Optical Activity Measurements Research Articles

Resolving old problems with layered polytungstates related to hexagonal tungsten bronze: phase formation, structures, crystal chemistry and some properties.

A 60-year-old problem with the atomic arrangements and exact compositions of alkali polytungstates related to hexagonal tungsten bronze (HTB) was solved. The systems A2WO4-WO3 (A = K, Rb) were restudied and the average monoclinic layered structures of stoichiometric polytungstates A4W11O35 (A = K, Rb, Cs, Tl) and A2W7O22 (A = K, Rb, Cs) were first successfully determined. The structures resemble those of "MoW11O36" and "MoW14O45" (J. Graham and A. D. Wadsley, Acta Crystallogr., 1961, 14, 379-383) and are derived from HTB by breaking into slabs parallel to (100) due to the ordered omission of some [WO]∞ chains along the hexagonal tunnels. The slabs in A4W11O35 (A = Cs, Tl) and A2W7O22 (A = Rb, Cs) are mutually shifted by the a/2 HTB unit cell axis. These data mainly confirmed our preliminary structural models of HTB-like alkali polytungstates (S. F. Solodovnikov, N. V. Ivannikova, Z. A. Solodovnikova and E. S. Zolotova, Inorg. Mater., 1998, 34, 845-853) and revealed a new similar thallium polytungstate. The structures of the HTB-like polytungstates and related compounds form a homologous series of layered complex oxides or fluorides An+2-xM3n+2X9n+8 where n = 2, 3 and 4 are equal to the numbers of HTB hexagonal tunnels across the polytungstate slab width for Tl2W4O13, A4W11O35 and A2W7O22 (A = K, Rb, Cs or Tl), respectively. The structures of the HTB-like polytungstates seem to intergrow with HTB-type AxWO3 to form, in particular, higher homologues of the series. Our group-supergroup analysis, measurements of nonlinear optical activity and electrical conductivity, and calculations of the bond-valence site energy barriers indicate possible ferroelectric/ferroelastic properties and moderate 2D oxide-ion mobility within the HTB-type slabs of the studied polytungstates.

Measuring optical activity with unpolarized light: Ghost polarimetry

Quantifying the optical activity of a sample requires the precise measurement of the rotation of the plane of linear polarization of the transmitted light. Central to this notion is that the sample needs to be exposed to light of a defined polarization state. We show that by using a polarization-entangled photon source we can measure optical activity whilst illuminating a sample with unpolarized light. This not only allows for low light measurement of optical activity but also allows for the analysis of samples that would otherwise be perturbed if subjected to polarized light.

Experimentally monitoring of the rapid biomolecular hydrolysis reaction with optical weak measurement

The real-time measurement of optical activity is of great significance for studying the chemical reactions of chiral molecules. In this study, we have built a set of linear common path optical weak measurement chiral sensor system in frequency domain, and the optical sensitivity of the system feasibly reaches 1524 for activity. Using this system, we have realized the state measurement of the ultrafast alkaline hydrolysis reaction of gluconolactone under different concentrations of catalyst. This study shows that the optical weak measurement chiral sensor can well monitor and compare the degree of hydrolysis reaction, and partly reflect the approximate change of the optical rotation in the real-time process of hydrolysis. Optical weak measurement chiral sensing does not require separation and labeling molecules, and has great application potential for the production and development of chiral compounds, especially chiral drugs.

Resolving Resonant Electronic States in Chiral Metal Complexes by Raman Optical Activity Spectroscopy.

Chiral metal complexes exhibit rich photophysical properties and are important for applications ranging from biosensing to photocatalysis. We present a combined experimental and computational approach leading to information about energies and transition moments of excited electronic states, documented on two chiral metal complexes. The experimental protocol for measurement of the resonance Raman optical activity comprises multiple techniques, i.e., absorption, circular dichroism, and polarized and differential Raman scattering. An accurate formula for subtraction of the interfering circular dichroism/polarized Raman scattering effect is given. An analysis of the spectra based on density functional theory calculations unveils the geometric and electronic structures of the molecules. Such insight into molecular electronic states of chromophores may be useful for understanding and tuning photochemical properties of metal-containing complexes, biomolecules, and supramolecules.

