Strongly Phosphorescent Palladium(II) Complexes of Tetradentate Ligands with Mixed Oxygen, Carbon, and Nitrogen Donor Atoms: Photophysics, Photochemistry, and Applications

Enhanced Emitting Dipole Orientation Based on Asymmetric Iridium(III) Complexes for Efficient Saturated-Blue Phosphorescent OLEDs

Three novel asymmetric Ir(III) complexes have been rationally designed to optimize their emitting dipole orientations (EDO) and enhance light outcoupling in blue phosphorescent organic light-emitting diodes (OLEDs), thereby boosting their external quantum efficiency (EQE). Bulky electron-donating groups (EDGs), namely: carbazole (Cz), di-tert-butyl carbazole (tBuCz), and phenoxazine (Pxz) are incorporated into the tridentate dicarbene pincer chelate to induce high degree of packing anisotropy, simultaneously enhancing their photophysical properties. Angle-dependent photoluminescence (ADPL) measurements indicate increased horizontal transition dipole ratios of 0.89 and 0.90 for the Ir(III) complexes Cz-dfppy-CN and tBuCz-dfppy-CN, respectively. Analysis of the single crystal structure and density functional theory (DFT) calculation results revealed an inherent correlation between molecular aspect ratio and EDO. Utilizing the newly obtained emitters, the blue OLED devices demonstrated exceptional performance, achieving a maximum EQE of 30.7% at a Commission International de l'Eclairage (CIE) coordinate of (0.140, 0.148). Optical transfer matrix-based simulations confirmed a maximum outcoupling efficiency of 35% due to improved EDO. Finally, the tandem OLED and hyper-OLED devices exhibited a maximum EQE of 44.2% and 31.6%, respectively, together with good device stability. This rational molecular design provides straightforward guidelines to reach highly efficient and stable saturated blue emission.

Modulation of Intermolecular Interactions in Organic Emitters for Highly Efficient Organic Light-Emitting Diodes

The adverse aggregated-caused quenching (ACQ) problem of most electroluminescent materials existing in highly doped thin films is one of the key factors impeding the commercialization of high-efficiency organic light-emitting diodes (OLEDs) panel. Whereas, by delicately constructing and modulating moderate intermolecular interactions, some aggregates have been demonstrated to present distinct luminescent properties such as tunable emission spectra, improved photoluminescence quantum yields, different emission mechanism and enhanced horizontal transition dipole ratio (Θ) of emitting layer, providing feasible solution for ACQ problem. The luminescence from newly generated emissive state in aggregates is different from the traditional "isolated" molecules in organic electronics and will possess novel properties and applications. Herein, we summarize the different types of intermolecular interactions within emitter aggregates exhibiting distinct luminescent mechanisms, as well as their effects on photoluminescent and electroluminescent properties, offering reliable reference for the advancement of highly efficient OLEDs utilizing aggregated emitters.

Pt-S Bond-Enabled Temperature-Dependent Phosphorescence in S-Heteroaryl Tetradentate Pt(S^C^N^O) Complexes

This study presents the rare examples of S-heteroaryl tetradentate Pt(S^C^N^O) luminescent complexes (PtSZ and PtSZtBu) containing a Pt-S bond. The presence of the Pt-S bond allows the novel Pt(S^C^N^O) complexes to exhibit temperature-dependent phosphorescent emission behavior. The PtSZtBu exhibits dual-emission phenomena and biexponential transient decay spectra above 250 K, indicating the presence of two minimal excited states in the potential energy surface (PES) of the T1 state. Through complementary experimental and computational studies, we have identified changes in orbital composition between Pt(dxy)-S(px) and Pt(dyz)-S(pz) in excited states with increasing temperature. This results in two energy minima, enabling the excited states to decay selectively and radiatively at different temperatures. Consequently, this leads to remarkable steady-state and transient emission spectra changes. Our work not only provides valuable insights for the development of novel Pt-S bond-based tetradentate Pt(II) complexes but also enhances our understanding of the distinctive properties governed by the Pt-S bond.

