Chiral morphologies and interfacial electronic structure of naphtho[2,3-a]pyrene on Au(111)

The adsorption of the two-dimensionally chiral naphtho[2,3-a]pyrene molecule has been studied on Au(111). Both structural and electronic properties of the naphtho[2,3-a]pyrene (NP)/Au(111) interface have been measured. Ultraviolet and X-ray photoelectron spectroscopy have been employed to measure the energies of the molecular orbitals of the NP film with respect to the gold Fermi level. A Schottky junction with a large interface dipole (0.99 eV) is formed between Au(111) and NP. Temperature-programmed desorption was used to determine that adsorbed NP has a binding energy of 102.2 kJ/mol. Chiral domains have been observed with scanning tunneling microscopy due to the spontaneous phase separation of the 2-D enantiomers. Two distinct structural polymorphs have been observed, one of which has homochiral paired molecular rows. Models of the 2D structure are proposed that are in excellent agreement with experimental measurements.     

Disappearing Enantiomorphs: Single Handedness in Racemate Crystals

Although crystallization is the most important method for the separation of enantiomers of chiral molecules in the chemical industry, the chiral recognition involved in this process is poorly understood at the molecular level. We report on the initial steps in the formation of layered racemate crystals from a racemic mixture, as observed by STM at submolecular resolution. Grown on a copper single‐crystal surface, the chiral hydrocarbon heptahelicene formed chiral racemic lattice structures within the first layer. In the second layer, enantiomerically pure domains were observed, underneath which the first layer contained exclusively the other enantiomer. Hence, the system changed from a 2D racemate into a 3D racemate with enantiomerically pure layers after exceeding monolayer‐saturation coverage. A chiral bias in form of a small enantiomeric excess suppressed the crystallization of one double‐layer enantiomorph so that the pure minor enantiomer crystallized only in the second layer.     

Quasi chiral phase separation in a two-dimensional orientationally disordered system: 6-nitrospiropyran on Au(111)

The adsorption and chiral expression of 6-nitrospiropyran (SP6) molecules on a Au(111) surface are studied by scanning tunneling microscopy (STM) in combination with density functional theory (DFT) calculations. Both the chirality and the adsorption orientation of each adsorbed SP6 molecule are determined. The racemic mixture of SP6 enantiomers forms two-dimensional (2D) domains with same close packed positional orders but different internal orientational structures due to the random distribution of two adsorption orientations in each domain. However, all these orientationally disordered 2D domains undergo spontaneous quasi chiral phase separation; the 2D SP6 domains separate into 1D homochiral chains in which the SP6 molecules adopt two orientations randomly. This novel phenomenon is attributed to the preferential formation of the energetic favorable configurations with both the C-H...O weak hydrogen bonds and the pi-stacking of the two moieties of each SP6 molecule.     

Expression of Chirality by Achiral Coadsorbed Molecules in Chiral Monolayers Observed by STM

The supramolecular packing mode of physisorbed monolayers built up by chiral isophthalic acid derivatives and coadsorbed achiral solvent molecules was imaged at the liquid/graphite interface with scanning tunneling microscopy (STM). The picture on the right shows the submolecularly resolved STM image of an enantiomorphous domain composed of the R enantiomer of the isophthalic acid derivative studied and 1-heptanol molecules; the latter express the chirality of the monolayer. Upon adsorption a racemic mixture is separated into enantiomorphous domains.     

Elektrochemische Untersuchungen über das enantiomerenselektive Verhalten von chiralen Ionophoren in flüssigen Membranen

Electrochemical Investigations of the Enantiomer Selectivity of Chiral Ionophores in Liquid Membranes The enantiomer selectivity of a series of chiral neutral ionophores was investigated by electrochemical methods using ionophore-solvent-polymer membranes and chiral ammonium salts as substrates. The potentiometrically determined enantiomer selectivities are in good agreement with the results of electrodialytic transport studies.     

