A Cyclic Alkyl(amino)carbene as Two-Atom π-Chromophore Leading to the First Phosphorescent Linear CuI Complexes

A Strongly Ambiphilic Ferrocene-Based Cyclic (Alkyl)(amino)carbene - Specific Decomposition to an Enamine by a 1,2-Phenyl Shift

The recently described crystalline cyclic (alkyl)(amino)carbene with a 1,1'-ferrocenylene (fc) backbone fc(CPh2-C-NMes) (A, Mes=mesityl) is highly reactive due to its particularly pronounced ambiphilicity and is thermally not stable in solution due to an intramolecular insertion of the divalent carbon atom into a methyl C-H bond of the Mes substituent. The closely related congener fc(CPh2-C-N-p-C6H4-tBu) (1) cannot undergo such an insertion reaction. Nevertheless, 1 is too short-lived for isolation due to a rapid 1,2-shift of a phenyl group, furnishing the isomeric cyclic enamine fc[C(Ph)=C(Ph)-N-p-C6H4-tBu] (1') in a specific decomposition process unprecedented for CAACs. Trapping of 1 was possible with carbon monoxide, elemental selenium and with [CuBr(SMe2)], respectively affording the aminoketene 1=C=O, the selenoamide 1=Se and the homoleptic CuI complex [Cu(1)2][CuBr2]. 1 is an even stronger ambiphile than A according to NMR spectroscopic data. Similar to A, 1 does not react with H2, because the experimentally observed intramolecular process is kinetically more favourable according to DFT results.

Exploring the reductive CO2 fixation with amines and hydrosilanes using readily available Cu(II) NHC-phenolate catalyst precursors

N-Methylation of amines is of great interest in the synthesis of pharmaceuticals and valuable compounds, and the possibility to perform this reaction with an inexpensive and non-toxic substrate like CO2 and its derivatives is quite appealing. Herein, the synthesis of four novel homoleptic Cu(II) complexes with hybrid NHC-phenolate (NHC = N-Heterocyclic Carbene) ligands is reported, and their use in the catalytic N-methylation of amines with CO2 in the presence of hydrosilanes is explored. Both bidentate or tetradentate ligands can be used in the preparation of the complexes provided that the structural requirement that the two NHC and the two phenolate donors in the metal coordination sphere are mutually in trans is fulfilled. A new reaction protocol to perform the N-methylation of secondary aromatic amines and dibenzylamine in high yield under mild reaction conditions is developed, using the ionic liquid [BMMIM][NTf2] (1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide) as solvent and the catalyst precursor [Cu(L2)2]. Reactivity studies indicate that the reaction follows two different pathways with different hydrosilanes, and that the starting Cu(II) complexes are reduced under the catalytic conditions.

A [2.2]Isoindolinophanyl-Based Carbene (iPC) Ligand: Synthesis, Electronic and Photophysical Properties, and Application in Photocatalysis

Cyclic amino(alkyl) and cyclic amino(aryl) carbenes (cAACs/cAArCs) have been established as very useful ligands for catalytic and photonic applications of transition metal complexes. Herein, we describe the synthesis of a structurally related sterically demanding, electrophilic [2.2]isoindolinophanyl-based carbene (iPC) that bears a [2.2]paracyclophane moiety. The latter leads to more delocalized frontier orbitals and intense green fluorescence of (HiPC)OTf (2) from an intra-ligand charge transfer (1ILCT) state in the solid state. Base-promoted synthesis of the free carbene led to an unusual ring expansion and subsequent dimerization reaction, but the beneficial ligand properties can be exploited by trapping in situ at a metal center. The iPC ligand is a very potent π-chromophore, which participates in low energy metal-to-ligand (ML)CT transitions in [RhCl(CO)2(iPC)] (4) and IL-"through-space"-CT transitions in [Au(iPC)2]OTf (5). The steric demand of the iPC leads to high stability of 5 against air, moisture, or solvent attack, and ultralong-lived green phosphorescence with a lifetime of 185 μs is observed in solution. The beneficial photophysical and electronic properties of the iPC ligand, including a large accessible π surface area, were exploited by employing highly efficient energy transfer (EnT) photocatalysis in a [2+2] styrene cycloaddition reaction using 5, which outperformed other established photocatalysts in comparison.

Synthesis and Application of Robust Spiro [Fluorene-9] CAAC Ruthenium Alkylidene Complexes for the "One-Pot" Conversion of Allyl Acetate to Butane-1,4-diol

A series of a novel CAAC ligands featuring a spiro-fluorene group have been synthesized and complexed with ruthenium alkylidenes, yielding the corresponding Hoveyda-type derivatives as a new family of olefin metathesis catalysts. The novel complexes have been characterized by XRD, HRMS and NMR measurements. The synthetised complexes were tested in catalysis and showed good activity in olefin metathesis, as demonstrated on diethyl diallylmalonate and allyl acetate substrates. The unique backbone in the ligand with the large, yet inflexible condensed system renders interesting properties to the catalyst, exemplified by the good catalytic performance and improved Z-selectivity. In addition, the complex can also serve as a hydrogenation catalyst in a consecutive (one-pot) reaction. The latter reaction can convert allyl acetate to butane-1,4-diol, a valuable chemical intermediate for biodegradable polybutylene succinate (PBS).

