Tuning the Excited State of Tetradentate Pd(II) Complexes for Highly Efficient Deep-Blue Phosphorescent Materials

Phosphorescent Dinuclear Pd-Pd Emitters in OLED Applications

Four phosphorescent dinuclear Pd-Pd complexes were synthesized by using the clamping ligand N,N'-diphenylformamidine and primary ligands of C∧N, which both have high ligand field strength to enhance the luminescence of the complexes. The single-crystal data confirmed their clamshell structures and revealed Pd-Pd distances between 2.829 and 2.864 Å. In addition, time-dependent density functional theory calculations revealed the phosphorescent nature of the metal-metal-to-ligand charge transfer and ligand-to-ligand charge transfer in these complexes. Moreover, complexes 3 and 4 were used to fabricate an OLED by a vapor phase deposition technique. At an optimal doping concentration of 5%, the 3-based device showed a maximum external quantum efficiency (EQEmax) of 8.79% for CIE coordinates of (0.49, 0.48) and the 4-based device showed an EQEmax of 13.10% for CIE coordinates of (0.61, 0.37).

High Efficiency and Narrow Emissions in Deep-Blue Pt(II) Emitters in Organic Light-Emitting Diodes via Anchor-Shaped Substituent Design

In this study, a tetradentate Pt(II) complex designed to have N-heterocyclic carbene ligands modified with an anchor-shaped 2,6-diisopropylphenyl (dip) group is described to enhance molecular rigidity for narrow emission and high efficiency. The tetradentate ligand with the dip group significantly hinders steric interactions and restricts π-conjugation from benzocarbene, leading to shallow lowest unoccupied molecular orbital levels and a consequent reduction in the triplet metal-to-ligand charge transfer character. These structural modifications result in narrow emission spectra and enhanced efficiency for blue organic light-emitting diodes (OLEDs) over wide doping concentration ranges. These blue OLEDs exhibit deep-blue emissions at 455 nm with color coordinates of (0.139, 0.090), a full width at half-maximum of 17 nm, and a high maximum external quantum efficiency of 24.1% at a doping concentration of 10 wt %. In addition, the OLED performance was stably maintained at a doping concentration of 20 wt %. Overall, the anchor-shaped dip group significantly enhances the rigidity of the ligand in the tetradentate Pt(II) complex, effectively reducing intermolecular interactions and allowing the complex to function as a highly efficient and pure deep-blue emitter.

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).

Antiprotozoal Pt(II) Complexes as Luminophores Bearing Monodentate P/As/Sb-Based Donors: An X-ray Diffractometric, Photoluminescence, and 121Sb-Mössbauer Spectroscopic Study with TD-DFT-Guided Interpretation and Predictive Extrapolation toward Bi

In this study, it is demonstrated that the radiative rate constant of phosphorescent metal complexes can be substantially enhanced using monodentate ancillary ligands containing heavy donor atoms. Thus, the chlorido coligand from a Pt(II) complex bearing a monoanionic tridentate C^N*N luminophore ([PtLCl]) was replaced by triphenylphosphane (PPh3) and its heavier pnictogen congeners (i.e., PnPh3 to yield [PtL(PnPh3)]). Due to the high tridentate-ligand-centered character of the excited states, the P-related radiative rate is rather low while showing a significant boost upon replacement of the P donor by heavier As- and Sb-based units. The syntheses of the three complexes containing PPh3, AsPh3, and SbPh3 were completed by unambiguous characterization of the clean products using exact mass spectrometry, X-ray diffractometry, bidimensional NMR, and 121Sb-Mössbauer spectroscopy (for [PtL(SbPh3)]) as well as steady state and time-resolved photoluminescence spectroscopies. Hence, it was shown that the hybridization defects of the Vth main-group atoms can be overcome by complexation with the Pt center. Notably, the enhancement of the radiative rate constants mediated by heavier coligands was achieved without significantly influencing the character of the excited states. A rationalization of the results was achieved by TD-DFT. Even though the Bi-based homologue was not accessible due to phenylation side reactions, the experimental data allowed a reasonable extrapolation of the structural features whereas the hybridization defects and the excited state properties related to the Bi-species and its phosphorescence rate can be predicted by theory. The three complexes showed an interesting antiprotozoal activity, which was unexpectedly notorious for the P-containing complex. This work could pave the road toward new efficient materials for optoelectronics and novel antiparasitic drugs.

