Chiral Dimers Research Articles

The Influence of the Supporting Substrate on Single‐Particle Circular Differential Scattering of DNA Assembled Nanorod Dimers

AbstractSingle‐particle measurements of chiral nanostructures have the potential to offer more detailed insights compared to the ensemble‐averaged signals obtained in ensemble circular dichroism spectroscopy. For instance, single‐particle circular differential scattering (CDS) studies have revealed the effects of structural heterogeneity in chiral plasmonic nanostructures. However, differential light‐matter interactions of 3D chiral nanostructures caused by diverse orientations on the supporting substrate remain largely unexplored. Here, the CDS of DNA origami‐templated twisted gold nanorod (AuNR) dimers on a glass support is investigated. With the help of correlated scanning electron microscopy and electromagnetic simulations, it is demonstrated how the broad diversity of spectral lineshapes and signal intensities as well as sign of the CDS originates mainly from four different orientations of the chiral dimer once deposited from solution on the substrate and dried. Most surprisingly, it is found that even for the same enantiomer a sign reversal is possible depending solely on the dimer orientation when deposited on the support. These results are important for the correct interpretation of single‐particle chiroptical studies and furthermore provide valuable insights into the design of substrate‐supported chiral plasmonic metamaterials.

Structures and Properties of Axially Chiral (2E,4E,6Z,8Z)-Nona-2,4,6,8-Tetraenoate Derivatives Highly Substituted by Aryl groups.

Unprecedented (2E,4E,6Z,8Z)-nona-2,4,6,8-tetraenoate derivatives highly substituted by aryl groups have been synthesized by the reaction of rhodium complexes having aryl-substituted hexa-1,3,5-trienyl ligands with acrylates. These compounds have potential axial chirality, and their enantiomers are isolable by the chiral HPLC technique. Although the racemization barrier of isolated enantiomers was not high, it was found that a cyclic dimer synthesized by head-to-tail transesterification of a modified analog has quite a stable axial chirality even at a high temperature. From a structural analogy with tetraphenylethene, those compounds are emissive in the solid state, and the chiral cyclic dimer exhibits solid-state circularly polarized luminescence (CPL) activity.

Bent-shaped dimers with chiral spacer – unravelling the potential of the naphthyl mesogenic unit

ABSTRACT Chiral liquid-crystalline dimers are becoming increasingly significant for fundamental research and practical applications due to their unique mesomorphism resulting in specific optical and stimuli-responsive properties. Here, we describe the mesomorphic and chiral properties of a novel series of racemic and corresponding chiral bent-shaped dimers featuring a chiral flexible spacer, which differ in the position and number of naphthyl groups within the mesogenic units. The synthesised dimers exhibit racemic and chiral forms of the nematic (N/N*) and twist-bend nematic (NTB/N*TB) phases. Incorporating the naphthyl group enhances the stability of both nematic phases and promotes the formation of a chiral hierarchical structure in the N*TB phase. Dimers with a naphthyl group adjacent to the chiral centre demonstrate a much shorter helical pitch of the N* phase and higher helical twisting power (HTP) than those with a phenyl group in the same position. Moreover, they show temperature-dependent helix inversion, a phenomenon rarely observed in liquid crystalline materials with a single chiral centre. Our findings reveal that the incorporation of the naphthyl moiety in our chiral dimer structures not only enhances stability but also imparts unique properties, making these dimers promising candidates for further research and potential applications.

Chiral liquid crystal dimers with smectic phases for stabilization of anticlinic state in surface-stabilised geometry

The growing demand for enhanced optical quality and faster electro-optical response in liquid crystal (LC) photonic devices has expanded the search for advanced materials capable of improving the properties of LCs beyond conventional ones. Among these compounds are LC dimers with ferroelectric and antiferroelectric properties which can be incorporated into LC mixtures to enhance performance. However, existing dimers often feature high melting temperatures, enthalpies, and other physicochemical characteristics that render them unsuitable as dopants in antiferroelectric liquid crystal (AFLC) mixtures targeted to modern photonic devices. Here, we show the design and physicochemical properties of novel dimers which are characterized by low melting points, high tilt angles and spontaneous polarizations, temperature-stable helical structures, and reasonably broad temperature ranges of ferroelectric or antiferroelectric phases. These features have facilitated the effective use of one dimer as a dopant in AFLCs. This eliminates the primary drawback of the electro-optical effect based on the surface-stabilised geometry of AFLCs by strongly promoting the anticlinic state in this effect. Importantly, for the first time, the above can be achieved without affecting any other physicochemical or optical properties of AFLCs. Thus, the obtained results represent a pivotal link between the longstanding concept of developing AFLCs that meet the stringent requirements for electro-optical effects based on surface-stabilised geometry and the realization of novel photonic devices demonstrating all the benefits of this effect.