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.

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.

A Rising Star: Luminescent Carbene-Metal-Amide Complexes

Coinage metal (gold, silver, and copper) complexes are attractive candidates to substitute the widely studied noble metal complexes, such as, iridium(III) and platinum(II), as luminescent materials in organic light-emitting diodes (OLEDs). However, the development of coinage metal complexes exhibiting high emission quantum yields and short exciton lifetimes is still a formidable challenge. In the past few years, coinage metal complexes featuring a carbene-metal-amide (CMA) motif have emerged as a new class of luminescent materials in OLEDs. Thanks to the coinage metal-bridged linear geometry, coplanar conformation, and the formation of excited states with dominant ligand-to-ligand charge transfer character and reduced metal d-orbital participation, most CMA complexes have high radiative rates via thermally activated delayed fluorescence. Currently, the family of CMA complexes have rapidly evolved and remarkable progresses in CMA-based OLEDs have been made. Here, a Concept article on CMA complexes is presented, with a focus on molecular design principles, the correlation between molecular structure/conformation and optoelectronic properties, as well as OLED performance. The future prospects of CMA complexes are also discussed.

Highly phosphorescent carbene-metal-carboranyl complexes of copper(I) and gold(I)

New phosphorescent "carbene-metal-carboranyl" (CMC) Cu(I) and Au(I) complexes based on the diamidocarbene (DAC) ligand show up to 68% photoluminescence quantum yield and microsecond range lifetimes. CMC organic light emitting diodes (OLEDs) emit sky-blue and warm white electroluminescence.

The Equilibrium Molecular Structure of Cyclic (Alkyl)(Amino) Carbene Copper(I) Chloride via Gas-Phase Electron Diffraction and Quantum Chemical Calculations

Copper-centered carbene-metal-halides (CMHs) with cyclic (alkyl)(amino) carbenes (CAACs) are bright phosphorescent emitters and key precursors in the synthesis of the highly promising class of the materials carbene-metal-amides (CMAs) operating via thermally activated delayed fluorescence (TADF). Aiming to reveal the molecular geometry for CMH phosphors in the absence of the intermolecular contacts, we report here the equilibrium molecular structure of the (CAAC)Cu(I)Cl (1) molecule in the gas-phase. We demonstrate that linear geometry around a copper atom shows no distortions in the ground state. The structure of complex 1 has been determined using the electron diffraction method, supported by quantum chemical calculations with RI-MP2/def2-QZVPP level of theory and compared with the crystal structure determined by X-ray diffraction analysis. Mean vibrational amplitudes, uij,h1, and anharmonic vibrational corrections (rij,e • rij,a) were calculated for experimental temperature T = 20 °C, using quadratic and cubic force constants, respectively. The quantum theory of atoms in molecules (QTAIM) and natural bond order (NBO) analysis of wave function at MN15/def2TZVP level of theory revealed two Cu…H, three H…H, and one three-center H…H…H bond paths with bond critical points. NBO analysis also revealed three-center, four-electron hyperbonds, (3c4e), [π(N-C) nπ(Cu) ↔ nπ(N) π(N-Cu)], or [N-C: Cu ↔ N: C-Cu] and nπ(Cu) → π(C-N)* hyperconjugation, that is the delocalization of the lone electron pair of Cu atom into the antibonding orbital of C-N bond.

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 .

N-Heterocyclic Carbene Copper (I) Complexes Incorporating Pyrene Chromophore: Synthesis, Crystal Structure, and Luminescent Properties

Luminescent N-heterocyclic carbene chloride copper (I) complexes incorporating pyrene chromophore (1-Pyrenyl-NHC-R)-Cu-Cl, (3, 4) have been prepared and fully characterized. Two complexes were prepared with R = methyl (3) and R = naphthyl groups (4) at the nitrogen center of the carbene unit to tune their electronic properties. The molecular structures of 3 and 4 have been elucidated by X-ray diffraction and confirm the formation of the target compounds. Preliminary results reveal that all compounds including the imidazole-pyrenyl ligand 1 are emissive in the blue region at room temperature in solution and in solid-state. All complexes display quantum yields comparable or higher when compared to the parent pyrene molecule. Interestingly replacement of the methyl by naphthyl group increases the quantum yield by almost two-folds. These compounds might show promise for applications as optical displays.

