Mechano-Stimulus and Environment-Dependent Circularly Polarized TADF in Chiral Copper(I) Complexes and Their Application in OLEDs

Achieving Circularly Polarized Phosphorescence through Noncovalent Clipping of Metallotweezers

Circularly polarized phosphorescent materials, based on host-guest complexation, have received significant attention due to their outstanding emission performance in solutions. Recent studies have primarily focused on macrocyclic host-guest complexes. To broaden the scope of this research, there is a keen pursuit of developing novel chiral phosphorescent host-guest systems. Metallotweezers with square-planar d8 transition metal complexes emerge as promising candidates for achieving this objective. Specifically, metallotweezers, comprising platinum(II) terpyridine and gold(III) diphenylpyridine pincers on a diphenylpyridine scaffold, have been designed and synthesized. Due to the preorganization effect rendered by the diphenylpyridine scaffold, the resulting metallotweezers are capable of complexing with each other and forming quadruple stacking structures. The phosphorescent emission is enhanced owing to the synergistic rigidifying and shielding effects. Meanwhile, the steric effect of chiral (1R) pinene units on the platinum(II) terpyridine pincers results in a stereospecific twist for the quadruple stacking structures. Thus, the chirality transfers from the molecular to the supramolecular level. By a combination of phosphorescent enhancement and supramolecular chirality for the clipping complex, circularly polarized phosphorescent emission is achieved. Overall, noncovalent clipping of metallotweezers exemplified in the current study presents a novel and effective approach toward solution-processable circularly polarized phosphorescent materials.

Activation of TADF in Photon Upconverting Crystals of Dinuclear Cu(I)-Iodide Complexes by Ligand Engineering

This work reports that ligand engineering can modulate the triplet harvesting mechanism in iodide-bridged rhombic Cu2I2 complexes. Complex-1, with a smaller Cu-Cu distance, exhibits phosphorescence from 3(M+X)LCT and 3CC states with 66% quantum yield, whereas an increased Cu-Cu distance in complex-2 results in a switch of the emission from phosphorescence to TADF, which occurs via 1/3(M+X)LCT states with 83% quantum yield. The TADF property of complex-2 has been utilized for the fabrication of a pc-LED emitting efficient warm white light. Moreover, the high charge-transfer nature of these complexes leads to the emergence of third-harmonic generation (THG). Interestingly, complex-1 exhibits efficient third-harmonic generation with a χ(3) value of 1.15 × 10-18 m2 V-2 and LIDT value of 14.73 GW/cm2. This work aims to provide a structure-property relationship to achieve effective harvestation of triplet excitons in iodide-bridged rhombic Cu2I2 complexes and their effective utilization in OLED device fabrication and nonlinear photon upconversion processes.

Two-Coordinate Thermally Activated Delayed Fluorescence Coinage Metal Complexes: Molecular Design, Photophysical Characters, and Device Application

Since the emergence of the first green light emission from a fluorescent thin-film organic light emitting diode (OLED) in the mid-1980s, a global consumer market for OLED displays has flourished over the past few decades. This growth can primarily be attributed to the development of noble metal phosphorescent emitters that facilitated remarkable gains in electrical conversion efficiency, a broadened color gamut, and vibrant image quality for OLED displays. Despite these achievements, the limited abundance of noble metals in the Earth's crust has spurred ongoing efforts to discover cost-effective electroluminescent materials. One particularly promising avenue is the exploration of thermally activated delayed fluorescence (TADF), a mechanism with the potential to fully harness excitons in OLEDs. Recently, investigations have unveiled TADF in a series of two-coordinate coinage metal (Cu, Ag, and Au) complexes. These organometallic TADF materials exhibit distinctive behavior in comparison to their organic counterparts. They offer benefits such as tunable emissive colors, short TADF emission lifetimes, high luminescent quantum yields, and reasonable stability. Impressively, both vacuum-deposited and solution-processed OLEDs incorporating these materials have achieved outstanding performance. This review encompasses various facets on two-coordinate TADF coinage metal complexes, including molecular design, photophysical characterizations, elucidation of structure-property relationships, and OLED applications.

