Robust phosphorescent platinum(II) complexes containing tetradentate O^N^C^N ligands: excimeric excited state and application in organic white-light-emitting diodes

Pt-S Bond-Enabled Temperature-Dependent Phosphorescence in S-Heteroaryl Tetradentate Pt(S^C^N^O) Complexes

This study presents the rare examples of S-heteroaryl tetradentate Pt(S^C^N^O) luminescent complexes (PtSZ and PtSZtBu) containing a Pt-S bond. The presence of the Pt-S bond allows the novel Pt(S^C^N^O) complexes to exhibit temperature-dependent phosphorescent emission behavior. The PtSZtBu exhibits dual-emission phenomena and biexponential transient decay spectra above 250 K, indicating the presence of two minimal excited states in the potential energy surface (PES) of the T1 state. Through complementary experimental and computational studies, we have identified changes in orbital composition between Pt(dxy)-S(px) and Pt(dyz)-S(pz) in excited states with increasing temperature. This results in two energy minima, enabling the excited states to decay selectively and radiatively at different temperatures. Consequently, this leads to remarkable steady-state and transient emission spectra changes. Our work not only provides valuable insights for the development of novel Pt-S bond-based tetradentate Pt(II) complexes but also enhances our understanding of the distinctive properties governed by the Pt-S bond.

Sterically Hindered Tetradentate [Pt(O^N^C^N)] Emitters with Radiative Decay Rates up to 5.3 × 105 s-1 for Phosphorescent Organic Light-Emitting Diodes with LT95 Lifetime over 9200 h at 1000 cd m-2

Described here are sterically hindered tetradentate [Pt(O^N^C^N)] emitters (Pt-1, Pt-2, and Pt-3) developed for stable and high-performance green phosphorescent organic light-emitting diodes (OLEDs). These Pt(II) emitters exhibit strong saturated green phosphorescence (λmax = 517-531 nm) in toluene and mCP thin films with emission quantum yields as high as 0.97, radiative rate constants (kr) as high as 4.4-5.3 × 105 s-1 and reduced excimer emission, and with a preferential horizontally oriented transition dipole ratio of up to 84%. Theoretical calculations show that p-(hetero)arene substituents at the periphery of the ligand scaffolds in Pt-1, Pt-2, and Pt-3 can i) enhance the spin-orbit coupling (SOC) between the lower singlet excited states and the T1 state, and S0→Sn (n = 1 or 2) transition dipole moment, and ii) introducing additional SOC activity and the bright 1ILCT[π(carbazole)→π*(N^C^N)] excited state (Pt-2 and Pt-3), which are the main contributors to the increased kr values. Utilizing these tetradentate Pt(II) emitters, green phosphorescent OLEDs are fabricated with narrow-band electroluminescence (FWHM down to 36 nm), high external quantum efficiency, current efficiency up to 27.6% and 98.7 cd A-1, and an unprecedented device lifetime (LT95) of up to 9270 h at 1000 cd m-2 under laboratory conditions.

Discrete Mononuclear Platinum(II) Complexes Realize High-Performance Red Phosphorescent OLEDs with EQEs of up to 31.8% and Superb Device Stability

Designing mononuclear platinum(II) complexes that do not rely on intermolecular aggregation for high-performance red organic light-emitting diodes remains a formidable challenge. In this work, three robust red-emitting Pt(II) complexes are created by utilizing a rigid 4-coordination configuration, where the ligands are formed by linking electron-donor of triphenylamine (TPA) moieties with electron-acceptor of pyridine, isoquinoline, and/or δ-carboline units. The thermal stability, electrochemical, and photophysical properties of the complexes are thoroughly examined. The complexes display efficient red phosphorescence, with high photoluminescence quantum yields and short excited lifetimes. The OLEDs dope with these complexes exhibit high maximum external quantum efficiencies (EQEs) of up to 31.8% with minimal efficiency roll-off even at high brightness. Significantly, the devices demonstrate exceptional long operational lifetime, with a T90 lifetime of over 14000 h at initial luminance of 1000 cd m-2 , indicating the potential for these complexes to be practically utilizes.