Mechanochemically interlocked cubane copper complex interface for WOLED

In the rapid development of organic light-emitting diodes (OLEDs), phosphorescent transition metal complexes have played a crucial role as the most promising candidates for next generation display and lighting applications. However, most devices are fabricated using iridium and platinum-based complexes which are expensive and available in very limited quantities, whereas using relatively abundant organometallic complexes for fabrication results mostly in inefficient performance results. To overcome these issues, we have synthesized tetra copper iodide with tetra triphenyl cage like structure (denoted as CIPh) as an emerging class of luminescent material by mechanochemical grinding followed by thermal treatment for application in white OLED. The CIPh complex exhibits considerable quantum yield and a millisecond decay lifetime. Phosphorescent OLEDs were fabricated using CIPh complex as emitter shows a remarkable performance with external quantum efficiency and current efficiency of 5.28 % and 22.76 cd/A, with a high brightness of 4200 cd m^-2, respectively. White OLEDs were also fabricated with a fluorescent blue and phosphorescent red emitted with (CIPh) as green emitter and achieved an impressive CRI of 82 with an EQE of over 3 %. This is the first ever attempt at fabricating WOLEDs using organocopper complex.

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

Molecular emitters that combine circularly polarized luminescence (CPL) and high radiative rate constants of the triplet exciton decay are highly attractive for electroluminescent devices (OLEDs) or next-generation photonic applications, such as spintronics, quantum computing, cryptography, or sensors. However, the design of such emitters is a major challenge because the criteria for enhancing these two properties are mutually exclusive. In this contribution, we show that enantiomerically pure {Cu(Cbz^R)[(S/R)-BINAP]} [R = H (1), 3,6-tBu (2)] are efficient thermally activated delayed fluorescence (TADF) emitters with high radiative rate constants of k_TADF up to 3.1 × 10⁵ s^-1 from ^1/3LLCT states according to our temperature-dependent time-resolved luminescence studies. The efficiency of the TADF process and emission wavelengths are highly sensitive to environmental hydrogen bonding of the ligands, which can be disrupted by grinding of the crystalline materials. The origin of this pronounced mechano-stimulus photophysical behavior is a thermal equilibrium between the ^1/3LLCT states and a ³LC state of the BINAP ligand, which depends on the relative energetic order of the excited states and is prone to inter-ligand C-H···π interactions. The copper(I) complexes are also efficient CPL emitters displaying exceptional dissymmetry values g_lum of up to ±0.6 × 10^-2 in THF solution and ±2.1 × 10^-2 in the solid state. Importantly for application in electroluminescence devices, the C-H···π interactions can also be disrupted by employing sterically bulky matrices. Accordingly, we have investigated various matrix materials for successful implementation of the chiral copper(I) TADF emitters in proof-of-concept CP-OLEDs.

Light emission mechanism in dimers of carbene-metal-amide complexes

Recently an efficient dual electroluminescence from monomers and dimers was observed among the structural examples of the emerging emitter class of carbene-metal-amides (CMAs), allowing the preparation of simple design white organic light emitting diodes (wOLEDs). Here we investigate in detail the light emission mechanism in the dimeric species of CMA emitters on the basis of a copper(I) complex TCP bearing thiazoline carbene and 10H-phenothiazine 5,5-dioxide (Ptz) ligands. The X-ray structure for crystals with dimer-only emission was obtained, revealing that emissive aggregates consist of face-to-face stacked molecular pairs with an intermolecular distance of 3.673 Å. The close packing is aided by reduced sterical bulk at the carbene ligand, as well as by a torsional twist between the carbene and amide fragments. Experimental and computational data show that the emission mechanism in aggregates is related to the formation of a persistent dimer, not the excimer. Radiative relaxation proceeds through an intermolecular charge transfer process between the carbene and amide ligands of the neighbouring molecules. In comparison to the monomer, the thermally activated delayed fluorescence (TADF) process in the dimer is characterized with significantly higher energy gaps (ΔE_ST) between the lowest singlet (S₁) and triplet (T₁) excited states. At the same time the aggregated species exhibit a significantly increased phosphorescence rate (τ = 12 μs at 10 K temperature) due to the presence of two metal atoms, resulting in a sixfold increase in the spin-orbit coupling (SOC) matrix element in comparison to the monomer.

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.

Luminescent Complexes of Platinum, Iridium, and Coinage Metals Containing N-Heterocyclic Carbene Ligands: Design, Structural Diversity, and Photophysical Properties

The employment of N-heterocyclic carbenes (NHCs) to design luminescent metal compounds has been the focus of recent intense investigations because of the strong σ-donor properties, which bring stability to the whole system and tend to push the d-d dark states so high in energy that they are rendered thermally inaccessible, thereby generating highly emissive complexes for useful applications such as organic light-emitting diodes (OLEDs), or featuring chiroptical properties, a field that is still in its infancy. Among the NHC complexes, those containing organic chromophores such as naphthalimide, pyrene, and carbazole exhibit rich emission behavior and thus have attracted extensive interest in the past five years, especially carbene coinage metal complexes with carbazolate ligands. In this review, the design strategies of NHC-based luminescent platinum and iridium complexes with large spin-orbit-coupling (SOC) are described first. Subsequent paragraphs illustrate the recent advances of luminescent coinage metal complexes with nucleophilic- and electrophilic-based carbenes based on silver, gold, and copper metal complexes that have the ability to display rich excited state emissions in particular via thermally activated delayed fluorescence (TADF). The luminescence mechanism and excited state dynamics are also described. We then summarize the advance of NHC-metal complexes in the aforementioned fields in recent years. Finally, we propose the development trend of this fast-growing field of luminescent NHC-metal complexes.

