**2. Platinum(II) complexes as phosphorescent OLED dopants**

Phosphorescent metal complexes remain mainstream OLED emitters, because relative to pure organic fluorophores, they can more efficiently harvest excitons for light emission. Phosphorescent metal complexes typically possess a transition metal ion with a high atomic number, e.g., Ru(II), Ir(III), or Pt(II), that can induce strong spin-orbit coupling (SOC), giving rise to ultrafast intersystem crossing (ISC) from the singlet to triplet states and promoting spin-forbidden triplet radiative decay. This triplet harvesting mechanism theoretically enables complete utilization of the excitons generated by electron-hole recombination for light emission, leading to a much higher efficiency and luminance. Reasonable phosphorescent metal OLED emitters should exhibit the following traits: (i) high phosphorescent quantum yields (i.e., >70%) when doped in a solid matrix, (ii) tunable emission color covering the blue, green, and red spectral regions (essential for full color displays), and (iii) superior thermal, chemical, and electrochemical stabilities for vacuum deposition and operation. The plethora of literature examples demonstrated that phosphors based on Ir(III) and Pt(II) can meet these requirements and generally outperform other metal phosphors [20–22]. In the past two decades, extensive research efforts on OLED emitters have been devoted to the development of phosphorescent Ir(III) and Pt(II) complexes and to investigations of their photophysical, electrochemical, and electroluminescent characteristics.

Phosphorescent Pt(II) complexes are noted for their desirable photophysical properties; they have a square planar coordination geometry, are coordinatively unsaturated, and exhibit diverse highly emissive excited states, including ligandcentered (LC), intra-ligand charge transfer (ILCT), ligand-to-ligand charge transfer (LLCT), metal-to-ligand charge transfer (MLCT), and excimeric and oligomeric metal-metal-to-ligand charge transfer (MMLCT) states [21, 22]. Tailoring the emission attributes (i.e., energy, quantum yield, lifetime, and radiative and nonradiative decay rate constants) to suit specific OLED applications can be achieved by the rational design of ligands, which allows regulation of (i) the energy levels of the metal d orbitals, the π and π\* orbitals of the ligands, and subsequently the composition of the frontier molecular orbitals, excited-state dynamics, and the nature of the emissive excited state and (ii) the intermolecular interactions that can contribute to emission from an aggregated state and/or emission quenching. As is the case for all phosphorescent OLED emitters, engineering the emissive excited state of Pt(II) emitters with high metal character to keep the emission lifetime short is important, since saturation of electroluminescence, severe efficiency roll-off at high luminance, and poor operational stability could otherwise result.

The planar coordination geometry renders platinum(II) complexes susceptible to self-assembly in ground and/or excited states through intermolecular ligand π-π and/or Pt-Pt interactions. This intrinsic property usually leads to a considerable redshift in the absorption and emission energies attributed to the generation of the low-energy emissive MMLCT excited states with enhanced radiative decay rate constants [21, 22], which could be harnessed to provide unique access to long-range ordered luminescent supramolecular structures, non-doped NIR OLEDs, and single-doped white OLEDs (WOLEDs). Nevertheless, this aggregation behavior could be unfavorable for applications in RGB OLED panels, especially when the Pt(II) emitters are doped at high concentrations, due to the possible occurrence of aggregate emission, triplet-triplet annihilation (TTA) and aggregation-caused

*Liquid Crystals and Display Technology*

the electron transporting layer (ETL) from the metal cathode, which generally has a low work function (e.g., Al or Ag). To facilitate this process, a 0.5- to 1.0-nm thick electron injection layer (EIL) of LiF or CsF is usually deposited between the cathode and the ETL. Electrons migrate by hopping toward the anode. Meanwhile, holes are injected from the anode, which usually consists of a metal oxide mixture of SnO2 (10%) and In2O3 (90%), namely, indium tin oxide (ITO). Following the anode, a hole injection layer (HIL) and a hole transporting layer (HTL) are typically required to promote hole transfer into the emission layer (EML), which includes the host matrix and dopant. Ideally, the recombination of electrons and holes takes place in the EML, subsequently populating the excited states that generate light emission. Obviously, the electron current must be well balanced with the hole current to avoid ohmic loss, which can be minimized by employing a hole blocking layer (HBL) between the ETL and EML and/or an electron blocking layer (EBL) between the HTL and EML. These blocking layers prevent the holes and electrons from leaving the EML without recombination. In addition, each layer requires materials with suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The development of auxiliary OLED materials (e.g., materials for the HTL, ETL, HBL, and EBL) has been the

subject of previous reviews and is not discussed in this chapter [10–13].

As described above, the electrons and holes recombine and form neutral excitons in the EML. According to spin statistics, the recombination will populate singlet and triplet excited states in a ratio of 1:3, meaning that 75% of the electrically excited states are triplet states [14]. Pure organic molecules usually do not show spin-forbidden triplet emission (i.e., phosphorescence) at room temperature [15, 16], and normally only the singlet exciton is emissive, resulting in limited quantum efficiency, which presents a challenge for improving the efficiency of OLED devices. In 1998, the first successful applications of organometallic complexes as emissive dopant material in electroluminescent devices to generate phosphorescence were independently reported by Ma and Che [17] as well as Thompson and Forrest [18]. This revolutionary appraoch offers a viable means for the maximum use of electically generated excitons for electroluminescence and allowed a substantial leap forward in OLED performance. Since then, there has been increasing

**164**

**Figure 1.**

*Structure of an OLED device.*

quenching (ACQ ). Therefore, the appropriate management of intermolecular interactions/aggregation by modulating the 3D morphology and electromagnetic properties of the complexes is crucial in the molecular design of Pt(II) emitters for specific OLED applications in order to achieve optimum device performances (i.e., high color purity, device efficiency, and long operational lifetime).

In the context of ligand architecture, the employment of tetradentate ligands in the design of platinum(II) emitters has clear advantages in terms of both chemical and thermal stabilities and luminescent efficiency compared to their bidentate and tridentate ligand counterparts. Tetradentate ligands provide a more stable scaffold for the coordination of platinum by offering the strong chelation effect, which could suppress ligand dissociation and demetalation. In addition, the rigid tetradentate ligand framework could largely restrict the excited-state metal-ligand structural distortion that in turn facilitates radiative deactivation of the emissive excited state, boosting the emission quantum yield of the Pt(II) emitter.

This chapter aims to provide an overview of mononuclear Pt(II) emitters containing tetradentate ligands reported in the literature. To keep the chapter to a reasonable size, we restrict our discussions to several selected classes of tetradentate platinum(II) emitters and apologize to the contributors to this field whose contributions are not mentioned herein.
