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.

Deep Blue Phosphorescence from Platinum Complexes Featuring Cyclometalated N-Pyridyl Carbazole Ligands with Monocarborane Clusters (CB11H12-)

The utilization of deep blue phosphorescent materials in high-performance displays and solid-state lighting requires high quantum efficiencies and color purities. Here, we describe the preparation and luminescent properties of novel platinum triplet emitters featuring cyclometalated N-pyridyl-carbazole ligands functionalized with closo-monocarborane clusters [CB₁₁H₁₂]^-. All reported complexes were fully characterized by using standard small molecule techniques (UV-vis, cyclic voltammetry, nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS)), and their solid-state structures were elucidated by X-ray diffraction. These platinum phosphors emit in the blue region of the visible wavelength spectrum in both the solid and solution states. Complex 4a exhibits the highest luminous efficiency at λ_em = 439 nm with a photoluminescent quantum yield (PLQY) of 60% by dispersing in a PMMA matrix. Electrochemical and computational studies of complexes 4a and 4b revealed that the blue phosphorescence originates mainly from intraligand ³π → π* (³ILCT) transitions with relatively small ³MLCT mixing. A deep-blue OLED containing 4a as the light-emitting dopant was successfully fabricated using a solution-processed method, and the device exhibited blue photoluminescence with CIE coordinates of (0.17, 0.15) and a maximum external quantum efficiency (EQE_max) value of 6.2%. This article represents the pioneering study of a deep blue PhOLED using a Pt complex bearing a closo-monocarborane anion substituent, providing a new avenue into the preparation of novel triplet emitters based on boron-rich cluster anions.

The Search for Efficient True Blue and Deep Blue Emitters: An Overview of Platinum Carbene Acetylide Complexes

Despite significant strides achieved in organic light emitting diode (OLED) based display technologies during the last decade, the search for highly stable and efficient true blue/deep blue emitters continues to remain elusive. During the past decade, emitters with the basic molecular scaffold consisting of Pt(II) acetylides and N-heterocyclic carbene (NHC) ligands have opened interesting opportunities to tune the emission properties with desired chromaticity in the blue and deep blue region. With an aim to achieve the desired CIE coordinates along with low device roll-off efficiencies and satisfactory color purity, a number of variations on the basic molecular fragment have been made. A number of NHC Pt(II) alkyne complexes bearing monodentate, bidentate and tridentate ligands have been synthesized and their photophysical investigations have been reported. Although NHC Pt(II) alkyne complexes have been explored in other areas of applications, much of the investigations have been primarily focused for their interesting emission properties appearing particularly in the shorter wavelength (450-495 nm) part of the electromagnetic spectrum for organic light emitting diode (OLED) devices. In this review, we provide an overview of the investigated NHC Pt(II) acetylide complexes by detailing their synthesis, luminescence properties, performance in devices and future perspectives.

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.

Luminescent Platinum(IV) Complexes Bearing Cyclometalated 1,2,3-Triazolylidene and Bi- or Terdentate 2,6-Diarylpyridine Ligands

The synthesis, structure, and photophysical properties of luminescent Pt^IV complexes that combine cyclometalated 1,2,3-triazolylidene and bi- or terdentate 2,6-diarylpyridine ligands are reported. The targeted complexes represent the first examples of Pt^IV species with a cyclometalated mesoionic aryl-NHC ligand. They exhibit moderate or weak emissions in fluid solution at 298 K arising from ³ LC states, which become very intense in poly(methyl methacrylate) (PMMA) matrices at 298 K. DFT and TD-DFT calculations confirm that the chromophoric ligand is the cyclometalated 2,6-diarylpyridine and show that the aryl-NHC ligand exerts a beneficial effect on the emission efficiencies of these derivatives by increasing the energy of deactivating LMCT excited states with respect to comparable Pt^IV complexes with cyclometalated 2-arylpyridine ligands.

Luminescence of Amphiphilic PtII Complexes Controlled by Confinement

The formation of hybrid silica-based systems to study the effect of the confinement on the emission properties of self-assembled platinum(II) complexes is reported. The complexes behave as surfactants since they possess a hydrophobic moiety and, on the ancillary ligand, a relatively long hydrophilic chain terminated with a positively charged group. The compounds, soluble in water, self-assemble, even at very low concentration, in supramolecular structures which display an orange luminescence. The properties of the assemblies have been studied in detail and in order to stabilize these supramolecular architectures and to enhance their emission properties hybrid silica porous nanoparticles have been prepared. In particular the Pt^II complexes have been employed as co-surfactant for the template formation of mesoporous silica nanoparticles (MSNs) using a sol gel synthesis. Interestingly, upon encapsulation in the silica pores, the platinum aggregates exhibit an emission profile similar in energy to the complexes assembled in solution, but the photoluminescence quantum yields of the hybrid systems are significantly higher (up to 45 %), and the excited state lifetimes much longer than those recorded in solution. Such enhancement of the photophysical properties together with the possibility to process the hybrid silica nanomaterials can pave the way to new type of emitters.

