Efficient Red Organic Light Emitting Diodes of Nona Coordinate Europium Tris(β-diketonato) Complexes Bearing 4'-Phenyl-2,2':6',2''-terpyridine

Europium-based β-hydroxyketone complexes: synthesis, optoelectronic, thermal and computational analyses

Six red-light-emitting Eu(III) complexes having a β-hydroxyketone as ligand and heterocyclic ring containing compounds as ancillary ligands were synthesized to explore their use in displays and optoelectronics. The coordinating behavior of complexes was determined by various techniques such as FTIR (Fourier transform infrared), 1H-NMR (Nuclear magnetic resonance), and 13C-NMR that establishes a bonding of ligand and ancillary ligand with the Eu(III) ion. Morphology and purity were investigated through XRD (X-ray diffraction), SEM (scanning electron microscopy) and EDS (energy-dispersive X-ray spectroscopy) analyses that suggest semicrystalline and pure complex formation. Thermal analysis of complexes by TGA/DTG (thermogravimetric/derivative thermogravimetric) indicates that complexes are stable upto 200 ºC temperature making them suitable for use in display devices. Analysis of the photophysical properties was carried out in both solid and solution states using PL (photoluminescence) studies, color parameters, J-O (Judd-Ofelt) analysis and bandgap. Most emissive transition (5D0 → 7F2) is responsible for the red emission in the complexes. The CIE (Commission International de I'Eclairage) coordinates of complexes also indicate the red emission on UV excitation. The bandgap which was obtained in the range of 2.54-3.02 eV reveals the semiconducting behavior of complexes. Values of J-O parameters and Ω2 in the complexes reflect asymmetric chemical environment around Eu (III) and less covalence and the Ω4 indicates that complexes are less rigid. Bandgap calculated through DFT (density function theory) for complexes is in range of 2.37-2.77 eV, and intensity parameters (J-O), energy transfer rates, and spherical coordinates were determined by LUMPAC software. The computational data are in good harmony with the experimental data. Further biological aspects of complexes were studied using antioxidant and antimicrobial studies.

Recent Progress in Phenoxazine-Based Thermally Activated Delayed Fluorescent Compounds and Their Full-Color Organic Light-Emitting Diodes

Third-generation organic light-emitting diodes (OLEDs) based on metal-free thermally activated delayed fluorescent (TADF) materials have sparked tremendous interest in the last decade due to their nearly 100% exciton utilization efficiency, which can address the low-efficiency issue of the first-generation fluorescent emitters and the high-cost issue of the second-generation organometallic phosphorescent emitters. Construction of efficient and stable TADF-OLEDs requires utilizing TADF materials with a narrow singlet-triplet energy gap (ΔEST), high photoluminescence quantum yield (PLQY) and short TADF lifetime. A small ΔEST is necessary for an efficient reverse intersystem crossing (RISC) process, which can be achieved through the effective spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). TADF emitters have been generally designed as intramolecular charge transfer (ICT) molecules with highly twisted donor-acceptor (D-A) molecular architectures. A wide variety of combinations of electron donors and acceptors have been explored. In this review, we shall focus on recent progress in organic TADF molecules incorporating strong electron-donor phenoxazine moiety and their application as emitting layer (EML) in OLEDs.

Synthesis, X-ray crystal structure and photophysics of butterfly shape orange and red emanating polynuclear complexes of tris(dibenzoylmethanato)Ln(III) (Ln = Sm/Eu) and exo-bidentate 4,4'-bipyridine

Reaction of two equivalents of [Ln(dbm)3(H2O)] (Ln = Sm/Eu/Gd) with one equivalent of 4,4'-bipyridine (4,4'-bpy) led to the formation of rare polynuclear complexes of the type [Ln(dbm)3(4,4'-bpy)]n (dbm is the anion of 1,3-diphenyl-1,3-propanedione) instead of symmetrically bridged dinuclear complexes. The structure of the complexes has been established by the single crystal X-ray diffraction (SC-XRD) method and shows that the coordination sphere is composed of a LnO6N2 core (octacoordinated). Shape analysis further revealed that the geometry around Ln(III) is distorted square anti-prismatic with SHAPE value 0.738 and 25.719 for [Sm(dbm)3(4,4'-bpy)]n and [Eu(dbm)3(4,4'-bpy)]n, respectively. Photoluminescence (PL) properties of [Sm(dbm)3(4,4'-bpy)]n and [Eu(dbm)3(4,4'-bpy)]n are discussed in the solid-state and PMMA hybrid film (w/w 6%). By employing theoretical modelling in conjunction with the experimental PL data and crystal structure and an energy transfer (ET) mechanism for the sensitized PL of [Eu(dbm)3(4,4'-bpy)]n is proposed and discussed in detail. Finally, the role of each ligand in sensitized PL of [Eu(dbm)3(4,4'-bpy)]n is calculated and discussed by the chemical partitions of the radiative decay.Graphical abstract.

High efficiency double light-emitting layer sky blue phosphor organic light-emitting diode with slow emission efficiency attenuation

In this study, by doping the iridium (III) double ((4,6-difluorobenzene) -pyridine-N, C2'Pyridinformate (FIrpic) into 4,4', 4″ three (carbazole) trioxilamine (TcTa) and 9 (4 tert-butyl-benzene) -3,6 double (triphenylsilyl) -9H carbazole (CzSi) layers as light emitting layers (EMLs) to achieve sky blue phosphor organic electroluminescence (EL) devices with very low efficiency roll-off. The dependence of the EL performance on the doping concentration of the light-emitting molecules and the different EML thicknesses are studied in detail. Experimental data show that the thin-film-doped TcTa layer is very important for the efficiency of the device. Finally, the optimal device achieves extremely high power efficiency, current efficiency, and external quantum efficiency (EQE), reaching 74.82 lm/W, 66.68 cd/A, and 31.9 %, respectively. At a certain luminance of 2000 cd/m2 (3.4 V), the same device retained power efficiency with current efficiency and external quantum efficiency of 57.71 lm/W, 62.46 cd/A and 29.3 %, respectively, based on theoretical discussion, the improved EL efficiency was attributed to the energy levels of the host and luminescent molecules, which helps to balance the charge carrier in the emitter molecule distribution and expand the recombination region.

Two new red-emitting ternary europium(III) complexes with high photoluminescence quantum yields and exceptional performance in OLED devices

Two new organo-europium complexes (OEuCs) [Eu(btfa)3(Bathphen)] (OEuC-1) and [Eu(tta)3(Bathphen)] (OEuC-2) where btfa and tta are the anions of 4,4,4-trifluoro-1-phenyl-1,3-butanedione and 2-thenoyltrifluoroacetone while Bathphen = Bathophenanthroline have been synthesized and characterized. Both complexes in the solid state exhibit strong red emissions with photoluminescence quantum yields (PLQYs) of 80% ± 10%. These complexes were tested as dopants in the emitting layer (EML) to fabricate red organic light emitting diodes (R-OLEDs). Through device engineering involving the amalgamation of appropriate host materials and complexes, we have achieved exceptional overall R-OLED performance for an OEuC-2 based device, with maximum current efficiency (CEmax) = 9.91 cd A-1, maximum power efficiency (PEmax) = 9.15 lm W-1, maximum external quantum efficiency (EQEmax.) = 6.24%, brightness (B) of 545 cd m-2, and (CIE)x,y = 0.620, 0.323 at J = 10 mA cm-2.