Simulation of Vibrational Circular Dichroism Spectra Using Second-Order Møller-Plesset Perturbation Theory and Configuration Interaction Doubles.

We present the first single-reference calculations of the atomic axial tensors (AATs) with wave function-based methods including dynamic electron correlation effects using second-order Møller-Plesset perturbation theory (MP2) and configuration interaction doubles (CID). Our implementation involves computing the overlap of numerical derivatives of the correlated wave functions with respect to both nuclear displacement coordinates and the external magnetic field. Out test set included three small molecules, including the axially chiral hydrogen molecule dimer and (P)-hydrogen peroxide, and the achiral H2O. For our molecular test set, we observed deviations of the AATs for MP2 and CID from that of the Hartree-Fock (HF) method upward of 49%, varying with the choice of basis set. For (P)-hydrogen peroxide, electron correlation effects on the vibrational circular dichroism (VCD) rotatory strengths and corresponding spectra were particularly significant, with maximum deviations of the rotatory strengths of 62 and 49% for MP2 and CID, respectively, using our largest basis set. The inclusion of dynamic electron correlation to the computation of the AATs can have a significant impact on the resulting rotatory strengths and VCD spectra.

Cryo-tomography and 3D Electron Diffraction Reveal the Polar Habit and Chiral Structure of the Malaria Pigment Crystal Hemozoin.

Detoxification of heme in Plasmodium depends on its crystallization into hemozoin. This pathway is a major target of antimalarial drugs. The crystalline structure of hemozoin was established by X-ray powder diffraction using a synthetic analog, β-hematin. Here, we apply emerging methods of in situ cryo-electron tomography and 3D electron diffraction to obtain a definitive structure of hemozoin directly from ruptured parasite cells. Biogenic hemozoin crystals take a striking polar morphology. Like β-hematin, the unit cell contains a heme dimer, which may form four distinct stereoisomers: two centrosymmetric and two chiral enantiomers. Diffraction analysis, supported by density functional theory analysis, reveals a selective mixture in the hemozoin lattice of one centrosymmetric and one chiral dimer. Absolute configuration has been determined by morphological analysis and confirmed by a novel method of exit-wave reconstruction from a focal series. Atomic disorder appears on specific facets asymmetrically, and the polar morphology can be understood in light of water binding. Structural modeling of the heme detoxification protein suggests a function as a chiral agent to bias the dimer formation in favor of rapid growth of a single crystalline phase. The refined structure of hemozoin should serve as a guide to new drug development.

Axially chiral dihydrophenanthrene dimers from Pholidota yunnanensis with anti-neuroinflammatory activities

Axially chiral compounds are well known in medicinal chemistry of natural products, but their absolute configurations and bioactivities are rarely reported and studied. In this study, eleven undescribed axially chiral dihydrophenanthrene dimers, as well as twenty-five known dihydrophenanthrenes, were isolated from the entire plant of Pholidota yunnanensis. Their structures were elucidated by comprehensive spectroscopic analysis. A method for determining the absolute configurations of enantiomers was developed based on the rotational barriers and calculated ECD spectra. Additionally, the activities of all isolated compounds were assessed in LPS-induced BV-2 microglial cells. Most dihydrophenanthrenes exhibited significant NO inhibitory activities, and compound 7 showed the most potent inhibitory effect with an IC50 value of 1.5 μM, compared to the positive control minocycline. The immunofluorescence and western blot results revealed that compound 7 suppressed the expression of Iba-1, iNOS and COX-2 in LPS-stimulated BV-2 microglial cells.

