1,721,211 research outputs found
Reply to Catalan: Double-proton-transfer dynamics of photo-excited 7-azaindole dimers
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Lifetimes and Lifetime-Associated Spectra for Reversible Excited Two-State Reactions
No full textPhotoinduced excited-state processes have been platforms for understanding the molecular mechanisms of many chemical and biological reactions. To elucidate associated chemical kinetics, time-resolved spectroscopic experiments have been performed tracking how the populations of reactants and products change during the reactions while reaction conditions such as the concentration of a reactant, temperature, and solvent properties change. Here, we simulate the lifetimes of a reactant and a product, and construct their lifetime-associated spectra with the various combinations of rate constants based on the analytical solutions of differential rate equations. Depending on the combinations of the rate constants, the results diverge, which has often been overlooked in previous works. To demonstrate the validity of our approach, the results are compared with the experimental results on diffusion-controlled excited-state proton transfer. The presented global analysis simulation can generally be applied to other excited two-state reactions.
Proton diffusion dynamics along a diol as a proton-conducting wire in a photo-amphiprotic model system
No full textWe investigated the dynamics of excited-state proton transfer (ESPT) of photo-amphiprotic 7-hydroxyquinoline (7HQ) in the presence of a hydrogen (H)-bond bridging diol in a polar aprotic medium. The formation of 1 : 1 H-bonded complexes of 7HQ with various diols of different alkane chain lengths was revealed using steady-state electronic spectroscopy. With femtosecond-resolved fluorescence spectroscopy, cyclic H-bonded 1 : 1 complexes were found to undergo facile ESPT from the acidic enol to the basic imine group of 7HQ via the H-bond bridge. Through quantum chemical calculations, we found that the proton-transfer rate of the well-configured H-bonded complex correlated with the intramolecular H-bond length of a H-bond wiring diol molecule. Noncyclic, singly H-bonded 7HQ with a diol molecule was observed to undergo ESPT once another diol molecule diffuses to the noncyclic complex and accomplishes the formation of a reactive cyclic H-bonded 7HQ-(diol)2 complex, which was evidenced by the observation that the overall proton-transfer rate constant decreases when a longer-chain diol was used as the bridging wire part. The kinetic isotope effect on the proton relay was investigated to confirm that the nature of the activation barrier for the proton diffusion along the wire is isotope-sensitive proton tunnelling, while for the non-cyclic configuration, the isotope-insensitive H-bond bridge formation is a prerequisite for ESPT.clos
Water-wire catalysis in photoinduced acid-base reactions
No full textThe pronounced ability of water to form a hyperdense hydrogen (H)-bond network among itself is at the heart of its exceptional properties. Due to the unique H-bonding capability and amphoteric nature, water is not only a passive medium, but also behaves as an active participant in many chemical and biological reactions. Here, we reveal the catalytic role of a short water wire, composed of two (or three) water molecules, in model aqueous acid-base reactions synthesizing 7-hydroxyquinoline derivatives. Utilizing femtosecond-resolved fluorescence spectroscopy, we tracked the trajectories of excited-state proton transfer and discovered that proton hopping along the water wire accomplishes the reaction more efficiently compared to the transfer occurring with bulk water clusters. Our finding suggests that the directionality of the proton movements along the charge-gradient H-bond network may be a key element for long-distance proton translocation in biological systems, as the H-bond networks wiring acidic and basic sites distal to each other can provide a shortcut for a proton in searching a global minimum on a complex energy landscape to its destination.
Chirped imaging pulses in four-dimensional electron microscopy: femtosecond pulsed hole burning
Full text linkThe energy and time correlation, i.e. the chirp, of imaging electron pulses in dispersive propagation is measured by time-slicing (temporal hole burning) using photon-induced near-field electron microscopy. The chirp coefficient and the degree of correlation are obtained in addition to the duration of the electron pulse and its energy spread. Improving temporal and energy resolutions by time-slicing and energy-selection is discussed here and we explore their utility in imaging with time and energy resolutions below those of the generated ultrashort electron pulse. Potential applications for these imaging capabilities are discussed.open7
4D imaging and diffraction dynamics of single-particle phase transition in heterogeneous ensembles
Full text linkIn this Letter, we introduce conical-scanning dark-field imaging in four-dimensional (4D) ultrafast electron microscopy to visualize single-particle dynamics of a polycrystalline ensemble undergoing phase transitions. Specifically, the ultrafast metal-insulator phase transition of vanadium dioxide is induced using laser excitation and followed by taking electron-pulsed, time-resolved images and diffraction patterns. The single-particle selectivity is achieved by identifying the origin of all constituent Bragg spots on Debye-Scherrer rings from the ensemble. Orientation mapping and dynamic scattering simulation of the electron diffraction patterns in the monoclinic and tetragonal phase during the transition confirm the observed behavior of Bragg spots change with time. We found that the threshold temperature for phase recovery increases with increasing particle sizes and we quantified the observation through a theoretical model developed for single-particle phase transitions. The reported methodology of conical scanning, orientation mapping in 4D imaging promises to be powerful for heterogeneous ensemble, as it enables imaging and diffraction at a given time with a full archive of structural information for each particle, for example, size, morphology, and orientation while minimizing radiation damage to the specimen.close1
Dynamics of Chemical Bonding Mapped by Energy-Resolved 4D Electron Microscopy
No full textChemical bonding dynamics are fundamental to the understanding of properties and behavior of materials and molecules. Here, we demonstrate the potential of time-resolved, femtosecond electron energy loss spectroscopy (EELS) for mapping electronic structural changes in the course of nuclear motions. For graphite, it is found that changes of milli - electron volts in the energy range of up to 50 electron volts reveal the compression and expansion of layers on the subpicometer scale (for surface and bulk atoms). These nonequilibrium structural features are correlated with the direction of change from sp 2 [two-dimensional (2D) graphene] to sp3 (3D-diamond) electronic hybridization, and the results are compared with theoretical charge-density calculations. The reported femtosecond time resolution of four-dimensional (4D) electron microscopy represents an advance of 10 orders of magnitude over that of conventional EELS methods.close676
Analysis of Risk Factor Dependency Applying Data Mining Techniques to Traffic Accident Data
No full textSurface versus Bulk: Charge Carriers Play by Different Rules
No full textIn this issue of Chem, El-Zohry et al. employ a novel time-resolved electron microscopy technique to report that photoinduced charge-carrier diffusion on a semiconducting cadmium telluride single-crystal surface exhibits unusual features that are not only distinct from those of the bulk crystal but also considerably dependent on the crystal facet orientation. ?? 2019 Elsevier Inc.In this issue of Chem, El-Zohry et al. employ a novel time-resolved electron microscopy technique to report that photoinduced charge-carrier diffusion on a semiconducting cadmium telluride single-crystal surface exhibits unusual features that are not only distinct from those of the bulk crystal but also considerably dependent on the crystal facet orientation
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