Carbene Metal Amide Research Articles

Cyclic(amino)(barrelene)carbene Metal Amide Complexes: Synthesis and Thermally Activated Delayed Fluorescence

Cyclic(amino)(barrelene)carbene Metal Amide Complexes: Synthesis and Thermally Activated Delayed Fluorescence

Understanding Spin-Triplet Excited States in Carbene-Metal-Amides.

Carbene-metal-amides (CMAs) are emerging delayed fluorescence materials for organic light-emitting diode (OLED) applications. CMAs possess fast, efficient emission owing to rapid forward and reverse intersystem crossing (ISC) rates. The resulting dynamic equilibrium between singlet and triplet spin manifolds distinguishes CMAs from most purely organic thermally activated delayed fluorescence emitters. However, direct experimental triplet characterization in CMAs is underutilized, limiting our detailed understanding of the ISC mechanism. In this work, we combine time-resolved spectroscopy with tuning of state energies through environmental polarity and metal substitution, focusing on the interplay between charge-transfer (3CT) and local exciton (3LE) triplets. Unlike previous photophysical work, we investigate evaporated host : guest films of CMAs and small-molecule hosts for increased device relevance. Transient absorption reveals an evolution in the triplet excited-state absorption (ESA) consistent with a change in orbital character between hosts with differing dielectric constants. Using quantum chemical calculations, we simulate ESAs of the lowest triplet states, highlighting the contribution of only 3CT and donor-moiety 3LE states to spectral features, with no strong evidence for a low-lying acceptor-centered 3LE. Thus, our work provides a blueprint for understanding the role of triplet excited states in CMAs which will enable further intelligent optimization of this promising class of materials.

Efficient Blue Photo- and Electroluminescence from CF3-Decorated Cu(I) Complexes.

Luminescent organometallic complexes of earth-abundant copper(I) have long been studied in organic light-emitting diodes (OLED). Particularly, Cu(I)-based carbene-metal-amide (CMA) complexes have recently emerged as promising organometallic emitters. However, blue-emitting Cu(I) CMA complexes have been rarely reported. Here we constructed two blue-emitting Cu(I) CMA emitters, MAC*-Cu-CF3Cz and MAC*-Cu-2CF3Cz, by introducing one or two CF3 substitutes into carbazole ligands. Both complexes exhibited high thermal stability and blue emission colors. Moreover, two complexes exhibited different emission origins rooting from different donor ligands: a distinct thermally activated delayed fluorescence (TADF) from ligand-to-ligand charge transfer excited states for MAC*-Cu-CF3Cz or a dominant phosphorescence nature from local triplet excited state of the carbazole ligand for MAC*-Cu-2CF3Cz. Inspiringly, MAC*-Cu-CF3Cz had high photoluminescence quantum yields of up to 94 % and short emission lifetimes of down to 1.2 μs in doped films, accompanied by relatively high radiative rates in the 105 s-1 order. The resultant vacuum-deposited OLEDs based on MAC*-Cu-CF3Cz delivered pure-blue electroluminescence at 462 nm together with a high external quantum efficiency of 13.0 %.

Understanding Spin‐Triplet Excited States in Carbene‐Metal‐Amides

Carbene‐metal‐amides (CMAs) are emerging delayed fluorescence materials for organic light‐emitting diode (OLED) applications. CMAs possess fast, efficient emission owing to rapid forward and reverse intersystem crossing (ISC) rates. The resulting dynamic equilibrium between singlet and triplet spin manifolds distinguishes CMAs from most purely organic thermally activated delayed fluorescence emitters. However, direct experimental triplet characterization in CMAs is underutilized, limiting our detailed understanding of the ISC mechanism. In this work, we combine time‐resolved spectroscopy with tuning of state energies through environmental polarity and metal substitution, focusing on the interplay between charge‐transfer (3CT) and local exciton (3LE) triplets. Unlike previous photophysical work, we investigate evaporated host:guest films of CMAs and small‐molecule hosts for increased device relevance. Transient absorption reveals an evolution in the triplet excited‐state absorption (ESA) consistent with a change in orbital character between hosts with differing dielectric constants. Using quantum chemical calculations, we simulate ESAs of the lowest triplet states, highlighting the contribution of only 3CT and donor‐moiety 3LE states to spectral features, with no strong evidence for a low‐lying acceptor‐centered 3LE. Thus, our work provides a blueprint for understanding the role of triplet excited states in CMAs which will enable further intelligent optimization of this promising class of materials.