Enhancing the circular dichroism of chiral dielectric nanostructure through the addition of a symmetric Au cylinder

Circular dichroism (CD) spectra play a crucial role in recognition, separation and detection of chiral molecules. Due to the inherent weak CD response of natural chiral molecules, researchers have endeavored to enhance CD signals through various artificial nanostructures. In this study, combining the advantages of both the dielectric and metal materials, we propose a hybrid dielectric-metal nanostructure consisting of a chiral Si nanorod dimer coupled with a symmetric Au cylinder to achieve robust CD responses. Owing to the plasmon resonance of the Au cylinder, the scattering-CD and absorption-CD of the hybrid system have been enhanced, which result in the enhanced extinction-CD response. Furthermore, the distributions of electric field, magnetic field and displacement current density of both the Si dimer and hybrid nanostructure have been meticulously crafted to elucidate the physical mechanisms underlying amplified CD signals. The synergistic coupling between the magnetic fields of dielectric materials and the electric fields of the Au cylinder leads to an increase in the electric field strength and the asymmetry of near-field distributions. Additionally, spatial overlaps between electric and magnetic fields occur. These factors contribute to the enhanced chiral response of the hybrid system. Meanwhile, the CD signal can be flexibly tuned by adjusting the size of the Au cylinder and Si nanorods. This design offers a versatile approach to enhancing the chiral response of dielectric nanostructures.

Search for Chirality in Hydrogenated Magnesium Nanosilicates: A DFT and TD-DFT Investigation.

The formation of silicate grains in the interstellar medium (ISM), especially those containing chiral surfaces such as clinopyroxenes, is poorly understood. Moreover, silicate interactions with various forms of hydrogen-proton (H+), neutral H (HI), and molecular hydrogen (H2) are of high importance as hydrogen comprises >90% of the ISM gas budget, and these species play important roles in the formation of new molecules in space. Furthermore, silicate surfaces catalyze the formation of H2 in the interstellar medium formed on dust grain surfaces by H-H association. The technical difficulty of in situ laboratory investigations of nanosilicate nucleation using astrophysically relevant environmental conditions makes computational chemistry a useful tool for studying potential nanosilicate structures. Furthermore, chiral surfaces interacting with chiral organic molecules could serve as templates that lead to the enantiomeric excess of l-amino acids and d-polyols detected in carbonaceous meteorites. However, in order for this effect to take place, an excess of one chiral form of a mineral is required to break the symmetry. This symmetry-breaking event could have been due to the asymmetric absorption of circularly polarized light by the nanosilicate as it traverses star-forming regions. We investigate this possibility using a metastable chiral form of an enstatite dimer as an input for density functional theory (DFT) and time-dependent (TD)-DFT calculations to obtain various properties and circular dichroism spectra. All in all, twenty-six magnesium nanosilicate structures were studied using varying degrees of hydrogenation: none, with HI, with H+, and with H2. The HSE06/aug-cc-pVQZ level of theory was used for the DFT calculations. TD-DFT calculations utilized the CAM-B3LYP/cc-pVQZ and ωB97X-D3/cc-pVQZ functional and basic set pairings. Results show that (1) all twenty-six structures have absorption bands that fall within the 0.6-28.3 μm range available with the newly launched James Webb Space Telescope and (2) there is a small enantioselective effect by UV-CPL on the eight chiral enstatite dimers (predicted g-values of up to 0.007). While the observed effect is small, it opens up the possibility that it is the inorganic material that becomes enantiomerically biased by UV-CPL, driving chiral enhancements in meteoric organic molecules.

Selective Formation of Homochiral Dimers by Intermolecular Charge Transfer on a hBN Nanomesh.

In this study, a comprehensive characterization was conducted on a chiral starburst molecule (C57H48N4, SBM) using scanning tunneling microscopy. When adsorbed onto the hBN/Rh(111) nanomesh, these molecules demonstrate homochiral recognition, leading to a selective formation of homochiral dimers. Further tip manipulation experiments reveal that the chiral dimers are stable and primarily controlled by strong intermolecular interactions. Density functional theory (DFT) calculations supported that the chiral recognition of SBM molecules is governed by the intermolecular charge transfer mechanism, different from the common steric hindrance effect. This study emphasizes the importance of intermolecular charge transfer interactions, offering valuable insights into the chiral recognition of a simple bimolecular system. These findings hold significance for the future advancement in chirality-based electronic sensors and pharmaceuticals, where the chirality of molecules can impact their properties.

The Interplay of Spacer Chirality and Parity in Mesogenic Dimers.

Introducing chirality into soft materials, including liquid crystals (LCs), profoundly impacts their self-organization and physical properties. In this study, we synthesized a novel series of LC dimers with a chiral center as part of their flexible spacer. The dimers were prepared in racemic and enantiomerically pure forms. Their spacer length and parity were varied to investigate the effect of spacer chirality and parity on mesomorphic behavior and on chiral induction in the nematic phase of achiral mesogens. Our results show that the even-membered chiral dimers only have chiral nematic phases. In contrast, the odd-membered dimers display rich mesomorphism, including the intriguing blue phase (BP) and chiral form of the twist-bend nematic phase (N*TB). The observed significant difference in the 3D surface morphology between the racemic and chiral forms of the N*TB phase suggests that the chiral moiety in the spacer promotes a chiral hierarchy. Furthermore, the chiral dimers show a prominent odd-even effect in the helical twisting power in nematic hosts. These findings highlight the importance of the position of the chiral group within the dimeric molecule and provide new insights into how intrinsic chirality in the spacer affects the overall structural chirality.