Bridging of Cove Regions: A Strategy for Realizing Persistently Chiral Double Heterohelicenes with Attractive Luminescent Properties

The racemization of chiral organic compounds is a common chemical phenomenon. However, it often poses configurational-stability issues to the application of this class of compounds. Achieving chiral organic compounds without the risk of racemization is fascinating, but it is challenging due to a lack of strategies. Here, we reveal the cove-regions bridging strategy for achieving persistently chiral multi-helicenes (incapable of racemization), based on the synthesized proof-of-concept double hetero[4]helicenes featuring macrocycle structures with a small 3D cavity. Additionally, we demonstrate that the strategy is also effective in tuning the electronic structures of multi-helicenes, resulting in a conversion from luminescence silence into thermally activated delayed fluorescence (TADF) for the present system. Furthermore, red circularly polarized TADF based on small double [4]helicene systems is achieved for the first time using this strategy. The disclosed cove-regions bridging strategy provides an opportunity to modulate the electronic structures and luminescent properties of multi-helicenes without concern for racemization, thus significantly enhancing the structural and property diversity of multi-helicenes for various applications.

Integration of fine-tuned chiral donor with hybrid long/short-range charge-transfer for high-performance circularly polarized electroluminescence

The synergistic integration of a fine-tuned chiral donor with a hybrid long/short-range charge-transfer mechanism offers an accessible pathway to construct highly efficient circularly polarized emitters. Consequently, a notable dissymmetry factor of 1.6 × 10-3, concomitantly with a record-setting maximum external quantum efficiency of 37.4%, is synchronously realized within a single embodiment.

Structural Control of Highly Efficient Thermally Activated Delayed Fluorescence in Carbene Zinc(II) Dithiolates

Luminescent metal complexes based on earth abundant elements are a valuable target to substitute 4d/5d transition metal complexes as triplet emitters in advanced photonic applications. Whereas CuI complexes have been thoroughly investigated in the last two decades for this purpose, no structure-property-relationships for efficient luminescence involving triplet excited states from ZnII complexes are established. Herein, we report on the design of monomeric carbene zinc(II) dithiolates (CZT) featuring a donor-acceptor-motif that leads to highly efficient thermally activated delayed fluorescence (TADF) with for ZnII compounds unprecedented radiative rate constants kTADF =1.2×106  s-1 at 297 K. Our high-level DFT/MRCI calculations revealed that the relative orientation of the ligands involved in the ligand-to-ligand charge transfer (1/3 LLCT) states is paramount to control the TADF process. Specifically, a dihedral angle of 36-40° leads to very efficient reverse intersystem-crossing (rISC) on the order of 109  s-1 due to spin-orbit coupling (SOC) mediated by the sulfur atoms in combination with a small ΔES1-T1 of ca. 56 meV.

From Mono- to Polynuclear 2-(Diphenylphosphino)pyridine-Based Cu(I) and Ag(I) Complexes: Synthesis, Structural Characterization, and DFT Calculations

A series of monometallic Ag(I) and Cu(I) halide complexes bearing 2-(diphenylphosphino)pyridine (PyrPhos, L) as a ligand were synthesized and spectroscopically characterized. The structure of most of the derivatives was unambiguously established by X-ray diffraction analysis, revealing the formation of mono-, di-, and tetranuclear complexes having general formulas MXL3 (M = Cu, X = Cl, Br; M = Ag, X = Cl, Br, I), Ag2X2L3 (X = Cl, Br), and Ag4X4L4 (X = Cl, Br, I). The Ag(I) species were compared to the corresponding Cu(I) analogues from a structural point of view. The formation of Cu(I)/Ag(I) heterobimetallic complexes MM'X2L3 (M/M' = Cu, Ag; X = Cl, Br, I) was also investigated. The X-ray structure of the bromo-derivatives revealed the formation of two possible MM'Br2L3 complexes with Cu/Ag ratios, respectively, of 7:1 and 1:7. The ratio between Cu and Ag was studied by scanning electron microscopy-energy-dispersive X-ray analysis (SEM-EDX) measurements. The structure of the binuclear homo- and heterometallic derivatives was investigated using density functional theory (DFT) calculations, revealing the tendency of the PyrPhos ligands not to maintain the bridging motif in the presence of Ag(I) as the metal center.

Synthesis, Structural Characterization, and Phosphorescence Properties of Trigonal Zn(II) Carbene Complexes