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.

Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion

Many Cu^I complexes have luminescent triplet charge-transfer excited states with diverse applications in photophysics and photochemistry, but for isoelectronic Zn^II compounds, this behavior is much less common, and they typically only show ligand-based fluorescence from singlet π-π* states. We report two closely related tetrahedral Zn^II compounds, in which intersystem crossing occurs with appreciable quantum yields and leads to the population of triplet excited states with intraligand charge-transfer (ILCT) character. In addition to showing fluorescence from their initially excited ¹ILCT states, these new compounds therefore undergo triplet-triplet energy transfer (TTET) from their ³ILCT states and consequently can act as sensitizers for photo-isomerization reactions and triplet-triplet annihilation upconversion from the blue to the ultraviolet spectral range. The photoactive ³ILCT state furthermore facilitates photoinduced electron transfer. Collectively, our findings demonstrate that mononuclear Zn^II compounds with photophysical and photochemical properties reminiscent of well-known Cu^I complexes are accessible with suitable ligands and that they are potentially amenable to many different applications. Our insights seem relevant in the greater context of obtaining photoactive compounds based on abundant transition metals, complementing well-known precious-metal-based luminophores and photosensitizers.

π-Extended Ligands in Two-Coordinate Coinage Metal Complexes

Two-coordinate carbene-M^I-amide (cMa, M^I = Cu, Ag, Au) complexes have emerged as highly efficient luminescent materials for use in a variety of photonic applications due to their extremely fast radiative rates through thermally activated delayed fluorescence (TADF) from an interligand charge transfer (ICT) process. A series of cMa derivatives was prepared to examine the variables that affect the radiative rate, with the goal of understanding the parameters that control the radiative TADF process in these materials. We find that blue-emissive complexes with high photoluminescence efficiencies (Φ_PL > 0.95) and fast radiative rates (k_r = 4 × 10⁶ s^-1) can be achieved by selectively extending the π-system of the carbene and amide ligands. Of note is the role played by the increased separation between the hole and electron in the ICT excited state. Analysis of temperature-dependent luminescence data and theoretical calculations indicate that the hole-electron separation exerts a primary effect on the energy gap between the lowest-energy singlet and triplet states (ΔE_ST) while keeping the radiative rate for the singlet state relatively unchanged. This interpretation provides guidelines for the design of new cMa derivatives with even faster radiative rates in addition to those with slower radiative rates and thus extended excited state lifetimes.

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

Molecular near-IR (NIR) triplet-state emitters are of importance for the development of new, organic-electronics-based telecommunication technologies as optical fibers operating in the corresponding spectral bands allow for data transfer over much longer distances due to the significantly lower attenuation. However, achieving such low-energy triplet excited states with good radiative rate constants is very challenging, and studies regarding the single-photon emission of organometallics in this energy range are scarce. We have prepared a series of trigonal Cu^I CAAC complexes bearing chelating ligands with O, N, S, and Se donor atoms and studied their photophysical properties in this context. The compounds show weak low-energy absorption in solution between 400 and 500 nm due to mixed Cu → CAAC ¹MLCT/LLCT states, resulting in yellow-green to orange appearance, which we have also correlated to the ¹⁵N NMR resonances of the π-accepting carbene ligand. In the solid state, phosphorescence from dominant ³(Cu → CAAC) CT states is observed at room temperature. The emission of the complexes is bathochromically shifted in comparison to structurally related linearly coordinated copper(I) CAAC complexes due to structural reorganization in the excited state to a T-shape. For [Cu(dbm)(CAAC^Me)], the broad phosphorescence with outstanding λ_max = 760 nm tailors out to ca. 1100 nm and leads to its proof-of-concept application as a nonclassical single-photon light source, constituting key functional units for the implementation of tap-proof data transfer.

Synthesis and Fluorescence Studies of Diborene Coinage Metal Complexes

Nine different coinage metal (Cu, Ag) π complexes of diborenes with various anionic diborene (aryl, heteroaryl) and metal substituents (Cl, Br, C₆F₅, C₂SiMe₃), stabilizing neutral donors (N-heterocyclic carbene = NHC, phosphine), configurations (cis/trans, acyclic/cyclic diborene), and charges (neutral, cationic) were synthesized and characterized by multinuclear NMR spectroscopy and X-ray crystallographic analyses. Their optical properties were investigated by UV-vis absorption and steady-state as well as time-resolved luminescence spectroscopy in solution and the solid state to gain insights into the excited-state behavior of this unusual class of photoactive compounds and to provide structure-property relationships. The structural and electronic modification of the (B═B)···M motif greatly influences not only the visible light absorption but also the photostability and quantum yields, which can reach high values of up to f = 0.42. The lifetimes are found in the nanosecond regime, providing estimated radiative rate constants over a wide range of k_r = 1.3-14 × 10⁷ s^-1, indicative of fluorescence. Intersystem crossing (ISC) is sufficiently slow for prompt emission from the S₁ state to be observed, while the spin-orbit coupling in the T₁ state is too weak for phosphorescence to occur at room temperature. ISC can be accelerated, however, by modifying diborene ligand substitution and the coinage metal center, hinting at the potential for exploiting the properties of long-lived triplet excited states of metal diborene complexes in the future.