Perimidocarbene-Based Tetradentate Platinum(II) Complexes with an Unexpectedly Negligible 3MLCT Character

Two novel six-membered perimidocarbene (PIC)-based tetradentate Pt(II) complexes were designed and successfully synthesized. Systematical experimental and theoretical studies suggest that the PIC moiety greatly affects the frontier orbitals, as well as the photophysical and excited-state properties of the Pt(II) complexes. PtYK2 has a broad emission spectrum peaking at 576 nm with a shoulder band at 620 nm, along with a full width at half-maximum (FWHM) value of 100.0 nm at 77 K in 2-MeTHF; however, the emission spectrum is slightly red-shifted with a dominant peak at 610 nm and a FWHM value of 125.0 nm at room temperature in a poly(methyl methacrylate) (PMMA) film. Time-dependent-density functional theory and natural transition orbital analyses reveal that PtYK2 has a 3LC (3πPIC* → πPIC)-dominated character with an unexpectedly negligible contribution of 3MLCT transition (0.68%) in the T1 state, which results in a broad emission spectrum and a relatively low quantum efficiency of 7.4% in the PMMA film.

The Novel Organic Emitters for High-Performance Narrow-Band Deep Blue OLEDs

Narrow-band deep-blue organic light-emitting diodes (OLEDs) have played a key role in the field of high-quality full-color displays. However, because of the considerable challenges of inherent band gaps, unbalanced carrier injection and the lack of molecular structures, narrow-band deep-blue emitters develop slowly compared with red- and green-emitting materials. Encouragingly, with the continuous efforts of scientists in recent years, great progress has been made in the molecule design and material synthesis of highly efficient narrow-band deep-blue emitters. The typical deep-blue emitters which exhibit narrow emission with a full width at half maximum of < 50 nm are summarized in this article. They are divided into the three categories: fluorescence, phosphorescence and thermally activated delayed fluorescence. The methods of molecular design for realizing narrow-band deep-blue emission are described in detail and future research directions are also discussed in this article.

8-Phenylquinoline-Based Tetradentate 6/6/6 Platinum(II) Complexes for Near-Infrared Emitters

A series of novel tetradentate 6/6/6 Pt(II) complexes containing an 8-phenylquinoline-benzo[d]imidazole-carbazole ligand was designed; the Pt(II) complexes could be synthesized by metalizing the corresponding ligand with K2PtCl4 in high isolated yields of 60-90%. Experimental and theoretical studies suggested that the ligand modification of the quinoline moieties of the Pt(II) complexes could tune their electrochemical, photophysical, and excited-state properties. Notably, all the Pt(II) complexes exhibited highly electrochemical stabilities with reversible redox processes except the quasi-reversible reduction of PtYL3. The large π-conjugation of the ligand together with increased metal-to-ligand charge-transfer (3MLCT) characters in T1 states enabled the Pt(II) complexes to show broad Gaussian-type NIR emission spectra with high photoluminescence quantum efficiencies of 1.2-1.5% and short τ of 0.8-1.5 μs in dichloromethane at room temperature. This work should provide a valuable reference for the design and development of monomer NIR emitters.

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.

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.

Fused 6/5/6 Metallocycle-Based Tetradentate Pt(II) Emitters for Efficient Green Phosphorescent OLEDs

Pt(II) complexes are promising phosphorescent materials for organic light-emitting diode (OLED) applications in the fields of display, lighting, healthcare, aerospace, and so on. A series of novel biphenyl (bp)-based tetradentate 6/5/6 Pt(II) emitters using oxygen or carbon as a linking atom was designed and developed. The intermolecular interactions in crystal packing, electrochemical, and photophysical properties of the bp-based Pt(II) emitters and also their excited-state properties were systematically studied, which could be effectively regulated by ligand modification through linking group control; however, their emission spectra nearly showed no change. All the bp-based Pt(II) emitters exhibited vibronically featured emission spectra with dominant peaks at 502-505 nm and photoluminescent quantum yields of 24-34% in dichloromethane solution. Green OLED using Pt(bp-12) as an emitter achieved a maximum brightness (L_max) of 16,644 cd/m².

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.