Achiral Structures Research Articles

Circularly Polarized Radioluminescence from Chiral Perovskite Scintillators for Improved X-ray Imaging.

Scintillators display radioluminescent properties when irradiated by high-energy photons and therefore play an essential role in radiation detection. The current inventory of scintillators is overwhelmingly represented by achiral structures, where the radioluminescence propagates isotropically after generation. Herein, we demonstrate that chiral perovskites of (R-3AP)PbBr3 Cl⋅H2 O (R-3APP) and (S-3AP)PbBr3 Cl⋅H2 O (S-3APP) emerge as a new type of scintillator displaying a distinct property of circularly polarized radioluminescence (CPRL). A high quantum yield of 27.6 %, high luminescence dissymmetric factors (glum ) of 4×10-2 , and high X-ray absorption coefficients were observed for these compounds. As proof of concept, we fabricated a polarized scintillator pair by assembling chiral scintillator crystals, which provides a new strategy to attenuate optical crosstalk between the scintillators by precisely controlling the radioluminescence propagation and improves the X-ray imaging quality at the boundary region.

Origin of circular dichroism in resonant elastic x-ray scattering from magnetic and polar chiral structures

Circular dichroism in resonant elastic x-ray scattering (CD-REXS) has recently been observed in chiral structures composed of multiferroic materials as well as magnetic moments or electric polarization vectors. In order to comprehensively understand the experimental results of these previous studies, we present here in detail the analytical formulation of CD-REXS for one-dimensional helices composed of magnetic moments and electric polarization vectors, respectively. In particular, by comparing CD-REXS for a proper-screw-shaped Bloch-type helix and cycloid-shaped N\'eel-type helix, we found that CD-REXS for both magnetic and polar helices can discriminate between both types of helices. We also found that the x-ray polarization factor depending on the scattering geometry is a significant factor in determining the characteristics of CD-REXS for chiral structures. The results obtained from the detailed formulas can be intuitively understood using the concept of mirror reflection. In particular, in this way it can be understood that Bloch- and N\'eel-type helices correspond to truly chiral and achiral structures, respectively, and why CD-REXS is able to distinguish between these two types of helices.

Azimuth-Resolved Circular Dichroism of Metamaterials.

Chiral optical metamaterials have attracted a great deal of attention due to their intriguing properties with respect to fundamental research and practical applications. For metamaterials with achiral structures, the system composed of metamaterials and obliquely incident light has extrinsic chirality and can produce circular dichroism (CD) effect. However, there have been few studies on the azimuth-dependent CD spectra of achiral metamaterials that have greatly improved our understanding of optical phenomena caused by external chirality. In this work, we experimentally studied the azimuth-dependent CD that originated from the extrinsic chirality of the metamaterials in an asymmetric-U shape and a U-bar-shape gold unit structure, separately. We explain the origin of the CD in the coupling of the macro-electric dipole and magnetic dipole, and the simulation results are in good agreement with the experiment. Our results provide a possible way to build an on-chip azimuth sensor based on azimuth-dependent CD spectra of metamaterials.

Evaluation of mechanical and vibration properties of chiral structure

In this study, the mechanical properties of chiral and achiral structures, which are mirror-image isomers, were compared with previous thesis by using FEM. In addition, a new method of equivalent shear modulus was investigated. As an application example, the vibration characteristics were investigated. Specifically, we predicted the natural frequencies due to in-plane vibrations considering the mechanical properties. In addition to the Bernoulli-Euler beam theory of bending vibration, a new method based on the Rayleigh-Ritz method is proposed. The Tetra-chiral structure is not suitable for vibration, while the Anti-chiral structure is suitable for vibration.

Realizing Spin-Conserved and Spin-Encrypted Hologram using Multipolar-modulated Meta-platform

The chiro-optical effects complemented with polarization conservation and wavefront shaping finds potential applications in advanced imaging, sorting and detection of enantiomers and quantum optics. Here, a unique design strategy proposed to manifest enormous chiro-optical effects using achiral structures (instead of conventional chiral antennas). The basic building block of the meta-platform contains a pair of achiral structures. The underlying mechanism behind the giant chiro-optical effects is explained by numerically calculating the multipolar resonances of scattering power. The designed diatomic meta-platform achieves absolute control over spin and wavefront of incident light to demonstrate the polarization-conserved and -encrypted meta-holograms.

