Recent Advances in Metal Triplet Emitters with d6, d8, and d10 Electronic Configurations

Efficient Deep-Blue Organic Light-Emitting Diodes Employing Doublet Sensitization

Fast and efficient exciton utilization is a crucial solution and highly desirable for achieving high-performance blue organic light-emitting diodes (OLEDs). However, the rate and efficiency of exciton utilization in traditional OLEDs, which employ fully closed-shell materials as emitters, are inevitably limited by spin statistical limitations and transition prohibition. Herein, a new sensitization strategy, namely doublet-sensitized fluorescence (DSF), is proposed to realize high-performance deep-blue electroluminescence. In the DSF-OLED, a doublet-emitting cerium(III) complex, Ce-2, is utilized as sensitizer for multi-resonance thermally activated delayed fluorescence emitter ν-DABNA. Experimental results reveal that holes and electrons predominantly recombine on Ce-2 to form doublet excitons, which subsequently transfer energy to the singlet state of ν-DABNA via exceptionally fast (over 108 s-1) and efficient (≈100%) Förster resonance energy transfer for deep-blue emission. Due to the circumvention of spin-flip in the DSF mechanism, near-unit exciton utilization efficiency and remarkably short exciton residence time of 1.36 µs are achieved in the proof-of-concept deep-blue DSF-OLED, which achieves a Commission Internationale de l'Eclairage coordinate of (0.13, 0.14), a high external quantum efficiency of 30.0%, and small efficiency roll-off of 14.7% at a luminance of 1000 cd m-2. The DSF device exhibits significantly improved operational stability compared with unsensitized reference device.

(P^N^C) Ligands to Stabilize Gold(III): A Straightforward Access to Hydroxo, Formate, and Hydride Complexes

A novel class of (P^N^C) pincer ligands capable of stabilizing elusive gold(III) species is reported here. Straightforward access to (P^N^C)gold(III) hydroxo, formate, and hydride complexes has been streamlined by first incorporating a cycloauration step devoid of toxic metals or harsh conditions. The resulting gold complexes exhibit remarkable stability in solution as well as in the solid state under ambient conditions, which enabled their characterization by X-ray diffraction analyses. Interestingly, the influence of the ligand allowed the preparation of gold(III)-hydrides using mild hydride donors such as H-Bpin, which contrasts with sensitive super hydrides or strong acids and cryogenic conditions employed in previous protocols. A detailed bonding characterization of these species is complemented by reactivity studies.

Structural and Photophysical Trends in Rhenium(I) Carbonyl Complexes with 2,2':6',2″-Terpyridines

This is the first comprehensive review of rhenium(I) carbonyl complexes with 2,2':6',2″-terpyridine-based ligands (R-terpy)-encompassing their synthesis, molecular features, photophysical behavior, and potential applications. Particular attention has been devoted to demonstrating how the coordination mode of 2,2':6',2″-terpyridine (terpy-κ2N and terpy-κ3N), structural modifications of terpy framework (R), and the nature of ancillary ligands (X-mono-negative anion, L-neutral ligand) may tune the photophysical behavior of Re(I) complexes [Re(X/L)(CO)3(R-terpy-κ2N)]0/+ and [Re(X/L)(CO)2(R-terpy-κ3N)]0/+. Our discussion also includes homo- and heteronuclear multicomponent systems with {Re(CO)3(R-terpy-κ2N)} and {Re(CO)2(R-terpy-κ3N)} motifs. The presented structure-property relationships are of high importance for controlling the photoinduced processes in these systems and making further progress in the development of more efficient Re-based luminophores, photosensitizers, and photocatalysts for modern technologies.

Phosphorescent Cyclometalated Palladium(II) and Platinum(II) Complexes Derived from Diaminocarbene Precursors

Metal-mediated self-assembly of isocyanides and methyl 4-aminopyrimidine-5-carboxylate leads to luminescent PdII and PtII complexes featuring C,N-cyclometalated acyclic diaminocarbene (ADC) ligands. The solid-state luminescent properties of these diaminocarbene derivatives are attributed to their triplet-state metal/metal-to-ligand charge-transfer (3MMLCT) nature, which is driven by attractive intermolecular M···M interactions further reinforced by the intramolecular π-π interactions even in the structure of the Pd compound, which is the first Pd-ADC phosphor reported.

