Highly Efficient Blue-Emitting Cyclometalated Platinum(II) Complexes by Judicious Molecular Design

Phosphorescent PdII-PdII Emitter-Based Red OLEDs with an EQEmax of 20.52

Three dinuclear Pd(II) complexes (1, 2, and 3) with intense red phosphorescence at room temperature are here synthesized using strong ligand field strength compounds. All three complexes are characterized by nuclear magnetic resonance, high-resolution mass spectrometry, and elemental analyses. Complexes 2 and 3 are characterized by single-crystal X-ray diffraction. The crystalline data of 2 and 3 reveal complex double-layer structures, with Pd-Pd distances of 2.8690(9) Å and 2.8584(17) Å, respectively. Furthermore, complexes 1, 2, and 3 show phosphorescence at room temperature in their solid states at the wavelengths of 678, 601, and 672 nm, respectively. In addition, they show phosphorescence at 634, 635, and 582 nm, respectively, in the 2 wt.% (PMMA) films, and phosphorescence at 670, 675, and 589 nm, respectively, in the deoxygenated CH2Cl2 solutions. Among three complexes, complex 1 shows red emission at 634 nm with phosphorescent quantum yield Ф = 67% in the 2 wt.% PMMA film. Furthermore, complex 1-based organic light-emitting diode is fabricated using a vapor-phase deposition process, and their maximum external quantum efficiency reaches 20.52%, which is the highest percentage obtained by using the dinuclear Pd(II) complex triplet emitters with the CIE coordinates of (0.62, 0.38).

Modified t-butyl in tetradentate platinum (II) complexes enables exceptional lifetime for blue-phosphorescent organic light-emitting diodes

In blue phosphorescent dopants, the tetradentate platinum(II) complex is a promising material showing high efficiency and stability in devices. However, metal-metal-to-ligand charge transfer (MMLCT) formation leads to low photo-luminescence quantum yields (PLQYs), wide spectra, and intermolecular interaction. To suppress MMLCT, PtON-tb-TTB and PtON-tb-DTB are designed using theoretical simulation by modifying t-butyl in PtON-TBBI. Both materials effectively suppress MMLCT and exhibit high PLQYs of 99% and 78% in 5 wt% doped film, respectively. The PtON-tb-TTB and PtON-tb-DTB devices have maximum external quantum efficiencies of 26.3% and 20.9%, respectively. Additionally, the PtON-tb-DTB device has an extended lifetime of 169.3 h with an initial luminescence of 1200 nit, which is 8.5 times greater than the PtON-TBBI device. Extended lifetime because of suppressed MMLCT and smaller displacement between the lowest triplet and triplet metal-centered states compared to other dopants. The study provides an effective approach to designing platinum(II) complexes for long device lifetimes.

High-Efficiency Simplified Orange and White Organic Light-Emitting Devices Based on a Platinum(II) Complex

We demonstrated efficient simplified orange and white organic light-emitting devices based on a platinum(II) complex Tetra-Pt-N. The maximum current efficiency achieved from the optimized orange device was 57.6 cd/A. The emission mechanism for the system of Tetra-Pt-N doped into 4,4'-bis(arbazole-9-yl)biphenyl was discussed. Moreover, a high-efficiency and simplified white device was fabricated by introducing an ultra-thin blue phosphorescent emission layer. The white device with a maximum current efficiency of 41.9 cd/A showed excellent stable spectra and low efficiency roll-off.

Cyclometalated Platinum(II) Complexes with Donor-Acceptor-Containing Bidentate Ligands and Their Application Studies as Organic Resistive Memories

A series of heteroleptic cyclometalated platinum(II) complexes, [Pt(C^N)(O^O)], (1-10) with various donors and acceptors has been synthesized and characterized by ¹ H NMR spectroscopy, elemental analyses, infrared spectroscopy and mass spectrometry. The X-ray structure of 2 has also been determined. The electrochemical and photophysical properties of the platinum(II) complexes were studied. These experimental results have been supported by computational studies. Furthermore, two of the complexes have been employed as the active material in the fabrication of resistive memory devices, exhibiting stable binary memory performance with low operating voltage, high ON/OFF ratio and long retention time.

