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When a boy and his aunt find that a bigot has written hurtful words on the sidewalk just outside the candy shop owned by \"Miz Chu\", a new immigrant from Taiwan, they set out to comfort her.

We correlate spatially resolved fluorescence (-lifetime) measurements with X-ray nanodiffraction to reveal surface defects in supercrystals of self-assembled cesium lead halide perovskite nanocrystals and study their effect on the fluorescence properties. Upon comparison with density functional modeling, we show that a loss in structural coherence, an increasing atomic misalignment between adjacent nanocrystals, and growing compressive strain near the surface of the supercrystal are responsible for the observed fluorescence blueshift and decreased fluorescence lifetimes. Such surface defect-related optical properties extend the frequently assumed analogy between atoms and nanocrystals as so-called quasi-atoms. Our results emphasize the importance of minimizing strain during the self-assembly of perovskite nanocrystals into supercrystals for lighting application such as superfluorescent emitters. By utilizing spatially resolved fluorescence (-lifetime) measurements and high precision X-ray nanodiffraction, the authors correlate the influence of structural misalignment and fluorescence (-lifetime) properties of all-inorganic CsPbX 3 (X – = Br – , Cl – ) perovskite superlattices.

A little boy who is not pleased with his own artistic efforts but treasures his great-grandmother's drawing goes on to collect art throughout his life.

In the present work, a novel concept for metallic metamaterials is presented, motivated by the creation of next-generation reversible damping systems that can be exposed to various environmental conditions. For this purpose, a unit cell is designed that consists of a parallel arrangement of a spring and snap-fit mechanism. The combination of the two concepts enables damping properties one order of magnitude higher than those of the constituting metal material. The spring element stores elastic energy while the snap-fit allows to absorb and dissipate energy and to reach a second stable state. Different configurations of single unit cells and connected cell assemblies are manufactured by laser powder bed fusion using Ti6Al4V powder. The dimensioning is supported by finite element modelling and the characteristic properties of the unit cells are studied in cyclic compression experiments. The metamaterial exhibits damping properties in the range of polymeric foams while retaining its higher environmental resistance. By variation of selected geometrical parameters, either bistable or self-recovering characteristics are achieved. Therefore, a metamaterial as an assembly of the described unit cells could offer a high potential as a structural element in future damping or energy storage systems operating at elevated temperatures and extreme environmental conditions.

The ring opening of a dithienylethene photoswitch incorporated in a bridged boron-dipyrromethene - dithienylethene molecular dyad was investigated with ultrafast spectroscopy. Coherent vibrations in the electronic ground state of the boron-dipyrromethene are triggered after selective photoexcitation of the closed dithienylethene indicating vibrational coupling although the two moieties are electronically isolated. A distribution of short-lived modes and a long-lived mode at 143 cm −1 are observed. Analysis of the theoretical frequency spectrum indicates two modes at 97 cm −1 and 147 cm −1 which strongly modulate the electronic transition energy. Both modes exhibit a characteristic displacement of the bridge suggesting that the mechanical momentum of the initial geometry change after photoexcitation of the dithienylethene is transduced to the boron-dipyrromethene. The relaxation to the dithienylethene electronic ground state is accompanied by significant heat dissipation into the surrounding medium. In the investigated dyad, the boron-dipyrromethene acts as probe for the ultrafast photophysical processes in the dithienylethene.