Copper(I) TADF exciplexes via Metal-Perturbed Through-Space charge transfer for efficient OLEDs

Organo-copper(I) emitters are promising cost-effective candidates as triplet-harvesting dopants in organic light-emitting diodes (OLEDs). To date, the mainstream design of Cu(I) emitters depends on ligand engineering to modify intramolecular charge transfer and thus modulate the related optoelectronic properties. However, little is known on luminescent Cu(I) systems based on intermolecular charge transfer. For the first time, the proof-of-concept Cu(I) exciplexes were developed employing a triazine-containing molecule DMIC-TRZ as an acceptor and several Cu(I)-based carbene-metal-amide (CMA) complexes as donors. The formation of these exciplexes is strongly associated with d-orbital participation of Cu atom on the highest occupied molecular orbital of CMA complexes to match with the acceptor DMIC-TRZ. These Cu(I) exciplexes not only displayed donor-dependent emission colors spanning from sky blue to red but also exhibited distinct thermally activated delayed fluorescence (TADF) via copper(I)-perturbed through-space charge transfer (TSCT). The OLEDs adopting the Cu(I) exciplexes as emitters delivered good external quantum efficiency (EQE) of up to 10.9%. Furthermore, the use of these Cu(I) exciplexes as hosts for multi-resonance TADF emitters delivered narrowband electroluminescence with high EQEs of up to 24.9%, superior to the reference OLEDs based on the common host.

Luminescent carbene-copper(i)-amide polymers for efficient host-free solution-processed OLEDs.

Luminescent metallopolymers have attracted broad interest in the fields of healthcare and organic electronics. However, polymeric emitters based on earth-abundant metal complexes are scarce. Here, two series of Cu(i) polymers, PMAC-x and PCAAC-x (x = 1-3) have been developed using two kinds of Cu(i)-based carbene-metal-amide (CMA) complexes as side-chain emitter units to combine with a nonconjugated polystyrene backbone. These Cu(i) polymers emit via distinct thermally activated delayed fluorescence or dominant phosphorescence, inherited from the grafted Cu(i)-based CMA units. Particularly, the PMAC-x polymers exhibit high photoluminescence quantum efficiencies of up to 0.78, short emission lifetimes of down to 0.66 μs, and fast radiative rates of up to 106 s-1 in neat films. Thanks to the good encapsulation effect of the polystyrene backbone, these Cu(i) polymers not only demonstrate favorable moisture stability but also show significant aggregation-induced emission. The resultant host-free solution-processed organic light-emitting diodes (OLEDs) achieve outstanding electroluminescence performance with a record external quantum efficiency of 13.8% at a practical luminance of ∼100 nits, representing state-of-the-art device efficiency for metallopolymer-based OLEDs. This work not only presents the first example of CMA polymers but also provides the future direction of polymeric emitters from earth-abundant metal complexes for the OLED application.

Synthesis of N-heterocyclic carbene (NHC)-Au/Ag/Cu benzotriazolyl complexes and their catalytic activity in propargylamide cycloisomerization and carbonyl hydrosilylation reactions.