Enhanced luminescence properties through heavy ancillary ligands in [Pt(C^N^C)(L)] complexes, L = AsPh3 and SbPh3

In the frame of our research aiming to develop efficient triplet-emitting materials, we are exploring the concept of introducing additional heavy atoms into cyclometalated transition metal complexes to enhance intersystem-crossing (ISC) and thus triplet emission through increased spin-orbit coupling (SOC). In an in-depth proof-of-principle study we investigated the double cyclometalated Pt(II) complexes [Pt(C^N^C)(PnPh3)] (HC^N^CH = 2,6-diphenyl-pyridine (H2dpp) or dibenzoacridine (H2dba); Pn = pnictogen atoms P, As, Sb, or Bi) through a combined experimental and theoretical approach. The derivatives containing Pn = P, As, and Sb were synthesised and characterised comprehensively using single crystal X-ray diffraction (scXRD), UV-vis absorption and emission spectroscopy, transient absorption (TA) spectroscopy and cyclic voltammetry (CV). Across the series P < As < Sb, a red-shift is observed concerning absorption and emission maxima as well as optical and electrochemical HOMO-LUMO gaps. Increased photoluminescence quantum yields ΦL and radiative rates kr from mixed metal-to-ligand charge transfer (MLCT)/ligand centred (LC) triplet states are observed for the heavier homologues. Transient absorption spectroscopy showed processes in the ps range that were assigned to the population of the T1 state by ISC. The heavy PnPh3 ancillary ligands are found to enhance the emission efficiency due to both higher Pt-Pn bond strength and stronger SOC related to increased MLCT character of the excited states. The experimental findings are mirrored in hybrid (TD-)DFT calculations. This allowed for extrapolation to the rather elusive Bi derivatives, which were synthetically not accessible. This shortcoming is attributed to the transmetalation of phenyl groups from BiPh3 to Pt, as supported by experimental NMR/MS as well as DFT studies.

Discrete Mononuclear Platinum(II) Complexes Realize High-Performance Red Phosphorescent OLEDs with EQEs of up to 31.8% and Superb Device Stability

Designing mononuclear platinum(II) complexes that do not rely on intermolecular aggregation for high-performance red organic light-emitting diodes remains a formidable challenge. In this work, three robust red-emitting Pt(II) complexes are created by utilizing a rigid 4-coordination configuration, where the ligands are formed by linking electron-donor of triphenylamine (TPA) moieties with electron-acceptor of pyridine, isoquinoline, and/or δ-carboline units. The thermal stability, electrochemical, and photophysical properties of the complexes are thoroughly examined. The complexes display efficient red phosphorescence, with high photoluminescence quantum yields and short excited lifetimes. The OLEDs dope with these complexes exhibit high maximum external quantum efficiencies (EQEs) of up to 31.8% with minimal efficiency roll-off even at high brightness. Significantly, the devices demonstrate exceptional long operational lifetime, with a T90 lifetime of over 14000 h at initial luminance of 1000 cd m-2 , indicating the potential for these complexes to be practically utilizes.

Highly efficient and stable thermally activated delayed fluorescent palladium(II) complexes for organic light-emitting diodes

Transition metal complexes exhibiting thermally activated delayed fluorescence (TADF) remain underdeveloped for organic light-emitting diodes (OLEDs). Here, we describe a design of TADF Pd(II) complexes featuring metal-perturbed intraligand charge-transfer excited states. Two orange- and red-emitting complexes with efficiencies of 82 and 89% and lifetimes of 2.19 and 0.97 μs have been developed. Combined transient spectroscopic and theoretical studies on one complex reveal a metal-perturbed fast intersystem crossing process. OLEDs using the Pd(II) complexes show maximum external quantum efficiencies of 27.5 to 31.4% and small roll-offs down to 1% at 1000 cd m^-2. Moreover, the Pd(II) complexes show exceptional operational stability with LT₉₅ values over 220 hours at 1000 cd m^-2, benefiting from the use of strong σ-donating ligands and the presence of multiple intramolecular noncovalent interactions beside their short emission lifetimes. This study demonstrates a promising approach for developing efficient and robust luminescent complexes without using the third-row transition metals.