Photoinduced energy transfer processes and reactions play an important role in many areas of chemistry, physics and biology. Among the most prominent examples are biological light-harvesting in photosynthesis and excitation energy transfer in functional materials. Here, we focus upon the second type of systems, which are used, e.g., in organic electronics as well as in a variety of tailored donor-acceptor units and switches. More specifically, we study two types of functional organic systems: First, oligo-para-phenylene-vinylene (OPV) and oligo-thiophene (OT) as building blocks for paradigm materials used in organic photovoltaics. Second, a small donor-acceptor dyad, i.e., dithienylethene boron-dipyrromethene (DTEBODIPY) which has been developed and investigated in collaboration with the experimental groups of K. Rück-Braun (TU Berlin) and J. Wachtveitl (Goethe University).In order to understand the relevant energy transfer mechanisms, we employ firstprinciples electronic structure and quantum dynamical studies. Parametrized model Hamiltonians based on high-level ab initio calculations are combined with high-dimensional quantum dynamical or mixed quantum-classical simulations. The parametrization of the Hamiltonian is carried out for small fragments which allow for an electronic-structure treatment, while the Hamiltonian as such can be used for much larger systems. The dynamical calculations rely either on the MultiConfiguration Time-Dependent Hartree (MCTDH) method which permits a full quantum treatment, or else on the semi-classical Ehrenfest method, which allows to treat larger systems. The Ehrenfest method was implemented in an independent code, including Langevin driving by an environment.In the oligomer (OPV, OT) systems, the dynamics in the presence of a structural defect was investigated. We aim to understand the dynamical phenomena induced by photoexcitation, which involve the migration of electronic excitations (excitons). We also aim to clarify the \"spectroscopic unit\" concept that postulates the confinement of photoinduced electronic excitations (excitons) due to geometric defects.The system is defined in a Frenkel exciton basis, where each basis function describes strongly localized electron-hole pairs on a monomer unit. Delocalized electronic excitations are represented as superpositions of such localized Frenkel states. Beyond the Frenkel picture, a generalized electron-hole basis including charge-transfer states, i.e., allowing for spatial separation of electrons and holes, is also addressed. The parametrization of the model Hamiltonian is based upon high level electronic structure calculations by the Algebraic Diagrammatic Construction (ADC(2)) method, in conjunction with a transition density analysis. This level of treatment allows a correct identification of the supermolecular states as Frenkel type exciton states, and their distinction from charge-transfer states. To include vibronic effects, low and high-frequency modes representing torsional and bond-length-alternation (BLA) coordinates of the system are included. To this end, potential energy surface (PES) cuts are carried out and mapped upon a global potential surface.With this setup, the quantum dynamical and mixed quantum-classical simulations for hexamer and 20-mer OPV and OT species were carried out. These calculations show that excitation energy transfer takes place on a sub-picosecond time scale and strongly correlates with structural defects. In the hexamer system, a coherent spreading of the exciton across a torsional defect is observed and simulations of a 20-mer system provide evidence for a „coherent hopping“ type mechanism of exciton migration.

In the summer of 1954, Jan Wahl, a young American student on a Fulbright scholarship to the University of Copenhagen, had the chance to spend some time with Danish director Carl Theodor Dreyer. The film Dreyer was shooting at the time was \"Ordet\" (\"The Word\"), subsequently acclaimed as one of the director's greatest--and one of the most moving explorations of faith in all cinema. Wahl's transcriptions of his conversations with Dreyer form the basis of a newly published memoir--\"Carl Theodor Dreyer and Ordet: My Summer with the Danish Filmmaker\"--of which an edited extract is provided.

In the summer of 1954, Jan Wahl, a young American student on a Fulbright scholarship to the University of Copenhagen, had the chance to spend some time with Danish director Carl Theodor Dreyer. The film Dreyer was shooting at the time was \"Ordet\" (\"The Word\"), subsequently acclaimed as one of the director's greatest--and one of the most moving explorations of faith in all cinema. Wahl's transcriptions of his conversations with Dreyer form the basis of a newly published memoir--\"Carl Theodor Dreyer and Ordet: My Summer with the Danish Filmmaker\"--of which an edited extract is provided.

We correlate spatially resolved fluorescence (-lifetime) measurements with X-ray nanodiffraction to reveal surface defects in supercrystals of self-assembled caesium lead halide perovskite nanocrystals and study their effect on the fluorescence properties. Upon comparison with density functional modelling, we show that a loss in structural coherence, an increasing atomic misalignment between adjacent nanocrystals, and growing compressive strain near the surface of the supercrystal are responsible for the observed fluorescence blueshift and decreased fluorescence lifetimes. Such surface defect-related optical properties extend the frequently assumed analogy between atoms and nanocrystals as so-called quasi-atoms. Our results emphasize the importance of minimizing strain during the self-assembly of perovskite nanocrystals into supercrystals for lighting application such as superfluorescent emitters.