**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 (Alq<sup>3</sup> ) 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

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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 solidstate lighting technologies.

judicious molecular design, bidentate platinum(II) complex can also emit strongly with

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

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

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

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

Cl<sup>2</sup>

rigid PMMA matrix. If more rigid carbazolyl-pyridine was incorporated and served as ancil-

plex 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

 [21] 484 15 2.6 0.6 3.3 53 6.0 [22] 490 60 3.8 1.6 1.1 73 5.7 [16, 23] 512 83 2.0 3.2 1.8 97 4.5 520 100b 4.2b 2.4 0.0 — —

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

 **at RT In PMMA at RT**

**/105 s−1 knr/105 s−1 ϕ/% τ/μs**

lary ligand, the ϕ could be further improved to 100% yield even in CH<sup>2</sup>

solution and be achieved to nearly unity in

Cl<sup>2</sup>

solution for com-

hybrid

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

lifetime in microsecond region, such as (ppy)Pt(acac) (**Table 1**) (1) [21]. Due to dsp<sup>2</sup>

solution at room temperature (RT).

stable platinum(II)-based phosphorescent emitters.

PtOO3 [16, 23] could be up to over 80% in CH<sup>2</sup>

ideal phosphorescent emitters for OLED applications.

**λmax/nm ϕ/%<sup>a</sup> τ/μs kr**

**Cl2**

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

**Comp. In CH2**

Absolute quantum efficiency.

a

b

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

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 pyridine derivatives.
