Supramolecular chirogenesis in zinc porphyrins: mechanism, role of guest structure, and application for the absolute configuration determination.

The achiral syn folded (face-to-face conformation) host molecule of the ethane-bridged bis(zinc porphyrin) transforms into the corresponding chiral extended anti bis-ligated species in the presence of enantiopure amine guests. The mechanism of the supramolecular chirogenesis is based upon the screw formation in bis(zinc porphyrin), arising from steric interactions between the largest substituent at the ligand's asymmetric carbon and peripheral alkyl groups of the neighboring porphyrin ring pointing toward the covalent bridge. The screw direction is determined by the guest's (amines) absolute configuration resulting in a positive chirality induced by (S)-enantiomers due to formation of the right-handed screw, and a negative chirality produced by the left-handed screw of (R)-enantiomers. The screw magnitude is strongly dependent upon the structure of the chiral guests. The amines with bulkier substituents result in stronger CD signals and larger (1)H NMR resonance splittings of enantiotopic protons. This system possesses a high degree of chiroptical activity, which allows the differentiation of one of the smallest homologous elements of organic chemistry, that is, the methyl and ethyl groups attached to the asymmetric carbon, and additionally, which senses a remote chiral center at a position beta to the amine binding group.     

Two-component supramolecular helical architectures: creation of tunable dissymmetric cavities for the inclusion and chiral recognition of the third components

The inclusion and chiral recognition of racemic arylalkanols by supramolecular helical architectures consisting of enantiopure primary amines and achiral carboxylic acids were thoroughly studied. Among the architectures examined, a supramolecular helical architecture composed of the salt of enantiopure erythro-2-amino-1,2-diphenylethanol (1 b) and benzoic acid (2 a) was found to include a wide variety of racemic arylalkanols with recognition of their chirality. The helical architecture gave a dissymmetric 1D groove in the salt crystal, and the arylalkanols were enantioselectively included in the groove. The size and shape of the groove were tunable by proper selection of the achiral carboxylic acid component. The origin of the chiral recognition with the combination 1 b/2 a is discussed on the basis of X-ray crystallographic analyses.     

Supramolecular chirogenesis in zinc porphyrins: interaction with bidentate ligands,formation of tweezer structures,and the origin of enhanced optical activity

The complexation behavior, binding properties, and spectral parameters of supramolecular chirality induction in the achiral host molecule, syn (face-to-face conformation) ethane-bridged bis(zinc porphyrin), upon interaction with chiral bidentate guests (diamines and amino alcohols) have been studied by means of UV-vis, CD, fluorescence, (1)H NMR, and ESI MS techniques. It was found that the guest structure plays a decisive role in the chirogenesis pathway. The majority of bidentate ligands (except those geometrically unsuitable) exhibit two major equilibria steps: the first guest ligation leading to formation of the 1:1 host-guest tweezer structure (K(1)) and the second guest molecule ligation (K(2)) forming the anti bis-ligated species (1:2). The second ligation is much weaker (K(1) > K(2)) due to the optimal geometry and stability of the 1:1 tweezer complex. The enhanced conformational stability of the tweezer complex ensures an efficient chirality transfer from the chiral guest to the achiral host, consequently inducing a remarkably high optical activity in the bis-porphyrin.     

Supramolecular chirogenesis in zinc porphyrins: equilibria, binding properties, and thermodynamics

Complexation mechanism, binding properties and thermodynamic parameters of supramolecular chirality induction in the achiral host molecule, syn (face-to-face conformation) ethane-bridged bis(zinc porphyrin), upon interaction with chiral monoamine and monoalcohol guests have been studied by means of the UV-vis, CD, (1)H NMR, and ESI MS techniques. It was found that the chirogenesis process includes three major equilibria steps: the first guest ligation to a zinc porphyrin subunit of the host (K(1)), syn to anti conformational switching (K(S)), and further ligation by a second guest molecule to the remaining ligand-free zinc porphyrin subunit (K(2)), thus forming the final bis-ligated species possessing supramolecular chirality. The validity of this equilibria model is confirmed by the excellent match between the calculated and experimentally observed spectral parameters of the bis-ligated species. The second ligation proceeds in a cooperative manner as K(2) > K(1) for all supramolecular systems studied, regardless of the structure of the chiral ligand used. The binding properties are highly dependent on the nature of the functional group (amines are stronger binders than alcohols) and on the structure of the chiral guests (primary and aliphatic amines have overall binding constant values greater than those of secondary and aromatic amines, respectively).     

