Daniel M. Zink

Synthesis, structure, and characterization of dinuclear copper(I) halide complexes with P^N ligands featuring exciting photoluminescence properties.

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.

From iridium and platinum to copper and carbon: new avenues for more sustainability in organic light-emitting diodes

Recently, the first commercially successful applications for organic light-emitting devices (OLEDs) have entered the lighting and display markets, especially in smaller devices such as tablets and smartphones. In this article, we analyse materials and techniques used in OLED manufacturing in terms of sustainability and highlight upcoming trends which are supposed to further enhance this technologies sustainability.

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) ).

Copper(I) Complexes Based on Five-Membered P∧N Heterocycles: Structural Diversity Linked to Exciting Luminescence Properties

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.

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.

Highly efficient photoluminescent Cu(I)-PyrPHOS-metallopolymers

The photoluminescence quantum efficiency as well as the processing properties of a series of brightly luminescent Cu(i)-metallopolymers strongly depended on the chosen synthetic approach. A monomeric, substituted styrenic complex features a photoluminescence quantum efficiency (PLQY) of only 4%, while its metallopolymeric thin film is over one order of magnitude more efficient.

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: efficient OLED devices with copper(I)iodide-NHetPHOS-emitters

The mass market application of OLEDs is currently hindered because i) the materials are too expensive and contain rare metals such as iridium and ii) current processing techniques are elaborate and cannot easily be up-scaled. Solution processable Cu(I)-complexes promise to solve both problems with one blow: Copper is an abundant metal, which offers new opportunities to develop materials for OLEDs. Due to their structural diversity, Cu(I) emitters allow for the design of materials with tunable properties. Beside this, it is also possible to adjust solution properties and introduce functionalities for cross-linking. The new materials feature exciting photophysical properties such as PLQY values close to unity and a tunable emission. The emission decay times are in the range of common emitters or lower, which is expected to reduce efficiency roll-off at high driving voltages. Cu(I)-complexes often feature thermally-activated delayed fluorescence (TADF). As a consequence, they can make use of triplet and singlet excitons in a process called Singlet Harvesting, which paves the way for high efficiencies. Unlike Ir(III)-complexes such as Irppy3, triplet-triplet annihilation does not occur when using Cu(I), even in very high doping concentrations. The feasibility of NHetPHOS-type Cu(I)-complexes is demonstrated as well as strategies that enable a smart crosslinking process, where the Cu(I) emitters themselves play an important role. In addition, high-brightness devices, which were operated at medium voltages, yielding 50.000 cd m-2 are shown. In a showcase example, we recently presented a device with an external quantum efficiency greater than 20% with a solution processed Cu(I)-PyrPHOS-device without using outcoupling techniques.

Are copper(I) complexes tough enough to be processed from solution

Organic light-emitting diodes (OLEDs) suggest fascinating possibilities: they promise to solve existing problems such as large-area lighting and display manufacturing in an elegant, efficient way. In so doing, they could enable innovative products such as flexible, transparent devices and smart packaging. However, OLEDs also present two key challenges: how to substitute commonly used compounds containing very rare metals (iridium and platinum) with readily available materials; and how to simplify a laborious vacuum process using wet-processing techniques such as coating and printing. The use of luminescent copper (Cu) complexes1, 2 could address both aspects, but there is a catch. Not every copper complex is suitable for solution processing. Owing to their electronic properties, only complexes with copper having the oxidation number +1 are suitable as emitters in luminescent devices. Such complexes contain one or more positively charged copper(I) ions that are connected to several ligands (binding molecules). Depending on the charge of the ligands, the complexes may contain counterions to obtain an electronically neutral compound. Some of these compounds are not air stable and are vulnerable to oxygen and water: see Figure 1(a). Dealing with this aspect is tedious during synthesis, but it has already been resolved. Both water and air must be carefully avoided in any event to produce OLED devices. In fact, some of the reported best copper(I) OLEDs contain substances very sensitive to air.3 Similarly, some copper(I) complexes show intrinsic weaknesses. They dissociate in solution, potentially forming other compounds and can therefore not be used in OLED devices.4 The key to properly controlling the structure and properties of Figure 1. Copper(I) complexes in solution degrade (a) owing to reaction with oxygen, water, or coordinating solvent molecules, or (b) because the formation of other complexes is sometimes favored. Cu: Copper. Br: Bromide. P: Phosphorus. Ph: Phenyl. n: Repeating unit.

Novel oligonuclear copper complexes featuring exciting luminescent characteristics

A series of highly luminescent mono-, di-, and trinuclear copper(I) complexes has been synthesized using modular ligand systems of easily accessible N^N, P^P or P^N ligands in order to show the rich structural diversity of copper(I) compounds. Those systems allow for the design of various emitting materials with desired photophysical properties, such as emission colors and high efficiencies. The complexes were characterized with well-established methods such as X-ray crystallographic studies or elemental analysis and, in addition, due to their interesting photoluminescence characteristics, their emission properties were further investigated by means of spectroscopic methods as well as DFT-calculations. In detail, various cationic and neutral mononuclear complexes have been synthesized in order to investigate the photophysical properties of this these different types of emitting compounds. It has been found that neutral copper(I) complexes show superior emission properties (with PLQY up to 89%) compared to their cationic counterparts. Furthermore, a series of dinuclear and trinuclear copper(I) complexes has been synthesized featuring an easy tunable emission maximum from sky blue to deep red (481 nm to 713 nm) with extraordinary high photoluminescence quantum yields up to 99%. In addition, a new crosslinking-technique has been developed to open up the door for a new way to fully solution processed OLED using these promising emitting compounds: Alkyne-substituted emitting complexes crosslink automatically with azide-polymers in a copper-catalyzed alkyne-azide Click reaction.