Phenylbenzothiazole-Based Platinum(II) and Diplatinum(II) and (III) Complexes with Pyrazolate Groups: Optical Properties and Photocatalysis

Based on 2-phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me2N-pbt), mononuclear [Pt(pbt)(R'2-pzH)2]PF6 (R'2-pzH = pzH 1a, 3,5-Me2pzH 1b, 3,5-iPr2pzH 1c) and diplatinum (PtII-PtII) [Pt(pbt)(μ-R'2pz)]2 (R'2-pz = pz 2a, 3,5-Me2pz 2b, 3,5-iPr2pz 2c) and [Pt(Me2N-pbt)(μ-pz)]2 (3a) complexes have been prepared. In the presence of sunlight, 2a and 3a evolve, in CHCl3 solution, to form the PtIII-PtIII complexes [Pt(R-pbt)(μ-pz)Cl]2 (R = H 4a, NMe2 5a). Experimental and computational studies reveal the negligible influence of the pyrazole or pyrazolate ligands on the optical properties of 1a-c and 2a,b, which exhibit a typical 3IL/3MLCT emission, whereas in 2c the emission has some 3MMLCT contribution. 3a displays unusual dual, fluorescence (1ILCT or 1MLCT/1LC), and phosphorescence (3ILCT) emissions depending on the excitation wavelength. The phosphorescence is lost in aerated solutions due to sensitization of 3O2 and formation of 1O2, whose determined quantum yield is also wavelength dependent. The phosphorescence can be reversibly photoinduced (365 nm, ∼ 15 min) in oxygenated THF and DMSO solutions. In 4a and 5a, the lowest electronic transitions (S1-S3) have mixed characters (LMMCT/LXCT/L'XCT 4a and LMMCT/LXCT/ILCT 5a) and they are weakly emissive in rigid media. The 1O2 generation property of complex 3a is successfully used for the photooxidation of p-bromothioanisol showing its potential application toward photocatalysis.

Further Insights into the Impact of Ligand-Localized Excited States on the Photophysics of Phenanthroline-Based Rhenium(I) Tricarbonyl Complexes

The present work shows the pivotal role of N-donor substituents attached to 1,10-phenanthroline at the 4,7-positions in perturbation of ground- and excited-state properties of fac-[ReCl(CO)3(R2phen)]. Excited-state processes occurring upon photoexcitation in the designed systems were thoroughly explored with a wide range of steady-state and time-resolved spectroscopic techniques, including transient absorption, as well as experimental results were complemented by theoretical studies based on the density functional theory (DFT). It was demonstrated that the attachment of six-membered heterocyclic amines (piperidine─ppr, morpholine─mor, and thiomorpholine─tmor) is a very effective tool for extending absorptivity and excited-state lifetimes of resulting fac-[ReCl(CO)3(R2phen)] due to the contribution of the excited state localized on the phenanthroline-based ligand. Both absorption and emission properties of these systems were attributed to configurationally mixed MLCT/IL excited states. Re(I) complexes with phenoxazine (pxz) and phenothiazine (ptz) substituents were shown to possess charge-separated excited states, clearly evidenced by the simultaneous presence of signals typical of phen-* and pxz+* or ptz+* in transient absorption spectra. Both complexes are rare examples of NIR light-emitting coordination compounds. The decoration of the phen framework with less polar 9,9-dimethyl-9,10-dihydroacridine (dmac) groups resulted in the formation of [ReCl(CO)3(R2phen)] with mixed 3MLCT/3ILCT triplet excited state.

