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

Mitochondria-targeted and near-infrared phosphorescent Ir(III) complexes for specific detection of Hg2+ and photodynamic therapy

S: Mercury ions (Hg2+) are highly toxic and prone to bioaccumulation, showing a strong attraction to proteins and enzymes that contain sulfur. Even minute quantities of Hg2+ can lead to severe health issues. Given that mitochondria are a primary target organelle of Hg2+, it is essential to create a probe that can accurately detect Hg2+ within intracellular mitochondria. In this study, we developed two innovative Ir(III) complex probes that emit near-infrared light. The crystal structure of Ir2 was determined using X-ray techniques, which reveals that Ir2 contains a pyridine group capable of recognizing Hg2+ and targeting mitochondria, allowing for the precise identification of Hg2+ both in vitro and within the mitochondria of living cells. Additionally, these two novel near-infrared phosphorescent Ir(III) complexes demonstrate significant capabilities in producing ROS including singlet oxygen, ·O2- and ·OH, which renders them effective photosensitizers under visible light exposure for photodynamic therapy (PDT). This research offers a promising approach for detecting Hg2+ in vitro and in the mitochondrial microenvironment of living cells, which have some implications for the future development of pertinent transition metal complexes for mitochondria-targeted photodynamic therapy in cancer cells.

Synthesis and Characterization of Copper(I) Halide Heteroleptic Complexes with Thermally Activated Delayed Fluorescence

In the global context of green chemistry and sustainable development, luminescent copper(I) halide complexes hold broad applications, attributed to their abundant resources and excellent photophysical properties. Herein, two novel copper(I) halide complexes were synthesized and systematically investigated using single-crystal X-ray diffraction, photophysical characterization, and theoretical calculations among other methods. Both solid-state complexes exhibit bright green luminescent emissions with low self-absorption rates. The photoluminescence quantum yields (PLQYs) are as high as 95% and 87%, respectively. Because their singlet-triplet energy gaps (ΔEST) are very small, both being 0.10 eV, these complexes can achieve thermally activated delayed fluorescence emission through efficient reverse intersystem crossing. Theoretical calculations revealed that their high-efficiency luminescence arises from the synergistic effects of metal-to-ligand charge transfer, halogen-to-ligand charge transfer, and ligand-to-ligand charge transfer.

Recent advances in active chromophores for detecting gamma-hydroxybutyric acid (GHB)-related illicit drugs

Gamma-hydroxybutyric acid (GHB) and its related illicit drugs are of particular forensic interest owing to their abuse as recreational drugs and implications in drug-facilitated sexual assault. The rapid and complete metabolism of GHB in the body results in a short evidence collection window for forensic experts, and challenges exist in simultaneously differentiating between exogenous addition in spiked drinking and low endogenous levels of GHB. Consequently, the development of real-time and on-site detection strategies for GHB plays vital roles in tackling drug-facilitated crimes. Recently, fluorescent and colorimetric strategies have emerged as promising approaches in this field, offering multiple merits of high sensitivity and specificity, ease of handling, and cumulative signaling effects. This minireview outlines the endogenous levels of GHB in the body and possible metabolism pathways, summarizes the recent advances in active chromophores, elucidates the corresponding sensing characteristics, and then exemplifies the developed sensing strips and detection kits based on the optimized chromophores mostly in the past five years. Additionally, the perspectives of the relevant studies are discussed in detail.

Molecular engineering and electrolyte optimization strategies for enhanced performance of Ru(ii) polypyridyl-sensitized DSSCs

Dye-sensitized solar cells (DSSCs) are a leading third-generation solar cell technology due to their low cost, ease of fabrication, and tunable photoelectrochemical properties. Among DSSC components, the photosensitizer plays a crucial role in light absorption and charge generation, with Ru(ii)-polypyridyl complexes standing out due to their superior photostability, broad absorption spectra, and efficient charge injection. This review provides a comprehensive analysis of molecular engineering strategies for Ru(ii)-polypyridyl photosensitizers, emphasizing ligand modifications to design and develop novel Ru(ii) photosensitizers with prolonged excited-state lifetimes, reduced charge recombination, enhanced light-harvesting capabilities, and improved overall solar-to-power conversion efficiency (PCE). In addition, cyclometallated polypyridyl Ru(ii) complexes are explored as promising alternatives to Ru(ii) complexes incorporating labile thiocyanate (SCN) ligands for DSSCs, which offer improved stability. The relationship between the molecular structure of Ru(ii) photosensitizers and their photovoltaic characteristics is analyzed by examining key factors that influence their photovoltaic performance, including light-harvesting efficiency, fine-tuning ground and excited state oxidation potentials (GSOP/ESOP), extending excited state lifetimes, and minimizing charge recombination. Additionally, the impact of co-adsorbents, electrolyte additives, and interfacial engineering on DSSC performance is explored. Emphasis is placed on optimizing redox electrolytes beyond conventional iodide/triiodide (I-/I- 3) systems to minimize energy loss and enhance PCE. By carefully considering those challenges, this review lays the groundwork for the rational design of next-generation DSSCs that are more efficient, stable, and commercially viable.

N-Alkyl Chain Induced Molecular Aggregation in Mononuclear Gold(I)-N-Heterocyclic Carbene Complexes for Blue Light Emitting Applications

The gold(I) N-heterocyclic carbene (NHC) molecule exhibits outstanding luminescent properties along with high thermal stability due to its strong carbene gold bond. Gold(I)-NHC complexes are known to emit green to yellow color, while blue emission is limited. Herewith, we report the photoluminescence and thermal stability correlation between blue emitting mononuclear gold(I)-NHC chloride complexes, N-(9-anthracenyl)-N'(n-alkyl)imidazol-2-ylidene gold(I) chloride (alkyl=n-butyl (1), n-pentyl (2), n-hexyl (3)). These complexes were synthesized by transmetallation route and characterized by FT-IR, NMR, TGA, and single crystal X-ray diffraction analysis. The solid-state study reveals a unique molecular packing due to hydrogen bonding, aurophilic interaction, and CH⋅⋅⋅π interactions in all 1-3. In addition, the π⋅⋅⋅π stacking has been found in 1. All these complexes show good thermal stability due to the strong metal-ligand bond strength along with crystal packing. In addition, 1-3 emits a blue colour in both solution and crystalline state. The quantum yield in the crystalline state was found to be higher for 1 (22.44 %) compared to 2 (9.90 %), and 3 (14.98 %). A similar high quantum yield for blue-emitting gold(I)-NHC chloride complex is rare. This promising quantum yield for 1 can be ascribed to the crystal packing effect. The theoretical calculations were carried out to understand the electronic and structural properties of 1-3. Computational studies revealed the origin of the luminescence behaviour of complexes. The blue light emitting thin film and LED are demonstrated using 1 exhibited the emission peak at the blue region with Commission Internationale de L'Eclairage (CIE) coordinates near the National Television Standards Committee (NTSC) value.

