Planar Chirality Research Articles

Circularly Polarized Luminescence of Some [2]Paracyclo[2](5,8)quinoliphane Derivatives with Planar and Central Chirality

AbstractWith the aim of finding and characterizing new simple organic molecules (SOM) generating circularly polarized luminescence (CPL), we test here nine [2]paracyclo[2](5,8)‐quinoliphane derivatives, whose circular dichroism (CD) spectra were previously measured at short wavelengths in a study encompassing also OR and VCD (CD in the IR). Completion of the CD spectra is carried out with attention paid to weaker and longer wavelength features. The observed signs of the CPL spectra are compared to the signs of the CD band at the longest wavelength and are found coincident except in two cases. The magnitudes of dissymmetry ratios g in luminescence are of the order 10−3, like the corresponding |g| values for the absorption case. TD‐DFT calculations for the two simplest cases provide details on the molecular orbitals involved in the transitions responsible for the observed signals of these molecules. The calculations allow us to provide also a first explanation of the two observed behaviors of the |glum|/|gabs| ratios.

Syntheses and Chiroptical Properties of Optically Active V‐Shaped Molecules Based on Planar Chiral [2.2]Paracyclophane

AbstractTwo optically active molecules were prepared using enantiopure planar chiral [2.2]paracyclophane as chiral building blocks, in which π‐electron systems were stacked at 120° to form a V‐shaped structure. The optical and chiroptical properties of the synthesized molecules were investigated experimentally and theoretically. The chiroptical properties depended strongly on the orientation of the stacked π‐electron systems; in other words, these properties could be regulated by changing the orientation of the stacked π‐electron systems.

Chiral response of spin-spiral states as the origin of chiral transport fingerprints of spin textures

The transport properties of nontrivial spin textures are coming under closer scrutiny as the amount of experimental data and theoretical simulations is increasing. To extend the commonly accepted yet simplifying and approximate picture of transport effects taking place in systems with spatially varying magnetization, it is important to understand the transport properties of building blocks for spin textures---the homochiral spin-spiral states. In this work, by referring to phenomenological symmetry arguments based on the gradient expansion, and explicit calculations within the Kubo framework, we study the transport properties of various types of spin-spirals in a two-dimensional model with strong spin-orbit interaction. In particular, we focus on the contributions to the magnetoconductivity, the planar Hall effect, and the anomalous Hall effect, which are sensitive to the sense of chirality of the spiral states. We analyze the emergence, symmetry, and microscopic properties of the resulting chiral magnetoconductivity, chiral planar Hall effect, and chiral Hall effect in terms of spin-spiral propagation direction, cone angle, spiral pitch, and disorder strength. Our findings suggest that the presence of spin-spiral states in magnets can be readily detected in various types of magnetotransport setups. Moreover, the sizable magnitude of chiral contributions to the conductivity of skyrmions estimated from homochiral spirals implies that chiral, as opposed to topological, magnetotransport can play a prominent role for the detection of nontrivial spin textures.

Synthesis of Bridged Indigos and Their Thermoisomerization and Photoisomerization Behaviors.

Bridged indigos were synthesized by bridging the two nitrogen atoms in the indigo structure with a carbon chain, and their properties were carefully examined. These bridged indigos have intrinsic planar chirality, and the enantiomers were separated using chiral high-performance liquid chromatography. When the chiral bridged indigos were subjected to thermo- and photoisomerization, the corresponding (Z)-indigo was not observed at all, and racemization was observed. This phenomenon is caused by the low activation energy of inversion due to the 1.5 bond order of the double bond of the indigo skeleton and the large energy difference between the ground states of (E)-indigo and (Z)-indigo.

