Photophysical and OLED performance data for
This chapter provides an overview of tetradentate platinum(II) emitters as a promising class of metal-organic phosphorescent dopants for organic light-emitting diodes (OLEDs). Tetradentate platinum(II) emitters showing blue, green, and red light emissions, which are essential for full color displays as well as white light emission, are reviewed and discussed in the context of molecular design and photophysical and electroluminescent properties. Emphasis is placed on the molecular structures, the nature of emissive excited states [including ligand-centered (LC), intra-ligand charge transfer (ILCT), metal-to-ligand charge transfer (MLCT), and excimeric and oligomeric metal-metal-to-ligand charge transfer (MMLCT)], the intermolecular interactions impacting photophysical attributes (e.g., emission energies, quantum yields, and decay times), and OLED device performances.
- organic light-emitting diodes
- metal complexes
- high efficiency
- operational lifetime
Organic light-emitting diodes (OLEDs) are solid-state devices based on organic films sandwiched between two electrodes that convert electricity to luminous energy. Since the pioneering work of electroluminescence (EL) in 1987 , OLEDs have been attracting considerable attention because of their light weight, low driving voltage, low power consumption, fast response speed, and high frame rate for displays, making them suitable for various applications [2, 3, 4, 5], e.g., wearable devices, virtual reality (VR), smart homes and cities, and imaging and sensing applications. Currently, products with OLED displays are found in several fields, ranging from micro-displays to TV applications, and notably in smartphones and personal computers.
Figure 1 shows the typical structure of an OLED device, including a number of thin layers, which individually facilitate charge transfer or light emission [6, 7, 8, 9]. During operation, when a suitable voltage is applied, electrons are injected into 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, 11, 12, 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 . 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  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 interest in the design and synthesis of new phosphorescent metal complexes, particularly those of Ru(II), Ir(III), and Pt(II), as OLED dopants [19, 20, 21, 22].
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, 21, 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 ligand-centered (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 non-radiative 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 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.
3. Tetradentate platinum(II) emitters
3.1 Platinum(II) porphyrin complexes
The first photophysical studies of Pt(II) porphyrins were reported in the early 1970s and were triggered by the physicochemical relevance of metalloporphyrins such as chlorophyll, haem, and vitamin B12, which serve key biological functions . This class of complexes is known for their high stability against heat, solvents, and extreme pH as a result of the strong coordination of Pt(II) ions in rigid porphyrin scaffolds. Pt(II) porphyrins exhibit intense absorptions in the visible region, and their electronic spectra are characterized by a Soret band at approximately 400 nm and two Q bands between 500 and 600 nm, which are attributed to porphyrin-centered 1π-π* electronic transitions. The triplet formation yields for Pt(II) porphyrins were reported to be close to unity due to an ultrafast intersystem crossing process that occurs on a sub-ps time scale . These complexes typically display strong saturated red to near infrared (NIR) phosphorescence, depending on the structures of macrocycles, with long decay times of tens of microseconds under anaerobic conditions because the emissive excited state is essentially 3LC (3π, π*) in nature localized in the porphyrin ligand. For this reason, the emission properties can be rationally tuned by modifying the porphyrin ligands.
In 1998, Thompson and Forest reported the first use of a Pt(II) porphyrin complex,
Che et al. found that with the introduction of electron-deficient pentafluorophenyl rings at the meso positions of the porphyrin scaffold,
Wang and co-workers designed a group of platinum(II) porphyrin dendrimers (
The emission of platinum(II) porphyrin complexes can be further shifted to the NIR region by extending the π conjugation of the porphyrin ligand . Schanze and co-workers developed a series of NIR-emitting platinum(II) di- and tetra-substituted benzoporphyrin complexes,
Pt(II) porphyrins as OLED emitters generally exhibit high thermal stability and outstanding performance in saturated red and NIR devices in terms of color purity and EL efficiencies attributed to their narrow emission band and high emission quantum yields. Nevertheless, the practical interest of this class of Pt(II) emitters is limited by the long emission decay times, which would result in substantial efficiency loss at higher luminance. It is hypothesized that careful device design and the use of appropriate auxiliary materials to mitigate TTA, e.g., by using a double host to broaden the recombination zone, could be a strategy for improving the practicality of these materials.
