Chiral recognition in surface explosion

The vast majority of chiral compounds crystallize into racemic crystals. It has been predicted and was experimentally established as a rule that chiral molecules on surfaces are more easily separated into homochiral domains due to confinement into a plane and lower entropic contributions. We investigated the formation and stability of two-dimensional tartrate crystals on a Cu(110) surface for the racemic mixture for the first time by means of temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). At low coverage, a bitartrate species becomes separated into homochiral domains, while at high coverage a monotartrate species forms a racemic mixture. At the same coverage and lateral arrangement, the thermally induced autocatalytic decomposition reaction occurs for the monotartrate racemate at a lower temperature than for the pure enantiomers. The stereochemistry in this so-called "surface explosion" reaction is explained by a higher stability of the enantiopure lattice due to lateral hydrogen-bond formation. The higher stability of the enantiopure two-dimensional lattice is in contrast to the higher stability of racemic three-dimensional tartaric acid crystals but is consistent with the observation that homochirality is preferred in hydrogen-bonded self-assembled biomolecular structures.     

Enantiospecific electrodeposition of chiral CuO films on single-crystal Cu(111)

Epitaxial films of monoclinic CuO have been electrodeposited on single-crystal Cu(111) from solutions containing either (S,S)- or (R,R)-tartrate. X-ray pole figure analysis reveals that the CuO film grown from (S,S)-tartrate exhibits a (1) out-of-plane orientation while the film grown from (R,R)-tartrate has a (11) orientation. Even though CuO does not crystallize within a chiral space group, the orientations obtained exhibit a surface chirality similar to that obtained from high index fcc metal surfaces. The films were shown to be enantioselective toward the catalytic oxidation of tartrate molecules by cyclic voltammetry. The technique should prove to be applicable to the electrodeposition of chiral surfaces of other low-symmetry materials on achiral substrates and should prove to be of use to those interested in the synthesis, separation, and detection of chiral molecules.     

Induction of homochirality in achiral enantiomorphous monolayers

We report the induction of homochirality in enantiomorphous layers of achiral succinic acid on a Cu(110) surface after doping with tartaric acid (TA) enantiomers. Succinic acid becomes chiral upon adsorption due to symmetry-breaking interactions with the Cu(110) surface. The doubly deprotonated bisuccinate forms mirror domains on the surface, which leads to a superposition of (11,-90) and (90,-11) patterns observed by low-energy electron diffraction (LEED). On average, however, the surface layer is racemic. An amount of 2 mol % of (R,R)- or (S,S)-tartaric acid in the monolayer, corresponding to an absolute coverage of 0.001 tartaric acid molecule per surface copper atom, is sufficient to make the LEED spots of one enantiomorphous lattice disappear. After thermally induced desorption of TA, the succinic acid lattice turns racemic again. In analogy to the "sergeants-and-soldiers" principle described for helical polymers, this effect is explained by a lateral cooperative interaction within the two-dimensional lattice.     

Solvent-induced dual chirality switching in the optical resolution of tropic acid via diastereomeric salt formation with (1R,2S)-2-amino-1,2-diphenylethanol

Solvent-induced chirality switching in the optical resolution of racemic tropic acid (TA) with (1R,2S)-2-amino-1,2-diphenylethanol has been demonstrated. Recrystallization of the diastereomeric salt mixture from i-PrOH or EtOH afforded the (S)-TA salt, while the (R)-TA salt was deposited from 1,4-dioxane and water-enriched alcohol solutions. Dual chirality switching was achieved by using two different types of solvents. The X-ray crystal structures of both diastereomeric salts showed that incorporation of the crystallization solvent played a crucial role in stabilizing each diastereomeric salt crystal. The mechanism of chirality switching has been discussed on the basis of the relative stability of the salt, as deduced from their structures.     

