Symmetry Of Charge Distribution Research Articles

Asymmetric Charge Distribution of Active Centers in Small Molecule Quinone Cathode Boosts High‐Energy and High‐Rate Aqueous Zinc‐Organic Batteries

AbstractSmall molecule quinone compounds are attractive cathode materials for rechargeable aqueous zinc‐organic batteries (AZOBs) because of their structural diversity and low‐cost merits. Among them, nonpolar quinones are dominant given the relatively low solubilities in aqueous electrolytes. However, their poor electronic conductivity and accumulated Coulombic repulsion lead to underutilized active sites and sluggish redox kinetics. Here, polar 2,6‐dimethoxy‐1,4‐benzoquinone (m‐DMBQ) works as an advanced AZOB cathode with unexpectedly superior performance over the nonpolar isomer of 2,5‐dimethoxy‐1,4‐benzoquinone (p‐DMBQ). The asymmetric charge distribution of active centers in the p−π conjugated backbone of m‐DMBQ induces reduced bandgap with improved electronic conductivity and redox activity, thus achieving a high specific capacity of 312 mAh g−1 approaching the theoretical limit. Additionally, the lowest unoccupied molecular orbital energy level is lowered for an increased average discharge voltage of 0.88 V. Characterizations and computational studies revealed boosted competitiveness of H+ relative to Zn2+ for significantly enhanced charge transfer kinetics and reversibility. As a result, the as‐fabricated AZOB achieves a high energy density of 275 Wh kg−1 based on m‐DMBQ along with high‐rate capability and long‐term cycling stability. This work provides a new molecular engineering strategy through regulating charge distribution symmetry for boosting charge storage in organic cathodes.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems.

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

Effect of edge on nonlinear optical property of graphene quantum dots

Graphene is a two-dimensional material with single-layer honeycomb lattice structure formed by sp<sup>2</sup> hybrid connection of carbon atoms. Graphene has excellent optical, electrical, thermal and mechanical properties, and it is considered to be an ideal material for future flexible optoelectronic devices. In recent years, the nonlinear optical properties and regulation of graphene nanostructures have attracted experimental and theoretical interest. Graphene has good delocalization of π-electrons and its unique plane structure, showing good nonlinear optical properties. Graphene quantum dots can be regarded as small graphene nanoflakes. Their unique electronic structure is closely related to the non-bond orbitals on the boundary/edge. Therefore, it is very important to study the boundary/edge effect on the electronic and optical properties of nanographene. In this paper, effects of the number of edge C=C double bonds and Borazine (B<sub>3</sub>N<sub>3</sub>) doping on the nonlinear optical properties and UV-Vis absorption spectrum of graphene quantum dots are studied by the quantum chemical calculation methods, respectively. It is found that the symmetry of hexagonal graphene quantum dots decreases and the symmetry of charge distribution is broken when C=C double bond is introduced into the armchair edge, which leads the second-order nonlinear optical activity to be enhanced. During the transition from armchair to zigzag edge, the polarizability and the second hyperpolarizability of hexagonal graphene quantum dots and B<sub>3</sub>N<sub>3</sub>-doped graphene quantum dots increase linearly with the number of introduced C=C double bonds incrrasing. In addition, the edge also has an important influence on the absorption spectrum of graphene quantum dots. For graphene quantum dots and B<sub>3</sub>N<sub>3</sub>-doped graphene quantum dots, the introduction of C=C double bond at the armchair edge increases the highest occupied molecular orbital energy level and also reduces the lowest unoccupied molecular orbital energy level, which reduces the energy gap between the frontier molecular orbitals, and thus resulting in the red-shift of the maximum absorption wavelength. The doping of B<sub>3</sub>N<sub>3</sub> ring will increase the energy gap between molecular frontier orbitals of graphene quantum dots, leading the UV-Vis absorption spectrum of graphene quantum dots to be blue-shifted. This study provides theoretical guidance for controlling the nonlinear optical response of graphene quantum dots by edge modification.

Rejection of events undergoing multiple interactions within a scintillation crystal array based on spatial charge spread discrimination for gamma-ray imaging

This work deals with the rejection of events interacting more than one time in a crystal array, by using the method introduced in the paper R. Scafè et al. (2016). In particular the grade of symmetry of charge distributions along the X and Y axes was successfully used for discrimination. Results are presented regarding a 10×10 LuYAP:Ce array of 2×2×10mm3 crystal-pixels coupled to a H10966 Hamamatsu 8×8 multi-anode assembly under gamma-ray irradiation from a Co-57 radioisotopic source. Filtered pulse-height spectra are shown characterized by relevant rejection of low-amplitude events. In this region of spectrum, asymmetrical charge distributions were measured due to lutetium and yttrium X-rays escape from lateral walls of crystal-pixels. Events from Lu-176 self activity above the Co-57 photoelectric peak were also rejected. Similar results are reasonably expected at PET photon energy.