Carbene-metal-amide (CMA) complexes of gold, silver, and copper have been studied extensively for their photochemical/photocatalytic properties and as potential (pre-)catalysts in organic synthesis. Herein, the design, synthesis, and characterization of five bench-stable Au-, Ag-, and Cu-NHC complexes bearing the benzotriazolyl anion as an amide donor, are reported. All complexes are synthesized in a facile and straightforward manner, using mild conditions. The catalytic activity of the Ag and Cu complexes was studied in propargylamide cycloisomerization and carbonyl hydrosilylation reactions. Both CMA-catalyzed transformations proceed under mild conditions and are highly efficient for a range of propargylamides and carbonyl compounds, respectively, affording the desired corresponding products in good to excellent yields.

Sulfonyl-functionalized benzo[d]imidazo[5,1-b]thiazole-based carbenes as building blocks for two-coordinate Cu(i) complexes exhibiting fast and efficient thermally activated delayed fluorescence

A peripherally-placed strong electron-accepting sulphonyl group reduces the HOMO–LUMO overlap in carbene–metal–amide TADF emitters.

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.

Thiazoline Carbene−Au(I)−Amide complexes exhibiting dimer electroluminescence

Two-coordinate coinage metal (Cu, Ag and Au) complexes with linear carbene–metal–amide (CMA) structure have recently emerged as promising thermally activated delayed fluorescence (TADF) emitters for OLED application. Herein we investigate a series of eight new Au(I) CMA complexes, which incorporate derivatives of 1,3-thiazoline as N-heterocyclic carbene (NHC) ligands. The complexes exhibit blue to green luminescence in thin films. A limitation of the torsional motions around the metal-ligand axis in a combination with the heavy gold atom effect enables highly efficient TADF with photoluminescence quantum yield (PLQY) of up to 0.99 and radiative rates nearing 1·106 s−1. A formation of emissive dimers of complex molecules was observed in vacuum processed OLEDs. Achieved dual electroluminescence from the redshifted (by 120 nm) dimeric species in a combination with co-occurring monomer emission opens potential application prospects in construction of single-emitter white OLEDs.

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.

Au(I)‐TADF Emitters for High Efficiency Full‐Color Vacuum‐Deposited OLEDs and TADF‐Sensitized Fluorescent OLEDs with Ultrahigh Brightness and Prolonged Operational Lifetime

AbstractHerein a class of structurally simple and operationally stable Au(I)‐TADF (TADF = thermally activated delayed fluorescence) materials, based on a carbene–metal–amide (CMA) molecular scaffold comprised of sterically bulky N‐heterocyclic carbene ligands with N‐heterocyclic π‐annulation, are reported. These CMA(Au) emitters are thermally stable, adopt coplanar or orthogonal geometry between the carbene and amide ligands, and show strong blue to deep red TADF emissions (466–666 nm) from thermally equilibrated singlet ligand‐to‐ligand‐charge‐transfer excited states with emission quantum yields of 0.63–0.99 and radiative decay rate constants of 0.68–3.2 × 106 s−1 in thin film samples at room temperature. The effects of increasing π‐extension and orthogonal molecular geometry are similarly manifested in the reduction of both singlet–triplet energy gap and S1 transition dipole moment. The vacuum‐deposited Au(I) organic light‐emitting diodes (OLEDs) display superior electroluminescence characterized by ultrahigh brightness up to 300 000 cd m−2 and external quantum efficiencies (EQEs) up to 26.2% with roll‐offs down to 2.6% at 1000 cd m−2 alongside record‐setting device lifetimes (LT95) up to 2082 h. Ultrapure‐green TADF‐sensitized fluorescent OLEDs employing the CMA(Au) emitter as sensitizer and a multiresonance terminal emitter achieve EQEs of up to 25.3%.

Tailoring Carbene-Metal-Amides for Thermally Activated Delayed Fluorescence: A Computationally Guided Study on the Effect of Cyclic (Alkyl)(amino)carbenes.

