Towards Printed Organic Light-Emitting Devices: A Solution-Stable, Highly Soluble CuI -NHetPHOS.

The development of iridium-free, yet efficient emitters with thermally activated delayed fluorescence (TADF) was an important step towards mass production of organic light-emitting diodes (OLEDs). Progress is currently impeded by the low solubility and low chemical stability of the materials. Herein, we present a CuI -based TADF emitter that is sufficiently chemically stable under ambient conditions and can be processed by printing techniques. The solubility is drastically enhanced (to 100 g L-1 ) in relevant printing solvents. The integrity of the complex is preserved in solution, as was demonstrated by X-ray absorption spectroscopy and other techniques. In addition, it was found that the optoelectronic properties are not affected even when partly processing under ambient conditions. As a highlight, we present a TADF-based OLED device that reached an efficiency of 11±2 % external quantum efficiency (EQE).

Bright coppertunities: multinuclear Cu(I) complexes with N-P ligands and their applications.

Easy come, easy go: the great structural diversity of Cu(I) complexes is an ambivalent trait. Apart from the well-known catalytic properties of Cu(I), a great number of potent luminescent complexes have been found in the last ten years featuring a plethora of structural motifs. The downside of this variety is the undesired formation of other species upon processing. In here, strategies to avoid this behavior are presented: Only one favorable structural unit often exists for multinuclear Cu(I) complexes with bridging ligands. In addition, these complexes exhibit favorable photophysical properties due to cooperative effects of the metal halide core. Furthermore, we demonstrate the broad range of applications of emitting Cu(I) compounds.

Copper(I) complexes based on five-membered P

Bridging P(^)N ligands bearing five-membered heterocyclic moieties such as tetrazoles, 1,2,4-triazoles, oxadiazoles, thiadiazoles, and oxazoles have been investigated regarding their complexation behavior with copper(I) iodide as metal salts. Different complex structures were found, depending either on the ligand itself or on the ligand-to-metal ratios used in the complexation reaction. Two different kinds of luminescent dinuclear complex structures and a kind of tetranuclear complex structure were revealed by X-ray single-crystal analyses and were further investigated for their photophysical properties. The emission maxima of these complexes are in the blue to yellow region of the visible spectrum for the dinuclear complexes and in the yellow to orange region for the tetranuclear complexes. Further investigations using density functional theory (DFT) show that the highest occupied molecular orbital (HOMO) is located mainly on the metal halide cores, while the lowest unoccupied molecular orbital (LUMO) resides mostly in the ligand sphere of the complexes. The emission properties were further examined in different environments such as neat powders, neat films, PMMA matrices, or dichloromethane solutions, revealing the high potential of these complexes for their application in organic light-emitting diodes. Especially complexes with 1,2,4-triazole moieties feature emission maxima in the blue region of the visible spectrum and quantum yields up to 95% together with short decay times of about 1-4 µs and are therefore promising candidates for blue-emitting materials in OLEDs.

Synthesis, structure, and characterization of dinuclear copper(I) halide complexes with P

A series of highly luminescent dinuclear copper(I) complexes has been synthesized in good yields using a modular ligand system of easily accessible diphenylphosphinopyridine-type P^N ligands. Characterization of these complexes via X-ray crystallographic studies and elemental analysis revealed a dinuclear complex structure with a butterfly-shaped metal-halide core. The complexes feature emission covering the visible spectrum from blue to red together with high quantum yields up to 96%. Density functional theory calculations show that the HOMO consists mainly of orbitals of both the metal core and the bridging halides, while the LUMO resides dominantly on the heterocyclic part of the P^N ligands. Therefore, modification of the heterocyclic moiety of the bridging ligand allows for systematic tuning of the luminescence wavelength. By increasing the aromatic system of the N-heterocycle or through functionalization of the pyridyl moiety, complexes with emission maxima from 481 to 713 nm are obtained. For a representative compound, it is shown that the ambient-temperature emission can be assigned as a thermally activated delayed fluorescence, featuring an attractively short emission decay time of only 6.5 µs at ϕPL = 0.8. It is proposed to apply these compounds for singlet harvesting in OLEDs.

Highly soluble fluorine containing Cu(i) AlkylPyrPhos TADF complexes.

Luminescent Cu(i) AlkylPyrPhos complexes with a butterfly-shaped Cu2I2 core and halogen containing ancillary ligands, with a special focus on fluorine, have been investigated in this study. These complexes show extremely high solubilities and a remarkable (photo)chemical stability in a series of solvents. A tunable emission resulting from thermally activated delayed fluorescence with high quantum yields was determined by luminescence and lifetime investigations in solvents and solids. Structures of the electronic ground states were analyzed by single crystal X-ray analysis. The structure of the lowest excited triplet state was determined by transient FTIR spectroscopy, in combination with quantum chemical calculations. With the obtained range of compounds we address the key requirement for the production of organic light emitting diodes based on solution processing.

Metal-Organic and Organic TADF-Materials: Status, Challenges and Characterization.

This section covers both metal-organic and organic materials that feature thermally activated delayed fluorescence (TADF). Such materials are especially useful for organic light-emitting diodes (OLEDs), a technology that was introduced in commercial displays only recently. We compare both material classes to show commonalities and differences, highlighting current issues and challenges. Advanced spectroscopic techniques as valuable tools to develop solutions to those issues are introduced. Finally, we provide an outlook over the field and highlight future trends.

Copper(I) Complexes for Thermally Activated Delayed Fluorescence: From Photophysical to Device Properties.

Molecules that exhibit thermally activated delayed fluorescence (TADF) represent a very promising emitter class for application in electroluminescent devices since all electrically generated excitons can be transferred into light according to the singlet harvesting mechanism. Cu(I) compounds are an important class of TADF emitters. In this contribution, we want to give a deeper insight into the photophysical properties of this material class and demonstrate how the emission properties depend on molecular and host rigidity. Moreover, we show that with molecular optimization a significant improvement of selected emission properties can be achieved. From the discussed materials, we select one specific dinuclear complex, for which the two Cu(I) centers are four-fold bridged to fabricate an organic light emitting diode (OLED). This device shows the highest efficiency (of 23 % external quantum efficiency) reported so far for OLEDs based on Cu(I) emitters.

Bridging the efficiency gap: fully bridged dinuclear Cu(I)-complexes for singlet harvesting in high-efficiency OLEDs.

The substitution of rare metals such as iridium and platinum in light-emitting materials is a key step to enable low-cost mass-production of organic light-emitting diodes (OLEDs). Here, it is demonstrated that using a solution-processed, fully bridged dinuclear Cu(I)-complex can yield very high efficiencies. An optimized device gives a maximum external quantum efficiency of 23 ± 1% (73 ± 2 cd A(-1) ).