Plasmon excitations in two-dimensional atomic cluster systems

Properties of plasmon excitations in two-dimensional (2D) atomic cluster systems are theoretically studied within an extended Hubbard model. The collective oscillation equations of charge, plasmon eigen-equations and the energy-absorption spectrum formula are presented. The calculated results show that different symmetries of plasmons exist in the cluster systems, and the symmetry of charge distribution in the plasmon resonance originate from the intrinsic symmetry of the corresponding eigen-plasmon modes, but not from the symmetry of applied external fields; however, the plasmon excitation with a certain polarization direction should be excited by the field in this direction, the dipole mode of plasmons can be excited by both uniform and non-uniform fields, but multipole ones cannot be excited by an uniform field. In addition, we show that for a given electron density, plasmon spectra are red-shifted with increasing size of the systems.

Symmetry effects in electrostatic interactions between two arbitrarily charged spherical shells in the Debye-Hückel approximation

Inhomogeneous charge distributions have important repercussions on electrostatic interactions in systems of charged particles but are often difficult to examine theoretically. We investigate how electrostatic interactions are influenced by patchy charge distributions exhibiting certain point group symmetries. We derive a general form of the electrostatic interaction energy of two permeable, arbitrarily charged spherical shells in the Debye-Hückel approximation and apply it to the case of particles with icosahedral, octahedral, and tetrahedral inhomogeneous charge distributions. We analyze in detail how charge distribution symmetry modifies the interaction energy and find that local charge inhomogeneities reduce the repulsion of two overall equally charged particles, while sufficient orientational variation in the charge distribution can turn the minimum interaction energy into an attraction. Additionally, we show that larger patches and thus lower symmetries and wave numbers result in bigger attraction given the same variation.

Spin and charge distribution symmetry dependence of stripe phases in two-dimensional electron systems confined to wide quantum wells

Measurements in clean two-dimensional electron systems confined to wide GaAs quantum wells in which two electric subbands are occupied reveal an unexpected rotation of the orientation of the stripe phase observed at a half-filled Landau level. Remarkably, the reorientation is sensitive to the spin of the half-filled Landau level and the symmetry of the charge distribution in the quantum well.

Reentrantν=1Quantum Hall State in a Two-Dimensional Hole System

We report the observation of a reentrant quantum Hall state at the Landau level filling factor ν=1 in a two-dimensional hole system confined to a 35-nm-wide (001) GaAs quantum well. The reentrant behavior is characterized by a weakening and eventual collapse of the ν=1 quantum Hall state in the presence of a parallel magnetic field component B(∥), followed by a strengthening and reemergence as B(∥) is further increased. The robustness of the ν=1 quantum Hall state during the transition depends strongly on the charge distribution symmetry of the quantum well, while the magnitude of B(∥) needed to invoke the transition increases with the total density of the system.

Fourier transform infrared spectroscopy of special pair bacteriochlorophylls in homodimeric reaction centers of heliobacteria and green sulfur bacteria

Heliobacteria and green sulfur bacteria have type I homodimeric reaction centers analogous to photosystem I. One remaining question regarding these homodimeric reaction centers is whether the structures and electron transfer reactions are truly symmetric or not. This question is relevant to the origin of the heterodimeric reaction centers, such as photosystem I and type II reaction centers. In this mini-review, Fourier transform infrared studies on the special pair bacteriochlorophylls, P798 in heliobacteria and P840 in green sulfur bacteria, are summarized. The data are reinterpreted in the light of the X-ray crystallographic structure of photosystem I and the sequence alignments of type I reaction center proteins, and discussed in terms of hydrogen bonding interactions and the symmetry of charge distribution over the dimer.

Correlated States of Electrons in Wide Quantum Wells at Low Fillings: The Role of Charge Distribution Symmetry

Magnetotransport measurements on electrons confined to a 57-nm-wide, GaAs quantum well reveal that the correlated electron states at low Landau level fillings (nu) display a remarkable dependence on the symmetry of the electron charge distribution. At a density of 1.93 x 10;{11} cm;{-2}, a developing fractional quantum Hall state is observed at the even-denominator filling nu = 1/4 when the distribution is symmetric, but it quickly vanishes when the distribution is made asymmetric. At lower densities, as we make the charge distribution asymmetric, we observe a rapid strengthening of the insulating phases that surround the nu = 1/5 fractional quantum Hall state.

