Strong Circular Dichroism Research Articles

Helicity-dependent resonant X-ray scattering in CuB2O4.

Exploitation of X-ray circular polarized beams to study forbidden Bragg reflections and new information that could be obtained in these experiments are discussed. It is shown that the intensities of such reflections can be different for the right- and left-circular polarizations (i.e. exhibiting circular dichroism) even for the dipole-dipole resonant transitions involved in the scattering process. This difference can be observed only in crystals having no center of inversion. Here, this approach is used to study helicity-dependent resonant diffraction in copper metaborate CuB2O4 single crystal, which is non-centrosymmetric but achiral. Nonetheless, a strong circular dichroism has been observed for hh0 forbidden reflections in the vicinity of the Cu K-edge. This effect is shown to originate from dipolar transitions in Cu atoms occupying the 8(d) Wyckoff position only.

Strong circular dichroism triggered by near-field perturbation

Circular dichroism of chiral structure can be used in biological sensing and analytical chemistry, which has been extensively explored. However, previous studies on chiral structures are mainly focused on geometric symmetry breaking, further investigation about the effect of the near-field perturbation on the chirality remains unrevealed. In this paper, circular dichroism of the honeycomb-shaped elliptical hole structure is investigated. By discussing the impact of the period of the structure on chirality, it is found that the near-field perturbation can trigger strong circular dichroism without breaking the rotational symmetry and mirror symmetry. Furthermore, the distributions of electric field and Poynting vector are studied to confirm the effect of the near-field perturbation. We believe that our work not only facilitates the understanding between the circularly polarized light and matter, but also provides a new approach for designing chiral structures. • Circular dichroism of the honeycomb-shaped elliptical hole structure is theoretically investigated. • The near-field perturbation can trigger strong circular dichroism without breaking symmetry. • The distributions of electric field and Poynting vector are studied to reveal the underlying mechanism. • This work provides a new approach for designing chiral structures.

Synthetic Tunability and Biophysical Basis for Fabricating Highly Fluorescent and Stable DNA Copper Nanoclusters.

The real motivation in the present work is to tune the synthesis variables that can result in a highly fluorescent and stable DNA copper nanocluster (CuNC) and also to understand the intricate mechanism behind this process. Here, carefully optimized concentrations of various reactants enabled the creation of a DNA-encapsulated CuNC for AT-DNA, displaying a size of <1.0 nm as confirmed by transmission electron microscopy and dynamic light scattering. The extremely small size of the AT-DNACuNC supports the discrete electronic transitions, also characterized by an exceptionally strong negative circular dichroism (CD) band around 350 nm, whose intensity is well correlated with the observed strong fluorescence emission intensity. This remarkably strong CD can open new applications in the detection and quantification of a specific DNACuNC. Further, time-dependent fluorescence analysis suggested stronger photostabilization of these DNACuNCs. The simulation study, based on Cu ion distribution, explained how AT-DNA is a better candidate for NC formation than GC-DNA. In conclusion, the better-tuned synthesis procedure has resulted in a highly compact, well-defined three-dimensional conformation that promotes a more favorable microenvironment to sequester a DNA-based CuNC with high brightness and outstanding photostability.

Optical activity and circular dichroism of a hybrid system composed of a chiral quantum dot and a dielectric nanoparticle

We study a hybrid system composed of a chiral quantum dot and a dielectric nanoparticle subjected to an electromagnetic wave. We aim to show that the hybrid system features synergistic properties caused by interactions between its components. We solve the Bloch equation for the density matrix of the chiral quantum dot modeled as a two-level system with transition electric and magnetic dipole moments. We show that the system acts as a bianisotropic meta-atom: The electric polarization (magnetization) can be induced by the incident magnetic (electric) field. The electromagnetic and magnetoelectric polarizability dyadics of the hybrid system are influenced by the nanoparticle size and separation and the intensity and polarization of the incident wave. Due to the self-interaction of the quantum dot in the presence of the nanoparticle, the system exhibits optical activity and circular dichroism spectra of different character in Fano, double peaks, weak transition, and strong transition regions of the parameter space. Notably, in the strong transition region, the optical activity and circular dichroism spectra become discontinuous and dependent on the initial state of the system. The magnetic dipole moment induced in the dielectric nanoparticle plays a key role: Indeed, unlike the isolated chiral quantum dot, the hybrid system exhibits strong circular dichroism in a broad frequency range. The chiral quantum dot-dielectric nanoparticle meta-atom offers unique opportunities for creating low-loss all-dielectric meta-surfaces operating in the visible spectral range.

