Direct observation of a doubly excited state of pleiadene

Evidence is presented that the second excited state of pleiadene (I) is of doubly excited nature. Such states are of interest in discussions of excited state and ground state reactivities and for further development of calculational methods.     

Direct observation for the cyclization of a diarylcarbonyl oxide

Trapping and laser flash spectroscopic experiments showed that the cyclization of diphenylcarbonyl oxide is turned into a very facile process by introducing p-methoxyl substituent, the lifetime of which is as short as 10⁻⁸ s. The trapping with diphenylsulfide and -sulfoxide suggested the intervention of a dioxirane intermediate.     

Direct observation of single-molecule generation at a solid-liquid interface

Direct observation of single-molecule generation from a chemical reaction was achieved at a solid-liquid interface. The reaction between fluorescamine and immobilized N'-(3-trimethoxysilylpropyl)diethylenetriamine (DETA) was studied at the single-molecule level. Time-lapse fluorescence images of single-molecule products, excited by the evanescent field generated at a quartz-liquid interface, were recorded to follow the chemical reaction to its completion. The reactions were restricted to the approximately 1 nm thick layer nearest to the interface. Analysis of the photoelectron intensity of the fluorescent product of the reaction and its distribution shows that the reaction kinetics goes through a transition from zeroth-order to first-order as the reaction proceeds. This approach offered a novel means to study single-molecule reactions at the solid-liquid interface. It also enabled the investigation of reaction kinetics and chemical mapping of surface heterogeneity at the single-molecule level.     

Nickel-catalyzed polymerization of a substituted sulfoxonium ylide

The homopolymerization of (dimethylamino)phenylsulfoxonium ethylide, a substituted sulfoxonium ylide, is reported. Treatment of the monomer with a readily available Ni(II) catalyst afforded poly[(1-butene)-ran-(2-butene)-ran-(ethylene)] in good yield and high molecular weight. Varying the initial monomer-to-catalyst feed ratio enabled control over the molecular weights of the polymers produced. The polymerization mechanism appears to proceed in a chain growth fashion that entails the addition of ethylide units to growing polymer chains in conjunction with the expulsion of (dimethylamino)phenyl sulfoxide as a byproduct.     

Direct observation of ion evaporation from a triply charged nanodroplet

Triply charged, highly solvated metal ions of the form [Ln(H(2)O)(n)](3+) can be generated using a commercial mass spectrometer, and CID studies on these highly charged metal-solvent clusters allow for the direct observation of a process best described as ion evaporation.     

Direct observation of peptide hydrogel self-assembly

The characterization of self-assembling molecules presents significant experimental challenges, especially when associated with phase separation or precipitation. Transparent window infrared (IR) spectroscopy leverages site-specific probes that absorb in the “transparent window” region of the biomolecular IR spectrum. Carbon–deuterium (C–D) bonds are especially compelling transparent window probes since they are non-perturbative, can be readily introduced site selectively into peptides and proteins, and their stretch frequencies are sensitive to changes in the local molecular environment. Importantly, IR spectroscopy can be applied to a wide range of molecular samples regardless of solubility or physical state, making it an ideal technique for addressing the solubility challenges presented by self-assembling molecules. Here, we present the first continuous observation of transparent window probes following stopped-flow initiation. To demonstrate utility in a self-assembling system, we selected the MAX1 peptide hydrogel, a biocompatible material that has significant promise for use in drug delivery and medical applications. C–D labeled valine was synthetically introduced into five distinct positions of the twenty-residue MAX1 β-hairpin peptide. Consistent with current structural models, steady-state IR absorption frequencies and linewidths of C–D bonds at all labeled positions indicate that these side chains occupy a hydrophobic region of the hydrogel and that the motion of side chains located in the middle of the hairpin is more restricted than those located on the hairpin ends. Following a rapid change in ionic strength to initiate self-assembly, the peptide absorption spectra were monitored as function of time, allowing determination of site-specific time constants. We find that within the experimental resolution, MAX1 self-assembly occurs as a cooperative process. These studies suggest that stopped-flow transparent window FTIR can be extended to other time-resolved applications, such as protein folding and enzyme kinetics.

To facilitate the characterization of phase-transitioning molecules, site-specific non-perturbative infrared probes are leveraged for continuous observation of the self-assembly of fibrils in a peptide hydrogel following stopped-flow initiation.     

Direct observation of the main-chain conformation of a combined liquid-crystal polymer

The conformation of the main-chain (backbone) of a combined main-chain/side-chain liquid crystalline polymer has been qualitatively determined by small angle neutron scattering in the oriented nematic, the smectic A and the smectic C phases. The polymer backbone presents only a weak anisotropy, of prolate shape, in the nematic and the smectic C phases. A stronger reorientation of the backbones in the direction of the applied magnetic field is measured for the S_A phase. However, this anisotropy remains small compared to the stretching of a main-chain liquid crystal polymer and the smectic structure results apparently from side-chain ordering. On the other hand, hydrodynamic measurements show that the combined polymer, in solvent, is as flexible as a polystyrene chain. This result is compatible with an explanation for the weak observed anisotropy.     

