Investigation of Luminescent Triplet States in Tetranuclear CuI Complexes: Thermochromism and Structural Characterization.

To develop new and flexible CuI containing luminescent substances, we extend our previous investigations on two metal-centered species to four metal-centered complexes. These complexes could be a basis for designing new organic light-emitting diode (OLED) relevant species. Both the synthesis and in-depth spectroscopic analysis, combined with high-level theoretical calculations are presented on a series of tetranuclear CuI complexes with a halide containing Cu4 X4 core (X=iodide, bromide or chloride) and two 2-(diphenylphosphino)pyridine bridging ligands with a methyl group in para (4-Me) or ortho (6-Me) position of the pyridine ring. The structure of the electronic ground state is characterized by X-ray diffraction, NMR, and IR spectroscopy with the support of theoretical calculations. In contrast to the para system, the complexes with ortho-substituted bridging ligands show a remarkable and reversible temperature-dependent dual phosphorescence. Here, we combine for the first time the luminescence thermochromism with time-resolved FTIR spectroscopy. Thus, we receive experimental data on the structures of the two triplet states involved in the luminescence thermochromism. The transient IR spectra of the underlying triplet metal/halide-to-ligand charge transfer (3 M/XLCT) and cluster-centered (3 CC) states were obtained and interpreted by comparison with calculated vibrational spectra. The systematic and significant dependence of the bridging halides was analyzed.

Real-time observation of molecular flattening and intersystem crossing in [(DPEPhos)Cu(i)(PyrTet)] via ultrafast UV/Vis- and mid-IR spectroscopy on solution and solid samples.

The primary photo-induced processes in the mononuclear, heteroleptic Cu(i) complex [(DPEPhos)Cu(PyrTet)] (1), relevant for OLED applications, were investigated in various solvents and in solid state samples via femtosecond (fs) time resolved UV/Vis and fs time resolved mid-IR transient absorption spectroscopy (TA) with MLCT excitation around 340 nm. UV/Vis fs-TA on 1 in solution reveals (i) a severe blue-shift of excited state absorption on the time scale of a few picoseconds (τ2) that is not observed in solids, and (ii), on the time scale of several tens of picoseconds (τ3), a process with very similar dynamics in all samples. Mid-IR fs-TA in solution indicates structural changes with τ2. Transient absorption anisotropy experiments temporally resolve a viscosity-dependent change of the excited state transition dipole moment orientation with τ2, as quantitatively predicted for the relaxed S1-state via TD-DFT. The results strongly suggest flattening distortion (FD) and structural rearrangement of the PyrTet-moiety to occur on the time scale of τ2 in liquid phase, and to be suppressed in solid phase. Moreover, intersystem crossing (ISC) is assigned to the process described by τ3, in line with its direct observation via time-resolved luminescence spectroscopy on 1 by Bergmann et al. (Sci. Adv., 2016, 2, e1500889). Overall, the use of structure-sensitive methods and the direct comparison of different preparations of 1 (i.e. solution vs. solid state), are a unique basis for a clear assignment of spectro-temporal characteristics to fundamental deactivation processes such as FD and ISC.

Solid-State Step-Scan FTIR Spectroscopy of Binuclear Copper(I) Complexes.

The structure in the ground and excited electronic state of two binuclear CuI N-heterocyclic phosphine complexes that are promising for implementation in organic light-emitting diodes is investigated by a combination of the time-resolved step-scan FTIR technique and quantum chemical calculations at the DFT level of theory. In contrast to the usual application of step-scan FTIR spectroscopy in solution, the herein-presented analyses are performed in a solid phase, that is, the CuI complexes are embedded in a KBr matrix (KBr pellet). The application of solid-state time-resolved step-scan FTIR spectroscopy is of great importance for transition metal complexes, since their photophysical properties often change on moving from solid to dissolved samples. The efficient applicability of the solid-state step-scan technique in a KBr matrix is demonstrated on the chosen CuI reference systems on nano- and microsecond timescales with an excitation wavelength of 355 nm. By comparison with theoretical results, the structure of the complexes in the electronic ground and lowest-lying electronically excited state can be determined.

Lanthanide Fluorobenzoates as Bio-Probes: a Quest for the Optimal Ligand Fluorination Degree.

The thorough study of fluorinated benzoates of lanthanides (Eu, Tb, Nd, Er, Yb, Gd, La, Lu) is reported. Their composition in single crystal and powder state revealed two predominant structural motifs. An in-depth luminescence study has been performed on the reported fluorobenzoates, showing, that terbium and europium complexes in solid state possess high luminescence intensity with the quantum yield of up to 69 %. High solubility in most organic solvents, as well as in water, combined with the high luminescence intensity in water solution and non-toxicity allowed the testing of europium complexes as bioprobes in cellulo. Among all tested fluorobenzoates, europium 2-fluorobenzoate dihydrate combined the best luminescent properties, thermodynamic stability, aqueous solubility, and non-toxicity, and was shown to be a viable bio-marker.