Raman Optical Activity of 1T-TaS2.

Measurements of optical activity can be readily performed in transparent matter by means of a rotation of transmitted light polarization. In the case of opaque bulk materials, such measurements cannot be performed, making it difficult to assess possible chiral properties. In this work, we present full angular polarization dependencies of the Raman modes of bulk 1T-TaS2, which has recently been suggested to have chiral properties after pulsed laser excitation. We found that a mechanical rotation of the sample does not alter polarization-resolved Raman spectra, which can only be explained by introducing an antisymmetric Raman tensor, frequently used to describe Raman optical activity (ROA). Raman spectra obtained under circularly polarized excitation demonstrate that 1T-TaS2 indeed shows ROA, providing strong evidence that 1T-TaS2 is chiral under the used conditions of laser excitation. Our results suggest that ROA may be used as a universal tool to study chiral properties of quantum materials.

Electronic Circular Dichroism-Circularly Polarized Raman (eCP-Raman): A New Form of Chiral Raman Spectroscopy.

This Concept article summarizes recent work on the development of a new form of chiral Raman spectroscopy, eCP-Raman, which combines two spectroscopies: electronic circular dichroism (ECD) and circularly polarized Raman (CP-Raman). First, some puzzling observations while carrying out Raman optical activity (ROA) measurements of several transition metal complexes under resonance are described, as well as the search for the mechanisms responsible. Then an equation for quantifying the eCP-Raman contribution is presented, followed by several examples of how eCP-Raman influences the IR -IL spectra of achiral and chiral solvent molecules and of a number of chiral solutes under resonance. The conditions to extract resonance ROA, when the eCP-Raman contribution is minimized, are also discussed. Finally, we comment on the potential applications of eCP-Raman.

Algorithms for calculating polarization direction based on spatial modulation of vector optical field

Polarization is an important property of electromagnetic waves, and measuring their polarization properties fast and precisely is a very important issue in many applications, such as skylight polarization navigation, optical activity measurement, imaging polarimetry, spectroscopic ellipsometry, and fluorescence polarization immunoassay . The polarization measurement method based on vector optical field modulation and image processing is a new type of spatial modulation polarization detection technology. The key step of this technique moving to practical application is determined by effective polarization measuring algorithms with high speed and accuracy. In order to find out the method of fast and precisely calculating polarization direction, the principle of polarization direction measurement based on vector optical field and spatial modulation is introduced briefly, and the basic characteristics of the spatially modulated intensity distribution images are analyzed. According to the properties of spatially modulated image, we propose and implement four different polarization direction calculation methods, which are the Radon transform, intensity modulation curve detection, radial integration, and image correlation detection, and also introduce their working principles and physical thoughts elaborately. To compare the detailed performances of these four algorithms, an experimental setup is constructed to collect the images and perform the algorithm verification, and the stabilities, speeds and accuracies of the four algorithms are compared. The research results indicate that all the four methods can achieve their stable and reliable polarization direction detections. The three methods, intensity modulation curve detection, radial integration and image correlation detection, can achieve the polarization direction measuring accuracy better than 0.01° . The intensity modulation curve detection and radial integration have relatively fast calculation speed and the best comprehensive performances , and are the most promising methods to realize real-time and high-precision polarization measurement.

Research on Algorithms for Calculating Polarization Direction Based on Spatial Modulation of Vector Optical Field