Recent Progress in Circularly Polarized Luminescence of [2.2]Paracyclophane Derivatives

Abstract[2.2]Paracyclophane (PCP) and its derivatives have attracted growing widespread interest in recent years due to their unique chemical structures, specific photophysical properties and potential chiroptical activities, especially in the study of circularly polarized luminescence (CPL). Among the CPL‐active small organic molecules, the introduction of a PCP skeleton with planar chirality is one of the most effective strategies to obtain emitters with both high photoluminescence quantum yields (PLQYs) and luminescent dissymmetry factors (glum). At present, the development of CPL emitters based on the PCP skeleton is still limited and only a few molecular design strategies have been reported. This review summarizes and discusses the corresponding reports, and the emitters are sorted according to their chemical structures. It is expected that this review can provide researchers with a practical perspective in CPL studies and can provide inspiration for future research on CPL.

Photoinduced Irreversible Intramolecular Proton Transfer of Arnebinones B, D, and E: The Case of Photoenolization at the p-Benzoquinone-CH2/CH-π System.

Arnebinones B, E, and D (1-3) have been found to be sensitive to light, generating complex and diverse proton transfer products when triggered by light. A unique two-step irreversible intramolecular proton transfer of 1 produced five scalemic mixtures, of which four possessed intriguing dual planar chirality. The unprecedented orientation epimerization equilibrium of the intra-annular double bond was first observed and researched in the homologous meroterpenoids by HPLC monitoring and DFT calculations. A "p-benzoquinone-CH2/CH-π" moiety in the structure was the common key feature for the occurrence of this type of photoenolization reaction. The product transformation processes and universality of this photoinduced irreversible proton transfer reaction were analyzed together with the cytotoxic activities of arnebinones B, D, and E, and their photoreaction products.

Synthesis of a Mechanically Planar Chiral and Axially Chiral [2]Rotaxane.

In this study, we synthesized a [2]rotaxane that was both mechanically planar chiral and axially chiral, comprising a symmetrical bis-crown ether featuring a biphenyl moiety (as the macrocyclic component) and a symmetrical bis-ammonium salt (as the dumbbell-shaped component).

Stimuli-Responsive Chiroptical Switching.

"Chirality" governs many fundamental properties in chemistry and biochemistry. While early investigations on stereochemistry are primarily dedicated to static chirality, there is an increasing interest in the field of dynamic chirality (chiral switches). These chiral switches are essential in controlling the directionality in molecular motors. Dynamic chiralities are equally crucial in switchable stereoselectivity, switchable asymmetric catalysis and enantioselective separation. Herein, we limit our discussion to recent advances on stimuli-induced chiroptical switching of axial, helical, and planar chirality in response to external stimuli. We also discuss a few examples of applications of the switchable chirality.

Synthesis and Chiroptical Properties of Planar Chiral Azahelicenes Based on [2.2]Paracyclophane.

Three pairs of planar chiral heteroarenes were synthesized using palladium-catalyzed Buchwald-Hartwig coupling and hypervalent iodine-mediated oxidative cyclization from optically pure 4-amino[2.2]paracyclophane. Among them, an enantiomer of planar chiral azahelicene was found to have circularly polarized luminescence activity that was remarkably stronger than that of planar chiral heteroarenes.

Metal-to-Metal Distance Modulation by Ligand Design: A Case Study of Structure-Property Correlation in Planar Chiral Cyclophanyl Metal Complexes.

Multinuclear metal complexes have seen tremendous progress in synthetic advances, their versatile structural features, and emerging applications. Here, we conceptualize Metal‐to‐Metal distance modulation in cyclophanyl metal complexes by bridging ligand design employing the co‐facially stacked cyclophanyl‐derived pseudo‐geminal, ‐ortho, ‐meta, and ‐para constitutional isomers grafted with N‐, O‐, and P‐ containing chelates that allow the installation of diverse (hetero)metallic moieties in a distance‐defined and spatially‐oriented relation to one another. Metal‐to‐Metal distance modulation and innate transannular “through‐space” π–π electronic interactions via the co‐facially stacked benzene rings in cyclophanyl‐derived complexes as well as their specific stereochemical structural features (element of planar chirality) are crucial factors that contribute to the tuning of structure‐property relationships, which stand at the very center from the perspective of cooperative effects in catalysis as well as emerging material applications.