Remarkable stability and enhanced optical activity of a chiral supramolecular bis-porphyrin tweezer in both solution and solid state

Interaction of the achiral syn (face-to-face) conformer of the ethane-bridged bis(zinc octaethylporphyrin) with the enantiopure 1,2-diaminocyclohexane results in the exclusive formation of a supramolecular chiral tweezer. This 1:1 host-guest complex exhibits remarkable stability in both solution (even upon photoexcitation) and solid-state phases, with a high degree of optical activity arising from the two-point interaction mode and optimal spatial geometry.     

Amplification of Dissymmetry for Circularly Polarized Photodetection by Cooperative Supramolecular Polymerization

Circularly polarized photodetectors require chiral light absorption materials with high sensing efficiency and low costs. Here readily accessible point chirality has been introduced to dicyanostilbenes as the chiral source, facilitating remote chirality transfer to the π-aromatic core by cooperative supramolecular polymerization. The single-handed supramolecular polymers display powerful circularly polarized photodetection capability with a dissymmetry factor value as high as 0.83, superior to those of π-conjugated small molecules and oligomers. Strong chiral amplification occurs between the enantiopure sergeants and the achiral soldiers. The resulting supramolecular copolymers exhibit comparable photodetection efficiency to those of the homopolymeric ones, with a 90 % decrease in the enantiopure compound consumption. Therefore, cooperative supramolecular polymerization provides an effective yet economical avenue toward circularly polarized photodetection applications.     

手性超分子组装研究进展

介绍了超分子手性的基本构筑方式及其特点,分别从手性分子组装、手性分子诱导非手性分子及非手性分子组装等3个方面对最近几年来在手性超分子组装领域内的重要成果及最新进展进行了综述,并对这一领域的发展前景作了展望。     

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.     

Total spontaneous resolution of chiral covalent networks from stereochemically labile metal complexes

Stereochemically labile copper and zinc complexes with the N,N'-dimethylethylenediamine ligand (dmeda) have been shown to be promising precursors for the total spontaneous resolution of chiral covalent networks. (N,N')-[Cu(NO3)2(dmeda)]infinity crystallises as a conglomerate and yields either enantiopure (R,R)-1 or enantiopure (S,S)-1. A mixed-valence copper(I/II) complex, [{Cu(II)Br2(dmeda)}3(Cu(I)Br)2]infinity (2), which crystallises as a pair of interpenetrating chiral (10,3)-a nets, is formed from CuBr, CuBr2 and dmeda. One net contains ligands with solely (R,R) configuration and exhibits helices with (P) configuration while the other has solely (S,S)-dmeda ligands and gives rise to a net in which the helices have (M) configuration. The whole crystalline arrangement is racemic, because the interpenetrating chiral nets are of opposite handedness. With zinc chloride (R,S)-[ZnCl(dmeda)2]2[ZnCl4] (3) is obtained, which is a network structure, although not chiral. Total spontaneous resolution of stereochemically labile metal complexes formed from achiral or racemic building blocks is suggested as a viable route for the preparation of covalent chiral networks. Once the absolute structure of the compound has been determined by X-ray crystallography, a quantitative determination of the enantiomeric excess of the bulk product can be undertaken by means of solid-state CD spectroscopy.     

Metastable Chiral Azobenzenes Stabilized in a Double Racemate

The self‐assembly of chiral organic chromophores is gaining huge significance due to the abundance of supramolecular chirality found in natural systems. We report an interdigitated molecular assembly involving axially chiral octabrominated perylenediimide (OBPDI) which transfers chiral information to achiral aromatic moieties. The crystalline two‐component assemblies of OBPDI and electron‐rich aromatic units were facilitated through π‐hole???π donor–acceptor interactions, and the charge‐transfer characteristics in the ground and excited states of the OBPDI cocrystals were established through spectroscopic and theoretical techniques. The OBPDI cocrystals entail a remarkable homochiral segregation of P and M enantiomers of both molecular entities in the same crystal system, leading to twisted double‐racemic arrangements. Synergistically engendered cavities with the stored chiral information of the twisted OBPDI stabilize higher‐energy P/M enantiomers of trans‐azobenzene through non‐covalent interactions.     

