Stable Tetradentate Gold(III)-TADF Emitters with Close to Unity Quantum Yield and Radiative Decay Rate Constant of up to 2 × 106 s-1 : High-Efficiency Green OLEDs with Operational Lifetime (LT90 ) Longer than 1800 h at 1000 cd m-2.

High maximum external quantum efficiency (EQEmax ), small efficiency roll-offs, and long operational lifetime at practical luminances are three crucial parameters for commercialization of organic light-emitting diodes (OLEDs). To simultaneously achieve these goals, it is desirable to have the radiative decay rate constant (kr ) as large as possible, which, for a thermally activated delayed fluorescent (TADF) emitter, requires both a large S1 →S0 radiative decay rate constant (kr S ) and a small singlet-triplet energy gap (ΔEST ). Here, the design of a class of tetradentate gold(III) TADF complexes for narrowing the ΔEST while keeping the kr S large is reported. The as-synthesized complexes display green emission with close to unity emission quantum yields, and kr approaching 2 × 106 s-1 in thin films. The vacuum-deposited green OLEDs based on 1 and 4 demonstrate maximum EQEs of up to 24 and 27% with efficiency roll-offs of 5.5 and 2.2% at 1000 cd m-2 , respectively; the EQEs maintain high at 10 000 cd m-2 (19% (1) and 24% (4)). A long LT90 device lifetime of 1820 h at 1000 cd m-2 for complex 1 is achieved, which is one of the longest device lifetimes of TADF-OLEDs reported in the literature.

Gold(I) Multi-Resonance Thermally Activated Delayed Fluorescent Emitters for Highly Efficient Ultrapure-Green Organic Light-Emitting Diodes.

Acceleration of singlet-triplet intersystem crossings (ISC) is instrumental in bolstering triplet exciton harvesting of multi-resonance thermally activated delayed fluorescent (MR-TADF) emitters. This work describes a simple gold(I) coordination strategy to enhance the spin-orbit coupling of green and blue BN(O)-based MR-TADF emitters, which results in a notable increase in rate constants of the spectroscopically observed ISC process to 3×109  s-1 with nearly unitary ISC quantum yields. Accordingly, the resultant thermally-stable AuI emitters attained large values of delayed fluorescence radiative rate constant up to 1.3×105 /1.7×105  s-1 in THF/PMMA film while preserving narrowband emissions (FWHM=30-37 nm) and high emission quantum yields (ca. 0.9). The vapor-deposited ultrapure-green OLEDs fabricated with these AuI emitters delivered high luminance of up to 2.53×105  cd m-2 as well as external quantum efficiencies of up to 30.3 % with roll-offs as low as 0.8 % and long device lifetimes (LT60 ) of 1210 h at 1000 cd m-2 .

Dinuclear PtII Complexes with Strong Blue Phosphorescence for Operationally Stable Organic Light-Emitting Diodes with EQE up to 23 % at 1000 cd m-2.

Here we describe the synthesis and characterization of a new class of dinuclear PtII complexes with blue phosphorescence. Bulky N-heterocyclic carbene and tethered bridging ligands were employed to suppress photo-induced structural changes and to improve thermal stability of the complexes. These complexes show mixed 3 IL/3 MLCT blue emission (≈460 nm) with emission quantum yields of up to 0.95, emission lifetimes of as low as 1.3 µs and radiative decay rate constants of up to 7.3×105  s-1 in 4 wt % doped PMMA films; the latter is attributed to a 1 MLCT excited state having high metal character (resulting in a large SOC) and a large transition dipole moment, based on DFT calculations. Phosphor-sensitized blue hyper-OLEDs with Commission Internationale de L'Eclairage (CIE) coordinates of (0.13, 0.12) showed a maximum EQE of 23.4 % with a full-width-at-half-maximum of 18 nm and a LT50 >250 h at an L0 of 1000 cd m-2 .

High Efficiency Sky-Blue Gold(III)-TADF Emitters*.

