Circular Intensity Difference Research Articles

Chiral-Induced Surface-Enhanced Raman Optical Activity on a Single-Particle Substrate.

Surface-enhanced Raman optical activity (SEROA) is a promising method for analyzing chiral molecules' molecular chirality and structural changes. However, conventional SEROA measurements face challenges related to substrate stability, signal uniformity, and interference from electronic circular dichroism (ECD). Therefore, in this study, we present a uniform and stable substrate for SEROA measurements by utilizing Au nanoparticles on the Au nanofilm structure to confine hotspots to the film-particle junctions and minimize ECD interference. This method also uses the induction of chirality from chiral molecules to achiral molecules to overcome the limitation of chiral molecules in SEROA measurements, specifically their lower signal efficiency. Successful chirality transfer is demonstrated through distinguishable SEROA signals when the l/d-alanine mixture is present. Enantiomeric discrimination of different l/d-alanine ratios was achieved with linear responses in the circular intensity difference (CID). Altogether, the proposed chiral-induced SEROA on the AuNP_on_AuNF substrate shows promising potential for detecting and characterizing structural changes in biomolecules, thus making it a valuable tool for various research applications.

On the use of property-oriented basis sets for the simulation of vibrational chiroptical spectroscopies

We computed vibrational circular dichroism (VCD) and Raman optical activity (ROA) spectra for a test set of six chiral compounds using two standard density-functionals and an array of basis sets. We analysed the performance of property-oriented basis sets using a quadruple-zeta basis as a reference against four key metrics. We find little qualitative difference between the spectra produced by the larger basis sets (ORP, LPolX, aug-cc-pVTZ, and aug-cc-pVQZ), though their quantitative metrics exhibit wide variations. The smaller basis sets (rDPS, augD-3-21G, augT3-3-21G, Sadlej-pVTZ, and aug-cc-pVDZ) performed better for VCD rotatory strengths than for the corresponding ROA circular intensity differences (CIDs). However, this trend diminishes as the basis-set size is increased, lending validity to the conclusion that more robust property-oriented basis sets are required for ROA spectral generation than that of VCD. We observed improved performance in the mid-infrared region compared to the high-frequency regime, as well as overestimation of VCD rotatory strengths in the latter region as compared to the reference. We conclude that the ORP and LPol-ds basis sets are the most efficient and effective choices of basis set for the prediction of VCD and ROA spectra, as they provide both highly accurate results at reduced computational expense.

Three-dimensional imaging of ferroaxial domains using circularly polarized second harmonic generation microscopy

The spontaneous symmetry breakdown of matter is one of the most important concepts in materials physics and leads to a phase transition into an ordered phase and domain formation in its consequence. The so-called ‘ferroaxial order’ characterized by a rotational structural distortion with an axial vector symmetry has gained growing interest as a new class of ordered state. However, the observation of ferroaxial domain states, that is, clockwise and counterclockwise rotational states, is not straightforward and has been little investigated. Here, we propose that the circular intensity difference in second harmonic generation (CID-SHG) offers an experimental technique to investigate ferroaxial order and its domain states through the transition process of higher-order multipoles such as magnetic-dipole and electric-quadrupole. By using CID-SHG microscopy, we successfully visualize three-dimensional images of ferroaxial domain structures in NiTiO3. Our results indicate that CID-SHG is a sensitive probe of ferroaxial order and opens possibilities for the use of ferroaxial materials in nonlinear optical manipulations.

Quantum interference directed chiral raman scattering in two-dimensional enantiomers

Raman scattering spectroscopy has been a necessary and accurate tool not only for characterizing lattice structure, but also for probing electron-photon and electron-phonon interactions. In the quantum picture, electrons at ground states can be excited to intermediate energy levels by photons at different k-points in the Brillouin zone, then couple to phonons and emit photons with changed energies. The elementary Raman processes via all possible pathways can interfere with each other, giving rise to intriguing scattering effects. Here we report that quantum interference can lead to significant chiral Raman response in monolayer transitional metal dichalcogenide with triclinic symmetry. Large circular intensity difference observed for monolayer rhenium dichalcogenide originates from inter-k interference of Raman scattering excited by circularly polarized light with opposite helicities. Our results reveal chiral Raman spectra as a new manifestation of quantum interference in Raman scattering process, and may inspire induction of chiral optical response in other materials.