An Account of Chiral Metal Surfaces and Their Enantiospecific Chemistry

ConspectusMolecular chirality has been of scientific interest since 1848 when Pasteur demonstrated its direct connection to the rotation of light by solutions of chiral compounds. In the 1960s the connection was made between the chirality of pharmaceutical compounds and their physiological impact; one enantiomer can be therapeutic while the other is toxic. That realization prompted enormous effort in the synthesis of enantiomerically pure compounds for bioactive use (a $300B/yr market). Until relatively recently, metals were ignored as potential substrates for asymmetric surface chemistry because metals have highly symmetric, achiral bulk structures and the premise was that they could not expose chiral surfaces. In 1996, we demonstrated that the high Miller index surfaces of metals can be chiral, existing in two enantiomeric forms M(hkl)R&S, and we hypothesized that they exhibit enantiospecific interactions with chiral adsorbates. Most such intrinsically chiral metal surfaces have ideal structural motifs based on low Miller index terraces separated by kinked monatomic steps. This Account begins with a short tutorial on the ideal and real structures of chiral metal surfaces to provide a firm basis for understanding the origin of their chirality. It then chronicles the evolution of our understanding of their enantiospecific interactions with chiral adsorbates.Detecting, quantifying, and understanding enantiospecific surface chemistry on intrinsically chiral metal surfaces has been far more challenging than coming to the realization that such surfaces exist. The first successes came from measurements and modeling of the enantiospecific adsorption energetics of small chiral molecules such as propylene oxide and trans-1,2-dimethylcyclopropane. These revealed one of the core challenges to observing enantiospecificity, the fact that the enantiospecificities of reaction energetics and barriers tend to be small, i.e., a few kJ/mol. Measurements of the enantiospecific adsorption energetics of R-3-methylcyclohexanone on seven different Cu(hkl)R&S surfaces demonstrated their sensitivity to surface structure, but again revealed variations of only a few kJ/mol. One of the most important advances in our understanding of chiral surface chemistry is that the limitations imposed by weakly enantiospecific interactions can be circumvented by processes with nonlinear kinetics or equilibria. As an example, the surface explosion mechanism of d- and l-tartaric acid decomposition on Cu(hkl)R&S surfaces leads to enantiospecific rates that differ by almost 2 orders of magnitude, in spite of the fact that the rate constants are only weakly enantiospecific. More surprising is the observation that equilibrium adsorption of nonracemic mixtures of d- and l-aspartic acid can lead to autoamplification of enantiomeric excess, even on achiral Cu(111) surfaces. Again, this arises from a nonlinear adsorption isotherm. Most recently, we have developed a high throughput method for identification of the most enantiospecific surface orientation for a given reaction from the continuum of Cu(hkl)R&S surface orientations. These developments, and others described in this Account, firmly establish some of the basic principles of chiral surface chemistry.

Nanocrystals as Model Systems for Studying the Interplay Between Crystallization and Chirality

AbstractThe interplay between crystallization and chirality has fascinated chemists for over 170 years now. Many questions deal with the transfer of chirality from molecular level building blocks and crystal capping ligands to the macroscopic morphologies of crystals. Also the interest in nanoscale chiral crystallization and the relationship between chiral ligands and inorganic nanocrystals has recently arose. In this review we highlight several different topics related to studies of the interplay between chirality and crystallization that involve inorganic nanocrystals. Specifically, we differentiate between the effects of chiral molecules on inorganic nanocrystals with chiral or achiral crystal structures. In the first case, chirality may appear both at the atomic‐scale and the particle shape, and in the second case chirality may be induced only at the particle shape level. We also discuss recent progress on the relationship between the atomic scale chirality and crystal morphology.