Electroluminescence of Tetradentate Pt(II) Complexes: O^N^N^O versus C^N^N^O Coordination

Alkylation of one of the phenolic hydroxyl groups in a salen-type tetradentate ligand changes the coordination mode from O^N^N^O to the cyclometallating C^N^N^O type. The ligand was used to synthesize a new cyclometalated luminescent Pt(II) complex 2. While in solution the complex is poorly luminescent, in the solid state the emission is reinstated, which allowed one to evaluate complex 2 as a phosphorescent emitter in organic light-emitting diodes. 2 displays external quantum efficiency (EQE) = 9.1% and a maximum luminance of 9000 cd m^-2 in a vacuum-deposited device. We carried out comparative analysis of photo- and electroluminescence of complex 2 with O^N^N^O complex 1 and demonstrated that the similar luminescent properties of the O^N^N^O and C^N^N^O complexes are rather coincidental because they display different excited-state landscapes. Surprisingly, the two complexes display a dramatically different electrochemical behavior, with O^N^N^O coordination leading to the formation of a stable electropolymer but C^N^N^O coordination fully preventing electropolymerization.

Luminescence and Palladium: The Odd Couple

The synthesis, photophysical properties, and applications of highly fluorescent and phosphorescent palladium complexes are reviewed, covering the period 2018–2022. Despite the fact that the Pd atom appears closely related with an efficient quenching of the fluorescence of different molecules, different synthetic strategies have been recently optimized to achieve the preservation and even the amplification of the luminescent properties of several fluorophores after Pd incorporation. Beyond classical methodologies such as orthopalladation or the use of highly emissive ligands as porphyrins and related systems (for instance, biladiene), new concepts such as AIE (Aggregation Induced Emission) in metallacages or in coordination-driven supramolecular compounds (CDS) by restriction of intramolecular motions (RIM), or complexes showing TADF (Thermally Activated Delayed Fluorescence), are here described and analysed. Without pretending to be comprehensive, selected examples of applications in areas such as the fabrication of lighting devices, biological markers, photodynamic therapy, or oxygen sensing are also here reported.

Electro-Synthesis of Organic Compounds with Heterogeneous Catalysis

Electro-organic synthesis has attracted a lot of attention in pharmaceutical science, medicinal chemistry, and future industrial applications in energy storage and conversion. To date, there has not been a detailed review on electro-organic synthesis with the strategy of heterogeneous catalysis. In this review, the most recent advances in synthesizing value-added chemicals by heterogeneous catalysis are summarized. An overview of electrocatalytic oxidation and reduction processes as well as paired electrocatalysis is provided, and the anodic oxidation of alcohols (monohydric and polyhydric), aldehydes, and amines are discussed. This review also provides in-depth insight into the cathodic reduction of carboxylates, carbon dioxide, CC, C≡C, and reductive coupling reactions. Moreover, the electrocatalytic paired electro-synthesis methods, including parallel paired, sequential divergent paired, and convergent paired electrolysis, are summarized. Additionally, the strategies developed to achieve high electrosynthesis efficiency and the associated challenges are also addressed. It is believed that electro-organic synthesis is a promising direction of organic electrochemistry, offering numerous opportunities to develop new organic reaction methods.

Room-temperature luminescence from Pd(II) and Pt(II) complexes: from mechanochromic crystals to flexible polymer matrices

A series of Pd(II) (PdLOMe, PdLOHex) and Pt(II) (PtLOMe, PtLOHex) complexes bearing tetradentate ligands as dianionic luminophores were synthesized. Hence, the cyclometallating chelators were alternatively decorated with two n-hexyloxy (LOHex) or two methoxy (LOMe) moieties to promote crystallization and processability. The new compounds were unambiguously characterized by means of multiple NMR spectroscopies and mass spectrometry as well as by single crystal X-ray diffractometric analysis (PtLOMe and PdLOMe). Steady state and time-resolved photoluminescence spectroscopic studies were carried out in crystalline phases, in fluid solutions at room temperature, in frozen glassy matrices at 77 K and in a flexible polymeric matrix (PMMA). PtLOMe presents an intriguing mechanochromism resulting from the responsive metal-metal interactions involving adjacent monomeric units. Incorporation of the Pd(II) complexes into the polymeric matrix boosts their photophysical properties by stiffening of the coordination environment while reducing non-radiative deactivation pathways mediated by dissociative metal-centred states, which also become thermally inaccessible at 77 K.