Irreducible Cartesian Tensor Analysis of Harmonic Scattering from Chiral Fluids

Symmetry principles of several distinct kinds are revealingly engaged in an analysis focussing on third harmonic scattering, a current focus of research on nonlinear optics in chiral media. Analysis in terms of irreducible Cartesian tensors elucidates the detailed electrodynamical origin and character of the corresponding material properties. Considerations of fundamental charge, parity and time reversal (CPT) symmetry reveal the conditions for an interplay of transition multipoles to elicit a chiral response using circularly polarised pump radiation, and the symmetry of quantised angular momentum underpins the associated selection rules and angular distribution. The intrinsic structural symmetry of chiral scatterers determines their capacity to exhibit differential response. Exploiting permutational index symmetry in the response tensors enables quantitative assessment of the boundary values for experimentally measurable properties, including circular intensity differentials.

Vibrational Raman optical activity of camphor: The importance of electric‐dipole—electric‐quadrupole polarizability contribution

AbstractThe importance of the electric‐dipole—electric‐quadrupole polarizability contribution to the vibrational Raman optical activity (VROA) and dimensionless circular intensity difference spectra of (1S)‐camphor is examined. The spectra are simulated with and without this tensor contribution using density functional theory calculations, and similarity is evaluated for each against the experimental spectra. Careful examination of the comparison between experimental and calculated spectra reveals multiple vibrational bands, in the ~1,130–950 cm−1 region, originating from −C–H bending vibrations that are dominated by the electric‐dipole—electric‐quadrupole polarizability contributions. The similarity overlap analysis also reveals that the similarity overlap between experimental and predicted spectra in the measured spectral range increases, by up to 12%, when electric‐dipole—electric‐quadrupole polarizability contributions are included. The negative VROA band at 1,125 cm−1 in the experimental spectrum of (1S)‐(‐)‐camphor can only be reproduced in the predicted spectra when electric‐dipole—electric‐quadrupole polarizability contribution is included. Investigations on additional molecules indicated that (a) two experimental VROA bands of (1S,4R)‐(+)‐fenchone at ~1,740 and 220 cm−1 originate from dominating electric‐dipole—electric‐quadrupole polarizability contribution and (b) the symmetric and antisymmetric ring deformation modes of dimethyloxirane have dominating electric‐dipole—electric‐quadrupole polarizability contribution. These observations establish the importance of electric‐dipole—electric‐quadrupole polarizability contribution to VROA for the first time.

Femtosecond Vibrational Sum-Frequency Generation Spectroscopy of Chiral Molecules in Isotropic Liquid.

Vibrationally resonant optically active (VOA) sum-frequency generation (SFG) is a second-order nonlinear process sensitive to the stereospecific vibrational structure of chiral molecules. We demonstrate that a femtosecond VOA SFG signal can be measured in the isotropic bulk of a chiral liquid. The chiral, achiral, and VOA SFG spectra of R- and S-limonene and their racemic mixture in the C-H stretching frequency region are characterized. In particular, it is shown that the observed circular intensity difference (CID) signal, which can provide distinguishable stereochemical vibrational information between enantiomers, arises from interference of the electric-dipole allowed antisymmetric Raman tensor-induced and Raman optical activity (ROA) tensor-induced SFG fields. Furthermore, we show that the CID and linear polarization intensity difference (LID) SFG spectra are connected to the real and imaginary parts of the effective chiral VOA SFG susceptibility, respectively. We anticipate that the present technique will be of use in transient chiroptical spectroscopy and stereochemical vibrational imaging studies.

Giant Optical Activity in an All-Dielectric Spiral Nanoflower.

Optical activity is an effect of prominent importance in stereochemistry, analytical chemistry, metamaterials, spin photonics, and astrobiology, but is naturally minuscule. Metallic nanostructures are commonly exploited as basic elements for artificially producing large optical activity by virtue of surface plasmon resonance (SPR) on the nanostructures. However, their intrinsic high ohmic loss amplified by the SPR results in low energy efficiency and large photothermal heat generation, severely limiting their performance and practical utility. Giant optical activity by inducing magnetic resonance in an all-dielectric spiral nanoflower (spiral-flower-shaped nanostructure) is demonstrated here. Specifically, a large circular-intensity difference of ≈35% is theoretically predicted and experimentally demonstrated by optimizing the magnetic quadrupole contribution of the nanoflower to scattered light. The nanoflower overcomes the bottleneck of the traditional metallic platforms and enables the development of diverse chiroptical devices and applications.

Anomalous Polarized Raman Scattering and Large Circular Intensity Differential in Layered Triclinic ReS2.