High-Efficiency Simplified Orange and White Organic Light-Emitting Devices Based on a Platinum(II) Complex

We demonstrated efficient simplified orange and white organic light-emitting devices based on a platinum(II) complex Tetra-Pt-N. The maximum current efficiency achieved from the optimized orange device was 57.6 cd/A. The emission mechanism for the system of Tetra-Pt-N doped into 4,4'-bis(arbazole-9-yl)biphenyl was discussed. Moreover, a high-efficiency and simplified white device was fabricated by introducing an ultra-thin blue phosphorescent emission layer. The white device with a maximum current efficiency of 41.9 cd/A showed excellent stable spectra and low efficiency roll-off.

Fused 6/5/6 Metallocycle-Based Tetradentate Pt(II) Emitters for Efficient Green Phosphorescent OLEDs

Pt(II) complexes are promising phosphorescent materials for organic light-emitting diode (OLED) applications in the fields of display, lighting, healthcare, aerospace, and so on. A series of novel biphenyl (bp)-based tetradentate 6/5/6 Pt(II) emitters using oxygen or carbon as a linking atom was designed and developed. The intermolecular interactions in crystal packing, electrochemical, and photophysical properties of the bp-based Pt(II) emitters and also their excited-state properties were systematically studied, which could be effectively regulated by ligand modification through linking group control; however, their emission spectra nearly showed no change. All the bp-based Pt(II) emitters exhibited vibronically featured emission spectra with dominant peaks at 502-505 nm and photoluminescent quantum yields of 24-34% in dichloromethane solution. Green OLED using Pt(bp-12) as an emitter achieved a maximum brightness (L_max) of 16,644 cd/m².

Thiazoline Carbene-Cu(I)-Amide complexes: Efficient White Electroluminescence from Combined Monomer and Excimer Emission

Luminescent carbene-metal-amide complexes bearing group 11 metals (Cu, Ag, Au) have recently attracted great attention due to their exceptional emission efficiency and high radiative decay rates (k_r). These materials provide a less costly alternative to organic light-emitting diode (OLED) emitters based on more scarce metals, such as Ir and Pt. Herein, a series of eight Cu(I) complexes bearing as yet unexplored 1,3-thiazoline carbenes have been investigated and analyzed with respect to their light emission properties and OLED application. For the first time among the class of copper-based organometallic compounds the formation of efficient electroluminescent excimers is demonstrated. The prevalence of electroluminescence (EL) from either the monomer (bluish green) or the excimer (orange-red) can be adjusted in vacuum-deposited emissive layers by altering the extent of steric encumbrance of the emitter or its concentration. Optimized conditions in terms of the emitter structure and mass fraction allowed a simultaneous EL from the monomer and excimer, which laid the basis for a preparation of a single-emitter white OLED (WOLED) with external quantum efficiency of 16.5% and a maximum luminance of over 40000 cd m^-2. Wide overlapping emission bands of the monomer and excimer ensure a device color rendering index (CRI) of above 80. In such a way the prospects of copper complexes as cost-effective materials for lighting devices are demonstrated, offering expense reduction through a cheaper emissive component and a simplified device architecture.

Luminescent cyclometalated alkynylplatinum(II) complexes with 1,3-di(pyrimidin-2-yl)benzene ligands: synthesis, electrochemistry, photophysics and computational studies

In this article, we report on a series of cyclometalated chloro- and alkynyl-platinum(II) complexes bearing various tridentate N^C^N-cyclometalated ligands derived from 1,3-bis(pyrimidin-2-yl)benzene. The X-ray crystal structures of two alkynyl-platinum(II) complexes were determined and other structures were DFT-calculated. Electrochemical and DFT-computational studies suggest a ligand-centred reduction on the R¹-substituted N^C^N ligand, whereas oxidation likely occurs either on the Pt-phenylacetylide moiety and/or the cyclometalated ligand. In CH₂Cl₂ solution at room temperature, the complexes show phosphorescent emissions ranging from green to orange, depending on the R¹ and R² substituents on the ligands. In KBr solid state matrix, excluding complexes bearing a trifluoromethyl substituted ligand, all compounds exhibit red emission. The presence of an alkynyl ancillary ligand has limited influence on absorption and emission spectra except in the case of the complex with the strongly electron-donating diphenylamino R² substituent on the alkynyl ligand, for which a significant red-shift was observed. The alkynyl Pt(II) complex with OMe groups as both R¹ and R² substituents shows the best emission quantum yield (0.81 in CH₂Cl₂ solution) in this series. The full series of DFT calculated band gaps correlated generally well with the electrochemical and absorption data and reasonably model the impact of the substituents on the electronics of these complexes.