Ultra-Long Lived Luminescent Triplet Excited States in Cyclic (Alkyl)(amino)carbene Complexes of Zn(II) Halides

The high element abundance and d10 electron configuration make ZnII -based compounds attractive candidates for the development of novel photoactive molecules. Although a large library of purely fluorescent compounds exists, emission involving triplet excited states is a rare phenomenon for zinc complexes. We have investigated the photophysical and -chemical properties of a series of dimeric and monomeric ZnII halide complexes bearing a cyclic (alkyl)(amino)carbene (cAAC) as chromophore unit. Specifically, [(cAAC)XZn(μ-X)2 ZnX(cAAC)] (X=Cl (1), Br (2), I (3)) and [ZnX2 (cAAC)(NCMe)] (X=Br (4), I (5)) were isolated and fully characterized, showing intense visible light photoluminescence under UV irradiation at 297 K and fast photo-induced transformation. At 77 K, the compounds exhibit improved stability allowing to record ultra-long lifetimes in the millisecond regime. DFT/MRCI calculations confirm that the emission stems from 3 XCT/LEcAAC states and indicate the phototransformation to be related to asymmetric distortion of the complexes by cAAC ligand rotation. This study enhances our understanding of the excited state properties for future development and application of new classes of ZnII phosphorescent complexes.

Realizing 1,1-Dehydration of Secondary Alcohols to Carbenes: Pyrrolidin-2-ols as a Source of Cyclic (Alkyl)(Amino)Carbenes

Herein we report secondary pyrrolidin-2-ols as a source of cyclic (alkyl)(amino)carbenes (CAAC) for the synthesis of CAAC-CuI -complexes and cyclic thiones when reacted with CuI -salts and elemental sulfur, respectively, under reductive elimination of water from the carbon(IV)-center. This result demonstrates a convenient and facile access to CAAC-based CuI -salts, which are well known catalysts for different organic transformations. It further establishes secondary alcohols to be a viable source of carbenes-realizing after 185 years Dumas' dream who tried to prepare the parent carbene (CH2 ) by 1,1-dehydration of methanol. Addressed is also the reactivity of water towards CAACs, which proceeds through an oxidative addition of the O-H bond to the carbon(II)-center. This emphasizes the ability of carbon-compounds to mimic the reactivity of transition-metal complexes: reversible oxidative addition and reductive elimination of the O-H bond to/from the C(II)/C(IV)-centre.

Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

A simple synthetic entryway into new families of NHC-gold-amido complexes and their in vitro antitumor activity

A simple synthetic pathway to Au-NHC amido complexes is described. Syntheses and isolation of [Au(NHC)(NR1R2)] complexes, bearing various NHC ligands and NH-containing heterocycles under mild conditions are reported. The in vitro anticancer activity of these gold-complexes was investigated on three human cancer cell lines. A number of these show comparable or even better antiproliferative activity than cisplatin. Noteworthy is the non-toxicity of most of the complexes on normal cells.

Linear Carbene Pyridine Copper Complexes with Sterically Demanding N,N'-Bis(trityl)imidazolylidene: Syntheses, Molecular Structures, and Photophysical Properties

The sterically demanding carbene ITr (N,N'-bis(triphenylmethyl)imidazolylidene) was used as a ligand for the preparation of luminescent copper(I) complexes of the type [(ITr)Cu(R-pyridine/R'-quinoline)]BF₄ (R = H, 4-CN, 4-CHO, 2,6-NH₂, and R' = 8-Cl, 6-Me). The selective formation of linear, bis(coordinated) complexes was observed for a series of pyridine and quinoline derivatives. Only in the case of 4-cyanopyridine a one-dimensional coordination polymer was formed, in which the cyano group of the cyanopyridine ligand additionally binds to another Cu atom in a bridging manner, thus leading to a trigonal planar coordination environment. In contrast, employing sterically less demanding monotrityl-substituted carbene 3, no (NHC)Cu-pyridine complexes could be prepared. Instead, a bis-carbene complex [(3)₂Cu]PF₆ was obtained which showed no luminescence. All linear pyridine/quinoline coordinated complexes show weak emission in solution but intense blue to orange luminescence doped with 10% in PMMA films and in the solid state either from triplet excited states with unusually long lifetimes of up to 4.8 ms or via TADF with high radiative rate constants of up to 1.7 × 10⁵ s^-1 at room temperature. Combined density functional theory and multireference configuration interaction calculations have been performed to rationalize the involved photophysics of these complexes. They reveal a high density of low-lying electronic states with mixed MLCT, LLCT, and LC character where the electronic structures of the absorbing and emitting state are not necessarily identical.

