• Energy Research

The Mn 1s near-edge absorption fine structure (NEXAFS) has been computed by means of transition-state gradient-corrected density functional theory (DFT) on four Mn(4)Ca clusters modeling the successive S(0) to S(3) steps of the oxygen-evolving complex (OEC) in photosystem II (PSII). The model clusters were obtained from a previous theoretical study where they were determined by energy minimization. They are composed of Mn(III) and Mn(IV) atoms, progressing from Mn(III)(3)Mn(IV) for S(0) to Mn(III)(2)Mn(IV)(2) for S(1) to Mn(III)Mn(IV)(3) for S(2) to Mn(IV)(4) for S(3), implying an Mn-centered oxidation during each step of the photosynthetic oxygen evolution. The DFT simulations of the Mn 1s absorption edge reproduce the experimentally measured curves quite well. By the half-height method, the theoretical IPEs are shifted by 0.93 eV for the S(0) → S(1) transition, by 1.43 eV for the S(1) → S(2) transition, and by 0.63 eV for the S(2) → S(3) transition. The inflection point energy (IPE) shifts depend strongly on the method used to determine them, and the most interesting result is that the present clusters reproduce the shift in the S(2) → S(3) transition obtained by both the half-height and second-derivative methods, thus giving strong support to the previously suggested structures and assignments.

The structural and dynamical properties of an individual molecule is much affected by changes in a surrounding solvent or protein environment. The focus of the present study is to investigate such changes between proteins and solvents, using as an illustrating example the structure and dynamics of 1,2-dichloroethane (DCE) within haloalkane dehalogenase (HAD) as protein and within water as solvent. We have studied DCE within HAD using Car-Parrinello molecular dynamics calculations in a quantum mechanics/molecular mechanics set-up. We find that the C-Cl bond length is shorter in HAD when compared to solution phase value, whereas the net atomic charges and dipole moment are significantly larger than the solution phase values. In contradiction to the usual trend that molecules in the vicinity of the proteins are less polar, we report the observation that the protein environment indeed polarizes the DCE solute more than the water solvent. Furthermore, within the protein environment we do not observe any conformational transition between gauche and trans conformers, and the DCE remains in the more polar gauche conformer during the entire simulation time scale. However, the trans conformer interconverts to the gauche conformer spontaneously within 0.4 ps, which clearly indicates that the trans conformer is unstable within the HAD protein. In contrast, the scatter diagram of total Kohn-Sham energy and dihedral angle between Cl-C-C-Cl atoms shows that the gauche and trans conformers have comparable energies in water. Overall, the present calculations show the within the protein not only the structure of DCE is altered but also that the conformational interconversion dynamics is affected very much. To investigate the confinement effect on the conformational equilibrium, we have also carried out force-field molecular dynamics calculations which show that the population of trans conformer is significantly lower within the protein when compared to that in water solvent.

Lanthanide-doped upconversion nanoparticles (UCNPs) have rich photophysics exhibiting complex luminescence kinetics. In this work, we thoroughly investigated the luminescence response of UCNPs to excitation pulse durations. Analyzing this response opens new opportunities in optical encoding/decoding and the assignment of transitions to emission peaks and provides advantages in applications of UCNPs, e.g., for better optical sectioning and improved luminescence nanothermometry. Our work shows that monitoring the UCNP luminescence response to excitation pulse durations (while keeping the duty cycle constant) by recording the average luminescence intensity using a low-time resolution detector such as a spectrometer offers a powerful approach for significantly extending the utility of UCNPs.

Cosensitizers were designed for simultaneous filling up of both absorption valleys of porphyrins, achieving a high cosensitized DSSC efficiency of 10.75%.