Asymmetrically Functionalized 1,3-Di(2-pyridyl)benzenes: Synthesis and Photophysical Studies

A convenient synthetic approach to asymmetrically functionalized 1,3-di(2-pyridyl)benzenes starting from 3-(3-bromophenyl)-1,2,4-triazines using sequential aza-Diels-Alder reactions and Stille cross-coupling is reported. Photophysical properties of the obtained compounds are studied.

Thiophene-Bipyridine Appended Diketopyrrolopyrrole Ligands and Platinum(II) Complexes

Straightforward palladium(II) catalyzed direct cross-coupling reaction between decyl, (S)-2-methyl-butyl, and dodecyl N-substituted diketopyrrolopyrrole thiophene (DPPT), including a 3-methoxy-thiophene derivative, and 6-bromo-2,2'-bipyridine afforded a series of mono- and bis-bipyridine substituted DPPT ligands 1-3. Complexation reactions with PtCl₂(DMSO)₂ provided ortho-metalated platinum(II) complexes 1-Pt and 2-Pt, together with the N^N^O complex 3d-Pt(N^N^O) resulted from the O-Me activation of the intermediary complex 3d-Pt(N^N). The ligand 1b and the mononuclear complexes 1a-Pt and 1b-Pt have been structurally characterized by single crystal X-ray structure, evidencing the establishment of numerous intermolecular π-π interactions in the solid state. Moreover, in the crystal structure of the model complex DMTB-Pt(N^N^O) (DMTB = 3,4-dimethoxy-(2,2'-bipyridine)) the chelating tridentate N^N^O mode is clearly evidenced. The chiral ligand 1b and its mononuclear complex 1b-Pt do not show any CD signal in solution, but they are CD active in the solid state with bisignate bands in the low energy region, opposite in sign between the ligand and the complex, suggesting helical supramolecular arrangement of the dpp chromophore in the solid state. Photophysical investigations demonstrate that all of the ligands are fluorescent with high quantum yields, while the emission is quenched for the complexes, except partially in 3d-Pt(N^N), very likely through an intersystem crossing mechanism promoted by the heavy metal. Density functional theory calculations support the differences observed between the absorption properties of the ligands, ortho- and non-ortho-metalated complexes. The highly fluorescent bipyridine ligands reported herein open the way toward multifunctional transition metal complexes and their use in organic electronics.

Tetradentate Gold(III) Complexes as Thermally Activated Delayed Fluorescence (TADF) Emitters: Microwave-Assisted Synthesis and High-Performance OLEDs with Long Operational Lifetime

Structurally robust tetradentate gold(III)-emitters have potent material applications but are rare and unprecedented for those displaying thermally activated delayed fluorescence (TADF). Herein, a novel synthetic route leading to the preparation of highly emissive, charge-neutral tetradentate [C^C^N^C] gold(III) complexes with 5-5-6-membered chelate rings has been developed through microwave-assisted C-H bond activation. These complexes show high thermal stability and with emission origin (³ IL, ³ ILCT, and TADF) tuned by varying the substituents of the C^C^N^C ligand. With phenoxazine/diphenylamine substituent, we prepared the first tetradentate gold(III) complexes that are TADF emitters with emission quantum yields of up to 94 % and emission lifetimes of down to 0.62 μs in deoxygenated toluene. These tetradentate Au^III TADF emitters showed good performance in vacuum-deposited OLEDs with maximum EQEs of up to 25 % and LT₉₅ of up to 5280 h at 100 cd m^-2 .