Iridium(III) Blue Phosphors with Heteroleptic Carbene Cyclometalates: Isomerization, Emission Tuning, and OLED Fabrications

Ir(III) complexes are particularly noted for their excellent photophysical properties in giving blue OLED phosphors. In this study, two distinctive carbene pro-chelates LAH2 + and LBH2 + (or LCH2 +) were employed in preparation of heteroleptic Ir(III) complexes, to which LAH2 + bears a cyano substituted benzoimidazolium along with N-mesityl appendage, while LBH2 + (or LCH2 +) carries the symmetrical benzoimidazolium entity. Notably, the reversible equilibration at high temperature was observed for m, f-ct14 and m, f-ct15 with a single LA chelate. In contrast, only the mer-substituted m-ct16 was obtained upon employing two LA chelates. All Ir(III) complexes exhibited blue photoluminescence (ΦPL ≥ ${\ge }$ 78 %) with short radiative lifetimes (τrad ≤ ${\le }$ 1.05 μs) in solution. The Ph OLED device with m-ct16 afforded an external quantum efficiency (EQE) of 22.8 % at 5000 cd ⋅ m-2. Moreover, the hyper-OLED based on m-ct16 and v-DABNA exhibited EQE1000 of 32.1 % (EQE recorded at 1000 cd ⋅ m-2) and J90 of 15.0 mA cm-2 (current density at 90 % of max. EQE). Its suppressed efficiency roll-off (EQE of 32.1 % and 27.7 % at 1000 cd ⋅ m-2 and 10000 cd ⋅ m-2) demonstrated a milestone in fabrication of blue OLED devices.

Deep Blue Tetradentate Pt(II) Emitter Coordinated With Fused Fluorenyl N-heterocyclic Carbene. High Efficiency, Narrow FWHM, and Superior Operational Lifetime LT95 of 290 h at 1000 cd m-2

Blue tetradentate Pt(II) emitters and the corresponding organic light-emitting diodes (OLEDs) are promising for next-generation high-resolution displays. However, developing blue Pt(II) emitters that simultaneously achieve high efficiency, high color purity, and excellent operational stability remains challenging. In this study, we developed two new high-performance deep blue tetradentate Pt(II) emitters (Pt1 and Pt2) whose ligands contain 1-(3,5-di-tert-butylphenyl)-9,9-dimethyl-1,9-dihydrofluoreno[2,3-d]imidazolium or 3-(3,5-di-tert-butylphenyl)-9,9-dimethyl-3,9-dihydrofluoreno[2,3-d]imidazolium carbene moiety (DMFI). These Pt(II) emitters display excellent photophysical characteristics including high photoluminescence quantum yield (PLQY) of up to 0.90, narrow full width at half maximum (FWHM), short emission lifetime, and high horizontal transition dipole ratio of up to 0.80. As a result, deep blue phosphorescent OLEDs are fabricated with a narrow FWHM of 21 nm, a high maximum external quantum efficiency (EQEmax) of 28.6%, and Commission Internationale de l'Éclairage (CIE) coordinates of (0.13, 0.15). Even more impressively, the devices based on the deuterated co-host system and different device structures achieved an unprecedented excellent device lifetime LT95 of 290 h at an initial luminance of 1000 cd m-2, an EQEmax of 20.8%, and CIE coordinates of (0.14, 0.17).

Carbazolylpyridine (cp)-based tetradentate platinum(II) complexes containing fused 6/5/6 metallocycles

A series of carbazolylpyridine (cp)-based 6/5/6 Pt(II) complexes featuring tetradentate ligands with nitrogen or oxygen atoms as bridging groups was designed and synthesized, and the bridging nitrogen atoms were derived from acridinyl (Ac), azaaceridine (AAc) and carbazole (Cz). Systematic experimental and theoretical studies reveal that the ligand structures have a significant effect on the electrochemical, photophysical and excited state properties of these complexes. Their oxidation processes mainly occur on the carbazole-Pt moieties, whereas the reduction processes typically occur on the electron-deficient pyridine (Py) moieties. Time-dependent density functional theory (TD-DFT) and natural transition orbital (NTO) calculations reveal that the cp-based Pt(II) complexes have a metal-to-ligand charge transfer (3MLCT) state mixed with ligand-centered (3LC) and intra-ligand charge-transfer (3ILCT) characteristics. Pt(cp-1) shows strong red luminescence with a dominant peak at 611 nm and an excited-state lifetime of 10.7 μs in dichloromethane at room temperature, 602 nm and 10.9 μs in toluene, and 602 nm and 8.2 μs in PMMA films. It also exhibits high photoluminescence quantum efficiencies of 85%, 84% and 60% in dichloromethane, toluene and PMMA, respectively. These studies indicate the potential application of the cp-based Pt(II) complexes as phosphorescent emitters in the field of organic light-emitting diodes (OLEDs).

Pushing the limits of electron donation for cis-chelating ligands via an alliance of phosphonium ylide and anionic abnormal NHC

The grafting of a -(CH2)2PR3+ moiety on an NHC ligand backbone in the Mn(I) complex [Cp(CO)2Mn(IMes)] followed by double deprotonation opens a route to bidentate ligands with extreme electron-donating character. Such remarkable electronic properties can even allow intramolecular sp2 C-H functionalization in typically inert square-planar Rh(I) dicarbonyl complexes.

NHC-Based Deep-Red Phosphorescent Iridium Complexes Featuring Three-Charge (0, -1, -2) Ligands

N-heterocyclic carbene (NHC)-based phosphorescent iridium complexes have attracted extensive attention due to their good optical properties and high stability in recent years. However, currently reported NHC-based iridium complexes can easily achieve emission of blue, green, or even ultraviolet light, while emission of red or deep-red light is relatively rare. Here, we report a new family of NHC-based deep-red iridium complexes (Ir1, Ir2, Ir3, and Ir4) featuring three-charge (0, -1, -2) ligands. The single-crystal structures confirm that all complexes exhibit a trans-C-N configuration between the NHC carbon atom of the monoanionic (-1) ligand and the nitrogen atom of the neutral (0) ligand and that there are abundant intermolecular and intramolecular interactions in the crystalline state. Notably, all complexes exhibited an effective deep-red emission (650-664 nm). Moreover, the iridium complexes (Ir2 and Ir4) based on benzimidazol-2-ylidene (pmb) exhibited a higher emission efficiency and longer emission lifetime than the corresponding iridium complexes (Ir1 and Ir2) based on imidazol-2-ylidene (pmi), respectively. Density functional theory calculations demonstrate that the pmb ligand of Ir3 and Ir4 is more involved in the excited state than the pmi ligand of Ir1 and Ir2, which is caused by the stronger electron-donating ability of the pmb ligand. Considering better optical properties, Ir2 and Ir4 were eventually used as dopant emitters of the optical light-emitting diode (OLED) devices to obtain good maximum external quantum efficiency (8.5 and 10.1%) in the deep-red region (628 and 624 nm) with Commission Internationale deL'Eclairage (CIE) coordinates of (0.65, 0.34) and (0.63, 0.36), respectively, with a low turn-on voltage (2.4 V). This research provides an important idea for the design and optoelectronic applications of NHC-based deep-red phosphorescent iridium complexes.

