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PubMed Journal Database | The journal of physical chemistry letters RSS

09:16 EDT 27th May 2019 | BioPortfolio

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Showing PubMed Articles 1–25 of 1,400+ from The journal of physical chemistry letters

A Heterogeneous Kinetics Model for Triplet Exciton Transfer in Solid-State Upconversion.

High internal quantum efficiency semiconductor nanocrystal (NC)-based photon upconversion devices are currently based on a single monolayer of active NCs. Devices are therefore limited in their external quantum efficiency based on the low number of photons absorbed. Increasing the number of photons absorbed is expected to increase the upconversion efficiency, yet experimentally increasing the number of layers does not appreciably increase the upconverted light output. We unravel this mystery by combining ki...

Pressure-Induced Tunable Electron Transfer and Auger Recombination Rates in CdSe/ZnS Quantum Dot-Anthraquinone Complexes.

Electron transfer (ET) and Auger recombination (AR) processes in quantum dots (QDs) are key mechanisms for the advance of QD-based devices. However, it still remains a challenge to promote ET and suppress AR simultaneously. Here, we use in situ high-pressure ultrafast transient absorption spectroscopy to explore the impact of pressure on the ET between CdSe/ZnS and anthraquinone (AQ) and AR dissolved in cyclohexane. Remarkably, under compression, ET lifetimes are shorten, while suppression of AR lifetimes i...

Single Electron Trapped Oxygen Vacancy on Ultrathin WO3·0.33H2O {100} Facets Suppressing Backward Reaction for Promoted H2 Evolution in Pure Water Splitting.

Solar water splitting to produce hydrogen is a promising solution for global energy issues. One of the main bottlenecks in this technology is the spontaneous fast backward reaction (2H2 + O2 → H2O, ΔG < 0), limiting the solar energy conversion efficiency. How to suppress backward reaction is vitally important but rarely reported. Here we found that single electron trapped oxygen vacancy (Vo•) can suppress spontaneous backward reaction in pure water splitting. Taking WO3·0.33H2O catalyst as an example,...

The Equal Footing of Thermal Expansion and Electron-Phonon Interaction in the Temperature Dependence of Lead Halide Perovskite Band Gaps.

Lead halide perovskites, which are causing a paradigm shift in photovoltaics, exhibit an atypical temperature dependence of the fundamental gap: it decreases in energy with decreasing temperature. Reports ascribe such a behavior to a strong electron-phonon renormalization of the gap, neglecting contributions from thermal expansion. However, high pressure experiments performed on the archetypal perovskite MAPbI (MA stands for methylammonium) yield a negative pressure coefficient for the gap of the tetragonal...

The Base Pair Contents and Sequences of DNA Double Helixes Differentiated by Surface-Enhanced Raman Spectroscopy.

Direct, label-free sequence analysis of DNA hybridization has been achieved by surface-enhanced Raman spectroscopy (SERS). In this work, aluminum-ions-aggregated and iodide-modified silver nanoparticles (Ag IANPs) were used as substrates for obtaining Raman spectra of the DNA strands with the same base composition but different sequences, which form random coils or various hairpin conformations. Upon DNA hybridization, reproducibly enhanced bands were easily observed, corresponding well to the formation of ...

The Role of Excited State Proton Relays on the Photochemical Dynamics of Water Nanodroplets.

In this work, we applied non-adiabatic excited state molecular dynamics in tandem with ab initio electronic structure theory to illustrate a complete mechanistic landscape underpinning the UV absorption initiated photochemical dynamics in water nanodroplets. The goal is to understand the nonequilibrium excited state molecular dynamics initiated by the relaxation of solvated photoelectron, and consequential photochemical processes. The lowest-lying excited state shows the proton dissociation for a single wat...

Pyrene, a Test Case for DeepUV Molecular Photophysics.

We determined the complete relaxation dynamics of pyrene in ethanol from the second bright state, employing experimental and theoretical broadband heterodyne detected transient grating and two-dimensional photon echo (2DPE) spectroscopy, using pulses with duration of 6 fs and covering a spectral range spanning from 250 to 300 nm. Multiple lifetimes are assigned to conical intersections through a cascade of electronic states, eventually leading to a rapid population of the lowest long living excited state an...

Ground-state Structural Disorder and Excited-state Symmetry Breaking in a Quadrupolar Molecule.

The influence of torsional disorder around the ethynyl π-bridges of a linear D-π-A-π-D molecule on the nature of its S excited state was investigated using ultrafast time-resolved infrared spectroscopy. By tuning the pump wavelength throughout the S←S absorption band, subpopulations with different extents of asymmetry could be excited. In non-polar solvents, the equilibrated S state is symmetric and quadrupolar independently of the initial degree of distortion. Photoexcitation of distorted molecules is...