Direct UHV-TEM observation of palladium clusters on a silicon surface.

About 1 monolayer of palladium was deposited onto a silicon (111) 7 x 7 surface at a temperature of about 550 K inside an ultrahigh vacuum transmission electron microscope, resulting in formation of Pd2Si nanoislands and a 1 x 1 surface layer. Pd clusters created from an excess of Pd atoms on the 1 x 1 surface layer were directly observed by in situ plan view high-resolution transmission electron microscopy. When an objective aperture was introduced so that electron diffractions less than 0.20 nm were filtered out, the lattice structure of the 1 x 1 surface with 0.33 nm spacing and the Pd clusters with a trimer shape were visualized. It was found that image contrast of the 1 x 1 lattice on the specific height terraces disappeared, and thereby an atomic structure of the Pd clusters was clearly observed. The appearance and disappearance of the 1 x 1 lattice was explained by the effect of the kinematical diffraction. It was identified that a Pd cluster was composed of three Pd atoms without a centered Si atom, which is consistent with the model proposed previously. The feature of the Pd clusters stuck at the surface step was also described.     

Direct observation of a 14-electron ruthenacyclobutane relevant to olefin metathesis

The 14-electron ruthenium phosphonium alkylidene complex [(IH2Mes)Cl2Ru=CH(PCy3)][B(C6F5)4], 1b, a highly active olefin metathesis catalyst, reacts with stoichiometric quantities of ethylene at -50 degrees C in CD2Cl2 to generate the ruthenacyclobutane complex [(IH2Mes)Cl2RuCH2CH2CH2], 2, and [CH2=CH(PCy3)][B(C6F5)4] in quantitative yield by NMR spectroscopy. 1H and 13C NMR spectroscopies on 2 and 2-13C3 are consistent with a symmetrical C2v structure, providing the first experimental information concerning this crucial intermediate in ruthenium-mediated olefin metathesis. At -50 degrees C, exchange with free ethylene takes place on the chemical time scale. Complex 2 decomposes in solution upon warming to room temperature, generating propene and unknown ruthenium product(s).     

Direct observation of a diblock copolymer-induced microemulsion at a polymer/polymer interface

We have studied the segregation of a block copolymer of poly(d₈-styrene-b-2-vinylpyridine) (dPS-PVP) at the interface between polystyrene and a random copolymer of poly(styreneran-4-hydroxystyrene) (PS-r-PPHS). Forward recoil spectrometry (FRES) was used to measure the equilibrium excess (z^*) of the dPS-PVP chains at the interface as a function of its volume fraction in the bulk PS phase (?_∞). It was found that there is a sharp increase in z^* at a critical value of ?_∞. This upturn indicates the formation of a microemulsion of PS and the random copolymer PS-r-PPHS due to a vanishing of the interfacial tension caused by the strong adsorption of the block copolymer. Cross-sectional transmission electron microscopy (TEM) of the interface shows that this microemulsion starts to form at the interface by forming a deeply corrugated structure where the “wavelength” of the corrugations is of the order of 50 nm. © 1995 John Wiley & Sons, Inc.     

The synthesis and structure of a stabilized nitrilium ylide

The X-ray structure of the first stable nitrilium ylide is reported showing the central ylidic system to be essentially linear with only slight bends at the central carbon and nitrogen (4°); however MINDO/3 calculations on model compounds suggest the ylide functionality is quite flexible with low deformation energies.     

Direct observation of nanoparticle embedding into the surface of a polymer melt

Direct embedding of metal nanoparticles (NPs) into the surface of a polymer melt is observed by TEM and a new embedding mechanism proposed. Upon annealing above the glass transition temperature of polystyrene (PS), NPs (20 nm gold) are rapidly covered by a thin PS wetting layer, h* approximately 1.3-1.8 nm (i.e., about two or three monomers). Because it creates capillary pressure on a NP, this "universal" wetting layer is proposed to be responsible for NP embedding. The value of h* is independent of the molecular weight of PS and constant during the embedding process. The value of h* is found to be similar to the equilibrium wetting layer thickness of a polymer melt spreading on a metal substrate. Using a model that includes the spreading coefficient, long-range van der Waals interactions, and a chain-stretching penalty, h* is shown to be independent of the molecular weight of the polymer. Using this model and the measured value of h*, the interfacial energy between Au NP and PS is estimated to be 8.7 J/m2.     

Direct observation of intramolecular coplanarity regulated polymorph emission of a tetraphenylethene derivative

Polymorphism makes it possible to clarify the relationship between emission property and crystal structure. However, based on the exact molecular conformation in tetraphenylethene polymorphisms, it is still challenging to evaluate the difference of intramolecular coplanarity without the support of calculation because of the complex combination of four different torsion angles between four peripheral benzenes and the central ethylene plane. Here, by using a di-formyl-functionalized tetraphenylethene derivative, two ideal polymorphisms with a consistent trend of the corresponding torsion angles have been obtained. For the first time, we explicitly demonstrated that intramolecular coplanarity is the underlying cause of the polymorphism-dependent emission of tetraphenylethene derivatives.