% even at 1000 cd/ m2 (**Table 7**); this device performance indicated that both the monomer and the excimer were highly efficient in the device settings, which were superior to that of the literature reporting bidentate and tridentate platinum(II) complexes, like FPt, Pt-4, and Pt-16 [46]. What's more is that the triplet-triplet annihilation (TTA) processes at high dopant concentrations, which were often observed in the iridium(III)-based devices, were also not significant for these complexes.

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/m<sup>2</sup> 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% Pt1O2me<sup>2</sup> -doped device exhibited a yellow emission; however, the operational lifetime LT80 could achieve over 400 h at an initial luminance of 1000 cd/m<sup>2</sup> , 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 Pt1O2me<sup>2</sup> -based device.