Chiral Induction and Amplification in Supramolecular Systems at the Liquid–Solid Interface

Chiral induction and amplification in surface‐confined supramolecular monolayers are investigated at the liquid–solid interface. Scanning tunneling microscopy (STM) proves that achiral molecules can self‐assemble into globally chiral patterns through a variety of approaches, including induction by chiral solvents or by a novel chiral amplification method. Our study demonstrates the aptness of both approaches, which have already been applied to (supramolecular) polymers in solution, to create chiral supramolecular monolayers at the liquid–solid interface.     

Chiral electrochemical recognition by very thin molecularly imprinted sol-gel films

Thin films with enantioselective properties for electrochemically active chiral probes were developed. Enantioselectivity was accomplished via molecular imprinting. The films were fabricated through the sol-gel technique and were spin-coated on ITO electrodes. The chiral selectivity recognition was detected using two enantiomer pairs: D- and L-3,4-dihydroxyphenylalanine (D- and L-dopa) and (R)- and (S)-N,N'-dimethylferrocenylethylamine [(R)-Fc and (S)-Fc]. A defined chiral cavity was obtained by selection of functional monomers that interact with the template molecule, followed by its removal. Chiral selection properties were measured by cyclic voltammetry and square wave voltammetry. For both template molecules, very good chiral recognition was revealed by electrochemical measurement. The nonspecific adsorption measured for reference nonimprinted films was negligible (less than 5%). Dopa imprinted films revealed both high sensitivity, by the detection of 1 nM (0.2 ppb) concentration, and excellent selectivity, when challenged with a series of catechol derivatives. Fc-imprinted films were able to detect ca. 2 ppm of the target molecule, with very good enantioselectivity and low nonspecific adsorption. To our knowledge, this is the first report of successful molecular imprinting of a ferrocene derivative.     

手性冠醚的研究进展

对近几年来的手性大环冠醚化合物的合成及应用研究进展作了较为全面的综述。以合成手性大环化合物的起始手性源为依据,对其进行分类,并分子介绍其合成方法;其次,介绍了手性冠醚的三个主要应用方面：(1)对映异构体的手性识别与拆分;(2)不对称催化作用;(3)离子选择性配位与萃取。     

分子印迹聚合物在毛细管电色谱拆分手性药物中的研究进展

手性药物通过与生物体内生物大分子之间的手性匹配与分子识别来发挥药理作用。两个对映体与体内手性环境相互作用的不同导致每个对映体表现出不同的药理活性、代谢过程、代谢速率及毒性等药代动力学特征。因此发展手性药物的拆分方法,对于手性药物的开发和生产过程的质量监控具有重要意义。分子印迹聚合物（MIPs）是以目标分子作为模板而制备的高分子聚合物,它具有特定的空间分子结构和官能团,对目标分子具有高度的特异性识别能力。基于该特点,MIPs非常适合于手性药物的拆分和纯化。毛细管电色谱（CEC）可同时基于毛细管电泳和液相色谱的分离机理对目标物进行分离,因此具有高分离效率和高选择性的特点。将MIPs材料作为CEC的固定相,可将这两种技术的优势结合,从而实现对手性药物的高效拆分。MIPs材料在1994年首次应用于CEC手性拆分,此后该研究领域开始获得关注和发展。MIPs材料主要通过4种模式在CEC中实现手性拆分,分别是作为开管柱、填充柱和整体柱的固定相以及分离介质中的准固定相。该综述以这4种模式作为分类基准,根据MIPs制备所需的材料和分离对象对其在CEC手性拆分中的应用进行了总结,揭示了MIPs在CEC手性拆分中的潜力,同时评述了这4种模式各自的优势与不足,并对将来MIPs在CEC手性拆分中的发展进行了展望。     

A new class of chiral calix[4]arenes as receptors with planar chirality

The modification of title compounds to chiral receptors and their characterization are reported. The two synthetic methods were developed. The racemates obtained could be resolved to each stable enantiomer by a chiral HPLC column. Chiral calixarenes were designed as the receptors with planar chirality. The (−)-receptor strongly forms 1:1 complex with (R)-(+)--phenylethylammonium picrate.     