Spectroscopic and Theoretical Studies of Hg(II) Complexation with Some Dicysteinyl Tetrapeptides.

Tetrapeptides containing a Cys-Gly-Cys motif and a propensity to adopt a reverse-turn structure were synthesized to evaluate how O-, N-, H-, and aromatic π donor groups might contribute to mercury(II) complex formation. Tetrapeptides Xaa-Cys-Gly-Cys, where Xaa is glycine, glutamate, histidine, or tryptophan, were prepared and reacted with mercury(II) chloride. Their complexation with mercury(II) was studied by spectroscopic methods and computational modeling. UV-vis studies confirmed that mercury(II) binds to the cysteinyl thiolates as indicated by characteristic ligand-to-metal-charge-transfer transitions for bisthiolated S-Hg-S complexes, which correspond to 1 : 1 mercury-peptide complex formation. ESI-MS data also showed dominant 1 : 1 mercury-peptide adducts that are consistent with double deprotonations from the cysteinyl thiols to form thiolates. These complexes exhibited a strong positive circular dichroism band at 210 nm and a negative band at 193 nm, indicating that these peptides adopted a β-turn structure after binding mercury(II). Theoretical studies confirmed that optimized 1 : 1 mercury-peptide complexes adopt β-turns stabilized by intramolecular hydrogen bonds. These optimized structures also illustrate how specific N-terminal side-chain donor groups can assume intramolecular interactions and contribute to complex stability. Fluorescence quenching results provided supporting data that the indole donor group could interact with the coordinated mercury. The results from this study indicate that N-terminal side-chain residues containing carboxylate, imidazole, or indole groups can participate in stabilizing dithiolated mercury(II) complexes. These structural insights on peripheral mercury-peptide interactions provide additional understanding of the chemistry of mercury(II) with side-chain donor groups in peptides.

Crystalline chirality and interlocked double hourglass Weyl fermion in polyhedra-intercalated transition metal dichalcogenides

Introducing crystalline chirality into transition metal dichalcogenides (TMDs) has attracted much attention due to its modulation effect on optical properties and the potential to reveal new forms of electronic states. Here, we predict a number of chiral materials by intercalating polyhedra into TMD lattices, finding a type of double hourglass Weyl fermion interlocked with crystalline chirality. The best candidate RhV3S6 (P6322) possesses the largest hourglass energy window of ~380 meV, as well as strong optical circular dichroism (CD) in the infrared regime, both of which are tunable by external strains. The chirality is originally induced by the configuration of intercalated polyhedra and then reduced by the rotational atomic displacements triggered by intercalation, as indicated by CD calculations. Our study opens the way of designing chiral materials with spin-split double hourglass Weyl fermions via structural unit intercalation in achiral crystals for future chiral-functionalized optoelectronic and spintronic devices.

Faraday rotations, ellipticity, and circular dichroism in magneto-optical spectrum of moiré superlattices* *Project supported by the National Natural Science Foundation of China (Grant Nos. 12050410228 and 12074260) and the Fund from the Science and Technology Commission of Shanghai Municipality, China (Grant No. 19ZR1436400).