Synthesis and photophysical properties of linear gold(I) complexes based on a CCC carbene

The reaction between allenylpyridine (L1) and (Me₂S)AuCl resulted in the quantitative formation of the (Indolizy)gold chloride complex 1 (Indolizy = indolizin-2-ylidene). The reaction of 1 with carbazole in the presence of KO^tBu affords the corresponding (Indolizy)Au(Cz) complex 2. Both compounds show high air- and temperature stability. The crystal structure of 2 confirmed the linear co-planar geometry. Complex 1 shows an intense low energy absorption of mixed character in the UV-vis spectrum, ascribed to intraligand and (M + Hal)L charge transfer processes, and exhibits bright yellow phosphorescence with an excited state lifetime of 62.8 μs in the crystal and a luminescence quantum yield up to 65%. On the other hand, the carbazolate complex 2 in a polystyrene matrix shows bright red delayed fluorescence at 617 nm with a sub-microsecond excited state lifetime and a quantum yield of 21.6%.

Luminescent First-Row Transition Metal Complexes

Precious and rare elements have traditionally dominated inorganic photophysics and photochemistry, but now we are witnessing a paradigm shift toward cheaper and more abundant metals. Even though emissive complexes based on selected first-row transition metals have long been known, recent conceptual breakthroughs revealed that a much broader range of elements in different oxidation states are useable for this purpose. Coordination compounds of V, Cr, Mn, Fe, Co, Ni, and Cu now show electronically excited states with unexpected reactivity and photoluminescence behavior. Aside from providing a compact survey of the recent conceptual key advances in this dynamic field, our Perspective identifies the main design strategies that enabled the discovery of fundamentally new types of 3d-metal-based luminophores and photosensitizers operating in solution at room temperature.

Strongly Red-Emissive Molecular Ruby [Cr(bpmp)2]3+ Surpasses [Ru(bpy)3]2

Gaining chemical control over the thermodynamics and kinetics of photoexcited states is paramount to an efficient and sustainable utilization of photoactive transition metal complexes in a plethora of technologies. In contrast to energies of charge transfer states described by spatially separated orbitals, the energies of spin-flip states cannot straightforwardly be predicted as Pauli repulsion and the nephelauxetic effect play key roles. Guided by multireference quantum chemical calculations, we report a novel highly luminescent spin-flip emitter with a quantum chemically predicted blue-shifted luminescence. The spin-flip emission band of the chromium complex [Cr(bpmp)₂]³⁺ (bpmp = 2,6-bis(2-pyridylmethyl)pyridine) shifted to higher energy from ca. 780 nm observed for known highly emissive chromium(III) complexes to 709 nm. The photoluminescence quantum yields climb to 20%, and very long excited state lifetimes in the millisecond range are achieved at room temperature in acidic D₂O solution. Partial ligand deuteration increases the quantum yield to 25%. The high excited state energy of [Cr(bpmp)₂]³⁺ and its facile reduction to [Cr(bpmp)₂]²⁺ result in a high excited state redox potential. The ligand's methylene bridge acts as a Brønsted acid quenching the luminescence at high pH. Combined with a pH-insensitive chromium(III) emitter, ratiometric optical pH sensing is achieved with single wavelength excitation. The photophysical and ground state properties (quantum yield, lifetime, redox potential, and acid/base) of this spin-flip complex incorporating an earth-abundant metal surpass those of the classical precious metal [Ru(α-diimine)₃]²⁺ charge transfer complexes, which are commonly employed in optical sensing and photo(redox) catalysis, underlining the bright future of these molecular ruby analogues.

Cyan-Emitting Cu(I) Complexes and Their Luminescent Metallopolymers

Copper complexes have shown great versatility and a wide application range across the natural and life sciences, with a particular promise as organic light-emitting diodes. In this work, four novel heteroleptic Cu(I) complexes were designed in order to allow their integration in advanced materials such as metallopolymers. We herein present the synthesis and the electrochemical and photophysical characterisation of these Cu(I) complexes, in combination with ab initio calculations. The complexes present a bright cyan emission (λem ~ 505 nm) in their solid state, both as powder and as blends in a polymer matrix. The successful synthesis of metallopolymers embedding two of the novel complexes is shown. These copolymers were also found to be luminescent and their photophysical properties were compared to those of their polymer blends. The chemical nature of the polymer backbone contributes significantly to the photoluminescence quantum yield, paving a route for the strategic design of novel luminescent Cu(I)-based polymeric materials.