Surface chiroselective assembly of enantiopure crystalline porous films containing bichiral building blocks

The development of chiral crystalline porous materials (CPMs) containing multiple chiral building blocks plays an important role in chiral chemistry and applications but is a challenging task. Herein, we report the first example of bichiral building block based enantiopure CPM films containing metal–organic cages (MOCs) and metal complexes. The functionalized substrate was immersed subsequently into homochiral metal complex (R)- or (S)-Mn(DCH)₃ (DCH = 1,2-diaminocyclohexane) and racemic Ti₄L₆ cage (L = embonate) solutions by a layer-by-layer growth method. During the assembly process, the substrate surface coordinated with (R)- or (S)-Mn(DCH)₃ can, respectively, layer-by-layer chiroselectively connect Δ- or Λ-Ti₄L₆ cages to form homochiral (R, Δ)- or (S, Λ)-CPM films with a preferred [111] growth orientation, tunable thickness and homogeneous surface. The resulting enantiopure CPM films show strong chirality, photoluminescence, and circularly polarized luminescence (CPL) properties as well as good enantioselective adsorption toward enantiomers of 2-butanol and methyl-lactate. The present in situ surface chiroselective strategy opens a new route to assemble homochiral CPM films containing multiple chiral building blocks for chiral applications.

Bichiral building block based enantiopure CPM films containing metal–organic cages (MOCs) and metal complexes are chiroselectively assembled on the substrate surface by a layer-by-layer method.     

Structures formed by the chiral assembly of racemic mixtures of enantiomers: iodination products of elaidic and oleic acids

The self-assembled monolayer structure of the products of elaidic acid iodination (the racemic mixture of 9,10-(9S,10R)-diiodooctadecanoic acid and 9,10-(9R,10S)-diiodooctadecanoic acid) and the products of oleic acid iodination (the racemic mixture of 9,10-(9R,10R)-diiodooctadecanoic acid and 9,10-(9S,10S)-diiodooctadecanoic acid) are studied by high-resolution scanning tunneling microscopy. For the iodination products of elaidic acid, the separation of enantiomers into distinct chiral domains during the formation of the 2-D crystal on the highly ordered pyrolytic graphite (HOPG) surface is not observed. Instead, within the diiodooctadecanoic acid SAM, each row of molecules is composed of opposite racemates. The two opposite racemates pack alternately inside a row, using different faces to adsorb on the surface. The unit cell is composed of a pair of opposite racemates, forming a heterochiral structure. For the iodination products of oleic acid, the racemic mixture is observed to exhibit quasi-phase separation during the formation of the 2-D crystal on the HOPG surface. Each row is composed of homochiral acid molecules, either the 9,10-(9R,10R)-diiodooctadecanoic acid (R) or the 9,10-(9S,10S)-diiodooctadecanoic acid (S). The R row and the S row pack alternately, with a unit cell composed of four molecules. Two of the molecules in the unit cell are the 9,10-(9R,10R)-diiodooctadecanoic acid (R) molecules; two are the 9,10-(9S,10S)-diiodooctadecanoic acid (S) molecules. In the unit cell, the two molecules that have the same chirality pack antiparallel inside the homochiral row, using different faces to adsorb on the surface. These results suggest that several different types of chiral assembly are possible. Enantiomers with opposite chirality exhibit many chiral assembly patterns, forming heterochiral structures on the surface in addition to separation to form macroscopic chiral domains. By using different conformations, similar enantiomers with opposite chirality will display many chiral assembly patterns to form heterochiral structures on the surface.     

Layer-by-layer growth of oriented metal organic polymers on a functionalized organic surface

A metal-organic coordination polymer based on benzenetricarboxylic acid ligands and Zn(II) ions was grown on a COOH-terminated organic surface in a stepwise fashion. The deposited films were characterized using a number of surface analysis techniques, including X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. IR measurements show that the metal-organic coordination polymer grows in a layer-by-layer fashion and can be reversibly loaded with NH3. The deposition is very selective and occurs only on COOH-terminated regions of an organic surface, as demonstrated by AFM measurements.     

β-Sheet-induced chirogenesis in polymerization of oligopeptides

The origin of long homochiral biopolymers in living systems has recently been the focus of intense research. In one particular research line, scientists studied, experimentally and theoretically, chiral amplification obtained during peptide formation by polymerization of amino acid building blocks. It was suggested that processes leading to temporal or spatial separation, and thus, to the growth of homochiral polymers at the expense of their heterochiral counterparts, can explain the chirality observed in larger molecules. We introduce a simple model and stochastic simulation for the polymerization of amino acids and β-sheet formation, showing the crucial effects of the β sheets on the distributions of peptide lengths. When chiral affinities are included, racemic β sheets of alternating homochiral strands lead to the formation of chiral peptides, the isotacticity of which increases with length, consistent with previous experimental results in aqueous solutions. The tendency to form isotactic peptides is shown for both initially racemic and initially nonracemic systems, as well as for closed and open systems. We suggest that these or similar mechanisms may explain the origin of chiroselectivity in prebiotic environments.     