3.2 Platinum(II) complexes supported by dianionic N2O2 ligands
3.2.1 Ligand systems and photophysical properties
Using “one-metal-one-ligand” approach to construct a stable luminescent platinum material, Che and co-workers developed the first non-porphyrin tetradentate aromatic N2O2 chelates,
|Complex||UV-Vis absorption in CH2Cl2, λabs (nm) (298 K) (ԑ, ×104 mol−1dm3cm−1)||λem (nm)||τem (μs)||
|291 (3.92), 315 (3.40), 325 (3.23), 352 (2.58), 375 (2.47), 420 (0.52), 488 (sh, 0.67), 504 (0.72)||586||5.3||0.60|
|253 (4.10), 313 (1.84), 397 (0.840), 479 (0.294), 504 (sh, 0.252)||595||1.9||0.12|
|319 (1.31), 344 (1.67), 420 (0.58), 440 (0.54)||542||3.7||0.27|
|253 (4.16), 318 (2.49), 366 (3.61), 382 (3.41), 462 (0.93), 503 (sh, 0.86), 535 (0.99)||618||3.6||0.20|
Schiff base ligands constitute another important class of N2O2 systems. The facile synthesis of Schiff base ligands, which can be prepared via one-pot multi-gram scale condensation reactions between substituted salicylic aldehydes and alkyl/aryl diamines, makes them an attractive ligand system for use in the synthesis of Pt(II) emitters. To elucidate structure-property relationships, Che and co-workers conducted a detailed investigation of a panel of Pt(II) Schiff base complexes with alkylene and arylene bridges (e.g.,
3.2.2 Chemical and thermal stability
Platinum(II) N2O2 complexes are generally stable in the solid state under ambient conditions. When dissolved in solution and exposed to light and air,
3.2.3 Electroluminescent properties
Devices with bis(2-(2-hydroxyphenyl)pyridine)beryllium (Bepp2) as the host and
The EL properties of platinum(II) Schiff base complexes were investigated. Figure 4 shows two additional complexes,
For red light-emitting materials,
Ease of synthesis, relatively short emission lifetime, high thermal stability, and decent emission quantum yield are traits that make platinum(II) N2O2 emitters attractive phosphorescent dopants, particularly for red OLEDs. Further research efforts in assessing and optimizing their operational stability in devices are anticipated.
3.3 Platinum(II) complexes supported by cyclometalated ligands
Incorporating anionic C-donor unit(s) into chromophoric ligands has been recognized as an effective strategy to enhance the luminescence of d6 and d8 transition metal complexes . The same principle generally holds for tetradentate Pt(II) emitters. The tetradentate cyclometalated Pt(II) emitters reported in the literature typically feature high phosphorescence quantum yields of up to unity, which could be attributed to the following combined effects: (i) the rigid tetradentate ligand scaffold may help suppress excited-state structural distortion, thereby disfavoring non-radiative deactivation of the emissive excited state, (ii) the strongly σ-donating carbanion may destabilize the antibonding Pt 5dx2-y2 orbitals to a great extent, thus reducing the quenching of emissive states via the 3d-d state, and (iii) the carbanion donor atom may also increase the metal character (e.g., 3MLCT) and hence the radiative decay rate of the emissive excited states.
3.3.1 Pt(II) emitters with [O^N^C^N] ligands
In 2013, Che et al. developed the first phosphorescent platinum(II) complexes supported by tetradentate [O^N^C^N] ligands for white OLED and polymer organic light-emitting diode (PLED) applications . These complexes were found to possess desirable physical properties as OLED emitters including high thermal stability with Td > 400°C and ease of sublimation for vacuum deposition. Several follow-up studies on this family of Pt(II) emitters for high-efficiency OLEDs have been reported by the same group [22, 37, 38, 39].
