Recent advances in copper complexes for electrical/light energy conversion

Steric and Electronic Influence of Excited-State Decay in Cu(I) MLCT Chromophores

ConspectusFor the past 11 years, a dedicated effort in our research group focused on fundamentally advancing the photophysical properties of cuprous bis-phenanthroline-based metal-to-ligand charge transfer (MLCT) excited states. We rationalized that, by gaining control over the numerous factors limiting the more widespread use of CuI MLCT photosensitizers, they would be readily adopted in numerous light-activated applications given the earth-abundance of copper and the extensive library of 1,10-phenanthrolines developed over the last century. Significant progress has been achieved by recognizing valuable structure-property concepts developed by other researchers in tandem with detailed ultrafast and conventional time-scale investigations, in-silico-inspired molecular designs to predict spectroscopic properties, and applying novel synthetic methodologies. Ultimately, we achieved a plateau in exerting cooperative steric influence to control CuI MLCT excited state decay. This led to combining sterics with π-conjugation and/or inductive electronic effects to further exert control over molecular photophysical properties. The lessons gleaned from our studies of homoleptic complexes were recently extended to heteroleptic bis(phenanthrolines) featuring enhanced visible light absorption properties and long-lived room-temperature photoluminescence. This Account navigates the reader through our intellectual journey of decision-making, molecular and experimental design, and data interpretation in parallel with appropriate background information related to the quantitative characterization of molecular photophysics using CuI MLCT chromophores as prototypical examples.Initially, CuI MLCT excited states, their energetics, and relevant structural conformation changes implicated in their photophysical decay processes are described. This is followed by a discussion of the literature that motivated our research in this area. This led to our first molecular design in 2013, achieving a 7-fold increase in excited state lifetime relative to the current state-of-the-art. The lifetime and photophysical property enhancement resulted from using 2,9-branched alkyl groups in conjunction with flanking 3,8-methyl substituents, a strategy we adapted from the McMillin group, which was initially described in the late 1990s. Applications of this newly conceived chromophore are presented in solar hydrogen-producing photocatalysis, photochemical upconversion, and photosensitization of [4 + 4] anthracene dimerization of potential interest in thermal storage of solar energy in metastable intermediates. Ultrafast transient absorption and fluorescence upconversion spectroscopic characterization of this and related CuI molecules inform the resultant photophysical properties and vice versa, so the most comprehensive structure-property understanding becomes realized when these experimental tools are collectively utilized to investigate the same series of molecules. Computationally guided structural designs generated newly conceived molecules featuring visible light-harvesting and 2,9-cycloalkane substituted complexes. The latter eventually produced record-setting excited state lifetimes in molecules leveraging both cooperative steric influence and electronic inductive effects. Using photoluminescence data from structurally homologous CuI MLCT excited states collected over 44 years, an energy gap correlation successfully modeled the data spanning a 0.3 eV emission energy range. Finally, a new research direction is revealed detailing structure-photophysical property relationships in heteroleptic CuI phenanthroline chromophores that are photoluminescent at room temperature.

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.

Twisted and Disconnected Chains: Flexible Linear Tetracuprous Arrays and a Decanuclear CuI Cluster as Blue- and Green/Yellow-Light Emitters

Defined arrays of transition metal ions embedded in tailored polydentate ligand scaffolds allow for a systematic design of their physical properties. Such molecular strings of closed-shell transition metal centers are particularly interesting for Group 11 metal ions in the oxidation state +1 if they undergo metallophilic d10···d10 contact interactions since these clusters are oftentimes efficient photoluminescence (PL) emitters. Copper is particularly attractive as a sustainable earth-abundant coinage metal source and because of the ability of several CuI complexes to serve as powerful thermally activated delayed fluorescence (TADF) emitters in molecular/organic light-emitting devices (OLEDs). Our combined synthetic, crystallographic, photophysical, and computational study describes a straight tetracuprous array possessing a centrally disconnected CuI2···CuI2 chain and a continuous helically bent CuI4 complex. This molecular helix undergoes a facile rearrangement in diethyl ether solution, yielding an unprecedented nanosized CuI10 cluster (2.9 × 2.0 nm) upon crystallization. All three clusters show either bright blue phosphorescence, TADF, or green/yellow multiband phosphorescence with quantum yields between 6.5 and 67%, which is persistent under hydrostatic pressure up to 30 kbar. Temperature-dependent PL investigations in combination with time-dependent density-functional theory (TD-DFT) calculations and void space analyses of the crystal packings complement a comprehensive correlation between the molecular structures and photoluminescence properties.

Thermally Activated Delayed Fluorescence (TADF) Materials Based on Earth-Abundant Transition Metal Complexes: Synthesis, Design and Applications

Materials exhibiting thermally activated delayed fluorescence (TADF) based on transition metal complexes are currently gathering significant attention due to their technological potential. Their application extends beyond optoelectronics, in particular organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs), and include also photocatalysis, sensing, and X-ray scintillators. From the perspective of sustainability, earth-abundant metal centers are preferred to rarer second- and third-transition series elements, thus determining a reduction in costs and toxicity but without compromising the overall performances. This review offers an overview of earth-abundant transition metal complexes exhibiting TADF and their application as photoconversion materials. Particular attention is devoted to the types of ligands employed, helping in the design of novel systems with enhanced TADF properties.

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 %.