High-Efficiency Solution-Processed Organic Light-Emitting Diodes with Tetradentate Platinum(II) Emitters

The realization of high-efficiency solution-processed organic light-emitting diodes (OLEDs) using phosphorescent tetradentate Pt(II) emitters and bipolar organic hosts is demonstrated in this work. To investigate the effect of organic host on the platinum dopant, the performances of solution-processed Pt-OLEDs with various combinations between four tetradentate Pt(II) emitters, including two newly developed tetra-Pt-S2 and tetra-Pt-S3 and three bipolar organic hosts m-TPAPy, o-TPAPy, and o-CzPy, have been analyzed and compared. Among the tetradentate Pt(II) complexes studied in this work, tetra-Pt-S3 exhibited the best electroluminescent performance attributable to its bulky molecular scaffold structure, high emission quantum yield, and good solubility in common organic solvents. High external quantum efficiencies (EQEs) of up to 22.4% were achieved in the solution-processed OLED with tetra-Pt-S3 emitter and m-TPAPy host at the dopant concentration of 4 wt %. At a high luminance of 1000 cd m^-2, the EQE of this device decreased slightly to 21.0%.

Influence of different ancillary ligand on the phosphorescent properties of platinum(II) complexes

In past two decades, lots of bidentate Pt(II) complexes are developed as potential organic light emitting diodes due to their simple synthetic process. The relative low quantum efficiency is one of the major blocks for their applications. Two new heteroleptic Pt(II) complexes bearing an n-hexyloxy substituted phenyllepidine-based ligand and either a picolinate (pic) (1) or acetylacetonate (acac) (2) ancillary ligand are synthesized as orange-red-emitter by Wawrzinek and coauthors. The quantum efficiency of 2 is much larger than that of 1 indicating that the variation of ancillary ligand has a great effect on the performance. Inspired by it, other two new bidentate Pt(II) complexes are theoretically designed with the same primary ligand along with pyrazolone (pzl) (3) or N-substituted carbazole (NCaz) (4) ancillary ligand. The phosphorescent properties are explored by density functional theory (DFT) and time dependent DFT (TDDFT) methods with the ultimate goal to explore the influence of ancillary ligand. Moreover, the emission rule is confirmed. Finally, the quantum yield is estimated according to the radiative rate constant (k_r) and nonradiative rate constant (k_nr). The smaller k_nr is the vital item to determine the high quantum yield.

Control of Dual Conformations: Developing Thermally Activated Delayed Fluorescence Emitters for Highly Efficient Single-Emitter White Organic Light-Emitting Diodes

In this work, we propose a novel concept to develop two fluorophores 2-(10 H-phenothiazin-10-yl)thianthrene 5,5,10,10-tetraoxide (PTZ-TTR) and 2-(4-(10 H-phenothiazin-10-yl)phenyl)thianthrene 5,5,10,10-tetraoxide (PTZ-Ph-TTR) showing dual conformations for highly efficient single-emitter white organic light-emitting diodes (WOLEDs). Both molecules exist in two stable conformations. Their nearly orthogonal forms own lower energy levels and show thermally activated delayed fluorescence (TADF) characteristics, whereas their nearly planar conformers possess higher energy levels and show only prompt fluorescence. These dual conformers were exploited for fabricating WOLEDs with complementary emission colors contributed by the two conformations. Moreover, the originally wasted triplet energy on the nearly planar conformation can be transferred to the nearly orthogonal one and then harvested via the TADF channel, realizing full exciton utilization. A PTZ-TTR-based single-emitter device exhibits standard white emission with a CIE coordinate of (0.33, 0.33) and a high color rendering index value of 92. On the other hand, the PTZ-Ph-TTR-based single-emitter device realizes an emission approaching warm white light and a high maximum external quantum efficiency of 16.34%. These results demonstrate an alternative approach for designing high-performance WOLEDs based on single TADF emitters.