Extrinsic Chirality by Interference between Two Plasmonic Modes on an Achiral Rectangular Nanostructure

The optical near field (NF) induced by circularly polarized light (CPL) is a hot scientific topic. We observed a chiral NF intensity distribution on a series of achiral gold nanorectangular structures (Au-NRs) under CPL irradiation by using multiphoton photoemission electron microscopy (MP-PEEM). Additionally, the differential NF spectra under left and right CPL irradiation, which represent the asymmetry of the NF intensity distribution, were investigated. We propose an interpretation that the chiral NF intensity distribution on an achiral metallic nanostructure is extrinsically generated by the interference between two plasmonic modes by combining state-of-the-art MP-PEEM techniques and the classical oscillator model. Our interpretation well explains both the experimental and simulation results. Furthermore, the intensity of the NF and its phase angle of each mode under linearly polarized light irradiation were revealed to be critical factors for the generation of extrinsic chirality in the NF intensity distribution.

Exploring the Geometric Space of Metal-Organic Polyhedrons (MOPs) of Metal-Oxo Clusters.

Metal organic polyhedra (MOPs) such as coordination cages and clusters are increasingly utilized across many fields, but their geometrically selective assembly during synthesis is nontrivial. When ligand coordination along these polyhedral edges is arranged in an unsymmetrical mode or the bridging ligand itself is nonsymmetric, a vast combinatorial space of potential isomers exists complicating formation and isolation. Here we describe two generalizable combinatorial methodologies to explore the geometrical space and enumerate the configurational isomers of MOPs with discrimination of the chiral and achiral structures. The methodology has been applied to the case of the octahedron {Bi6Fe13L12} which has unsymmetrical coordination of a carboxylate ligand (L) along its edges. For these polyhedra, the enumeration methodology revealed 186 distinct isomers, including 74 chiral pairs and 38 achiral. To explore the programming of these, we then used a range of ligands to synthesize several configurational isomers. Our analysis demonstrates that ligand halo-substituents influence isomer symmetry and suggests that more symmetric halo-substituted ligands counterintuitively yield lower symmetry isomers. We performed mass spectrometry studies of these {Bi6Fe13L12} clusters to evaluate their stability and aggregation behavior in solution and the gas phase showing that various isomers have different levels of aggregation in solution.

Plasmon-Mediated Chiroptical Second Harmonic Generation from Seemingly Achiral Gold Nanorods.

Throughout nature, simple rules explain complex phenomena, such as the selective interaction of chiral objects with circularly polarized light. Here, we demonstrate chiroptical signals from gold nanorods, which are seemingly achiral structures. Shape anisotropy due to atomic-level faceting and rounding at the tips of nanorods, which are free of chiral surface ligands, induces linear-to-circular polarization modulation during second harmonic generation. The intrinsic nanorod chiroptical response is increased by plasmon-resonant excitation, which preferentially amplifies circularly polarized harmonic signals. This structure-plasmon interplay is uniquely resolved by polarization-resolved second harmonic generation measurements. The material's second-order polarizability is the product of the structure-dependent lattice-normal susceptibility and local surface plasmon field vectors. Synthetically scalable plasmon-supporting nanorods that amplify small circular dichroism signals provide a simple, assembly-free platform for chiroptical transduction.

Hierarchical Microphase Behaviors of Chiral Block Copolymers under Kinetic and Thermodynamic Control

Hierarchical Microphase Behaviors of Chiral Block Copolymers under Kinetic and Thermodynamic Control

Detection of scorpion venom by optical circular dichroism method

Various efforts have been made to detect minimum amounts of any toxic materials in water or the neurotoxic effect of venom (Odontobuthus Doriae Scorpion) in the human’s blood serum nerve by high-sensitivity, accurate, and low-cost sensors in order to enhance life style. Therefore, the present study was done to investigate reliability of two-dimensional plasmonic structure and circular dichroism (CD) in toxic samples in order to measure and determine venom concentrations and its neurotoxic effect on humans҆ blood serum Neurotransmitter analytes. Our results confirmed dependency of CD signal to neurotoxic effect of venom concentrations and good sensitivity of this sensor with the help of achiral plasmonic structure.