Ligand-mediate exciton allocation enables efficient cluster-based white light-emitting diodes via single and heavy doping

Despite potential in high-resolution and low-cost displays and lighting, multi-doping structures and low concentrations (<1%) limit repeatability and stability of single-emissive-layer white light-emitting devices. Herein, we report a singly doped white-emitting system of blue thermally activated delayed fluorescence host matrix (CzAcSF) doped by yellow Cu4I4 cluster ([tBCzDppy]2Cu4I4). CzAcSF:x% [tBCzDppy]2Cu4I4 films realize photo- and electro-luminescence colors from cool white to warm white at x = 20-40. The external quantum efficiency of 23.5% was achieved at x = 30, indicating the record-high efficiency among solution-processed analogs and the largest doping concentration among efficient white light-emitting devices. It shows that di(tert-butyl)carbazole moieties in [tBCzDppy]2Cu4I4 provide high-lying excited energy levels at~2.6 eV to mediate energy transfer from CzAcSF (2.9 eV) to coordinated Cu4I4 (2.2 eV). Our results demonstrate the antenna effect of ligands on optimizing charge and energy transfer in organic-cluster systems and superiority of white cluster light-emitting diodes in practical applications.

Mechanoresponsive luminescence triggered by phase transition of a supercooled copper(I) complex

Under mechanical stimulation, a copper(I) complex in its supercooled liquid state transforms into a crystalline phase, showing a dramatic emission color change from red to green that is accompanied by a 20-fold increase in the photoluminescence quantum yield up to 87%. This reversible phase transition relies on the intriguing ability of this copper complex to form a supercooled metastable state.

Enhanced Visible Light Absorption in Heteroleptic Cuprous Phenanthrolines

This work presents a series of Cu(I) heteroleptic 1,10-phenanthroline chromophores featuring enhanced UVA and visible-light-harvesting properties manifested through vectorial control of the copper-to-phenanthroline charge-transfer transitions. The molecules were prepared using the HETPHEN strategy, wherein a sterically congested 2,9-dimesityl-1,10-phenanthrolne (mesPhen) ligand was paired with a second phenanthroline ligand incorporating extended π-systems in their 4,7-positions. The combination of electrochemistry, static and time-resolved electronic spectroscopy, 77 K photoluminescence spectra, and time-dependent density functional theory calculations corroborated all of the experimental findings. The model chromophore, [Cu(mesPhen)(phen)]+ (1), lacking 4,7-substitutions preferentially reduces the mesPhen ligand in the lowest energy metal-to-ligand charge-transfer (MLCT) excited state. The remaining cuprous phenanthrolines (2-4) preferentially reduce their π-conjugated ligands in the low-lying MLCT excited state. The absorption cross sections of 2-4 were enhanced (εMLCTmax = 7430-9980 M-1 cm-1) and significantly broadened across the UVA and visible regions of the spectrum compared to 1 (εMLCTmax = 6494 M-1 cm-1). The excited-state decay mechanism mirrored those of long-lived homoleptic Cu(I) phenanthrolines, yielding three distinguishable time constants in ultrafast transient absorption experiments. These represent pseudo-Jahn-Teller distortion (τ1), singlet-triplet intersystem crossing (τ2), and the relaxed MLCT excited-state lifetime (τ3). Effective light-harvesting from Cu(I)-based chromophores can now be rationalized within the HETPHEN strategy while achieving directionality in their respective MLCT transitions, valuable for integration into more complex donor-acceptor architectures and longer-lived photosensitizers.

Allochroic cluster light-emitting diodes based on unique μ3-tetraphosphine Cu3X3 crowns with tunable excited states

Multicomponent excited states endow copper iodide clusters with allochroic properties under diverse stimuli. However, crystal states are required, and cluster stimulus sensitivity hampers electroluminochromism. We developed PhQPCu3X3 (X = Cl, Br, and I) with the first μ3-bridging tetraphosphine ligand, whose Cu3X3 crowns were exposed to external stimulus. The increased proportion of Cu3X3 results in equal contributions of cluster- and ligand-centered components to excited states, the former of which is highly sensitive to grind, vapor, and, especially, electric stimuli, due to semi-exposed Cu3X3. Through vacuum evaporation and vapor fumigation of cluster-based emissive layers, the diodes' electroluminescence colors changed from yellow to white. Joule heat during device operation induced further color variation to orange, corresponding to Commission Internationale de l'Eclairage coordinates of PhQPCu3I3 changed from (0.44 ± 0.1, 0.34 ± 0.1) to (0.57 ± 0.1, 0.42 ± 0.1). These results demonstrate the superiority of luminescent clusters in accurate excited-state modulation, holding promise for wide applications.

Air- and Water-Stable Heteroleptic Copper (I) Complexes Bearing Bis(indazol-1-yl)methane Ligands: Synthesis, Characterisation, and Computational Studies

A series of four novel heteroleptic Cu(I) complexes, bearing bis(1H-indazol-1-yl)methane analogues as N,N ligands and DPEPhos as the P,P ligand, were synthesised in high yields under mild conditions and characterised by spectroscopic and spectrometric techniques. In addition, the position of the carboxymethyl substituent in the complexes and its effect on the electrochemical and photophysical behaviour was evaluated. As expected, the homoleptic copper (I) complexes with the N,N ligands showed air instability. In contrast, the obtained heteroleptic complexes were air- and water-stable in solid and solution. All complexes displayed green-yellow luminescence in CH2Cl2 at room temperature due to ligand-centred (LC) phosphorescence in the case of the Cu(I) complex with an unsubstituted N,N ligand and metal-to-ligand charge transfer (MLCT) phosphorescence for the carboxymethyl-substituted complexes. Interestingly, proper substitution of the bis(1H-indazol-1-yl)methane ligand enabled the achievement of a remarkable luminescent yield (2.5%) in solution, showcasing the great potential of this novel class of copper(I) complexes for potential applications in luminescent devices and/or photocatalysis.

