Understanding Attenuated Solvent Reorganization Energies near Electrode Interfaces

This manuscript studies the position-dependent profile of Marcus-like outer-sphere reorganization energy near electrode interfaces. We establish the relation between the reorganization energy and the fluctuation in the Madelung potential of redox ions in the electrolyte. The distribution of such potential narrows significantly within the screening length of the electrode, leading to the minimization of electron transfer barrier. In addition, we present a statistical field model that can capture the attenuation of reorganization energy, indicating the generality of this effect near electrode interfaces regardless the molecular details

Resonance Tunability and Ultra-Long Range Enhancement of Plasmon-Coupled Resonance Energy Transfer Facilitated by Silver Nanorods (I): An Overview via Computational Study

Resonance energy transfer (RET) between molecules or quantum dots is an important process in many energy related applications. Different environmental structures have been studied and demonstrated to be able to enhance the RET rate between a nearby donor-acceptor pair. In particular, cylindrical silver nanorods and nanowires have shown extraordinary ability to transfer energy along its longitudinal axis over large distances. However, the mechanism of such transfer and the exact effects on molecular RET process are yet elusive. In this study, we use the recently developed computational tool based on the plasmon-coupled resonance energy transfer (PC-RET) method to systematically study the effects of nanorods with different dimensions on RET rates. We find that highly frequency-dependent coupling factor (CF) spectra, whose amplitudes determine RET rates, can be obtained due to the localized surface plasmon polariton (LSPP) modes of the rods with nanoscale dimensions. Simple phenomenological models can be derived for the wavelengths of CF peaks in relation to the length and width of the nanorods, providing easy tunability for enhancing RET rate in specific wavelength ranges. When coupled to longer rods with mesoscale lengths, exponential decay of the CF over long donor-acceptor distances with a small decay constant is observed, leading to the possibility of long-range RET processes. Furthermore, drumhead resonance modes emerge on the flat ends of the rod when the rod\u27s diameter reaches 300~nm, resulting in extra enhancement to RET rate compared to certain thinner rods. These findings shed new light on the mechanism of plasmonic enhancement with silver nanorods and establish design principles for how to optimally utilize these structures to manipulate RET processes for various applications

Plasmon-Coupled Resonance Energy Transfer

In this study, we overview resonance energy transfer between molecules in the presence of plasmonic structures and derive an explicit Förster-type expression for the rate of plasmon-coupled resonance energy transfer (PC-RET). The proposed theory is general for energy transfer in the presence of materials with any space-dependent, frequency-dependent, or complex dielectric functions. Furthermore, the theory allows us to develop the concept of a generalized spectral overlap (GSO) <i>J̃</i> (the integral of the molecular absorption coefficient, normalized emission spectrum, and the plasmon coupling factor) for understanding the wavelength dependence of PC-RET and to estimate the rate of PC-RET <i>W</i>_ET. Indeed, <i>W</i>_ET = (8.785 × 10^–25 mol) ϕ_Dτ_D^–1<i>J̃</i>, where ϕ_D is donor fluorescence quantum yield and τ_D is the emission lifetime. Simulations of the GSO for PC-RET show that the most important spectral region for PC-RET is not necessarily near the maximum overlap of donor emission and acceptor absorption. Instead a significant plasmonic contribution can involve a different spectral region from the extinction maximum of the plasmonic structure. This study opens a promising direction for exploring exciton transport in plasmonic nanostructures, with possible applications in spectroscopy, photonics, biosensing, and energy devices

Resolution of a Challenge for Solvation Modeling: Calculation of Dicarboxylic Acid Dissociation Constants Using Mixed Discrete–Continuum Solvation Models

First and second dissociation constants (p<i>K</i>_a values) of oxalic acid, malonic acid, and adipic acid were computed by using a number of theoretical protocols based on density functional theory and using both continuum solvation models and mixed discrete–continuum solvation models. We show that fully implicit solvation models (in which the entire solvent is represented by a dielectric continuum) fail badly for dicarboxylic acids with mean unsigned errors (averaged over six p<i>K</i>_a values) of 2.4–9.0 log units, depending on the particular implicit model used. The use of water–solute clusters and accounting for multiple conformations in solution significantly improve the performance of both generalized Born solvation models and models that solve the nonhomogeneous dielectric Poisson equation for bulk electrostatics. The four most successful models have mean unsigned errors of only 0.6–0.8 log units

Investigating the bearing performance of the foundation under the combined effects of flood scouring and soaking

Abstract Bearing capacity degradation of foundations under the impact of the flood is one of the major reasons responsible for the collapse and damage to the rural buildings, posing a serious threat to the local village societies. Based on a case study of a rural building foundation had been destroyed by flooding. This paper investigated the deterioration process of rural building foundations under the combined effect of dynamic scouring and static soaking caused by flooding. Using the two-dimensional shallow water equation, erosion depth was calculated for different flood velocities. Then, the bearing capacity degradation under the combined scouring-soaking effect was analyzed using the finite element method. Finally, investigating the influence of inflow direction and building group masking on the foundation's bearing capacity. The results indicate that under the combined effect, the bearing capacity of village building foundations decreases by 47.88%, with scouring slightly more impactful than soaking. Inflow angle has minimal effect on bearing performance, while the masking effect of the building group provides better protection for the foundation of rear buildings