Electroluminescence and hyperphosphorescence from stable blue Ir(III) carbene complexes with suppressed efficiency roll-off

Efficient Förster energy transfer from a phosphorescent sensitizer to a thermally activated delayed fluorescent terminal emitter constitutes a potential solution for achieving superb blue emissive organic light-emitting diodes, which are urgently needed for high-performance displays. Herein, we report the design of four Ir(III) metal complexes, f-ct1a ‒ d, that exhibit efficient true-blue emissions and fast radiative decay lifetimes. More importantly, they also undergo facile isomerization in the presence of catalysts (sodium acetate and p-toluenesulfonic acid) at elevated temperature and, hence, allow for the mass production of either emitter without decomposition. In this work, the resulting hyper-OLED exhibits a true-blue color (Commission Internationale de I'Eclairage coordinate CIEy = 0.11), a full width at half maximum of 18 nm, a maximum external quantum efficiency of 35.5% and a high external quantum efficiency 20.3% at 5000 cd m‒2, paving the way for innovative blue OLED technology.

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.

Polymorph-Dependent Phosphorescence of Cyclometalated Platinum(II) Complexes and Its Relation to Non-covalent Interactions

Cyclometalated platinum(II) complexes [Pt(ppy)Cl(CNAr)] (ppy = 2-phenylpyridinato-C²,N; Ar = C₆H₄-2-I 1, C₆H₄-4-I 2, C₆H₃-2-F-4-I 3, and C₆H₃-2,4-I₂ 4) bearing ancillary isocyanide ligands were obtained by the bridge-splitting reaction between the dimer [Pt(ppy)(μ-Cl)]₂ and 2 equiv any one of the corresponding CNAr. Complex 2 was crystallized in two polymorphic forms, namely, 2 ^I and 2 ^II, exhibiting green (emission quantum yield of 0.5%) and orange (emission quantum yield of 12%) phosphorescence, respectively. Structure-directing non-covalent contacts in these polymorphs were verified by a combination of experimental (X-ray diffraction) and theoretical methods (NCIplot analysis, combined electron localization function (ELF), and Bader quantum theory of atoms in molecules (QTAIM analysis)). A noticeable difference in the spectrum of non-covalent interactions of 2 ^I and 2 ^II is seen in the Pt···Pt interactions in 2 ^II and absence of these metallophilic contacts in 2 ^I. The other solid luminophores, namely, 1, 3 ^I-II, 4, and 4·CHCl₃, exhibit green luminescence; their structures include intermolecular C-I···Cl-Pt halogen bonds as the structure-directing interactions. Crystals of 1, 2 ^I, 3 ^I, 3 ^II, 4, and 4·CHCl₃ demonstrated a reversible mechanochromic color change achieved by mechanical grinding (green to orange) and solvent adsorption (orange to green).

Luminescence and Length Control in Nonchelated d8 -Metallosupramolecular Polymers through Metal-Metal Interactions

Supramolecular polymers (SPs) of d8 transition metal complexes have received considerable attention by virtue of their rich photophysical properties arising from metal-metal interactions. However, thus far, the molecular design is restricted to complexes with chelating ligands due to their advantageous preorganization and strong ligand fields. Herein, we demonstrate unique pathway-controllable metal-metal-interactions and remarkable 3 MMLCT luminescence in SPs of a non-chelated PtII complex. Under kinetic control, self-complementary bisamide H-bonding motifs induce a rapid self-assembly into non-emissive H-type aggregates (1A). However, under thermodynamic conditions, a more efficient ligand coplanarization leads to superiorly stabilized SP 1B with extended Pt⋅⋅⋅Pt interactions and remarkably long 3 MMLCT luminescence (τ77 K =0.26 ms). The metal-metal interactions could be subsequently exploited to control the length of the emissive SPs using the seeded-growth approach.

Copper(I)-Pyrazolate Complexes as Solid-State Phosphors: Deep-Blue Emission through a Remote Steric Effect

We describe a novel manifestation of rigidochromic behavior in a series of tetranuclear Cu(I)-pyrazolate (Cu₄pz₄) macrocycles, with implications for solid-state luminescence at deep-blue wavelengths (<460 nm). The Cu₄pz₄ emissions are remarkably sensitive to structural effects far from the luminescent core: when 3,5-di-tert-butylpyrazoles are used as bridging ligands, adding a C4 substituent can induce a blue shift of more than 100 nm. X-ray crystal and computational analyses reveal that C4 units influence the conformational behavior of adjacent tert-butyl groups, with a subsequent impact on the global conformation of the Cu₄pz₄ complex. Emissions are mediated primarily through a cluster-centered triplet (³CC) state; compression of the Cu₄ cluster into a nearly close-packed geometry prevents the reorganization of its excited-state structure and preserves the ³CC energy at a high level. The remote steric effect may thus offer alternative strategies toward the design of phosphors with rigid excited-state geometries.