Fluorescent zinc(II) thione and selone complexes for light-emitting applications

Three 1-(anthracene-9-ylmethyl)-3-isopropyl-imidazol-2-thione Zn(II) halide complexes (1-3) and one 1-(anthracene-9-ylmethyl)-3-isopropyl-imidazol-2-selone Zn(II) dichloride complex (4) were synthesized and characterized. Complexes 2, 3, and 4 exhibited distorted tetrahedral geometries, while complex 1 adopted a regular tetrahedral geometry. All these complexes displayed emission in the crystalline state, with complex 3 emitting in the yellow region and complex 1 and 4 in the blue region, while complex 2 gave a bluish-green emission. The ligands L1 and L2, however, showed no emission in the solution and crystalline state. The photophysical properties of these four complexes were studied, and their quantum yields in the crystalline states were determined. Complex 1 exhibited the highest quantum yield of 7.72%, and complexes 2 and 3 demonstrated 5.95% and 5.07% yield, respectively. Complex 4 exhibited a relatively lower quantum yield of 3.87%. The crystalline state quantum yields for the complexes were found to vary with the variation of the halide ion coordinated to the metal center, following the trend Cl- > Br- > I-. The quantum yield for the thione complexes 1-3 was found to be superior to that of the selone complex 4. Density functional theory calculations were performed to study their structural properties and emissive nature. TD-DFT natural transition orbital calculations revealed that the observed emission behaviour is primarily driven by intra-ligand charge transfer (1ILCT) mediated through the metal center.

Aurophilic interaction-based aggregation of gem-digold(I) aryls towards high spin-orbit coupling and strong phosphorescence

Luminescent gold(I) compounds have attracted intensive attention due to anticipated strong spin-orbit coupling (SOC) resulting from heavy atom effect of gold atoms. However, some mononuclear gold(I) compounds are barely satisfactory. Here, we unveil that low participation of gold in transition-related orbitals, caused by 6s-π symmetry mismatch, is the cause of low SOCs in monogold(I) compounds. To address this issue, we have developed a series of acceptor-donor organogold(I) luminescent compounds by incorporating a gem-digold moiety with various aryl donors. These compounds demonstrate wide-range tunable emission colors and impressive photoluminescence quantum yields of up to 78%, among the highest reported for polynuclear gold(I) compounds. We further reveal that the integration of the gem-digold moiety allows better interaction of gold 6s orbitals with aryl π orbitals, facilitates aryl-to-gold electron transfer, and reduces Pauli repulsion between digold units, finally engendering the formation of aurophilic interaction-based aggregates. Moreover, the strength of such intermolecular aurophilic interaction can be systematically regulated by the electron donor nature of aryl ligands. The formation of those aurophilic aggregates significantly enhances SOC from <10 to 239 cm-1 and mainly accounts for high-efficiency phosphorescent emission in solid state.

Lanthanide-Sensitized Upconversion Iridium Complex via Triplet Energy Transfer

Cyclometalated iridium (Ir) complexes demonstrate impressive capabilities across a range of fields, including biology and photocatalysis, due to their tunable optical characteristics and structure flexibility. However, generating upconversion luminescence of Ir complexes under near-infrared light excitation is challenging. Herein, by employing lanthanide-doped upconversion nanoparticles (UCNPs) as the sensitizer, a new strategy is demonstrated to gain upconversion luminescence of Ir complexes via triplet energy transfer. This design relies on a rationally designed hybrid of core-shell structured NaYbF4:Tb@NaTbF4 UCNPs and new Ir phosphonate complexes, in which UCNPs can migrate upconverted energy to the surface of nanoparticles through Tb3+-mediated energy migration and then sensitize the upconversion luminescence of Ir complexes upon 980 nm excitation. Both experimental and theoretical investigations highlight the significance of triplet energy transfer from excited Tb3+ ions to the triplet state of Ir complexes in the sensitization of upconversion luminescence of Ir complexes. These findings may open exciting avenues for fabricating hybrid Ir materials with new functions and driving the development of UCNP-based nanomaterials.

Synthesis, Luminescence, and Electrochemistry of Tris-Chelate Platinum(IV) Complexes with Cyclometalated N-Heterocyclic Carbene Ligands and Aromatic Diimines

Dicationic, C2-symmetrical, tris-chelate Pt(IV) complexes of general formula [Pt(trz)2(N∧N)](OTf)2, bearing two cyclometalated 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) ligands and one aromatic diimine [N∧N = 2,2'-bipyridine (bpy, 2), 4,4'-di-tert-butyl-2,2'-bipyridine (dbbpy, 3), 4,4'-dimethoxi-2,2'-bipyridine (dMeO-bpy, 4), 1,10-phenanthroline (phen, 5), 4,7-diphenyl-1,10-phenanthroline (bphen, 6), dipyrido[3,2-a:2',3'-c]phenazine (dppz, 7), or 2,3-diphenylpyrazino[2,3-f][1,10]phenanthroline (dpprzphen, 8)] are obtained through chloride abstraction from [PtCl2(trz)2] (1) using AgOTf in the presence of the corresponding diimine. Complexes 2-4 show long-lived phosphorescence from 3LC excited states involving the diimine ligand, with quantum yields that reach 0.18 in solution and 0.58 in the solid matrix at room temperature for 3. Derivatives with more extended aromatic systems show dual phosphorescent/fluorescent emissions (5, 6) or mainly fluorescence (7, 8) in solution. Comparisons with similar complexes bearing cyclometalated 2-arylpyridines instead of aryl-N-heterocyclic carbenes indicate that the {Pt(trz)2} subunit is crucial to enable efficient emissions from diimine-centered excited states. It is also shown that the introduction of protective bulky substituents on the diimine, such as the tert-butyl groups in 3, is a key strategy to reach higher emission efficiencies. The new compounds represent rare examples of luminescent Pt(IV) complexes that show quasi-reversible one-electron reductions, indicating an unusually high redox stability.