We study the magneto-optical conductivity of a number of van der Waals heterostructures, namely, twisted bilayer graphene, AB–AB and AB–BA stacked twisted double bilayer graphene and monolayer graphene and AB-stacked bilayer graphene on hexagonal boron nitride. As the magnetic field increases, the absorption spectrum exhibits a self-similar recursive pattern reflecting the fractal nature of the energy spectrum. Whilst twisted bilayer graphene displays only weak circular dichroism, the other four structures display strong circular dichroism with monolayer graphene and AB-stacked bilayer graphene on hexagonal boron nitride being particularly pronounced owing to strong inversion symmetry breaking properties of the hexagonal boron nitride layer. As the left and right circularly polarized light interact with these structures differently, plane-polarized incident light undergoes a Faraday rotation and gains an ellipticity when transmitted. The size of the respective angles is on the order of a degree.

Phase-change metasurface with tunable and switchable circular dichroism.

Metasurfaces with tunable/switchable circular dichroism (CD) response have great potential to serve as important elements for plenty of advanced applications. In this work, we proposed a novel metasurface absorber integrated with periodic ${{\rm Ge}_2}{{\rm Sb}_2}{{\rm Te}_5}$ (GST) resonators and numerically demonstrated its capability in reconfiguration of the CD effect. Due to the strong chiral plasmonic resonance, a strong CD of about 0.75 can be achieved in a prescribed spectrum. Additionally, the phase transition of GST resonators enables the quasi-linearly modification of CD strength in a broad range (from 0.03 to 0.75). Furthermore, reversible chirality of the metasurface absorber can be realized by controlling the states of the left- and right-hand GST resonators separately, enabling the CD signal to be readily switched between on-, off-, and reverse-state.

Porphyrin with circularly polarized luminescence in aggregated states

Porphyrin with circularly polarized luminescence (CPL) in aggregated state was seldom presented due to the strong aggregation-caused quenching (ACQ) effect at aggregation. This work designed and synthesized a novel porphyrin derivative (CPL-P) with good CPL behaviour in aggregated states based on the strong AIE-FRET effect and good chirality transfer of liquid crystalline porphyrin, which was constructed by using porphyrin unit as core with four AIE cyanostilbene groups bearing peripheral chiral alkyl chains. CPL-P exhibited orderly hexagonal columnar mesophase between 62.5 °C and 113.2 °C. It displayed excellent fluorescence emission at 640–760 nm in aggregated states due to the AIE-FRET effect between cyanostilbene unit and porphyrin moiety. Moreover, CPL-P showed the strong circular dichroism and CPL properties in aggregated states based on the effective chiral transfer of the spiral liquid crystalline self-assembly. The |glum| value for mesophase attained 6.6 × 10−3 based on the combining effect of AIE-FRET and chiral transfer. This research supplied a new strategy for transferring typical achiral porphyrin with ACQ effect to luminescent molecule with good CPL behaviour in aggregated state.

Dynamically reversible and strong circular dichroism based on Babinet-invertible chiral metasurfaces.

We propose a Babinet-invertible chiral metasurface for achieving dynamically reversible and strong circular dichroism (CD). The proposed metasurface is composed of a VO2-metal hybrid structure, and when VO2 transits between the dielectric state and the metallic state, the metasurface unit cell switches between complementary structures that are designed according to Babinet's principle. This leads to a large and reversible CD tuning range between ±0.5 at 0.97 THz, which is larger than the one found in the literature. We attribute the CD effect to extrinsic chirality of the proposed metasurface. We envision that the Babinet-invertible chiral metasurface proposed here will advance the engineering of active and tunable chiro-optical devices and promote their applications.

Thermally switching between perfect absorber and asymmetric transmission in vanadium dioxide-assisted metamaterials.

