An Isosteric Triaza Analogue of a Polycyclic Aromatic Hydrocarbon Monkey Saddle

Since a few years, the interest in negatively-curved fused polycyclic aromatic hydrocarbons (PAHs) has significantly increased. Recently, the first chiral negatively-curved PAH with the topology of a monkey saddle was introduced. Herein the synthesis of its triaza congener is reported. The influence of this CH↔N exchange on photophysical and electrochemical properties is studied as well as the isomerization process of the enantiomers. The aza analogue has a significantly higher inversion barrier, which makes it easier to handle at room temperature. All experimental results are underpinned by theoretical DFT calculations.     

Influence of terminal alkyl chain length on helical twisting property of chiral 1,2-propanediol derivatives

In this study,a novel series of chiral 1,2-propanediol derivatives with different terminal alkyl chain length were synthesized and characterized by FT-IR,~1H NMR and DSC.After doped into a nematic liquid crystal host,all the chiral dopants induced chiral nematic liquid crystals exhibiting a helix inversion with temperature variation.The results indicate that terminal alkyl chain length has a prominent influence on helical twisting property of the chiral dopants.With increasing the terminal alkyl chain le...     

Catalytic and Enantioselective Synthesis of Chiral Multisubstituted Tribenzothiepins by Intermolecular Cycloadditions

The first catalytic and highly enantioselective synthesis of tribenzothiepin derivatives was achieved. Two types of intermolecular cycloadditions using either diphenyl‐sulfide‐tethered diynes or 2‐phenyl sulfanylbenzene‐tethered diynes with a monoalkyne successfully gave chiral multisubstituted tribenzothiepins in good to excellent ee values under mild conditions. The inversion energy of this saddle‐shaped molecule was calculated by measurement of the racemization rate of chiral tribenzothiepins using the Eyring kinetic equation under heating conditions. The present protocol could also be used to prepare a chiral tribenzoselenepin.     

Tuning the helical twisting power of nematic liquid crystals induced by chiral 1, 2-propanediol derivatives using varied substituents

In this study,a novel series of chiral 1,2-propanediol derivatives with different electron-donating and electron-withdrawing groups were synthesized and characterized by FT-IR and ~1H NMR.The helical twisting properties of all the chiral dopants were investigated by doping the chiral dopants into a nematic liquid crystal host(SLC-1717).The results indicate that the donor-acceptor electron effect have a prominent influence on helical twisting property of the chiral nematic phase induced by the chiral dopants. Introducing electron-withdrawing groups into the terminal ends of chiral 1,2-propanediol can decrease the absolute values of the helical twisting power.In addition,the helix inversion temperatures of the induced chiral nematic phase are variational with the change of terminal groups.     

Chiral separation of disease biomarkers with 2‐hydroxycarboxylic acid structure

Chiral 2‐hydroxycarboxylic acids are compounds that have been linked to particular diseases and are putative biomarkers with some diagnostic potential. The importance of identifying whether a particular enantiomer is related to certain diseases has been encouraged recently. However, in many cases it has not yet been elucidated whether there are stereochemical implications with respect to these biomarkers and whether their enantioselective analysis provides new insights and diagnostic potential. In this study 13 disease‐related chiral 2‐hydrocarboxylic acids were studied for their chiral separation by high‐performance liquid chromatography on three cinchona alkaloid‐derived chiral stationary phases. From a subgroup of eight 2‐hydroxymonocarboxylic acids, baseline resolution could be achieved and inversion of elution order by exchanging tert‐butylcarbamoyl quinidine chiral stationary phase (Chiralpak QD‐AX) for the corresponding quinine analogue (Chiralpak QN‐AX) is shown for seven of them. Furthermore, conditions for chiral separation of the 2‐hydroxydicarboxylic acids, citramalic acid, 2‐isopropylmalic acid, and 2‐hydroxyadipic acid are reported and compared to the previous reported conditions for 2‐hydroxyglutaric acid and malic acid.     

Mechanical Motion of Chiral Azobenzene Crystals with Twisting upon Photoirradiation

The photomechanical motion of chiral crystals of trans‐azobenzene derivatives with an (S)‐ and (R)‐phenylethylamide group was investigated and compared with a racemic crystal. Changes in the UV/Vis absorption spectra of the powdered crystals before and after UV irradiation were measured by using an optical waveguide spectrometer, showing that the lifetime of the cis‐to‐trans thermal back‐isomerization of the chiral crystals was faster than that of the racemic crystals. Upon UV irradiation, a long plate‐like chiral microcrystal bent away from the light source with a twisting motion. A square‐like chiral microcrystal curled toward the light with some twisting. Reversible bending of a rod‐like chiral microcrystal was repeatable over twenty‐five cycles. In contrast, bending of a plate‐like racemic microcrystal was small. A possible mechanism for the bending and twisting motion was discussed based on the optimized cis conformer determined by using calculations, showing that the bending motion with twisting is caused by elongation along the b axis and shrinkage along the a axis.     