188.8.131.52 Molecular design strategies
In general, for monochromic blue to yellow OLEDs, emission from monomeric Pt(II) complexes should be dominant, and aggregate emission should be minimized for achieving a high color purity (Figure 5). Early works showed that platinum(II) [O^N^C^N] complexes are prone to excimeric emission at elevated concentration. In attempts to address the excimer issues for realizing monchromic green and yellow OLEDs, Che and co-workers proposed a strategy to bolster the 3D configuration of the moelcular strucutre of the complexes to supress the intermolecular interactions via the introduction of rigid and bulky substituents, such as t-Bu groups and a norbornene moiety, to the ligand periphery, and the incorporation of bridging tertiary arylamine units or biphenyl groups with spiro linkages to the ligand frameworks. These modifications were found to effectively disfavor intermolecular interactions, evident by the diminished emission self-quenching and excimeric emissions in solution and in thin film at high concentrations. In addition, the corresponding devices showed improved device efficiencies and diminished efficiency roll-offs. Additionally, Pt(II) [O^N^C^N] emitters bearing a cross-shaped molecular structure (i.e., a spiro linkage) may also cause molecular entanglement in the amorphous state, which help prevent recrystallization of the emissive layer and prolong operational lifetimes.
Through deliberate molecular design and variations in the doping concentration, the extent of intermolecular interactions and aggregations of platinum(II) [O^N^C^N] emitters could be controlled and manipulated for red and NIR as well as white OLED applications based on aggregation and monomer/aggregation emissions, respectively (Figure 5). Instead of tuning the 3D configuration to limit intermolecular interactions, for these applications, adopting a relatively planar ligand scaffold and introducing fluorine substituent(s) at specific position(s), which favor intermolecular π-π and/or Pt-Pt interactions for low-energy aggregate emission from the excited states of dimers or oligomers, are preferred.
184.108.40.206 Photophysical properties and OLEDs based on monomer emission
The cyclometalated [O^N^C^N] ligand system is useful for the construction of robust and highly efficient Pt(II) emitters. Che et al. developed a panel of platinum(II) [O^N^C^N] emitters, i.e.,
|Complex||UV-Vis absorption in CH2Cl2, λabs (nm) (ԑ, ×104 mol−1dm3cm−1)||λem (nm)||τem (μs)||
|282 (4.5), 304 (sh, 3.3), 336 (sh, 1.8), 372 (1.9), 400 (sh, 1.1), 430 (sh, 0.8)||522||4.0||0.77|
|283 (4.4), 298 (sh, 3.7), 362 (sh, 1.6), 373 (1.7), 400 (sh, 1.0), 435 (sh, 0.69)||522||4.0||0.77|
|286 (4.4), 303 (sh, 3.2), 265 (sh, 1.5), 376 (1.8), 405 (sh, 0.98), 440 (sh, 0.75)||543||5.5||0.95|
|261 (5.1), 288 (5.4), 361 (sh, 1.5), 376 (2.2), 410 (sh, 1.2), 450 (sh, 0.85)||570||2.3||0.23|
|262 (4.4), 295 (sh, 3.5), 330 (2.2), 370 (sh, 1.1), 450 (sh, 0.27), 481 (sh, 0.21)||551||4.3||0.90|
|261 (sh, 5.0), 279 (5.4), 301 (sh, 3.6), 329 (1.8), 356 (1.7), 393 (0.72), 431 (sh, 0.38)||517||5.1||0.80|
|PE (lm W−1)||CE (cd A−1)||EQE (%)|
|Complex (wt%) a||Max.||At 104 cd m−2||Max.||At 104 cd m−2||Max.||At 104 cd m−2||CIE (x, y)|
The intense absorption bands of
The EL properties of
The EL properties of
220.127.116.11 Aggregation-induced red and NIR OLEDs
The emission of Pt(II) complexes in aggregation forms is dramatically redshifted from that of monomers. Because of the increased metal character in the excited states (e.g., MMLCT) leading to the enhanced radiative decay rates, the emission lifetimes of aggregated Pt(II) emitters are usually short, in the range of 0.1–1 μs, which is fundamentally important for addressing the efficiency roll-off and the operational lifetime issues of phosphorescent OLEDs. In addition, this aggregation emission can be manipulated by tuning the doping concentration; this is particularly useful for the design of high-performance red and NIR OLEDs.