Super-cooled chiral fluorescent liquid crystal dimers with reduced symmetry

We report on the design and synthesis of new chiral fluorescent liquid crystal dimers. The molecular design involves attaching two cholesteryl moieties to a resorcinol molecule, resulting in a structure with reduced symmetry and a bent angle of 120°, accompanied by a lateral cyano group. The influence of reduced symmetry, chirality, and bent angle on the mesophase of 4-cyano-1,3-phenylene bis(n-cholesteryloxy)alkanoates dimers are assessed. The synthesized dimers are characterized using standard spectroscopic techniques. The mesomorphic properties of these chiral dimers are evaluated through polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction measurements. Remarkably, all the dimers exhibit either the N*/ SmA phase, or both. The dimers demonstrate fluorescence and organize themselves into H-aggregates, as confirmed by UV-visible and fluorescence spectroscopic investigation. On further development, chiral fluorescence could be utilized for real-time visualization and tracking of cholesterol distribution within cell membranes, as well as in lipid-protein interactions.

Chiral cyanobiphenyl dimers – significance of the linking group for mesomorphic properties and helical induction

Chiral cyanobiphenyl dimers display rarely observed room-temperature twist-bend nematic (NTB) phase when the linking group is an ester, while the amide analogues are strong inducers of helical organization in the chiral nematic phase.

Solvent-free synthesis and chiroptical properties of a C-N axially chiral cruciform dimer of benzo[b]phenoxazine.

A novel C-N axially chiral molecule composed of two tert-butyl-substituted benzo[b]phenoxazine (BPO) was synthesized via solvent-free reactions. The absolute configurations of the enantiomers were determined by X-ray single-crystal analysis. The enantiomers had a sufficiently high racemization barrier to ignore racemization at room temperature (149 ± 20 kJ mol-1), and the solutions exhibited dual circularly polarized emissions stemming from fluorescence and phosphorescence of |gCPL| = ca. 1 × 10-3.

Chiral Gold Nanorods with Five‐Fold Rotational Symmetry and Orientation‐Dependent Chiroptical Properties of Their Monomers and Dimers

AbstractChiral plasmonic nanoparticles have attracted much attention because of their strong chiroptical responses and broad scientific applications. However, the types of chiral plasmonic nanoparticles have remained limited. Herein we report on a new type of chiral nanoparticle, chiral Au nanorod (NR) with five‐fold rotational symmetry, which is synthesized using chiral molecules. Three different types of Au seeds (Au elongated nanodecahedrons, nanodecahedrons, and nanobipyramids) are used to study the growth behaviors. Different synthesis parameters, including the chiral molecules, surfactant, reductant, seeds, and Au precursor, are systematically varied to optimize the chiroptical responses of the chiral Au NRs. The chiral scattering measurements on the individual chiral Au NRs and their dimers are performed. Intriguingly, the chiroptical signals of the individual chiral Au NRs and their end‐to‐end dimers are similar, while those of the side‐by‐side dimers are largely reduced. Theoretical calculations and numerical simulations reveal that the different chiroptical responses of the chiral NR dimers are originated from the coupling effect between the plasmon resonance modes. Our study enriches chiral plasmonic nanoparticles and provides valuable insight for the design of plasmonic nanostructures with desired chiroptical properties.

Chiral Gold Nanorods with Five-Fold Rotational Symmetry and Orientation-Dependent Chiroptical Properties of Their Monomers and Dimers.

Chiral plasmonic nanoparticles have attracted much attention because of their strong chiroptical responses and broad scientific applications. However, the types of chiral plasmonic nanoparticles have remained limited. Herein we report on a new type of chiral nanoparticle, chiral Au nanorod (NR) with five-fold rotational symmetry, which is synthesized using chiral molecules. Three different types of Au seeds (Au elongated nanodecahedrons, nanodecahedrons, and nanobipyramids) are used to study the growth behaviors. Different synthesis parameters, including the chiral molecules, surfactant, reductant, seeds, and Au precursor, are systematically varied to optimize the chiroptical responses of the chiral Au NRs. The chiral scattering measurements on the individual chiral Au NRs and their dimers are performed. Intriguingly, the chiroptical signals of the individual chiral Au NRs and their end-to-end dimers are similar, while those of the side-by-side dimers are largely reduced. Theoretical calculations and numerical simulations reveal that the different chiroptical responses of the chiral NR dimers are originated from the coupling effect between the plasmon resonance modes. Our study enriches chiral plasmonic nanoparticles and provides valuable insight for the design of plasmonic nanostructures with desired chiroptical properties.