Gold-centered carbene-metal-amides (CMAs) containing cyclic (alkyl)(amino)carbenes (CAACs) are promising emitters for thermally activated delayed fluorescence (TADF). Aiming at the design and optimization of new TADF emitters, we report a density functional theory study of over 60 CMAs with various CAAC ligands, systematically evaluating computed parameters in relation to photoluminescence properties. The CMA structures were primarily selected based on experimental synthesis prospects. We demonstrate that TADF efficiency of the CMA materials originates from a compromise between oscillator strength coefficients and exchange energy (ΔEST). The latter is governed by the overlap of HOMO and LUMO orbitals, where HOMO is localized on the amide and LUMO over the Au-carbene bond. The S0 ground and excited T1 states of the CMAs adopt approximately coplanar geometry of carbene and amide ligands, but rotate perpendicular in the excited S1 states, resulting in degeneracy or near-degeneracy of S1 and T1, accompanied by a decrease in the S1-S0 oscillator strength from its maximum at coplanar geometries to near zero at rotated geometries. Based on the computations, promising new TADF emitters are proposed and synthesized. Bright CMA complex (Et2CAAC)Au(carbazolide) is obtained and fully characterized in order to demonstrate that excellent stability and high radiative rates up to 106 s-1 can be obtained for the gold-CMA complexes with small CAAC-carbene ligands.

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 (ΔEST) 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.

Mesyl and Triflyl Functionalized N-Heterocyclic Carbenes as Acceptor Fragments in Luminescent Carbene-Metal-Amide Complexes

AbstractSynthetic procedures providing access to mesyl and triflyl functionalized derivatives of 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr) have been provided in detail. New luminescent carbene-metal-amide (CMA) Cu(I) complexes based on acceptor group functionalized SIPr have been prepared. The effect of the LUMO energy in the sulfonyl functionalized N-heterocyclic carbene (NHC) series on the emissive properties of the CMAs has been investigated.

Aggregation‐Dependent Circularly Polarized Luminescence and Thermally Activated Delayed Fluorescence from Chiral Carbene‐CuI‐Amide Enantiomers

AbstractThe field of luminescent carbene‐metal‐amide (CMA) complexes and chiroptical‐active materials has been blossoming in recent years, although chiroptical‐active CMA complexes have not been reported so far. For the first time, a pair of chiral CuI‐based CMA enantiomers, (R,R)‐PSIPr*‐Cu‐DMAC and (S,S)‐PSIPr*‐Cu‐DMAC, have been developed by using chiral phenyl‐substituted N‐heterocyclic carbenes as acceptor ligands in the CMA motif. The CuI‐based CMA enantiomers exhibited aggregation‐induced circularly polarized luminescence with a large luminescence dissymmetry factor of up to +0.027, the first reported for CMA complexes. This success originates from the limited ligand‐ligand rotation freedom and asymmetrical packing pattern (helical structure) of the CMA enantiomers in the crystals. Moreover, these CuI enantiomers displayed inspiring aggregation‐dependent thermally activated delayed fluorescence properties. These findings bring new insights into the optical properties of chiral CMA complexes from the perspective of aggregation states.

Aggregation-Dependent Circularly Polarized Luminescence and Thermally Activated Delayed Fluorescence from Chiral Carbene-CuI -Amide Enantiomers.

The field of luminescent carbene-metal-amide (CMA) complexes and chiroptical-active materials has been blossoming in recent years, although chiroptical-active CMA complexes have not been reported so far. For the first time, a pair of chiral CuI -based CMA enantiomers, (R,R)-PSIPr*-Cu-DMAC and (S,S)-PSIPr*-Cu-DMAC, have been developed by using chiral phenyl-substituted N-heterocyclic carbenes as acceptor ligands in the CMA motif. The CuI -based CMA enantiomers exhibited aggregation-induced circularly polarized luminescence with a large luminescence dissymmetry factor of up to +0.027, the first reported for CMA complexes. This success originates from the limited ligand-ligand rotation freedom and asymmetrical packing pattern (helical structure) of the CMA enantiomers in the crystals. Moreover, these CuI enantiomers displayed inspiring aggregation-dependent thermally activated delayed fluorescence properties. These findings bring new insights into the optical properties of chiral CMA complexes from the perspective of aggregation states.