Functional Materials Based on Cyclometalated Platinum(II) β-Diketonate Complexes: A Review of Structure-Property Relationships and Applications

Square planar organoplatinum(II) complexes have garnered immense interest in the area of materials research. The combination of the Pt(II) fragment with mono-, bi- tri- and tetradentate organic ligands gives rise to a large variety of complexes with intriguing properties, especially cyclometalated Pt(II) complexes in which ligands are connected through covalent bonds demonstrate higher stability, excellent photoluminescence properties, and diverse applications. The properties and applications of the Pt(II)-based materials can be smartly fine-tuned via a judicious selection of the cyclometalating as well as ancillary ligands. In this review, attempts have been made to provide a brief review of the recent developments of neutral Pt(II) organometallic complexes bearing bidentate cyclometalating ligands and β-diketonate ancillary ligands, i.e., (C^N)Pt(O^O) and (C^C)Pt(O^O) derivatives. Both small (monomeric, dimeric) and large (polymeric) materials have been considered. We critically assessed the role of functionalities (ligands) on photophysical properties and their impact on applications.

A Brief History of OLEDs-Emitter Development and Industry Milestones

Organic light-emitting diodes (OLEDs) have come a long way ever since their first introduction in 1987 at Eastman Kodak. Today, OLEDs are especially valued in the display and lighting industry for their promising features. As one of the research fields that equally inspires and drives development in academia and industry, OLED device technology has continuously evolved over more than 30 years. OLED devices have come forward based on three generations of emitter materials relying on fluorescence (first generation), phosphorescence (second generation), and thermally activated delayed fluorescence (third generation). Furthermore, research in academia and industry toward the fourth generation of OLEDs is in progress. Excerpts from the history of green, orange-red, and blue OLED emitter development on the side of academia and milestones achieved by key players in the industry are included in this report.

Synthesis of N-Alkyl and N-H-Carbazoles through SN Ar-Based Aminations of Dibenzothiophene Dioxides

Alkyl amines have become available for the synthesis of diverse N-alkyl carbazoles through twofold S_N Ar aminations of dibenzothiophene dioxides by using alkali metal bases. Of particular importance is the choice of counter cations on alkali metal bases, that is, i) the use of Li base for the efficient intermolecular reaction and ii) the sequential addition of heavier alkali metal bases (Na, K, or Cs) to promote intramolecular cyclization in a one-pot manner. This protocol also enables the cascade synthesis of N-H-carbazoles by using 2-phenylethylamine by removal of the 2-phenethyl group from N-(2-phenethyl) carbazoles in a single operation.

Sky-Blue Triplet Emitters with Cyclometalated Imidazopyrazine-Based NHC-Ligands and Aromatic Bulky Acetylacetonates

Platinum(II) complexes with an N-heterocyclic carbene and a cyclometalating phenyl ligand (C^C*) are excellent candidates as efficient blue triplet emitters for OLED applications. The electronic and photophysical properties of these complexes can be fine-tuned with the objective to increase the quantum yields and lower the phosphorescence decay times. We found that platinum complexes with an imidazopyrazine C^C* ligand and bulky acetylacetonates are sky-blue triplet emitters, characterised by an almost unitary quantum yield and short phosphorescence decay times.