Thermo-/Mechano-Chromic Chiral Coordination Dimer: Formation of Switchable and Metastable Discrete Structure through Chiral Self-Sorting.

Although strong chiral self-sorting often emerges in extended covalent or supramolecular polymers, the phenomenon is generally weak in discrete assemblies (e.g., dimers and oligomers) of small molecules due to the lack of a cooperative growth mechanism. Consequently, chiral self-sorting has been overlooked in the design of switchable and metastable discrete supramolecular structures. Here, we report a butyl-benzo[h]quinoline-based iridium(III) complex (Bu-Ir) with helical chirality at its metal center, which forms preferentially a homochiral dimer and exhibits thermo-/mechano-chromism based on a monomer-dimer transformation. While a five-coordinate monomer is formed in a racemic or an enantiopure Bu-Ir solution at 25 °C, a six-coordinate homochiral dimer complex is formed almost exclusively at low temperatures, with a higher degree of dimerization in enantiopure Bu-Ir solution. Estimation of apparent dimerization binding constants (K) and thermodynamic parameters (ΔH and ΔS) based on variable temperature ultraviolet-visible (UV-vis) and 1H NMR spectra reveals a strong preference for homochiral dimerization (largest known value for the coordination complex, Khomo/Khetero > 50). Notably, crystals of the homochiral dimer are metastable, undergoing a distinct color change upon grinding (from yellow to red) due to mechanical cleavage of coordination bonds (i.e., a dimer to monomer transformation). A comparison with control compounds having different substituents (proton, methyl, isopropyl, and phenyl groups) reveals that Bu-Ir dimerization involves both strong homochiral self-sorting preference and connected thermo-/mechano-chromic behavior, which is based on matched propeller-shaped chirality and subtle steric repulsion between alkyl substituents that render the homochiral dimer switchable and metastable. These findings provide substantial insights into the emergence of dynamic functionality based on the rational design of discrete chiral assemblies.

Collective modes, relaxations, de Vries behavior and field influence in SmA and SmC*deVr phases of chiral liquid crystal dimer C-10Bms with siloxy spacer.

Investigations in chiral liquid crystal dimer with trisiloxy unit on spacer unit, viz. C-10Bms, are reported with respect to its resolved dipole moment. Occurrence of SmA and SmC*deVr LC phases was characterized by POM and DSC. Relatively enhanced thermal range of LC phases in dimer configuration was detailed. Tilt angle, hysteresis, polarization, collective modes and other low-frequency relaxations were investigated. Primary order parameter revealed validity of critical field model. Moderate tilt for viewing angle and electro-clinic response for switch speed in dimer were addressed. Hysteresis in SmC*deVr ensued storage capability for memories. Slightly higher activation energy Ea was attributed to siloxy chain on spacer. Goldstone and Soft modes were identified in SmC*deVr phase. Field influence on Goldstone mode was analyzed. Equivalence of thermal/Arrhenius and field activation energies was observed. Curie-Weiss law for SM revealed strong collective ferroelectric response. Temperature and field trends of dielectric relaxation parameters were addressed in terms of chiral center-based out-of-phase dipole moment. Figure of merit parameters in SmC*deVr estimated by dielectric loss revealed its relative performance far below the SmA-SmC*deVr phase transition.

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers.

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies gabs of up to 2.6 × 10-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.

Circularly polarized luminescence from porphyrin-containing (supra)molecular systems

Functional dyes showing circularly polarized luminescence (CPL) have attracted considerable attention. Porphyrins and porphyrinoids have characteristic properties useful for the development of CPL materials, although the examples of CPL-active porphyrins and porphyrinoids are still limited. Here we have summarized recent progress in CPL-active systems containing porphyrins or phthalocyanines, which are classified into the following four categories: (i) porphyrins with chiral sources; (ii) axially chiral porphyrin dimers; (iii) self-assembled porphyrins; (iv) porphyrin-sensitized CPL dyes.