Recent Progress in Metal-Catalyzed [2+2+2] Cycloaddition Reactions

AbstractMetal-catalyzed [2+2+2] cycloaddition is a powerful tool that allows rapid construction of functionalized 6-membered carbo- and heterocycles in a single step through an atom-economical process with high functional group tolerance. The reaction is usually regio- and chemoselective although selectivity issues can still be challenging for intermolecular reactions involving the cross-[2+2+2] cycloaddition of two or three different alkynes and various strategies have been developed to attain high selectivities. Furthermore, enantioselective [2+2+2] cycloaddition is an efficient means to create central, axial, and planar chirality and a variety of chiral organometallic complexes can be used for asymmetric transition-metal-catalyzed inter- and intramolecular reactions. This review summarizes the recent advances in the field of [2+2+2] cycloaddition.1 Introduction2 Formation of Carbocycles2.1 Intermolecular Reactions2.1.1 Cyclotrimerization of Alkynes2.1.2 [2+2+2] Cycloaddition of Two Different Alkynes2.1.3 [2+2+2] Cycloaddition of Alkynes/Alkenes with Alkenes/Enamides2.2 Partially Intramolecular [2+2+2] Cycloaddition Reactions2.2.1 Rhodium-Catalyzed [2+2+2] Cycloaddition2.2.2 Molybdenum-Catalyzed [2+2+2] Cycloaddition2.2.3 Cobalt-Catalyzed [2+2+2] Cycloaddition2.2.4 Ruthenium-Catalyzed [2+2+2] Cycloaddition2.2.5 Other Metal-Catalyzed [2+2+2] Cycloaddition2.3 Totally Intramolecular [2+2+2] Cycloaddition Reactions3 Formation of Heterocycles3.1 Cycloaddition of Alkynes with Nitriles3.2 Cycloaddition of 1,6-Diynes with Cyanamides3.3 Cycloaddition of 1,6-Diynes with Selenocyanates3.4 Cycloaddition of Imines with Allenes or Alkenes3.5 Cycloaddition of (Thio)Cyanates and Isocyanates3.6 Cycloaddition of 1,3,5-Triazines with Allenes3.7 Cycloaddition of Aldehydes with Enynes or Allenes/Alkenes3.8 Totally Intramolecular [2+2+2] Cycloaddition Reactions4 Conclusion

Chiroptical Properties of Mechanically Interlocked Molecules

AbstractMechanically Interlocked Molecules (MIMs), such as catenanes, knots, and rotaxanes, are a class of molecules that possess mechanical bonds. These bonds constitute an entanglement in space between components of the MIMs that cannot be separated without breaking a participating covalent bond. Although it is possible to synthesize chiral MIMs with covalent stereogenic centers, this new bond opens up the possibilities of axial, planar and helical chirality, as well as topologically chiral compounds. A growing number of chiral MIMs have been investigated and employed in numerous applications, ranging from sensing to catalysis. While the investigation of the properties of MIMs is highly developed, it is also essential to study their chiroptical properties. In this review, we focus on the chiroptical properties of chiral MIMs. We discuss their electronic circular dichroism (ECD) since these properties have been analyzed in some detail in relation to their structures and stereochemistry. Although only a few examples have been described to date, we discuss the encouraging circularly polarized luminescence (CPL) properties of MIMs. The review also includes a recent study of vibrational circular dichroism (VCD) in mechanically planar chiral rotaxanes suggesting that this technique is a promising tool for analyzing the structures of MIMs. The study of the chiroptical properties of MIMs can have a large impact on their use in materials science or as catalysts. One of the key advantage of chiral MIMs is that they provide a means of generating efficient chiroptical switches that have a bright future in the context of multiple applications.

Principle of Chirality Hierarchy in Three-Blade Propeller Systems.