Photokinetic study on remarkable excimer phosphorescence from heteroleptic cyclometalated platinum(ii) complexes bearing a benzoylated 2-phenylpyridinate ligand

Novel heteroleptic cyclometalated platinum(ii) complexes consisting of 5'-benzoylated 2-phenylpyridinate (ppy) cyclometalated and acetylacetonate ancillary ligands were synthesized, and their photoluminescence (PL) properties were investigated. The 5'-benzoylated complex without any other substituents exhibited phosphorescence-based monomer emission at 479 nm in dichloromethane (10 μM, rt) with a PL quantum yield of 0.28. On the other hand, in poly(methyl methacrylate) (PMMA) film, remarkable excimer emission additionally emerged at ca. 600 nm with a relatively high PL quantum yield of 0.47 as the doping level increased to 0.20 mmmol g^-1, which was comparably intense in comparison with the monomer emission. In the case of the complexes with unsubstituted, 4'-benzoylated, and 5'-fluorinated ppy cyclometalated ligands, excimer emission was modestly generated at the same doping level, and thus the introduction of a benzoyl group to the 5'-position is effective to obtain remarkable excimer emission. The combination of benzoyl and fluoro groups was more effective at inducing excimer emission, and the intensity of excimer emission of the 2-(5-benzoyl-4,6-difluorophenyl)pyridinate-based complex was 3.5 times larger than that of monomer emission at a doping level of 0.20 mmmol g^-1 in PMMA. From the analysis of PL lifetimes at varying concentrations, photokinetic profiles were fully analyzed according to the model system for the irreversible excimer formation, and the excimer formation rate constant of the 5'-benzoylated complex was determined in dichloromethane as 2.2 × 10⁹ M^-1 s^-1, which is 4.4 times larger than that of the unsubstituted complex. We also fabricated an organic light-emitting diode using the 2-(5-benzoyl-4,6-difluorophenyl)pyridinate-based complex as a single emitter. The device exhibited pseudo-white EL with the Commission internationale de l'éclairage chromaticity coordinates of (0.42, 0.42).

Benzothiazole-Based Cycloplatinated Chromophores: Synthetic, Optical, and Biological Studies

Cycloplatinated complexes based on 2-(4-substituted)benzothiazole ligands of type [Pt(R-PBT-κC,N)Cl(L)] (PBT=2-phenylbenzothiazole; R=Br (1), Me₂ N (2); L=dimethyl sulfoxide (DMSO; a), 1,3,5- triaza-7-phosphaadamantane (PTA; b), triphenylphosphine 3,3',3''-trisulfonate (TPPTS; c)) and [Pt(Br-PBT-κC)Cl(PTA)₂ ] (3) are presented. On the basis of the photophysical data and time-dependent (TD)-DFT calculations (1 a and 2 a), the low-lying transitions (absorption and emission) were associated with ligand-center (LC) charge transfer, with minor metal-to-ligand charge transfer (MLCT), and intraligand charge transfer (ILCT) [Me₂ N-PBT→PBT] excited states, respectively. Simultaneous fluorescence/phosphorescence bands were found in fluid solutions (and also in the solid state for 2 a), which become dominated by triplet emission bands in rigid media at 77 K. The effect of the concentration on emissive behavior of 2 a, b indicated the occurrence of aggregation-induced luminescence properties related to the occurrence of metal-metal and π⋅⋅⋅π interactions, which are more enhanced in 2 a because of the less bulky DMSO ligand. The behavior of 2 a toward para-toluenesulfonic acid (PTSA) in aerated acetonitrile and to hydrogen chloride gas in the solid state has been evaluated, thus showing a clear reversible change between the ¹ ILCT and ³ LC/³ MLCT states due to protonation of the NMe₂ group (theoretical calculations on 2 a-H⁺ ). Solid 2 a undergoes a surprising oxidation of the Pt^II center to Pt^IV with concomitant deoxygenation of DMSO, under prolonged reaction with hydrogen chloride gas to afford the Pt^IV /dimethyl sulfide complex (mer-[Pt(Me₂ N-PBT-κC,N)Cl₃ (SMe₂ )]; mer-4), which evolves in solution to fac-4, as confirmed by X-ray studies. Cytotoxic activity studies on A549 and HeLa cell lines indicated cytotoxic activity of 1 b and 2 a, b. In addition, fluorescent cell microscopy revealed cytoplasmic staining, more visible in perinuclear areas. Inhibition of tubulin polymerization by 1 b in both cells is presented as a preliminary mechanism of its cytotoxic action.