Highly efficient sky-blue luminescent gold(III) complexes with emission quantum yields up to 82 %, lifetimes down to 0.67 µs and emission peak maxima at 470-484 nm were prepared through a consideration of pincer gold(III) donor-acceptor complexes. Photophysical studies and time-dependent density functional theory (TDDFT) calculations revealed that the emission nature of these gold(III) complexes is most consistent with TADF. Solution-processed OLEDs with these gold(III) complexes as dopants afforded electroluminescence maxima at 465-473 nm with FWHM of 64-67 nm and maximum external quantum efficiencies (EQEs) of up to 15.25 %. This research demonstrates the first example of gold(III)-OLEDs showing electroluminescence maxima at smaller than 470 nm, and highlights the potential of using gold(III)-TADF emitters in the development of high efficiency blue OLEDs and blue emissive dopant in WOLEDs.

Luminescent tungsten(vi) complexes as photocatalysts for light-driven C-C and C-B bond formation reactions.

The realization of photocatalysis for practical synthetic application hinges on the development of inexpensive photocatalysts which can be prepared on a large scale. Herein an air-stable, visible-light-absorbing photoluminescent tungsten(vi) complex which can be conveniently prepared at the gram-scale is described. This complex could catalyse photochemical organic transformation reactions including borylation of aryl halides, such as aryl chloride, reductive coupling of benzyl bromides for C-C bond formation, reductive coupling of phenacyl bromides, and decarboxylative coupling of redox-active esters of alkyl carboxylic acid with high product yields and broad functional group tolerance.

Recent Advances in Metal-TADF Emitters and Their Application in Organic Light-Emitting Diodes.

In this contribution, recent advances in new classes of efficient metal-TADF complexes, especially those of Au(I), Au(III), and W(VI), and their application in OLEDs are reviewed. The high performance (EQE = 25%) and long device operational lifetime (LT95 = 5,280 h) achieved in an OLED with tetradentate Au(III) TADF emitter reflect the competitiveness of this class of emitters for use in OLEDs with practical interest. The high EQE of 15.6% achieved in solution-processed OLED with W(VI) TADF emitter represents an alternative direction toward low-cost light-emitting materials. Finally, the design strategy of metal-TADF emitters and their next-stage development are discussed.

Excitation-Wavelength-Dependent and Auxiliary-Ligand-Tuned Intersystem-Crossing Efficiency in Cyclometalated Platinum(II) Complexes: Spectroscopic and Theoretical Studies.

Understanding the factors affecting the intersystem-crossing (ISC) rate constant (kISC) of transition-metal complexes is crucial to material design with tailored photophysical properties. Most of the works on ISC to date focused on the influence by the chromophoric ligand and the understanding of the ISC efficiency were mainly drawn from the steady-state fluorescence to phosphorescence intensity ratio and ground-state calculations, with only a few high-level calculations on kISC that take excited-state structural change and solvent reorganization into account for quantitative comparisons with the experimental data. In this work, a series of [Pt(thpy)X)]+ complexes were prepared [Hthpy = 2-(2'-thienyl)pyridine, where X = auxiliary ligands] and characterized by both steady-state and time-resolved luminescence spectroscopies. A panel of auxiliary ligands with varying σ-donating/π-accepting character have been used. For comparison, analogues of [Pt(ppy)(P^P)]+ (Hppy = 2-phenylpyridine and P^P = diphosphino ligand) were also examined. The [Pt(thpy)(P^P)]+ complexes exhibit dual fluorescence-phosphorescence emission, with their ISC rate constants varied with the electronic characteristics of the auxiliary ligand: the more electron-donating ligand induces faster ISC from the S1 excited state to the triplet manifold. Density functional theory (DFT)/time-dependent DFT calculations of kISC(S1→T2) at the optimized excited-state geometries give excellent quantitative agreement with the femtosecond time-resolved fluorescence measurements; it was revealed that the more electron-donating auxiliary ligand increases metal contributions to both occupied and virtual orbitals and decreases the energy gap of the coupling excited states, leading to a decrease in the activation energy and an increase in spin-orbit coupling. Furthermore, the ISC rate constants of [Pt(thpy)(P^P)]+ complexes are found to depend on the excitation wavelengths. The deviation from Kasha-Vavilov's rule upon photoexcitation at λexc < 350 nm is due to the ultrafast S2 → T2 and S2 → T3 ISCs, as demonstrated by the calculated τISC < 100 fs, giving hints as to why S2 → S1 internal conversion (τIC ∼ ps) is not competitive with this hyper-ISC.