The Raman tensor of a crystal is the derivative of its polarizability tensor and is dependent on the symmetries of the crystal and the Raman-active vibrational mode. The intensity of a particular mode is determined by the Raman selection rule, which involves the Raman tensor and the polarization configurations. For anisotropic two-dimensional (2D) layered crystals, polarized Raman scattering has been used to reveal the crystalline orientations. However, due to its complicated Raman tensors and optical birefringence, the polarized Raman scattering of triclinic 2D crystals has not been well studied yet. Herein, we report the anomalous polarized Raman scattering of 2D layered triclinic rhenium disulfide (ReS2) and show a large circular intensity differential (CID) of Raman scattering in ReS2 of different thicknesses. The origin of CID and the anomalous behavior in polarized Raman scattering were attributed to the appearance of nonzero off-diagonal Raman tensor elements and the phase factor owing to optical birefringence. This can provide a method to identify the vertical orientation of triclinic layered materials. These findings may help to further understand the Raman scattering process in 2D materials of low symmetry and may indicate important applications in chiral recognition by using 2D materials.

Vibrational Raman optical activity of bicyclic terpenes: comparison between experimental and calculated vibrational Raman, Raman optical activity, and dimensionless circular intensity difference spectra and their similarity analysis

Subsequent to publication of Polavarapu et al.1 in Journal of Raman Spectroscopy Volume 48 Issue 2 February 2017, we noticed that we incorrectly specified the absolute configurations of 2-carene and 3-carene and the following changes are to be noted: The (+)-(1R,6S) label associated with both 2-carene and 3-carene in Tables 1, 2 and 3 and in legends of Figures 1, 5, 6 and 7 should be (+)-(1S,6R). In Figures 5 and 6, the experimental ROA spectra of (+) enantiomers of 2-carene and 3-carene were compared with those predicted for 2-carene and 3-carene with (1S,6R) configuration. The later were obtained by multiplying the ROA spectra calculated for (1R,6S) configuration with -1.

CDSpecTech: A single software suite for multiple chiroptical spectroscopic analyses.

The program CDSpecTech was developed to facilitate the analysis of chiroptical spectra, which include the following: vibrational circular dichroism (VCD) and corresponding vibrational absorption (VA) spectra; vibrational Raman optical activity (VROA) and corresponding vibrational Raman spectra; electronic circular dichroism (ECD) and corresponding electronic absorption (EA) spectra. In addition, the program allows for generating optical rotatory dispersion (ORD) as the Kramers-Kronig transform of ECD spectra. The simulation of theoretical spectra from transition strengths can be achieved using different bandshape profiles. The experimental and simulated theoretical spectra can be visually compared by displaying them together. A unique feature of CDSpecTech is performing spectral analysis using the ratio spectra; i.e., the dimensionless dissymmetry factor (DF) spectrum, which is the ratio of CD to absorption spectra, and the dimensionless circular intensity difference (CID) spectrum, which is the ratio of VROA to vibrational Raman spectra. The quantitative agreement between experimental and simulated theoretical spectra can also be assessed from the numerical similarity overlap between them. Two different similarity overlap methods are available. The program uses a graphical user interface which allows for ease of use and facilitates the analysis. All these features make CDSpecTech a valuable tool for the analysis of chiroptical spectra. The program is freely available on the World Wide Web.

Gauge-origin independent calculations of electric-field-induced second-harmonic generation circular intensity difference using London atomic orbitals

ABSTRACTWe present the first gauge-origin independent calculations of the circular intensity difference (CID) in electric-field-induced second-harmonic generation (EFISHG), including all contributions up to the electric-quadrupole–magnetic-dipole level. A recursive, open-ended response theory framework in combination with the use of London atomic orbitals allows us to ensure gauge-origin independent results. We apply this approach to study EFISHG-CID in a collection of chiral amino acids. We demonstrate that diffuse polarising basis functions are critical in order to obtain accurate CIDs, and that a basis set of at least aug-cc-pVTZ quality is needed in order to obtain results close to the basis-set limit. The use of London orbitals does not lead to significantly faster basis set convergence, although the improved basis set convergence allows the aug-cc-pVDZ basis set to be used with some confidence for larger molecules.