The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

Silylene-Copper-Amide Emitters: From Thermally Activated Delayed Fluorescence to Dual Emission

Herein, we report the inaugural instance of N-heterocyclic silylene (NHSi)-coordinated copper amide emitters (2-5). These complexes exhibit thermally activated delayed fluorescence (TADF) and singlet-triplet dual emission in anaerobic conditions. The NHSi-Cu-diphenylamide (2) complex demonstrates TADF with a very small ΔEST gap (0.01 eV), an absolute quantum yield of 11 %, a radiative rate of 2.55×105 s-1, and a short τTADF of 0.45 μs in the solid state. The dual emissive complexes (3-5) achieve an absolute quantum yield of up to 20 % in the solid state with a kISC rate of 1.82×108 s-1 and exhibit room temperature phosphorescence (RTP) with lifetimes up to 9 ms. The gradual decrease in the intensity of the triplet state of complex 3 under controlled oxygen exposure demonstrates its potential for future oxygen-sensing applications. Complexes 2 and 3 have been further utilized to fabricate converted LEDs, paving the way for future OLED production using newly synthesized NHSi-Cu-amides.

Cyclometalated N-Difluoromethylbenzimidazolylidene Platinum(II) Complexes with Built-in Secondary Coordination Spheres: Photophysical Properties and Bioimaging

Bidentate Pt(II) complexes with cyclometalated N-heteroarene or N-heterocyclic carbene (NHC) ligands have been extensively studied as phosphorescent emitters over the past two decades. Herein, we introduce a difluoromethyl group (CF2H) into the wingtip of NHCs, where CF2H acts as a lipophilic hydrogen bond (HB) donor. Their cyclometalated Pt(II) complexes show excellent PLQYs (up to 93%) and phosphorescence lifetimes mainly due to the rigid structure with hydrogen bonding between the CF2H group and the adjacent O atom at the β-diketonate ligand. Bioimaging studies demonstrate high cellular uptake efficiency and deep tumor penetration capability of complex 7 in HeLa cells and multicellular tumor spheroids, highlighting their potential as bioimaging probes.

Structural Engineering of Iridium(III) Phosphors with Imidazo[4,5-b]pyrazin-2-ylidene Cyclometalates for Efficient Blue Electroluminescence

Iridium(III) complexes are particularly noted for their excellent potentials in fabrication of blue organic light-emitting diodes (OLEDs), but the severe efficiency roll-off largely hampered their practical applications. To reveal the underlying characteristics, three Ir(III) complexes, namely f-ct5c, f-ct5d, and f-ct11, bearing imidazo[4,5-b]pyrazin-2-ylidene cyclometalates are prepared and characterized in detail. Both f-ct5c and f-ct5d (also their mixture f-ct5mix) gave intensive blue emissions peaking at ≈465 nm with short radiative lifetimes of 1.76 and 2.45 µs respectively, in degassed toluene. Alternatively, f-ct11 with two 4-tert-butylphenyl substituents on each imidazo[4,5-b]pyrazin-2-ylidene entity, possessed a bluish-green emission (508 nm) together with an extended radiative lifetime of 34.3 µs in the dispersed PMMA matrix. Consequently, the resulting solution-processed OLED with f-ct11 delivered a maximum external quantum efficiency (EQEmax) of 6.5% with serious efficiency roll-offs. In contrast, f-ct5mix based device achieved a high EQEmax of 27.2% and the EQE maintained at 23.0% of 1000 cd m-2. Furthermore, the hyper-OLEDs with f-ct5mix as the sensitizer and v-DABNA as the terminal emitter afford narrowed emission with a considerably high EQEmax exceeding 32%, affirming the potential of f-ct5mix to serve as both the emitter and sensitizer in OLEDs.

Advancing Heterogeneous Organic Synthesis With Coordination Chemistry-Empowered Single-Atom Catalysts

For traditional metal complexes, intricate chemistry is required to acquire appropriate ligands for controlling the electron and steric hindrance of metal active centers. Comparatively, the preparation of single-atom catalysts is much easier with more straightforward and effective accesses for the arrangement and control of metal active centers. The presence of coordination atoms or neighboring functional atoms on the supports' surface ensures the stability of metal single-atoms and their interactions with individual metal atoms substantially regulate the performance of metal active centers. Therefore, the collaborative interaction between metal and the surrounding coordination environment enhances the initiation of reaction substrates and the formation and transformation of crucial intermediate compounds, which imparts single-atom catalysts with significant catalytic efficacy, rendering them a valuable framework for investigating the correlation between structure and activity, as well as the reaction mechanism of catalysts in organic reactions. Herein, comprehensive overviews of the coordination interaction for both homogeneous metal complexes and single-atom catalysts in organic reactions are provided. Additionally, reflective conjectures about the advancement of single-atom catalysts in organic synthesis are also proposed to present as a reference for later development.

Tetradentate Pt(II) Complexes Based on Xylenylamino Linked Dual Pyrazolate Chelates for Organic Light Emitting Diodes

In this work, we report the syntheses of three Pt(II) emitters, namely, Pt4N1, Pt4N2, and Pt4N3, to which their tetradentate chelates were assembled by linking two pyrazolate chelates with a single xylenylamino entity. Functionalization of Pt4N1 was achieved upon the addition of electronegative CF3 substituent on pyridinyl groups and switching to more electron-deficient pyrazinyl groups in giving Pt4N2 and Pt4N3, respectively. The vertically arranged xylenylamino entity has effectively suppressed the inter-molecular π-π stacking and Pt⋅⋅⋅Pt interaction, as shown by the single crystal X-ray structural analyses. Upon fabrication of OLED devices, Pt4N2 and Pt4N3 based devices delivered efficient cyan and green emission, with an EQEmax of 15.2 % and 11.2 %, respectively, affirming the successfulness of the tetradentate chelating strategy.

Chiral Organometallic Complexes Derived from Helicenic N-Heterocyclic Carbenes (NHCs): Design, Structural Diversity, and Chiroptical and Photophysical Properties