By combining results and analysis from cylindrical microjet photoelectron spectroscopy (cMJ-PES) and theoretical simulations, we unravel the microscopic properties of ethanol-water solutions with respect to structure and intermolecular bonding patterns following the full concentration scale from 0 to 100% ethanol content. In particular, we highlight the salient differences between bulk and surface. Like for the pure water and alcohol constituents, alcohol-water mixtures have attracted much interest in applications of X-ray spectroscopies owing to their potential of combining electronic and geometric structure probing. The water mixtures of the two simplest alcohols, methanol and ethanol, have generated particular attention due to their delicate hydrogen bonding networks that underlie their structural and thermodynamic properties. Macroscopically ethanol-water seems to mix very well, however microscopically this is not true. The aberrant thermodynamics of water-alcohol mixtures have been suggested to be caused by energy differences of hydrogen bonding between water-water, alcohol-alcohol and alcohol-water molecules. These networks may perturb the local character of the interaction between X-rays and matter, calling for analysis that go beyond the normally applied local selection and building block rules and that can combine the effects of light-matter, intra- and intermolecular interactions. However, despite decades of ongoing research there are still controversies of the precise nature of hydrogen bonding networks that underlie the mixing of these simple molecules. Our combined analysis indicates that at low concentration ethanol molecules form a film at the surface since ethanol at the surface can expose its hydrophobic part to the vacuum retaining its two (or three) possible hydrogen bonds, while water at the surface cannot retain all its four possible hydrogen bonds. Thus, ethanol at the surface becomes energetically favorable. Ethanol molecules show a tilting angle variation of the C-C axis with respect to the surface normal as large as 60° at very low concentration. In bulk, around ca. ten %, the ethanol oxygen atoms tend to make a third acceptor hydrogen bond to water molecules. At ca. 20 %, there is a U-shaped change in the CH3 to CH2OH binding energy (BE) shift indicating the presence of ring-like agglomerates called clathrate structures. At the surface, between 5 and 25%, ethanol forms a closely packed layer with the smallest C-C tilting angle variation down to ∼20°. Above 25% and below the azeotrope at the surface, ethanol shows an increase in the tilting angle variation, while at very high ethanol concentrations water tends to move to the surface so giving a microscopic explanation of the azeotrope effect. This migration is connected to the presence of longer (shorter) ethanol chains in the bulk (surface). A brief comparison with discussions and predictions from other spectroscopic techniques is also given. We emphasize the execution of an integrated approach that combines molecular structural dynamics with quantum predictions of the core electronic chemical shift, so establishing a protocol with considerable interpretative as well as predictive power for cMJ-PES measurements. We believe that this protocol can valorize cMJ-PES for studies of properties of other alcohol mixtures as well as of binary solutions in general.

The luminescence efficiency of lanthanide-doped upconversion nanoparticles is of particular importance for their embodiment in biophotonic and photonic applications. Here, we show that the upconversion luminescence of typically used NaYF4:Yb3+30%/Tm3+0.5% nanoparticles can be enhanced by ~240 times through a hierarchical active core/active shell/inert shell (NaYF4:Yb3+30%/Tm3+0.5%)/NaYbF4/NaYF4 design, which involves the use of directed energy migration in the second active shell layer. The resulting active core/active shell/inert shell nanoparticles are determined to be about 11 times brighter than that of well-investigated (NaYF4:Yb3+30%/Tm3+0.5%)/NaYF4 active core/inert shell nanoparticles when excited at ~980 nm. The strategy for enhanced upconversion in Yb3+/Tm3+-codoped NaYF4 nanoparticles through directed energy migration might have implications for other types of lanthanide-doped upconversion nanoparticles.

In this work, we report on efficient visible and near-IR upconversion emissions in colloidal hexagonal-phase core/shell NaYF4:Er(3+)/NaYF4 nanoparticles (∼38 nm) under IR laser excitation at 1523 nm. Varying amounts of Er(3+) dopants were introduced into the core NaYF4:Er(3+) nanoparticles, revealing an optimized Er(3+) concentration of 10% for the highest luminescent efficiency. An inert epitaxial shell layer of NaYF4 grown onto the core of the NaYF4:Er(3+) 10% nanoparticle increased its upconversion emission intensity fivefold due to suppression of surface-related quenching mechanisms, yielding the absolute upconversion efficiency to be as high as ∼3.9±0.3% under an excitation density of 18 W/cm(2). The dependence of the intensity of upconversion emission peaks on laser excitation density in the core/shell nanoparticle displayed "saturation effects" at low excitation density in the range of 1.5-18 W/cm(2), which again demonstrates high upconversion efficiency.

Type-II quantum dots (QDs) were applied for QDs-sensitized solar cells for the first time and showed prominent absorbed photon to current conversion efficiency.