Proton Conduction in a MIL-53(Al) Metal–Organic Framework: Confinement versus Host/Guest Interaction

In this contribution, we present and discuss results from a computational study of proton transfers between imidazole molecules confined in a MIL-53­(Al) metal–organic framework. We combined molecular-dynamics simulations and a density-functional tight-binding method. The extensive analysis of trajectories resulted in two main competing effects: on the one hand, the one-dimensional channel structure of MIL-53­(Al) arranges the imidazole molecules to allow proton exchange by hopping transport; on the other hand, the interactions between the MIL-53­(Al) host system and the imidazole molecules influence the free movement retaining the molecules. We find that the retaining leads to an increase in proton transfers, when both vehicle mechanisms and hopping events are considered. Thus, a well-balanced relationship between these two effects is necessary for efficient proton transport in metal–organic frameworks. Furthermore, the lifetime of the transition state could be estimated to be on the order of 100 fs

Intramolecular Polarization Induces Electron–Hole Charge Separation in Light-Harvesting Molecular Triads

Artificial light-harvesting supramolecular structures reproduce the light-to-electrochemical energy transduction mechanisms observed in natural photosynthesis. Among them the prototypical carotenoid­(C)–porphyrin­(P)–fullerene­(C₆₀) type of structures have been the most studied. Several experiments performed in such structures, and others alike, have shown that the photoexcited state <i>C</i>– ¹<i>P</i>–<i>C</i>₆₀ decays to the metastable charge-separated state <i>C</i>– <i>P</i>^•+ −<i>C</i>₆₀^•– within a few picoseconds, whereas the final charge-separated state, <i>C</i>^•+– <i>P</i> −<i>C</i>₆₀^•–, is obtained within hundreds of picoseconds. This paper introduces a nonlinear polarizable extended Hückel Hamiltonian that describes the charge dynamics and charge-separation effects in such triads by means of quantum dynamics simulations performed on the photoexcited electron–hole pair. The results are interpreted on the basis of the discrete self-trapping equation and enlighten the role played by the polarizability on charge-separation phenomena

Effect of Surface Properties on the Microstructure, Thermal, and Colloidal Stability of VB₂ Nanoparticles

Recent years have seen an increasing research effort focused on nanoscaling of metal borides, a class of compounds characterized by a variety of crystal structures and bonding interactions. Despite being subject to an increasing number of studies in the application field, comprehensive studies of the size-dependent structural changes of metal borides are limited. In this work, size-dependent microstructural analysis of the VB₂ nanocrystals prepared by means of a size-controlled colloidal solution synthesis is carried out using X-ray powder diffraction. The contributions of crystallite size and strain to X-ray line broadening is separated by introducing a modified Williamson–Hall method taking into account different reflection profile shapes. For average crystallite sizes smaller than ca. 20 nm, a remarkable increase of lattice strain is observed together with a significant contraction of the hexagonal lattice decreasing primarily the cell parameter <i>c</i>. Exemplary density-functional theory calculations support this trend. The size-dependent lattice contraction of VB₂ nanoparticles is associated with the decrease of the interatomic boron distances along the <i>c</i>-axis. The larger fraction of constituent atoms at the surface is formed by boron atoms. Accordingly, lattice contraction is considered to be a surface effect. The anisotropy of the size-dependent lattice contraction in VB₂ nanocrystals is in line with the higher compressibility of its macroscopic bulk structure along the <i>c</i>-axis revealed by theoretical calculations of the respective elastic properties. Transmission electron microscopy indicates that the VB₂ nanocrystals are embedded in an amorphous matrix. X-ray photoelectron spectroscopy analysis reveals that this matrix is mainly composed of boric acid, boron oxides, and vanadium oxides. VB₂ nanocrystals coated with these oxygen containing amorphous species are stable up to 789 °C as evidenced by thermal analysis and temperature dependent X-ray diffraction measurements carried out under Ar atmosphere. Electrokinetic measurement indicates that the aqueous suspension of VB₂ nanoparticles with hydroxyl groups on the surface region has a good stability at neutral and basic pH arising from electrostatic stabilizatio