Dinuclear PtII Complexes with Strong Blue Phosphorescence for Operational Stable Organic Light-Emitting Diodes with EQE up to 23% at 1000 cd m-2

Here we describe the synthesis and characterization of a new class of dinuclear PtII complexes with blue phosphorescence. Bulky N-heterocyclic carbene and tethered bridging ligands were employed to suppress photo-induced structural changes and to improve thermal stability of the complexes. These complexes show mixed 3IL/3MLCT blue emission (~460 nm) with emission quantum yields of up to 0.95, emission lifetimes of as low as 1.3 µs and radiative decay rate constants of up to 7.3 × 105 s-1 in 4 wt% doped PMMA films; the latter is attributed to a 1MLCT excited state having high metal character (resulting in large SOC) and a large transition dipole moment, based on DFT calculations. Phosphor-sensitized blue hyperOLEDs with Commission Internationale de L'Eclairage (CIE) coordinates of (0.13, 0.12) showed maximum EQE of 23.4% with full-width-at-half-maximum of 18 nm and LT50 > 250 h at an L0 of 1000 cd m-2.

Metal- and oxidant-free electrochemically promoted oxidative coupling of amines

The selective oxidation of amines into imines is a priority research topic in organic synthesis and has attracted much attention over the past few decades. However, the oxidation of amines generally suffers from the drawback of transition-metal, even noble-metal catalysts. Thus, the strategy of metal- and oxidant-free selective synthesis of imines is highly desirable yet largely unmet. This paper unravels a metal-free and external oxidant-free electrochemical strategy for the oxidative coupling methodology of amines. This general transformation is compatible with various functional amines and led to functionalized imines in moderate to satisfactory yields.

Application of Triplet-Triplet Annihilation Upconversion in Organic Optoelectronic Devices: Advances and Perspectives

Organic semiconductor materials have been widely used in various optoelectronic devices due to their rich optical and/or electrical properties, which are highly related to their excited states. Therefore, how to manage and utilize the excited states in organic semiconductors is essential for the realization of high-performance optoelectronic devices. Triplet-triplet annihilation (TTA) upconversion is a unique process of converting two non-emissive triplet excitons to one singlet exciton with higher energy. Efficient optical-to-electrical devices can be realized by harvesting sub-bandgap photons through TTA-based upconversion. In electrical-to-optical devices, triplets generated after the combination of electrons and holes also can be efficiently utilized via TTA, which resulted in a high internal conversion efficiency of 62.5%. Currently, many interesting explorations and significant advances have been demonstrated in these fields. In this review, a comprehensive summary of these intriguing advances on developing efficient TTA upconversion materials and their application in optoelectronic devices is systematically given along with some discussions. Finally, the key challenges and perspectives of TTA upconversion systems for further improvement for optoelectronic devices and other related research directions are provided. This review hopes to provide valuable guidelines for future related research and advancement in organic optoelectronics.

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.

Tetradentate Platinum(II) and Palladium(II) Complexes Containing Fused 6/6/6 or 6/6/5 Metallocycles with Azacarbazolylcarbazole-Based Ligands