The sterically demanding N-heterocyclic carbene ITr (N,N'-bis(triphenylmethyl)imidazolylidene) was employed for the preparation of novel trigonal zinc(II) complexes of the type [ZnX2(ITr)] [X = Cl (1), Br (2), and I (3)], for which the low coordination mode was confirmed in both solution and solid state. Because of the atypical coordination geometry, the reactivity of 1-3 was studied in detail using partial or exhaustive halide exchange and halide abstraction reactions to access [ZnLCl(ITr)] [L = carbazolate (4), 3,6-di-tert-butyl-carbazolate (5), phenoxazine (6), and phenothiazine (7)], [Zn(bdt)(ITr)] (bdt = benzene-1,2-dithiolate) (8), and cationic [Zn(μ2-X)(ITr)]2[B(C6F5)4]2 [X = Cl (9), Br (10), and I (11)], all of which were isolated and structurally characterized. Importantly, for all complexes 4-11, the trigonal coordination environment of the ZnII ion is maintained, demonstrating a highly stabilizing effect due to the steric demand of the ITr ligand, which protects the metal center from further ligand association. In addition, complexes 1-3 and 8-11 show long-lived luminescence from triplet excited states in the solid state at room temperature, according to our photophysical studies. Our quantum chemical density functional theory/multireference configuration interaction (DFT/MRCI) calculations reveal that the phosphorescence of 8 originates from a locally excited triplet state on the bdt ligand. They further suggest that the phenyl substituents of ITr are photochemically not innocent but can coordinate to the electron-deficient metal center of this trigonal complex in the excited state.

Deep-Blue Narrowband Hetero[6]helicenes Showing Circularly Polarized Thermally Activated Delayed Fluorescence Toward High-Performance OLEDs

Helicenes exhibit substantial potential as circularly polarized luminescence (CPL) active molecules. However, their application in circularly polarized organic light-emitting diodes (CP-OLEDs) is typically hindered by the challenge of integrating both high color purity and efficient triplet-harvesting capability, particularly in the blue spectral region. Herein, a series of hetero[6]helicene-based emitters that is strategically engineered through the helical extension of a deep-blue double-boron-based multiple resonance thermally activated delayed fluorescence (MR-TADF) motif, is introduced. Importantly, the helical extension does not cause apparent structural deformation or perturb frontier molecular orbitals; thus, preserving the deep-blue emission and MR-TADF characteristics of the parent molecule. This approach also leads to reduced reorganization energy, resulting in emitters with narrower linewidth and higher photoluminescence quantum yield. Further, the helical motif enhances the racemization barrier and leads to improved CPL performance with luminescence dissymmetry factor values up to 1.5 × 10-3 . Exploiting these merits, devices incorporating the chiral dopants demonstrate deep-blue emission within the Broadcast Service Television 2020 color-gamut range, record external quantum efficiencies (EQEs) up to 29.3%, and have distinctive circularly polarized electroluminescence (CPEL) signals. Overall, the authors' findings underscore the helical extension as a promising strategy for designing narrowband chiroptical materials and advancing high-definition displays.

Towards Fast Circularly Polarized Luminescence in 2-Coordinate Chiral Mechanochromic Copper(I) Carbene Complexes

A series of chiral mechanochromic copper(I) cAAC (cAAC=cyclic (alkyl)(amino)carbene) complexes with a variety of amide ligands have been studied with regard to their photophysical and chiroptical properties to elucidate structure-property relationships for the design of efficient triplet exciton emitters exhibiting circularly polarized luminescence. Depending on the environment, which determines the excited state energies, either thermally activated delayed fluorescence (TADF) from 1/3 LLCT states or phosphorescence from 3 LLCT/LC states occurs. However, neither chiral moieties at the carbene nor at the carbazolate ligands provide detectable luminescence dissymmetries glum . An exception is [Cu(phenoxazinyl)(cAAC)], showing orange to deep red TADF with λmax =601-715 nm in solution, powders and in PMMA. In this case, the amide ligand can undergo distortions in the excited state. This design motif leads to the first linear, non-aggregated CPL-active copper(I) complex with glum of -3.4 ⋅ 10-3 combined with a high radiative rate constant of 6.7 ⋅ 105  s-1 .

Thermally Activated Delayed Fluorescence Coinage Metal Cluster Scintillator

X-ray scintillators are widely used in medical imaging, industrial flaw detection, security inspection, and space exploration. However, traditional commercial scintillators are usually associated with a high use cost because of their substantial toxicity and easy deliquescence. In this work, an atomically precise Au-Cu cluster scintillator (1) with a thermally activated delayed fluorescence (TADF) property was facilely synthesized, which is environmentally friendly and highly stable to water and oxygen. The TADF property of 1 endows it with an ultrahigh exciton utilization rate. Combined with the effective absorption of X-ray caused by the heavy-atom effect and a limited nonradiative transition caused by close packing in the crystal state, 1 exhibits an excellent radioluminescence property. Moreover, 1 has good processability for fabricating a large, flexible thin-film device (10 cm × 10 cm) for high-resolution X-ray imaging, which can reach 40 μm (12.5 LP mm^-1). The properties mentioned earlier make the coinage metal cluster promising for use as a substitute for traditional commercial scintillators.