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.     

Observation of hierarchical chiral structures in 8-nitrospiropyran monolayers

The adsorption and self-organization of racemic mixture of 8-nitrospiropyran (SP8) molecules on Au(111) surfaces was studied by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). The SP8 enantiomers, in spite of their low-symmetric and nonplanar molecular structures, formed well-ordered monolayers on Au(111). In the monolayers, we found two types of enantiomorphous, i.e., mirror-imaged, 2D chiral domains, denoted as lambda and delta phases. Both phases consist of periodically packed chiral quatrefoils. In the lambda domain, the quatrefoils are counterclockwise folded, while in the delta domain, the quatrefoils are clockwise folded. High-resolution STM images revealed that each chiral quatrefoil contains four heterochiral dimers and that each dimer is composed of two antiparallelly packed homochiral SP8 molecules. Therefore both of the two mirror-imaged 2D chiral structures are not chirally pure but racemic 2D crystals. A domain boundary, which serves as the glide reflection line between a lambda domain and a delta domain, was also observed along the [11] direction of the Au(111) substrate.     

Pair of Diastereomeric Uranyl Salen Cavitands Displaying Opposite Enantiodiscrimination of α-Amino Acid Ammonium Salts

A pair of diastereomeric salen cavitands and their uranyl complexes combine a chiral (R,R) salen bridge and an inherent chiral tris-bridged quinoxaline cup within the same molecule. Whereas the free ligands show a preference for the same enantiomer of an α-amino acid pair, the corresponding UO(2) complexes display opposite enantiodiscrimination and exceptionally high enantioselectivities (K(D)/K(L) = 26.4).     

Chiral pair monolayer adsorption of iodine-substituted octadecanol molecules on graphite

High-resolution scanning tunneling microscopy has been used to examine the adsorbate structures formed when a racemic mixture of (9R,10R)-9,10-diiodooctadecan-1-ol and (9S,10S)-9,10-diiodooctadecan-1-ol is adsorbed at the basal plane of highly ordered pyrolytic graphite. The herringbone structure characteristic of the adsorption of long-chain molecules on graphite is observed. Close examination of the micrographs indicates a unique structure in which the chiral molecules adsorb in pairs, with one enantiomer filling half of the unit cell, and the other enantiomer filling the other half. Instead of forming separate chiral domains, as is sometimes observed when a racemic mixture adsorbs on an achiral surface, chiral pairs are formed and the pairs form an ordered monolayer, exposing opposite faces of the same molecule. An achiral racemic mixture is observed to form a chiral structure on an achiral surface in the regions of the surface examined here.     

Hierarchical chiral framework based on a rigid adamantane tripod on Au(111)

We have investigated the tripod-shaped bromo adamantane trithiol (BATT) molecule on Au(111) using scanning tunneling microscopy (STM) at 4.7 K. Adsorption of BATT leads to formation of highly ordered self-assembled monolayers (SAMs) with three-point contacts on Au(111). The structure of these SAMs has been found to have a two-tiered hierarchical chiral organization. The self-assembly of achiral monomers produces chiral trimers, which then act as the building blocks for chiral hexagonal supermolecules. SAMs begin to form from the racemic mixture of assembled molecules in ribbon-shaped islands, followed by the transformation to enantiomeric domains when SAM layers develop two-dimensionally across hcp domains. Such a chiral phase transition at the two-dimensional domain can arise from a subtle balance between molecule-substrate and intermolecular interactions. Two structural factors, the S atom (stabilization) and the methylene groups (chirality) located just above the S atom, induce the chiral ordering of BATT on Au(111).     