A Luminescent Two-Coordinate AuI Bimetallic Complex with a Tandem-Carbene Structure: A Molecular Design for the Enhancement of TADF Radiative Decay Rate

A luminescent bimetallic Au^I complex comprised of N-heterocyclic carbene (NHC) and carbazole (Cz) ligands, that is, (NHC')Au(NHC)AuCz has been synthesized and studied. Both carbene ligands in the bimetallic complex act as electron acceptors in tandem to increase the energy separation between the ground and excited state, which is higher than those found in either monometallic analogue, (NHC)AuCz and (NHC')AuCz. A coplanar geometry designed into the tandem complex ensures sufficient electronic coupling between the π-orbitals of the ligands to impart a strong oscillator strength to the singlet intra-ligand charge-transfer (¹ ICT) transition. Theoretical modelling indicates that the emissive ICT excited state involves both NHC ligands. The tandem complex gives blue luminescence (λ_max =480 nm) with a high photoluminescent quantum yield (Φ_PL =0.80) with a short decay lifetime (τ=0.52 μs). Temperature-dependent photophysical studies indicate that emission is via thermally assisted delayed fluorescence (TADF) and give a small singlet-triplet energy difference (ΔE_ST =50 meV, 400 cm^-1 ) consistent with the short TADF lifetime.

A Brief History of OLEDs-Emitter Development and Industry Milestones

Organic light-emitting diodes (OLEDs) have come a long way ever since their first introduction in 1987 at Eastman Kodak. Today, OLEDs are especially valued in the display and lighting industry for their promising features. As one of the research fields that equally inspires and drives development in academia and industry, OLED device technology has continuously evolved over more than 30 years. OLED devices have come forward based on three generations of emitter materials relying on fluorescence (first generation), phosphorescence (second generation), and thermally activated delayed fluorescence (third generation). Furthermore, research in academia and industry toward the fourth generation of OLEDs is in progress. Excerpts from the history of green, orange-red, and blue OLED emitter development on the side of academia and milestones achieved by key players in the industry are included in this report.

Understanding the luminescence properties of Cu(i) complexes: a quantum chemical perusal

Electronic structures and excited-state properties of Cu(i) complexes with varying coordination numbers have been investigated by means of advanced quantum chemical methods. The computational protocol employs density functional-based methods for geometry optimizations and vibrational analyses including solvent effects through continuum models. Excitation energies, spin-orbit couplings and luminescence properties are evaluated using multireference configuration interaction methods. Rate constants of spin-allowed and spin-forbidden transitions have been determined according to the Fermi golden rule. The computational results for the 4-coordinate (DPEPhos)Cu(PyrTet), the 3-coordinate [IPr-Cu-Py₂]⁺, and the linear CAAC^Me₂-Cu-Cl complexes agree well with experimental absorption and emission wavelengths, intersystem crossing (ISC) time constants, and radiative lifetimes in liquid solution. Spectral shifts on the ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT) transitions caused by the polarity of the environment are well represented by the continuum models whereas the shifts caused by pseudo-Jahn-Teller distortions in the MLCT states are too pronounced in comparison to solid-state data. Systematic variation of the ligands in linear Cu(i) carbene complexes shows that only those complexes with S₁ and T₁ states of LLCT character possess sufficiently small singlet-triplet energy gaps ΔE_ST to enable thermally activated delayed fluorescence (TADF). Complexes whose S₁ and T₁ wavefunctions are dominated by MLCT excitations tend to emit phosphorescence instead. Unlike the situation in metal-free TADF emitters, the presence of low-lying locally excited triplet states does not promote ISC. These states rather hold the danger of trapping the excitation with nonradiative deactivation being the major deactivation channel.

Isocyanate Insertion into a La-P Phosphide Bond: A Versatile Route to Phosphaureate-Bridged Heterobimetallic Lanthanide-Coinage-Metal Complexes

A new route to heterobimetallic lanthanide-coinage-metal complexes is disclosed. The selective insertion of organic substrates such as phenyl iso(thio)cyanate into the La-P bond of the primary phosphido complex (PN)2La(PHMes) (1) (with PN- = (N-(2-(diisopropylphosphanyl)-4-methylphenyl)-2,4,6-trimethylanilide) yields the phospha(thio)ureate complexes (PN)2La(OC(NPh)(PHMes)) (2) and (PN)2La(SC(NPh)(PHMes)) (3) with retention of the PH protons. Subsequent deprotonation of the phosphaureate complex 2 with potassium hexamethyldisilazide (KHMDS, K[N(SiMe3)2]) leads to the polymeric complex [K{(PN)2La(OC(NPh)(PMes))}]n (4). Complex 4 was found to be an excellent precursor for salt metathesis reactions with copper(I) and gold(I) chlorides supported by an N-heterocyclic carbene (NHC, 5 and 6) or a cyclic alkyl amino carbene (CAAC, 7 and 8). This resulted in the unprecedented formation of heterobimetallic lanthanum-coinage-metal complexes, containing the first example of a μ,κ2(O,N):κ1(P)-phosphaureate bridging ligand. For an alternative route to complex 8 a direct protonolysis protocol between a new basic gold(I) precursor, namely (MeCAAC)Au(HMDS), and 2 was also investigated. The complexes have been characterized by multinuclear NMR spectroscopy, IR spectroscopy, and X-ray crystallography (except for 8).