Recently, two series of platinum [O^N^C^N] complexes (Figures 6 and 7), i.e., type-I (
18.104.22.168 WOLEDs based on a single emitter
WOLED devices typically employ two or more co-dopants with different emission colors in the EML. Nevertheless, broad-band white light emission with a single Pt(II) emitter could be achieved when both the high-energy monomer emission and low-energy aggregation emission are harvested. In this case, a fine balance of the concentration of excited state monomers and excited state aggregation species is desired. Complex
3.3.2 Pt(II) emitters with [N^C^C^N] ligands
In 2013, Li et al. developed two efficient blue-emitting tetradentate platinum complexes with a carbazolyl-pyridine motif integrated into the ligand scaffold. These complexes show emission quantum yields of up to 0.89, and the corresponding devices achieved excellent EQEs of up to 25%, highlighting the potential of these platinum emitters for blue OLED applications . Subsequent works by the same group demonstrated that the carbazolyl-pyridine entity is also a versatile modular building block for various tetradentate dianionic cyclometalated N^C^C^N ligands, providing access to several new classes of efficient blue-, green-, and red-emitting platinum(II) complexes [43, 44, 45, 46].
22.214.171.124 Molecular design strategies
The emission energies of the complexes in this family can be rationally and readily tuned by modifying the modular ligand scaffold, which consists of a cyclometalated chromophoric C^N unit and an auxiliary carbazolyl-pyridine group connected by a heteroatom or the heteroatom itself may be part of the chromophoric unit, as shown in Figure 8. Complex
This class of Pt[N^C^C^N] complexes was reported to be free from excimer-based emission, which was proposed to be a consequence of distortion of the molecular structure from planarity that disfavors intermolecular interactions . Recently, Li and co-workers conducted a systematic photophysical study on derivatives of
126.96.36.199 Red-emitting complexes and devices
The EL properties of
188.8.131.52 Green-emitting complexes and devices
To investigate the EL properties of
184.108.40.206 Blue-emitting complexes and devices
Breaking the π conjugation of ligand scaffolds can increase the T1 energy for harvesting blue emission. By having all-six-membered chelate rings to interrupt the π conjugation, the O-bridged carbazolyl-pyridyl complex
Devices with the structure [ITO/HATCN (10 nm)/NPB (40 nm)/EBL/10%
Overall, tetradentate cyclometalated Pt(II) emitters have been demonstrated to exhibit high versatility in emission color tuning across RGB colors and white light, as well as superior photophysical and electroluminescent efficiencies and respectable operational lifetimes at practical luminance levels. While the performance metrics of this class of Pt(II) emitters are comparable to that of the best reported Ir(III) emitters in many aspects, more focused efforts should be directed at reducing the radiative lifetimes of these emitters by careful molecular design, which will be instrumental in further improving the operational stability of these complexes to meet the stringent standards required for commercialization.
Substantial room for innovation remains in OLED materials research, and the development of robust, high efficiency emitters for diverse applications remains a challenge both in academia and industry. While in the past decade, tris-(bidentate chelate) iridium(III) complexes have been seemingly edging out other classes of metal phosphors, it is remarkable that tetradentate platinum(II) emitters have demonstrated high performance and are being increasingly recognized by academia and industry as a competitive alternative. Importantly, the unique aggregation behavior and the associated photophysical properties afforded by their planar coordiantion geometry distinguish platinum(II) emitters from octahedral iridium(III) emitters. The unique photophysical properties of platinum(II) emitters render them well suited for some OLED applications using simple device structures such single-dopant WOLEDs and aggregation-based red and NIR OLEDs as covered in this review. In addition, appropriate molecular design of the ligand scaffold allows the regulation of the emissive excited states and the intermolecular interactions, which consequently offers flexibility in manipulating the emission characteristics of platinum(II) emitters to cater to various OLED applications. Indeed, sustained and concerted efforts between academia and industry have already realized successful application of tetradentate Pt(II) emitters in OLED devices in an industrial setting. It is without doubt that Pt(II) emitters, after full optimization, will meet the technical specifications including operational stability, required for commercialization. We hope the perspective described herein will spur interest among stakeholders and drive further development of tetradentate Pt(II) emitters for display and lighting applications.
This work was supported by the Major Program of Guangdong Basic and Applied Research (2019B030302009), Innovation and Technology Fund (ITS/224/17FP), Hong Kong Research Grants Council (HKU 17330416), the Basic Research Program of Shenzhen (JCYJ20170412140251576, JCYJ20170818141858021, and JCYJ20180508162429786), the National Key Basic Research Program of China (2013CB834802), Innovation and Technology Commission, Centre of Machine Learning for Energy Materials and Devices, a major initiative—Artificial Intelligence and Robotics cluster under InnoHK (AIR@InnoHK).
Conflict of interest
The authors declare no conflict of interest.
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