Control of excimer phosphorescence by steric effects in cyclometalated platinum(ii) diketonate complexes bearing peripheral carbazole moieties towards application in non-doped white OLEDs

Non-doped multilayer OLEDs employing bulky platinum(ii) complexes exhibited white electroluminescence due to the combination of blue monomer and orange excimer emissions.

Electronic structure and luminescence properties of unique complexes: cyclometalated iridium(iii) chelated by o-carboranyl-pyridine ligands

Iridium(iii) complexes that use o-carborane as a chelating ligand have been theoretically investigated for the first time. The superior properties of complexes will potentially promote the discovery of efficient blue-emitting complexes.

Efficient and stable single-doped white OLEDs using a palladium-based phosphorescent excimer

A tetradentate Pd(ii) complex, Pd3O3, which exhibits highly efficient excimer emission is synthesized and characterized. Pd3O3 can achieve blue emission despite using phenyl-pyridine emissive ligands which have been a mainstay of stable green and red phosphorescent emitter designs, making Pd3O3 a good candidate for stable blue or white OLEDs. Pd3O3 exhibits strong and efficient phosphorescent excimer emission expanding the excimer based white OLEDs beyond the sole class of Pt complexes. Devices of Pd3O3 demonstrate peak external quantum efficiencies as high as 24.2% and power efficiencies of 67.9 Lm per W for warm white devices. Furthermore, Pd3O3 devices in a carefully designed stable structure achieved a device operational lifetime of nearly 3000 h at 1000 cd m-2 without any outcoupling enhancement while simultaneously achieving peak external quantum efficiencies of 27.3% and power efficiencies over 81 Lm per W.

Heteroleptic Cycloplatinated N-Heterocyclic Carbene Complexes: A New Approach to Highly Efficient Blue-Light Emitters

New heteroleptic compounds of platinum(II)-containing cyclometalated N-heterocyclic carbenes, [PtCl(R-C^C*)(PPh₃)] [R-CH^C*-κC* = 3-methyl-1-(naphthalen-2-yl)-1H-imidazol-2-ylidene (R-C = Naph; 1A), 1-[4-(ethoxycarbonyl)phenyl]-3-methyl-1H-imidazol-2-ylidene (R = CO₂Et; 1B), and [Pt(R-C^C*)(py)(PPh₃)]PF₆ (py = pyridine; R-C = Naph, 2A; R = CO₂Et, 2B], have been prepared and fully characterized. All of them were obtained as the trans-(C*,PPh₃) isomer in high yields. The selectivity of their synthesis has been explained in terms of the degree of transphobia (T) of pairs of ligands in trans positions. X-ray diffraction studies on both 2A and 2B revealed that only in 2A, containing a C^C* with a more extended π system, do the molecules assemble themselves into head-to-tail pairs through intermolecular π···π contacts. The photophysical properties of 2A and 2B and those of the related compounds [Pt(NC-C^C*)(PPh₃)L]PF₆ [NC-CH^C*-κC* = 1-(4-cyanophenyl)-3-methyl-1H-imidazol-2-ylidene; L = pyridine (py; 2C), 2,6-dimethylphenylisocyanide (CNXyl; 3C), and 2-mercapto-1-methylimidazole (MMI; 4C)] have been examined to analyze the influence of the R substituent on R-C^C* (R-C = Naph; R = CO₂Et, CN) and that of the ancillary ligands (L) on them. Experimental data and time-dependent density functional theory calculations showed the similarity of the electronic features associated with R-C^C* (R = CN, CO₂Et) and their difference with respect to R-C^C* (R-C = Naph). All of the compounds are very efficient blue emitters in poly(methyl methacrylate) films under an argon atmosphere, with QY values ranging from 68% (2B) to 93% (2C). In the solid state, the color of the emission changes to yellowish-orange for compounds 2A (λ_max = 600 nm) and 3C (λ_max = 590 nm) because of the formation of aggregates through intermolecular π···π interactions. 2C and 3C were chosen to fabricate fully solution-processed electroluminescent devices with blue-light (2C), yellow-orange-light (3C), and white-light (mixtures of 2C and 3C) emission from neat films of the compounds as emitting layers.