Active terahertz spin state and optical chirality in liquid crystal chiral metasurface

Dynamic control of photonic spin state and chirality plays a vital role in various applications, such as polarization control, polarization-sensitive imaging, and biosensing. Here, we present a scheme for the flexible and dynamic manipulation of terahertz spin state conversion and optical chirality by combining two achiral structures: an asymmetric metasurface and a layer of anisotropic liquid crystal. The proposed asymmetric metasurface can realize the polarization conversion effect. For the circularly polarized incidence, it exhibits the asymmetric transmission of the spin-flipped states but no spin-locked optical chirality since its geometry is mirror symmetric along with the wave propagation. The introduction of the liquid crystal makes the composite metasurface not only exhibit the spin state conversion but also spin-locked chirality and spin-flipped chirality on account of breaking mirror symmetry, which realizes an electrically active terahertz chiral device. The experimental results show that the asymmetric transmission of the terahertz spin states can be dynamically manipulated, resulting in a large controllable range 83.8% to \ensuremath{-}30.7% of spin-locked circular dichroism at 0.76 THz and \ensuremath{-}98.2% to 44.7% of spin-flipped circular dichroism at 0.73 THz. This work paves the way for the development of terahertz meta devices capable of enabling active photonic spin state and chirality manipulation.

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Theoretical Prediction of Structures, Vibrational Circular Dichroism, and Infrared Spectra of Chiral Be4B8 Cluster at Different Temperatures.

Lowest-energy structures, the distribution of isomers, and their molecular properties depend significantly on geometry and temperature. Total energy computations using DFT methodology are typically carried out at a temperature of zero K; thereby, entropic contributions to the total energy are neglected, even though functional materials work at finite temperatures. In the present study, the probability of the occurrence of one particular Be4B8 isomer at temperature T is estimated by employing Gibbs free energy computed within the framework of quantum statistical mechanics and nanothermodynamics. To identify a list of all possible low-energy chiral and achiral structures, an exhaustive and efficient exploration of the potential/free energy surfaces is carried out using a multi-level multistep global genetic algorithm search coupled with DFT. In addition, we discuss the energetic ordering of structures computed at the DFT level against single-point energy calculations at the CCSD(T) level of theory. The total VCD/IR spectra as a function of temperature are computed using each isomer’s probability of occurrence in a Boltzmann-weighted superposition of each isomer’s spectrum. Additionally, we present chemical bonding analysis using the adaptive natural density partitioning method in the chiral putative global minimum. The transition state structures and the enantiomer–enantiomer and enantiomer–achiral activation energies as a function of temperature evidence that a change from an endergonic to an exergonic type of reaction occurs at a temperature of 739 K.

Extremize Optical Chiralities through Polarization Singularities.

Chiral optical effects are generally quantified along some specific incident directions of exciting waves (especially for extrinsic chiralities of achiral structures) or defined as direction-independent properties by averaging the responses among all structure orientations. Though of great significance for various applications, chirality extremization (maximized or minimized) with respect to incident directions or structure orientations has not been explored, especially in a systematic manner. In this study we examine the chiral responses of open photonic structures from perspectives of quasinormal modes and polarization singularities of their far-field radiations. The nontrivial topology of the momentum sphere secures the existence of generic singularity directions along which mode radiations are either circularly or linearly polarized. When plane waves are incident along those directions, the reciprocity ensures ideal maximization and minimization of optical chiralities, for corresponding mode radiations of circular and linear polarizations, respectively. For directions of general elliptical polarizations, we have unveiled the subtle equality of a Stokes parameter and the circular dichroism, showing that an intrinsically (geometrically) chiral structure can unexpectedly exhibit no optical chirality at all or even optical chiralities of opposite handedness for different incident directions. The framework we establish can be applied to not only finite scattering bodies but also infinite structures, encompassing both intrinsic and extrinsic optical chiralities. We have effectively merged two vibrant disciplines of chiral and singular optics, which can potentially trigger more optical chirality-singularity related interdisciplinary studies.

