Tunable Chiroptical Properties from the Plasmonic Band to Metal–Ligand Charge Transfer Band of Cysteine‐Capped Molybdenum Oxide Nanoparticles

Understanding the interactions between a semiconducting nanocrystal surface and chiral anchoring molecules could resolve the mechanism of chirality induction in nanoscale and facilitate the rational design of chiral semiconducting materials for chiroptics. Now, chiral molybdenum oxide nanoparticles are presented in which chirality is transferred via a bio‐to‐nano approach. With facile control of the amount of chiral cysteine molecules under redox treatment, circular dichroism (CD) signals are generated in the plasmon region and metal–ligand charge‐transfer band. The obtained enhanced CD signals with tunable lineshapes illustrate the possibility of using chiral molybdenum oxide nanoparticles as potentials for chiral semiconductor nanosensors, optoelectronics, and photocatalysts.     

A Quintuple [6]Helicene with a Corannulene Core as a C5‐Symmetric Propeller‐Shaped π‐System

The synthesis and structural analysis of a quintuple [6]helicene with a corannulene core is reported. The compound was synthesized from corannulene in three steps including a five‐fold intramolecular direct arylation. X‐ray crystallographic analysis revealed a C₅‐symmetric propeller‐shaped structure and one‐dimensional alignment in the solid state. The enantiomers of the quintuple [6]helicene were successfully separated by HPLC, and the chirality of the two fractions was identified by CD spectroscopy. A kinetic study yielded a racemization barrier of 34.2 kcal mol^?1, which is slightly lower than that of pristine [6]helicene. DFT calculations indicate a rapid bowl‐to‐bowl inversion of the corannulene moiety and a step‐by‐step chiral inversion pathway for the five [6]helicene moieties.     

Chiral molecular glass: synthesis and characterization of enantiomerically pure thiophene-based [7]helicene

Synthesis of thiophene-based [7]helicenes, which are functionalized for both design of organic chiral glasses with strong chiroptical properties and for further homologation to higher [n]helicenes, is reported. The key synthetic transformations are kinetic resolution of the intermediate diketone and the annelation step forming the center benzene ring by means of an intramolecular McMurry reaction. Based upon X-ray crystallographic determinations of the absolute configurations for (+)-enantiomers of the diketone and the [7]helicene, stereochemical correlation between the (R) axial chirality of the diketone and the (M) helical chirality of the [7]helicene is established. One such enantiopure trimethylsilyl-substituted [7]helicene possesses enchanced chiroptical properties and forms a chiral molecular glass.     

Helicenophyrins: Expanded Carbaporphyrins Incorporating Aza[5]helicene and Heptacyclic S‐Shaped Aza[5]helicene Motifs

Incorporation of phenanthrene into a hexaphyrin(1.1.1.1.1.0) frame resulted in intramolecular ring fusion, thus giving rise to chiral helicenophyrins. These molecules contain helicene and porphyrin features by incorporating either an aza[5]helicene or heptacyclic S‐shaped aza[5]helicene.     

Water‐Dependent Optical Activity Inversion of Chiral DNA–Silica Assemblies

Chirality is widely found in nature and is expressed hierarchically in many organic–inorganic hybrid materials. Optical activity (OA) is the most fundamental attribute of these chiral materials. In this study, we found that the OA of impeller‐like chiral DNA–silica assemblies (CDSAs) was inverted with the addition of water. The state of DNA under dry and wet conditions, and the dual chirality of chiral DNA layers and twisted helical arrays of opposite handedness in CDSAs were considered to exert predominant effects on the OAs. The circular dichroism (CD) responses for the dry CDSAs were mostly attributed to the chiral arrangement of DNA layers, whereas the opposite CD responses for the wet CDSAs primarily originated from twisted helical arrays of DNA molecules. The observed CD signals were a super‐position of the two opposing OA responses. The increase in the longitudinal relation of DNA molecules due to the recovery of a double‐helical structure of DNA in the presence of water was considered to be the reason for the increase in intensity of the CD signals that originated from the twisted helical array, which led to the inversion of OA of the CDSAs. The inversion of the plasmon‐resonance‐based OAs for the chiral‐arranged achiral Ag nanoparticles (NPs) located in the channels of the CDSAs in dry and wet states further confirmed the dual chirality of DNA packing. Such research on DNA assemblies and metal NPs with dual, opposite chirality assists in the understanding of DNA hierarchical chirality in living systems and the creation of macroscopic ordered helical materials and biosensors.     