A series of novel tetradentate Pt(II) and Pd(II) complexes containing fused 6/6/6 or 6/6/5 metallocycles employing azacarbazolylcarbazole (ACzCz)-based ligands was developed. Systematic experimental and theoretical studies suggest that both the ligand structures and the central metal ions have great influences on the electrochemical and photophysical properties of the complexes. The time-dependent density functional theory (TD-DFT) calculations and natural transition orbital (NTO) analyses reveal that the Pt(II) complexes possess 10.8-15.2% metal-to-ligand charge transfer (³MLCT) mixed with ligand-centered (³LC) characters, by contrast, the Pd(II) complexes exhibit significantly decreased 4.2-7.1% ³MLCT characters and enhanced ³LC compositions. All of the Pt(II) and Pd(II) complexes possess various channels for the intersystem crossing (ISC) on the basis of small energy gaps ΔE_S1-Tn and matching transition orbital compositions; moreover, Pd(ACzCz-1) and Pd(ACzCz-2) also possess efficient reverse intersystem crossing (RISC) to show both delayed fluorescence (DF) and phosphorescence in PMMA films at room temperature (RT). Pt(ACzCz-3) has Φ_PL values of 57% with a τ of 5.1 μs in dichloromethane at RT and 50% with 3.9 μs in PMMA at RT. Notably, Pd(ACzCz-1) exhibits ultralong low-temperature phosphorescence with a τ of 1307 μs. Pt(ACzCz-2)-based green OLED employing 26mCPy as the host demonstrated a peak EQE of 8.2% and a L_max of 24065 cd/m².

Exceptionally fast radiative decay of a dinuclear platinum complex through thermally activated delayed fluorescence

A novel dinuclear platinum(ii) complex featuring a ditopic, bis-tetradentate ligand has been prepared. The ligand offers each metal ion a planar O^N^C^N coordination environment, with the two metal ions bound to the nitrogen atoms of a bridging pyrimidine unit. The complex is brightly luminescent in the red region of the spectrum with a photoluminescence quantum yield of 83% in deoxygenated methylcyclohexane solution at ambient temperature, and shows a remarkably short excited state lifetime of 2.1 μs. These properties are the result of an unusually high radiative rate constant of around 4 × 105 s-1, a value which is comparable to that of the very best performing Ir(iii) complexes. This unusual behaviour is the result of efficient thermally activated reverse intersystem crossing, promoted by a small singlet-triplet energy difference of only 69 ± 3 meV. The complex was incorporated into solution-processed OLEDs achieving EQEmax = 7.4%. We believe this to be the first fully evidenced report of a Pt(ii) complex showing thermally activated delayed fluorescence (TADF) at room temperature, and indeed of a Pt(ii)-based delayed fluorescence emitter to be incorporated into an OLED.

Synthesis, Study, and Application of Pd(II) Hydrazone Complexes as the Emissive Components of Single-Layer Light-Emitting Electrochemical Cells

For the first time, square planar Pd(II) complexes of hydrazone ligands have been investigated as the emissive components of light-emitting electrochemical cells (LECs). The neutral transition metal complex, [Pd(L¹)₂]·2CH₃OH (1), (HL¹ = (E)-N'-(phenyl(pyridin-2-yl)methylene)isonicotinhydrazide), was prepared and structurally characterized. Complex 1 displays quasireversible redox properties and is emissive at room temperature in solution with a λ_max of 590 nm. As a result, it was subsequently employed as the emissive material of a single-layer LEC with configuration FTO/1/Ga/In, where studies reveal that it has a yellow color with CIE(x, y) = (0.33, 0.55), a luminance of 134 cd cm^-2, and a turn-on voltage of 3.5 V. Protonation of the pendant pyridine nitrogen atoms of L¹ afforded a second ionic complex [Pd(L¹H)₂](ClO₄)₂ (2) which is also emissive at room temperature with a λ_max of 611 nm, resulting in an orange LEC with CIE(x, y) = (0.43, 0.53). The presence of mobile anions and cations in the second inorganic transition metal complex resulted in more efficient charge injection and transport which significantly improved the luminance and turn-on voltage of the device to 188.6 cd cm^-2 and 3 V, respectively. This study establishes Pd(II) hydrazone complexes as a new class of materials whose emissive properties can be chemically tuned and provides proof-of-concept for their use in LECs, opening up exciting new avenues for potential applications in the field of solid state lighting.

