Planar Chiral Structures Research Articles

Enantioselective Synthesis of Planar Chiral Macrocyclic Metacyclophanes by Pd-Catalyzed C–O Cross-Coupling

Despite the presence of planar chiral elements in natural products and in ligands for asymmetric catalysis, catalytic enantioselective methods for installing planar chirality remain underdeveloped relative to methods for installing central and axial chirality. Here, we report an enantioselective Pd-catalyzed C–O bond-forming macrocyclization to access enantioenriched planar chiral macrocyclic metacyclophanes incorporating 2,3,4-trisubstituted pyridines. A variety of bridging chains and substituents can be incorporated in the metacyclophanes under the same reaction conditions (14 examples, 59–85% yields, 79–92% ee) to generate enantioenriched meta-, metaortho-, and metapara- cyclophanes. The syn-/anti-conformational preferences of metacyclophanes were elucidated by the X-ray structures, 1H NMR analyses, and computational studies. We also found that the two enantiomers of the metacyclophane can be obtained using the same enantiomer of the chiral ligand by starting from two regioisomeric linear precursors. This work provides an entry into the design of enantioselective cross-coupling macrocyclizations to access planar chiral structures for applications in medicinal chemistry and catalysis.

Micropolar modeling of a typical bending-dominant lattice comprising zigzag beams

Within the framework of micropolar theory, we have studied the continuum homogenization procedure of a typical kind of planar chiral structures, in which each two neighboring joints interact through a zigzag beam. Both the triangular and square lattices are taken as the demonstrating examples. Usage of zigzag beams makes the lattices bending-dominant, and thus calls for micropolar theory to predict the constitutive behaviors. According to the equivalence of microstructure- and continuum-level strain energies, the micropolar-type elastic stiffness constants are expressed in terms of the microstructural geometric and material parameters. By considering uniaxial tension, Young's modulus and Poisson's ratio are determined in an analytical form. Particularly, for square lattices, there arises a strong coupling between stretch and shear, meaning that the lattice under uniaxial tension is subject to a shearing deformation along the direction perpendicular to the tension, and vice versa. The theoretical results are validated by comparing with finite element simulations and experiments done on 3D printed specimens. We have also analyzed the influences of microstructural geometric parameters on the homogenized Young's modulus and Poisson's ratio. Furthermore, in this study the micropolar characteristic lengths are given explicitly in terms of microstructural parameters, providing the convenience and guidance of tuning the related size effects.

Research progress of electromagnetic properties of tunable chiral metasurfaces

Chiral metasurfaces are ultra-thin metamaterials composed of planar chiral cell structures with specific electromagnetic responses. They have attracted great attention due to their singular ability to control electromagnetic waves at will. With tunable materials incorporated into the metasurfaces design, one can realize tunable/reconfigurable metadevices with functionalities controlled by external stimuli, opening a new platform to dynamically manipulate electromagnetic waves. In this paper, we introduce some theoretical foundations of the electromagnetic properties of tunable/reconfigurable chiral metasurfaces. When a linearly polarized light enters a tunable chiral metasurface, it can be decomposed into left-handed circularly polarized (LCP) wave and right-handed circularly polarized (RCP) wave. By changing the dielectric constant and magnetic permeability of the medium through the external environment, the metadevices can dynamically control the response characteristics to various polarized lights, especially circularly polarized lights such as refractive index, dichroism, optical rotation, asymmetric transmission, etc. According to the properties of negative refractive index, circular dichroism, optical rotation, and asymmetric transmission controlled by the tunable chiral metasurfaces, we review the latest research progress. Finally, we put forward our own opinions on the possible future development directions and existing challenges of the rapidly developing field of the tunable chiral metasurface.