Polarization is an important property of electromagnetic waves, and measuring their polarization properties fast and precisely is a very important issue in many applications, such as skylight polarization navigation, optical activity measurement, imaging polarimetry, spectroscopic ellipsometry, fluorescence polarization immunoassay and so on. The polarization measurement method based on vector optical field modulation and image processing is a new type of spatial modulation polarization detection technology. The key step of this technique moving to practical application is determined by the effective polarization measuring algorithms with high speed and accuracy. In order to find out the fast and high precision polarization direction calculation methods, the principle of polarization direction measurement based on vector optical field and spatial modulation is introduced briefly, and the basic characteristics of the spatially modulated intensity distribution images are analyzed. According to the properties of spatially modulated image, we have designed and implemented four different polarization direction calculation methods, which are the Radon transform, intensity modulation curve detection, radial integration and image correlation detection, and their working principles and physical thoughts are introduced elaborately. To compare the detailed performance of these four algorithms, an experimental setup has been constructed to collect the images and perform the algorithm verification, and the stability, speed and accuracy of the four algorithms are compared respectively. The research results indicate that all the four methods can achieve stable and reliable polarization direction detection. The three methods of intensity modulation curve detection, radial integration and image correlation detection, can obtain the measurement accuracy of polarization direction better than 0.01 degrees. The intensity modulation curve detection and radial integration own the fast calculation speed relatively, and their overall performances are the best, and they are the most promising methods for realizing real-time and high-precision polarization measurement.

Broadband Optical Activity Spectroscopy with Interferometric Fourier-Transform Balanced Detection.

Spectrally resolved measurements of optical activity, such as circular dichroism (CD) and optical rotatory dispersion (ORD), are powerful tools to study chiroptical properties of (bio)molecular and nanoplasmonic systems. The wider utilization of these techniques, however, has been impeded by the bulky and slow design of conventional spectropolarimeters, which have been limited to a narrowband scanning approach for more than 50 years. In this work, we demonstrate broadband measurements of optical activity by combining a balanced detection scheme with interferometric Fourier-transform spectroscopy. The setup utilizes a linearly polarized light field that creates an orthogonally polarized weak chiral free-induction-decay field, along with a phase-locked achiral transmitted signal, which serves as the local oscillator for heterodyne amplification. By scanning the delay between the two fields with a birefringent common-path interferometer and recording their interferogram with a balanced detector that measures polarization rotation, broadband CD and ORD spectra are retrieved simultaneously with a Fourier transform. Using an incoherent thermal light source, we achieve state-of-the-art sensitivity for CD and ORD across a broad wavelength range in a remarkably simple setup. We further demonstrate the potential of our technique for highly sensitive measurements of glucose concentration and the real-time monitoring of ground-state chemical reactions. The setup also accepts broadband pulses and will be suitable for broadband transient optical activity spectroscopy and broadband optical activity imaging.

Chiral Charge Carriers Chiralions for Polyaniline Examined with Optical Rotation Spectroelectrochemistry

In this study, the optical active polyaniline film was examined with optical rotatory dispersion (ORD) through change in potential with spectroelectrochemical method. Polyaniline is one of the representative conducting polymers. Preparation of polyaniline is convenient because synthesis of polyaniline can perform in water medium. Conducting polymers shows electrochromism via change in the electronic state upon electrochemical redox process. Polyaniline forms various electronic states such as emeraldine base, emeraldine salt, and pernigraniline. This change allows to color tuning through electrochemical method. Further, optically active polyanilines have been synthesized in the presence of (+) or (-)-camphorsulfonic acid (CSA) by electrochemical polymerization or chemical polymerization with ammonium persulfate (oxidizing agent). Both electrochemical method and chemical method produce polyaniline through radical polymerization process. Polyaniline emeraldine salt as prepared form of polyaniline is doped with CSA, showing optical activity in the circular dichroism. In this research, we carried out to synthesis of polyaniline in the presence of optically active CSA, according to previously reported method (by Wallace et al). Here, we have found that the polyaniline thus prepared showed optically active electrochromism via electrochemical redox process in circular dichroism spectroscopy measurements. In our previous study, we calculated the ORD spectra for the polymer by Kramars - Kronich (K-K) conversion from circular dichromism. In the present research, we successfully obtained experimental data of optical rotation-spectroelectrochemical result, as a first example of optical rotation control of natural optical activity for conjugated polymers based on electrochemical method. In the case of low potential for the polymer in the electrochemical redox process, the polymer shows intense optical rotation at visible wavelength range. On the other hands, higher potential at around 1 V affords to give doping process for the polymer, resulting generation of polaron as charge carrier, which shows optical activity at near infrared region. We referred this optical charge carrier as “chiral charge carrier chiralions”. The electrochemical oxidation equivalents to electrochemical doping from the polymer. In other words, electrochemical oxidation gives electrochemical doping which results change in degree of optical rotation for the polymer. The control of optical ration for materials can be driven by magnetic field (Faraday rotation), and electric field (Kerr rotation). We propose a new phenomenon as ‘‘electrochemical optical rotation’’. The electro-conductivity of the polymer is measured with four-probe method. The polymer which is electronic doped state with (+)-CSA as electric dopant for production of radical cation (polarons) or dication (bipolarons) as charge carriers and chiral inducer for twisting main-chain. The polymer film was synthesized as followed. First, electrochemical polymerization of aniline as a monomer was carried out in excessive optically active (+)-CSA with three electrode system consisting of working electrode (indium-tin oxide coated glass), counter electrode (platinum wire), and reference electrode (SCE) in water. The polymerization reaction is rapidly progressed with formation of polymer thin film onto the counter electrode. The polymer was washed with excess amount of water, acetone. The electroactive polymer film was thus deposited on the electrode was used for optical activity measurements. Cyclic voltammetry (CV) measurements were carried out for the resultant polymer (monomer free) deposited on to the ITO coated glass electrode. The CV results with change in scan rate from 10-100 mV/s indicates electroactive. Increase of scan rate increases current density for the polymer due to increase of diffusion of ion in the electrolyte solution. Oxidation signals of the polymer are observed due to change in electronic state of the polymer. Corresponding reduction signals against oxidation process are observed, indicating reversible redox activity for the polymer. ORD measurement reveled that the polymer film thus obtained is optically active. The polyaniline can be formed helical structure due to doping of optically active CSA. We carried out in-situ ORD measurement with potential change, evaluating electrochemical driven change in optical rotation. Chemical structure of the double layer polymer is examined with Fourie transform infrared optical absorption spectroscopy measurements which reveled this polymer is polyaniline, and CSA as a dopant. Ultra-violet optical absorption measurements for the polymer film indicates intense and wide range absorptions from invisible to infrared range as charge carriers the delocalized along the main-chain, indicating the polymers are conductive materials. The advantages of the polymer film are formation of self-standing redox activity, tuning of optical activity and color coordination. This paper reports novel function of control optical rotation with electrochemical method examined with spectroelectrochemistry. This concept can be a first example as optical rotation effect of electro-optically active conducting polymers. This paper reports the charge carries of helical polyaniline as chiralions can tune via electrochemical method.

Phase-stable optical activity measurement by common-path spectral interferometry.

A robust optical activity (OA) spectrometer covering the visible and near-infrared regimes was designed and built via a combination of a linear polarizer and a birefringent plate. The OA spectrometer relies on common-path spectral interferometry, where the two interfering fields travel common optical paths, and ensures signal reproducibility over several hours. By detecting OA without polarization switching, the data acquisition time is shortened to 1 s, enabling real-time monitoring of the chiral complex formation. The present configuration also allows OA measurement with broadband pulses, which is promising for probing ultrafast circular dichroism and optical rotatory dispersion.

Spectroradiometric detection of competitor diatoms and the grazer Poteriochromonas in algal cultures

To address challenges in early detection of pond pests, we have extended a spectroradiometric monitoring method, initially demonstrated for measurement of pigment optical activity and biomass, to the detection of algal competitors and grazers. The method relies upon measurement and interpretation of pond reflectance spectra spanning from the visible into the near-infrared. Reflectance spectra are acquired every 5 min with a multi-channel, fiber-coupled spectroradiometer, providing monitoring of algal pond conditions with high temporal frequency. The spectra are interpreted via numerical inversion of a reflectance model, in which the above-water reflectance is expressed in terms of the absorption and backscatter coefficients of the cultured species, with additional terms accounting for the pigment fluorescence features and for the water-surface reflection of sunlight and skylight. With this method we demonstrate detection of diatoms and the predator Poteriochromonas in outdoor cultures of Nannochloropsis oceanica and Chlorella vulgaris, respectively. The relative strength of these signatures is compared to microscopy and sequencing analysis. Spectroradiometric detection of diatoms is then further assessed on beaker-contained mixtures of Microchloropsis salina with Phaeodactylum tricornutum, Thalassiosira weissflogii, and Thalassiosira pseudonana, respectively, providing an initial evaluation of the sensitivity and specificity of detecting pond competitors.

CONSTRUCTION OF A LOW-COST POLARIMETER FOR EDUCATIONAL PURPOSES

This paper describes the construction of a polarimeter for the identification of chemical substances. The device was built with low-cost materials, measures the phenomenon of optical activity and has educational applicability. The measurement of optical activity is achieved through the measurement of the resulting polarized radiation in a photodiode connected to a voltmeter after passing through polarizing and analyzing filters.

Solution Structure of Mannobioses Unravelled by Means of Raman Optical Activity.

Structural analysis of carbohydrates is a complicated endeavour, due to the complexity and diversity of the samples at hand. Herein, we apply a combined computational and experimental approach, employing molecular dynamics (MD) and density functional theory (DFT) calculations together with NMR and Raman optical activity (ROA) measurements, in the structural study of three mannobiose disaccharides, consisting of two mannoses with varying glycosidic linkages. The disaccharide structures make up the scaffold of high mannose glycans and are therefore important targets for structural analysis. Based on the MD population analysis and NMR, the major conformers of each mannobiose were identified and used as input for DFT analysis. By systematically varying the solvent models used to describe water interacting with the molecules and applying overlap integral analysis to the resulting calculational ROA spectra, we found that a full quantum mechanical/molecular mechanical approach is required for an optimal calculation of the ROA parameters. Subsequent normal mode analysis of the predicted vibrational modes was attempted in order to identify possible marker bands for glycosidic linkages. However, the normal mode vibrations of the mannobioses are completely delocalised, presumably due to conformational flexibility in these compounds, rendering the identification of isolated marker bands unfeasible.

Raman Optical Activity on a Solid Sample: Identification of Atropisomers of Perfluoroalkyl Chains Having a Helical Conformation and No Chiral Center.

Perfluoroalkyl (Rf) chains have a specific helical conformation due to the steric repulsion between the adjacent CF2 units. Although Rf chains have no chiral center, two chiral structures, i.e., the right-handed (R) and left-handed (L) helices, are available as the most stable conformations, which are atropisomers to each other. According to the stratified dipole array (SDA) theory, the helical structure about the chain axis plays a key role in the spontaneous molecular aggregation of Rf chains in a two-dimensional manner, and the Rf chains having the same chirality tend to be aggregated spontaneously to generate molecular domains. This implies that an Rf compound in a solid state should be a mixture of the R and L domains, and each domain should exhibit distinguishable optical activity. To identify molecular domains with different atropisomers, in this study, Raman optical activity (ROA) measurements were performed on a Raman imaging spectrometer. Through the ROA measurements of recrystallized solid samples of an Rf compound, each particle exhibits an apparent optical activity, and the two atropisomers were readily distinguished. As a result, an Rf compound with the same helicity is found to be spontaneously aggregated as expected by the SDA theory.

ONE-POT DIASTEREOSELECTIVE SYNTHESIS OF CHIRAL TRICYCLIC l-CYSTEINE AND d-PENICILLAMINE DERIVATIVES: A LABORATORY EXPERIMENT

A one-pot diastereoselective synthesis of thiazolidine-ring fused systems derived from enantiomerically pure amino acids, L-cysteine or D-penicillamine, and achiral succindialdehyde is described as an experiment to be carried out by upper-division undergraduate students in a laboratory classroom. Reactions were performed under mild conditions, the products were isolated through simple experimental procedures and fully characterized. This study combines organic synthesis, determination of the purity of compounds (TLC analysis and melting point measurements), optical activity measurements as well as structural analysis (interpretation of 1D NMR and 2D NMR spectra). It offers a platform for the discussion of important organic chemistry concepts such as diastereoselectivity, kinetic control vs thermodynamic control and cyclization reactions via nucleophilic addition to imines/iminium cations.