Pyrenyl-Substituted Imidazo[4,5-f][1,10]phenanthroline Rhenium(I) Complexes with Record-High Triplet Excited-State Lifetimes at Room Temperature: Steric Control of Photoinduced Processes in Bichromophoric Systems

Photochemical applications based on intermolecular photoinduced energy triplet state transfer require photosensitizers with strong visible absorptivity and extended triplet excited-state lifetimes. Using a bichromophore approach, two Re(I) tricarbonyl complexes with 2-(1-pyrenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (pyr-imphen) and 1-(4-(methyl)phenyl)-2-(1-pyrenyl)-imidazo[4,5-f][1,10]phenanthroline (pyr-tol-imphen) showing extraordinary long triplet excited states at room temperature (>1000 μs) were obtained, and their ground- and excited-state properties were thoroughly investigated by a wide range of spectroscopic methods, including femtosecond transient absorption (fs-TA). It is worth noting that the designed [ReCl(CO)3(pyr-imphen)] (1) and [ReCl(CO)3(pyr-tol-imphen)] (2) complexes form a unique pair differing in the mutual chromophore arrangement due to introduction of a 4-(methyl)phenyl substituent into the imidazole ring at the H1-position, imposing an increase in the dihedral angle between the pyrene and {ReCl(CO)3(imphen)} chromophores. The magnitude of the electronic coupling between the pyrene and {ReCl(CO)3(imphen)} chromophores was found to be an efficient tool to tune the photophysical properties of 1 and 2. The usefulness of designed Re(I) compounds as triplet photosensitizers was successfully verified by examination of their abilities for 1O2 generation and triplet-triplet annihilation upconversion. The phosphorescence lifetimes, ∼1800 μs for 1 and ∼1500 μs for 2, are the longest lifetimes reported for Re(I) diimine carbonyl complexes in solution at room temperature.

Cubane Dimerization: Cu4 vs Cu8 Copper Iodide Clusters

Copper(I) halides are well-known for their structural diversity and rich photoluminescence properties, showing great potential for the development of solid-state lighting technology. A series of four molecular copper iodide clusters based on the [Cu4I4] cubane geometry is reported. Among them, [Cu8I8] octanuclear clusters of rare geometry resulting from dimerization of the tetranuclear counterparts were also synthesized. Two different phosphine ligands were studied, bearing either a styrene or an ethyl group. Therefore, the effect of the dimerization and of the ligand nature on the photophysical properties of the resulting clusters is investigated. The structural differences were analyzed by single-crystal X-ray diffraction (SCXRD), solid-state nuclear magnetic resonance (NMR), infrared, and Raman analyses. Compared to the ethyl group, the styrene function appears to greatly impact the photophysical properties of the clusters. The luminescence thermochromic properties of the ethyl derivatives and the intriguing photophysical properties of the clusters with styrene function were rationalized by density functional theory (DFT) calculations. Thus, the styrene group significantly lowers in energy the vacant orbitals and consequently affects the global energetic layout of the clusters. From this study, it was found that the nuclearity of copper iodide clusters eventually has less influence on the photophysical properties than the nature of the ligand. The design of proper ligands should therefore be considered when developing materials for specific lighting applications.

Design Strategies for Luminescent Titanocenes: Improving the Photoluminescence and Photostability of Arylethynyltitanocenes

Complexes that undergo ligand-to-metal charge transfer (LMCT) to d0 metals are of interest as possible photocatalysts. Cp2Ti(C2Ph)2 (where C2Ph = phenylethynyl) was reported to be weakly emissive in room-temperature (RT) fluid solution from its phenylethynyl-to-Ti 3LMCT state but readily photodecomposes. Coordination of CuX between the alkyne ligands to give Cp2Ti(C2Ph)2CuX (X = Cl, Br) has been shown to significantly increase the photostability, but such complexes are not emissive in RT solution. Herein, we investigate whether inhibition of alkyne-Ti-alkyne bond compression might be responsible for the increased photostability of the CuX complexes by investigating the decomposition of a structurally constrained analogue, Cp2Ti(OBET) (OBET = o-bis(ethynyl)tolane). To investigate the mechanism of nonradiative decay from the 3LMCT states in Cp2Ti(C2Ph)2CuX, the photophysical properties were investigated both upon deuteration and upon rigidifying in a poly(methyl methacrylate) film. These investigations suggested that inhibition of structural rearrangement may play a dominant role in increasing emission lifetimes and quantum yields. The bulkier Cp*2Ti(C2Ph)2CuBr was prepared and is emissive at 693 nm in RT THF solution with a photoluminescent quantum yield of 1.3 × 10-3 (τ = 0.18 μs). Time-dependent density functional theory (TDDFT) calculations suggest that emission occurs from a 3LMCT state dominated by Cp*-to-Ti charge transfer.

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.

Bulk Passivation Enables Hundredfold-Enhanced Electroluminescence of Monophosphine Cu4 I4 Cubes

Electroluminescent (EL) clusters emerged rapidly, owing to their organic-inorganic hybrid character useful for comprehensive performance integration and the potential for large-scale display and lighting applications. However, despite their good photoluminescent (PL) properties, until present, no efficient EL monodentate ligand-based clusters were reported due to structural variation during processing and excitation and exciton confinement on cluster-centered quenching states. Here we demonstrate an effective bulky passivation strategy for efficient cluster light-emitting diodes with a monophosphine Cu4 I4 cube named [TMeOPP]4 Cu4 I4 . With terminal pyridine groups, an active matrix named TmPyPB supports an effective host-cluster interplay for configuration fixation, structural stabilization, and exciton-confinement optimization. Compared to common inactive hosts, the passivation effects of TmPyPB markedly reduce trap-state densities by 24-40 % to suppress nonradiative decay, resulting in state-of-the-art PL and EL quantum yields reaching 99 % and 15.6 %, respectively, which are significantly improved by about 7-fold. TmPyPB simultaneously increases EL luminance to 104 nits, which is ≈100-fold that of the non-doped analogue.

