Subterahertz phonon dynamics in acoustic nanocavities

We report a direct determination of the dynamic behavior of confined acoustic phonons in nanocavities by picosecond acoustics. We provide the broadband, high resolution transmission amplitude curve in the subterahertz range, and we give evidence of resonant transmission peaks in three successive stop bands, in quantitative agreement with acoustic simulations. We furthermore demonstrate transit times in the nanosecond range at the cavity peaks reflecting the strong confinement of resonant phonons within the cavity layer. On the other hand, picosecond transit times are measured in the stop band, shorter than in any of the constituting materials, a tunneling effect well known both in photonic crystals and in macroscopic phononic systems.     

Implicit multifunction theorems in complete metric spaces

In this paper, we establish some new characterizations of metric regularity of implicit multifunctions in complete metric spaces by using lower semicontinuous envelopes of the distance functions to set-valued mappings. Through these new characterizations it is possible to investigate implicit multifunction theorems based on coderivatives and on contingent derivatives as well as the perturbation stability of implicit multifunctions.     

Control of Metal–Organic Framework Crystallization by Metastable Intermediate Pre‐equilibrium Species

There is an increasing amount of interest in metal–organic frameworks (MOFs) for a variety of applications, from gas sensing and separations to electronics and catalysis. However, the mechanisms by which they crystallize remain poorly understood. Herein, an important new insight into MOF formation is reported. It is shown that, prior to network assembly, crystallization intermediates in the canonical ZIF‐8 system exist in a dynamic pre‐equilibrium, which depends on the reactant concentrations and the progress of reaction. Concentration can, therefore, be used as a synthetic handle to directly control particle size, with potential implications for industrial scale‐up and gas sorption applications. These findings enable the rationalization of apparent contradictions between previous studies of ZIF‐8 and opens up new opportunities for the control of crystallization in network solids more generally.     

Intramolecular,Interrupted Homo-Nazarov Cascade Biscyclizations to Angular (Hetero)Aryl-Fused Polycycles

Herein, a SnCl₄-catalyzed intramolecular, interrupted homo-Nazarov cascade biscyclization to access angular (hetero)aryl-fused polycycles is reported. Subsequent decarboxylation of the readily enolizable products afforded the angular products in up to 71 % yield over two steps, with the trans-diastereomers as the major products. The cyclopropyl homo-Nazarov cyclization precursors were formed using a scalable and modular synthetic route that, ultimately, offers access to 6,6,6-, 6,6,5-, 6,5,6-, 6,6,5,6-, and 6,6,6,5-fused angular polycyclic products. To showcase the rigor and utility of the method, an 8-step total synthesis of (±)-1-oxoferruginol, an antibacterial aromatic abietane diterpenoid, was disclosed.     

Physical Evidence of Stress-Induced Conformational Changes in Polymers

Background

Polymer mechanics and characterization is an active area of research where a keen effort is directed towards gaining a predictive and correlative relationship between the applied loads and the specific conformational motions of the macromolecule chains.

Objective

Therefore, the objective of this research is to introduce the preliminary results based on a novel technique to in situ probe the mechanical properties of polymers using non-invasive, non-destructive, and non-contact terahertz spectroscopy.

Methods

A dielectric elastomer actuator (DEA) structure is used as the loading mechanism to avoid obscuring the beam path of transmission terahertz time-domain spectroscopy. In DEAs, the applied voltage results in mechanical stresses under the active electrode area with far-reaching stretching in the passive area. Finite element analysis is used to model and simulate the DEA to quantify the induced stresses at the observation site over a voltage range spanning from 0 V to 3000 V. Additionally, a novel analysis technique is introduced based on the Hilbert-Huang transform to exploit the time-domain signals of the ultrathin elastomeric film and to defy the limits set forth by the current state-of-the-art analysis techniques.

Results

The computational result shows a nonlinear relationship between the effective stresses and the applied voltage. Analysis of the terahertz time-domain signals shows a shift in the delay times and a decrease in signal peak amplitudes, whereas these characteristics are implicitly related to the change in the index of refraction.

Conclusions

In all, the results evidentially signify the interrelationship between the conformational changes and applied mechanical stress.

     

A nanocomposite prepared from FeOOH and N-doped carbon nanosheets as a peroxidase mimic,and its application to enzymatic sensing of glucose in human urine

A method is described for the synthesis of a nanocomposite containing FeOOH and N-doped carbon nanosheets. The nanocomposite was synthesized by a hydrothermal method using a Fe₃O₄/chitosan nanocomposite as the precursor. The nanocomposite displays peroxidase-like activity and catalyzes the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by H₂O₂. This results in the formation of a blue colored product with an absorption maximum at 652 nm in the UV-vis spectra. Based on these findings, colorimetric assays were worked out for both hydrogen peroxide and glucose. The H₂O₂ assay works in the 5 to 19 μM concentration range, and the limit of detection is 5 nM. The glucose assay works in the 8 μM to 0.8 mM concentration range and has a 0.2 μM detection limit. The method was successfully applied to the determination of glucose in human urine.

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Graphical abstract Schematic of the hydrothermal synthesis of a FeOOH/N-doped carbon nanocomposite. It was used to replace peroxidase enzyme for the catalytic oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in a visual colorimetric test for glucose in human urine.

     

The β‐Agostic Structure in (C5Me5)2Sc(CH2CH3): Solid‐State NMR Studies of (C5Me5)2Sc−R (R=Me,Ph, Et)

Multinuclear solid‐state NMR studies of Cp*₂Sc?R (Cp*=pentamethylcyclopentadienyl; R=Me, Ph, Et) and DFT calculations show that the Sc?Et complex contains a β‐CH agostic interaction. The static central transition ⁴⁵Sc NMR spectra show that the quadrupolar coupling constants (C_q) follow the trend of Ph≈Me>Et, indicating that the Sc?R bond is different in Cp*₂Sc?Et compared to the methyl and phenyl complexes. Analysis of the chemical shift tensor (CST) shows that the deshielding experienced by Cβ in Sc?CH₂CH₃ is related to coupling between the filled σ_C‐C orbital and the vacant orbital.     

Nido‐Carboranes: Donors for Thermally Activated Delayed Fluorescence

An approach to the design of nido‐carborane‐based luminescent compounds that can exhibit thermally activated delayed fluorescence (TADF) is proposed. 7,8‐Dicarba‐nido‐undecaboranes (nido‐carboranes) having various 8‐R groups (R=H, Me, i‐Pr, Ph) are appended to the meta or para position of the phenyl ring of the dimesitylphenylborane (PhBMes₂) acceptor, forming donor–acceptor compounds (nido‐ m1 – m4 and nido‐ p1 – p4 ). The bulky 8‐R group and meta substitution of the nido‐carborane are essential to attain a highly twisted arrangement between the donor and acceptor moieties, leading to a very small energy splitting between the singlet and triplet excited states (ΔE_ST <0.05 eV for nido‐ m2 , ‐ m3 , and ‐ p3 ). These compounds exhibit efficient TADF with microsecond‐range lifetimes. In particular, nido‐ m2 and ‐ m3 display aggregation‐induced emission (AIE) with TADF properties.     

Carbon-coated LiFePO4–carbon nanotube electrodes for high-rate Li-ion battery

Journal of Solid State Electrochemistry - Olivine LiFePO4 (LFP) is a promising cathode material for high-rated lithium-ion batteries. However, olivine faced a severe disadvantage of low...