What accounts for the color purity of tetradentate Pt complexes? A computational analysis

In order to improve the texture of human visual perception and broaden the range of certain optical applications, many phosphorescent complexes exhibiting narrow emission spectra have been prepared through reasonable molecular design. For example, by adding a particular group such as tert-butyl (tbu) to a suitable position of PtON1 and PtON7, the peak width of a relevant vibronic band caused by the specific vibrational normal modes could be dramatically restrained in the emission spectra at room temperature. For the purpose of finding an effective approach to replace the trial-and-error manner, the microscopic mechanism of such high color purity was elucidated by computational investigation. In this study, we aim to identify the reason that causes sharp emission associated with the relevant vibrational normal modes. Here, these modes can be labeled to the emission peak by the vibrationally resolved emission spectra. Based on the displacement vectors of relevant normal modes and the vibrationally resolved spectra, the most possible reason for the higher color purity is that tbu in a specific location can restrain the structural deformation between the first triplet excited state (T₁) and the ground state (S₀). That is to say, the relevant Huang-Rhys factor (S_k) of specific vibrational modes would be decreased. For these compounds, the total bandwidth and the height of the intermediate and high-frequency regions which are in direct proportion to S_k would be decreased to obtain the higher color purity by tbu in a particular position. What is more, the best position for tbu in order to suppress the structural deformation was also considered. In the meantime, radiative (k_r) and nonradiative (k_nr) decay rates of T₁ were investigated to seek the effective phosphorescent complexes.

Dielectric Relaxation and Beyond Limiting Behavior of Alternating-Current Conductivity in a Supermolecular Ferroelectric

A cyclen-based hybrid supermolecule crystal, [(FeCl₂ )(cyclen)]Cl (1), where cyclen=1,4,7,10-tetraazacyclododecane, was prepared using a liquid-liquid diffusion approach. The variable crystal structures exhibit that compound 1 belongs to an orthorhombic crystal system, Pna2₁ space group (point group C_2V ) in the temperature range of 150-400 K. This hybrid supermolecule shows a dielectric relaxation behavior around room temperature, and the ferroelectric nature of 1 has been directly verified by hysteresis measurements. In addition, the AC (alternating current) conductivity study reveals that the 1 displays a beyond limiting behavior. These interesting findings are for the first time reported in the field of supermolecular ferroelectrics. This study may open a new way to construct supermolecular ferroelectrics and give insights into their conductor behavior.

Accurate computations to simulate the phosphorescence spectra of large transition complexes: simulated colors match experiment

Herein, an ab initio investigation on the luminescence properties of three iridium(iii) complexes is reported.

Induction of Strong Long-Lived Room-Temperature Phosphorescence of N-Phenyl-2-naphthylamine Molecules by Confinement in a Crystalline Dibromobiphenyl Matrix

The design and preparation of metal-free organic materials that exhibit room-temperature phosphorescence (RTP) is a very attractive topic owing to potential applications in organic optoelectronic devices. Herein, we present a facile approach to efficient and long-lived organic RTP involving the doping of N-phenylnaphthalen-2-amine (PNA) or its derivatives into a crystalline 4,4'-dibromobiphenyl (DBBP) matrix. The resulting materials showed strong and persistent RTP emission with a quantum efficiency of approximately 20 % and a lifetime of a few to more than 100 milliseconds. Bright white dual emission containing blue fluorescence and yellowish-green RTP from the PNA-doped DBBP crystals was also confirmed by Commission Internationale de l'Eclairage (CIE) coordinates of (x=0.29-0.31, y=0.38-0.41).

Tridentate Complexes of Palladium(II) and Platinum(II) Bearing bis-Aryloxide Triazole Ligands: A Joint Experimental and Theoretical Investigation

