Identifying lifetime as one of the key parameters responsible for the low brightness of lanthanide-based OLEDs

The increase of europium-based OLED luminance through reducing the excited state lifetime by mixed-ligand complex formation

An approach to the luminance increase of the europium-based OLED is proposed through the formation of the mixed-ligand complex. The introduction of two diverse anionic ligands around one europium ion forming a mixed-ligand complex is confirmed by powder X-ray diffraction, ¹H and ¹⁹F NMR spectroscopy, MALDI MS spectroscopy, and luminescence spectroscopy. A decrease in the symmetry of the coordination environment leads to a 50% reduction of the lifetime of the excited state. The obtained OLEDs based on mixed ligand europium complexes are significantly superior in luminance to OLEDs based on individual complexes.

Perfluoroalkyl Chain Length Effect on Crystal Packing and [LnO8] Coordination Geometry in Lanthanide-Lithium β-Diketonates: Luminescence and Single-Ion Magnet Behavior

Functionalized perfluoroalkyl lithium β-diketonates (LiL) react with lanthanide(III) salts (Ln = Eu, Gd, Tb, Dy) in methanol to give heterobimetallic Ln-Li complexes of general formula [(LnL₃)(LiL)(MeOH)]. The length of fluoroalkyl substituent in ligand was found to affect the crystal packing of complexes. Photoluminescent and magnetic properties of heterobimetallic β-diketonates in the solid state are reported. The effect of the geometry of the [LnO₈] coordination environment of heterometallic β-diketonates on the luminescent properties (quantum yields, phosphorescence lifetimes for Eu, Tb, Dy complexes) and single-ion magnet behavior (U_eff for Dy complexes) is revealed.

Impact of chiral ligands on photophysical and electro-optical properties of β-diketonate europium complexes in circularly polarized OLEDs

Luminescent lanthanide complexes exhibiting chiroptical properties are attracting attention for their application in chiral optoelectronics and photonics, thanks to their unique optical properties, allied to intraconfigurational f-f transitions, which are generally electric-dipole-forbidden and can be magnetic dipole-allowed, which in an appropriate environment can lead to high dissymmetry factors and strong luminescence, in the presence of an antenna ligand. However, because luminescence and chiroptical activity are governed by different selection rules, their successful application in commonly used technologies is still an expectation. Recently, we showed that europium complexes bearing β-diketonates acting as luminescence sensitizers, and chiral bis(oxazolinyl) pyridine derivatives as the chirality inducer, reasonably perform in circularly polarized (CP) organic light-emitting devices (OLEDs). Indeed, europium β-diketonate complexes are an interesting molecular starting point, given their strong luminescence and their established use in conventional (i.e., nonpolarized) OLEDs. In this context, it is interesting to investigate in detail the impact of the ancillary chiral ligand on complex emission properties and the performances of corresponding CP-OLEDs. Here we show that, by incorporating the chiral compound as emitter in the architecture of solution processed electroluminescent devices, CP emission is retained, and the efficiency of the device is comparable to reference unpolarized OLED. The observed remarkable dissymmetry values strengthen the position of chiral lanthanide-OLEDs as CP-emitting devices.

Solution-Processed OLED Based on a Mixed-Ligand Europium Complex

An approach to increase the efficiency of europium-based OLEDs was proposed through the formation of a mixed-ligand complex. The design of a series of europium complexes, together with an optimization of the solution deposition, including the host selection, as well as the variation of the solvent and deposition parameters, resulted in a noticeable increase in OLED luminance. As a result, the maximum luminance of the Eu-based OLED reached up to 700 cd/m², which is one of the highest values for an Eu-based solution-processed OLED. Finally, its stability was investigated.

Investigation progresses of rare earth complexes as emitters or sensitizers in organic light-emitting diodes

Due to unique photo-physical characteristics, rare earth (RE) complexes play important roles in many fields, for example, telecommunications, life science, and organic light-emitting diodes (OLEDs). Especially, thanks to narrow emission bandwidth and 100% theoretical internal quantum efficiency (IQE), the study of RE complexes in the electroluminescence field has been a hot research topic in recent 30 years. As a leading technology in solid-state light source fields, OLEDs have attracted great interest from academic researchers and commercial endeavors. In the last decades, OLED-based products have trickled into the commercial market and developed quickly into portable display devices. Here, we briefly introduce the luminescent characteristics and electroluminescent (EL) study of RE complexes in material synthesis and device design. Moreover, we emphatically reveal the innovative application of RE complexes as sensitizers in OLEDs. Through experimental validation, the application of RE complexes as sensitizers can realize the complementary advantages of RE complexes and transition metal complexes, leading to significantly improved performances of OLEDs. The application of RE complexes as sensitizers provides a new strategy for designing and developing novel high performances OLEDs.

Record efficiency of 1000 nm electroluminescence from a solution-processable host-free OLED

New ytterbium complexes K(Solv)_x[Yb(Lⁿ)₂] (Solv = ethanol and/or water) with 2-tosylaminobenzylidene-aryloylhydrazones (H₂L¹, aryloyl = benzoyl; H₂L², aryloyl = 2-naphthoyl) demonstrated high solubility and hole mobility (ca. 2.6 × 10^-6 cm² V^-1 s^-1), while their electron mobility and PLQY were different. The substitution of a benzoyl substituent with naphthoyl resulted in a significant increase of the electron mobility (6.9 × 10^-7vs. 1.7 × 10^-6 cm² V^-1 s^-1) and a decrease of the quantum yield (1.2% vs. 0.6%). As a result, the optimized OLEDs based on the K[Yb(Lⁿ)₂] layer demonstrated efficiencies up to 385 μW W^-1 and 441 μW W^-1, indicating the superior importance of charge mobility over the quantum yield. These are the highest efficiencies of the Yb electroluminescence.

Crystal Structures, Thermal and Luminescent Properties of Gadolinium(III) Trans-1,4-cyclohexanedicarboxylate Metal-Organic Frameworks

Four new gadolinium(III) metal-organic frameworks containing 2,2′-bipyridyl (bpy) or 1,10-phenanthroline (phen) chelate ligands and trans-1,4-cyclohexanedicarboxylate (chdc2−) were synthesized. Their crystal structures were determined by single-crystal X-ray diffraction analysis. All four coordination frameworks are based on the binuclear carboxylate building units. In the compounds [Gd2(bpy)2(chdc)3]·H2O (1) and [Gd2(phen)2(chdc)3]·0.5DMF (2), the six-connected {Ln2(L)2(OOCR)6} blocks form a 3D network with the primitive cubic (pcu) topology. In the compounds [Gd2(NO3)2(phen)2(chdc)2]·2DMF (3) and [Gd2Cl2(phen)2(chdc)2]·0.3DMF·2.2dioxane (4), the four-connected {Ln2(L)2(X)2OOCR)4} units (where X = NO3− for 3 or Cl− for 4) form a 2D square-grid (sql) network. The solid-state luminescent properties were investigated for the synthesized frameworks. Bpy-containing compound 1 shows no luminescence, possibly due to the paramagnetic quenching by Gd3+ cation. In contrast, the phenathroline-containing MOFs 2–4 possess yellow emission under visible excitation (λex = 460 nm) with the tuning of the characteristic wavelength by the coordination environment of the metal center.