Trigonal Copper(I) Complexes with Cyclic (Alkyl)(amino)carbene Ligands for Single-Photon Near-IR Triplet Emission

Organo Chalcogenone-Triggered Luminescent Copper(I) Clusters for Light Emitting Applications

A novel organo sulfur and selenium-controlled emission behavior in discrete copper(I) clusters has been demonstrated for the first time. The pentanuclear [Cu5Br5(L1)2] (1), trinuclear [Cu3Br3(L2)2] (2), dinuclear [Cu2I2(L1)2] (3), and tetranuclear [Cu4I4(L2)2CH3CN] (4) copper(I) discrete clusters have been synthesized from the reaction between L1 [L1 = 1-isopropyl-3-(pyridin-2-yl)-imidazol-2-thione] or L2 [L2 = 1-isopropyl-3-(pyridin-2-yl)-imidazol-2-selone] chelating ligands and corresponding copper(I) halide salts. These new clusters have been characterized by FT-IR, UV-visible, thermogravimetric analysis, and fluorescence spectroscopy techniques. Single-crystal X-ray diffraction studies reveal that 1-4 consists of abundant d10-d10 interactions. The structural and bonding features of clusters have been investigated using density functional theory calculations. Notably, the L2-ligated 2 and 4 are poorly emissive, while L1-ligated 1 and 3 showed strong emission in the orange and green regions, respectively. The time-dependent density functional theory natural transition orbital calculations of 1 and 3 reveal the nature of the transitions contributed by 3MLCT/3LLCT/3ILCT. Photoluminescence quantum yields of 1 and 3 are 19 and 11%, with average lifetimes of 21.55 and 6.57 μs, respectively. 1 and 3 were coated on prototype LED bulbs for light-emitting performance.

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.

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.

Binuclear Copper(I) Complexes for Near-Infrared Light-Emitting Electrochemical Cells

Two binuclear heteroleptic CuI complexes, namely Cu-NIR1 and Cu-NIR2, bearing rigid chelating diphosphines and π-conjugated 2,5-di(pyridin-2-yl)thiazolo[5,4-d]thiazole as the bis-bidentate ligand are presented. The proposed dinuclearization strategy yields a large bathochromic shift of the emission when compared to the mononuclear counterparts (M1-M2) and enables shifting luminescence into the near-infrared (NIR) region in both solution and solid state, showing emission maximum at ca. 750 and 712 nm, respectively. The radiative process is assigned to an excited state with triplet metal-to-ligand charge transfer (3 MLCT) character as demonstrated by in-depth photophysical and computational investigation. Noteworthy, X-ray analysis of the binuclear complexes unravels two interligand π-π-stacking interactions yielding a doubly locked structure that disfavours flattening of the tetrahedral coordination around the CuI centre in the excited state and maintain enhanced NIR luminescence. No such interaction is present in M1-M2. These findings prompt the successful use of Cu-NIR1 and Cu-NIR2 in NIR light-emitting electrochemical cells (LECs), which display electroluminescence maximum up to 756 nm and peak external quantum efficiency (EQE) of 0.43 %. Their suitability for the fabrication of white-emitting LECs is also demonstrated. To the best of our knowledge, these are the first examples of NIR electroluminescent devices based on earth-abundant CuI emitters.

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 .

High-temperature negative thermal quenching phosphors from molecular-based materials

High-temperature negative thermal quenching (NTQ) phosphors are crucial to high-performance light-emitting devices. Herein, we report the high-temperature NTQ effect in deep-red to near-infrared (NIR) emitting copper iodide cluster-based coordination polymers as unconventional phosphors, whose NTQ operating temperature can reach as high as 500 K, the highest temperature reached by NTQ molecular-based materials.

An Air- and Moisture-stable Zinc(II) Carbene Dithiolate Dimer Showing Fast Thermally Activated Delayed Fluorescence and Dexter Energy Transfer Catalysis

A dimeric Zn^II carbene complex featuring bridging and chelating benzene-1,2-dithiolate ligands is highly stable towards air and water. The donor-Zn-acceptor structure leads to visible light emission in the solid state, solution and polymer matrices with λ_max between 577-657 nm and, for zinc(II) complexes, unusually high radiative rate constants for triplet exciton decay of up to k_r =1.5×10⁵  s^-1 at room temperature. Variable temperature and DFT/MRCI studies show that a small energy gap between the ^1/3 LL/LMCT states of only 79 meV is responsible for efficient thermally activated delayed fluorescence (TADF). Time-resolved luminescence and transient absorption studies confirm the occurrence of long-lived, dominantly ligand-to-ligand charge transfer excited states in solution, allowing for application in Dexter energy transfer photocatalysis.

Photoactive Copper Complexes: Properties and Applications

Photocatalyzed and photosensitized chemical processes have seen growing interest recently and have become among the most active areas of chemical research, notably due to their applications in fields such as medicine, chemical synthesis, material science or environmental chemistry. Among all homogeneous catalytic systems reported to date, photoactive copper(I) complexes have been shown to be especially attractive, not only as alternative to noble metal complexes, and have been extensively studied and utilized recently. They are at the core of this review article which is divided into two main sections. The first one focuses on an exhaustive and comprehensive overview of the structural, photophysical and electrochemical properties of mononuclear copper(I) complexes, typical examples highlighting the most critical structural parameters and their impact on the properties being presented to enlighten future design of photoactive copper(I) complexes. The second section is devoted to their main areas of application (photoredox catalysis of organic reactions and polymerization, hydrogen production, photoreduction of carbon dioxide and dye-sensitized solar cells), illustrating their progression from early systems to the current state-of-the-art and showcasing how some limitations of photoactive copper(I) complexes can be overcome with their high versatility.