Enantioselective, chirally templated sol-gel thin films

Enantioselective surfactant-templated thin films were fabricated through the sol-gel (SG) process. The enantioselectivity is general in the sense that it discriminates between pairs of enantiomers not used for the imprinting process. The chiral cationic surfactant (-)-N-dodecyl-N-methylephedrinium bromide (1) was used as the surfactant template, and after its extraction chiral domains were created. The chiral discriminative feature of these films was examined by challenging with pure enantiomer solutions for rebinding. Selective adsorption was shown using (R)- and (S)-propranolol, (R)-2 and (S)-2, respectively, and (R)- and (S)-2,2,2-trifluoro-1-(9-anthryl)ethanol, (R)-3 and (S)-3, respectively, as the chiral probes. The selective adsorption was measured by fluorescence analysis, and the chiral selectivity factors were found to be 1.6 for 2 and 2.25 for 3. In both cases, (R)-enantiomer was adsorbed preferably. The resulting material was characterized by transmission electron microscopy, by diffraction, and by surface area measurements, and was found to be semicrystalline with short-range ordered domains (50 A) of hexagonal symmetry.     

Selective reflection and textural studies of free-standing films of a chiral liquid crystal

Abstract

We present a detailed study of selective reflection from free-standing films of chiral smectic phases. Free-standing films are drawn of the chiral compound 2-(4-hexyloxyphenyl)-5-[4-(1-chloro-3-methylbutanoyloxy)phenyl]pyrimidine (2f-6) possessing a chiral smectic C and the recently discovered chiral smectic M phase, both phases showing the phenomenon of selective reflection of visible light in a certain temperature range. With linearly polarized incident light two selective reflection maxima are observed which are caused by the presence of π-walls in the film. The wavelength of the selective reflection maxima is independent of the film thickness, whereas the intensity decreases with decreasing film thickness. The dependence of the reflection maxima on temperature and enantiomeric excess is studied.     

Fabrication of chiral Langmuir-Schaefer films of achiral amphiphilic Schiff base derivatives through an interfacial organization

Supramolecular chirality in the Langmuir-Schaefer (LS) films of two achiral amphiphilic Schiff bases, 2-(2'-benzimidazolyliminomethyl)-4-octadecyloxyphenol (BSC18) and 2-(2'-benzthiazolyliminomethyl)-4-octadecyloxyphenol (TSC18), was investigated. Both of these amphiphiles could form LS films from the water surface or coordinate with Ag(I) in the subphase to form Ag(I)-coordinated LS films. Although both of these amphiphiles were achiral, TSC18 formed a chiral LS film from the water surface, while BSC18 formed a chiral Ag(I)-coordinated LS film from the aqueous AgNO3 subphase. The supramolecular chirality in these LS films was suggested to be due to a cooperative stereoregular pi-pi stacking of the functional groups together with the long alkyl chains in a helical sense. The relationship between the chirality of the LS films and the molecular structures of TSC18 and BSC18 as well as their H-bond or coordination behaviors was discussed. The Schiff base films showed a reversible color change upon exposure to HCl and NH3 gas alternatively; however, the supramolecular chirality was irreversible during these processes.     

Enantioselective crystallization of histidine on chiral self-assembled films of cysteine

In this paper, the preparation and use of chiral surfaces derived from enantiomerically pure crystals of amino acids are described. For this purpose, a self-assembly process to grow thin chiral films of (+)-L- or (-)-D-cysteine on gold surfaces was chosen. These chiral films were utilized as crystallization catalysts in the crystallization of enantiomers from solutions. To demonstrate the chiral discrimination power of the chiral surfaces in crystallization processes, the crystallization of racemic histidine onto the chiral films was investigated. Our study demonstrates the potential application of chiral films to control chirality throughout crystallization, where one enantiomer crystallizes onto the chiral surfaces with relative high enantiomeric excess. In addition, crystallization of pure histidine enantiomers onto chiral films results in strong crystal morphology modification with preferred orientation.     

PHOTOINDUCED HOLOGRAPHIC PHASE GRATINGS BURIED IN AZOBENZENE SIDE-CHAIN POLYMER FILMS WITH A CHIRAL GROUP

An optically active polymer (PM1) containing azobenzene moieties with a chiral group (s-2-methyl-butyl) wassynthesized by homopolymerization of monomer, 4-[2-(methacryloyloxy)ethyloxy-4'-(s-2-methyl-1-butyl oxycarbonyl)azobenzene, using the free radical polymerization method. The polymer dissolved in tetrahydrofuran (THF) could be easilyprocessed into high optical quality films. The optical anisotropy of the polymer films was investigated by polarizing opticalmicroscopy (POM). The experimental results showed that irradiation with a circularly polarized beam could align theorientation of the molecules in the polymer films. Moreover, the holographic phase gratings of photo-induced polymer filmswere detected by atomic force microscopy (AFM) and POM. In comparison with polymer containing no chiral group, it wasfound from the preliminary measurement of the photo-induced holographic phase gratings that PM1 containing a chiralgroup could form holographic phase gratings buried in the films.     

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.     

手性取代聚(对亚苯基亚乙炔基-alt-间亚苯基亚乙炔基)的设计合成及其手性光性能研究

设计合成了两种主链结构相同、但取代方式不同的带有(-)-trans-桃金娘烷氧基手性侧基的规整连接聚(对亚苯基亚乙炔基-altr-间亚苯基亚乙炔基)(Myr-PMPE-1和Myr-PMPE-2),其中Myr-PMPE-1分子中手性取代基在分子链上均匀分布,而Myr-PMPE-2分子中手性取代基的分布则交替地存在局部"拥挤"和局部"松散".利用紫外-可见吸收光谱、荧光光谱和圆二色谱对两种聚合物膜的手性光性能进行了研究.结果表明,退火处理对Myr-PMPE-1膜的UV吸收和荧光光谱以及光学活性的影响都不明显,其最大不对称因子|gmax|为1.62×10-4;而退火处理对Myr-PMPE-2膜影响显著,未经退火处理或退火温度≤100℃时,Myr-PMPE-2膜几乎不显示光学活性,但当退火温度高于120℃时,随着退火温度的升高和退火时间的延长,Myr-PMPE-2膜的UV吸收和荧光光谱均稍有红移,特别是膜的光学活性显著提高,经140℃退火处理4 h后,其|gmax|可达3.07×10-3,比Myr-PMPE-1膜的高出1个数量级.     

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.     

Alignment and switching of nematic liquid crystals embedded in porous chiral thin films

Porous thin films with engineered microstructures have been fabricated using glancing angle deposition (GLAD). GLAD films with chiral microstructures have been previously shown to exhibit unique chiral optical response. The pores of these films were embedded with (non-chiral) nematic liquid crystals (LCs) to produce a new composite optical material wherein the GLAD film induces chiral nematic-like LC orientation. We demonstrate here reversible electro-optic switching of the LC component of these hybrid films. Unaddressed, cells of GLAD/LC hybrid films show enhanced chiral optic response compared with the unfilled GLAD film. When addressed, the chiral optic response vanishes.     

Remarkable enantioselectivity of molecularly imprinted TiO2 nano-thin films

TiO(2) nano-thin films with imprinted (R)- and (S)-enantiomers of propranolol, 1,1'-bi-naphthol, and 2-(4-isobutylphenyl)-propionic acid were fabricated on quartz plates by spin-coating their solutions with Ti(O-(n)Bu)(4) in a toluene-ethanol mixture (1:1, v/v). After template removal, the imprinted films showed better binding for original templates than to the corresponding enantiomers. The assessment of template incorporation, template removal, and re-binding was conducted through UV-vis measurements. Significant enhancement of enantioselectivity was achieved by optimization of the film thickness and by heat-treatment of the imprinted films. After subtraction of non-specific binding, the optimized films provided chiral recognition with the enantioselectivity of almost 100% for (R)-propranolol and 95% for (S)-propranolol.     

OPTICAL PHASE CONJUGATION RESPONSE OF PHOTOINDUCED POLYMER FILMS CONTAINING AZOBENZENE MOIETIES WITH CHIRAL GROUP

An optically active monomer containing azobenzene moieties with chiral group (s-2-methyl-butyl), 4-[2-(methacryloyloxy)ethyloxy]-4'-(s-2-methyl-1-butyloxycarbonyl) azobenzene(Ml) was synthesized. Polymer (PMl)possessing optical phase conjugated response was obtained by homopolymerzation of the optically active monomer (Ml)using free radical polymerization. The polymer was very soluble in common solvents and good optical quality films could beeasily fabricated by spin coating. The optical phase conjugated responses of the polymer PM1 were measured by degeneratefour-wave mixing (DFWM). In comparison with polymer containing no chiral group, it was found from the preliminarymeasurement of photoisomeric change that optical phase conjugated response of the PM1 in the long-range order hexagonalsymmetry microstructure could be easily controlled by choosing the appropriate polarization direction of the irradiatingbeams (514.5 nm) and the irradiating number, presumably due to the chiral group in the PM1 molecular structure. For thecase of the polymer investigated here, a chiral group side chain was introduced to increase optical phase conjugated responseintensity with different polarization directions of the irradiating beams, which aims originally at searching for a new photo-ative material.     

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.     

Imprinting of chiral molecular recognition sites in thin TiO2 films associated with field-effect transistors: novel functionalized devices for chiroselective and chirospecific analyses

(R)- or (S)-2-Methylferrocene carboxylic acids, (R)-1 or (S)-1, (R)- or (S)-2-phenylbutanoic acid, (R)-2 or (S)-2, and (R)- or (S)-2-propanoic acid, (R)-3 or (S)-3, can be imprinted in thin TiO2 films on the gate surface of ion-sensitive field-effect transistor (ISFET) devices. The imprinting is performed by hydrolyzing the respective carboxylate TiIV butoxide complex on the gate surface, followed by washing off the acid from the resulting TiO2 film. The imprinted sites reveal chiroselectivity only towards the sensing of the imprinted enantiomer. The chiral recognition sites reveal not only chiroselectivity but also chirospecificity and, for example, the (R)-2-imprinted film is active in the sensing of (R)-2, but insensitive towards the sensing of (R)2-phenylpropanoic acid, (R)-3, which exhibits a similar chirality. Similarly, the (R)-3-imprinted film is inactive in the analysis of (R)-2. The chiroselectivity and chirospecificity of the resulting imprinted films are attributed to the need to align and fit the respective substrates in precise molecular contours generated in the cross-linked TiO2 films upon the imprinting process.     

X-Ray Diffraction Study of the Smectic I, F, J and G Phases of—4—2-Methy1Butyl Phenyl 4-n-Octybiphenyl-4-Carboxylate

The smectic I, F, J and G phases of the chiral ferroelectric compound (+)-(4-(2'-methylbutyl)phenyl 4'-n-octylbiphenyl-4-carboxylate) (+ 2M4P8BC) were studied by means of optical microscopy and X-ray diffraction. The free-standing film method was used. The films were additionally ordered with a d. c. electric field and true monodomain samples were prepared and examined.     

Chiral permselectivity in surface-modified nanoporous opal films

Nanoporous 7 mum thin opal films comprising 35 layers of 200 nm diameter SiO2 spheres were assembled on Pt electrodes and modified with chiral selector moieties on the silica surface. Diffusion of chiral redox species through the opals was studied by cyclic voltammetry. The chiral opal films demonstrate high selectivity for transport of one enantiomer over the other. This chiral permselectivity is attributed to the surface-facilitated transport utilizing noncovalent interactions between the chiral permeant molecules and surface-bound chiral selectors.     

New chiral photochromic menthone-containing homopolymers and copolymers - synthesis, phase behaviour and photo-optical properties

A new approach for the preparation of photosensitive materials for coloured data recording and storage is advanced. This approach involves the synthesis of copolymers containing nematogenic and combined (-)-arylidene-p-menthan-3-one chiral photochromic fragments in one monomer unit. The conditions for the formation of an LC phase in the chiral photochromic homopolymers as a function of the structure of the side groups have been identified; for a new series of copolymers, the effect of their composition on the phase behaviour and photooptical properties is considered. Planarly oriented films of the copolymers show selective light reflection in the UV, visible, and near IR spectral regions. The photochemical behaviour of the homopolymers and copolymers in dilute solution and as films has been studied. In these systems, under UV radiation, the E-Z isomerization of the (-)-arylidene-p-menthan-3-one fragment is the dominating process. The kinetic features of the photoprocess are revealed, and the effective quantum yields calculated. The isomerization process leads to dramatic changes in the anisometry of the chiral side groups, and the helical twisting power decreases. Therefore, as a result of UV radiation of films of the copolymers, the selective light reflection peak is shifted to a longer wavelength spectral region. This means that such polymeric films can be considered as promising materials for colour data recording and as storage media.     

Surface chirality of CuO thin films

We present X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD) investigations of CuO thin films electrochemically deposited on an Au(001) single-crystal surface from a solution containing chiral tartaric acid (TA). The presence of enantiopure TA in the deposition process results in a homochiral CuO surface, as revealed by XPD. On the other hand, XPD patterns of films deposited with racemic tartaric acid or the "achiral" meso-tartaric acid are completely symmetric. A detailed analysis of the experimental data using single scattering cluster calculations reveals that the films grown with l(+)-TA exhibit a CuO(1) orientation, whereas growth in the presence of d(-)-TA results in a CuO(11) surface orientation. A simple bulk-truncated model structure with two terminating oxygen layers reproduces the experimental XPD data. Deposition with alternating enantiomers of tartaric acid leads to CuO films of alternating chirality. Enantiospecifity of the chiral CuO surfaces is demonstrated by further deposition of CuO from a solution containing racemic tartaric acid. The pre-deposited homochiral films exhibit selectivity toward the same enantiomeric deposition pathway.     

Surface-induced enantiomorphic crystallization of achiral fullerene derivatives in thin films

The chirality of organic semiconductors is important for various applications in optoelectronics and spintronics. Here, we propose a new strategy to induce structural chirality in achiral organic semiconductors in thin films. Enantiomeric fullerene derivatives (S)-pSi and (R)-pSi, which have oligo(dimethylsiloxane) as a low-surface-energy moiety, were synthesized and used as surface-segregated monolayers (SSMs) in spin-coated films of several achiral fullerene derivatives. Upon thermal annealing, the presence of the chiral SSMs led to the crystallization of the fullerenes in the films as an SSM-induced crystal phase at lower temperatures. The crystallized films showed circular dichroism ascribed to the fullerene absorption, the sign and the intensity of which depended on the handedness of the SSM molecules and the film thickness, respectively. These results indicate that the achiral fullerene derivatives in the films were induced by the SSMs to crystallize into enantiomorphic crystals. Our approach to inducing chirality in organic thin films is compatible with many device applications.

Chiral induction: surface-segregated monolayers of chiral molecules induce the enantiomorphic crystallization of achiral fullerene derivatives in thin films.     

Imaging chirality with surface second harmonic generation microscopy

Chirality is a fundamental construct in nature which arises from an antisymmetric arrangement of atoms, molecules, or larger structures, resulting in the formation of nonsuperimposable mirror images. Bulk chiral effects can easily be measured using circular dichroism (CD) or optical rotary dispersion (ORD). However, the imaging of chirality originating from molecular surface films cannot be obtained with these linear optical methods. By using chiral second harmonic generation (C-SHG), with its inherent surface sensitivity and ability to discriminate between the symmetry of surface adsorbed species in combination with a counter-propagating optical geometry, we have developed the first nonlinear chiral microscope. In the study presented here, the intrinsic chirality of R- and S-(+)-1,1'-bi-2-naphthol (RBN, SBN) has been used to image a patterned planar supported lipid bilayer (PSLB) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using C-SHG. Spatial resolution of the patterned PSLB is visible when either RBN or SBN is intercalated into the membrane. No image is observed when a racemic mixture of RBN and SBN is present. The C-SHG images are compared with those obtained from fluorescence microscopy to verify the C-SHG imaging technique. The results presented here demonstrate that C-SHG possesses the requisite surface selectivity and sensitivity to detect interfacial chirality and provides a direct route for the visualization of chirality originating from molecular surface films.     

Inherently chiral electrodes: the tool for chiral voltammetry

2,2′-Bis[2-(5,2′-bithienyl)]-3,3′-bithianaphthene oligomers are a model case of electroactive films endowed with “inherent chirality”, originating from a stereogenic element coinciding with the whole electroactive backbone, thus resulting in impressive manifestations. This study highlights their applicative potentialities as low-cost and easy-to-prepare artificial enantiopure electrode surfaces, which display an unprecedented ability to pronouncedly separate voltammetry peaks of enantiomers of quite different chiral probes of applicative interest, concurrently with linear dynamic ranges for peak currents, affording enantiomer excess determination. Thus inherently chiral enantiopure electrodes can indeed be regarded as a key to chiral voltammetry.