A novel class of palladium(II) and platinum(II) complexes bearing tridentate bis-aryloxide triazole ligands was prepared by using straightforward and high-yielding synthetic routes. The complexes were fully characterized and the molecular structures of four derivatives were unambigously determined by single-crystal X-ray diffractometric analyses. For the most promising luminescent Pt(II) derivatives, further experimental investigations were carried out to characterize their photophysical features and to ascertain the nature of the emitting excited state by means of electronic absorption, steady-state, and time-resolved emission techniques in different conditions. In degassed fluid solution the complexes displayed broad and featureless photoluminescence with λ(em) =522-585 nm, excited-state lifetime up to few microseconds and quantum yield (PLQY) up to 17%, depending on the nature of both ancillary ligand and substituent on the tridentate ligand. Computational investigation using density functional theory and time-dependent DFT were performed to gain insight into the electronic processes responsible for optical transitions and structure-photoluminescence relationship. Jointly, experimental and theoretical characterization indicated that the radiative transition arises from an excited state with admixed triplet-manifold metal-to-ligand charge transfer and ligand-centered ((3)MLCT/(3)LC) character. We elucidated the modulation of the photophysical properties upon variation of substituents for this new family of complexes.

Luminescent pincer platinum(II) complexes with emission quantum yields up to almost unity: photophysics, photoreductive C-C bond formation, and materials applications

Luminescent pincer-type Pt(II)  complexes supported by C-deprotonated π-extended tridentate RC^N^NR' ligands and pentafluorophenylacetylide ligands show emission quantum yields up to almost unity. Femtosecond time-resolved fluorescence measurements and time-dependent DFT calculations together reveal the dependence of excited-state structural distortions of [Pt(RC^N^NR')(CC-C6 F5 )] on the positional isomers of the tridentate ligand. Pt complexes [Pt(R-C^N^NR')(CC-Ar)] are efficient photocatalysts for visible-light-induced reductive CC bond formation. The [Pt(R-C^N^NR')(CC-C6 F5 )] complexes perform strongly as phosphorescent dopants for green- and red-emitting organic light-emitting diodes (OLEDs) with external quantum efficiency values over 22.1 %. These complexes are also applied in two-photon cellular imaging when incorporated into mesoporous silica nanoparticles (MSNs).

Blue phosphorescent N-heterocyclic carbene chelated Pt(ii) complexes with an α-duryl-β-diketonato ancillary ligand

Blue phosphorescent Pt(ii) complexes that display bright blue emission in the solid state have been obtained employing NHC-based C^∧C*-chelate ligands and an α-duryl-β-diketonato ancillary ligand that provides steric blocking to minimize intermolecular interactions.

Theoretical study and design of multifunctional phosphorescent platinum(ii) complexes containing triarylboron moieties for efficient OLED emitters

TD-DFT including SOC calculations were employed to evaluated the Φ_PL of phosphorescence OLED emitters.

Luminescence color tuning of Pt(II) complexes and a kinetic study of trimer formation in the photoexcited state

We investigated the luminescence properties and color tuning of [Pt(dpb)Cl] (dpbH=1,3-di(2-pyridyl)benzene) and its analogues. An almost blue emission was obtained for the complex [Pt(Fmdpb)CN] (FmdpbH=4-fluoro-1,3-di(4-methyl-2-pyridyl)benzene), modified by the introduction of F and CH3 groups to the dpb ligand and the substitution of Cl by CN. As the concentration of the solution was increased, the color of the emission varied from blue to white to orange. The color change resulted from a monomer-excimer equilibrium in the excited state. A broad emission spectrum around 620 nm was clearly detected along with a structured monomer emission around 500 nm. Upon further increases in concentration, another broad peak appeared in the longer wavelength region of the spectrum. We assigned the near-infrared band to the emission from an excited trimer generated by the reaction of the excimer with the ground-state monomer. The emission lifetimes of the monomer, dimer, and trimer were evaluated as τM =12.8 μs, τD =2.13 μs, and τT =0.68 μs, respectively, which were sufficiently long to allow association with another Pt(II) complex and dissociation into a lower order aggregate. Based on equilibrium constants determined from a kinetic study, the formation of the excimer and the excited trimer were concluded to be exothermic processes, with ΔG*D =-24.5 kJ mol(-1) and ΔG*T =-20.4 kJ mol(-1) respectively, at 300 K.