There are different kinds of molecular chirality, such as zero-dimensional point chirality, one-dimensional axial chirality, 2D planar chirality, and 3D chirality. When they coexist in one system, such as in helical structures of proteins and DNA, they form a chirality hierarchy. Earlier, we showed that the chirality propensity of a lower level in a hierarchy is dictated by that of a higher level and henceforth proposed the Principle of Chirality Hierarchy. In this work, we confirm the validity of this principle in the three-blade propeller molecular system. Our results show that the preference of the 0D chirality of a functional group in the propeller system is determined by the 1D chirality, and homochirality is also a remarkable feature for this system. The establishment and confirmation of the Principle of Chirality Hierarchy from this work should find important applications in asymmetric synthesis, macromolecular assembly, and many others.

Stereospecific on-Surface Cyclodehydrogenation of Bishelicenes: Preservation of Handedness from Helical to Planar Chirality.

Flattening helices while keeping the handedness: On-surface cyclodehydrogenation of bishelicene enantiomers leads stereospecifically to (M,M) and (P,P) chiral planar polyaromatic hydrocarbons. This is followed by their homochiral aggregation into a 2D conglomerate. Thermally induced cyclodehydrogenation proceeds stereospecifically to chiral, planar coronocoronene. Such a reaction is a special example of topochemistry in which enantiospecific conversion is supported by the alignment of the reactant by the surface.

Hexaphyrin-cyclodextrin hybrids: Getting larger, getting confused

The diversity of hexaphyrin-cyclodextrin hybrids (HCD), previously formed from the covalent assembly of regular hexaphyrin and [Formula: see text]-cyclodextrin subunits, has been increased following two main directions: enlarging the confined space provided by the cyclodextrin and tuning the N-core of the hexaphyrin aiming at bimetallic complexes. The larger [Formula: see text]-cyclodextrin unit was selectively functionalized with aldehyde linkers on its primary rim which were further reacted with 2 eq. of pentafluorophenyl tripyrrane in acidic conditions, followed by DDQ oxidation. Doubly-linked HCD hybrids were obtained with structural variations in the hexaphyrin macrocycles depending on the MSA concentration (2 vs. 200 mM) producing either regular ([26] rectangular/[28] Möbius) or doubly N-confused dioxo ([26] rectangular) scaffolds. For the latter, two isomers were isolated featuring mirror transoid skeletons and long-side meso-linking patterns, giving rise to planar chirality at the origin of intense and opposite electronic circular dichroism (ECD) spectra. Zn(II) complexation was then investigated leading to two main findings. (i) The regular [28] Möbius HCD afforded a mixture of monometallic Möbius Zn(II) complexes in the presence of acac ligand. ECD spectroscopy indicated a chirality transfer from the [Formula: see text]-cyclodextrin favoring a [Formula: see text] Möbius twist, but opposite to that arising from the narrower [Formula: see text]-cyclodextrin congener ([Formula: see text] twist favored). (ii) Owing to N3O coordination boxes, dissymmetric bimetallic Zn(II) complexes were readily formed with the doubly N-confused dioxo HCDs through a positive cooperative process. Both metal centers bind a DMAP ligand in axial positions, which is of interest for the

Simultaneous construction of axial and planar chirality by gold/TY-Phos-catalyzed asymmetric hydroarylation

The simultaneous construction of two different chiralities via a simple operation poses considerable challenge. Herein a cationic gold-catalyzed asymmetric hydroarylation of ortho-alkynylaryl ferrocenes derivatives is developed, which enable the simultaneous construction of axial and planar chirality. The here identified TY-Phos derived gold complex is responsible for the high yield, good diastereoselectivity (>20:1 dr), high enantioselectivities (up to 99% ee) and mild conditions. The catalyst system also shows potential application in the synthesis of chiral biaryl compounds. The cause of high enantioselectivity of this hydroarylation is investigated with density functional theory caculation.