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 AuIII 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%.

Thermally Stable Donor-Acceptor Type (Alkynyl)Gold(III) TADF Emitters Achieved EQEs and Luminance of up to 23.4% and 70 300 cd m-2 in Vacuum-Deposited OLEDs.

Thermally stable, strongly luminescent gold-TADF emitters are the clue to realize practical applications of gold metal in next generation display and lighting technology, a scarce example of which is herein described. A series of donor-acceptor type cyclometalated gold(III) alkynyl complexes with some of them displaying highly efficient thermally activated delayed fluorescence (TADF) with Φ up to 88% in thin films and emission lifetimes of ≈1-2 µs at room temperature are developed. The emission color of these complexes is readily tunable from green to red by varying the donor unit and cyclometalating ligand. Vacuum-deposited organic light-emitting diodes (OLEDs) with these complexes as emissive dopants achieve external quantum efficiencies (EQEs) and luminance of up to 23.4% and 70 300 cd m-2, respectively.

Strongly Luminescent Tungsten Emitters with Emission Quantum Yields of up to 84 %: TADF and High-Efficiency Molecular Tungsten OLEDs.

Metal-TADF (thermally activated delayed fluorescence) emitters hold promise in the development of next generation light-emitting materials for display and lighting applications, examples of which are, however, largely confined to CuI and recently AuI , AgI , and AuIII emitters. Herein is described the design strategy for an unprecedented type of metal-TADF emitter based on inexpensive tungsten metal chelated with Schiff base ligand that exhibit high emission quantum yields of up to 56 % in solutions and 84 % in thin-film (5 wt % in 1,3-bis(N-carbazolyl)benzene, mCP) at room temperature. Femtosecond time-resolved emission (fs-TRE) spectroscopy and DFT calculations were undertaken to decipher the TADF properties. Solution-processed OLEDs fabricated with the W-TADF emitter demonstrated external quantum efficiency (EQE) and luminance of up to 15.6 % and 16890 cd m-2 , respectively.

Highly Luminescent Pincer Gold(III) Aryl Emitters: Thermally Activated Delayed Fluorescence and Solution-Processed OLEDs.

Herein are described the synthesis, photophysical properties and applications of a series of luminescent cyclometalated AuIII complexes having an auxiliary aryl ligand. These complexes show photoluminescence with emission quantum yields of up to 0.79 in solution and 0.84 in thin films (4 wt % in PMMA) at room temperature, both of which are the highest reported values among AuIII complexes. Thermally activated delayed fluorescence (TADF) is the emission origin for some of these complexes. Solution-processed OLEDs made with these complexes showed sky-blue to green electroluminescence with external quantum efficiencies (EQEs) of up to 23.8 %, current efficiencies of up to 70.4 cd A-1 , and roll-off of down to 1 %, highlighting the bright prospect of AuIII -TADF emitters in OLEDs.

Strongly Luminescent Cyclometalated Gold(III) Complexes Supported by Bidentate Ligands Displaying Intermolecular Interactions and Tunable Emission Energy.