A Double Resonance CEST Experiment to Study Multistate Protein Conformational Exchange: An Application to Protein Folding.

Despite the importance of protein dynamics to function, studying exchange between multiple conformational states remains a challenge because sparsely populated states are invisible to conventional techniques. CEST NMR experiments can detect minor states with lifetimes between 5 and 200 milliseconds populated to just 1%. However CEST often cannot provide the exchange mechanism for processes involving three or more states leaving the role of the detected minor states unknown. Here a double resonance CEST expe...

Origin of Unexpectedly Simple Oscillatory Responses in the Excited-State Dynamics of Disordered Molecular Aggregates.

Unraveling the many facets of coherent and incoherent exciton motion in an ensemble of chromophores is an inherently complex quantum mechanical problem thττat has triggered a vivid debate on the role of quantum effects in molecular materials and biophysical systems. Here the dynamics of a statistical ensemble of molecular aggregates consisting of identical chromophores is investigated within a new theoretical framework. Taking account of intrinsic properties of the system, the Hamiltonian of the aggregate...

Chiral Water Superstructures Around Anti-parallel β-sheets Observed by Chiral Vibrational Sum Frequency Generation Spectroscopy.

Hydration modulates every aspect of protein structure and function. However, studying water structures in hydration shells remains challenging mostly due to overwhelming background from bulk water. We used vibrational sum frequency generation (SFG) spectroscopy to characterize hydrated films of an anti-parallel β-sheet peptide (LKβ) adsorbed on glass slides. The hydrated films give chiral SFG response from water only when the peptide self-assembles into anti-parallel β-sheets. Experiments of isotopic lab...

Multi-Photon Excitation of CsPbBr Perovskite Quantum Dots: How Many Electrons Can One PQD Donate to Multiple Molecular Acceptors?

Metastable multi-excitonic states (MES) of semiconductor quantum dots can be involved in multi-electron transfer reaction, which opens new perspectives in nanomaterials-based opto-electronic applications. Herein, we demonstrate the generation of MES in CsPbBr3 perovskite quantum dots (PQD) and its dissociation dynamics through multiple electrons transfer to molecular electron acceptors, anthraquinones (AQs), bound to the PQD surface by carboxylic anchor. As many as 14 excitons are produced at excitation den...

Exotic Hydrogen Bonding in Compressed Ammonia Hydrides.

Hydrogen-rich compounds attract significant fundamental and practical interest for their ability to accommodate diverse hydrogen bonding patterns and their promise as superior energy storage materials. Here we report on an intriguing discovery of exotic hydrogen bonding in compressed ammonia hydrides and identify two novel ionic phases in an unusual stoichiometry NH7. The first is a hexagonal R-3m phase containing NH3-H+-NH3, H- and H2 structural units stabilized above 25 GPa. The exotic NH3-H+-NH3 unit c...

Surface-Templated Nanobubbles Protect Proteins from Surface-Mediated Denaturation.

In this Letter, we report that surface-bound nanobubbles reduce protein denaturation on methylated glass by irreversible protein shell formation. Single-molecule total internal reflection fluorescence (SM-TIRF) microscopy was combined with intramolecular Förster resonance energy transfer (FRET) to study the conformational dynamics of nitroreductase (NfsB) on nanobubble-laden methylated glass surfaces, using reflection brightfield microscopy to register nanobubble locations with NfsB adsorption. First, NfsB...

The Base Side of Noble Metal Clusters: Efficient Route to Captamino-Gold, Au(-S(CH)N(CH)), n = 25 - 144.

Monolayer protected clusters (MPCs), typified by the (Au, Ag)-thiolates, share dimensions and masses with aqueous globu-lar proteins (enzymes), yet efficient bioanalytical methods have not proved applicable to MPC analytics. Here we demon-strate that direct facile ESI(+)MS analysis of MPCs succeeds, at the few-picomol level, for aqueous basic amino-terminated thiolates. Specifically, captamino-gold clusters, Au(SR), wherein -R = -(CH)N(CH), are prepared quantitatively via a direct 1-phase (aq/EtOH) method, ...

Cavity Hydration and Competitive Binding in Methylated β-Cyclodextrin.

) spectroscopy is used to measure the vibrational spectrum of water molecules in the cavity of an aqueous methylated β-cyclodextrin (Me-β-CD), as well as to quantify the competitive expulsion of those waters by benzene. The Me-β-CD cavity is found to contain 5-6 water molecules whose structure is remarkably similar to bulk water, although slightly more tetrahedral and with fewer weak hydrogen bonds. The binding constant of benzene to Me-β-CD, obtained using Raman-MCR, is found to be similar to that of b...

Temperature-Dependent Transient Absorption Spectroscopy Elucidates Trapped-Hole Dynamics in CdS and CdSe Nanorods.

