Chiral Switch Research Articles

Chirality‐Based Biosensors

AbstractThe fabrication of chiral nanostructures gives rise to characteristic chiroptical activity, which can be used for chirality‐based biosensors. Great progress is made in the use of nanoassemblies for the construction of chiral nanoparticle dimers, pyramids, helices, and twisted structures, and their chiroptical activities correlate with diverse structural geometries and enantiomeric configurations. In DNA‐hybridization‐based chiral nanoassemblies, the assembly parameters, such as the components, gaps, multicomponents, and the aftergrowth of metal, can result in multiple bands and enhanced chiroptical effects. Based on known chiral nanostructures, the existing chiral nanoassembly‐based biosensors together with their targets and signal amplification strategies are reviewed. Chirality involves multiple signals, and multitarget biosensors are introduced with newly developed chiral architectures for the accurate and reliable monitoring of biomarkers in living cells. The interactions between chiral nanoarchitectures and biosystems are also highlighted, which are important not only in the chiral dynamic switching of nano‐objects for biomonitoring, but also in manipulating cell growth, proliferation, and adhesion. The future perspectives on chiral fabrication and its use in biosensors are also comprehensively discussed.

Torsion-induced vortex switching and skyrmion-like state in ferroelectric nanodisks

The controllability of vortex state in ferroelectric nanodisks under the effect of external torsion is investigated in this work based on phase-field simulation. We discover a novel Bloch skyrmion-like state in ferroelectric nanodisks due to the combining effect of the torsion and the depolarization field. Moreover, a new strategy is proposed to achieve deterministic switching of the vortex chirality in the ferroelectric nanodisks. On the one hand, if a fixed external electric field is applied to the nanodisk, the vortex chirality can be switched by the torsion force. On the other hand, if we apply a fixed torsion force to the nanodisk, the vortex chirality can be readily switched by an external electric field. The feasibility of both mechanical and electrical switching of the vortex in the ferroelectric nanodisks is based on the trilinear coupling between the toroidization, polarization and shear strain of the system. The influences of temperature, electric field, torsion, and size of the nanodisk on the control of the vortex state are further revealed. Our findings shed light on the practical control and application of ferroelectric dipole vortices.

Driving a Liquid Crystal Phase Transition Using a Photochromic Hydrazone.

The dynamic manipulation of the properties of soft matter can lead to adaptive functional materials that can be used in advanced applications. Here we report on a new chiral dopant, built on an isosorbide scaffold attached to two bistable hydrazone-based light switches that can be used to control the self-assembly, and hence photophysical properties, of nematic liquid crystals (LCs). The bistability of the switch allows kinetic trapping of various helical assemblies as a function of the photostationary states, resulting in the reflection of different wavelengths of light. Surprisingly, doping 5CB with the chiral switch, followed by irradiation with blue light, triggers an isothermal phase change from the helical cholesteric phase to the untwisted lamellar smectic A* phase. This transition was used to modulate the transparency of a LC film, resulting in a light-gated optical window.

Chiral switching in biomineral suprastructures induced by homochiral l-amino acid.

How homochiral l-biomolecules in nature induce a chiral switch in biomineralized architectures is unknown, although chiral switching is common in many calcium carbonate-hardened structures found in marine and terrestrial organisms. We created hierarchically organized, chiral biomineral structures of calcium carbonate, whose chirality can be switched by a single l-enantiomer of an amino acid. The control of this chiral switching involves two stages: a calcium carbonate (vaterite) platelet layer inclination stage, followed by a platelet layer rotation stage, the latter stage being responsible for successional chiral switching events within the biomineralized structures. The morphology of the synthesized chiral vaterite structures remarkably resembles pathologic chiral vaterite otoconia found in the human inner ear. In general, these findings describe how a single-enantiomer amino acid might contribute to biomineral architectures having more than one chirality as is commonly seen in biology, and more specifically, they suggest how pathologic chiral malformations may arise in humans.