Isolable Chiral Aggregates of Achiral π‐Conjugated Carboxylic Acids

The induced aggregation of achiral building blocks by a chiral species to form chiral aggregates with memorized chirality has been observed for a number of systems. However, chiral memory in isolated aggregates of achiral building blocks remains rare. One possible reason for this discrepancy could be that not much is understood in terms of designing these chiral aggregates. Herein, we report a strategy for creating such isolable chiral aggregates from achiral building blocks that retain chiral memory after the facile physical removal of the chiral templates. This strategy was used for the isolation of chiral homoaggregates of neutral achiral π‐conjugated carboxylic acids in pure aqueous solution. Under what we have termed an “interaction–substitution” mechanism, we generated chiral homoaggregates of a variety of π‐conjugated carboxylic acids by using carboxymethyl cellulose (CMC) as a mediator in acidic aqueous solutions. These aggregates were subsequently isolated from the CMC templates whilst retaining their memorized supramolecular chirality. Circular dichroism (CD) spectra of the aggregates formed in the acidic CMC solution exhibited bisignated exciton‐coupled signals of various signs and intensities that were maintained in the isolated pure homoaggregates of the achiral π‐conjugated carboxylic acids. The memory of the supramolecular chirality in the isolated aggregates was ascribed to the substitution of COOH/COOH hydrogen‐bonding interaction between the carboxylic acid groups within the aggregates for the hydrogen‐bonding interactions between the COOH groups of the building blocks and the chiral templates. We expect that this “interaction–substitution” procedure will open up a new route to isolable pure chiral aggregates from achiral species.     

Self-twisting for macrochirality from an achiral asterisk molecule with fluorescence-phosphorescence dual emission

A self-progressing chiral self-assembly form an achiral and C₆-symmetric molecule, resulting in a chiral amplification with prolonging the time. The system shows three distinct luminescent colors with the change of time in the same solution system.     

Origin of second harmonic generation optical activity of a tryptophan derivative at the air/water interface

Second harmonic generation optical activity (SHG-OA) of chiral monolayers of the tryptophan derivative N(alpha)-(tert-butoxycarbonyl)-tryptophan (BOC-Trp) at an air/water interface has been studied in detail. In combination with previously reported experimental measurements with the fundamental frequency variant Planck's 'h/' omega=2.20 eV (lambda=564 nm), new measurements with lambda=564 and 800 nm fully characterize the nonlinear susceptibility tensors of chiral and achiral (racemic) monolayers under two-photon resonant and nonresonant conditions of the fundamental frequency. A realistic computational approach including semiempirical, intermediate neglect of differential overlap (ZINDO/S) calculations has been used to calculate the nonlinear susceptibilities of model achiral and chiral monolayers composed of indole chromophores. There is satisfactory agreement between calculated and observed nonlinear susceptibilities, which constrains certain structural parameters of the monolayers including the absolute orientation of the long molecular axis of indole at the air/water interface. The origin of SHG-OA of BOC-Trp monolayers is discussed with reference of two distinct mechanisms at the microscopic level, designated type I or chiral assembly and type II or electronic coupling. Both mechanisms are studied in detail within the framework of ZINDO/S calculations. The dominant effect for the BOC-Trp monolayers is type I, involving chiral assembly of indole chromophores.     

Induction of chirality in an achiral monolayer at the liquid/solid interface by a supramolecular chiral auxiliary

An achiral oligo(p-phenylene vinylene) derivative with a ureido-triazine hydrogen bonding unit self-assembles into rows of hydrogen bonded dimers at the liquid/solid interface. Scanning tunneling microscopy reveals the formation of chiral domains, but overall, the surface remains racemic. Addition of a chiral auxiliary which is able to interact with the dimers through hydrogen bonding, showed that global organizational chirality could be achieved since a majority of the domains show the same handedness. After removing the chiral auxiliary with a volatile solvent, the global organizational chirality could be trapped, revealing a memory effect. With this straightforward supramolecular approach, we were able to create a chiral surface with preferred handedness composed of achiral molecules at the air/solid interface.     

Expression of molecular chirality and two-dimensional supramolecular self-assembly of chiral, racemic, and achiral monodendrons at the liquid-solid interface

We have investigated the two-dimensional ordering of chiral and achiral monodendrons at the liquid-solid interface. The chiral molecules self-assemble into extended arrays of dimers. As expected, the R enantiomer forms the mirror image type pattern of the chiral two-dimensional structure formed by the S enantiomer. A racemic mixture applied from solution onto the substrate undergoes spontaneous segregation: the enantiomers separate on the surface and appear in different domains. In contrast to the chiral molecules, the achiral analogue self-assembles into cyclic tetramers. Moreover, the pattern formed by the achiral molecule strongly depends on the solvent used. In the case of 1-phenyloctane, solvent molecules are coadsorbed in a 2:1 (dendron:solvent) ratio whereas in 1-octanol, no solvent molecules are coadsorbed. By the appropriate solvent choice, the distance between the potential "supramolecular containers" can be influenced.     