Enhanced circular dichroism and biosensing application of planar chiral nanostructure by covering graphene nanobelts

Circular dichroism (CD) of plasmonic nanostructures has broad application prospects, including negative refraction, chemical analysis, and biosensing. Compared with three-dimensional chiral structures, planar chiral structures with easy preparation show a weak CD effect. In this study, graphene nanobelts (GNs) were introduced into planar chiral structures composed of the IL-shaped chiral nanostructures (ILCNs) to enhance the effect of CD. The simulation results showed that the enhancement of CD was obtained from the plasmonic coupling between ILCNs and GNs. It led to different enhancements of the absorption under different circularly polarized lights. The bonding mode, anti-bonding mode, and trapping mode were used to interpret the coupling between the nanorods in ILCNs/GNs. The electromagnetic loss distributions of ILCNs/GNs showed that the electromagnetic loss was transferred from the metal part to the graphene part. The CD spectra of ILCNs with different graphene nanostructures revealed that the enhancements of absorption and CD mostly depended on the graphene belt located in the gaps of ILCNs. Meanwhile, the CD effect strongly depended on the geometric parameters of ILCNs/GNs. Besides, the CD spectra for chiral molecules could be enhanced by ILCNs/GNs under a different handedness. The maximum enhancement factor was 3200 times in the near-infrared band. These results might help explore the mechanism of CD enhancement and promote the application of the planar chiral structure in biosensing.

Planar chiral [2.2]paracyclophanyl-based boron fluoride complexes: synthesis, crystal structure and photophysical properties.

Planar chiral [2.2]paracyclophanyl-based boron fluoride complexes (3a-3d) were designed and facilely synthesized. The X-ray structure study, theoretical calculations and CD spectra reveal the intense emission and planar chiral structures of these complexes. In particular, 3a-3d show moderate quantum yields and large Stokes shifts both in solution and solid state. Furthermore, the blue-shifted mechanochromic properties of 3a and 3b were both investigated in the solid state. This work is the first study on planar chiral boron monofluoride complexes within the boron fluoride complex field.

Planar Achiral Metasurfaces-Induced Anomalous Chiroptical Effect of Optical Spin Isolation.

Planar chiral structures respond differently for oppositely handed incident light, and thus can produce extraordinary chiroptical effects such as circular conversion dichroism (CCD) and asymmetric transmission (AT). Such chiroptical effects are powerful tools to realize the fundamental principle of optical spin isolation, which leads to a plethora of applications such as optical conversion diodes, chiral imaging, and sensing. Here, we demonstrate the chiroptical effects of simultaneous CCD and AT through meticulously designed single-layered achiral nanofins. Our metamolecule consists of four achiral hydrogenated amorphous silicon (a-Si:H) nanofins that are carefully oriented and optimized to exhibit considerable CCD and AT. The device demonstrates a circular conversion dichroism of 55% and an asymmetric transmission of 58% at a wavelength of 633 nm. Right-hand circularly polarized light (RHCP) is completely absorbed, while left-hand circularly polarized light (LHCP) is transmitted with a polarization conversion, making it a perfect circular polarization wave isolator with negligible backscattering (due to low reflectance). This unique design and its underlying working mechanism are described comprehensively with three different techniques. These methods validate the proposed design and its methodology. For practical applications such as imaging, the proposed design realizes the Pancharatnam-Berry (PB) phase, achieving a 0-2π phase coverage for transmitted circular polarization. For the proof of concept, a metahologram is designed and demonstrated by employing the achieved full-phase control. The measured response of the fabricated metadevice not only validates the CCD and AT but also exhibits a simulated polarization conversion efficiency of up to 71% and measured efficiency up to 52%, comparable to state-of-the-art metahologram demonstrations.