A series of charge-neutral AuIII complexes, which comprise a dicarbanionic C-deprotonated biphenyl ligand and bidentate ancillary ligands ([Au(C^C)(L^X)]; L^X=ß-diketonate and relatives (O^O), quinolinolate and relatives (N^O), and diphosphino (P^P) ligands), were prepared. All the complexes are emissive in degassed CH2 Cl2 solutions and in thin-film samples with Φem up to 18 and 35 %, respectively, except for 5 and 6, which bear (N^O)-type ancillary ligands. Variation of the electronic characteristics of the ß-diketonate ancillary ligand was demonstrated to be a viable route for tuning the emission color from blue-green (peak λem at ca. 466 nm for 1 and 2; 501 nm for 4 a and 4 b) to orange (peak λem at 585 nm for 3), in contrast to the common observations that the ancillary ligand has a negligible effect on the excited-state energy of the AuIII complexes reported in the literature. DFT/time-dependent (TD) DFT calculations revealed that the energies of the 3 ππ*(C^C) and the 3 ILCT(O^O) excited states (ILCT=intraligand charge transfer) switch in order on going from O^O=acetylacetonate (acac) to aryl-substituted ß-diketonate ligands. Solution-processed and vacuum-deposited organic light-emitting diode (OLED) devices of selected complexes were prepared. The vacuum-deposited OLED fabricated with 2 displays a sky-blue emission with a maximum external quantum efficiency (EQE) of 6.71 % and CIE coordinates of (0.22, 0.40). The crystal structures of 7 and 9 reveal short intermolecular AuIII ⋅⋅⋅AuIII contacts, with intermetal distances of 3.408 and 3.453 Å, respectively. DFT/TDDFT calculations were performed on 7 and 9 to account for the noncovalent interactions. Solid samples of 1, 3, and 9 exhibit excimeric emission at room temperature, which is rarely reported in AuIII complexes.

The interplay between fluorescence and phosphorescence with luminescent gold(i) and gold(iii) complexes bearing heterocyclic arylacetylide ligands.

The photophysical properties of a series of gold(i) [LAu(C[triple bond, length as m-dash]CR)] (L = PCy3 (1a-4a), RNC (5a), NHC (6a)) and gold(iii) complexes [Au(C^N^C)(C[triple bond, length as m-dash]CR)] (1b-4b) bearing heterocyclic arylacetylide ligands with narrow band-gap are compared. The luminescence of both series are derived from an intraligand transition localized on the arylacetylide ligand (ππ*(C[triple bond, length as m-dash]CR)) but 1a-3a displayed prompt fluorescence (τ PF = 2.7-12.0 ns) while 1b-3b showed mainly phosphorescence (τ Ph = 104-205 µs). The experimentally determined intersystem crossing (ISC) rate constants (k ISC) are on the order of 106 to 108 s-1 for the gold(i) series (1a-3a) but 1010 to 1011 s-1 for the gold(iii) analogues (1b-3b). DFT/TDDFT calculations have been performed to help understand the difference in the k ISC between the two series of complexes. Owing to the different oxidation states of the gold ion, the Au(i) complexes have linear coordination geometry while the Au(iii) complexes are square planar. It was found from DFT/TDDFT calculations that due to this difference in coordination geometries, the energy gap between the singlet and triplet excited states (ΔE ST) with effective spin-orbit coupling (SOC) for Au(i) systems is much larger than that for the Au(iii) counterparts, thus resulting in the poor ISC efficiency for the former. Time-resolved spectroscopies revealed a minor contribution (<2.9%) of a long-lived delayed fluorescence (DF) (τ DF = 4.6-12.5 µs) to the total fluorescence in 1a-3a. Attempts have been made to elucidate the mechanism for the origins of the DF: the dependence of the DF intensity with the power of excitation light reveals that triplet-triplet annihilation (TTA) is the most probable mechanism for the DF of 1a while germinate electron-hole pair (GP) recombination accounts for the DF of 2a in 77 K glassy solution (MeOH/EtOH = 4 : 1). Both 4a and 4b contain a BODIPY moiety at the acetylide ligand and display only 1IL(ππ*) fluorescence with negligible phosphorescence being observed. Computational analyses attributed this observation to the lack of low-lying triplet excited states that could have effective SOC with the S1 excited state.

Luminescent Cyclometalated Gold(III) Alkyl Complexes: Photophysical and Photochemical Properties.

A series of luminescent cyclometalated gold(III) complexes having alkyls as auxiliary ligands has been prepared. The alkyl ligand was found to effectively increase the emission quantum yields and lifetimes of luminescent cyclometalated gold(III) complexes by circumventing the population of LLCT excited states that are found in complexes supported by arylacetylide ligands. These gold(III) alkyl complexes exhibit emission quantum yields and lifetimes of up to 0.40 and 180 µs, respectively, in solution at room temperature. The triplet emission color of these complexes is tunable from yellow to sky blue by modifying the cyclometalating ligand.