ConspectusRecently, helicene derivatives have emerged as an important class of molecules with potential applications spanning over asymmetric catalysis, biological activity, magnetism, spin filtering, solar cells, and polymer science. To harness their full potential, especially as emissive components in circularly polarized organic light-emitting diodes (CP-OLEDs), generating structural chemical diversity and understanding the resulting photophysical and chiroptical properties are crucial. In this Account, we shed light on chemical engineering combining helicene and N-heterocyclic carbene (NHC) chemistries to create transition-metal complexes with unique architectures and describe their photophysical and chiroptical attributes. The σ-donating and π-accepting capabilities of the helically chiral π-conjugated NHCs endow the complexes with remarkable structural and electronic features. These characteristics manifest in phenomena such as chirality induction, very long-lived phosphorescence, and strong chiroptical signatures (electronic circular dichroism and circularly polarized luminescence).We describe the different classes of ligands primarily developed in our group by classifying them according to their connection between the helicenic moiety and the imidazole precursor. This connection is essential in determining the degree of π-conjugation and characterizing the emissive state. We comprehensively discuss 6-coordinate, 4-coordinate, and 2-coordinate complexes, delving into their structural nuances and examining how the interplay between metals and auxiliary ligands shapes their photophysical properties, with interpretations enriched by DFT calculations. Helicenes are known to promote intersystem crossing thanks to strong spin-orbit coupling, while metals offer robust frameworks leading to a variety of molecular architectures with specific topologies together with distinct excited-state properties. The electronic configurations and energy levels of the ligand and metal orbitals thus significantly modulate the photophysical and chiroptical behaviors of these complexes. In-depth analysis of chiroptical properties, notably electronic circular dichroism and circularly polarized luminescence, emphasizes the influence of different stereogenic elements on the chiroptical responses across various energy ranges with appealing "match-mismatch" effects. Finally, we describe future prospects of helicene NHCs, particularly in the context of emerging research on cost-effective and abundant transition metals for materials science and for photocatalysis. Indeed, the inherent long-lived MLCT, excited-state delocalization, structural rigidity, and intrinsic chirality of these complexes present intriguing avenues for future investigations.

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.

Robust Luminescent Pyrene-Based Metal-Organic Framework Hydrogel as a pH-Responsive Fluorescence Emitter for Sensitive Immunoassay of Cardiac Troponin I

Despite many luminescent advantages including outstanding absorption coefficient and high quantum yield, pyrene and its derivatives have been suffering from a dramatic aggregation-caused quenching (ACQ) effect. Although the dramatic ACQ effect of pyrene-based fluorophores has been restrained in pyrene-doped metal-organic frameworks (MOFs), the low loading of fluorescent (FL) units substantially impedes the improved luminescent behaviors. Herein, pyrene-based MOFs hydrogel was synthesized with a high loading of pyrene as the unique organic linker blocks instead of a dopant in MOFs. The gel matrix contributed to rigidifying the location of the FL emitters and achieving intensive FL emission and high luminescent stability and therefore efficiently overcoming the ACQ effect. Furthermore, the protonation of pyrene in the MOFs hydrogel remarkably decreased the luminescent intensity, which endowed the FL hydrogel with highly pH-responsive activity in the broad range (pH 4-10). Interestingly, glucose oxidase was immobilized into ZIF-8 as a highly efficient luminescent quencher, which contributed to catalyzing the form of gluconic acid and thus drastically quenching the FL signal of the MOFs hydrogel. Furthermore, the emitter-quencher pair of pyrene-based MOFs hydrogel and glucose oxidase was successfully employed to develop an ultrasensitive FL immunoassay platform for cardiac troponin I (as a model analyte). The limit of detection for cardiac troponin I was 5.2 pg/mL (3σ). The proof-of-principle study demonstrated the thrilling auxiliary effect of tailorable MOFs hydrogel on boosting the feasibility of aqueous insoluble FL chromophores for trace analysis.

Luminescence Behavior of Cationic and Neutral CuI Complexes of Phosphine and Pyridine Embedded 1,2,3-Triazole

Synthesis of a potentially polydentate, phosphine and pyridine embedded 1,2,3-triazole, o-Ph2P(C6H4)C(CH)-1,2,3-N3(CH2)(Py) (1) (here onward referred to as "P∩N3∩N") and its copper complexes are described. Reactions of 1 with CuX yielded mononuclear [Cu{(P∩N3∩N)2-κ2-P,N}]X (2 - 4; X = I, CuBr2 and CuCl2) and dinuclear [Cu2{(P∩N3∩N)2-κ4-P,N,N,N}]X (5 X = OTf, 6 X = BF4) complexes. Interestingly, the cationic complex [Cu{(P∩N3∩N)2-κ2-P,N}]I (2) in acetonitrile changes into neutral complex [Cu3(μ2-I)2(μ3-I)(NCCH3){(P∩N3∩N)-κ4(μ2-P,N)(μ2-N,N)}](7), which on addition of dichloromethane reverts back to the cationic form. The photoluminescent characteristics of cationic complexes are significantly impacted by the nature of counteranions and hence the corresponding photoluminescence quantum yields. Cationic complex 2 showed an increase in quantum yield and lifetime on changing over to neutral complex 7. TD-DFT calculations also assisted in assessing the photophysical properties.

A Cyclometalated Iridium(III) Complex Exerts High Anticancer Efficacy via Fatty Acid Beta-Oxidation Inhibition and Sphingolipid Metabolism Reprogramming

Given the extensive role of lipids in cancer development, there is substantial clinical interest in developing therapies that target lipid metabolism. In this study, we identified one cyclometalated iridium complex (Ir2) that exhibits potent antiproliferation activity in MIA PaCa-2 cells by regulating fatty acid metabolism and sphingolipid metabolism simultaneously. Ir2 also efficiently overcomes cisplatin resistance in vitro. Satisfyingly, the generated Ir2@F127 carriers, as a temperature-sensitive in situ gelling system of Ir2, showed effective cancer treatment with minimal side effects in an in vivo xenograft study. To the best of our knowledge, Ir2 is the first reported cyclometalated iridium complex that exerts anticancer activity in MIA PaCa-2 cells by intervening in lipid metabolism, which provides an alternative pathway for the anticancer mechanism of cyclometalated iridium complexes.

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.

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

Luminescence imaging is a powerful and versatile technique for investigating cell physiology and pathology in living systems, making significant contributions to life science research and clinical diagnosis. In recent years, luminescent transition metal complexes have gained significant attention for diagnostic and therapeutic applications due to their unique photophysical and photochemical properties. In this Review, we provide a comprehensive overview of the recent development of luminescent transition metal complexes for bioimaging and biosensing applications, with a focus on transition metal centers with a d6, d8, and d10 electronic configuration. We elucidate the structure-property relationships of luminescent transition metal complexes, exploring how their structural characteristics can be manipulated to control their biological behavior such as cellular uptake, localization, biocompatibility, pharmacokinetics, and biodistribution. Furthermore, we introduce the various design strategies that leverage the interesting photophysical properties of luminescent transition metal complexes for a wide variety of biological applications, including autofluorescence-free imaging, multimodal imaging, organelle imaging, biological sensing, microenvironment monitoring, bioorthogonal labeling, bacterial imaging, and cell viability assessment. Finally, we provide insights into the challenges and perspectives of luminescent transition metal complexes for bioimaging and biosensing applications, as well as their use in disease diagnosis and treatment evaluation.

Genuine carbene versus carbene-like reactivity

Singlet carbenes are not always isolable and often even elude direct detection. When they escape observation, their formation can sometimes be evidenced by in situ trapping experiments. However, is carbene-like reactivity genuine evidence of carbene formation? Herein, using the first example of a spectroscopically characterized cyclic (amino)(aryl)carbene (CAArC), we cast doubt on the most common carbene trapping reactions as sufficient proof of carbene formation.