Spontaneous transmission of chirality through multiple length scales

The hierarchical transfer of chirality in nature, from the nano‐, to meso‐, to macroscopic length scales, is very complex, and as of yet, not well understood. The advent of scanning probes has allowed chirality to be monitored at the single molecule or monolayer level and has opened up the possibility to track enantiospecific interactions and chiral self‐assembly with molecular‐scale detail. This paper describes the self‐assembly of a simple, model molecule (naphtho[2,3‐a]pyrene) that is achiral in the gas phase, but becomes chiral when adsorbed on a surface. This polyaromatic hydrocarbon forms a stable and reversibly ordered system on Cu(111) in which the transmission of chirality from single surface‐bound molecules to complex 2D chiral architectures can be monitored as a function of molecular packing density and surface temperature. In addition to the point chirality of the surface‐bound molecule, the unit cell of the molecular domains was also found to be chiral due to the incommensurate alignment of the molecular rows with respect to the underlying metal lattice. These molecular domains always aggregated in groups of three, all of the same chirality, but with different rotational orientations, forming homochiral “tri‐lobe” ensembles. At a larger length scale, these tri‐lobe ensembles associated with nearest‐neighbor tri‐lobe units of opposite chirality at lower packing densities before forming an extended array of homochiral tri‐lobe ensembles at higher converges. This system displayed chirality at a variety of size scales from the molecular (≈1 nm) and domain (≈5 nm) to the tri‐lobe ensemble (≈10 nm) and extended array (>25 nm) levels. The chirality of the tri‐lobe ensembles dictated how the overall surface packing occurred and both homo‐ and heterochiral arrays could be reproducibly and reversibly formed and interchanged as a function of surface coverage. Finally, these chirally templated surfaces displayed remarkable enantiospecificity for naphtho[2,3‐a]pyrene molecules adsorbed in the second layer. Given its simplicity, reversibility, and rich degree of order, this system represents an ideal test bed for the investigation of symmetry breaking and the hierarchical transmission of chirality through multiple length scales.     

Inversion of the Supramolecular Chirality of Nanofibrous Structures through Co‐Assembly with Achiral Molecules

To understand the behavior of chiral nanostructures, it is of critical importance to study how achiral molecules regulate the chirality of such nanostructures and what the main driving forces for the regulation processes are. In this work, the supramolecular chirality of helical nanofibers consisting of phenylalanine‐based enantiomers is inverted by achiral bis(pyridinyl) derivatives through co‐assembly. This inversion is mainly mediated by intermolecular hydrogen bonding interactions between the achiral additives and the chiral molecules, which may induce stereoselective interactions and different reorientations for the assembled molecules, as confirmed by calculations. This work not only exemplifies a feasible method to invert the helicity of chiral nanostructures by the addition of achiral molecules, but also provides a method to explore their functions in environments where chiral and achiral molecules are in close proximity.     

Calcul de chiralité quantitative par la méthode des moindres carrés

It is shown how the root mean square chiral index is computed without enumerating all the permutations of a set of n atoms. As an example, the quantitative measure of chirality is achieved for various helicene derivatives. It is also shown that the chiral index of a protein backbone is computable without enumeration.     

Rhodium‐Catalyzed Highly Diastereo‐ and Enantioselective Synthesis of a Configurationally Stable S‐Shaped Double Helicene‐Like Molecule

An S‐shaped double helicene‐like molecule (>99 % ee), possessing stable helical chirality, has been synthesized by the rhodium(I)/difluorphos complex‐catalyzed highly diastereo‐ and enantioselective intramolecular double [2+2+2] cycloaddition of a 2‐naphthol‐ and benzene‐linked hexayne. The collision between two terminal naphthalene rings destabilizes the helical chirality of the S‐shaped double helicene‐like molecule, but the introduction of two additional fused benzene rings significantly increases the configurational stability. Thus, no epimerization and racemization were observed even at 100 °C. The enantiopure S‐shaped double helicene‐like molecule forms a trimer through the multiple C?H???π and C?H???O interactions in the solid‐state. The trimers stack to form columnar packing structures, in which neighboring stacks have opposite dipole directions. The accumulation of helical structures in the same direction in the S‐shaped double helicene‐like molecule enhanced the chiroptical properties.     

Chirality of Anisotropic Metal Nanowires with a Distinct Multihelix

Two‐dimensional (2D) anisotropic silver nanowire (AgNW) arrays, fabricated inside chiral mesoporous silica (CMS), exhibited strong and tunable plasmon circular dichroism (CD) signals in the visible and near‐IR regions due to collective dipole coupling between the anisotropic AgNWs. The multihelix with a helical channel orientation and helical arrays of opposite handedness in CMS played a predominant effects on the transversal and longitudinal chirality of the AgNWs, respectively.This behavior differs from both isotropic‐nanoparticle and single‐helix‐induced CD responses. This system will provide new insight into the optical activity of metal inorganic nanoparticles capped with chiral organic molecules and assembled in chiral environments.     