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).

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.

A Modular Class of Fluorescent Difluoroboranes: Synthesis, Structure, Optical Properties, Theoretical Calculations and Applications for Biological Imaging.

Ten borylated bipyridines (BOBIPYs) have been synthesized and selected structural modifications have been made that allow useful structure-optical property relationships to be gathered. These systems have been further investigated using DFT calculations and spectroscopic measurements, showing blue to green fluorescence with quantum yields up to 41 %. They allow full mapping of the structure to determine where selected functionalities can be implemented, to tune the optical properties or to incorporate linking groups. The best derivative was thus functionalised with an alkyne linker, which would enable further applications through click chemistry and in this optic, the stability of the fluorophores has been evaluated.

Highly Luminescent, Water-Soluble Lanthanide Fluorobenzoates: Syntheses, Structures and Photophysics, Part I: Lanthanide Pentafluorobenzoates.

Highly luminescent, photostable, and soluble lanthanide pentafluorobenzoates have been synthesized and thoroughly characterized, with a focus on Eu(III) and Tb(III) complexes as visible emitters and Nd(III) , Er(III) , and Yb(III) complexes as infrared emitters. Investigation of the crystal structures of the complexes in powder form and as single crystals by using X-ray diffraction revealed five different structural types, including monomeric, dimeric, and polymeric. The local structure in different solutions was studied by using X-ray absorption spectroscopy. The photoluminescence quantum yields (PLQYs) of terbium and europium complexes were 39 and 15 %, respectively; the latter value was increased almost twice by using the heterometallic complex [Tb0.5 Eu0.5 (pfb)3 (H2 O)] (Hpfb=pentafluorobenzoic acid). Due to the effectively utilized sensitization strategy (pfb)(-) →Tb→Eu, a pure europium luminescence with a PLQY of 29 % was achieved.

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) ).

Labile or stable: can homoleptic and heteroleptic PyrPHOS-copper complexes be processed from solution?

Luminescent Cu(I) complexes are interesting candidates as dopants in organic light-emitting diodes (OLEDs). However, open questions remain regarding the stability of such complexes in solution and therefore their suitability for solution processing. Since the emission behavior of Cu(I) emitters often drastically differs between bulk and thin film samples, it cannot be excluded that changes such as partial decomposition or formation of alternative emitting compounds upon processing are responsible. In this study, we present three particularly interesting candidates of the recently established copper-halide-(diphenylphosphino)pyridine derivatives (PyrPHOS) family that do not show such changes. We compare single crystals, amorphous bulk samples, and neat thin films in order to verify whether the material remains stable upon processing. Solid-state nuclear magnetic resonance (MAS (31)P NMR) was used to investigate the electronic environment of the phosphorus atoms, and X-ray absorption spectroscopy at the Cu K edge provides insight into the local electronic and geometrical environment of the copper(I) metal centers of the samples. Our results suggest that--unlike other copper(I) complexes--the copper-halide-PyrPHOS clusters are significantly more stable upon processing and retain their initial structure upon quick precipitation as well as thin film processing.

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.

How the quantum efficiency of a highly emissive binuclear copper complex is enhanced by changing the processing solvent.

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.

Di-µ-iodido-bis-{[(R)-(+)-2,2'-bis-(di-phenyl-phosphan-yl)-1,1'-binaphthyl-κ(2)P,P']copper(I)} 0.67-hydrate.

The structure of the title compound, [Cu(2)I(2)(C(44)H(32)P(2))(2)]·0.67H(2)O, has been determined because of its inter-esting catalytic and optical features. The mol-ecule, which has non-crystallographic C2-symmetry, consists of a core structure of two Cu(I) ions, bridged by two iodide ions. Each Cu(I) ion is also coordinated by one equivalent of the chiral bidentate (R)-BINAP ligand [BINAP = 2,2'-bis-(diphenyl-phosphan-yl)-1,1'-binaphth-yl]. Thus, both cations show a distorted tetra-hedral geometry being surrounded by two I atoms and two P atoms from the (R)-BINAP ligands. The complex consists of isolated butterfly-shaped mol-ecules featuring an angle of 146.11 (2)° between adjacent CuI(2) planes. The structure displays intra-molecular C-H⋯I hydrogen bonding and contains disordered water. The absolute configuration of this chiral complex was determined by anomalous dispersion effects.