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 %.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Dual-Emissive Iridium(III) Complexes and Their Applications in Biological Sensing and Imaging

Phosphorescent iridium(III) complexes are widely recognized for their unique properties in the excited triplet state, making them crucial for various applications including biological sensing and imaging. Most of these complexes display single phosphorescence emission from the lowest-lying triplet state after undergoing highly efficient intersystem crossing (ISC) and ultrafast internal conversion (IC) processes. However, in cases where these excited-state processes are restricted, the less common phenomenon of dual emission has been observed. This dual emission phenomenon presents an opportunity for developing biological probes and imaging agents with multiple emission bands of different wavelengths. Compared to intensity-based biosensing, where the existence and concentration of an analyte are indicated by the brightness of the probe, the emission profile response involves modifications in emission color. This enables quantification by utilizing the intensity ratio of different wavelengths, which is self-calibrating and unaffected by the probe concentration and excitation laser power. Moreover, dual-emissive probes have the potential to demonstrate distinct responses to multiple analytes at separate wavelengths, providing orthogonal detection capabilities. In this concept, we focus on iridium(III) complexes displaying fluorescence-phosphorescence or phosphorescence-phosphorescence dual emission, along with their applications as biological probes for sensing and imaging.

Concatenating Structural Constraint Effects at Tin for the Sequential Generation, Stabilization, and Transfer of Acyclic Aminocarbenes

Structural constraint approaches have been employed toward different ends in recent years, from augmenting the nucleophilicity in pyramidalized low-valent p-block compounds to enhancing the Lewis acidities at planarized tetravalent p-block elements. While previous studies exploited these effects separately, this work introduces a strategy to concatenate structural constraint approaches at individual stages of a reaction sequence in a row to unlock a synthetic path unattainable by conventional methodologies. The boosted nucleophilicity resulting from the constrained tetracoordinated calix[4]pyrrolato stannate(II) dianion enables the reductive formation of sterically unprotected acyclic aminocarbenes. These amino carbenes are stabilized at the concomitantly formed square-planar stannane(IV) as air-stable adducts. Transfer of the carbenes onto copper(I) by cooperativity of the calix[4]pyrrole ligand finalizes this protocol to hitherto unreported yet prototypical carbene complexes. Detailed spectroscopic and quantum theoretical analyses establish the synergy of structural constraints and element-ligand cooperation as the linchpin to this reaction path and its selectivity.

Phosphorescent Platinum(II) Complexes with a Spiro-fused Xanthene Unit: Synthesis and Photophysical Properties

Novel platinum(II) complexes, derived from the spiro[fluorene-9,9'-xanthene] (SFX) motif, were synthesized and combined with different auxiliary ligands such as acetylacetonate (acac), bis(2,4,6-trimethylphenyl)propane-1,3-dionate (mesacac) and dihydrobis(3,5-dimethylpyrazole-1-yl) borate. The final products were obtained in yields of up to 36 % and characterized by NMR, X-ray and combustion analysis. These complexes have structured green-blue emission spectra with Commission Internationale de l'Éclairage (CIExy) coordinates of (0.21;0.46). Excellent photoluminescence quantum yields (PLQYs) ranging from 87 %-91 % were found. The emission lifetimes vary from 33 μs to 43 μs. Calculations on the B3LYP/6-311++G** level of theory reveal, that the nature of the emissive state is dependent on the positional regioisomerism of the SFX motif. The 2-SFX complexes demonstrate ligand-centered (3LC) emission, while the 2'-SFX regioisomer with the mesacac ligand shows a strong 3MLCT character.

Temperature-Induced Sign Inversion of Circularly Polarized Luminescence of Binaphthyl-Bridged Tetrathiapyrenophanes

D2-symmetric (R)-binaphthyl-bridged pyrenophanes containing thioether bonds were synthesized. The pyrenophanes exhibited the temperature-induced sign inversion of circularly polarized luminescence (CPL) while maintaining the emission wavelength and reversibility. The Δglum value reached 0.02, and the FL quenching by heat was negligible. The sign inversion of CPL originates from the inversion of intramolecular excimer chirality associated with excitation dynamics. The two pyrenes form a kinetically trapped left-handed twist excimer at low temperatures, while they form a thermodynamically favored right-handed twist excimer at high temperatures. The thioether linkers can impart flexibility suitable for the inversion of chirality of the excimers.

Blue Electrophosphorescence from Iridium(III) Phosphors Bearing Asymmetric Di-N-aryl 6-(trifluoromethyl)-2H-imidazo[4,5-b]pyridin-2-ylidene Chelates

Efficient blue phosphors remain a formidable challenge for organic light-emitting diodes (OLEDs). To circumvent this obstacle, a series of Ir(III)-based carbene complexes bearing asymmetric di-N-aryl 6-(trifluoromethyl)-2H-imidazo[4,5-b]pyridin-2-ylidene chelates, namely, f-ct6a‒c, are synthesized, and their structures and photophysical properties are comprehensively investigated. Moreover, these emitters can undergo interconversion in refluxing 1,2,4-trichlorobenzene, catalyzed by a mixture of sodium acetate (NaOAc) and p-toluenesulfonic acid monohydrate (TsOH·H2O) without decomposition. All Ir(III) complexes present good photoluminescence quantum yield (ΦPL = 83-88%) with peak maximum (max.) at 443-452 nm and narrowed full width at half maximum (FWHM = 66-73 nm). Among all the fabricated OLED devices, f-ct6b delivers a max. external quantum efficiency (EQE) of 23.4% and Commission Internationale de L'Eclairage CIEx , y coordinates of (0.14, 0.12), whereas the hyper-OLED device based on f-ct6a and 5H,9H,11H,15H-[1,4] benzazaborino [2,3,4-kl][1,4]benzazaborino[4',3',2':4,5][1,4]benzazaborino[3,2-b]phenazaborine-7,13-diamine, N7,N7,N13,N13,5,9,11,15-octaphenyl (ν-DABNA) exhibits max. EQE of 26.2% and CIEx , y of (0.12, 0.13). Finally, the corresponding tandem OLED with f-ct6b as dopant gives a max. luminance of over 10 000 cd m-2 and max. EQE of 42.1%, confirming their candidacies for making true-blue OLEDs.