Single Molecule Rectification Induced by the Asymmetry of a Single Frontier Orbital

A mechanism for electronic rectification under low bias potentials is elucidated for the prototype molecule HS-phenyl-amide-phenyl-SH. We apply density functional theory (DFT) combined with the nonequilibrium Green’s function formalism (NEGF), as implemented in the TranSIESTA computational code to calculate transport properties. We find that a single frontier orbital, the closest to the Fermi level, provides the dominant contribution to the overall transmission and determines the current. The asymmetric distribution of electron density in that orbital leads to rectification in charge transport due to its asymmetric response, shifting toward (or away from) the Fermi level under forward (or reverse) applied bias voltage. These findings provide a simple design principle to suppress recombination in molecular assemblies of dye-sensitized solar cells (DSSCs) where interfacial electron transfer is mediated by frontier orbitals with asymmetric character

High-Conductance Conformers in Histograms of Single-Molecule Current–Voltage Characteristics

Understanding charge transport across single molecular junctions is essential for the rational design and optimization of molecular device components. However, the correlation between calculated and experimental transport properties of single molecules probed by current–voltage (<i>I</i>–<i>V</i>) characteristics is often uncertain. Part of the challenge is that molecular conductance is sensitive to several factors that are difficult to control, including molecular orientation, conformation, aggregation, and chemical stability. Other challenges include the limitations of computational methodologies. Here, we implement the Σ-Extended Hückel (EH) nonequilibrium Green’s function (NEGF) method to analyze the histogram of <i>I</i>–<i>V</i> curves of 4,4′-diaminostilbene probed by break-junction experiments. We elucidate the nature of the molecular conformations with a widespread distribution of <i>I</i>–<i>V</i> curves, typically probed under experimental conditions. We find maximum conductance for molecules that are not at the minimum energy configuration but rather are aligned almost parallel to the transport direction. The increased conductance is due to the more favorable electronic coupling between the transport channel state and the electronic states in the contacts, as indicated by the broadening of bands in the transmission function near the Fermi level. These findings provide valuable guidelines for the design of anchoring groups that stabilize conformations of molecular assemblies with optimal charge transport properties

Thiocyanate linkage isomerism in a ruthenium polypyridyl complex

Ruthenium polypyridyl complexes have seen extensive use in solar energy applications. One of the most efficient dye-sensitized solar cells produced to date employs the dye-sensitizer N719, a ruthenium polypyridyl thiocyanate complex. Thiocyanate complexes are typically present as an inseparable mixture of N-bound and S-bound linkage isomers. Here we report the synthesis of a new complex, [Ru(terpy)(tbbpy)SCN][SbF6] (terpy = 2,2′;6′, 2′-terpyridine, tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine), as a mixture of N-bound and S-bound thiocyanate linkage isomers that can be separated based on their relative solubility in ethanol. Both isomers have been characterized spectroscopically and by X-ray crystallography. At elevated temperatures the isomers equilibrate, the product being significantly enriched in the more thermodynamically stable N-bound form. Density functional theory analysis supports our experimental observation that the N-bound isomer is thermodynamically preferred, and provides insight into the isomerization mechanism. © 2011 American Chemical Society

An Integrated Approach for Simulating Debris-Flow Dynamic Process Embedded with Physically Based Initiation and Entrainment Models

Recent studies have indicated that the accurate simulation of debris flows depends not only on the selection of numerical models but also on the availability of precise data on the initial source location and depth. Unfortunately, it is currently difficult to obtain quantitative data on source locations and depths during field investigations or model experiments of debris flow disasters. Therefore, in this study, we propose an integrated approach for simulating the debris-flow dynamic process that includes the physically based slope initiation source estimation and the entrainment-incorporated process simulation. We treat the potential slip surfaces’ locations and depths as random variables to search for the critical surface corresponding to the minimum stability factor by Monte Carlo simulation. Using the spatial variation interval of the soil parameters, we estimate the range of possible critical slip surfaces and the interval of the initiation source volume. Moreover, we propose a wet/dry front treatment method applied to the finite difference scheme and integrate it into our entrainment-incorporated model to improve the stability and accuracy of the numerical solution over complex topography. The effectiveness of the method is demonstrated through a case study of the 2010 Hongchun debris flow event in Yingxiu town. The result indicates that our method is effective in simulating debris flow dynamics, including slope initiation source estimation and dynamic process simulation

Electrode-Ligand Interactions Dramatically Enhance CO₂ Conversion to CO by the [Ni(cyclam)](PF₆)₂ Catalyst

Dramatic enhancement of electrochemical CO₂ conversion to CO, catalyzed by [Ni­(cyclam)]­(PF₆)₂ is observed on mercury/gold electrodes. We find that Hg provides favorable noncovalent dispersive interactions with the cyclam ligand. As a result, the Hg surface destabilizes the poisoned CO-bound form of the catalyst, leading to enhanced reaction kinetics. These findings are particularly relevant to the design of ligands that improve the electrocatalytic performance of transition-metal complexes on interaction with metallic surfaces under cell operating conditions