Ligand-controlled and nanoconfinement-boosted luminescence employing Pt(ii) and Pd(ii) complexes: from color-tunable aggregation-enhanced dual emitters towards self-referenced oxygen reporters

In this work, we describe the synthesis, structural and photophysical characterization of four novel Pd(ii) and Pt(ii) complexes bearing tetradentate luminophoric ligands with high photoluminescence quantum yields (Φ L) and long excited state lifetimes (τ) at room temperature, where the results were interpreted by means of DFT calculations. Incorporation of fluorine atoms into the tetradentate ligand favors aggregation and thereby, a shortened average distance between the metal centers, which provides accessibility to metal-metal-to-ligand charge-transfer (3MMLCT) excimers acting as red-shifted energy traps if compared with the monomeric entities. This supramolecular approach provides an elegant way to enable room-temperature phosphorescence from Pd(ii) complexes, which are otherwise quenched by a thermal population of dissociative states due to a lower ligand field splitting. Encapsulation of these complexes in 100 nm-sized aminated polystyrene nanoparticles enables concentration-controlled aggregation-enhanced dual emission. This phenomenon facilitates the tunability of the absorption and emission colors while providing a rigidified environment supporting an enhanced Φ L up to about 80% and extended τ exceeding 100 μs. Additionally, these nanoarrays constitute rare examples for self-referenced oxygen reporters, since the phosphorescence of the aggregates is insensitive to external influences, whereas the monomeric species drop in luminescence lifetime and intensity with increasing triplet molecular dioxygen concentrations (diffusion-controlled quenching).

Phosphorescent κ3 -(N^C^C)-Gold(III) Complexes: Synthesis, Photophysics, Computational Studies and Application to Solution-Processable OLEDs

Efficient OLED devices have been fabricated using organometallic complexes of platinum group metals. Still, the high material cost and low stability represent central challenges for their application in commercial display technologies. Based on its innate stability, gold(III) complexes are emerging as promising candidates for high-performance OLEDs. Here, a series of alkynyl-, N-heterocyclic carbene (NHC)- and aryl-gold(III) complexes stabilized by a κ³ -(N^C^C) template have been prepared and their photophysical properties have been characterized in detail. These compounds exhibit good photoluminescence quantum efficiency (η_PL ) of up to 33 %. The PL emission can be tuned from sky-blue to yellowish green colors by variations on both the ancillary ligands as well as on the pincer template. Further, solution-processable OLED devices based on some of these complexes display remarkable emissive properties (η_CE 46.6 cd.A^-1 and η_ext 14.0 %), thus showcasing the potential of these motifs for the low-cost fabrication of display and illumination technologies.

Fluorescent perylenylpyridine complexes: an experimental and theoretical study

The perylene derivative 2-(3-perylenyl)-4-methylpyridine (HPerPy) was prepared and used to synthesize [Ag(HPerPy)(PPh3)(OClO3)], with the perylene ligand bonded to the metal centre only by the pyridine nitrogen. The treatment of HPerPy with [Pd(OAc)2] in methanol or acetic acid led to acetate bridged dimers (μ-OOCCH3)2[Pd(PerPy)]2, six-membered or five-membered cycled at the perylenyl fragment. Substitution reactions afforded mononuclear compounds [Pd(PerPy)(acac)] (six-member or five-member cycled) and [Pd(PerPy)(S2COMe)] (six-member or five-member cycled). The reaction of HPerPy with a platinum(ii) fragment led to a five-membered cyclometallated Pt(ii) complex [Pt(PerPy)(acac)]. The oxidative addition with MeI gave the corresponding cyclometallated Pt(iv) compound [Pt(PerPy)(acac)MeI]. X-ray single crystal studies of compounds [Ag(HPerPy)(PPh3)(OClO3)], (μ-OOCCH3)2[Pd(PerPy)]2-five-membered, [Pd(PerPy)(acac)]-six-membered, [Pd(PerPy)(S2COMe)]-five-membered, [Pt(PerPy)(acac)]-five-membered, and [Pt(PerPy)(acac)MeI]-five-membered confirmed the proposed structures. The UV-Vis spectra show one intense absorption with vibronic coupling in the visible region with maxima in the range of 448-519 nm. DFT calculations were carried out for the absorption spectra of the HPerPy molecule and representative complexes [M(PerPy)(acac)] (M: Pd, Pt; five and six-membered isomers) and [Pt(PerPy)(acac)MeI], showing that the lowest energy most intense transition in the complexes corresponds to the HOMO → LUMO transition in the perylene moiety, although affected by the metallacycle size and the metal nature. All the compounds are fluorescent in solution, due to the perylene fragment. The emission spectra display maxima in the range of 468-549 nm, with quantum yields from 1.1 to 82%. The attenuation of the intensity of fluorescence by the presence of heavy atoms and the formation of metallacycles has been experimentally determined and sequenced.