Investigations on exciton recombination and annihilation in TmPyPB-ETL OLEDs using magnetic field effects

Although the effect of the electron transport layer (ETL) material TmPyPb on the electroluminescence performance of organic light-emitting diodes (OLEDs) has been extensively studied, the process of TmPyPb regulating exciton recombination and annihilation within the device is still unclear. Here, we fabricated devices of various TmPyPb thicknesses with and without ETL. Subsequently, we measured the magneto-electroluminescence (MEL) of these devices. Specifically, at the same luminance, the triplet-charge annihilation (TQA) process is more likely to occur as the thickness of TmPyPb increases, resulting in a decrease in the maximum luminance of devices. Due to electron leakage and exciton recombination region moving towards the cathode, leading to a decrease in luminance efficiency at first and then an enhancement with an increase in the thickness of TmPyPb. Furthermore, at room temperature, the application of a large bias voltage suppresses singlet fission (SF) processes by modulating the dissociation of singlet polaron pairs (PPS) and the concentration of triplet exciton (T1). This leads to the conversion of SF to the TQA process. At low temperatures, the bias voltage and temperature can regulate the concentration and lifetime of PPS and T1. Therefore, as the temperature decreases, the transition of SF → TQA → triplet-triplet annihilation (TTA) and TQA coexistence → TTA process occurs. Moreover, MEL responses of the TmPyPb-ETL device show a W-linear pattern owing to the combined effect of the hyperfine interaction (HFI) and Zeeman splitting at 145 K. Accordingly, we explored the electroluminescence (EL) performance of TmPyPB-ETL OLEDs and investigated the evolution of SF, TQA, and TTA processes using MEL. Our study revealed the effect of exciton recombination and annihilation in OLEDs with varying thicknesses of TmPyPb.

Changing Directions: Influence of Ligand Electronics on the Directionality and Kinetics of Photoinduced Charge Transfer in Cu(I)Diimine Complexes

A key challenge to the effective utilization of solar energy is to promote efficient photoinduced charge transfer, specifically avoiding unproductive, circuitous electron-transfer pathways and optimizing the kinetics of charge separation and recombination. We hypothesize that one way to address this challenge is to develop a fundamental understanding of how to initiate and control directional photoinduced charge transfer, particularly for earth-abundant first-row transition-metal coordination complexes, which typically suffer from relatively short excited-state lifetimes. Here, we report a series of functionalized heteroleptic copper(I)bis(phenanthroline) complexes, which have allowed us to investigate the directionality of intramolecular photoinduced metal-to-ligand charge transfer (MLCT) as a function of the substituent Hammett parameter. Ultrafast transient absorption suggests a complicated interplay of MLCT localization and solvent interaction with the Cu(II) center of the MLCT state. This work provides a set of design principles for directional charge transfer in earth-abundant complexes and can be used to efficiently design pathways for connecting the molecular modules to catalysts or electrodes and integration into systems for light-driven catalysis.

Interfacial Passivation Enormously Enhances Electroluminescence of Triphenylphosphine Cu4 I4 Cube

Defect is one of the key factors limiting optoelectronic performances of organic-inorganic hybrid systems. Although high-efficiency bidentate ligands based electroluminescent (EL) clusters reported, until present, only few EL clusters based on monodentate ligands are realized since their structural instability induces more surface/interface defects. Herein, this bottleneck is first overcome in virtue of interfacial passivation by electron transporting layers (ETL). Through using TmPyPB with meta-linked pyridines as ETL, photoluminescent (PL) and EL quantum efficiencies of the simplest monophosphine Cu4 I4 cube [TPP]4 Cu4 I4 are greatly improved by ≈2 and 23 folds, respectively, as well as ≈200 folds increased luminance, corresponding to a huge leap from nearly unlighted (<20 nits) to highly bright (>3000 nits). The passivation effect of TmPyPB on surface defects of cluster layer is embodied as preventing interfacial charge trapping and suppressing exciton nonradiation.

Mechanochromic and Selective Vapochromic Solid-State Luminescence of a Dinuclear Cuprous Complex

The unraveling of the stimuli-responsive mechanism is crucial to the design and precise synthesis of stimuli-responsive luminescent materials. We report herein the mechanochromic and selective vapochromic solid-state luminescence properties of a new bimetallic cuprous complex [{Cu(bpmtzH)}2(μ-dppm)2](ClO4)2 (1), and the corresponding response mechanisms are elucidated by investigating its two different solvated polymorphs 1·2CH2Cl2 (1-g) and 1·2CHCl3 (1-c). Green-emissive 1-g and cyan-emissive 1-c can be interconverted upon alternate exposure to CHCl3 and CH2Cl2 vapors, which is principally attributable to a combined alteration of both intermolecular NHbpmtzH···OClO3- hydrogen bonds and intramolecular "triazolyl/phenyl" π···π interactions induced by different solvents. Solid-state luminescence mechanochromism present in 1-g and 1-c is mainly ascribed to the grinding-induced breakage of the NHbpmtzH···OClO3- hydrogen bonds. It is suggested that intramolecular π···π-triazolyl/phenyl interactions are affected by different solvents but not by grinding. The results provide new insights into the design and precise synthesis of multi-stimuli-responsive luminescent materials by the comprehensive use of intermolecular hydrogen bonds and intramolecular π···π interactions.