Bis-tridentate Ir(III) Phosphors and Blue Hyperphosphorescence with Suppressed Efficiency Roll-Off at High Brightness

Narrowband blue emitters are indispensable in achieving ultrahigh-definition OLED displays that satisfy the stringent BT 2020 standard. Hereby, a series of bis-tridentate Ir(III) complexes bearing electron-deficient imidazo[4,5-b]pyridin-2-ylidene carbene coordination fragments and 2,6-diaryloxy pyridine ancillary groups were designed and synthesized. They exhibited deep blue emission with quantum yields of up to 89% and a radiative lifetime of 0.71 μs in the DPEPO host matrix, indicating both the high efficiency and excellent energy transfer process from the host to dopant. The OLED based on Irtb1 showed an emission at 468 nm with a maximum external quantum efficiency (EQE) of 22.7%. Moreover, the hyper-OLED with Irtb1 as a sensitizer for transferring energy to terminal emitter v-DABNA exhibited a narrowband blue emission at 472 nm and full width at half-maximum (FWHM) of 24 nm, a maximum EQE of 23.5%, and EQEs of 19.7, 16.1, and 12.9% at a practical brightness of 100, 1000, and 5000 cd/m2, respectively.

Half-sandwich Ni(II) complexes bearing enantiopure bidentate NHC-carboxylate ligands: efficient catalysts for the hydrosilylative reduction of acetophenones

Chiral nickel complexes containing NHC-carboxylate chelate ligands derived from the (S)-isomeric form of amino acids have been synthesised from the corresponding imidazolium salt and nickelocene. The presence of the carboxylate on the N-side arm of the heterocycle results in the competing formation of mixtures of mono- and bis-NHC complexes (i.e., [Ni(η5-Cp)(κ2-C,O-NHC)] and [Ni(κ2-C,O-NHC)2]), both of which retain the (S)-configuration of the stereogenic center and which can be separated by chromatography. Both the 18e- and 16e- complexes are found to be very stable and cannot be interconverted. The composition of the resulting mixtures depends mainly on the entity of the amino acid residue and, of more practical interest, on the reaction conditions. Thus, microwave heating and MeCN as a solvent favor the formation of the half-sandwich nickel complexes, rather than the bis-NHC compounds. Some of the [Ni(η5-Cp)(κ2-C,O-NHC)] complexes turn out to be among the best nickel catalysts for the hydrosilylative reduction of p-acetophenones described to date, although without chiral induction, in the absence of activating additives and under mild catalytic conditions.

Recent Advances in Organometallic NIR Iridium(III) Complexes for Detection and Therapy

Iridium(III) complexes are emerging as a promising tool in the area of detection and therapy due to their prominent photophysical properties, including higher photostability, tunable phosphorescence emission, long-lasting phosphorescence, and high quantum yields. In recent years, much effort has been devoted to develop novel near-infrared (NIR) iridium(III) complexes to improve signal-to-noise ratio and enhance tissue penetration. In this review, we summarize different classes of organometallic NIR iridium(III) complexes for detection and therapy, including cyclometalated ligand-enabled NIR iridium(III) complexes and NIR-dye-conjugated iridium(III) complexes. Moreover, the prospects and challenges for organometallic NIR iridium(III) complexes for targeted detection and therapy are discussed.

High-Efficiency Pure Blue Circularly Polarized Phosphorescence from Chiral N-Heterocyclic-Carbene-Stabilized Copper(I) Clusters

Circularly polarized luminescence (CPL) materials have attracted considerable attention for their promising applications in encryption, chiral sensing, and three-dimensional (3D) displays. However, the preparation of high-efficiency, pure blue CPL materials remains challenging. In this study, we reported an enantiomeric pair of triangle copper(I) clusters (R/S-Cu3) rigidified by employing chiral N-heterocyclic carbene (NHC) ligands with two pyridine-functionalized wingtips. These chiral clusters emitted pure blue phosphorescence that overlapped with that of the commercial blue phosphor having Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of (0.14, 0.10), and the films exhibited an unprecedented photoluminescence quantum yield (PLQY) of ∼70.0%. Additionally, the solutions showed very bright circularly polarized phosphorescence (CPP) with a dissymmetry factor of ±2.1 × 10-3. The excellent solubility and photostability endowed these pure-blue-emitting chiral clusters with promising applications as pure blue CPP inks for 3D printing white objects, such as precise-atomic-enlarged models of metal clusters and a lovely white stereoscopic "rabbit". The intricate mechanism underlying blue phosphorescence in this small cluster and across various states is elucidated through a comprehensive approach that integrates thorough analysis of luminescence properties, controlled experiments, and theoretical calculations. For the first time, we propose that the dominant high-energy emission center is constituted by delocalized hybrid orbitals over multiple atomic centers, encompassing both the metal and the coordinated atoms. This challenges stereotypical assumptions that the cluster center solely supports low-energy emissions. This work expands the currently limited range of CPP functional materials and provides a new direction for CPP applications involving NHC-stabilized metal clusters.

Iridium metal complex targeting oxidation resistance 1 protein attenuates spinal cord injury by inhibiting oxidative stress-associated reactive oxygen species

Oxidative stress is a key factor leading to profound neurological deficits following spinal cord injury (SCI). In this study, we present the development and potential application of an iridium (iii) complex, (CpxbiPh) Ir (N^N) Cl, where CpxbiPh represents 1-biphenyl-2,3,4,5-tetramethyl cyclopentadienyl, and N^N denotes 2-(3-(4-nitrophenyl)-1H-1,2,4-triazol-5-yl) pyridine chelating agents, to address this challenge through a mechanism governed by the regulation of an antioxidant protein. This iridium complex, IrPHtz, can modulate the Oxidation Resistance 1 (OXR1) protein levels within spinal cord tissues, thus showcasing its antioxidative potential. By eliminating reactive oxygen species (ROS) and preventing apoptosis, the IrPHtz demonstrated neuroprotective and neural healing characteristics on injured neurons. Our molecular docking analysis unveiled the presence of π stacking within the IrPHtz-OXR1 complex, an interaction that enhanced OXR1 expression, subsequently diminishing oxidative stress, thwarting neuroinflammation, and averting neuronal apoptosis. Furthermore, in in vivo experimentation with SCI-afflicted mice, IrPHtz was efficacious in shielding spinal cord neurons, promoting their regrowth, restoring electrical signaling, and improving motor performance. Collectively, these findings underscore the potential of employing the iridium metal complex in a novel, protein-regulated antioxidant strategy, presenting a promising avenue for therapeutic intervention in SCI.

Mononuclear copper(I) complexes bearing a 3-phenyl-5-(pyridin-4-yl)-1,2,4-triazole ligand: synthesis, crystal structure, TADF-luminescence, and mechanochromic effects

Three new mononuclear heteroleptic copper(I) halide complexes, [CuL(PPh3)2X] (X = Cl, Br, I), based on 3-phenyl-5-(pyridin-4-yl)-1,2,4-triazole (L) and triphenylphosphine (PPh3) ligands, have been prepared by reaction of CuX (X = Cl, Br, I), L and PPh3 in a molar ratio of 1 : 1 : 2 in MeCN solutions. The synthesized complexes exhibit blue light emission in solutions and bright green emission in the crystal state with quantum yields of up to 100%. The luminescence decay analysis and density functional theory calculations revealed that the emission of solid samples at room temperature corresponds to the thermally activated delayed fluorescence, while that at 77 K is assigned to phosphorescence. Utilizing the studied complexes in OLED heterostructures resulted in high-performing green-emitting devices with an external quantum efficiency of up to 13.4%.

