Non-radiative decay and stability of NN-heterocyclic carbene iridium(III) complexes

Devices based on deep-blue emitting iridium (III) complexes with N-heterocyclic carbene (NHC) ligands have recently been shown to give excellent performance as phosphorescent organic light-emitting diodes (PHOLEDs). To facilitate the design of even better deep-blue phosphorescent emitters we carried out density functional theory (DFT) calculations of the lowest triplet ($T_1$) potential-energy surfaces (PES) upon lengthening the iridium-ligand (Ir-C) bonds. Relativistic time dependent-DFT (TDDFT) calculations demonstrate that this changes the nature of $T_1$ from a highly-emissive metal-to-ligand charge transfer ($^3$MLCT) state to a metal centered ($^3$MC) state where the radiative decay rate is orders of magnitude slower than that of the $^3$MLCT state. We identify the elongation of an Ir-C bond on the NHC group as the pathway with lowest energy barrier between the $^3$MLCT and $^3$MC states for all complexes studied and show that the barrier height is correlated with the experimentally measured non-radiative decay rate. This suggests that the thermal population of $^3$MC states is the dominant non-radiative decay mechanism at room temperature. We show that the $^3$MLCT $\rightarrow$ $^3$MC transition is reversible, in marked contrast to other deep blue phosphors containing coordinating nitrogen atoms, where the population of $^3$MC states breaks Ir-N bonds. This suggests that, as well as improved efficiency, blue PHOLEDs containing phosphors where the metal is only coordinated by carbon atoms will have improved device lifetimes.Comment: 15 pages, 4 figures, 3 table

Aggregates of diketopyrrolopyrrole dimers in solution

Dimers based on diketopyrrolopyrrole (DPP) chromophores have gained tremendous interest as an excellent material building block for organic solar cells and photodiodes. However, a counterintuitive blue-shift in their solution absorption spectra occurs with an increasing number of thiophene units bridging the DPP moieties. We allocate this to aggregates in solution, which might hinder adequate mixing in blends, leading to poor film forming quality and reduced charge generation in solution processed devices. Hence, identification of such aggregates is crucial in order to find measures for device optimisation. In this study, we present synthesis and characterisation of three pyridyl end-capped DPP dimers of different conjugation length using thiophene linkers and compare their parent monomer to evidence the behaviours of aggregates in solution. We employ conventional and temperature dependent UV–Vis spectroscopy, fluorescence and excitation-emission spectroscopy as well as TD-DFT calculations to show that such DPP dimers predominantly form aggregates in solution even at low concentrations. By disentangling the spectroscopic behaviour of both aggregated and non-aggregated species, we refute literature's explanation that the apparent blue shift in absorption arises from a reduced conjugation length due to more molecular flexibility. Instead, absorption and emission signals of non-aggregated dimers are mostly masked by their aggregated species. This work provides a tool set using common laboratory spectroscopic equipment to identify and characterise solution aggregates—information particularly important towards optimisation of organic electronics processed from solution

Multi-scale diff-changed feature fusion network for hyperspectral image change detection.

For hyperspectral images (HSI) change detection (CD), multi-scale features are usually used to construct the detection models. However, the existing studies only consider the multi-scale features containing changed and unchanged components, which is difficult to represent the subtle changes between bi-temporal HSIs in each scale. To address this problem, we propose a multi-scale diff-changed feature fusion network (MSDFFN) for HSI CD, which improves the ability of feature representation by learning the refined change components between bi-temporal HSIs under different scales. In this network, a temporal feature encoder-decoder sub-network, which combines a reduced inception module and a cross-layer attention module to highlight the significant features, is designed to extract the temporal features of HSIs. A bidirectional diff-changed feature representation module is proposed to learn the fine changed features of bi-temporal HSIs at various scales to enhance the discriminative performance of the subtle change. A multi-scale attention fusion module is developed to adaptively fuse the changed features of various scales. The proposed method can not only discover the subtle change of bi-temporal HSIs but also improve the discriminating power for HSI CD. Experimental results on three HSI datasets show that MSDFFN outperforms a few state-of-the-art methods

Overexpression of Class III β-tubulin, Sox2, and nuclear Survivin is predictive of taxane resistance in patients with stage III ovarian epithelial cancer

Failed root canal treatment is best addressed primarily with the provision of repeat endodontic treatment with thorough irrigation under isolation. If a post is present in the root of the tooth it needs to be removed first. This paper is the second in a series of two which provide an overview of techniques for post removal. Specifically designed post removal devices and the removal of fibre posts are described. Post removal device techniques are illustrated with a series of clinical case figures

Effect of n-propyl substituents on the emission properties of blue phosphorescent iridium(iii) complexes