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.     

A Single-Enantiomer Emitter Enabled Superstructural Helix Inversion for Upconverting and Downshifting Luminescence with Bidirectional Circular Polarization

The helical twisting tendency of liquid crystals (LCs) is generally governed by the inherent configuration of the chiral emitter. Here, we introduce the multistage inversion of supramolecular chirality as well as circularly polarized luminescence (CPL) by manipulating the ratio of single enantiomeric emitters (R-PCP) to LC monomers (5CB). Increasing the content of R-PCP from 1 wt % to 3 wt % inverted the helix of LCs from left-handed to right-handed, accompanying a CPL sign changed from positive to negative. The biaxiality of chiral emitters, as well as the steric effect of chiral-chiral and chiral-achiral interaction, were identified as the reasons for helical sense inversion. Due to the strong helical twisting power, 4 wt % R-PCP drove the photonic band gap (PBG) of chiral LCs to match up with their emission range, leading to an inversion of the CPL again with a high dissymmetry factor (≈1.2). Directly adjusting the PBG using chiral emitters is seldom achieved in cholesteric LCs. On this basis, an achiral sensitizer PtTPBP was assembled into the helical superstructure. The generation of triplet-triplet annihilation-induced upconverted CPL from R-PCP and the downshifting CPL from PtTPBP with opposite rotation was achieved in a single chiral LC system by tuning the position of the PBG.     

A Decatwistacene with an Overall 170° Torsion

Two different lengths of twistacenes, namely hexatwistacene and decatwistacene, induced by steric hindrance between imide groups and neighboring annulated benzene rings, were synthesized by bottom‐up synthesis of palladium‐catalyzed Suzuki cross‐coupling and C−H activation. Single‐crystal X‐ray analyses revealed that decatwistacene, which is the longest twistacene reported, exhibits an astonishing overall end‐to‐end torsion angle of about 170°, the largest torsion angle reported. Both twistacenes have an enhanced solubility and stability with respect to light and oxygen owing to their large twisting deformations together with much lower LUMO levels caused by the introduction of imide groups, opening a window to the narrowest chiral graphene nanoribbons with good stability and processability.     

Controllability of dynamic double helices: quantitative analysis of the inversion of a screw-sense preference upon complexation

We describe a quantitative analysis of the complexation-induced inversion of a screw-sense preference based on a conformationally dynamic double-helix structure in a macrocycle. The macrocycle is composed of two twisting units (terephthalamide), which are spaced by two strands (1,3-bis(phenylethynyl)benzene), and is designed to generate a double-helix structure through twisting about a C₂ axis in a conrotatory manner. The attachment of chiral auxiliaries to the twisting units induces a helical preference for a particular sense of (M)- or (P)-helicity through the intramolecular transmission of chirality to dynamic double helices. The twisting unit can also act as a binding site for capturing a guest molecule, and, in a complexed state, the preferred screw sense of the dynamic double-helix structure is reversed to exhibit the contrary preference. We quantitatively monitored the complexation-induced inversion of the screw-sense preference using ¹H NMR spectroscopy, which enabled us to observe independently two species with (M)- or (P)-helicity in both the absence and presence of a guest molecule. Inversion of the screw-sense preference was induced upon complexation with an achiral guest as well as a chiral guest.     

Solid‐state conformation of the 1,3,2‐oxazaphospholane ring in 2‐dialkylamino‐3,4‐dimethyl‐5‐phenyl‐1,3,2‐oxazaphospholane sulfides

Dialkylamino‐3,4‐dimethyl‐5‐phenyl‐1,3,2‐oxazaphospholane sulfides (where the alkyl is an isopropyl, ethyl or methyl group) was obtained by sulfurization of the respective phospholanes. The structures of these compounds were determined by X‐ray crystal structure analysis. Five‐membered rings exist in the envelope conformation in the crystalline state. The planar part of the 1,3,2‐oxazaphospholane rings undergoes twisting with the enlargement of a substituent. The environment of the P atom has the geometry of a distorted tetrahedron. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:146–150, 2001     

Light‐Directed Dynamic Chirality Inversion in Functional Self‐Organized Helical Superstructures