Optical Quenching of Magnetic Vortex Visualized In Situ by Lorentz Electron Microscopy

Understanding the fundamental dynamics of topological vortex and antivortex naturally formed in micro/nanoscale ferromagnetic building blocks under external perturbations is crucial to magnetic vortex based information processing and spintronic devices. All previous studies have focused on magnetic vortex-core switching via external magnetic fields, spin-polarized currents, or spin waves, which have largely prohibited the investigation of novel spin configurations that could emerge from the ground states in ferromagnetic disks and their underlying dynamics. Here, we report in situ visualization of femtosecond laser quenching induced magnetic vortex change in various symmetric ferromagnetic Permalloy disks by Lorentz phase imaging using 4D electron microscopy. Besides the switching of magnetic vortex chirality and polarity, we observed with distinct occurrence frequencies a plenitude of complex magnetic structures that have never been observed by magnetic field or current assisted switching. These complex magnetic structures consist of a number of newly created topological magnetic defects (vortex and antivortex) strictly conserving the topological winding number, demonstrating the direct impact of topological invariant on the magnetization dynamics in ferromagnetic disks. Their spin configurations show mirror or rotation symmetry due to the geometrical confinement of the disks. Combined micromagnetic simulations with the experimental observations reveal the underlying magnetization dynamics and formation mechanism of the optical quenching induced complex magnetic structures. Their distinct occurrence rates are pertinent to their formation-growth energetics and pinning effects at the disk edge. Based on these findings, we propose a paradigm of optical-quenching-assisted fast switching of vortex cores for the control of magnetic vortex based information recording and spintronic devices.

Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch.

We describe the synthesis and properties of 13,14-dimethylcethrene, a prototype of a chiral diradicaloid photochemical switch that can be transformed reversibly via conrotatory electrocyclization to its more stable closed form by light (630 nm) or heat and back to its open form by light (365 nm). This system illustrates how the chemical reactivity of a diradicaloid molecule can be translated into a switching function, which alters substantially all electronic parameters, namely, the HOMO–LUMO and the singlet–triplet (ST) energy gaps, and the degree of helical twist. As a result, distinct changes in the optical and chiroptical properties of this system were observed, which allowed us to monitor the switching process by a variety of spectroscopic techniques, including NMR, UV–vis, and CD. In comparison to the previously reported parent molecule cethrene, this system benefits from two methyl substituents installed in the fjord region, which account for the stability of the closed form against oxidation and racemization. The methyl substituents increase the ST energy gap of 13,14-dimethylcethrene by ∼4 kcal mol–1 in comparison to cethrene. Our DFT calculations reveal that the larger ST gap is a result of electronic and geometric effects of the methyl substituents and show the potential of related systems to act as magnetic switches at room temperature.

Spontaneous [formula omitted]-symmetry breaking and exceptional points in cavity quantum electrodynamics systems

Spontaneous symmetry breaking has revolutionized the understanding in numerous fields of modern physics. Here, we theoretically demonstrate the spontaneous time-reversal symmetry breaking in a cavity quantum electrodynamics system in which an atomic ensemble interacts coherently with a single resonant cavity mode. The interacting system can be effectively described by two coupled oscillators with positive and negative mass, when the two-level atoms are prepared in their excited states. The occurrence of symmetry breaking is controlled by the atomic detuning and the coupling to the cavity mode, which naturally divides the parameter space into the symmetry broken and symmetry unbroken phases. The two phases are separated by a spectral singularity, a so-called exceptional point, where the eigenstates of the Hamiltonian coalesce. When encircling the singularity in the parameter space, the quasi-adiabatic dynamics shows chiral mode switching which enables topological manipulation of quantum states.

Large-Voltage Tuning of Dzyaloshinskii-Moriya Interactions: A Route toward Dynamic Control of Skyrmion Chirality.