Charge-carrier traps play a central role in the excited-state dynamics of semiconductor nanocrystals, but their influence is often difficult to measure directly. In CdS and CdSe nanorods of nonuniform width, spatially-separated electrons and trapped holes display recombination dynamics that follow a power-law function in time that is consistent with trapped-hole diffusion-limited recombination. However, power-law relaxation can originate from mechanisms other than diffusion. Here we report transient absorpt...

Accurate and Efficient ab Initio Calculations for Supramolecular Complexes: Symmetry-Adapted Perturbation Theory with Many-Body Dispersion.

Symmetry-adapted perturbation theory (SAPT) provides a chemically meaningful energy decomposition scheme for non-bonded interactions that is useful for interpretive purposes. Although formally a dimer theory, we have previously introduced an "extended" version (XSAPT) that incorporates many-body polarization via self-consistent charge embedding. Here, we extend the XSAPT methodology to include nonadditive dispersion, using a modified form of the many-body dispersion (MBD) method of Tkatchenko and co-workers...

Po-containing Molecules in Fusion and Fission Reactors.

Fission and fusion reactors can only play a role in the future energy landscape if they are inherently safe by design. For some reactor concepts, a major remaining issue is the undesired production of radiotoxic Po-210. To filter out the volatile Po species, information on their molecular composition is needed. An experimental characterization is very challenging due to the large required amount of radioactive Po. An alternative quantum chemistry approach was taken to predict the T-dependent stability of re...

Insights into Direct Methods for Predictions of Ionization Potential and Electron Affinity in Density Functional Theory.

Vertical ionization potential (IP) and electron affinity (EA) are fundamental molecular properties, while the Δ method and the direct method are the widely used approaches to compute these properties. The Δ method is calculated by taking the total energy difference of the initial and final states, whose reliability is seriously affected by the issue associated with the imbalanced treatment of these two states. The direct method based on the derivatives involving only one single state calculation can yield...

Do Protons in Trapped Water Molecules Hop or Tunnel?

Water molecules trapped in crystals of barium chlorate monohydrate have been investigated by magic angle spinning proton NMR spectroscopy in the temperature range 110-300 K. At high temperatures, a single spinning sideband pattern is observed. Below 150 K however, two interleaved spinning sideband manifolds appear, with distinct centerbands that do not coincide with the average isotropic chemical shift seen at high temperatures. This hitherto unknown "cross-term splitting" results from the interplay of the ...

Broadband Ultraviolet CD-Spectroscopy of Ultrafast Peptide Backbone Conformational Dynamics.

The far UV spectral window widely used for the conformational analysis of biomolecules is not easily covered with broadband lasers. This has made it difficult to use CD spectroscopy to directly follow fast structure changes. By combining transient CD spectroscopy in the deep UV with thioamide substitution, we demonstrate a method to overcome this difficulty. We investigated a dipeptide whose two carbonyl oxygen atoms were replaced by sulfur, red-shifting the strong lowest-lying ππ* transitions into the mo...

Spectral- and Structural Variations of Biomimetic Light-Harvesting Nanotubes.

Bio-inspired, self-assembled, nanotubes have been investigated by low-temperature, polarisation-resolved single-tube spectroscopy. These assemblies are based on zinc chlorin monomers, and are considered as model systems that resemble the secondary structural elements in the natural light-harvesting systems of green-(non) sulfur bacteria. Compared to the natural systems, the spectral parameters extracted from the single-nanotube spectra feature distributions with significantly smaller widths, which is ascrib...

Enhancing Spin-Orbit Coupling by Introducing Lone Pair Electron with p-Orbital Character in Thermally Activated Delayed Fluorescence Emitter: Photophysics and Devices.

Reverse intersystem crossing (RISC) is the rate-limited step for the radiative process of thermally activated delayed fluorescence (TADF) materials, which is important to achieve ideal photoluminescence and electroluminescence efficiency. Herein, we propose a new strategy of introducing lone pair (n) electron with p orbital character to enhance spin-orbit coupling (SOC) for promoting RISC process. A proof-of-concept TADF molecule with p orbital lone pair, namely MoCz-PCN, was developed and three counterpart...

Stability of Cubic FAPbI from X-ray Diffraction, Anelastic and Dielectric Measurements.

Among the hybrid metal-organic perovskites for photovoltaic applications FAPbI (FAPI) has the best performance regarding efficiency and the worst regarding stability, even though the reports on its stability are highly contradictory. In particular, since at room temperature the cubic α phase, black and with high photovoltaic efficiency, is metastable against the yellow hexagonal δ phase, it is believed that α-FAPI spontaneously transform into δ-FAPI within a relatively short time. We performed X-ray dif...


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