Non-emissive RuII Polypyridyl Complexes as Efficient and Selective Photosensitizers for the Photooxidation of Benzylamines

Five new Ru^II polypyridyl complexes bearing N-(arylsulfonyl)-8-amidoquinolate ligands and three of their biscyclometalated Ir^III congeners have been prepared and employed as photocatalysts (PCs) in the photooxidation of benzylamines with O₂ . In particular, the new Ru^II complexes do not exhibit photoluminescence, rather they harvest visible light efficiently and are very stable in solution under irradiation with blue light. Their non-emissive behavior has been related to the low electrochemical energy gaps and rationalized on the basis of theoretical calculations (DFT analysis) that predict low S₀ ←T₁ energy values. Moreover, the Ru^II complexes, despite being non-emissive, display excellent activities in the selective photocatalytic transformation of benzylamines into the corresponding imines. The presence of an electron-withdrawing group (-CF3) on the arene ring of the N-(arylsulfonyl)-8-amidoquinolate ligand improves the photocatalytic activity of the corresponding photocatalyst. Furthermore, all the experimental evidence, including transient absorption spectroscopy measurements suggest that singlet oxygen is the actual oxidant. The Ir^III analogues are considerably more photosensitive and consequently less efficient photosensitizers (PSs).

Highly Efficient Phosphorescent Tetradentate Platinum(II) Complexes Containing Fused 6/5/6 Metallocycles

A series of neutral tetradentate Pt(II) complexes with fused 6/5/6 metallocycles and biphenyl (bp)-containing ligands have been designed and synthesized. All bridging atoms adopt nitrogens designed as an acridinyl group (Ac), an aza acridinyl group (AAc), and an aza carbazolyl group (ACz), which can effectively tune their LUMO energy levels. Their HOMO energy levels can be well-controlled through molecular modifications on the bp moieties with electron-donating and electron-withdrawing groups. These molecular modifications also have profound effects on the electrochemical and photophysical properties and photostabilities of the Pt(II) complexes. The ground-states and excited states are systematically studied by density functional theory (DFT), time-dependent density functional theory (TD-DFT), and natural transition orbital (NTO) calculations. All the Pt(II) complexes exhibit admixed ³(LC/MLCT) characters in T₁ states with various proportions, which are strongly structure-dependent. These 6/5/6 Pt(II) complexes demonstrate high quantum efficiencies in dichloromethane solutions (Φ_PL = 27-51%) and in doped PMMA films (Φ_PL = 36-52%) at room temperature with short luminescence lifetimes of 1.6-9.5 μs and 7.6-9.0 μs, respectively. They emit green light with dominant peaks of 512-529 nm in solutions and 512-524 nm in doped PMMA films, respectively. Importantly, Pt(bp-2) exhibits highly stable emission colors with the same dominant peaks at 512 nm in various matrixes and also demonstrates a long photostability lifetime, LT₈₀, at 80% of initial luminance, of 190 min, which is doped in polystyrene films (5 wt %) excited by UV light of 375 nm at 500 W/m². These studies indicate that these 6/5/6 Pt(II) complexes can act as good phosphorescent emitters for OLED applications and should provide a viable route for the development of efficient and stable Pt(II)-based phosphorescent emitters.