Coinage-Metal-Based Cyclic Trinuclear Complexes with Metal-Metal Interactions: Theories to Experiments and Structures to Functions

Among the d¹⁰ coinage metal complexes, cyclic trinuclear complexes (CTCs) or trinuclear metallocycles with intratrimer metal-metal interactions are fascinating and important metal-organic or organometallic π-acids/bases. Each CTC of characteristic planar or near-planar trimetal nine-membered rings consists of Au(I)/Ag(I)/Cu(I) cations that linearly coordinate with N and/or C atoms in ditopic anionic bridging ligands. Since the first discovery of Au(I) CTC in the 1970s, research of CTCs has involved several fundamental areas, including noncovalent and metallophilic interaction, excimer/exciplex, acid-base chemistry, metalloaromaticity, supramolecular assemblies, and host/guest chemistry. These allow CTCs to be embraced in a wide range of innovative potential applications that include chemical sensing, semiconducting, gas and liquid adsorption/separation, catalysis, full-color display, and solid-state lighting. This review aims to provide a historic and comprehensive summary on CTCs and their extension to higher nuclearity complexes and coordination polymers from the perspectives of synthesis, structure, theoretical insight, and potential applications.

Mononuclear Cu Complexes Based on Nitrogen Heterocyclic Carbene: A Comprehensive Review

During the last decade, organometallic, coordination, and catalytic chemistry of the three-dimensional metals such as copper (Cu) has been greatly affected by the emergence of nitrogen heterocyclic carbene (NHC) complexes. The NHCs, and in particular the mononuclear Cu^I-based ones, have been proven vastly useful in several applications such as in biosynthesis, catalysis, photochemistry, etc. This review tries to thoroughly describe a series of mononuclear Cu^I NHC complexes and their subcategories such as heteroleptics, and bidentate and tridentate heteroatom complexes, and give some detailed insights on their development, emergence, and applications. A brief outlook is also disclosed to enable other researchers to further develop a platform for future advances and studies in the field of Cu^I-based NHCs.

Cyclic (Amino)(aryl)carbenes Enter the Field of Chromophore Ligands: Expanded π System Leads to Unusually Deep Red Emitting CuI Compounds

A series of copper(I) complexes bearing a cyclic (amino)(aryl)carbene (CAArC) ligand with various complex geometries have been investigated in great detail with regard to their structural, electronic, and photophysical properties. Comparison of [CuX(CAArC)] (X = Br (1), Cbz (2), acac (3), Ph₂acac (4), Cp (5), and Cp* (6)) with known Cu^I complexes bearing cyclic (amino)(alkyl), monoamido, or diamido carbenes (CAAC, MAC, or DAC, respectively) as chromophore ligands reveals that the expanded π-system of the CAArC leads to relatively low energy absorption maxima between 350 and 550 nm in THF with high absorption coefficients of 5-15 × 10³ M^-1 cm^-1 for 1-6. Furthermore, 1-5 show intense deep red to near-IR emission involving their triplet excited states in the solid state and in PMMA films with λ^em_max = 621-784 nm. Linear [Cu(Cbz)(^DippCAArC)] (2) has been found to be an exceptional deep red (λ_max = 621 nm, ϕ = 0.32, τ_av = 366 ns) thermally activated delayed fluorescence (TADF) emitter with a radiative rate constant k_r of ca. 9 × 10⁵ s^-1, exceeding those of commercially employed Ir^III- or Pt^II-based emitters. Time-resolved transient absorption and fluorescence upconversion experiments complemented by quantum chemical calculations employing Kohn-Sham density functional theory and multireference configuration interaction methods as well as temperature-dependent steady-state and time-resolved luminescence studies provide a detailed picture of the excited-state dynamics of 2. To demonstrate the potential applicability of this new class of low-energy emitters in future photonic applications, such as nonclassical light sources for quantum communication or quantum cryptography, we have successfully conducted single-molecule photon-correlation experiments of 2, showing distinct antibunching as required for single-photon emitters.

Revealing High-Lying Intersystem Crossing in Brightly Luminescent Cyclic Trinuclear CuI/AgI Complexes

The increased attention to luminescent copper(I) complexes, mostly mononuclear and dinuclear ones, in the past few years was mainly due to the new pathways established in the intersystem crossing (ISC) for highly efficient singlet/triplet harvesting, which showed great potential in light-emitting devices. Governing the photophysical processes of planar cyclic trinuclear complexes is more challenging owing to the rich intra- and intermolecular metal-metal interactions involved, but new opportunities also accompany this. Herein reported is a hidden route to the ultra-long-lived, highly efficient phosphorescence of cyclic trinuclear two-coordinate Cu^I-pyrazolate complexes through pushing the unfavorable metal-to-ligand charge transfer events to the high-lying ISC pathways. Moreover, an anomaly of much higher quantum yields of a trinuclear Ag^I-pyrazolate complex relative to its Cu^I analogue is observed.