Ligand substitution is often used for tuning the emission color of phosphorescent iridium(iii) complexes that are used in organic light-emitting diodes. However, in addition to tuning the emission color, the substituents can also affect the radiative and non-radiative decay rates of the excited state and hence the photoluminescence quantum yield. Understanding the substituent effect is therefore important for the design of new iridium(iii) complexes with specific emission properties. Using (time dependent) density functional methods, we investigate the substituent effect of n-propyl groups on the structure, emission color, and emission efficiency of fac-tris(1-methyl-5-phenyl-[1,2,4]triazolyl)iridium(iii) based phosphorescent complexes by comparing the calculated results for structural models with and without the n-propyl substituents. We find that attachment of the n-propyl groups increases the length of three Ir–N bonds, and although the emission color does not change significantly, the radiative and non-radiative rates do, leading to a prediction of enhanced blue phosphorescence emission efficiency. Furthermore, the calculations show that the attachment of the n-propyl groups leads to a larger activation energy to degradation and the formation of dark states

How to choose the frozen density in Frozen-Density Embedding Theory-based numerical simulations of local excitations?

According to Frozen-Density Embedding Theory, any observable evaluated for the embedded species is a functional of the frozen density (ρ B —the density associated with the environment). The environment-induced shifts in the energies of local excitations in organic chromophores embedded in hydrogen-bonded environments are analyzed. The excitation energies obtained for ρ B , which is derived from ground-state calculations for the whole environment applying medium quality basis sets (STO-DZP) or larger, vary in a narrow range (about 0.02eV which is at least one order of magnitude less than the magnitude of the shift). At the same time, the ground-state dipole moment of the environment varies significantly. The lack of correlation between the calculated shift and the dipole moment of the environment reflects the fact that, in Frozen-Density Embedding Theory, the partitioning of the total density is not unique. As a consequence, such concepts as "environment polarization” are not well defined within Frozen-Density Embedding Theory. Other strategies to generate ρ B (superposition of densities of atoms/molecules in the environment) are shown to be less robust for simulating excitation energy shifts for chromophores in environments comprising hydrogen-bonded molecules

Predicted Electronic Markers for Polytypes of LaOBiS\u3csub\u3e2\u3c/sub\u3e Examined via Angle-Resolved Photoemission Spectroscopy

The natural periodic stacking of symmetry-inequivalent planes in layered compounds can lead to the formation of natural superlattices; albeit close in total energy, (thus in their thermodynamic stability), such polytype superlattices can exhibit different structural symmetries, thus have markedly different electronic properties which can in turn be used as “structural markers”. We illustrate this general principle on the layered LaOBiS2 compound where density-functional theory (DFT) calculations on the (BiS2)/(LaO)/(BiS2) polytype superlattices reveal both qualitatively and quantitatively distinct electronic structure markers associated with the Rashba physics, yet the total energies are only ∼ 0.1 meV apart. This opens the exciting possibility of identifying subtle structural features via electronic markers. We show that the pattern of removal of band degeneracies in different polytypes by the different forms of symmetry breaking leads to Rashba “minigaps” with characteristic Rashba parameters that can be determined from spectroscopy, thereby narrowing down the physically possible polytypes. By identifying these distinct DFT-predicted fingerprints via angle-resolved photoemission spectroscopy (ARPES) measurements on LaBiOS2 we found the dominant polytype with small amounts of mixtures of other polytypes. This conclusion, consistent with neutron scattering results, establishes ARPES detection of theoretically established electronic markers as a powerful tool to delineate energetically quasidegenerate polytypes

Predicted electronic markers for polytypes of LaOBiS2 examined via angular resolved photoemission spectroscopy

The natural periodic stacking of symmetry-inequivalent planes in layered compounds can lead to the formation of natural superlattices; albeit close in total energy, (thus in their thermodynamic stability), such polytype superlattices can exhibit different structural symmetries, thus have markedly different electronic properties which can in turn be used as "structural markers". We illustrate this general principle on the layered LaOBiS2 compound where density-functional theory (DFT) calculations on the (BiS2)/(LaO)/(BiS2) polytype superlattices reveal both qualitatively and quantitatively distinct electronic structure markers associated with the Rashba physics, yet the total energies are only ~ 0.1 meV apart. This opens the exciting possibility of identifying subtle structural features via electronic markers. We show that the pattern of removal of band degeneracies in different polytypes by the different forms of symmetry breaking leads to new Rashba "mini gaps" with characteristic Rashba parameters that can be determined from spectroscopy, thereby narrowing down the physically possible polytypes. By identifying these distinct DFT-predicted fingerprints via ARPES measurements on LaBiOS2 we found the dominant polytype with small amounts of mixtures of other polytypes. This conclusion, consistent with neutron scattering results, establishes ARPES detection of theoretically established electronic markers as a powerful tool to delineate energetically quasidegenerate polytypes.Comment: 13 pages, 4 figure