Effects of external stress on defect complexes in semiconductors

Crystal field gradients that exist at lattice sites in solids depend on the symmetry of chargedistribution around atomic sites. The charge symmetry could be broken either by thepresence of impurity complexes in the host matrix or by external stress on the samples,which leads to an observable magnitude of electric field gradients (EFGs). The perturbedγ–γ angular correlation (PAC) method is employed here to investigate the effect of uniaxial stress on111Cd sites in crystalline doped semiconductors.

Molecular structure—two-photon absorption property relations in polymethine dyes

We performed a comprehensive experimental investigation of two-photon absorption (2PA) spectra of a series of 12 symmetrical and asymmetrical cationic polymethine dyes, including complete one-and two-photon excitation anisotropy measurements. Quantum-chemical calculations were performed with the goal of understanding the nature of 2PA bands and of uncovering structure-property relations. We found that there are 2PA bands in the spectral region between the first absorption band and that for twice its energy. A weakly allowed 2PA band within the short-wavelength shoulder of the first absorption band was observed owing to the effects of vibrational and charge distribution symmetry breaking. The nature of the strongest 2PA band is connected to the electron transition from the molecular orbital localized at the benzene rings of the terminal groups to the lowest unoccupied molecular orbital (LUMO). Structure-property relations revealed that the 2PA cross section tends to be enhanced by either an increase in the length of the polymethine chromophore or an increase in the donor strengths in the terminal groups.

Vibrational spectra of BEDT-TTF based 2D charge ordering systems

We measured the polarized Raman spectra of θ-(BEDT-TTF) 2RbZn(SCN) 4 (RbZn) and (BEDT-TTF) 3CuBr 4 (CuBr) to investigate the detailed property of charge ordering (CO), and discussed an approach for analyzing the vibrational spectra of CO systems. The Raman spectra of the two compounds showed multiple peaks around the CC stretching region. With the help of isotope-shift measurements, the multiple peaks of RbZn were assigned to two symmetric CC stretching modes, called ν 2 and ν 3. We used the vibronic signals of ν 3 to characterize the spatial symmetry of charge distribution, and employed the peak frequencies of ν 2 with weaker vibronic coupling effect to the estimation of charge disproportionation ratio. Similar spectral analysis was applied to CuBr salt to verify whether the approach can be applied to other systems.

Spin dynamical properties and orbital states of the layered perovskiteLa2−2xSr1+2xMn2O7 (0.3<~x<0.5)

Low-temperature spin dynamics of the double-layered perovskite La_2-2x_Sr_1+2x_Mn_2_O_7 (LSMO327) was systematically studied in a wide hole concentration range (0.3 <= x < 0.5). The spin-wave dispersion, which is almost perfectly 2D, has two branches due to a coupling between layers within a double-layer. Each branch exhibits a characteristic intensity oscillation along the out-of-plane direction. We found that the in-plane spin stiffness constant and the gap between the two branches strongly depend on x. By fitting to calculated dispersion relations and cross sections assuming Heisenberg models, we have obtained the in-plane (J_para), intra-bilayer (J_perp) and inter-bilayer (J') exchange interactions at each x. At x=0.30, J_para=-4meV and J_perp=-5meV, namely almost isotropic and ferromagnetic. Upon increasing x, J_perp rapidly approaches zero while |J_para| increases slightly, indicating an enhancement of the planar magnetic anisotropy. At x=0.48, J_para reaches -9meV, while J_perp turns to +1meV indicating an antiferromagnetic interaction. Such a drastic change of the exchange interactions can be ascribed to the change of the relative stability of the d_x^2-y^2 and d_3z^2-r^2 orbital states upon doping. However, a simple linear combination of the two states results in an orbital state with an orthorhombic symmetry, which is inconsistent with the tetragonal symmetry of the crystal structure. We thus propose that an ``orbital liquid'' state realizes in LSMO327, where the charge distribution symmetry is kept tetragonal around each Mn site.

Chiral electrostatic interaction and cholesteric liquid crystals of DNA

Explicit expressions for electrostatic interaction between stiff DNA duplexes of finite length are obtained. These expressions allow for the helical symmetry of charge distribution on DNA molecules and reveal chiral and non-chiral interaction terms. Asymptotic expressions at small twist angles are applied to evaluate the cholesteric pitch and the twist elastic constant and their dependence on the length of DNA fragments. These estimates suggest an explanation for the large value of the cholesteric pitch and its nonmonotonic variation with the density of the liquid crystal. An analysis of biaxial correlations rationalizes the driving force of the transition from the cholesteric to hexagonal phase upon dehydration.