Enantiomer separation of chiral pharmaceuticals by capillary electrochromatography

Enantiomer separation of chiral pharmaceuticals by capillary electrochromatography (CEC) is achieved with open-tubular capillaries (o-CEC), with packed capillaries (p-CEC) or with monolithic capillaries. In o-CEC, capillaries are coated with a thin film containing cyclodextrin derivatives, cellulose, proteins, poly-terguride or molecularly imprinted polymers as chiral selectors. In p-CEC, typical chiral HPLC stationary phases such as silica-bonded cyclodextrin or cellulose derivatives, proteins, glycoproteins, macrocyclic antibiotics, quinine-derived and 'Pirkle' selectors, polyacrylamides and molecularly imprinted polymers are used as chiral selectors. Chiral monolithic stationary phases prepared by in situ polymerization into the capillary were also developed for electrochromatographic enantiomer separation.     

Halogen bonds as stabilizing interactions in a chiral self-assembled molecular monolayer

We report the formation of highly-ordered self-assembled monolayers of an achiral organic semiconductor molecule. STM results show spontaneous formation of very large single domains of ordered chiral monolayers. DFT calculations support the identification of halogen bonds as the primary interactions that steer molecular self-assembly, leading to organizational chirality.     

Self-assembly and two-dimensional spontaneous resolution of cyano-functionalized [7]helicenes on cu(111)

Birds of a feather flock together: STM and DFT studies provide the first example of spontaneous chiral resolution of a helicene on a surface. Racemic 6,13-dicyano[7]helicene forms fully segregated domains of pure enantiomers (2D conglomerate) on Cu(111). The propensity of the system to optimize intermolecular CN???HC(Ar) hydrogen bonding and CN???CN dipolar interactions translates into chiral recognition with preferential assembly of homochiral molecules.     

Chiral recognition capabilities of melamine and cyanuric acid supramolecular structures

Currently, there are numerous papers that discuss local chiral domains in supramolecular structures of achiral molecules established using the STM method, and by using DFT calculations. However, there are no data regarding the obtainment of macroscopically chiral 2D-supramolecular structures from achiral molecules. In this study, melamine and cyanuric acid supramolecular structures were self-assembled on a graphitized carbon black surface, which had a surface structure that was identical to HOPG, and also on the surface of an inert solid support for chromatography. Chirality induction according to the Kondepudi effect was used. For the supramolecular structures, MD calculations showed the possibility of obtaining a chiral structure. To establish macroscopic chirality, we proposed the use of the difference in enantiomer adsorption on the modified adsorbents. For this, two indirect methods were used: static adsorption with a polarimetric control and gas chromatography. Both methods indicated the chiral recognition ability of the adsorbents used.     

Sensing Chiral Drugs by Using CdSe/ZnS Nanoparticles Capped with N‐Acetyl‐L‐Cysteine Methyl Ester

Chiral quantum dots (QDs), differing in their core or shell size and, consequently, in their optical properties, were synthesized by the treatment of commercially available amine‐capped quantum dots with methyl ester N‐acetyl‐L ‐cysteine (CysP). Interestingly, their colloidal methanol solutions remain stable for several months. Their NMR and IR spectra were in accordance with CysP binding to the QD surface through two anchoring groups; its thiolate (strongly bound) and the carbonyl group of its ester (weaker bound) group, whereas their circular dichroism (CD) spectra showed a new broad redshifted band, suggesting that the attachment to the QD surface modified the conformational equilibrium towards conformer(s) with optical activity in this region. These QDs were sufficiently fluorescent to perform studies of the chiral recognition of drugs, in particular the aryl propionic acids (APAs) ketoprofen (KP), naproxen (NP), flurbiprofen (FP), and ibuprofen (IP). We used different drug concentration ranges, depending on the QD solubility. All the assayed drugs quenched the QD emission in a concentration‐dependent mode. Quenching fluorescence assays with the chiral QDs (CS@CysP) showed their extraordinary capacity for the chiral recognition of KP, NP, and FP, and particularly in the case of KP and FP, a remarkable positive allosteric effect was detected for the R enantiomer. By using a drug/CS@CysP molar ratio of 5000:1 and 2500:1, the changes of intensity and the sign of the CD spectrum of the drug evidenced the dissociation of the drug carboxylic group in the presence of the QD.