The Tail Wags the Dog: The Far Periphery of the Coordination Environment Manipulates the Photophysical Properties of Heteroleptic Cu(I) Complexes

In this work we show, using the example of a series of [Cu(Xantphos)(N^N)]+ complexes (N^N being substituted 5-phenyl-bipyridine) with different peripheral N^N ligands, that substituents distant from the main action zone can have a significant effect on the physicochemical properties of the system. By using the C≡C bond on the periphery of the coordination environment, three hybrid molecular systems with -Si(CH3)3, -Au(PR3), and -C2HN3(CH2)C10H7 fragments were produced. The Cu(I) complexes thus obtained demonstrate complicated emission behaviour, which was investigated by spectroscopic, electrochemical, and computational methods in order to understand the mechanism of energy transfer. It was found that the -Si(CH3)3 fragment connected to the peripheral C≡C bond changes luminescence to long-lived intra-ligand phosphorescence, in contrast to MLCT phosphorescence or TADF. The obtained results can be used for the design of new materials based on Cu(I) complexes with controlled optoelectronic properties on the molecular level, as well as for the production of hybrid systems.

Visible-emitting Cu(i) complexes with N-functionalized benzotriazole-based ligands

Bidentate benzotriazole-based N-ligands are suited for the preparation of luminescent heteroleptic copper(i) complexes with noticeable emissions related to 3MLCT transitions.

Tuning the luminescence of transition metal complexes with acyclic diaminocarbene ligands

Organometallics featuring acyclic diaminocarbene ligands have recently emerged as powerful emitters for use in electroluminescent technologies.

Efficient long-range triplet exciton transport by metal-metal interaction at room temperature

Efficient and long-range exciton transport is critical for photosynthesis and opto-electronic devices, and for triplet-harvesting materials, triplet exciton diffusion length ( [[EQUATION]] ) and coefficient ( [[EQUATION]] ) are key parameters in determining their performances. Herein, we observed that PtII and PdII organometallic nanowires exhibit long-range anisotropic triplet exciton LD of 5-7 μm along the M-M direction using direct photoluminescence (PL) imaging technique by low-power continuous wave (CW) laser excitation. At room temperature, via a combined triplet-triplet annihilation (TTA) analysis and spatial PL imaging, an efficient triplet exciton diffusion was observed for the PtII and PdII nanowires with extended close M-M contact, while is absent in nanowires without close M-M contact. Two-dimensional electronic spectroscopy (2DES) and calculations revealed a significant contribution of the delocalized 1/3[dσ*(M-M)→π*] excited state during the exciton diffusion modulated by the M-M distance.

Phosphorescent NIR emitters for biomedicine: applications, advances and challenges

Application of NIR (near-infrared) emitting transition metal complexes in biomedicine is a rapidly developing area of research. Emission of this class of compounds in the "optical transparency windows" of biological tissues and the intrinsic sensitivity of their phosphorescence to oxygen resulted in the preparation of several commercial oxygen sensors capable of deep (up to whole-body) and quantitative mapping of oxygen gradients suitable for in vivo experimental studies. In addition to this achievement, the last decade has also witnessed the increased growth of successful alternative applications of NIR phosphors that include (i) site-specific in vitro and in vivo visualization of sophisticated biological models ranging from 3D cell cultures to intact animals; (ii) sensing of various biologically relevant analytes, such as pH, reactive oxygen and nitrogen species, RedOx agents, etc.; (iii) and several therapeutic applications such as photodynamic (PDT), photothermal (PTT), and photoactivated cancer (PACT) therapies as well as their combinations with other therapeutic and imaging modalities to yield new variants of combined therapies and theranostics. Nevertheless, emerging applications of these compounds in experimental biomedicine and their implementation as therapeutic agents practically applicable in PDT, PTT, and PACT face challenges related to a critically important improvement of their photophysical and physico-chemical characteristics. This review outlines the current state of the art and achievements of the last decade and stresses the most promising trends, major development prospects, and challenges in the design of NIR phosphors suitable for biomedical applications.