Interfacial assembly of cinnamoyl-terminated bolaamphiphiles through the air/water interface: headgroup-dependent assembly, supramolecular nanotube and photochemical sewing

A series of cinnamoyl-terminated bolaamphiphiles were synthesized and their assemblies at the air/water interface were investigated. It was found that the assembly behaviour depended on the substituted groups on the cinnamoyl unit. The bolaamphiphile with 4-hydroxycinnamoyl head groups (HCDA) was found to assemble into a supramolecular nanotube, while the others formed only layer-structured films. Moreover, the nanotube formed from HCDA showed supramolecular chirality due to the symmetry breaking. Both the layered films and the nanotubes showed photochemical dimerization upon UV irradiation, which were studied from the UV-Vis, FT-IR spectral and MALDI-TOF MS analysis. Interestingly, such dimerization behavior of the cinnamoyl group could be used to stabilize the nanotube of HCDAvia photochemical sewing. During such a process both the supramolecular chirality and the tubular shapes were kept. Remarkably, such a photochemical sewed chiral nanotube could further induce the chirality of an achiral porphyrin derivative assembled on it, and produced the induced chirality without using any chiral molecules.     

Molecular basis for water-promoted supramolecular chirality inversion in helical rosette nanotubes

Helical rosette nanotubes (RNTs) are obtained through the self-assembly of the GwedgeC motif, a self-complementary DNA base analogue featuring the complementary hydrogen bonding arrays of both guanine and cytosine. The first step of this process is the formation of a 6-membered supermacrocycle (rosette) maintained by 18 hydrogen bonds, which then self-organizes into a helical stack defining a supramolecular sextuple helix whose chirality and three-dimensional organization arise from the chirality, chemical structure, and conformational organization of the GwedgeC motif. Because a chiral GwedgeC motif is predisposed to express itself asymmetrically upon self-assembly, there is a natural tendency for it to form one chiral RNT over its mirror image. Here we describe the synthesis and characterization of a chiral GwedgeC motif that self-assembles into helical RNTs in methanol, but undergoes mirror image supramolecular chirality inversion upon the addition of very small amounts of water (<1% v/v). Extensive physical and computational studies established that the mirror-image RNTs obtained, referred to as chiromers, result from thermodynamic (in water) and kinetic (in methanol) self-assembly processes involving two conformational isomers of the parent GwedgeC motif. Although derived from conformational states, the chiromers are thermodynamically stable supramolecular species, they display dominant/recessive behavior, they memorize and amplify their chirality in an achiral environment, they change their chirality in response to solvent and temperature, and they catalytically transfer their chirality. On the basis of these studies, a detailed mechanism for supramolecular chirality inversion triggered by specific molecular interactions between water molecules and the GwedgeC motif is proposed.     

Scanning tunneling microscopy investigation of a supramolecular self-assembled three-dimensional chiral prism on a Au(111) surface

Spontaneous symmetry-breaking of a racemic mixture of supramolecular triginal prisms into chiral domains on a Au(111) surface is observed by scanning tunneling microscopy (STM). High-resolution STM analysis enables the structural aspects of each enantiomeric domain to be elucidated. The ability to resolve chirality on achiral surfaces has potential applications in heterogeneous stereoselective synthesis and catalysis.     

Isolable chiral aggregates of achiral π-conjugated carboxylic acids

The induced aggregation of achiral building blocks by a chiral species to form chiral aggregates with memorized chirality has been observed for a number of systems. However, chiral memory in isolated aggregates of achiral building blocks remains rare. One possible reason for this discrepancy could be that not much is understood in terms of designing these chiral aggregates. Herein, we report a strategy for creating such isolable chiral aggregates from achiral building blocks that retain chiral memory after the facile physical removal of the chiral templates. This strategy was used for the isolation of chiral homoaggregates of neutral achiral π-conjugated carboxylic acids in pure aqueous solution. Under what we have termed an "interaction-substitution" mechanism, we generated chiral homoaggregates of a variety of π-conjugated carboxylic acids by using carboxymethyl cellulose (CMC) as a mediator in acidic aqueous solutions. These aggregates were subsequently isolated from the CMC templates whilst retaining their memorized supramolecular chirality. Circular dichroism (CD) spectra of the aggregates formed in the acidic CMC solution exhibited bisignated exciton-coupled signals of various signs and intensities that were maintained in the isolated pure homoaggregates of the achiral π-conjugated carboxylic acids. The memory of the supramolecular chirality in the isolated aggregates was ascribed to the substitution of COOH/COOH hydrogen-bonding interaction between the carboxylic acid groups within the aggregates for the hydrogen-bonding interactions between the COOH groups of the building blocks and the chiral templates. We expect that this "interaction-substitution" procedure will open up a new route to isolable pure chiral aggregates from achiral species.     

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.