Synthesis, Structure, and Photophysical Properties of Platinum Compounds with Thiophene-Derived Cyclohexyl Diimine Ligands

Platinum(II) and platinum(IV) compounds were prepared by the stereoselective and regioselective reactions of thiophene-derived cyclohexyl diimine C^N^N-ligands with [Pt2Me4(μ-SMe2)2]. Newly synthesized ligands were characterized by NMR spectroscopy and elemental analysis, and Pt(II)/Pt(IV) compounds were characterized by NMR spectroscopy, elemental analysis, high-resolution mass spectrometry, and single-crystal X-ray diffraction. UV-vis absorbance and photoluminescence measurements were performed on newly synthesized complexes, as well as structurally related Pt(II)/Pt(IV) compounds with benzene-derived cyclohexyl diimine ligands, in dichloromethane solution, as solids, and as 5% by weight PMMA-doped films. DFT and TD-DFT calculations were performed, and the results were compared with the observed spectroscopic properties of the newly synthesized complexes. X-ray total scattering measurements and real space pair distribution function analysis were performed on the synthesized complexes to examine the local- and intermediate-range atomic structures of the emissive solid states.

Polymeric copper(I)-NHC complexes with bulky bidentate (N^C) ligands: synthesis and solid-state luminescence

Starting from imidazolium chlorides bearing bulky nitrogen donors, a series of four complexes, mainly [Cu(C^N)Cl]n coordination polymers were obtained directly as luminescent species by simple filtration from the aqueous reaction medium, highlighting a simple, eco-friendly, robust and reproducible synthetic procedure. Additionally, we have shown on the most efficient example that chloride could be exchanged very easily by other halides/pseudohalides (Br-, I-, NCS-, N3-) allowing to slightly modulate the emitted colour while conserving the polymeric structure, except for azide for which a dimer was obtained. The combination of chemical analyses, of photoluminescence studies in the solid state including quantum yield measurement and X-ray diffraction on single crystals and as-synthesized microcrystalline powders highlighted that the polymeric luminescent species was indeed obtained directly by simple filtration and that no major alteration of the structure was observed upon recrystallisation. Samples of all polymeric complexes displayed remarkable stability towards air oxidation remaining unchanged upon storage for several months and partially retaining their photoluminescence properties even after a thermal treatment at 100 °C for 24 h.

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.

Dimetal-Binding Scaffold 2-(Pyridin-2-yl)imidazo [1,5-b]pyridazine-7-ylidene: Synthesis of Trinuclear Heterobimetallic Complexes Involving Gold-Metal Interactions

As a dimetal-binding rigid scaffold, 2-(pyridin-2-yl)imidazo[1,5-b]pyridazine-7-ylidene was introduced. The scaffold was first converted into a meridional Au,N,N-tridentate ligand through binding of a Au(I)Cl moiety at the carbene center. The Au(I) center and the N,N-chelating moiety were expected to function as metallophilic and 4e-σ-donative interaction sites, respectively, in the binding of the second metal center. In this manner, various trinuclear heterobimetallic complexes were synthesized with different 3d-metal sources, such as cationic CuI , CuII , NiII , and CoII salts. SC-XRD analysis showed that the mono-3d-metal di-gold(I) trinuclear heterobimetallic complexes were constructed through gold(I)-metal interactions. Metallophilic interactions were also investigated by quantum chemical calculations including the AIM and IGMH methods.

Dual-emissive Iridium(III) Complexes as Phosphorescent Probes with Orthogonal Responses to Analyte Binding and Oxygen Quenching

Phosphorescent probes often show sensitive response toward analytes at a specific wavelength. However, oxygen quenching usually occurs at the same wavelength and thus hinders the accurate detection of analytes. In this study, we have developed dual-emissive iridium(III) complexes that exhibit phosphorescence responses to copper(II) ions at a wavelength distinct from that where oxygen quenching occurs. The complexes displayed colorimetric phosphorescence response in aqueous solutions under different copper(II) and oxygen conditions. In cellular imaging, variation in oxygen concentration over a large range from 5 % to 80 % can modulate the intensity and lifetime of green phosphorescence without affecting the response of red phosphorescence toward intracellular copper(II) ions.

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.

Engineering of Cyano Functionalized Benzo[d]imidazol-2-ylidene Ir(III) Phosphors for Blue Organic Light-Emitting Diodes

In this study, we designed and synthesized three series of blue emitting homoleptic iridium(III) phosphors bearing 4-cyano-3-methyl-1-phenyl-6-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (mfcp), 5-cyano-1-methyl-3-phenyl-6-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (ofcp), and 1-(3-(tert-butyl)phenyl)-6-cyano-3-methyl-4-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (5-mfcp) cyclometalates, respectively. These iridium complexes exhibit intense phosphorescence in the high energy region of 435-513 nm in the solution state at RT, to which the relatively large T₁ → S₀ transition dipole moment is beneficial for serving as a pure emitter and an energy donor to the multiresonance thermally activated delayed fluorescence (MR-TADF) terminal emitters via Förster resonance energy transfer (FRET). The resulting OLEDs achieved true blue, narrow bandwidth EL with a max EQE of 16-19% and great suppression of efficiency roll-off with ν-DABNA and t-DABNA. We obtained the FRET efficiency up to 85% using titled Ir(III) phosphors f-Ir(mfcp)₃ and f-Ir(5-mfcp)₃ to achieve true blue narrow bandwidth emission. Importantly, we also provide analysis on the kinetic parameters involved in the energy transfer processes and, accordingly, propose feasible ways to improve the efficiency roll-off caused by the shortened radiative lifetime of hyperphosphorescence.

Cyclometalated platinum(II) complexes with acyclic diaminocarbene ligands for OLED application

A novel series of cyclometalated platinum(II) complexes bearing acyclic diaminocarbene (ADC) ancillary ligands were designed and prepared. Their photophysical properties were systematically studied through experimental and theoretical investigations. All complexes exhibit green phosphorescence with a quantum efficiency of up to 45% in 2 wt% doped PMMA film at room temperature. The complexes are used as light-emitting dopants for organic light-emitting diode (OLED) fabrication. The devices displayed a green emission with a maximum current efficiency of 2.9 cd A^-1 and a luminance of 2700 cd m^-2. These results show that these cyclometalated platinum(II) complexes can be used as efficient green emitting components of OLED devices.