Efficient fluorescence-enhanced probe for cyanide ions based on a tetraphenylethene pyridine coordinated copper-iodide complex

An efficient fluorescence-enhanced probe was developed for detecting cyanide ions (CN^-) based on a tetraphenylethene coordinated copper-iodide complex (named CIT-Z). The coordination polymers (CPs) prepared were (Z)-1,2-diphenyl-1,2-bis[4-(pyridin-3-ylmethoxy)phenyl]ethene (1Z) and a CuI cluster, where the tetraphenylethylene (TPE) pyridine derivatives acted as organic ligands and the CuI cluster acted as a metal center. The higher-dimensional CIT-Z exhibited a 3-fold-interpenetrating network structure with excellent optical properties and chemical stability. This study also provides insights into the mechanism behind the fluorescence enhancement, which is attributed to the competitive coordination between CN^- and the ligands. The probe showed high selectivity and sensitivity towards CN^-, with a detection limit of 0.1 μM and good recovery in the real water samples.

Intramolecular charge transfer enables highly-efficient X-ray luminescence in cluster scintillators

Luminescence clusters composed of organic ligands and metals have gained significant interests as scintillators owing to their great potential in high X-ray absorption, customizable radioluminescence, and solution processability at low temperatures. However, X-ray luminescence efficiency in clusters is primarily governed by the competition between radiative states from organic ligands and nonradiative cluster-centered charge transfer. Here we report that a class of Cu₄I₄ cubes exhibit highly emissive radioluminescence in response to X-ray irradiation through functionalizing biphosphine ligands with acridine. Mechanistic studies show that these clusters can efficiently absorb radiation ionization to generate electron-hole pairs and transfer them to ligands during thermalization for efficient radioluminescence through precise control over intramolecular charge transfer. Our experimental results indicate that copper/iodine-to-ligand and intraligand charge transfer states are predominant in radiative processes. We demonstrate that photoluminescence and electroluminescence quantum efficiencies of the clusters reach 95% and 25.6%, with the assistance of external triplet-to-singlet conversion by a thermally activated delayed fluorescence matrix. We further show the utility of the Cu₄I₄ scintillators in achieving a lowest X-ray detection limit of 77 nGy s^-1 and a high X-ray imaging resolution of 12 line pairs per millimeter. Our study offers insights into universal luminescent mechanism and ligand engineering of cluster scintillators.

Mono-, Bis-, and Tris-Chelate Zn(II) Complexes with Imidazo[1,5-a]pyridine: Luminescence and Structural Dependence

New mono-, bis-, and tris-chelate Zn(II) complexes have been synthesized starting from different Zn(II) salts and employing a fluorescent 1,3-substituted-imidazo[1,5-a]pyridine as a chelating ligand. The products have been characterized by single-crystal X-ray diffraction; mass spectrometry; and vibrational spectroscopy. The optical properties have been investigated to compare the performances of mono-, bis-, and tris-chelate forms. The collected data (in the solid state and in solution) elucidate an important modification of the ligand conformation upon metal coordination; which is responsible for a notable increase in the optical performance. An intense modification of the emission quantum yield along the series in the solid state is observed comparing mono-, bis-, and tris-chelate adducts; independently from the anionic ligand introduced by ionic exchange.

DFT study of UV-vis-properties of thiophene-containing Cu(β-diketonato)2 - Application for DSSC

Experimental and theoretically calculated UV-vis properties of three Cu(β-diketonato)₂ complexes are presented. The Cu(β-diketonato)₂ contains β-diketones without (β-diketone = acetylacetone, (CH₃)COCH₂CO(CH₃), complex (1)), with one (β-diketone = thenoyltrifluoroacetone, (CF₃)COCH₂CO(C₄H₃S), complex (2)) and with two thiophene (β-diketone = (CF₃)COCH₂CO(C₄H₂S) (C₄H₃S), complex (3)) groups. More thiophenes on the β-diketonato ligand of Cu(β-diketonato)₂, lead to a red shift of the experimental absorbance maxima of the UV-vis of the complex, from 295 nm for complex (1), to 340 nm for complex (2) to 390 nm for complex (3). Theoretical time dependant density functional theory calculations indicate that both the two strongest absorbance peaks of the ultraviolet-visible spectrum of Cu(acetylacetonato)₂ are mainly ligand-to-metal charge-transfer excitations. However, the absorbance maxima of the UV-vis of thiophene-containing Cu(β-diketonato)₂ are mainly ligand-to-ligand charge-transfer excitations. Calculated properties such as light harvesting energy (LHE = 0.47, 0.94 and 0.99 for (1)-(3) respectively), driving force for electron injection (ΔG_inject = 1.43, 0.76 and 0.63 for (1)-(3) respectively), and driving force of dye regeneration (ΔG_regenerate = 1.85, 2.16 and 1.49 for (1)-(3) respectively), are favourable for (1)-(3) to be considered as dyes in DSSCs. However, some structural modifications are needed to prevent intramolecular charge recombination after excitation.

Perspectives on the Structural Design and Luminescent Behavior of Liquid Crystalline Materials Based on Copper(I) Complexes

This paper provides insight into the various studies that have been carried out to date on liquid crystalline materials based on copper(I) complexes. Although the study of copper(I) complexes with respect to their liquid crystalline property is quite limited, metallomesogens prepared with different structural components and ligands from groups such as azamacrocycles, alkythiolates, ethers, isocyanides, phenanthroline, Schiff bases, pyrazolates, phosphines, biquinolines, and benzoylthioureas are reported and summarized in this review. A special section is dedicated to the discussion of emission properties of copper(I) metallomesogens.