Luminescent Tungsten(VI) Complexes: Photophysics and Applicability to Organic Light-Emitting Diodes and Photocatalysis.

The synthesis, excited-state dynamics, and applications of two series of air-stable luminescent tungsten(VI) complexes are described. These tungsten(VI) complexes show phosphorescence in the solid state and in solutions with emission quantum yields up to 22 % in thin film (5 % in mCP) at room temperature. Complex 2 c, containing a 5,7-diphenyl-8-hydroxyquinolinate ligand, displays prompt fluorescence (blue-green) and phosphorescence (red) of comparable intensity, which could be used for ratiometric luminescent sensing. Solution-processed organic light-emitting diodes (OLEDs) based on 1 d showed a stable yellow emission with an external quantum efficiency (EQE) and luminance up to 4.79 % and 1400 cd m-2 respectively. These tungsten(VI) complexes were also applied in light-induced aerobic oxidation reactions.

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.

Luminescent pincer platinum(II) complexes with emission quantum yields up to almost unity: photophysics, photoreductive C-C bond formation, and materials applications.

Luminescent pincer-type Pt(II)  complexes supported by C-deprotonated π-extended tridentate RC^N^NR' ligands and pentafluorophenylacetylide ligands show emission quantum yields up to almost unity. Femtosecond time-resolved fluorescence measurements and time-dependent DFT calculations together reveal the dependence of excited-state structural distortions of [Pt(RC^N^NR')(CC-C6 F5 )] on the positional isomers of the tridentate ligand. Pt complexes [Pt(R-C^N^NR')(CC-Ar)] are efficient photocatalysts for visible-light-induced reductive CC bond formation. The [Pt(R-C^N^NR')(CC-C6 F5 )] complexes perform strongly as phosphorescent dopants for green- and red-emitting organic light-emitting diodes (OLEDs) with external quantum efficiency values over 22.1 %. These complexes are also applied in two-photon cellular imaging when incorporated into mesoporous silica nanoparticles (MSNs).

A theoretical investigation into the luminescent properties of d8-transition-metal complexes with tetradentate Schiff base ligands.

A theoretical investigation on the luminescence efficiency of a series of d(8) transition-metal Schiff base complexes was undertaken. The aim was to understand the different photophysics of [M-salen](n) complexes (salen = N,N'-bis(salicylidene)ethylenediamine; M = Pt, Pd (n = 0); Au (n = +1)) in acetonitrile solutions at room temperature: [Pt-salen] is phosphorescent and [Au-salen](+) is fluorescent, but [Pd-salen] is nonemissive. Based on the calculation results, it was proposed that incorporation of electron-withdrawing groups at the 4-position of the Schiff base ligand should widen the (3)MLCT-(3)MC gap (MLCT = metal-to-ligand charge transfer and MC = metal centered, that is, the dd excited state); thus permitting phosphorescence of the corresponding Pd(II) Schiff base complex. Although it is experimentally proven that [Pd-salph-4E] (salph = N,N'-bis(salicylidene)-1,2-phenylenediamine; 4E means an electron-withdrawing substituent at the 4-position of the salicylidene) displays triplet emission, its quantum yield is low at room temperature. The corresponding Pt(II) Schiff base complex, [Pt-salph-4E], is also much less emissive than the unsubstituted analogue, [Pt-salph]. Thus, a detailed theoretical analysis of how the substituent and central metal affected the photophysics of [M-salph-X] (X is a substituent on the salph ligand, M = Pt or Pd) was performed. Temperature effects were also investigated. The simple energy gap law underestimated the nonradiative decay rates and was insufficient to account for the temperature dependence of the nonradiative decay rates of the complexes studied herein. On the other hand, the present analysis demonstrates that inclusions of low-frequency modes and the associated frequency shifts are decisive in providing better quantitative estimates of the nonradiative decay rates and the experimentally observed temperature effects. Moreover, spin-orbit coupling, which is often considered only in the context of radiative decay rate, has a significant role in determining the nonradiative rate as well.