Excited-State Lifetime Modulation by Twisted and Tilted Molecular Design in Carbene-Metal-Amide Photoemitters.

Carbene–metal–amides (CMAs) are an emerging class of photoemitters based on a linear donor–linker–acceptor arrangement. They exhibit high flexibility about the carbene–metal and metal–amide bonds, leading to a conformational freedom which has a strong influence on their photophysical properties. Herein we report CMA complexes with (1) nearly coplanar, (2) twisted, (3) tilted, and (4) tilt-twisted orientations between donor and acceptor ligands and illustrate the influence of preferred ground-state conformations on both the luminescence quantum yields and excited-state lifetimes. The performance is found to be optimum for structures with partially twisted and/or tilted conformations, resulting in radiative rates exceeding 1 × 106 s–1. Although the metal atoms make only small contributions to HOMOs and LUMOs, they provide sufficient spin–orbit coupling between the low-lying excited states to reduce the excited-state lifetimes down to 500 ns. At the same time, high photoluminescence quantum yields are maintained for a strongly tilted emitter in a host matrix. Proof-of-concept organic light-emitting diodes (OLEDs) based on these new emitter designs were fabricated, with a maximum external quantum efficiency (EQE) of 19.1% with low device roll-off efficiency. Transient electroluminescence studies indicate that molecular design concepts for new CMA emitters can be successfully translated into the OLED device.

Theoretical studies on excited‐state properties and luminescence mechanism of a Carbene–Metal–Amide Au(I) complex with thermally activated delayed fluorescence

AbstractTwo‐coordinate Carbene−Metal−Amide (CMA) complexes with thermally activated delayed fluorescence (TADF) have attracted much attention owing to their excellent luminescence properties and potential applications in organic light‐emitting devices. However, the luminescence mechanism remains unclear. Herein, we took one CMA Au(I) complex as an example and investigate its relevant photophysics using both density functional theory (DFT) and time‐dependent DFT methods with a polarizable continuum model. The calculated absorption and emission spectra agree with the experimental data and the S1 and T1 states show mixed ligand to ligand charge transfer (CT) and ligand to metal CT characters. Small spatial overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) minimizes the energy difference between the S1 and T1 states (ΔEST). Properly large spin‐orbit coupling promotes the reverse intersystem crossing (rISC) process. At 300 K, the rISC process is much more efficient than the T1 phosphorescent emission, which leads to the TADF emission.

Donor N‐Substitution as Design Principle for Fast and Blue Luminescence in Carbene‐Metal‐Amides

AbstractA series of gold‐centered carbene‐metal‐amide (CMA) complexes are synthesized with the carbazole donor ligand modified by substitution with nitrogen atoms in varying positions. The luminescence of new aza‐CMA complexes shows a significant blueshift depending on the position of the N atom, to provide bright blue‐green (500 nm), sky‐blue (478 nm), blue (450 nm) and deep‐blue (419 nm) light‐emitters. The impact of the electron‐withdrawing aza‐group on the nature of the luminescence and the excited state energies of the locally excited (LE) or charge transfer (CT) states have been interpreted with the help of transient absorption, in‐depth photoluminescence experiments and theoretical calculations. By considering the orbital characters of the lowest CT and LE states, we develop a new concept for simultaneous energy tuning for both of these states with a single aza‐substitution, allowing for fast and blue CT emission. This concept allows the interference of 3LE phosphorescence to be avoided at room temperature. The approach is extended to two N substitutions at the optimal location in the 3‐ and 6‐positions of the carbazole skeleton. These results suggest a practical molecular design towards the development of bright and deep‐blue emitting CMA materials to tackle the stability problem of energy‐efficient deep‐blue OLEDs.