A new subclass of copper(I) hybrid emitters showing TADF with near-unity quantum yields and a strong solvatochromic effect

We introduce here a new subclass of copper(I) hybrid emitters simultaneously containing [Cu_xI_y]^z- anions and Cu⁺ cations, separated in space by a Janus head ligand. When UV-irradiated at 298 K, these unique "Two-In-One" hybrids exhibit a short-lived green TADF with near-unity quantum yield and a strong solvatochromic effect. Moreover, they manifest a strong radioluminescence upon X-ray irradiation. These findings open up new possibilities for the design of highly performing TADF materials.

A family of CuI-based 1D polymers showing colorful short-lived TADF and phosphorescence induced by photo- and X-ray irradiation

Exploiting 2-(alkylsulfonyl)pyridines as 1,3-N,S-ligands, herein we have constructed 1D CuI-based coordination polymers (CPs) bearing unprecedented (CuI)_n chains and possessing remarkable photophysical properties. At room temperature, these CPs show efficient TADF, phosphorescence or dual emission in the deep-blue to red range with outstandingly short decay times of 0.4-2.0 μs and good quantum performance. Owing to great structural diversity, the CPs demonstrate a variety of emissive mechanisms, spanning from TADF of ¹(M + X)LCT type to ³CC and ³(M + X)LCT phosphorescence. Moreover, the designed compounds emit strong X-ray radioluminescence with the quantum efficiency of up to an impressive 55% relative to all-inorganic BGO scintillators. The presented findings push the boundaries in designing TADF and triplet emitters with very short decay times.

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.

A stable superatomic Cu6(SMPP)6 nanocluster with dual emission

We have synthesized single crystals of a highly stable Cu₆ nanocluster protected by six ligands of 2-mercapto-5-n-propylpyrimidine (SMPP). This Cu₆(SMPP)₆ cluster has a quasi-octahedral superatomic Cu₆ core, with the Cu atoms being protected by both -S- and N-bidentate coordination of the SMPP ligands. Interestingly, each Cu atom is linked with an N atom, while the two neighboring Cu atoms on the same triangular facet are linked by the -S- bridge of the ligand. Single-crystal parsing results show that the altered orientation of the SMPP ligands give rise to three packing modes (named as 1, 2, and 3) of the Cu₆(SMPP)₆ NCs. Apart from the well-organized coordination, this Cu₆(SMPP)₆ nanocluster exhibits superatomic stability with a metallic core of 4 valence electrons (1S²2S²||3S²), enabling to largely balance the interactions between the polynuclear core and delocalized electrons. Interestingly, the Cu₆(SMPP)₆ NCs display dual emissions in both ultraviolet-visible (UV-Vis) and near-infrared (NIR) regions. First-principles calculations well reproduce the experimental spectrum, shedding light on the nature of excitation states and metal-ligand interactions in the Cu₆(SMPP)₆ cluster.

Emission and Luminescent Vapochromism Control of Octahedral Cu4 I4 Complexes by Conformationally Restricted P,N Ligands

A conformationally restricted P,N-ligand capable of the design of polynuclear copper(I) complexes was synthesized via the reaction of primary pyridylphosphine, paraformaldehyde, and benzhydrylamine. The reaction of the ligand with copper(I) iodide leads to the tetranuclear copper(I) complex with the octahedral type of copper-iodide core. Different orientation of coordination bonds of the ligands relative to the P,N₂ -heterocyclic fragments and to the Cu₄ I₄ cores leads to the existence of two types of conformers of the complex with "compact" or "stretched" geometry of the Cu₄ I₄ cluster. This lability of the complex allowed for obtaining two crystalline phases displaying green or red luminescence. The TDDFT computations along with XRD structural analysis gave a strong interpretation of the green emission belonging to the "compact" form of the complex and belonging of the red emission to the "stretched" form. Moreover, both crystalline phases demonstrate the strong vapochromic responses of luminescence on the vapors of wide range of solvents.

Structure and emission properties of dinuclear copper(i) complexes with pyridyltriazole

A new series of five highly emissive binuclear heteroleptic pyridyltriazole-Cu(i)-phosphine complexes 1-5 was synthesized and examined by different experimental (IR, elemental and thermogravimetric analysis, single crystal X-ray diffraction technique, UV-vis and fluorescence spectroscopy) and quantum chemical aproaches. Complexes 1-5 exhibited excellent stimuli-responsive photoluminescent performance in the solid state at room temperature (quantum yield (QY) = 27.5-52.0%; lifetime (τ) = 8.3-10.7 μs) and when the temperature was lowered to 77 K (QY = 38.3-88.2; τ = 17.8-134.7 μs). The highest QY was examined for complex 3 (52%) that can be explained by the small structural changes between the ground S₀ and exited S₁ and T₁ states leading to the small S₁-T₁ triplet gap and efficient thermally-activated delayed fluorescence. Moreover, complex 4 demonstrates reversible mechanochromic and excitation dependent luminescence.