In this paper, we propose a switchable bi-functional metamaterial device based on a hybrid gold-vanadium dioxide (VO2) nanostructure. Utilizing the property of a metal-to-insulator transition in VO2, perfect absorption and asymmetric transmission (AT) can be thermally switched for circularly polarized light in the near-infrared region. When VO2 is in the metallic state, the designed metamaterial device behaves as a chiral-selective plasmonic perfect absorber, which can result in an optical circular dichroism (CD) response with a maximum value ∼ 0.7. When VO2 is in the insulating state, the proposed metamaterial device exhibits a dual-band AT effect. The combined hybridization model and electromagnetic field distributions are presented to explain the physical mechanisms of chiral-selective perfect absorption and AT effect, respectively. The influences of structure parameters on CD response and AT effect are also discussed. Moreover, the proposed switchable bi-functional device is robust against the incident angle for obtaining perfect absorption and strong CD response as well as the AT effect. Our work may provide a promising path for the development of multifunctional optoelectronic devices, such as thermal emitters, optical modulators, CD spectroscopy, optical isolator, etc.

Enhanced asymmetric transmission of linearly polarized light based on all-dielectric stereometamaterial

Reciprocal asymmetric transmission (AT) phenomenon has received much attention due to various polarization applications, however, high-efficiency AT still remains out of reach in the optical range. In order to improve AT of linearly polarized light, low-loss dielectric metamaterial with twisted resonators should be designed and investigated. The stereometamaterial is constructed by an array of bilayer 90° twisted H-shaped Ge dimers. Based on the transmission matrix, numerical simulation and multipolar resonances analysis, the polarization responses in all-dielectric stereometamaterial are characterized. The simulated results show that the AT parameter reaches up to 98% around 194 THz. In addition, the AT can be engineered via geometrical parameters. The proposed stereometamaterial also exhibits strong circular dichroism that is direction-independent. The multipole decomposition calculations can be utilized to understand the resonant mechanism.

Broadband Enhanced Chirality with Tunable Response in Hybrid Plasmonic Helical Metamaterials

AbstractDesigning broadband enhanced chirality is of strong interest to the emerging fields of chiral chemistry and sensing, or to control the spin orbital momentum of photons in recently introduced nanophotonic chiral quantum and classical optical applications. However, chiral light‐matter interactions have an extremely weak nature, are difficult to control and enhance, and cannot be made tunable or broadband. In addition, planar ultrathin nanophotonic structures to achieve strong, broadband, and tunable chirality at the technologically important visible to ultraviolet spectrum still remain elusive. Here, these important problems are tackled by experimentally demonstrating and theoretically verifying spectrally tunable, extremely large, and broadband chiroptical response by nanohelical metamaterials. The reported new designs of all‐dielectric and dielectric‐metallic (hybrid) plasmonic metamaterials permit the largest and broadest ever measured chiral Kuhn's dissymmetry factor achieved by a large‐scale nanophotonic structure. In addition, the strong circular dichroism of the presented bottom‐up fabricated optical metamaterials can be tuned by varying their dimensions and proportions between their dielectric and plasmonic helical subsections. The currently demonstrated ultrathin optical metamaterials are expected to provide a substantial boost to the developing field of chiroptics leading to significantly enhanced and broadband chiral light‐matter interactions at the nanoscale.

Supramolecular Chirogenesis in Bis-Porphyrin: Crystallographic Structure and CD Spectra for a Complex with a Chiral Guanidine Derivative

The complexation of (3aR,7aR)-N-(3,5-bis(trifluoromethyl)phenyl)octahydro-2H-benzo[d]imidazol-2-imine (BTI), as a guest, to ethane-bridged bis(zinc octaethylporphyrin), bis(ZnOEP), as a host, has been studied by means of ultraviolet-visible (UV-Vis) and circular dichroism (CD) absorption spectroscopies, single crystal X-ray diffraction, and computational simulation. The formation of 1:2 host-guest complex was established by X-ray diffraction and UV-Vis titration studies. Two guest BTI molecules are located at the opposite sides of two porphyrin subunits of bis(ZnOEP) host, which is resting in the anti-conformation. The complexation of BTI molecules proceed via coordination of the imine nitrogens to the zinc ions of each porphyrin subunit of the host. Such supramolecular organization of the complex results in a screw arrangement of the two porphyrin subunits, inducing a strong CD signal in the Soret (B) band region. The corresponding DFT computational studies are in a good agreement with the experimental results and prove the presence of 1:2 host-guest complex as the major component in the solution (97.7%), but its optimized geometry differs from that observed in the solid-state. The UV-Vis and CD spectra simulated by using the solution-state geometry and the TD-DFT/ωB97X-D/cc-pVDZ + SMD (CH2Cl2) level of theory reproduced the experimentally obtained UV-Vis and CD spectra and confirmed the difference between the solid-state and solution structures. Moreover, it was shown that CD spectrum is very sensitive to the spatial arrangement of porphyrin subunits.