Wideband and high-efficiency planar chiral structure design for asymmetric transmission and linear polarization conversion

The phenomenon of asymmetric transmission (AT) means the existence of a giant difference in transmittance after the incident waves pass through the special slab in two opposite directions, which is quite useful for realizing irreversible electromagnetic waves and has attracted much attention in recent years. However, it is difficult to achieve wideband asymmetric transmission and linear polarization conversion simultaneously using the bilayer transmission-mode structure. In this work, we propose and numerically investigate a high-efficiency transmission-mode metamaterial that consists of twisted multigap rings on both sides of a dielectric slab. Benefiting from the antisymmetric design of the split ring bilayer transmitarray element, the proposed structure excites the electric and magnetic dipole resonances with mutual interference, thereby simultaneously achieving asymmetric transmission and linear polarization conversion. The proposed structure is designed, fabricated, and measured, which can realize a good agreement between the measurement results and the anticipation. Within the frequency range from 12.7 to 17 GHz, the AT parameter can be close to 0.9 and the polarization conversion ratio can be more than 93%.

Design of a novel chiral fractal resonator

In this paper, we presented a compact resonator using a planar chiral fractal tree structure, where the chirality and the fractal form can combine several features at once. The chiral properties of this resonator, which are discussed based on their optical activity, circular dichroism, and the effect of the number of the iteration. Our chiral fractal structure is of major interest in scientific research, their characteristics have given rise to interesting new applications. Numerical results are given and discussed to confirm.

Enhancing circular dichroism by super chiral hot spots from a chiral metasurface with apexes

Manipulating light spin (or circular polarization) is an important research field and may find broad applications from sensors, display technology, to quantum computing and communication. To this end, planar metasurfaces with larger circular dichroism are strongly demanded. However, current planar chiral metasurface structures suffer from either fabrication challenge, especially from near-infrared to visible spectrum, or insufficient circular dichroism. Here, we report a chiral metasurface composed of achiral nanoholes which allow for precisely creating apexes in the designed structure. Our investigation indicates that the apexes act as super chiral hot spots and enable the highly concentrated near-field optical chirality leading to a remarkable enhancement of circular dichroism in the far-field. A 4-fold enhancement of the circular dichroism and a strong optical activity of ∼15 degrees have been experimentally achieved. Besides the enhanced chirality, our design genuinely overcomes the nanofabrication challenge faced in existing planar chiral metasurfaces.

Rotation of the polarization plane by double-layer planar-chiral structures. Review of the results of theoretical and experimental studies

This article provides examples that illustrate the search for different two-layer metamaterials that provide rotation of the polarization plane (“optical activity”). Selected objects show a twenty-year history of the search for a new principle of creation of polarization rotators based on planar metamaterials that were implemented in the form of thin-layered periodic structures. The manifestation of optical activity, presence or absence of satisfactory or perfect matching, the possibility of a multiband phenomena, the role of high spatial harmonics in “electromagnetics” of this effect are explained by the features of the eigen-oscillations that are excited in the gap of the multilayer structure.

Tunable asymmetric transmission of THz wave through a graphene chiral metasurface

In this paper, we show numerically that the asymmetric transmission of circularly polarized waves through a graphene chiral metasurface can be observed in the THz range. The relative enantiomeric difference in the total transmission varies with the change of graphene’s Fermi level. The plasmonic excitation in the graphene metasurface is enantiomerically sensitive, which is asymmetric for opposite propagating directions. This phenomenon will deepen our understanding of light-matter interactions in planar chiral structures and may find applications in polarization-sensitive devices, sensors, detectors and other areas.

Controllable thermal expansion of large magnitude in chiral negative Poisson's ratio lattices

AbstractLattices of controlled thermal expansion are presented based on planar chiral lattice structure with Poisson's ratio approaching . Thermal expansion values can be arbitrarily large positive or negative. A lattice was fabricated from bimetallic strips and the properties analyzed and studied experimentally. The effective thermal expansion coefficient of the lattice is about K. This is much larger in magnitude than that of constituent metals. Nodes were observed to rotate as temperature was changed corresponding to a Cosserat thermoelastic solid.

Enhanced rotation of the polarization of a light beam transmitted through a silver film with an array of perforated S-shaped holes.