Preparation of a nanocomposite film from poly(diallydimethyl ammonium chloride) and gold nanoparticles by in-situ electrochemical reduction, and its application to SERS spectroscopy and sensing of ascorbic acid

A nanocomposite film is described that is composed of alternating layers of poly(diallydimethyl ammonium chloride) and gold nanoparticles that interact through electrostatic forces. The films of varying thickness were prepared by the layer-by-layer technique, and Au-NPs were generated by electrochemical reduction of hexachloroauric acid. The composite films were characterized by UV?Cvis spectroscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. Most nanocomposite films exhibit linear, uniform, and regular layer-by-layer growth during the process of formation. The films exhibit unique performance in terms of surface enhanced Raman scattering and electrocatalytic activitiy towards the oxidation of ascorbic acid.

Direct chiral resolution of tartaric acid by ion-pair capillary electrophoresis using an aqueous background electrolyte with (1R,2R)-(-)-1,2-diaminocyclohexane as a chiral counterion

Chiral resolution of native DL-tartaric acid was achieved by ion-pair capillary electrophoresis (CE) using an aqueous-ethanol background electrolyte with (1R,2R)-(-)-1,2-diaminocyclohexane (R-DACH) as a chiral counterion. Factors affecting chiral resolution and migration time of tartaric acid were studied. By increasing the viscosity of the background electrolyte and the ion-pair formation, using organic solvents with a lower relative dielectric constant, resulted in a longer migration time. The optimum conditions for both high resolution and short migration time of tartaric acid were found to be a mixture of 65% v/v ethanol and 35% v/v aqueous solution containing 30 mM R-DACH and 75 mM phosphoric acid (pH 5.1) with an applied voltage of -30 kV at 25 degrees C, using direct detection at 200 nm. By using this system, the resolution (Rs) of racemic tartaric acid was approximately 1. The electrophoretic patterns of tartaric and malic acids suggest that two carboxyl groups and two hydroxyl groups of tartaric acid are associated with the enantioseparation of tartaric acid by the proposed CE method.     

Global and local expression of chirality in serine on the Cu{110} surface

Establishing a molecular-level understanding of enantioselectivity and chiral resolution at the organic-inorganic interfaces is a key challenge in the field of heterogeneous catalysis. As a model system, we investigate the adsorption geometry of serine on Cu{110} using a combination of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The chirality of enantiopure chemisorbed layers, where serine is in its deprotonated (anionic) state, is expressed at three levels: (i) the molecules form dimers whose orientation with respect to the substrate depends on the molecular chirality, (ii) dimers of L- and D-enantiomers aggregate into superstructures with chiral (-1 ?2; 4 0) lattices, respectively, which are mirror images of each other, and (iii) small islands have elongated shapes with the dominant direction depending on the chirality of the molecules. Dimer and superlattice formation can be explained in terms of intra- and interdimer bonds involving carboxylate, amino, and β-OH groups. The stability of the layers increases with the size of ordered islands. In racemic mixtures, we observe chiral resolution into small ordered enantiopure islands, which appears to be driven by the formation of homochiral dimer subunits and the directionality of interdimer hydrogen bonds. These islands show the same enantiospecific elongated shapes those as in low-coverage enantiopure layers.     

In situ growth of CuS thin films on functionalized self-assembled monolayers using chemical bath deposition

Nano-structured CuS thin films were deposited on the functionalized -NH(2)-terminated self-assembled monolayers (SAMs) surface by chemical bath deposition (CBD). The deposition mechanism of CuS on the -NH(2)-terminated group was systematically investigated using field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscope (XPS), UV-vis absorption. The optical, electrical and photoelectrochemical performance of CuS thin films incorporating with the X-ray diffraction (XRD) analysis confirmed the nanocrystalline nature of CuS with hexagonal crystal structure and also revealed that CuS thin film is a p-type semiconductor with high electrical conductivity (12.3Ω/□). The functionalized SAMs terminal group plays a key role in the deposition of CuS thin films. The growth of CuS on the varying SAMs surface shows different deposition mechanisms. On -NH(2)-terminated surfaces, a combination of ion-by-ion growth and cluster-by-cluster deposition can interpret the observed behavior. On -OH- and -CH(3)-terminated surfaces, the dominant growth mechanism on the surface is cluster-by-cluster deposition in the solution. According to this principle, the patterned CuS microarrays with different feature sizes were successfully deposited on -NH(2)-terminated SAMs regions of -NH(2)/-CH(3) patterned SAMs surface.     

Surface investigation of intermetallic PdGa(111)

The intermetallic PdGa is a highly selective and potent catalyst in the semihydrogenation of acetylene, which is attributed to the surface stability and isolated Pd atom ensembles. In this context PdGa single crystals of form B with (111) orientation were investigated by means of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), X-ray photoelectron diffraction (XPD), and low-energy electron diffraction (LEED) to study the electronic and geometric properties of this surface. UPS and thermal desorption spectroscopy (TDS) were used to probe the chemisorption behavior of CO. The PdGa(111) surface exhibits a (1 × 1) LEED and a pronounced XPD pattern indicating an unreconstructed bulk-truncated surface. Low-temperature STM reveals a smooth surface with a (1 × 1) unit cell. No segregation occurs, and no impurities are detected by XPS. The electronic structure and the CO adsorption properties reveal PdGa(111) to be a bulk-truncated intermetallic compound with Pd-Ga partial covalent bonding.     

Three-dimensional homochiral transition-metal camphorate architectures directed by a flexible auxiliary ligand

Reported here are four homochiral and one racemic chiral compounds assembled from transition metals (Fe2+, Co2+, and Ni2+), rigid enantiopure camphoric acid, and flexible 4,4'-trimethylenedipyridine; they feature an unusual homochiral three-dimensional supramolecular assembly (resulting from catenation of 4-connected bilayers) as well as a homochiral 3-fold interpenetrating diamond net and a noninterpenetrating primitive cubic net.     

Direct chiral resolution of tartaric acid in food products by ligand exchange capillary electrophoresis using copper(II)-D-quinic acid as a chiral selector.

Chiral resolution of native DL-tartaric acid was performed by ligand-exchange capillary electrophoresis using copper(II)-D-quinic acid as a chiral selector. Factors affecting chiral resolution, migration time, and peak area of tartaric acid were studied. The running conditions for optimum separation of tartaric acid were found to be 1 mM copper(II) sulfate-10 mM D-quinic acid (pH 5.0) with an effective voltage of -15 kV at 30 degrees C, using direct detection at 250 nm, and resolution of racemic tartaric acid was approximately 1.3. With this system, chiral resolution of DL-tartaric acid in food products was conducted successfully.     

Chiral discrimination of a gemini-type surfactant with rigid spacer at the air-water interface

Spontaneous separation of chiral phases was observed in the monolayers of a racemate of gemini-type twin-tailed, twin-chiral amphiphiles, (2R,3R)-(+)-bis(decyloxy)succinic acid and (2S,3S)-(-)-bis(decyloxy)succinic acid. The pressure-area isotherms of the interfacial monolayers formed at the liquid-air interface, and the 2D lattice structures studied through surface probe measurements revealed that the racemate exhibits a homochiral discrimination of the enantiomers in two dimensions. An enantiomeric excess (e,e) of 20% was sufficient to break the chiral symmetry at the air-water interface for a homochiral interaction. Langmuir monolayers on ZnCl2 and CaCl2 subphases manifested chiral discrimination with Zn2+ evidencing homochiral interaction with a chelate-type complex, whereas Ca2+ resulted in a heterochiral interaction forming an ionic-type complex. For the chiral asymmetric units, oblique and rectangular unit cells of the racemic monolayer had exclusive requirements of homo- and heterochiral recognitions for Zn2+ and Ca2+ ions, respectively. Monolayers transferred from the condensed phase at 25 mN/m onto hydrophilic Si(100) and quartz substrates revealed the formation of bilayers through transfer-induced monolayer buckling. The emergence of homochiral discrimination was explained using the effective-pair-potential (EPP) approach.     

