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

Abstract

As one of the most important phosphorescent emitters, tetradentate cyclometalated platinum(II) complexes have attracted much attention in recent years, because of the high luminescent efficiency, emission spectra, and color tuned easily, especially for the development of high-efficient deep-blue and "pure" blue emitters and single-doped white organic light-emitting diodes (OLEDs). Also, some platinum(II)-based OLEDs exhibited superior operational stability, indicating their potentials in full-color display and solidstate lighting applications. In this chapter, we will introduce the recent advances of the tetradentate cyclometalated platinum(II) complexes, including pyrazole, N-heterocyclic carbene, imidazole and pyridine-based complexes, molecular design, photophysical properties, and some of their device performances.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 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][7][8][9][10][11][12][13][14][15][16][17][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.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 ...

N-Heterocyclic Carbene-Based Tetradentate Pd(II) Complexes for Deep-Blue Phosphorescent Materials

“…1 μs in a host-guest matrix and demonstrated an efficient and extremely deep-blue OLED with Commission International de l’Éclairage (CIE) coordinates of (0.154, 0.052) . Recently, many Pt­(II) − ,− and Au­(III) − complexes have also demonstrated promising properties as emitters in OLED applications, such as short excited-state lifetime (τ), − , high photoluminescence quantum yields (PLQYs), − and long device operational lifetime. − Several Au­(III) C^C^N complexes with strong σ-donating ligands showed a short τ of <0.5 μs in degassed toluene solution and demonstrated long estimated half-lifetimes in a device setting . Tetradentate Pt­(II) complexes also demonstrated superior operational stability in a device setting .…”

“…The Pd­(II) complexes emit deep-blue to yellow lights in PMMA film, and the τ values are also large at a range of 42–61 μs except for Pd­(1- ptz Ph) with a relatively short τ of 37 μs . NHC is a strong σ-donor and a relatively weak π-acceptor, which can shorten the metal–carbene bond length, shallow the LUMO energy level, and raise the d-d level of the excited state in the metal complexes; these will be beneficial for the suppression of the thermally activated nonradiative decay and the enhancement of PLQY and radiative decay. In fact, NHC-based ligands have been adopted for the Pd(0) complexes by Zysman-Colman and co-workers in 2015, and a PLQY of 70.1% was achieved in toluene solution with a very short τ of 2.93 μs, although these Pd(0) complexes were sensitive to molecular oxygen in air to afford Pd­(II) peroxo adducts due to the full filled d 10 electronic structures of the Pd(0).…”

Phosphorescent Tetradentate Platinum(II) Complexes Containing Fused 6/5/5 or 6/5/6 Metallocycles

“…An organic light-emitting device (OLED) is a key technology for full-color display application in high-end electronics. , The design and development of light-emitting materials play a critical role in this field and have been attracting great attention in both academia and industry. − Heavy metal complexes-based phosphorescent materials have a potential ability to harvest both electrogenerated singlet and triplet excitons to achieve unity internal quantum efficiency (IQE) and act as an important kind of light-emitting material in OLED fabrications. , With the efforts of the past three decades, many phosphorescent transition metal complexes, such as Ir­(III), Pt­(II), Au­(III), Pd­(II), Rh­(III), and Ru­(II) complexes, − have been developed, and OLEDs using the phosphorescent metal complexes as emitters have demonstrated good device performances with high external quantum efficiencies (EQEs), − high color purities, ,,− or long operational lifetimes, ,,,− indicating the potential applications of the phosphorescent metal complexes in full-color display and solid-state lighting. However, the phosphorescent OLEDs that can meet the commercial application for electronics are scarce.…”

Novel Carbazole/Fluorene-Based Host Material for Stable and Efficient Phosphorescent OLEDs

“…Within the past three decades, significant progress in OLED development has been achieved, − paving the way for OLEDs to become a main-stream display technology for commercial consumer electronic devices. Despite the advancements in the field, the development of efficient and stable blue and deep blue OLEDs has remained a challenge which requires further material development efforts. ,− Efficient Ir­(III), − Pt­(II), − and Au­(III) ,, -based phosphorescent emitters and thermally activated delayed fluorescent (TADF) emitters − have been developed. However, as a high concentration can increase the probability of triplet–triplet and triplet–polaron annihilation events, resulting in reduced device efficiency and expedited device degradation processes, consequently, host–guest systems are commonly adopted for an emissive layer (EML) design in the OLED architecture .…”