Ultrafast electron diffraction and direct observation of transient structures in a chemical reaction

Ultrafast electron diffraction is a unique method for the studies of structural changes of complex molecular systems. In this contribution, we report direct ultrafast electron diffraction study of the evolution of short-lived intermediates in the course of a chemical change. Specifically, we observe the transient intermediate in the elimination reaction of 1,2-diiodotetrafluoroethane (C2F4I2) to produce the corresponding ethylene derivative by the breakage of two carbon-iodine, C---I, bonds. The evolution of the ground-state intermediate (C2F4I radical) is directly revealed in the population change of a single chemical bond, namely the second C---I bond. The elimination of two iodine atoms was shown to be nonconcerted, with reaction time of the second C---I bond breakage being 17 +/- 2 ps. The structure of the short-lived C2F4I radical is more favorable to the classical radical structure than to the bridged radical structure. This leap in our ability to record structural changes on the ps and shorter time scales bodes well for many future applications in complex molecular systems.     

Structural Dynamics of C2F4I2 in Cyclohexane Studied via Time-Resolved X-ray Liquidography

The halogen elimination of 1,2-diiodoethane (C₂H₄I₂) and 1,2-diiodotetrafluoroethane (C₂F₄I₂) serves as a model reaction for investigating the influence of fluorination on reaction dynamics and solute–solvent interactions in solution-phase reactions. While the kinetics and reaction pathways of the halogen elimination reaction of C₂H₄I₂ were reported to vary substantially depending on the solvent, the solvent effects on the photodissociation of C₂F₄I₂ remain to be explored, as its reaction dynamics have only been studied in methanol. Here, to investigate the solvent dependence, we conducted a time-resolved X-ray liquidography (TRXL) experiment on C₂F₄I₂ in cyclohexane. The data revealed that (ⅰ) the solvent dependence of the photoreaction of C₂F₄I₂ is not as strong as that observed for C₂H₄I₂, and (ⅱ) the nongeminate recombination leading to the formation of I₂ is slower in cyclohexane than in methanol. We also show that the molecular structures of the relevant species determined from the structural analysis of TRXL data provide an excellent benchmark for DFT calculations, especially for investigating the relevance of exchange-correlation functionals used for the structural optimization of haloalkanes. This study demonstrates that TRXL is a powerful technique to study solvent dependence in the solution phase.     

Bionanosphere Lithography via Hierarchical Peptide Self‐Assembly of Aromatic Triphenylalanine

A nanolithographic approach based on hierarchical peptide self‐assembly is presented. An aromatic peptide of N‐(t‐Boc)‐terminated triphenylalanine is designed from a structural motif for the β‐amyloid associated with Alzheimer's disease. This peptide adopts a turnlike conformation with three phenyl rings oriented outward, which mediate intermolecular π–π stacking interactions and eventually facilitate highly crystalline bionanosphere assembly with both thermal and chemical stability. The self‐assembled bionanospheres spontaneously pack into a hexagonal monolayer at the evaporating solvent edge, constituting evaporation‐induced hierarchical self‐assembly. Metal nanoparticle arrays or embossed Si nanoposts could be successfully created from the hexagonal bionanosphere array masks in conjunction with a conventional metal‐evaporation or etching process. Our approach represents a bionanofabrication concept that biomolecular self‐assembly is hierarchically directed to establish a straightforward nanolithography compatible with conventional device‐fabrication processes.     

Cover Picture: Liquid Crystalline Peptide Nanowires (Adv. Mater. 22/2007)

Liquid‐crystalline peptide nanowires consisting of aromatic dipeptides are easily prepared by a sonication in a volatile organic solvent, report Hyotcherl Ihee, Sang Ouk Kim, and co‐workers on p. 3924. The colloidal nematic liquid crystalline phase of the rigid nanowires allows for a macroscopically ordered morphology of the nanowires under an external electric field. The cover image presents a broad field view of nanowires oriented by an electric field between two electrodes.     

In situ TEM observation of heterogeneous phase transition of a constrained single-crystalline Ag2Te nanowire

Laterally epitaxial single crystalline Ag2Te nanowires (NWs) are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these Ag2Te NWs via in situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (bcc) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of bcc phase penetrating into the face-centered cubic matrix at 200 °C by high-resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices.