Plasmonic Interactions at Close Proximity in Chiral Geometries: Route toward Broadband Chiroptical Response and Giant Enantiomeric Sensitivity

Chiral metamaterials can have diverse technological applications, such as engineering strongly twisted local electromagnetic fields for sensitive detection of chiral molecules, negative indices of refraction, broadband circular polarization devices, and many more. These are commonly achieved by arranging a group of noble-metal nanoparticles in a chiral geometry, which, for example, can be a helix, whose chiroptical response originates in the dynamic electromagnetic interactions between the localized plasmon modes of the individual nanoparticles. A key question relevant to the chiroptical response of such materials is the role of plasmon interactions as the constituent particles are brought closer, which is investigated in this paper through theoretical and experimental studies. The results of our theoretical analysis, when the particles are brought in close proximity are dramatic, showing a large red shift and enhancement of the spectral width and a near-exponential rise in the strength of the chiroptical response. These predictions were further confirmed with experimental studies of gold and silver nanoparticles arranged on a helical template, where the role of particle separation could be investigated in a systematic manner. The ``optical chirality'' of the electromagnetic fields in the vicinity of the nanoparticles was estimated to be orders of magnitude larger than what could be achieved in all other nanoplasmonic geometries considered so far, implying the suitability of the experimental system for sensitive detection of chiral molecules.

Shape Memory Alloy based Active Chiral Composite Cells

Wing morphing is one of the emerging methodology towards improving aerodynamic efficiency of flight vehicle structures. In this paper a morphing structural element is designed and studied which has its origin in the well known chiral structures. The new aspect of design and functionality explored in this paper is that the chiral cell is actuated using thermal Shape Memory Alloy (SMA) actuator wires to provide directional motion. Such structure utilizes the potential of different actuations concepts based on actuator embedded in the chiral structure skin. This paper describes a new class of chiral cell structure with integrated SMA wire for actuation. Chiral topological constructs are obtained by considering passive and active load path decoupling and sub-optimal shape changes. Single cell of chiral honeycomb with actuators are analyzed using finite element simulation results and experiments. To this end, a multi-cell plan-form is characterized showing interesting possibilities in structural morphing applications. The applicability of the developed chiral cell to flexible wing skin, variable stiffness based design and controlling longitudinal-to-transverse stiffness ratio are discussed.

A versatile ternary ionic complex for chiral discrimination of molecules with diverse functionalities using H-1 NMR

This study reports a simple, efficient and versatile protocol developed for NMR spectroscopic enantiodiscrimination of molecules containing diverse functional -groups, such as amino alcohols, secondary alcohols, cyanohydrins, oxazolidones, diols, thiones and epoxides, using a phosphorous based three component mixture. The simple mixing and shaking of enantiopure 1,1'-binaphthyt-2,2'-diyl hydrogenphosphate (BNPA), 4-(dimethylamino)pyridine (DMAP) and a chiral analyte in the solvent CDCl3 served as a chiral solvating agent and resulted in well dispersed peaks for each enantiomer in the H-1 NMR spectrum. Discrimination could be achieved not only for the proton at the chiral centre, but also for multiple proton sites. The devised approach also permitted the precise measurement of the enantiomeric excess (ee).

VIBRATIONAL CHARACTERISTICS OF ZIGZAG, ARMCHAIR AND CHIRAL CANTILEVER SINGLE-WALLED CARBON NANOTUBES

Finite element analysis has been performed to study vibrational characteristics of cantilever single walled carbon nanotubes. Finite element models are generated by specifying the C-C bond rigidities, which are estimated by equating energies from molecular mechanics and continuum mechanics. Bending, torsion, and axial modes are identified based on effective mass for armchair, zigzag and chiral cantilever single walled carbon nanotubes, whose Young's modulus is evaluated from the bending frequency. Empirical relations are provided for frequencies of bending, torsion, and axial modes.

Synthesis and Antioxidant Activities of Novel Chiral Ebselen Analogues

Novel chiral analogues of the antioxidant, anti-inflammatory organoselenium drug ebselen were synthesized. The reaction proceeded readily from 2-(chloroseleno)benzoyl chloride with chiral amino compounds. The chiral substituent on the nitrogen atom did not provide a substantial increase in activities and the newly synthesized compounds showed similar activities to ebselen.

Recent NMR Methodological Developments for Chiral Analysis in Isotropic Solutions

Chiral auxiliaries are used for NMR spectroscopic study of enantiomers. Often the presence of impurities, severe overlap of peaks, excessive line broadening and complex multiplicity pattern restricts the chiral analysis using 1D H-1 NMR spectrum. There are few approaches to resolve the overlapped peaks. One approach is to use suitable chiral auxiliary, which induces large chemical shift difference between the discriminated peaks (Delta delta(R,S)) and minimize the overlap. Another direction of approach is to design appropriate NMR experiments to circumvent some of these problems, viz, enhancing spectral resolution, unravelling the superimposed spectra of enantiomers, and reduction of spectral complexity. Large number of NMR techniques, such as two dimensional selective F-1 decoupling, RES-TOCSY, multiple quantum detection, frequency selective homodecoupling, band selective homodecoupling, broadband homodecoupling, etc. have been reported for such a purpose. Many of these techniques have aided in chiral analysis for molecules of diverse functionality in the presence of chiral auxiliaries. The present review summarizes the recently reported NMR experimental methodologies, with a special emphasis on the work carried out in authors' laboratory.

Charge Transfer Induces Formation of Stimuli-Responsive, Chiral, Cohesive Vesicles-on-a-String that Eventually Turn into a Hydrogel

A charge transfer (CT) mediated two-component, multistimuli responsive supergelation involving a L-histidine-appended pyrenyl derivative (PyHisOMe) as a donor and an asymmetric bolaamphiphilic naphthalene-diimide (Asym-NDI) derivative as an acceptor in a 2: 1 mixture of H2O/MeOH was investigated. Asym-NDI alone self-assembled into pH-responsive vesicular nanostructures in water. Excellent selectivity in CT gel formation was achieved in terms of choosing amino acid appended pyrenyl donor scaffolds. Circular di-chroism and morphological studies suggested formation of chiral, interconnected vesicular assemblies resembling ``pearls-on-a-string'' from these CT mixed stacks. XRD studies revealed the formation of monolayer lipid membranes from these CT mixed stacks that eventually led to the formation of individual vesicles. Strong cohesive forces among the interconnected vesicles originate from the protrusion of the oxyethylene chains from the surfaces of the chiral vesicles.

Chiral discrimination of secondary alcohols and carboxylic acids by NMR spectroscopy

The manuscript reports two novel ternary ion-pair complexes, which serve as chiral solvating agents, for enantiodiscrimination of secondary alcohols and carboxylic acids. The protocol for discrimination of secondary alcohols is designed by using one equivalent mixture each of enantiopure mandelic acid, 4-dimethylaminopyridine (DMAP) and a chiral alcohol. For discrimination of carboxylic acids, the ternary complex is obtained by one equivalent mixture each of enantiopure chiral alcohol, DMAP and a carboxylic acid. The designed protocols also permit accurate measurement of enantiomeric composition. Copyright (C) 2014 John Wiley & Sons, Ltd.

RNA nucleosides as chiral sensing agents in NMR spectroscopy

The study reports chiral sensing properties of RNA nucleosides. Adenosine, guanosine, uridine and cytidine are used as chiral derivatizing agents to differentiate chiral 1 degrees-amines. A three component protocol has been adopted for complexation of nucleosides and amines. The chiral differentiating ability of nucleosides is examined for different amines based on the H-1 NMR chemical shift differences of diastereomers (Delta delta(R,S)). Enantiomeric differentiation has been observed at multiple chemically distinct proton sites. Adenosine and guanosine exhibit large chiral differentiation (Delta delta(R,S)) due to the presence of a purine ring. The diastereomeric excess (de) measured by using adenosine is in good agreement with the gravimetric values.

Ag+-induced reverse vesicle to helical fiber transformation in a self-assembly by adjusting the keto-enol equilibrium of a chiral salicylideneaniline

A new chiral amphiphilic salicylideneaniline bearing a terminal pyridine was synthesized. It formed reverse vesicles in toluene. The addition of Ag+, however, reversibly transforms these reverse vesicles into left-handed nanohelices accompanied by spontaneous gel formation at room temperature.

Efficient and practical synthesis of modular chiral beta-organochalcogeno amines, ArYCH2CH(R)NH2, and single crystal structures of (S) - MsOCH2CH(Bz)NH3+ center dot Cl- and (R)-MsOCH2CH(Ph)NH3+ center dot Cl-

Modular chiral I3-organochalcogeno amines, ArYCH2CH(R)NH2 (4a-4g) where R = Me, Bz, Ph; and ArY = PhS, BzSe and 4-MeOC6H4Te respectively have been synthesized and characterized. Compounds 4a-4g were synthesized (Method II) from chiral aminoalkyl 13-methanesulfonate hydrochlorides, MsOCH2CH(R)NH3+ center dot Cl- (2a-2c) through nucleophilic displacement of MsO- with organochalcogenolate (ArY-). In another attempt (Method I) chiral beta-organotelluro amines (4a-4c) were prepared by deprotection of chiral N-boc I3-organotelluro amides, 4-MeOC6H4TeCH2CH(R)NH-Boc (3a-3c), which in turn, 13,-,1 were made from chiral N-boc 13-methanesulfonate amides (la-lc) and ArTeNa. 1H, and FTIR spectra of all the compounds (3a-3c and 4a-4g) were characteristic. The composition of 3a-3c was determined by elemental analysis. The a]TD values of 3b-3c and 4a-4g were determined. The single crystal structures of (S)-2b and (R)-2c were determined by X-Ray diffraction studies. Both (S)-2b and (R)2c were crystallized in orthorhombic system and the Flack parameter x was found 0.08(12) and 0.00(2) respectively. The crystal of (S)-2b contain two asymmetric units with gauche (A) and staggered (B) conformations. There are NH Cl-, NH-O and CH-O intra and intermolecular secondary interactions in (S)-2b and (R)-2c resulting in supramolecular structures. (C) 2015 Elsevier By. All rights reserved.

First donor-acceptor interaction promoted gelation of organic fluids

In recent years there has been considerable interest in developing new types of gelators of organic solvents.1 Despite the recent advances, a priori design of a gelator for gelling a given solvent has remained a challenging task. Various noncovalent interactions like hydrogen-bonding,2 metal coordination3 etc. have been used as the driving force for the gelation process. A special class of cholesterol-based gelators were reported by Weiss,4 and by Shinkai.5 Gels derived from these molecules have been used for chiral recognition/sensing,6 for studying photo- and metal-responsive functions,7 and as templates to make hollow fiber silica.8 Other types of organogels have been used for designing polymerized 9 and reverse aerogels,10 and in molecular imprinting.11 Hanabusa’s group has recently reported organogels with a bile acid derivative.12 This has prompted us to disclose our results on a novel electron donor–acceptor (EDA) interaction mediated two-component13 gelator system based on the bile acid14 backbone.

Constraining the low-energy pion electromagnetic form factor with space-like and phase of time-like data

The Taylor coefficients c and d of the EM form factor of the pion are constrained using analyticity, knowledge of the phase of the form factor in the time-like region, 4m(pi)(2) <= t <= t(in) and its value at one space-like point, using as input the (g - 2) of the muon. This is achieved using the technique of Lagrange multipliers, which gives a transparent expression for the corresponding bounds. We present a detailed study of the sensitivity of the bounds to the choice of time-like phase and errors present in the space-like data, taken from recent experiments. We find that our results constrain c stringently. We compare our results with those in the literature and find agreement with the chiral perturbation-theory results for c. We obtain d similar to O(10) GeV-6 when c is set to the chiral perturbation-theory values.

Enantiospecific total synthesis of ent-10, 11-thapsan-10-ol

First enantiospecific total synthesis of optical antipode of the sesquiterpene 10,11-epoxythapsan-10-ol has been described. (R)-Carvone has been employed as the chiral starting material and a combination of intramolecular alkylation and Criegee fragmentation are employed for intramolecular stereospecific transfer of the chirality. An intramolecular diazoketone cyclopropanation and regioselective cyclopropane ring cleavage reactions have been employed for the creation of the three requisite contiguous quaternary carbon atoms.

Impressive Gelation in Organic Solvents by Synthetic, Low Molecular Mass, Self-Organizing Urethane Amides of L-Phenylalanine

Synthetic routes leading to 12 L-phenylalanine based mono- and bipolar derivatives (1-12) and an in-depth study of their structure-property relationship with respect to gelation have been presented. These include monopolar systems such as N-[(benzyloxy)carbonyl]-L-phenylalanine-N-alkylamides and the corresponding bipolar derivatives with flexible and rigid spacers such as with 1,12-diaminododecane and 4,4'-diaminodiphenylmethane, respectively. The two ends of the latter have been functionalized with N-[(benzyloxy)carbonyl]-L-phenylalanine units via amide connection. Another bipolar molecule was synthesized in which the middle portion of the hydrocarbon segment contained polymerizable diacetylene unit. To ascertain the role of the presence of urethane linkages in the gelator molecule protected L-phenylalanine derivatives were also synthesized in which the (benzyloxy)carbonyl group has been replaced with (tert-butyloxy)carbonyl, acetyl, and benzoyl groups, respectively. Upon completion of the synthesis and adequate characterization of the newly described molecules, we examined the aggregation and gelation properties of each of them in a number of solvents and their mixtures. Optical microscopy and electron microscopy further characterized the systems that formed gels. Few representative systems, which showed excellent gelation behavior was, further examined by FT-IR, calorimetric, and powder X-ray diffraction studies. To explain the possible reasons for gelation, the results of molecular modeling and energy-minimization studies were also included. Taken together these results demonstrate the importance of the presence of (benzyloxy)carbonyl unit, urethane and secondary amide linkages, chiral purities of the headgroup and the length of the alkyl chain of the hydrophobic segment as critical determinants toward effective gelation.