Hydration effects turn a highly stretched polymer from an entropic into an energetic spring

Polyethylene glycol (PEG) is a structurally simple and nontoxic water-soluble polymer that is widely used in medical and pharmaceutical applications as molecular linker and spacer. In such applications, PEG’s elastic response against conformational deformations is key to its function. According to text-book knowledge, a polymer reacts to the stretching of its end-to-end separation by a decrease in entropy that is due to the reduction of available conformations, which is why polymers are commonly called entropic springs. By a combination of single-molecule force spectroscopy experiments with molecular dynamics simulations in explicit water, we show that entropic hydration effects almost exactly compensate the chain conformational entropy loss at high stretching. Our simulations reveal that this entropic compensation is due to the stretching-induced release of water molecules that in the relaxed state form double hydrogen bonds with PEG. As a consequence, the stretching response of PEG is predominantly of energetic, not of entropic, origin at high forces and caused by hydration effects, while PEG backbone deformations only play a minor role. These findings demonstrate the importance of hydration for the mechanics of macromolecules and constitute a case example that sheds light on the antagonistic interplay of conformational and hydration degrees of freedom

Blue cadmium-free and air-fabricated quantum dot light-emitting diodes

The article processing charge was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 491192747 and the Open Access Publication Fund of Humboldt-Universität zu Berlin.Quantum dot (QD) materials have found increasing use in display applications because of their high color purity and fluorescence quantum yield, enabling devices with higher brightness and efficiency. However, to access large-area printing and coating methods that are carried out in ambient conditions, it is necessary to, first, move away from toxic cadmium, and second, to target materials that can be air-processed. Herein, we synthesize zinc selenide-based blue QD material and air-fabricate light-emitting diodes (LEDs) and single-carrier devices. The encapsulated devices were also measured under ambient conditions. Multi-shell-structured ZnSeTe/ZnSe/ZnS (core/shell/shell) QDs show pure deep blue/purple fluorescence emission with a high photoluminescence quantum yield of 78%. The blue QD-LED devices are fabricated in a conventional structure with bottom light emission with two electron transport materials (ZnO and ZnMgO). The QD-LED devices with ZnO electron transport layer show a maximum luminance of ∼6200 cd m−2 at 9 V with a turn-on voltage of 3.5 V and current efficacy of 0.38 cd A−1, while with ZnMgO electron transport layer, the devices show a maximum luminance of 3000 cd m−2 at 7 V with a turn-on voltage of 3 V and current efficacy of 0.6 cd A−1. Electron-only and hole-only devices were fabricated to show and confirm the underlying charge transport mechanisms. To our knowledge, these results show for the first-time air-fabricated ZnSe-based QD-LEDs, paving the way for scaling up display applications and moving toward high-performance printed electronics.Peer Reviewe