Near-Infrared Chiral Plasmonic Microwires through Precision Assembly of Gold Nanorods on Soft Biotemplates

Directing the assembly of plasmonic nanoparticles into chiral superstructures has diverse applications including, chiroptical sensing, nonlinear optics, and biomedicine. Though soft template-mediated assemblies of both spherical and nonspherical gold nanoparticles have made significant progress, most approaches require sophisticated chemical synthesis or advanced methodologies. Besides, reports of structurally precise chiral plasmonic assemblies beyond nanoscale are limited. Here, we propose an efficient yet simple strategy to grow such precision assemblies up to mesoscale, which is beneficial for a broader community. Briefly, cationic gold nanorods (AuNRs) are allowed to systematically assemble along atomically precise, chiral, rodlike tobacco mosaic virus (TMV) particles via electrostatic attraction under ambient condition. This leads to spontaneous formation of helical hybrid microwires with high structural precision, as evidenced by cryogenic transmission electron microscopy and tomography. Resulting composite superstructures show a strong circular dichroism response at the plasmon wavelength of the AuNRs, which is supported by simulations using discrete dipole approximation. Further, chirality of the system is investigated at a single-microwire level using polarized dark-field scattering microscopy. An alternative chiral template, negatively charged colloidal cellulose nanocrystals, also arrange AuNRs into similar chiral microstructures. Thus, our report proposes a generic methodology to obtain chiral plasmonic response at the NIR region using inexpensive templates that will encourage the exploration of a wider range of nanoscale templates for creating hybrid mesostructures with emerging optoelectronic properties.

Using Circular Dichroism to Control Energy Transfer in Multiphoton Ionization.

Chirality causes symmetry breaks in a large variety of natural phenomena ranging from particle physics to biochemistry. We investigate one of the simplest conceivable chiral systems, a laser-excited, oriented, effective one-electron Li target. Prepared in a polarized p state with |m|=1 in an optical trap, the atoms are exposed to co- and counterrotating circularly polarized femtosecond laser pulses. For a field frequency near the excitation energy of the oriented initial state, a strong circular dichroism is observed and the photoelectron energies are significantly affected by the helicity-dependent Autler-Townes splitting. Besides its fundamental relevance, this system is suited to create spin-polarized electron pulses with a reversible switch on a femtosecond timescale at an energy resolution of a few meV.

Giant and dynamically tunable plasmonic circular dichroism in graphene ribbons and Z-shaped metal metamaterials

Plasmonic chirality has drawn a lot of attention owing to its strong and tunable circular dichroism (CD) effects and its desirable applications in bio-sensing, chiral molecules analysis, etc. In this work, the CD response in the mid-infrared region of a metamaterial consisting of Z-shaped metal array and graphene ribbons has been studied theoretically. Benefiting from symmetry breaking and electromagnetic coupling effect, the strong CD signal is generated by the metamaterial and the maximum CD value could reach 22%, which is much larger than that of reported graphene-based chiral systems. In particular, the intensity and peak position of the CD response could be dynamically controlled by the Fermi level in a large range. Meanwhile, the CD response also could be actively controlled by changing the number and the width of graphene ribbons, the distance between the graphene ribbons and Z-shaped metal array, and so on. Furthermore, the CD effect is sensitive to the ambient medium. Therefore, the graphene ribbons and metal metamaterial could be potentially utilized in bio-sensing and related fields.