Effect of non-covalent self-dimerization on the spectroscopic and electrochemical properties of mixed Cu(i) complexes

A series of six new Cu(i) complexes with ([Cu(N-{4-R}pyridine-2-yl-methanimine)(PPh₃)Br]) formulation, where R corresponds to a donor or acceptor p-substituent, have been synthesized and were used to study self-association effects on their structural and electrochemical properties. X-ray diffraction results showed that in all complexes the packing is organized from a dimer generated by supramolecular π stacking and hydrogen bonding. ¹H-NMR experiments at several concentrations showed that all complexes undergo a fast-self-association monomer-dimer equilibrium in solution, while changes in resonance frequency towards the high or low field in specific protons of the imine ligand allow establishing that dimers have similar structures to those found in the crystal. The thermodynamic parameters for this self-association process were calculated from dimerization constants determined by VT-¹H-NMR experiments for several concentrations at different temperatures. The values for K _D (4.0 to 70.0 M^-1 range), ΔH (-1.4 to -2.6 kcal mol^-1 range), ΔS (-0.2 to 2.1 cal mol^-1 K^-1 range), and ΔG ₂₉₈ (-0.8 to -2.0 kcal mol^-1 range) are of the same order and indicate that the self-dimerization process is enthalpically driven for all complexes. The electrochemical profile of the complexes shows two redox Cu(ii)/Cu(i) processes whose relative intensities are sensitive to concentration changes, indicating that both species are in chemical equilibrium, with the monomer and the dimer having different electrochemical characteristics. We associate this behaviour with the structural lability of the Cu(i) centre that allows the monomeric molecules to reorder conformationally to achieve a more adequate assembly in the non-covalent dimer. As expected, structural properties in the solid and in solution, as well as their electrochemical properties, are not correlated with the electronic parameters usually used to evaluate R substituent effects. This confirms that the properties of the Cu(i) complexes are usually more influenced by steric effects than by the inductive effects of substituents of the ligands. In fact, the results obtained showed the importance of non-covalent intermolecular interactions in the structuring of the coordination geometry around the Cu centre and in the coordinative stability to avoid dissociative equilibria.

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.

Effect of the dangling aromatic ring on neutral luminescent bis(phosphine) Cu(i)/Ag(i) complexes with the asymmetric pyridyl-tetrazolate ligands

A series of neutral luminescent bis(phosphine) Cu(i) complexes of pyridyl-tetrazolate ligands (L1-L3) with the general formula [CuI(L n )(P^P)] (1-6) were synthesized, which have been well characterized by IR, UV/vis, CV, 1H NMR and 31P NMR. For comparison, an Ag(i) complex [AgI(L2)(PPh3)2] (7) was also synthesized. The crystal structures of 2 and 7 have been further determined by X-ray crystallography. All these Cu(i) compounds show bright luminescence in the solid state with photoluminescence quantum yields (PLQYs) in the range of 25.8% to 85.0%. More interestingly, the Cu(i) complexes bearing an additional dangling aromatic ring on the diimine ligands exhibit enhanced luminescent performance in various solutions and their PLQYs are significantly higher than those of related Cu(i) complexes without steric protection. Compared with 1, the Cu(i) complexes with an additional dangling tetrazole moiety show a significant solvatochromic effect, which is uncommon for luminescent Cu(i) complexes. Moreover, [CuI(L2)(PPh3)2] (2) was further designed as an OLED material, which showed a high external quantum efficiency of 7.7%.

Intense Photoinduced Intervalence Charge Transfer in High-Valent Iron Mixed Phenolate/Carbene Complexes

We report high-valent iron complexes supported by N-heterocyclic carbene (NHC)-anchored, bis-phenolate pincer ligands that undergo ligand-to-metal charge transfer (LMCT) upon photoexcitation. The resulting excited states - with a lifetime in the picosecond range - feature a ligand-based, mixed-valence system and intense intervalence charge transfer bands in the near-infrared region. Upon oxidation of the complex, corresponding intervalence charge transfer absorptions are also observed in the ground state. We suggest that the spectroscopic hallmarks of such LMCT states provide useful tools to decipher excited-state decay mechanisms in high-valent NHC complexes. Our observations further indicate that NHC-anchored, bis-phenolate pincer ligands are not sufficiently strong donors to prevent the population of excited metal-centered states in high-valent iron complexes.

Synthesis of a Novel Unexpected Cu(II)–Thiazolidine Complex—X-ray Structure, Hirshfeld Surface Analysis, and Biological Studies

An unexpected trinuclear Cu(II)–thiazolidine complex has been synthesized by mixing CuCl₂·2H₂O with the Schiff base ligand, 1-(((4,5-dihydrothiazol-2-yl)ethylidene)hydrazono)methyl)phenol L, in ethanol. Unexpectedly, the reaction proceeded via the hydrolysis of the Schiff base L, followed by cyclization to afford 3-methyl-5,6-dihydrothiazolo[3,2-c][1,2,3]triazole (L^a), then complexation with the Cu(II) salt, forming the trinuclear [Cu₃(L^a)₄(Cl)₆] complex. The complex was characterized by means of FTIR spectra, elemental analysis, and X-ray crystallography. In the trinuclear [Cu₃(L^a)₄(Cl)₆] complex, there are two crystallographically independent hexa- and penta-coordinated Cu(II) sites, where the thiazolidine ligand L^a units act as a monodentate ligand and a linker between the Cu(II) centers. The crystal packing of the [Cu₃(L^a)₄(Cl)₆] complex is primarily affected by the weak non-covalent C-H∙∙∙Cl interactions. In accordance with Hirshfeld surface analysis, the Cl∙∙∙H, H∙∙∙H, S∙∙∙H, and N∙∙∙H percentages are 31.9%, 27.2%, 13.5%, and 9.9%, respectively. X-ray photoelectron spectroscopy confirmed the oxidation state of copper as Cu(II), as well as the presence of two different coordination environments around copper centers. The complex showed interesting antibacterial activity against the Gram-positive bacteria S. subtilis, with MIC = 9.7 µg/mL compared to MIC = 4.8 µg/mL for the control, gentamycin. Moreover, the Cu(II) complex showed an equal MIC (312.5 µg/mL) against C. albicans compared to ketoconazole. It also exhibits a very promising inhibitory activity against colon carcinoma (IC₅₀ = 3.75 ± 0.43 µg/mL).