Tuning Achiral to Chiral Supramolecular Helical Superstructures

The design and synthesis of achiral organic functional molecules which can assemble into a chiral with selective handedness in the absence of chiral substances is an important in understanding the role chirality plays within these systems. In this review, we described general approaches towards supramolecular chiral molecules the synthesis and self‐assembly of achiral molecule to active chiral molecules to investigate controlled supramolecular chiral nanostructures with their photoluminescent properties for rapid, sensitive and selective detection of analytes of choice. Various small molecules have been discussed for achiral to chiral along with induction of chirality and controlled chiral helical structures in detail. We discussed few examples where stimuli used to control the chirality such as temperature, pH etc. Finally, we will also explore on the photo responsive helicity properties of the aggregation induced emission active molecule such as tetraphenylethene conjugates.     

Autoamplification of Molecular Chirality through the Induction of Supramolecular Chirality

The novel concept for the autoamplification of molecular chirality, wherein the amplification proceeds through the induction of supramolecular chirality, is presented. A solution of prochiral, ring‐open diarylethenes is doped with a small amount of their chiral, ring‐closed counterpart. The molecules co‐assemble into helical fibers through hydrogen bonding and the handedness of the fibers is biased by the chiral, ring‐closed diarylethene. Photochemical ring closure of the open diarylethene yields the ring‐closed product, which is enriched in the template enantiomer.     

Self‐Replication and Amplification of Enantiomeric Excess of Chiral Multifunctionalized Large Molecules by Asymmetric Autocatalysis

Self‐replication of large chiral molecular architectures is one of the great challenges and interests in synthetic, systems, and prebiotic chemistry. Described herein is a new chemical system in which large chiral multifunctionalized molecules possess asymmetric autocatalytic self‐replicating and self‐improving abilities, that is, improvement of their enantioenrichment in addition to the diastereomeric ratio. The large chiral multifunctionalized molecules catalyze the production of themselves with the same structure, including the chirality of newly formed asymmetric carbon atoms, in the reaction of the corresponding achiral aldehydes and reagent. The chirality of the large multifunctionalized molecules controlled the enantioselectivity of the reaction in a highly selective manner to construct multiple asymmetric stereogenic centers in a single reaction.     

Lifting the mirror symmetry of metal surfaces: decoupling the electronic and physical manifestations of surface chirality

Naturally occurring metal surfaces possess planes of mirror symmetry on the nanometer-length scale. This mirror symmetry can be lifted and chirality "physically" conveyed onto a surface by adsorbing a chiral molecule. Until now, it has not been known whether the conveying of chirality is limited to just the physical structure or whether it goes deeper and permeates the electronic structure of the underlying surface. By using optically active second harmonic generation (OA-SHG), it is demonstrated that the adsorption of some, but not all, chiral molecules can reversibly, and without significant structural rearrangement, measurably lift the mirror symmetry of the surface electronic structure of a metal. It is proposed that the ability of a chiral molecule to place a significant "chiral perturbation" on the electronic structure of a surface is correlated to its adsorption geometry. The microscopic origins of the observed optical activity are also discussed in terms of classical models of chirality. The results of the study challenge current models of how chiral adsorbates induce enantioselectivity in the chemical/physical behavior of heterogeneous systems, which are based on geometric/stereochemical arguments, by suggesting that chiral electronic perturbations could play a role.     

Pillararene Host–Guest Complexation Induced Chirality Amplification: A New Way to Detect Cryptochiral Compounds

The contradiction between the rising demands of optical chirality sensing and the failure in chiral detection of cryptochiral compounds encourages researchers to find new methods for chirality amplification. Inspired by planar chirality and the host–guest recognition of pillararenes, we establish a new concept for amplifying CD signals of cryptochiral molecules by pillararene host–guest complexation induced chirality amplification. The planar chirality of pillararenes is induced and stabilized in the presence of the chiral guest, which makes the cryptochiral molecule detectable by CD spectroscopy. Several chiral guests are selected in these experiments and the mechanism of chiral amplification is studied with a non‐rotatable pillararene derivative and density functional theory calculations. We believe this work affords deeper understanding of chirality and provides a new perspective for chiral sensing.     

Chiral Amplification in Nature: Studying Cell‐Extracted Chiral Carotenoid Microcrystals via the Resonance Raman Optical Activity of Model Systems

Carotenoid microcrystals, extracted from cells of carrot roots and consisting of 95 % of achiral β‐carotene, exhibit a very intense chiroptical (ECD and ROA) signal. The preferential chirality of crystalline aggregates that consist mostly of achiral building blocks is a newly observed phenomenon in nature, and may be related to asymmetric information transfer from the chiral seeds (small amount of α‐carotene or lutein) present in carrot cells. To confirm this hypothesis, we synthesized several model aggregates from various achiral and chiral carotenoids. Because of the sergeant‐and‐soldier behavior, a small number of chiral sergeants (α‐carotene or astaxanthin) force the achiral soldier molecules (β‐ or 11,11′‐[D₂]‐β‐carotene) to jointly form supramolecular assemblies of induced chirality. The chiral amplification observed in these model systems confirmed that chiral microcrystals appearing in nature might consist predominantly of achiral building blocks and their supramolecular chirality might result from the co‐crystallization of chiral and achiral analogues.     