Luminescent halido(aryl) Pt(IV) complexes obtained via oxidative addition of iodobenzene or diaryliodonium salts to bis-cyclometalated Pt(II) precursors

The synthesis of bis-cyclometalated halido(aryl) Pt(IV) complexes [PtX(Ar)(C^N)₂], with C^N = cyclometalated 4-(tert-butyl)-2-phenylpyridine (bppy), 2-(p-tolyl)pyridine (tpy), 2-(2-thienyl)pyridine (thpy), or 1-phenylisoquinoline (piq), X = I, Cl, or F, and Ar = Ph (for all C^N ligands) or t-BuPh (for C^N = tpy), and the photophysical properties of the chlorido and fluorido series is reported. The oxidative addition of iodobenzene to cis-[Pt(C^N)₂] precursors is demonstrated to occur in MeCN under irradiation with visible light to give complexes [PtI(Ph)(C^N)₂], presumably involving radical species that also produce the activation of the solvent to give cyanomethyl complexes [PtI(CH₂CN)(C^N)₂]. The introduction of an aryl ligand can also be achieved by reacting cis-[Pt(C^N)₂] with (Ar₂I)PF₆ (Ar = Ph, t-BuPh), which affords cationic intermediates of the type [Pt(Ar)(C^N)₂(NCMe)]⁺. The subsequent addition of an iodide or chloride salt gives the corresponding iodido- or chlorido(aryl) complexes. The fluorido(aryl) derivatives can be obtained from the iodido complexes by halide exchange using AgF. The chlorido- and fluorido(aryl) complexes display intense phosphorescence in deaerated CH₂Cl₂ solution and poly(methyl methacrylate) (PMMA) films at 298 K from triplet excited states primarily localized on the cyclometalated ligands (³LC) with a small MLCT admixture. Compared with the chlorido complexes, the fluorido derivatives consistently present significantly shorter emission lifetimes and higher radiative and nonradiative rate constants due to a greater MLCT contribution to the emissive state. In contrast, the introduction of the t-BuPh group did not induce significant changes in radiative rates with respect to the phenyl complexes.

Diversity of Luminescent Metal Complexes in OLEDs: Beyond Traditional Precious Metals

Organic light-emitting diodes (OLED) have attracted increasing attention due to their excellent properties, such as self-luminosity, high color gamut and flexibility, and potential applications in display, wearable devices and lighting. The emitters are the most important composition in OLEDs, mainly classified into fluorescent compounds (first generation), metal phosphorescent complexes (second generation), and thermally activated delayed fluorescence (TADF) materials (third generation). In this review, we summarize the advances of novel emitters of organic metal complexes in the last decade, focusing on coinage metals (Cu, Ag, and Au) and non-precious metals (Al, Zn, W, and alkali metal). Also, the design strategy of d¹⁰ and Au(III) complexes was discussed. We aim to provide guidance for exploring efficient metal complexes beyond traditional phosphorescent complexes.

Recent Advances of Near-Infrared (NIR) Emissive Metal Complexes Bridged by Ligands with N- and/or O-Donor Sites

Near-infrared (NIR) emissive metal complexes have shown potential applications in optical communication, chemosensors, bioimaging, and laser and organic light-emitting diodes (OLEDs) due to their structural tunability and luminescence stability. Among them, complexes with bridging ligands that exhibit unique emission behavior have attracted extensive interests in recent years. The target performance can be easily achieved by NIR light-emitting metal complexes with bridging ligands through molecular structure design. In this review, the luminescence mechanism and design strategies of NIR luminescent metal complexes with bridging ligands are described firstly, and then summarize the recent advance of NIR luminescent metal complexes with bridging ligands in the fields of electroluminescence and biosensing/bioimaging. Finally, the development trend of NIR luminescent metal complexes with bridging ligands are proposed, which shows an attractive prospect in the field of photophysical and photochemical materials.