Photoexcited Pd(ii) auxiliaries enable light-induced control in C(sp3)-H bond functionalisation

Herein we report the photophysical and photochemical properties of palladacycle complexes derived from 8-aminoquinoline ligands, commonly used auxiliaries in C–H activation. Spectroscopic, electrochemical and computational studies reveal that visible light irradiation induces a mixed LLCT/MLCT charge transfer providing access to synthetically relevant Pd(iii)/Pd(iv) redox couples. The Pd(ii) complex undergoes photoinduced electron transfer with alkyl halides generating C(sp³)–H halogenation products rather than C–C bond adducts. Online photochemical ESI-MS analysis implicates participation of a mononuclear Pd(iii) species which promotes C–X bond formation via a distinct Pd(iii)/Pd(iv) pathway. To demonstrate the synthetic utility, we developed a general method for inert C(sp³)–H bond bromination, chlorination and iodination with alkyl halides. This new strategy in auxiliary-directed C–H activation provides predictable and controllable access to distinct reactivity pathways proceeding via Pd(iii)/Pd(iv) redox couples induced by visible light irradiation.

Visible light irradiation of 8-aminoquinoline Pd(ii) complexes initiates photoinduced electron transfer with alkyl halides, affording C–H halogenation over C–C bond adducts. A method for inert C(sp³)–H bond halogenation (Br, Cl and I) is reported.

Synthetic access to a phosphorescent non-palindromic pincer complex of palladium by a double oxidative addition – comproportionation sequence

A highly phosphorescent non-palindromic (C^C^N) palladium complex may be prepared by means of a double oxidative addition – comproportionation sequence, which is a new approach for the synthesis of non-palindromic pincer complexes.

Isomeric Tetradentate Ligand-Containing Cyclometalated Gold(III) Complexes

A simple one-pot two bond-forming reaction for the rapid construction of cyclometalated gold(III) complexes with fully π-conjugated tetradentate ligand is reported. The coupling of the bifunctional gold(III) precursor with the bifunctional aromatic compound has led to the formation of two regioisomers with either C- or N-coordination. Through monitoring by high-throughput high performance liquid chromatography, the regioselectivity of the reaction has been effectively tuned toward the formation of a single isomer, allowing easy separation of the metal complexes. The structures of the complexes have been determined by X-ray crystallography, and the photophysical, electrochemical, and electroluminescence (EL) studies have been carried out. Computational study has been performed to provide insights into the nature of the excited states. Isomeric effect has been shown to have a significant influence on the EL behavior of the organic light-emitting devices.

Unusual near infrared (NIR) fluorescent palladium(ii) macrocyclic complexes containing M-C bonds with bioimaging capability

Near infrared (NIR) luminescent metal complexes are promising probes in bioimaging and biosensing, however they generally suffer from oxygen interference arising from heavy metal effects. We designed new tetradentate macrocyclic benzitripyrrin (C^N^N^N) ligands by combination of M-C bond formation and reducing the π-conjugation to achieve NIR fluorescent Pd complexes (700-1000 nm) with quantum yields up to 14%. To understand the origin of NIR fluorescence, detailed analyses by density functional theory/time-dependent density functional theory (DFT/TDDFT) calculations together with femtosecond and nanosecond transient absorption spectroscopies suggest that M-C bond formation indeed leads to destabilization of the d-d excited state and less effective quenching of emission; and importantly, small spin-orbital coupling (SOC) and the large singlet-triplet energy gap are the primary causes of the non-population of triplet states. Comparison of PdII and PtII analogues shows that the non-radiative channel of the out-plane vibration of the tripyrrin plane effectively quenches the fluorescence of the PtII complex but not the PdII congener. We also demonstrate the proof-of-concept applications of PdII complexes (Pd-1 and Pd-3) encapsulated in silica nanoparticles, in both in vitro and in vivo bioimaging experiments without oxygen interference. Moreover, pH-induced reversible switching of NIR fluorescence was achieved even intracellularly using the Pd complex (Pd-2), which shows the potential to further develop perspective stimuli-responsive NIR materials.