Influence of Charge Distribution on the Properties of Keggin- and Dawson-Type Heteropolyanions

The cyclic voltammetric behavior of selected Keggin- and Dawson-type heteropolyanions has been studied as a function of electrolyte composition. The buffer capacity of the electrolyte is shown to be an important parameter that can be used to reveal the symmetry or dissymetry in charge distribution within each oxometalate. A fully symmetrical heteropolyanion like [P2W18O62]6− exhibits a uniform protonability, while its α1 and α2 lacunary derivatives present a donor set of hard oxide ions with stepwise acid-base equilibria. The behavior of these sites is revealed readily in unbuffered or poorly buffered media at pH = 3, and is ascribed to the change in symmetry of charge distribution in the molecules. It is shown that cations other than the proton, even in very high excess, fail to give rise to the classical voltammograms of lacunary heteropolyanions. The same behavior also appears in α2[Mo2P2W15O61]10−, α[PW11O39]7−, α[SiW11O39]8−, [H2P2W12O48]12−, and, to a lesser extent, in α2[Ni(H2O)P2W17O61]8− and α2[Zn(H2O)P2W17O61]8−. The analogy in the cyclic voltammograms of most dication-substituted heteropolyanions with those of the precursor lacunary species could thus be explained.

Charge Distribution and Second-Order Nonlinear Optical Response of Charged Centrosymmetric Chromophore Aggregates. An ab Initio Electronic Structure Study of p-Nitroaniline Dimers

This contribution presents a quantum chemical investigation of the nonlinear optical response of charged centrosymmetric aggregates of the archetypical chromophore p-nitroaniline using an ab initio supermolecular approach. Localized hole states, responsible for aggregate nonlinearity, are predicted to be more stable than delocalized states. The calculated aggregate hyperpolarizabilities are consistently larger than in the isolated chromophore, and the mechanism giving rise to the nonlinearity is essentially “intramolecular” in nature. The aggregate formation is predicted to stabilize the positive charge and to enhance the nonlinearity. Moreover, the calculations reveal that a (formally) centrosymmetric charged aggregate can possess a substantial dipole moment the principal axis of which is oriented perpendicular to the principal hyperpolarizability tensor; the dipole moment arises because electron localization breaks the charge distribution symmetry. These results explain a number curious features of chromophoric polymer films which are electric field poled with an “in-plane” electrode geometry.

A new model of the third-order nonlinear optical systems which utilizes both through-bond and through-space charge-transfer effects: Polymeric systems with polar side chains

The third-order organic nonlinear optical systems can be divided into four groups. The criteria of the classification are the symmetry of charge distributions (centrosymmetric and noncentrosymmetric charge distributions) and the types of the interactions which induce the charge-transfer (CT) effects (through-bond and through-space interactions). In this paper we propose the new polymeric systems with polar side chains which are expected to utilize both the through-bond and through-space interactions. The coupled-Hartree-Fock (CHF) method is employed to calculate the third-order hyperpolarizabilities for these systems. The γ density analysis is also carried out to obtain the spatial contributions of γ values. It is found that the contributions of the side-chain regions are as large as those of the main-chain.

Protein transfer from an aqueous phase into reversed micelles. The effect of protein size and charge distribution.

Proteins are spontaneously transferred from an aqueous solution into reversed micelles, provided the aqueous phase has the proper composition. Besides the composition of the aqueous phase, the composition of the organic phase and the properties of the proteins also play a role. We studied uptake profiles of 19 proteins as a function of pH of the aqueous solution. The organic phase consisted of trioctylmethylammonium chloride and nonylphenol pentaethoxylate (Rewopal HV5) as surfactant, octanol as cosurfactant and isooctane as continuous phase. In all cases, except for rubredoxin, proteins were transferred at pH values above their isoelectric point. The pH where maximal solubilization takes place can be described by the relationship: pHoptimum = isoelectric point +0.11 x 10(-3) Mr -0.97. So, the larger the protein, the more charge is needed to provide the energy required for the adaptation of the micellar size to the protein size. For protein transfer into sodium di-(2-ethylhexyl)sulphosuccinate (AOT) reversed micelles a similar relationship was found. The percentage of protein transferred could be related to the symmetry of charge distribution over the protein. This symmetry was expressed as the % of random electric moments on a protein that is larger than the effective electric moment of the protein (% S) [Barlow, D. J. and Thornton, J. M. (1986) Biopolymers 25, 1717]. The larger the value of % S, the more homogeneously the charges are distributed and the lower the percentage transfer.

Some physicochemical properties of plasma membrane vesicles.

The plasma membranes of many animal cells can be disrupted into small sealed vesicles that can be purified centrifugally and utilized for studies on membrane transport. The vesicles behave as micro-osmometers. However, the presence of charges fixed at the internal and external surfaces of the membrane walls produce pH levels at these surfaces that deviate considerably from bulk pH. Transverse symmetry of charge distribution further leads to transverse asymmetry of surface pH. Finally, charges fixed at the internal membrane surface produced significant Donnan osmotic effects that depend upon membrane composition and ionic environment.