Chiral separation of helical chromenes with chloromethyl phenylcarbamate polysaccharide‐based stationary phases

Two chloromethyl phenylcarbamate‐based chiral stationary phases, one containing an amylose‐type chiral selector (Lux Amylose 2, from Phenomenex) and the other a cellulose‐type one (Lux Cellulose‐4, from Phenomenex), were successfully used for the chiral resolution of three helical chromenes featuring a helicene‐like structure. The compound bearing a phenyl substituent on the helicene‐like structure was enantioresolved at 25°C with Lux Cellulose‐4 and a n‐hexane/1‐propanol 99:1 v/v eluent. With a n‐hexane/2‐propanol 99.8:0.2 v/v mobile phase, the same column (operated at 35°C) provided the separation of the four isomers of the compound having a hexyl residue on the helicene‐like motif and an additional asymmetric carbon. Lux Amylose‐2 was necessary for the enantioseparation of the compound having the sole hexyl residue on the helical scaffold. For the last compound a n‐hexane/2‐propanol 99.8:0.2 v/v eluent was used, and the column temperature was fixed at 5°C. The enantiomer elution order was appraised by using electronic circular dichroism and theoretical calculations. Notably, different thermodynamics of retention and enantioseparation were observed for molecules with pronounced structural similarity, that is, the enantiomer pairs of the compound containing the additional asymmetric carbon atom. Indeed, both entropically and enthalpically controlled adsorption and separation processes were observed.     

Enantiomorphic Microvortex‐Enabled Supramolecular Sensing of Racemic Amino Acids by Using Achiral Building Blocks

Chiral analysis of bioactive molecules is of increasing significance in chemical and life sciences. However, the quantitative detection of a racemic mixture of enantiomers is a challenging task, which relies on complicated and time‐consuming multiple steps of chiral derivatization, chiral separation, and spectroscopic measurement. Herein, we show that, without the use of chiral molecules or pretreatment steps, the co‐assembly of amino acids with achiral TPPS₄ monomers controlled by enantiomorphic microvortices allows quantitative detection of racemic or enantiomeric amino acids, through analysis of the sign and magnitude of supramolecular chirality in different outlets of a microfluidic platform. A model demonstrates that chiral microvortices can induce an initial chiral bias by bending the sheet structure, resulting in supramolecular self‐assembly of TPPS₄ and amino acids of compatible chirality by the self‐sorting. This sensing system may find versatile applications in chiral sensing.     

Methods of studies on quantitative structure–activity relationships for chiral compounds

Chiral compounds are very important in drug development, organic synthesis, materials science, toxicology, or environmental chemistry. Therefore, for creating new drugs, several methods have been suggested in recent years. In several laboratories in the world, some new methods for the derivations of the parameters were constructed and used for studies on quantitative structure–activity/property relationships of chiral molecules. The algorithms reviewed in this paper involve Zargeb group chiral indices, chiral molecular connectivity index, chiral topological charge index, chiral A_m index, chiral indices based on the matrixes, chiral indices based on chiral product, conformation‐independent chirality code, conformation‐dependent chirality code, quantitative two‐dimensional chirality degrees of benzenoids, and so on. Copyright © 2012 John Wiley & Sons, Ltd.     

Chiral Gold Nanoclusters: Atomic Level Origins of Chirality

Chiral nanomaterials have received wide interest in many areas, but the exact origin of chirality at the atomic level remains elusive in many cases. With recent significant progress in atomically precise gold nanoclusters (e.g., thiolate‐protected Au_n (SR)_m ), several origins of chirality have been unveiled based upon atomic structures determined by using single‐crystal X‐ray crystallography. The reported chiral Au_n (SR)_m structures explicitly reveal a predominant origin of chirality that arises from the Au–S chiral patterns at the metal–ligand interface, as opposed to the chiral arrangement of metal atoms in the inner core (i.e. kernel). In addition, chirality can also be introduced by a chiral ligand, manifested in the circular dichroism response from metal‐based electronic transitions other than the ligand's own transition(s). Lastly, the chiral arrangement of carbon tails of the ligands has also been discovered in a very recent work on chiral Au₁₃₃(SR)₅₂ and Au₂₄₆(SR)₈₀ nanoclusters. Overall, the origins of chirality discovered in Au_n (SR)_m nanoclusters may provide models for the understanding of chirality origins in other types of nanomaterials and also constitute the basis for the development of various applications of chiral nanoparticles.