In this Letter, we realized an enhanced optical rotation of the zero-order transmitted light through a silver film with an array of perforated S-shaped holes. Different from the previous studies, this effect results from the contribution of both the localized surface plasmons and surface plasmon polaritons (SPPs). The rotation angle can be modulated with the thickness due to the phase retardation of the SPPs when tunneling to the emitted surface. With a sample thickness 245 nm, a near-complete cross-polarization conversion (90° optical rotation) can be achieved, representing a major advance in performance compared to the previously reported planar chiral structures.

Resonant features of planar Faraday metamaterial with high structural symmetry

The transmission of electromagnetic wave through a planar chiral structure, loaded with the gyrotropic medium being under an action of the longitudinal magnetic field, is studied. The frequency dependence of the metamaterial resonance and the angle of rotation of the polarization plane are obtained. We demonstrate both theoretically and experimentally a resonant enhancement of the Faraday rotation. The ranges of frequency and magnetic field strength are defined, where the angle of polarization plane rotation for the metamaterial is substantially higher than that one for a single ferrite slab.

3,3]Paracyclophanes as planar chiral scaffolds for the synthesis of new phosphoric acids

Cyclic phosphoric acids displaying planar chiral paracyclophane structures, which include a 1,1'-ferrocenediyl unit, have been designed as a new class of chiral organocatalysts. Their synthesis, optical resolution, structural characterization and preliminary catalytic tests are reported.

Asymmetric transmission in planar chiral split-ring metamaterials: Microscopic Lorentz-theory approach

The electronic Lorentz theory is employed to explain the optical properties of planar split-ring metamaterials. Starting from the dynamics of individual free carriers, the electromagnetic response of an individual split-ring meta-atom is determined, and the effective permittivity tensor of the metamaterial is calculated for normal incidence of light. Whenever the split ring lacks in-plane mirror symmetry, the corresponding permittivity tensor has a crystallographic structure of an elliptically dichroic medium, and the metamaterial exhibits optical properties of planar chiral structures. Its transmission spectra are different for right-handed versus left-handed circular polarization of the incident wave, so the structure changes its transmittance when the direction of incidence is reversed. The magnitude of this change is shown to be related to the geometric parameters of the split ring. The proposed approach can be generalized to a wide variety of metal-dielectric metamaterial geometries.

Through-space conjugated polymers consisting of planar chiral pseudo-ortho-linked [2.2]paracyclophane

Herein, we describe the synthesis and chiroptical properties of through-space conjugated polymers with enantiopure planar chiral (R)- and (S)-pseudo-ortho-disubstituted [2.2]paracyclophane skeletons. The polymers exhibited intense circularly polarized luminescence in dilute solutions due to their conformationally stable planar chiral zigzag structures.

Investigation on strong optical activity and negative refractive index in double-layer metallic wires planar chiral structure

A planar chiral structure composed of double-layer metallic wires with fourfold symmetry is proposed. The rotation angle and the dielectric layer thickness between double-layer metallic wires is optimized. The numerical simulation results show that the exceptionally strong polarization rotation and the large chiral parameter are found at the infrared wavelength, and show a negative refraction index in a certain band.

Asymmetric Optical Second-Harmonic Generation from ChiralG-Shaped Gold Nanostructures

We present a new electromagnetic phenomenon-the asymmetric second-harmonic generation from planar chiral structures. The effect consists in distinguishing the handedness of a chiral material by rotating the sample in an experiment involving solely linearly polarized light. This phenomenon originates in the surface plasmon resonance of chiral gold nanostructures, where homodyne interference of anisotropic and chiral electric and/or magnetic multipoles appears to play an important role.

Elliptical dichroism: operating principle of planar chiral metamaterials

We employ a homogenization technique based on the Lorentz electronic theory to show that planar chiral structures (PCSs) can be described by an effective dielectric tensor similar to that of biaxial elliptically dichroic crystals. Such a crystal is shown to behave like a PCS insofar as it exhibits its characteristic optical properties, namely, corotating elliptical polarization eigenstates and asymmetric, direction-dependent transmission for left- or right-handed incident wave polarization.