Macrochirality of Self-Assembled and Co-assembled Supramolecular Structures of a Pair of Enantiomeric Peptides.

Although macrochirality of peptides’ supramolecular structures has been found to play important roles in biological activities, how macrochirality is determined by the molecular chirality of the constituted amino acids is still unclear. Here, two chiral peptides, Ac-LKLHLHLQLKLLLVLFLFLALK-NH2 (KK-11) and Ac-DKDHDHDQDKDL DVDFDFDADK-NH2 (KKd-11), which were composed entirely of either L- or D-amino acids, were designed for studying the chiral characteristics of the supramolecular microstructures. It was found that monocomponent KK-11 or KKd-11 self-assembled into right- or left-handed helical nanofibrils, respectively. However, when they co-assembled with concentration ratios varied from 1:9 to 9:1, achiral nanowire-like structures were formed. Both circular dichroism and Fourier transform infrared spectra indicated that the secondary structures changed when the peptides co-assembled. MD simulations indicated that KK-11 or KKd-11 exhibited a strong propensity to self-assemble into right-handed or left-handed nanofibrils, respectively. However, when KK-11 and KKd-11 were both presented in a solution, they had a higher probability to co-assemble instead of self-sort. MD simulations indicated that, in their mixtures, they formed nanowires without handedness feature, a good agreement with experimental observation. Our results shed light on the molecular mechanisms of the macrochirality of peptide supramolecular microstructures.

Self-Templated, Enantioselective Assembly of an Amyloid-like Dipeptide into Multifunctional Hierarchical Helical Arrays.

Chiral self-assembly of peptides has attracted great interest owing to their promising applications in biomedicine, chemistry, and materials science. However, compared with the rich knowledge about their chiral self-assembly at the molecular or nanoscale, the formation of long-range-ordered hierarchical helical arrays (HHAs) from simple peptides remains a formidable challenge. Herein, we report the self-templated assembly of an amyloid-like dipeptide into long-range-ordered HHAs by their spontaneous fibrillization and hierarchical helical assembly within a confined film. The chiral interactions between the peptide and diamines result in geometry frustration and the phase transition of self-assembling peptide films from achiral spherulite structures into chiral HHAs. By changing the chirality and enantioselective interactions, we can control the phase behavior, handedness, and chiroptics of the self-assembled HHAs precisely. Moreover, the redox activity of the HHAs allows the in situ decoration of nanoparticles with high catalytic activity. These results provide insights into the chiral self-assembly of peptides and the fabrication of highly ordered materials with complex architectures and promising applications in chiroptics and catalysis.

Crystalline Porous Materials for Nonlinear Optics.

Crystalline porous materials have been extensively explored for wide applications in many fields including nonlinear optics (NLO) for frequency doubling, two-photon absorption/emission, optical limiting effect, photoelectric conversion, and biological imaging. The structural diversity and flexibility of the crystalline porous materials such as the metal-organic frameworks, covalent organic frameworks, and polyoxometalates provide numerous opportunities to orderly organize the dipolar chromophores and to systemically modify the type and concentration of these dipolar chromophores in the confined spaces, which are highly desirable for NLO. Here, the recent advances in the crystalline porous NLO materials are discussed. The second-order NLO of crystalline porous materials have been mainly devoted to the chiral and achiral structures, while the third-order NLO crystalline porous materials have been categorized into pure organic and hybrid organic/inorganic materials. Some representative properties and applications of these crystalline porous materials in the NLO regime are highlighted. The future perspective of challenges as well as the potential research directions of crystalline porous materials have been also proposed.

Synthesis of bis(benzoxazole) frameworks chiralized by planar chiral [2.2]Paracyclophane

In this study, a new class of chiral bis(benzoxazole) has been synthesized. The bis(benzoxazole) framework, which has an achiral structure was chiralized by a planar chiral [2.2]paracyclophane moiety. Both enantiomeric forms of the bis(benzoxazole) derivatives were successfully obtained in 99% ee. Even though the starting material was not completely enantiomerically pure, enantiomeric enrichment was afforded by the Horeau principle in the synthesis route.