Binding forces in metallo-supramolecular coordination compounds

Multivalente Wechselwirkungen sind in diversen biomolekularen und supramolekularen Systemen anzutreffen. Gewöhnlich werden sie durch ihre thermische Stabilität charakterisiert. Doch auch das mechanische Reißverhalten ist relevant: Ein System mit großer Reißlänge (Verformbarkeit) weist zwar eine geringere Reißkraft auf, kann aber besser auf äußere Einflüsse ohne Bindungsbruch reagieren. Daher besteht ein zunehmendes Interesse an Modellen zur Vorhersage der mechanischen Stabilität multivalenter Wechselwirkungen. Einzelmolekül-Kraftspektroskopie (SMFS) ist eine nützliche Methode, um den Reißprozess nichtkovalenter Wechselwirkungen zu studieren. Im Rahmen dieser Dissertation wurden mono- und bivalenten Pyridine, komplexiert und verbunden durch Cu(II) und Zn(II), entworfen und untersucht. Die drei bivalenten Pyridine wiesen unterschiedlich flexible Rückgratstrukturen auf (flexibel, teilflexibel, steif). Überraschenderweise wurde ein anderer Trend für die Verformbarkeiten gemessen (flexibel > steif > teilflexibel). Durch Vergleich von experimentellen Daten mit ab-initio Berechnungen konnten komplexe Reißmechanismen vorgeschlagen werden: Das Lösungsmittel war entscheidend und führte zu wasserverbrückten Zwischenprodukten, was die Verformbarkeit aller Systeme stark erhöhte. Im bivalente System mit teilflexiblem Rückgrat, koordiniert durch Cu(II), rissen beide Bindungen gleichzeitig bei vergleichsweise großen Kräften. Die beiden anderen Systeme mit Cu(II) wurden in zweistufigen Prozessen voneinander getrennt, was kleinere Reißkräfte zur Folge hatte. Insbesondere das flexible System war zwar thermisch stabiler, brach aber leichter als das monovalente System. Damit wurde zum ersten Mal der große Einfluss des Rückgrats, bei sonst gleicher Art von Wechselwirkung, auf die mechanische Stabilität bivalenter Wechselwirkungen gezeigt. Außerdem ist das entwickelte Modellsystem sehr nützlich für weiterführende Untersuchungen in biologisch relevanten wässrigen Lösungsmitteln.Multivalent interactions are ubiquitous in biomolecular and supramolecular systems. They are commonly characterized by their thermal stability in terms of average bond lifetime or equilibration constant. However, also mechanical stabilities are relevant: A system with high rupture length (malleability) has a lower rupture force, but can more easily adopt to external constraints without rupture. Thus it is of ever-increasing interest to find appropriate models that allow predictions on the mechanical stability of multivalent interactions. Single-molecule force spectroscopy (SMFS) is a powerful tool to study the rupture process of non-covalent interactions. In the present thesis, a comprehensive study on the mechanical stability of bivalent pyridine coordination compounds with the metal ions Cu(II) and Zn(II) was performed. Surprisingly, three different backbone flexibilities (high, intermediate, low) did not correlate with the measured malleabilities (high > low > intermediate). Instead, comparison between experimental results and ab-initio calculations revealed more complex underlying rupture mechanisms: Due to the aqueous environment, hydrogen bound complexes were formed and important intermediate structures that strongly increased malleabilities. Both interactions of the intermediately flexible bivalent system with Cu(II) broke simultaneous, yielding comparatively large rupture forces. The bivalent interactions of high and low backbone flexibility with Cu(II) broke stepwise at smaller forces. Although being thermally more stable, the highly flexible system even broke at lower forces than the monovalent system. Thereby it was shown for the first time, that rupture forces of similar systems can be tuned over a broad range, just by changing the connecting backbone structure. Furthermore, the developed approach is a rich toolkit to study further the balanced interplay between rupture force and malleability in biologically relevant aqueous solvents

Mechanical stability of bivalent transition metal complexes analyzed by single-molecule force spectroscopy

Multivalent biomolecular interactions allow for a balanced interplay of mechanical stability and malleability, and nature makes widely use of it. For instance, systems of similar thermal stability may have very different rupture forces. Thus it is of paramount interest to study and understand the mechanical properties of multivalent systems through well-characterized model systems. We analyzed the rupture behavior of three different bivalent pyridine coordination complexes with Cu2+ in aqueous environment by single-molecule force spectroscopy. Those complexes share the same supramolecular interaction leading to similar thermal off-rates in the range of 0.09 and 0.36 s−1, compared to 1.7 s−1 for the monovalent complex. On the other hand, the backbones exhibit different flexibility, and we determined a broad range of rupture lengths between 0.3 and 1.1 nm, with higher most-probable rupture forces for the stiffer backbones. Interestingly, the medium-flexible connection has the highest rupture forces, whereas the ligands with highest and lowest rigidity seem to be prone to consecutive bond rupture. The presented approach allows separating bond and backbone effects in multivalent model systems

Nanoscopic Properties and Application of Mix-and-Match Plasmonic Surfaces for Microscopic SERS