Helical superstructures are widely observed in nature, in synthetic polymers, and in supramolecular assemblies. Controlling the chirality (the handedness) of dynamic helical superstructures of molecular and macromolecular systems by external stimuli is a challenging task, but is of great fundamental significance with appealing morphology‐dependent applications. Light‐driven chirality inversion in self‐organized helical superstructures (i.e. cholesteric, chiral nematic liquid crystals) is currently in the limelight because inversion of the handedness alters the chirality of the circularly polarized light that they selectively reflect, which has wide potential for application. Here we discuss the recent developments toward inversion of the handedness of cholesteric liquid crystals enabled by photoisomerizable chiral molecular switches or motors. Different classes of chiral photoresponsive dopants (guests) capable of conferring light‐driven reversible chirality inversion of helical superstructures fabricated from different nematic hosts are discussed. Rational molecular designs of chiral molecular switches toward endowing handedness inversion to the induced helical superstructures of cholesteric liquid crystals are highlighted. This Review is concluded by throwing light on the challenges and opportunities in this emerging frontier, and it is expected to provide useful guidelines toward the development of self‐organized soft materials with stimuli‐directed chirality inversion capability and multifunctional host–guest systems.     

Lamellar‐Twisting‐Induced Circular Dichroism of Chromophore Moieties in Banded Spherulites with Evolution of Homochirality

Banded spherulites are formed by crystallization of a chiral polymer that is end‐capped with chromophore. Induced circular dichroism (ICD) of the chromophore can be found in the crystallized chiral polymers, giving exclusive optical response of the ICD. The ICD signals are presumed to be driven by the lamellar twisting in the crystalline spherulites, and the exclusive optical activity is attributed to the chirality transfer from molecular level to macroscopic level. To verify the suggested mechanism, the sense of the lamellar twisting in the crystalline spherulite is determined using PLM for the comparison with the ICD signals of the chromophore in the electron circular dichroism spectrum. The conformational chirality of the chiral polymer is determined by the vibrational circular dichroism spectrum. On the basis of the chiroptical results, evolution of homochirality from helical polymer chains (conformational chirality) to lamellar twisting in the banded spherulite (hierachical chirality) is suggested.     

BN‐Embedded Polycyclic Aromatic Hydrocarbon Oligomers: Synthesis,Aromaticity, and Reactivity

BN‐embedded oligomers with different pairs of BN units were synthesized by electrophilic borylation. Up to four pairs of BN units were incorporated in the large polycyclic aromatic hydrocarbons (PAHs). Their geometric, photophysical, electrochemical, and Lewis acidic properties were investigated by X‐ray crystallography, optical spectroscopy, and cyclic voltammetry. The B?N bonds show delocalized double‐bond characteristics and the conjugation can be extended through the trans‐orientated aromatic azaborine units. Calculations reveal the relatively lower aromaticity for the inner azaborine rings in the BN‐embedded PAH oligomers. The frontier orbitals of the longer oligomers are delocalized over the inner aromatic rings. Consequently, the inner moieties of the BN‐embedded PAH oligomers are more active than the outer parts. This is confirmed by a simple oxidation reaction, which has significant effects on the aromaticity and the intramolecular charge‐transfer interactions.     

Glycosyl‐Templated Chiral Helix Stapling of Ethynylpyridine Oligomers by Alkene Metathesis between Inter‐Pitch Side Chains

Ethynylpyridine polymers and oligomers consisting of 4‐substituted pyridine rings linked by acetylene bonds at the 2‐ and 6‐positions have been investigated. Ethynylpyridine oligomers covalently linked with a glycosyl chiral template form chiral helical complexes by intramolecular hydrogen bonding, in which the chirality of the template is translated to the helix. With a view to fixation of the chiral architecture, D /L ‐galactosyl‐ and D /L ‐mannosyl‐linked ethynylpyridine oligomers have been developed with 4‐(3‐butenyloxy)pyridine units having alkene side chains. The helical structures are successfully stapled by alkene metathesis of the side chains. Subsequent removal of the chiral templates by acidolysis produces template‐free stapled oligomers. The chiral, template‐free, stapled oligomers show chiral helicity, which is resistant to polar solvents and heating.     