Chirality Switching in Ferromagnetic Nanostructures Via Nanosecond Electric Pulses

AbstractThe stability of magnetism in reduced dimensions has become a major scientific agenda in the pursuit of implementing magnetic nanostructures as functional components in spintronic devices. Methods to probe and control magnetization states of such structures in a deterministic manner include use of spin polarized currents, photon absorption, and relatively recently, electric fields that tailor magnetoelectric coupling in multiferroic based structures. In theory, a short electric pulse is able to generate localized magnetic fields that can couple to the local magnetic dipoles electrodynamically. Here, using the Landau–Lifshitz–Gilbert formalism of magnetism dynamics combined with continuum Maxwell relations, the response of a ferromagnetic permalloy nanodisc to nanosecond electric field pulses is studied. The dynamics of the magnetic order of the nanodiscs during this process are examined and discussed. Ferromagnet nanodiscs, when below a critical size and in the absence of any external field, relax to a vortex phase as the ground state due to the demagnetizing field. Simulations demonstrate that the planar chirality of such a ferromagnet nanodisc can be switched via a time‐wise asymmetric electric field pulse on the order of a few ns duration that generates radially varying tangential magnetic fields. These fields couple to the vortex state of the nanodisc ferromagnet electrodynamically, revealing an effective and robust method to control chirality.

Multilevel Chirality Transfer from Amino Acid Derivatives to Circularly Polarized Luminescence-Active Nanoparticles in Aqueous Medium.

Chirality at different levels is widely observed in nature, but the clue to connect it all together, and the way chirality transfers among different levels are still obscure. Herein, a l-/d-lysine-based self-assembly system was constructed, in which two-step chirality transfer among three different levels was observed in aqueous solution. The chirality originated from the point chirality of amino acid derivatives l-/d-PyLys hydrochloride, and was transferred to the planar conformational chirality of water-soluble pillar[5]arene pR-/pS-WP5. Then, with the aid of pR-/pS-WP5, nanoparticles were formed that exhibited L-/R-handed circularly polarized luminescence with a dissymmetry factor of up to ±0.001, arising from pyrene chiral excimers. This multilevel chirality transfer not only provides a perspective to trace potential clues, and to pursue certain ways by which the chirality transfers, but also offers a strategy to create controllable CPL emission in aqueous media.

Ferroelectric polarization reversal in multiferroic MnWO4 via a rotating magnetic field up to 52 T

A long-standing issue in multiferroic ${\mathrm{MnWO}}_{4}$ is the electric polarization reversal when magnetic field is applied along the magnetic easy axis $(H||u)$. In this work, we performed comprehensive ferroelectric polarization measurements on ${\mathrm{MnWO}}_{4}$ in fields up to 52 T and with rotating field direction in the $ac$ plane. When the field is rotated from $H||c$ to $H||u$, the induced high-field phase $(\ensuremath{-}P||b)$ favors a sign reversal relative to the low-field $+P||b$ phase, which is attributed to the vector spin chirality changing from one side of the spin connecting vector to the other. Particularly, further slight deviation of the field from the magnetic easy axis towards $H||a$ causes an abrupt reversal of the high-field phase from $\ensuremath{-}P||b$ to $+P||b$. We assume that this unusual angular dependence of polarization behavior can be understood by magnetic control of the helicity of the chiral plane with opposite conical spin structures. In addition, we also propose electric-field control of the chiral plane close to the ferroelectric phase transition at high fields. This study results in an overall understanding of this important multiferroic material and sheds light on field-induced polarization switching in related compounds.

Planar Chiral B-N Heteroarenes Based on [2.2]Paracyclophane as Circularly Polarized Luminescence Emitters.

Planar chiral boron-nitrogen heteroarenes based on [2.2]paracyclophane were successfully synthesized in a few steps as a new family of circularly polarized luminescence emitters. It represents the first case of boron-nitrogen heteroarenes with planar chirality. Those compounds have been demonstrated to exhibit strong circularly polarized luminescence signals and high quantum yields, in both solution and doped film (with glum up to 5.0 × 10-3 and Φsolution up to 73%).