Gold and silver nanoparticles can be immobilized on glass slides using aminosilane linkers. Here, we demonstrate that particle monolayer surfaces can also be generated by simultaneous immobilization of both gold and silver nanoparticles with the same organosilane linker. These new surfaces display surface-enhanced Raman scattering (SERS) enhancement typical for gold or silver monolayers, depending on the ratio of the two types of nanoparticles and, at the same time, have the capability to probe complex analytes composed from various molecules which adsorb at only one of the metals. The reported results from scanning electron microscopy, scanning force microscopy, and UV/vis absorbance for surfaces containing one or two types of nanoparticles indicate that an enhancement level above 10⁴ is related to nanoaggregates that form in the 2D plane. High and stable enhancement factors over a wide range of analyte concentrations along with high homogeneity of the enhancement at the microscopic scale make the plasmonic nanoparticle mix-and-match surfaces ideal substrates for use in microscopic SERS sensing

Mechanical Rupture of Mono- and Bivalent Transition Metal Complexes in Experiment and Theory

Biomolecular systems are commonly exposed to a manifold of forces, often acting between multivalent ligands. To understand these forces, we studied mono- and bivalent model systems of pyridine coordination complexes with Cu²⁺ and Zn²⁺ in aqueous environment by means of scanning force microscopy based single-molecule force spectroscopy in combination with <i>ab initio</i> DFT calculations. The monovalent interactions show remarkably long rupture lengths of approximately 3 Å that we attribute to a dissociation mechanism involving a hydrogen-bound intermediate state. The bivalent interaction with copper dissociates also via hydrogen-bound intermediates, leading to an even longer rupture length between 5 and 6 Å. Although the bivalent system is thermally more stable, the most probable rupture forces of both systems are similar over the range of measured loading rates. Our results prove that already in small model systems the dissociation mechanism strongly affects the mechanical stability. The presented approach offers the opportunity to study the force-reducing effects also as a function of different backbone properties

Light-Controlled “Molecular Zippers” Based on Azobenzene Main Chain Polymers

Single strands of azobenzene main chain polymers exhibiting alkyl side chains can be largely and reversibly contracted and extended with light. We show that upon self-assembly in a thin layered film they act as “molecular zippers” that can be opened and closed with UV- and blue light, respectively. Simultaneously <i>in situ</i> recorded time-resolved X-ray diffraction and optical spectroscopy measurements, together with scanning force microscopy show that upon the light-induced <i>E → Z</i> isomerization of the main chain azobenzenes the layered film morphology remains, while the initially highly ordered alkyl side chains become disordered. Already the <i>E → Z</i> isomerization of about 20% of all azobenzene chromophores triggers a complete disorder of the alkyl chains. The kinetics of this partial amorphization of the film is about 18 times slower than the ensemble kinetics of the initial azobenzene photoisomerization. This is the first demonstration of a rigid main chain polymer film with reversibly photoswitchable side chain crystallinity

Lattice Matching as the Determining Factor for Molecular Tilt and Multilayer Growth Mode of the Nanographene Hexa-<i>peri</i>-hexabenzocoronene

The microstructure, morphology, and growth dynamics of hexa-<i>peri</i>-hexabenzocoronene (HBC, C₄₂H₁₈) thin films deposited on inert substrates of similar surface energies are studied with particular emphasis on the influence of substrate symmetry and substrate–molecule lattice matching on the resulting films of this material. By combining atomic force microscopy (AFM) with X-ray diffraction (XRD), X-ray absorption spectroscopy (NEXAFS), and in situ X-ray reflectivity (XRR) measurements, it is shown that HBC forms polycrystalline films on SiO₂, where molecules are oriented in an upright fashion and adopt the known bulk structure. Remarkably, HBC films deposited on highly oriented pyrolytic graphite (HOPG) exhibit a new, substrate-induced polymorph, where all molecules adopt a recumbent orientation with planar π-stacking. Formation of this new phase, however, depends critically on the coherence of the underlying graphite lattice since HBC grown on defective HOPG reveals the same orientation and phase as on SiO₂. These results therefore demonstrate that the resulting film structure and morphology are not solely governed by the adsorption energy but also by the presence or absence of symmetry- and lattice-matching between the substrate and admolecules. Moreover, it highlights that weakly interacting substrates of high quality and coherence can be useful to induce new polymorphs with distinctly different molecular arrangements than the bulk structure