Electric control of magnetism is a prerequisite for efficient and low-power spintronic devices. More specifically, in heavy metal-ferromagnet-insulator heterostructures, voltage gating has been shown to locally and dynamically tune magnetic properties such as interface anisotropy and saturation magnetization. However, its effect on interfacial Dzyaloshinskii-Moriya Interaction (DMI), which is crucial for the stability of magnetic skyrmions, has been challenging to achieve and has not been reported yet for ultrathin films. Here, we demonstrate a 130% variation of DMI with electric field in Ta/FeCoB/TaO x trilayer through Brillouin Light Spectroscopy (BLS). Using polar magneto-optical Kerr-effect microscopy, we further show a monotonic variation of DMI and skyrmionic bubble size with electric field with an unprecedented efficiency. We anticipate through our observations that a sign reversal of DMI with an electric field is possible, leading to a chirality switch. This dynamic manipulation of DMI establishes an additional degree of control to engineer programmable skyrmion-based memory or logic devices.

Hidden Order and Haldane-Like Phase in Molecular Chains Assembled from Conformation-Switchable Molecules.

Topological properties of matters have attracted tremendous interest in the past years due to the scientific and technological importance. It is of great interest to discover the analogs of topological phases in molecular architectures, if the key constituents of the phases are properly mimicked. Using scanning tunneling microscopy, we demonstrate that quasi-1D molecular chains assembled from conformation-switchable dibenzo[ g, p]chrysene molecules show hidden antiparallel order analogous to the hidden antiferromagnetic order in the Haldane phase, a known topological phase of quantum spin-1 chains. This is realized by mimicking the spin degree of freedom with the intramolecular helicene chiral switches and by emulating the interspin antiferromagnetic coupling with intermolecular homochiral coupling. The discovery of the molecular analog of topological matters may inspire the search of molecular architectures with nontrivial topological properties.

Treadmilling analysis reveals new insights into dynamic FtsZ ring architecture.

FtsZ, the primary protein of the bacterial Z ring guiding cell division, has been recently shown to engage in intriguing treadmilling dynamics along the circumference of the division plane. When coreconstituted in vitro with FtsA, one of its natural membrane anchors, on flat supported membranes, these proteins assemble into dynamic chiral vortices compatible with treadmilling of curved polar filaments. Replacing FtsA by a membrane-targeting sequence (mts) to FtsZ, we have discovered conditions for the formation of dynamic rings, showing that the phenomenon is intrinsic to FtsZ. Ring formation is only observed for a narrow range of protein concentrations at the bilayer, which is highly modulated by free Mg2+ and depends upon guanosine triphosphate (GTP) hydrolysis. Interestingly, the direction of rotation can be reversed by switching the mts from the C-terminus to the N-terminus of the protein, implying that the filament attachment must have a perpendicular component to both curvature and polarity. Remarkably, this chirality switch concurs with previously shown inward or outward membrane deformations by the respective FtsZ mutants. Our results lead us to suggest an intrinsic helicity of FtsZ filaments with more than one direction of curvature, supporting earlier hypotheses and experimental evidence.

2D Chiroptical Nanostructures for High‐Performance Photooxidants

AbstractModulation of the chirality of solid‐like nanoscale membranous structures used as selective photooxidants is an important goal of chemical and materials science. Here, the fabrication of a chiral plasmonic nanoparticle monolayer film which is a highly selective photooxidant under circularly polarized light (CPL) in the visible‐light region is reported. The chiroptical activity of the film can be controlled by altering the amount and stereochemistry of amino acids. The experiments disclose that this stable and reusable catalyst is active in the selective oxidation of glucose enantiomers and CPL of opposite polarization gives around 10.3‐fold increase in conversion rates. The results reveal that the handedness of polarized light dominates the catalytic activity of the chiral film. It is demonstrated that the specific chiral binding of the amino acid ligands and the local field enhancement in the light‐limited regime regulates the selective photocatalytic performance, as confirmed by first‐principles density functional theory and physical field simulations. With the catalyst's signature ability for chiral recognition and switching of handedness of polarized light, the discovery provides a foundation for designing and tailoring chiral inorganic photooxidants. This research also sets an example for the development of light–matter interactions and polarized optics.