Conformational properties of thiophene oligomers

The molecular geometries and the torsional potentials about the inter‐ring C‐C bond in α‐oligothiophenes (α‐nTh, n=2–4) have been calculated by means of conventional ab initio and density functional theory (DFT) calculations employing the hybrid B3LYP and BH&HLYP functionals. The position and the energetics of the critical points in the potential energy curve generated by rotation about the inter‐ring C? C bond are shown to be dependent on the computational method. DFT calculations, in comparison with MP2 calculations, favour conjugative interactions, while steric and coulombic interactions are equally treated by both methods. On oligomerization the electron delocalisation increases slightly, the p‐charge being preferentially confined within the rings, although it is sufficient to move the molecular structure towards co‐planarization and to increase the barrier through the perpendicular conformation. The IR and Raman spectra on the relevant rotamers of α‐2Th have been computed at HF/6–31G* and B3LYP/6–31G* levels. The comparison with the experiment is excellent. It has been found that small twisting from the planar conformation has no apparent effects, while 90° twisting and isomerization to the syn‐gauche form produce significant frequency and intensity variations which could be useful probes in conformational studies. The simulated IR and Raman spectra of the α‐2Th rotamers are consistent with a smallπ‐electron delocalisation between the rings.     

Odd-Even Effects on Transport Properties of Polycyclic Arene Molecular Devices with Decreasing Numbers of Benzene Rings

The electron transport properties of polycyclic aromatic hydrocarbons (PAHs) with different numbers of benzene rings tethered to narrow zigzag graphene nanoribbon (ZGNR) electrodes have been investigated. Results show that the transport properties of PAHs are dependent on whether the number of benzene rings in the width direction is odd or even. This effect is strong for narrow width PAHs, but its strength decreases as the width of the PAH is increased. PAHs with an odd number of rings exhibit poor transport properties, whereas the ones having an even number of rings show excellent transport properties coupled with a negative differential resistance (NDR) effect. Moreover, the linkage points and the structure of the molecules have a noticeable effect on the transport properties of the PAH, making the odd-even effect weaker or disappear entirely. Although the PAH with three benzene rings displays poor transport capabilities, it shows excellent rectification behavior compared to the other examined molecules. These studies present a feasible avenue for designing molecular devices with enhanced performance by the careful manipulation of the PAH molecular structure.     

1H NMR spectral properties and conformational preferences of some 1,2,9,10,17,18,25,26-octathia[2.2.2.2]-metacyclophanes

The stereochemistry in solution of three octathia[2.2.2.2]metacyclophanes has been investigated by means of ¹H NMR spectroscopy. The results lead to the conclusion that the saddle or crown conformation is preferred for these compounds, depending on the substitution pattern of the constituent aromatic rings.     

Synthesis of a Helical Bilayer Nanographene

A rigid, inherently chiral bilayer nanographene has been synthesized as both the racemate and enantioenriched M isomer (with 93 % ee) in three steps from established helicenes. This folded nanographene is composed of two hexa‐peri‐hexabenzocoronene layers fused to a [10]helicene, with an interlayer distance of 3.6 Å as determined by X‐ray crystallography. The rigidity of the helicene linker forces the layers to adopt a nearly aligned AA‐stacked conformation, rarely observed in few‐layer graphene. By combining the advantages of nanographenes and helicenes, we have constructed a bilayer system of 30 fused benzene rings that is also chiral, rigid, and remains soluble in common organic solvents. We present this as a molecular model system of bilayer graphene, with properties of interest in a variety of potential applications.     

Influence of a Change in Helical Twisting Power of Photoresponsive Chiral Dopants on Rotational Manipulation of Micro‐Objects on the Surface of Chiral Nematic Liquid Crystalline Films

Herein we report a group of five planar chiral molecules as photon‐mode chiral switches for the reversible control of the self‐assembled superstructures of doped chiral nematic liquid crystals. The chiral switches are composed of an asymmetrically substituted aromatic moiety and a photoisomerizing azobenzene unit connected in a cyclic manner through methylene spacers of varying lengths. All the molecules show conformational restriction in the rotation of the asymmetrically substituted aromatic moiety in both the E and Z states of the azobenzene units resulting in planar chirality with separable enantiomers. Our newly synthesized compounds in pure enantiomeric form show high helical twisting power (HTP) in addition to an improved change in HTP between the E and Z states. The molecule with a diphenylnaphthalene unit shows the highest ever known initial helical twisting power among chiral dopants with planar chirality. In addition to the reversible tuning of reflection colors, we employed the enantiomers of these five compounds in combination with four nematic liquid crystalline hosts to study their properties as molecular machines; the change in HTP of the chiral dopant upon photoisomerization induces rotation of the texture of the liquid crystal surfaces. Importantly, this study has revealed a linear dependence of the ratio of the difference between HTPs before and after irradiation against the absolute value of the initial HTP, not the absolute value of the change in helical twisting power between two states, on the angle of rotation of micro‐objects on chiral nematic liquid crystalline films. This study has also revealed that a change in irradiation intensity does not affect the maximum angle of rotation, but it does affect the speed of rotational reorganization of the cholesteric helix.