Phosphorescent Pt(II) and Pd(II) Complexes for Efficient, High-Color-Quality, and Stable OLEDs

Phosphorescent organic light-emitting diodes (OLEDs) are leading candidates for next-generation displays and solid-state lighting technologies. Much of the academic and commercial pursuits in phosphorescent OLEDs have been dominated by Ir(III) complexes. Over the past decade recent developments have enabled square planar Pt(II) and Pd(II) complexes to meet or exceed the performance of Ir complexes in many aspects. In particular, the development of N-heterocyclic carbene-based emitters and tetradentate cyclometalated Pt and Pd complexes have significantly improved the emission efficiency and reduced their radiative lifetimes making them competitive with the best reported Ir complexes. Furthermore, their unique and diverse molecular design possibilities have enabled exciting photophysical attributes including narrower emission spectra, excimer -based white emission, and thermally activated delayed fluorescence. These developments have enabled the fabrication of efficient and "pure" blue OLEDs, single-doped white devices with EQEs of over 25% and high CRI, and device operational lifetimes which show early promise that square planar metal complexes can be stable enough for commercialization. These accomplishments have brought Pt complexes to the forefront of academic research. The molecular design strategies, photophysical characteristics, and device performance resulting from the major advancements in emissive Pt and Pd square planar complexes are discussed.

The phosphorescence properties of a series of diarylethene-containing platinum complexes: the effect of ligand photoisomerization

Benefitting from isomerization in the ancillary-ligand diarylethene, a control of phosphorescence efficiency becomes available.

Highly luminescent palladium(ii) complexes with sub-millisecond blue to green phosphorescent excited states. Photocatalysis and highly efficient PSF-OLEDs

Palladium(ii) complexes supported by tetradentate [N^C^C^N] and [O^N^C^N] ligand systems display sky blue to red phosphorescence with emission quantum yields and emission lifetimes up to 0.64 and 272 μs, respectively. Femtosecond time-resolved fluorescence (fs-TRF) measurements on these Pd(ii) complexes reveal a fast intersystem crossing from singlet to triplet manifolds with time constants of 0.6-21 ps. DFT/TDDFT calculations revealed that, as a result of the spiro-fluorene and bridging tertiary amine units of the ligands, the T1 excited state is more ligand-localized and has smaller structural distortion, leading to slower non-radiative decay as well as radiative decay of T1 → S0 transition and thereby highly emissive, long-lived triplet excited states. The Pd(ii) complexes have been found to be efficient catalysts for visible light-driven, reductive C-C bond formation from unactivated alkyl bromides with conversions and yields of up to 90% and 83%, respectively. These complexes have also been employed as photosensitizers for [2 + 2] cycloaddition of styrenes, with conversions and yields comparable to those of the reported Ir(iii) complexes. Both green and sky blue organic-light emitting devices (OLEDs) have been generated with these Pd(ii) complexes as guest emitters. Maximum external quantum efficiencies (EQE) of up to 16.5% have been achieved in the sky blue OLEDs. The long emission lifetimes render the Pd(ii) complexes good sensitizers for phosphor-sensitized fluorescent OLEDs (PSF-OLEDs). By utilizing these phosphorescent Pd(ii) complexes as sensitizers, highly efficient green and yellow PSF-OLEDs having high EQE (up to 14.3%), high colour purity and long operation lifetimes, with 90% of initial luminance (LT90) for more than 80 000 h, have been realized.