Dynamics of vortex domain walls in ferromagnetic nanowires – A possible method for chirality manipulation

The dynamic behaviors of vortex domain walls (VDWs) in ferromagnetic nanowires driven by a magnetic field above Walker breakdown field (Hw) were investigated using micromagnetic simulation. It was found when nanowire has proper geometrical dimensions, the VDW may oscillate in a chirality invariant mode or a chirality switching mode depending on applied field and damping constant. At fixed damping constant, the oscillation mode can be controlled by applied field – with the increase of applied field, the oscillation of VDW change from a chirality invariant mode to a variant one. As the oscillation of VDW changes from chirality invariant regime to chirality switching regime, the oscillation frequency and amplification will undergo an abnormal change, which may offer a fingerprint for the switch of oscillation mode. Our finding proposes a simple way to control the chirality of a VDW by properly manipulating nanowire geometry and applied field, which may have important applications in VDW-based devices.

Defect-mediated vortex multiplication and annihilation in ferroelectrics and the feasibility of vortex switching by stress

The possibility of switching the direction of the dipole toroidal moment in ferroelectrics provides exciting opportunities for development of novel nanoscale memory and logic devices. However, a practical control of vortex chirality is rather challenging at present stage, not to mention via mechanical methods. In this paper, we performed the phase-field simulations to show that mechanical switching of vortex chirality can be achieved in ferroelectric nanoplatelet via defect engineering. After introducing a void defect in the nanoplatelet, relative stability of single-vortex state and multi-vortices state is found to be altered. Importantly, during stress-induced vortex multiplication process, the void is a favored nucleation core of new vortex; meanwhile, vortices tend to annihilate away from the void during a vortex annihilation process. As the favored regions of vortex nucleation and annihilation are not the same, a deterministic mechanical switching of vortex chirality can be achieved. The effects of temperature, shape of the nanoplatelet, void size, as well as void position, on the defect-mediated vortex switching behaviors are systematically revealed. Our study demonstrates the feasibility of vortex switching by mechanical loads and provides a route to control and develop electromechanical devices based on ferroic vortices.

Chiral Switches: Piecewise Phototuning of Self‐Organized Helical Superstructures (Adv. Mater. 8/2018)

Chiral Switches: Piecewise Phototuning of Self‐Organized Helical Superstructures (Adv. Mater. 8/2018)

Photochemically and Thermally Driven Full-Color Reflection in a Self-Organized Helical Superstructure Enabled by a Halogen-Bonded Chiral Molecular Switch.

Supramolecular approaches toward the fabrication of functional materials and systems have been an enabling endeavor. Recently, halogen bonding has been harnessed as a promising supramolecular tool. Herein we report the synthesis and characterization of a novel halogen-bonded light-driven axially chiral molecular switch. The photoactive halogen-bonded chiral switch is able to induce a self-organized, tunable helical superstructure, that is, cholesteric liquid crystal (CLC), when doped into an achiral liquid crystal (LC) host. The halogen-bonded switch as a chiral dopant has a high helical twisting power (HTP) and shows a large change of its HTP upon photoisomerization. This light-driven dynamic modulation enables reversible selective reflection color tuning across the entire visible spectrum. The chiral switch also displays a temperature-dependent HTP change that enables thermally driven red, green, and blue (RGB) reflection colors in the self-organized helical superstructure.