Chiral Pentaphenylpyrrole Derivatives with Aggregation-Induced Emission Enhancement and Aggregation Induced Circular Dichroism and Their Helical Self-assembly

A pair of enantiomers (S)-P5PdiAM and (R)-P5PdiAM containing two chiral substituents, and their raceme rac-P5PdiAM were synthesized by introducing (R)-/(S)- or racemic 1-phenylethylamine into 1-biphenyl-2,3,4,5-tetraphenylpyrrole. The target compounds all show weak aggregation-induced emission enhancement (AIEE) characteristics in the DMSO/H2O mixed system, indicating that the introduction of chirality hardly affect the photophysical properties of the compound. In addition, the chiral compounds (S)-P5PdiAM and (R)-P5PdiAM show strong circular dichroism (CD) signals in THF/H2O mixed solution with a water fraction of 70%, that is, they exhibit aggregation-induced circular dichroism characteristics (AICD). Because the aggregates of (S)-P5PdiAM and (R)-P5PdiAM emit weakly, there is no circularly polarized luminescence (CPL) signals upon aggregation. The chiral compounds (S)-P5PdiAM and (R)-P5PdiAM can self-assemble to form helical nanofibers in a THF/H2O mixture with a water fraction of 70%, while the racemic rac-P5PdiAM assembled to form nano-particles, indicating that chiral substituents have a great influence on the self-assembly and the morphologies of the nano-aggregates.

Enhanced circular dichroism of plasmonic system in the strong coupling regime* *Project supported by the National Key R&D Program of China (Grant No. 2016YFA0301300), the Fundamental Research Funds for the Central Universities, China (Grant No. 2019XD-A09), and the National Natural Science Foundation of China (Grant No. 11574035).

The circular dichroism (CD) signal of a molecule is usually weak, however, a strong CD signal in optical spectrum is desirable because of its wide range of applications in biosensing, chiral photo detection, and chiral catalysis. In this work, we show that a strong chiral response can be obtained in a hybridized system consisting of an artificial chiral molecule and a nanorod in the strong coupling regime. The artificial chiral molecule is composed of six quantum dots in a helix assembly, and its CD signal arises from internal Coulomb interactions between quantum dots. The CD signal of the hybridized system is highly dependent on the Coulomb interactions and the strong coupling progress through the electromagnetic interactions. We use the coupled oscillator model to analyze strong coupling phenomenon and address that the strong coupling progress can amplify the CD signal. This work provides a scenario for designing new plasmonic nanostructures with a strong chiral optical response.

A Chiral Metal-Organic 1D-Coordination Polymer Upon Complexation of Phenylene-Bridged Bipyrrole and Palladium (II) Ion.

Metal-organic 1D-coordination polymers, having unique electronic and optical properties, are expected to be a novel advanced functional material capable of fabricating smart plastics, films, and fibers. In this study, we have synthesized a novel metal-organic 1D-coordination polymer composed of a phenylene-bridged bipyrrole bearing N-alkylimino groups (BPI) and palladium(II) ion. The BPI and Pd(II) form square planar bis(bidentate) complex to form a metal coordinated π-conjugation polymer (Poly-BPI/Pd). It is stable in solutions at room temperature, and allowed measurement of its average molecular weight in SEC (Mw = 106,000 and Mn = 18,000, Mw/Mn = 5.88). It also provided a reversible multi redox profile in cyclic voltammetry, most likely originating from strong π-electronic interactions between the BPI components via Pd ion. A variety of substituent groups can be attached to the imino-nitrogens of BPI. A coordination polymer composed of a BPI derivative bearing chiral alkyl chains and Pd(II) showed strong circular dichroism (CD) in the solution due to the unidirectional chiral conformation of the BPI components in the polymer backbone.