Platinum and Gold Complexes for OLEDs

Encouraging efforts on the design of high-performance organic materials and smart architecture during the past two decades have made organic light-emitting device (OLED) technology an important competitor for the existing liquid crystal displays. Particularly, the development of phosphorescent materials based on transition metals plays a crucial role for this success. Apart from the extensively studied iridium(III) complexes with d(6) electronic configuration and octahedral geometry, the coordination-unsaturated nature of d(8) transition metal complexes with square-planar structures has been found to provide intriguing spectroscopic and luminescence properties. This article briefly summarizes the development of d(8) platinum(II) and gold(III) complexes and their application studies in the fabrication of phosphorescent OLEDs. An in-depth understanding of the nature of the excited states has offered a great opportunity to fine-tune the emission colors covering the entire visible spectrum as well as to improve their photophysical properties. With good device engineering, high performance vacuum-deposited OLEDs with external quantum efficiencies (EQEs) of up to 30 % and solution-processable OLEDs with EQEs of up to 10 % have been realized by modifying the cyclometalated or pincer ligands of these metal complexes. These impressive demonstrations reveal that d(8) metal complexes are promising candidates as phosphorescent materials for OLED applications in displays as well as in solid-state lighting in the future.

Highly phosphorescent platinum(ii) emitters: photophysics, materials and biological applications

In recent years a blossoming interest in the synthesis, photophysics and application of phosphorescent Pt(ii) complexes, particularly on their uses in bioimaging, photocatalysis and phosphorescent organic light-emitting diodes (OLEDs), has been witnessed. The superior performance of phosphorescent Pt(ii) complexes in these applications is linked to their diverse spectroscopic and photophysical properties, which can be systematically modulated by appropriate choices of auxiliary ligands. Meanwhile, an important criterion for the practical application of phosphorescent metal complexes is their stability which is crucial for biological utilization and industrial OLED applications. Taking both the luminescence properties and stability into consideration, chelating ligands having rigid scaffolds and with strong σ-donor atoms are advantageous for the construction of highly robust phosphorescent Pt(ii) complexes. The square-planar coordination geometry endows Pt(ii) complexes with the intriguing spectroscopic and photophysical properties associated with their intermolecular interactions in both the ground and excited states. In this article, we discuss the design and synthesis of phosphorescent Pt(ii) complexes with elaboration on the effects of ligands on the structure and luminescence properties. Based on their photophysical and emission properties, we intend to shed light on the great promise of highly robust phosphorescent Pt(ii) emitters in an array of applications from molecular materials to biosensors.

Stable luminescent iridium(iii) complexes with bis(N-heterocyclic carbene) ligands: photo-stability, excited state properties, visible-light-driven radical cyclization and CO2 reduction, and cellular imaging

Excited state properties, photo-catalysis and cellular imaging of photo-stable bis-NHC Ir(iii) complexes are described.

A new class of cyclometalated Ir(iii) complexes supported by various bidentate C-deprotonated (C^N) and cis-chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been synthesized. These complexes display strong emission in deaerated solutions at room temperature with photoluminescence quantum yields up to 89% and emission lifetimes up to 96 μs. A photo-stable complex containing C-deprotonated fluorenyl-substituted C^N shows no significant decomposition even upon irradiation for over 120 h by blue LEDs (12 W). These, together with the strong absorption in the visible region and rich photo-redox properties, allow the bis-NHC Ir(iii) complexes to act as good photo-catalysts for reductive C–C bond formation from C(sp³/sp²)–Br bonds cleavage using visible-light irradiation (λ > 440 nm). A water-soluble complex with a glucose-functionalized bis-NHC ligand catalysed a visible-light-driven radical cyclization for the synthesis of pyrrolidine in aqueous media. Also, the bis-NHC Ir(iii) complex in combination with a cobalt catalyst can catalyse the visible-light-driven CO₂ reduction with excellent turnover numbers (>2400) and selectivity (CO over H₂ in gas phase: >95%). Additionally, this series of bis-NHC Ir(iii) complexes are found to localize in and stain endoplasmic reticulum (ER) of various cell lines with high selectivity, and exhibit high cytotoxicity towards cancer cells, revealing their potential uses as bioimaging and/or anti-cancer agents.

Theoretical study and design of highly efficient platinum(ii) complexes bearing tetradentate ligands for OLED

Theoretical study for improving the efficiency of a novel tetradentate platinum(ii) complexes in OLED by molecular modification.