Photochemically and Thermally Driven Full‐Color Reflection in a Self‐Organized Helical Superstructure Enabled by a Halogen‐Bonded Chiral Molecular Switch

AbstractSupramolecular approaches toward the fabrication of functional materials and systems have been an enabling endeavor. Recently, halogen bonding has been harnessed as a promising supramolecular tool. Herein we report the synthesis and characterization of a novel halogen‐bonded light‐driven axially chiral molecular switch. The photoactive halogen‐bonded chiral switch is able to induce a self‐organized, tunable helical superstructure, that is, cholesteric liquid crystal (CLC), when doped into an achiral liquid crystal (LC) host. The halogen‐bonded switch as a chiral dopant has a high helical twisting power (HTP) and shows a large change of its HTP upon photoisomerization. This light‐driven dynamic modulation enables reversible selective reflection color tuning across the entire visible spectrum. The chiral switch also displays a temperature‐dependent HTP change that enables thermally driven red, green, and blue (RGB) reflection colors in the self‐organized helical superstructure.

Red, Green, and Blue Reflections Enabled in an Electrically Tunable Helical Superstructure

AbstractMaterials with tunable optical properties in the visible spectrum are intriguingly important in science and technology. Such desired tunability can be conveniently accomplished by controlling optical materials with electric field. Now an organic film of self‐organized helical superstructure producing electrically tunable reflection of visible light is introduced in this work. The composite is a dual‐frequency thermosensitive cholesteric (DFTC) material consisting of a liquid crystal host doped with a binary chiral‐dopant mixture including a thermosensitive chiral switch. The red, green, and blue reflections in the DFTC film can be obtained by varying the frequencies of a 45 Vrmsapplied voltage to induce dielectric heating. The resulting variation in temperature and, in turn, in helical pitch length, enables various wavelengths of reflection to render full‐color display in a single film without the need of film stacking. As far as it is known, this is the first work to demonstrate broad electrical tuning of cholesteric reflection in red, green, and blue.

Efficient visible-light full-color tuning of self-organized helical superstructures enabled by fluorinated chiral switches.

Light-driven chiral switches have the ability to tune and control the self-organized helical superstructures of cholesteric liquid crystals (CLCs), resulting in the photo-induced reflection wavelength shift of the CLCs. A new type of axially chiral switch functionalized with fluorine atoms ortho to the azobenzene moiety is found to exhibit reversible visible-light-driven photoisomerization due to a separation of the n–π* absorption bands of the trans and cis isomers. These chiral switches all have high HTP values and the doped CLCs with 15.8 wt% concentration demonstrates reversible dynamic tuning of the reflection color within the entire visible spectrum driven by 530 nm and 445 nm visible light. It is also noteworthy that the thermal stability is improved thanks to the cis form of the fluorinated azobenzenes possessing a remarkably long half-life. The newly designed visible-light-driven chiral switches may broaden the application of CLCs, especially in the fields where high energy UV light is unfavorable.

Correction: Guest-regulated chirality switching of planar chiral pseudo[1]catenanes.

Correction for 'Guest-regulated chirality switching of planar chiral pseudo[1]catenanes' by Ya-Fen Yang et al., Org. Biomol. Chem., 2018, DOI: 10.1039/c8ob00156a.

Chirality switching of the self-assembled CuPc domains induced by electric field.

Chiral switching of the self-assembled domains of CuPc molecules on the Cd(0001) surface has been investigated by means of a low temperature scanning tunneling microscopy (STM). With the coverage increasing, the CuPc molecules show the structural evolutions from an initial gas-like state to a network phase, a square phase, and finally to a compact phase at full monolayer. In the network and square phases, the achiral CuPc molecules reveal both the point chirality and chiral domains. In particular, the chirality of network domain can be switched from one enantiomer to another driven by the electric filed from a STM tip, which can also lead to the lattice rotation of network phase. These results demonstrate that (i) there is strong interaction between the CuPc molecules and STM tip; (ii) the adsorbed CuPc molecules carry considerable net charge or polarizability due to the charge transfer; (iii) the network phase has a low barrier for the interconversion between right- and left-handed domains. Our findings are significant for the understanding and control of the domain's chirality in the self-assembled structures.