Twins in YAl3(BO3)4 and K2Al2B2O7 Crystals as Revealed by Changes in Optical Activity

Many borate crystals feature nonlinear optical properties that allow for efficient frequency conversion of common lasers down into the ultraviolet spectrum. Twinning may degrade crystal quality and affect nonlinear optical properties, in particular if crystals are composed of twin domains with opposing polarities. Here, we use measurements of optical activity to demonstrate the existence of inversion twins within single crystals of YAl 3 (BO 3 ) 4 (YAB) and K 2 Al 2 B 2 O 7 (KABO). We determine the optical rotatory dispersion of YAB and KABO throughout the visible spectrum using a spectrophotometer with rotatable polarizers. Space-resolved measurements of the optical rotation can be related to the twin structure and give estimates on the extent of twinning. The reported dispersion relations for the rotatory power of YAB and KABO may be used to assess crystal quality and to select twin-free specimens.

Quantum Design for Ultrafast Probing of Molecular Chirality through Enantiomer-Specific Coherent π-Electron Angular Momentum

Probing molecular chirality, right-handed or left-handed chiral molecules, is one of the central issues in chemistry and biochemistry. The conventional theory of optical activity measurements such as circular dichroism has been derived in the second-order processes involving electric and magnetic dipole moments, and the signals are very weak. We propose an efficient enantiomer-probing scenario for chiral aromatic ring molecules based on photoinduced coherent π-electron rotations. In our model, the resultant laser-induced currents themselves produce a strong magnetic field. The principle for probing molecular chirality is a utilization of dynamic Stark effects of two electronic excited states. These electronic states subjected to strong nonresonant linearly polarized UV lasers become degenerate to create enantiomer-specific electronic angular momentum. A pair of enantiomers of phenylalanine was taken as an example. Enantiomer-specific coherent magnetic fluxes on the order of a few teslas can be generated in several tens of femtoseconds. The direct detection of strong coherent magnetic fluxes could be carried out by time-resolved magnetic force microscopy experiments. The results provide important implications for the measurement of effective probing of chiral aromatic molecules.

Amplified measurement of weak optical activity using a spin-phase-gradient beam.

Development of alternate techniques to polarimetry for the measurement of weak optical rotation, with improved sensitivity, is becoming increasingly important as one understands the role of chirality in drug design and synthesis and the fundamentals of chiral light-matter interaction. We demonstrate here an optical amplification scheme using a spin-phase-gradient beam to measure ultra-small optical rotation angle (4mdeg), with a sensitivity of 220μdeg/μm, due to dilute (mg/mL) dextro-rotatory sugar solution. A Soleil-Babinet compensator is used to generate a tunable spin-phase-gradient beam which enables us to achieve high measurement sensitivities. Theoretical formalism of the technique leads us to the possibility to realize much higher measurement sensitivity of up to 10μdeg/μm by tuning-in the experimental parameters.

Composite materials containing chromophores with 3,7-(di)vinylquinoxalinone π-electron bridge doped into PMMA: Atomistic modeling and measurements of quadratic nonlinear optical activity

Two novel chromophores with dibutylaniline donor moiety, 3,7-(di)vinylquinoxalinone π-electron bridge and tricyanofuranyl or dicyanovinyl acceptor moieties are synthesized and their properties were systematically investigated. Atomistic modeling of PMMA-based composites with the developed chromophores as guests is performed and nonlinear optical activity is studied for the first time. Atomistic modeling performed in the amorphous cell has shown that chromophore content 20 wt% may be considered most favorable for achievement of optimal d33 values for 7-DBA-VQonV-TCF/PMMA material. Various non-covalent interactions both between chromophores and polymer matrix and chromophores inter se were revealed in the course of modeling; these interactions could contribute to the relaxation stability of orientational order and thus quadratic NLO response. Composite material 7-DBA-VQonV-TCF/PMMA is characterized by nonlinear-optical coefficient, d33, values of 97, 111 and 47 pm/V at chromophore content of 10, 20 and 30 wt%, respectively. Thus chromophore content 20 wt% provides maximal value of NLO coefficient, what is in agreement with the obtained theoretical data. The d33 value for 7-DBA-VQonV-DCV/PMMA is 2.5 times smaller than that for 7-DBA-VQonV-TCF/PMMA at the same chromophore load (20 wt%).