New Approach toward Dual-Emissive Organic-Inorganic Hybrids by Integrating Mn(II) and Cu(I) Emission Centers in Ionic Crystals

Inorganic-organic hybrid luminescent materials have received great attention for their potential applications in a wide range of clean/renewable energy-related areas, including photovoltaics and solid-state lighting. Herein, we present a unique and general "Mn + Cu" approach by blending two earth-abundant luminogenic metals, manganese and copper, within a single ionic structure to construct a remarkable family of low-cost and multifunctional hybrid materials featuring dual emission, as well as triboluminescence and second-harmonic generation response. The novel hybrid materials are made of diphosphine dioxide-chelated [Mn(O∧O)₃]²⁺ cations and various anionic [Cu_xI_y]^(y-x)- clusters, ensuring manifestation of dual phosphorescence streamed from octahedral Mn²⁺ ions (605-648 nm) and iodocuprate anions (480-728 nm). Noteworthily, the relative ratio of the emission bands, and hence a resulting emission chromaticity, can be tuned in a wide range through modification of cluster [Cu_xI_y]^(y-x)- modules. The structural diversity, enhanced robustness, and up to 100% luminescence quantum yield make the designed materials promising phosphors for lighting and sensing applications.

Photochemistry and Photophysics of Charge-Transfer Excited States in Emissive d10/d0 Heterobimetallic Titanocene Tweezer Complexes

Transition-metal complexes that undergo ligand-to-metal charge transfer (LMCT) to d⁰ metals are of interest as possible photocatalysts due to the lack of deactivating d-d states. Herein, the synthesis and characterization of nine titanocene complexes of the formula Cp₂Ti(C₂Ar)₂·MX (where Ar = phenyl, dimethylaniline, or triphenylamine; and MX = CuCl, CuBr, or AgCl) are presented. Solid-state structural characterization demonstrates that MX coordinates to the alkyne tweezers and CuX coordination has a greater structural impact than AgCl. All complexes, including the parent complexes without coordinated MX, are brightly emissive at 77 K (emission max between 575 and 767 nm), with the coordination of MX redshifting the emission in all cases except for the coordination of AgCl into Cp₂Ti(C₂Ph)₂. TDDFT investigations suggest that emission is dominated by arylalkynyl-to-titanium ³LMCT in all cases except Cp₂Ti(C₂Ph)₂·CuBr, which is dominated by CuBr-to-Ti charge transfer. In room-temperature fluid solution, only Cp₂Ti(C₂Ph)₂ and Cp₂Ti(C₂Ph)₂·AgCl are emissive, albeit with photoluminescent quantum yields ≤2 × 10^-4. The parent complexes photodecompose in room-temperature solution with quantum yields, Φ_rxn, between 0.25 and 0.99. The coordination of MX decreases Φ_rxn by two to three orders of magnitude. There is a clear trend that Φ_rxn increases as the emission energy increases. This trend is consistent with a competition between energy-gap-law controlled nonradiative decay and thermally activated intersystem crossing between the ³LMCT state and the singlet transition state for decomposition.

Thermally activated delayed fluorescence in luminescent cationic copper(i) complexes

In this work, the photophysical characteristics of [Cu(N^N)2]+ and [Cu(N^N)(P^P)]+ complexes were described. The concept of thermally activated delayed fluorescence (TADF) and its development throughout the years was also explained. The importance of ΔE (S1-T1) and spin-orbital coupling (SOC) values on the TADF behavior of [Cu(N^N)2]+ and [Cu(N^N)(P^P)]+ complexes is discussed. Examples of ΔE (S1-T1) values reported in the literature were collected and some trends were proposed (e.g. the effect of the substituents at the 2,9 positions of the phenanthroline ligand). Besides, the techniques (or calculation methods) used for determining ΔE (S1-T1) values were described. The effect of SOC in TADF was also discussed, and examples of the determination of SOC values by DFT and TD-DFT calculations are provided. The last chapter covers the applications of [Cu(N^N)2]+ and [Cu(N^N)(P^P)]+ TADF complexes and the challenges that are still needed to be addressed to ensure the industrial applications of these compounds.

Overcoming Efficiency Limitation of Cluster Light-Emitting Diodes with Asymmetrically Functionalized Biphosphine Cu4I4 Cubes

Electroluminescent (EL) nanoclusters holding promise for new-generation cluster light-emitting devices (CLEDs) rapidly emerge. However, slow radiation and serious quenching of cluster emitters largely limit the device performance. Herein, we report two monofunctionalized biphosphine chelated Cu₄I₄ clusters [DMACDBFDP]₂Cu₄I₄ and [DPACDBFDP]₂Cu₄I₄. The asymmetric modification and electron-donating effect of acridine groups lead to the iodine-to-ligand charge transfer predominant excited states of the clusters, which feature thermally activated delayed fluorescence with markedly improved singlet radiative rate constants and reduced triplet nonradiative rate constants. As consequence, compared to the nonfunctionalized parent cluster, [DPACDBFDP]₂Cu₄I₄ achieves 16-fold increased photoluminescence (81%) and 20-fold increased EL (19.5%) quantum efficiencies. Such new-record efficiencies make CLEDs achieve the state-of-the-art performance of all kinds of EL technologies.