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.
A series of highly luminescent, heteroleptic copper(I) complexes has been synthesized using a modular approach based on easily accessible P^N ligands, triphenylphosphine, and copper(I) halides, ...allowing for an independent tuning of the emission wavelength with low synthetic efforts. The molecular structure has been investigated via X-ray analysis, confirming a dinuclear copper(I) complex consisting of a butterfly shaped metal-halide cluster and two different sets of ligands. The bidentate P^N ligand bridges the two metal centers and can be used to tune the energy of the frontier orbitals and therefore the photophysical characteristics, as confirmed by emission spectroscopy and theoretical investigations, whereas the two monodentate triphenylphosphine ligands on the periphery of the cluster core mainly influence the solubility of the complex. By using electron-rich or electron-poor heterocycles as part of the bridging ligand, emission colors can be adjusted, respectively, between yellow (581 nm) and deep blue (451 nm). These complexes have been further investigated in particular with regard to their photophysical properties in thin films and polymer matrix as well as in solution. Furthermore, the suitability of this class of materials for being applied in organic light-emitting diodes (OLEDs) has been demonstrated in a solution-processed device with a maximum current efficiency of 9 cd/A and a low turn-on voltage of 4.1 V using a representative complex as an emitting compound.
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).
Polymorphism is often linked to the choice of processing solvents. Packing effects or the preference of one certain conformer as possible causes of this phenomenon are strongly dependent on solvents ...and especially on their polarity. Even in amorphous solids, the microstructure can be controlled by the choice of solvents. Polymorphs or amorphous solids featuring different packing densities can exhibit different properties in terms of stability or optical effects. The influence of these effects on a binuclear, strongly luminescent copper(I) complex was investigated. Many possible applications for luminescent, amorphous coordination compounds, such as organic light-emitting diodes, sensors, and organic lasers, rely on photophysical properties like quantum efficiency to be repeatable. The effect of processing solvents in this context is often underestimated, but very relevant for utilization in device manufacturing and should therefore be understood more deeply. In this work, theoretical derivations, DFT calculations, X-ray-diffraction, photoluminescence spectroscopy, and the time-dependent single-photon-counting-technique (TDSPC) were used to understand this phenomenon more deeply. The influence of five different solvents on Cu2I2(MePyrPHOS)3 was probed. This resulted in a modulation of the photoluminescence quantum yield ϕ between 0.5 and 0.9 in amorphous solid state. A new polymorph of the material with slightly reduced values for ϕ has been identified. The reduced efficiency could be correlated with a higher porosity and a reduced packing density. Dense packing reduces nonradiative decay by geometrical fixation and thus increases the quantum efficiency. The existence of similar effects on aluminum and iridium compounds has been confirmed by application of different processing solvents on Alq3 and Ir(ppy)3. These results show that a tuning of the efficiency of a emissive metal complexes by choosing a proper processing solvent is possible. If highly efficient materials for practical applications are desired, an evaluation of multiple solvents has to be considered.
•Organic light emitting diodes (OLEDs) produced from metal complexes.•Sustainability issues are imminent.•Focus on copper and zinc complexes.•Complex preparation.•Cooperativity, chirality, and ...printing.
Organic light-emitting diodes (OLEDs) produced from metal complexes play an important role in modern electroluminescent devices. While OLEDs are being used in display various applications such as TVs, smartphones and wearables already, a drastic increase in the production volume in the next years is being expected as soon as OLED lighting applications and printed OLEDs hit the market. Given that thus far, phosphorescent iridium compounds are required to make these products, sustainability issues are imminent. To review viable alternatives, we highlight the current status of sustainable metal complexes with a special focus on copper and zinc complexes. Ligand structures and complex preparation were highlighted. We also briefly address features like cooperativity, chirality, and printing.
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).
A new NHetPHOS–CuI emitter was synthesized and characterized. With synchrotron‐based X‐ray absorption spectroscopy, proof is provided that changes of the photophysics in solution compared with solid‐state results are not a result of dissociation. The evidence that the material can be processed in the presence of ambient air with inkjet‐printing, even in the absence of a host material, is presented. In a refined device, efficiency values of more than 11 % EQE were obtained (see scheme).
Imaging is – even these days – still restricted to of a few classes of robust dyes. A demand for switchable tags led us to the design of a new class of pre-fluorophores. We achieved this by using a ...non-fluorescent N -(4-azidophenyl)-carbazole tag which turns fluorescent by click reaction with alkynes and cyclooctynes. The spectral properties of the labelled dyes were investigated. Our results suggest that a twisted internal charge transfer (TICT) transition is responsible for the emission. DFT calculations and single-crystal X-ray diffraction of selected examples support this explanation. The feasibility of the new dyes for biological application has also been tested via confocal microscopy.
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.
Easy come, easy go: the great structural diversity of CuI complexes is an ambivalent trait. Apart from the well‐known catalytic properties of CuI, 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 CuI 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 CuI compounds.
Easy come, easy go: The great structural diversity of CuI complexes is an ambivalent trait. Apart from the well‐known catalytic properties of CuI, a great number of potent luminescent complexes (see figure) 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. Strategies to avoid this behavior are presented.
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