Improvement of the Photophysical Performance of Platinum-Cyclometalated Complexes in Halogen-Bonded Adducts

Three groups of luminescent platinum complexes [Pt(C^N)(L)(Y)] [C^N=benzothienyl-pyridine (1), bezofuryl-pyridine (2), phenyl-pyridine (3); L/Y=DMSO/Cl (a), PPh₃ /Cl (b), PPh₃ /CN (c)] have been probed as halogen-bond (XB) acceptors towards iodofluorobenzenes (IC₆ F₅ and I₂ C₆ F₄ ). Compounds 1 a and 2 a (L/Y=DMSO/Cl) afford the adducts 1 a⋅⋅⋅I₂ C₆ F₄ and 2 a⋅⋅⋅I₂ C₆ F₄ , which feature I⋅⋅⋅S_btpy /I⋅⋅⋅π_btpy and I⋅⋅⋅O_DMSO /I⋅⋅⋅Cl short contacts, respectively. The phosphane-cyanide derivatives 1 c and 2 c (L/Y=PPh₃ /CN) co-crystallise with both IC₆ F₅ and I₂ C₆ F₄ . None of the phpy-based species 3 a-3 c participated in XB interactions. Although the native complexes are rather poor luminophores in the solid state (Φ_em =0.023-0.089), the adducts exhibit an up to 10-fold increase of the intensity with a minor alteration of the emission energy. The observed gain in the quantum efficiency is mainly attributed to the joint influence of non-covalent interactions (halogen/hydrogen bonding, π-π stacking), which govern the crystal-packing mode and diminish the radiationless pathways for the T₁ →S₀ transition by providing a rigid environment around the chromophore.

Dinuclear Cu(I) Complex with Combined Bright TADF and Phosphorescence. Zero-Field Splitting and Spin-Lattice Relaxation Effects of the Triplet State

The three-fold bridged dinuclear Cu(I) complex Cu₂(μ-I)₂(1 N- n-butyl-5-diphenyl-phosphino-1,2,4-triazole)₃, Cu₂I₂(P^N)₃, shows bright thermally activated delayed fluorescence (TADF) as well as phosphorescence at ambient temperature with a total quantum yield of 85% at an emission decay time of 7 μs. The singlet (S₁)-triplet (T₁) energy gap is as small as only 430 cm^-1 (53 meV). Spin-orbit coupling induces a short-lived phosphorescence with a decay time of 52 μs ( T = 77 K) and a distinct zero-field splitting (ZFS) of T₁ into substates by ∼2.5 cm^-1 (0.3 meV). Below T ≈ 10 K, effects of spin-lattice relaxation (SLR) are observed and agree with the size of ZFS. According to the combined phosphorescence and TADF, the overall emission decay time is reduced by ∼13% as compared to the TADF-only process. The compound may potentially be applied in solution-processed OLEDs, exploiting both the singlet and triplet harvesting mechanisms.

Self-assemble nanoparticles based on polypeptides containing C-terminal luminescent Pt-cysteine complex

The growing attention to the luminescent nanocarriers is strongly stimulated by their potential application as drug delivery systems and by the necessity to monitor their distribution in cells and tissues. In this communication we report on the synthesis of amphiphilic polypeptides bearing C-terminal phosphorescent label together with preparation of nanoparticles using the polypeptides obtained. The approach suggested is based on a unique and highly technological process where the new phosphorescent Pt-cysteine complex serves as initiator of the ring-opening polymerization of α-amino acid N-carboxyanhydrides to obtain the polypeptides bearing intact the platinum chromophore covalently bound to the polymer chain. It was established that the luminescent label retains unchanged its emission characteristics not only in the polypeptides but also in more complicated nanoaggregates such as the polymer derived amphiphilic block-copolymers and self-assembled nanoparticles. The phosphorescent nanoparticles display no cytotoxicity and hemolytic activity in the tested range of concentrations and easily internalize into living cells that makes possible in vivo cell visualization, including prospective application in time resolved imaging and drug delivery monitoring.

Dibenzothiophene-platinated complexes: probing the effect of ancillary ligands on the photophysical performance

Alteration of the ancillary mono- and bidentate ligands was used to influence the photoemission behaviour of a family of cyclometalated platinum complexes.

Sonogashira (Cu and amine free) and Suzuki coupling in air catalyzed via nanoparticles formed in situ from Pd(ii) complexes of chalcogenated Schiff bases of 1-naphthaldehyde and their reduced forms

The activation of the coupling reactions with the Pd(ii)-complexes (0.05–0.01 mol% loading) is significant in 1–2 h under mild conditions.

A curious interplay in the films of N-heterocyclic carbene Pt(II) complexes upon deposition of alkali metals

The recently synthesized series of Pt^II complexes containing cyclometallating (phenylpyridine or benzoquinoline) and N-heterocyclic carbene ligands possess intriguing structures, topologies, and light emitting properties. Here, we report curious physicochemical interactions between in situ PVD-grown films of a typical representative of the aforementioned Pt^II complex compounds and Li, Na, K and Cs atoms. Based on a combination of detailed core-level photoelectron spectroscopy and quantum-chemical calculations at the density functional theory level, we found that the deposition of alkali atoms onto the molecular film leads to unusual redistribution of electron density: essential modification of nitrogen sites, reduction of the coordination Pt^II centre to Pt⁰ and decrease of electron density on the bromine atoms. A possible explanation for this is formation of a supramolecular system “Pt complex-alkali metal ion”; the latter is supported by restoration of the system to the initial state upon subsequent oxygen treatment. The discovered properties highlight a considerable potential of the Pt^II complexes for a variety of biomedical, sensing, chemical, and electronic applications.

A cyclometalated (C^C*) platinum(ii) NHC complex decorated via different carboranes to tune the photodeactivation mechanism: a theoretical investigation

Carboranes with different electronic properties are employed to tune the photodeactivation mechanism of a cyclometalated (C^C*) platinum(ii) NHC complex, including radiative and nonradiative processes.

Theoretical study and design of cyclometalated platinum complexes bearing innovatively a highly-rigid terdentate ligand with carboranyl as a chelating unit

The phosphorescence quantum efficiency can be greatly enhanced through suppression of the non-emissive ³MC population by modifications in the ancillary ligand.