Methyl Radical in Clathrate Silica Voids. The Peculiar Physisorption Features of the Guest–Host Molecular Dynamics Interaction

EPR line shape simulations of CH₃/SiO₂ clathrates and comparison to CH₃/N₂O and CH₃/SiO₂ experiments reveal the motional conditions of the CH₃ radical up to the unusual regime of its stability, the high-temperature diffusional regime, at 300 K. In the low-temperature region, the CH₃ in clathrates is found to rotate around the in-plane axes even at as low temperatures as 3.8 K. However, nonrotating methyls performing only libration about the <i>C</i>₂-axes as well as around the <i>C</i>₃-axis are also found, proving the existence of special sites in the clathrate voids that begin to accumulate a significant fraction of methyl radicals at temperatures below approximately 7 K. A distinctive feature in the spectrum anisotropy and line width temperature profiles is found nearby 25 K, which is interpreted as the radical physisorption inside the voids that occurs with the sample temperature lowering. The unusual increase of the CH₃/SiO₂ clathrate EPR spectral width with temperature over approximately 120 K has its origin in repeated angular momentum vector alterations due to frequent collisions with the clathrate void walls between periodical free rotation periods. This relaxation mechanism resembles to spin–rotation interaction known only for small molecular species in nonviscous fluids but unknown earlier for methyl hosted in solids

Structure of the FeBTC Metal–Organic Framework: A Model Based on the Local Environment Study

The local environment of iron in FeBTC, a metal organic framework commercially known as Basolite F300, is investigated combining XANES and EXAFS studies of the iron K-edge. The building block of the FeBTC can be described as an iron acetate moiety. Dehydration induces a change in the coordination of the first shell while preserving the network. We propose that the local structure around Fe atoms does not undergo a rearrangement, thus, leading to the formation of an open site. The analysis conveys that the FeBTC is a disordered network of locally ordered blocks

β‑C₃N₄ Nanocrystals: Carbon Dots with Extraordinary Morphological, Structural, and Optical Homogeneity

Carbon nanodots are known for their appealing optical properties, especially their intense fluorescence tunable in the visible range. However, they are often affected by considerable issues of optical and structural heterogeneity, which limit their optical performance and limit the practical possibility of applying these nanoparticles in several fields. Here we developed a synthesis method capable of producing a unique variety of carbon nanodots displaying an extremely high visible absorption strength (ε > 3 × 10⁶ M­(dot)⁻¹ cm^–1) and a high fluorescence quantum yield (73%). The high homogeneity of these dots reflects in many domains: morphological (narrow size distribution), structural (quasi-perfect nanocrystals with large electronic bandgaps), and optical (nontunable fluorescence from a single electronic transition). Moreover, we provide the proof of principle that an aqueous solution of these dots can be used as an active medium in a laser cavity, displaying a very efficient laser emission with dye-like characteristics, which reflects the benefits of such a highly homogeneous type of carbon-based nanodots

Light-Induced Formation of Pb³⁺ Paramagnetic Species in Lead Halide Perovskites

Hybrid halide perovskites are soft materials processed at room temperature, revolutionary players in the photovoltaic field. Nowadays, investigation of the nature and role of defects is seen as one of the key challenges toward full comprehension of their behavior and achievement of high device stability under working conditions. We reveal the reversible generation, under illumination, of paramagnetic Pb³⁺ defects in CH₃NH₃PbI₃, synthesized in ambient conditions, induced by the presence of Pb–O defects in the perovskite structure that may trap photogenerated holes, possibly mediated by the concomitant oxidation and migration of ions. According to the mechanism that we hypothesize, one charge is trapped for each paramagnetic center generated; thus, it does not contribute to the photocurrent, potentially limiting the solar cell performance. Our study, based on combined experimental/theoretical approach, reveals the dynamic evolution of the perovskite characteristics under illumination that needs to be considered when investigating the material physical–chemical properties