Ion scattering and X-ray photoelectron spectroscopy of copper overlayers vacuum deposited onto mercaptohexadecanoic acid self-assembled monolayers

Metal overlayers deposited in vacuum onto self-assembled monolayer (SAM) systems serve as models for more complex metalized polymers. Metals (M) deposited onto SAMs with different organic functional end groups exhibit a wide range of behavior, ranging from strong chemical interactions with the end group to complete penetration of the metal through the SAM. In this work, we have characterized the interactions of Cu with the ---COOH of mercaptohexadecanoic acid (MHA, HOOC(CH₂)₁₅SH) SAMs self assembled on gold films by using X-ray photoelectron spectroscopy (XPS) to examine the chemical interactions, and a combination of XPS and ion scattering spectroscopy (ISS) to deduce the growth mode and penetration rate of the deposited Cu. We found that submonolayer amounts of Cu react with HOOC, whereas the rest of the Cu remains metallic and penetrates beneath the SAM surface to the SAM  Au interface. Considerable amounts of Cu (5 nm or more) will penetrate beneath the SAM layer, which is ca. 2.5 nm thick, despite the submonolayer presence of Cu at the SAM surface. The penetration rate depends strongly on the Cu deposition rate. Depositing copper onto MHA at 220 K or less, or using faster Cu deposition rates, results in slower or even completely suppressed penetration of the Cu through the SAM layer, whereas exposure to X-rays greatly enhances the penetration rate of large amounts of Cu through the SAM layer. The reacted copper is, based on the XPS 2p and LMM peaks, in the +2 oxidation state, but cannot be identified with a simple, stoichiometric oxide such as Cu₂O, CuO, or Cu (OH)₂.     

Preparation and characterization of poly(l-histidine)/poly(l-glutamic acid) multilayer on silicon with nanometer-sized surface structures

The specific design and modification of surfaces is of great interest, especially for functional surfaces and medical applications. In order to obtain films on a surface, the layer-by-layer deposition of polyelectrolytes represents a well-established methodology. The alternating deposition of poly(l-histidine) and poly(l-glutamic acid) results in a defined, continuous surface coating that was thoroughly characterized using X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, ellipsometry, X-ray reflectometry, atomic force microscopy, scanning electron microscopy, contact angle, and electrokinetic measurements. Surface charge, film growth, and final thickness were measured and cross-validated. Additionally, the chemical composition and distribution of polyelectrolytes in the layerstack were determined. Finally, the optical parameters were specified and the surface topography was visualized by several methods. These characterizations revealed a coating with embedded spheroids forming from the bottom layers. This rough surface formed by (PLH/PGA)(8) was highly reproducible and might provide unique features for the design of tailored surfaces.     

Colloid surface engineering via deposition of multilayered thin films from polyelectrolyte blend solutions

Multilayer thin films were constructed on polystyrene colloidal particles by depositing alternating layers of poly(allylamine hydrochloride) (PAH) at pH 7.5 and varying composition blends of poly(acrylic acid) (PAA) and poly(styrenesulfonate) (PSS) at pH 3.5. Following the deposition of each layer, microelectrophoresis experiments showed alternating zeta-potentials, suggesting the formation of multilayered films on the particles. Scanning and transmission electron microscopy were used to examine the surface morphology of the colloidal particles, with homogeneous surface coatings apparent for films deposited from PAA/PSS blend solutions containing up to 90 wt % PAA. The colloidal stability of these particles is greater than those coated with individual PAH and PAA layers. In the case of the blend PAA/PSS = 25:75 wt %, up to 20 layers were assembled without compromising the colloidal stability of the dispersion. The results demonstrate that the deposition of layers from PE blend solutions containing a strong and weak PE can be used as a facile method for controlling the surface properties and hence the colloidal stability of core-shell particles, as well as the thickness and morphology of the coatings. Control of these parameters is important for subsequent processing and application of these particles in controlled delivery, photonics, catalytic, and separation applications.     

Tuning the Surface Properties of Oxygen-Rich and Nitrogen-Rich Plasma Polymers: Functional Groups and Surface Charge

Controlling the concentration and nature of functional groups in plasma polymer films by adjusting the flow ratio of constituent precursor gases can be exploited to tune the surface charge of the resulting coating. Plasma polymer films containing various concentrations of nitrogen and oxygen functional groups were deposited in a low-pressure capacitively-coupled glow discharge reactor by plasma polymerization of binary gas mixtures of a hydrocarbon (ethylene or butadiene) and a heteroatom source gas (ammonia and/or carbon dioxide). Increasing the flow ratio of heteroatom to hydrocarbon gases increased the concentration of bonded nitrogen or oxygen, including that of primary amine or carboxylic groups as determined by X-ray photoelectron spectroscopy and chemical derivatization procedures. The zeta potential of samples was measured using an electro-kinetic analyser in a diluted sodium chloride solution. The deposition parameters controlled the composition of the coatings, allowing to tune the surface charge to either positive (ammonia based films)—or negatively (carbon dioxide base films) values at physiological pH.