1,721,309 research outputs found
Cathodoluminescence in Ultrafast Electron Microscopy
No full textImplementing the modern technologies of light-emitting devices, light harvesting, and quantum information processing requires the understanding of the structure-function relations at spatial scales below the optical diffraction limit and time scales of energy and information flows. Here, we distinctively combine cathodoluminescence (CL) with ultrafast electron microscopy (UEM), termed CL-UEM, because CL and UEM synergetically afford the required spectral and spatiotemporal sensitivities, respectively. For color centers in nanodiamonds, we demonstrate the measurement of CL lifetime with a local sensitivity of 50 nm and a time resolution of 100 ps. It is revealed that the emitting states of the color centers can be populated through charge transfer among the color centers across diamond lattices upon high-energy electron beam excitation. The technical advance achieved in this study will facilitate the specific control over energy conversion at nanoscales, relevant to quantum dots and single-photon sources
Reply to Catalan: Double-proton-transfer dynamics of photo-excited 7-azaindole dimers
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Ultrafast Excited-State Proton Transfer of a Cationic Superphotoacid in a Nanoscopic Water Pool
No full textThe excited-state proton transfer (ESPT) of a cationic superphotoacid, N-methyl-7-hydroxyquinolium, was studied within the water pool of an anionic aerosol-OT (AOT), bis(2-ethylhexyl) sulfosuccinate, reverse micelle (RM). Previously, we had found that the cationic photoacid residing at the anionic AOT interface was conducive to ESPT to the bound water having concentric heterogeneity on the time scale of hundreds of picoseconds to nanoseconds. In our present study, on the time scale of hundreds of femtoseconds to a few tens of picoseconds, the photoacid underwent an ultrafast ESPT influenced by mobile water constituting the core of the RM. The two subpopulations of the core water molecules that determine the ultrafast biphasic deprotonation of the photoacid on time scales differing by an order of magnitude were identified. The core water molecules solvating the counteranion of the photoacid showed a higher basicity than typical water clusters in bulk resulting in ESPT on a subpicosecond time scale. Bare water clusters sensed by the photoacid showed a slower ESPT, over several picoseconds, as typically limited by the rotational motion of water molecules for similar types of the photoacid
4D Electron Tomography
No full textTemporal Tomography
Tomography is a widely used technique for visualizing three-dimensional objects by algorithmic reconstruction from multiple two-dimensional images from distinct vantage points. However, its application has largely been restricted to static imaging.
Kwon and Zewail
(p.
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) have now adapted an ultrafast electron microscope to perform tomography with subpicosecond resolution. The method relies on systematically varying the tilt angle of the sample with respect to the incoming electron beam, and enabled assembly of a detailed frame-by-frame record of the response of a curled carbon nanotube to sudden heating.
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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. © the Owner Societies 20161111sciescopu
Macromolecular structural dynamics visualized by pulsed dose control in 4D electron microscopy
No full textMacromolecular conformation dynamics, which span a wide range of time scales, are fundamental to the understanding of properties and functions of their structures. Here, we report direct imaging of structural dynamics of helical macromolecules over the time scales of conformational dynamics (ns to subsecond) by means of four-dimensional (4D) electron microscopy in the single-pulse and stroboscopic modes. With temporally controlled electron dosage, both diffraction and real-space images are obtained without irreversible radiation damage. In this way, the order-disorder transition is revealed for the organic chain polymer. Through a series of equilibrium-temperature and temperature-jump dependencies, it is shown that the metastable structures and entropy of conformations can be mapped in the nonequilibrium region of a "funnellike" free-energy landscape. The T-jump is introduced through a substrate (a "hot plate" type arrangement) because only the substrate is made to absorb the pulsed energy. These results illustrate the promise of ultrafast 4D imaging for other applications in the study of polymer physics as well as in the visualization of biological phenomena.close111
Hydrogen-Bond Dynamics and Energetics of Biological Water
Full text linkWater molecules in the immediate vicinity of biomacromolecules and biomimetic organized assemblies often exhibit a markedly distinct behavior from that of their bulk counterparts. The overall sluggish behavior of biological water substantially affects the stability and integrity of biomolecules, as well as the successful execution of various crucial water???mediated biochemical phenomena. In this minireview, we provide insights into the features of truncated hydrogen???bond networks that grant biological water its unique characteristics. In particular, we present the experimental and theoretical efforts, based on chemical kinetics, that have shed light on the dynamics and energetics governing such characteristics. We emphasize how such details help frame our understanding of the energetics of biological water, an aspect relatively less explored than its dynamics. For instance, when biological water at hydrophilic or charged protein surfaces was explored, the free energy of H???bond breakage was found to be of the order of 0.4 kcal??mol ???1 higher than that of bulk water
Carbon Dots: Bottom-Up Syntheses, Properties, and Light-Harvesting Applications
No full textThe development of cost-effective and environmentally friendly photocatalysts and photosensitizers has received tremendous attention because of their potential utilization in solar-light-harvesting applications. In this respect, carbon dots (CDs) prepared by bottom-up methods have been considered to be promising light-harvesting materials. Through their preparation from various molecular precursors and synthetic methods, CDs exhibit excellent optical and charge-transfer properties. Furthermore, their photophysical properties can be readily optimized and enhanced by means of doping, functionalization, and post-synthetic treatment. In this review, we summarize the recent progress in CDs synthesized using bottom-up approaches. These CDs exhibit strong light absorption and unique electron donor/acceptor capabilities for light-harvesting applications. We anticipate that this review will provide new insights into novel types of photosensitizers and photocatalysts for a wide range of applications
(C) 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim11sciescopu
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