Vibrational Raman optical activity of bicyclic terpenes: comparison between experimental and calculated vibrational Raman, Raman optical activity, and dimensionless circular intensity difference spectra and their similarity analysis

Experimental vibrational Raman, vibrational Raman optical activity (VROA), and dimensionless circular intensity difference (CID) spectra are analyzed with quantum chemical (QC) predictions of the corresponding spectra using the B3LYP functional and the aug-cc-pVDZ basis set, for five bicyclic terpenes as test cases. The experimentally measured vibrational Raman and VROA spectra have arbitrary y-axis intensities, because of their dependence on instrumental parameters. As a result, the absolute magnitudes of intensities in experimental and calculated vibrational Raman or vibrational ROA spectra are not normally analyzed. However, the experimental dimensionless CID spectrum is independent of the instrumental parameters and can be evaluated quantitatively with corresponding QC predicted CID spectrum. The similarity overlaps between experimental and quantum chemical predictions of vibrational Raman, VROA, and dimensionless CID spectra are evaluated. The quantum chemical predictions of dimensionless CID spectral magnitudes at B3LYP/aug-ccpVDZ level match well with the corresponding experimental magnitudes. The individual anisotropic contributions to VROA and CID spectra are also presented. The anisotropic contribution involving electric dipole-electric quadrupole polarizability derivative tensor is relatively smaller than that involving electric dipole-magnetic dipole polarizability derivative tensor. A new format, which contains all of the information needed for the spectral analysis, has been advanced for presenting the vibrational Raman, VROA, and CID spectra and their similarity analysis with corresponding QC-predicted spectra. Copyright © 2016 John Wiley & Sons, Ltd.

Strongly Enhanced Raman Optical Activity in Molecules by Magnetic Response of Nanoparticles

An analytical theory for the surface-enhanced Raman optical activity (SEROA) with the magnetic response of the substrate particle has been presented. We have demonstrated that the SEROA signal is proportional to the magnetic polarizability of the substrate particle, which can be significantly enhanced due to the existence of the magnetic response. At the same time, a large circular intensity difference (CID) for the SEROA can also be achieved in the presence of the magnetic response. Taking Si nanoparticles as examples, we have found that the CID enhanced by a Si nanoparticle is 10 times larger than that of Au. Furthermore, when the molecule is located in the hotspot of a Si dimer, CID can be 60 times larger. The phenomena originate from large magnetic fields concentrated near the nanoparticle and boosted magnetic dipole emission of the molecule. The symmetric breaking of the electric fields caused by the magnetic dipole response of the nanoparticle also plays an important role. Our findings provide a new way to tailor theRaman optical activity by designing metamaterials with the strong magnetic response.

Diamagnetic Raman Optical Activity of Chlorine, Bromine, and Iodine Gases

AbstractMagnetic Raman optical activity of gases provides unique information about their electric and magnetic properties. Magnetic Raman optical activity has recently been observed in a paramagnetic gas (Angew. Chem. Int. Ed. 2012, 51, 11058; Angew. Chem. 2012, 124, 11220). In diamagnetic molecules, it has been considered too weak to be measurable. However, in chlorine, bromine and iodine vapors, we could detect a significant signal as well. Zeeman splitting of electronic ground‐state energy levels cannot rationalize the observed circular intensity difference (CID) values of about 10−4. These are explicable by participation of paramagnetic excited electronic states. Then a simple model including one electronic excited state provides reasonable spectral intensities. The results suggest that this kind of scattering by diamagnetic molecules is a general event observable under resonance conditions. The phenomenon sheds new light on the role of excited states in the Raman scattering, and may be used to probe molecular geometry and electronic structure.

Diamagnetic Raman Optical Activity of Chlorine, Bromine, and Iodine Gases.

Magnetic Raman optical activity of gases provides unique information about their electric and magnetic properties. Magnetic Raman optical activity has recently been observed in a paramagnetic gas (Angew. Chem. Int. Ed. 2012, 51, 11058; Angew. Chem. 2012, 124, 11220). In diamagnetic molecules, it has been considered too weak to be measurable. However, in chlorine, bromine and iodine vapors, we could detect a significant signal as well. Zeeman splitting of electronic ground-state energy levels cannot rationalize the observed circular intensity difference (CID) values of about 10(-4). These are explicable by participation of paramagnetic excited electronic states. Then a simple model including one electronic excited state provides reasonable spectral intensities. The results suggest that this kind of scattering by diamagnetic molecules is a general event observable under resonance conditions. The phenomenon sheds new light on the role of excited states in the Raman scattering, and may be used to probe molecular geometry and electronic structure.

Detection of Circularly Polarized Luminescence of a Cs‐EuIII Complex in Raman Optical Activity Experiments

AbstractCircularly polarized luminescence (CPL) spectra are extremely sensitive to molecular structure. However, conventional CPL measurements are difficult and require expensive instrumentation. As an alternative, we explore CPL using Raman scattering and Raman optical activity (ROA) spectroscopy. The cesium tetrakis(3‐heptafluoro‐butylryl‐(+)‐camphorato) europium(III) complex was chosen as a model as it is known to exhibit very large CPL dissymmetry ratio. The fluorescent bands could be discriminated from true Raman signals by comparison of spectra acquired with different laser excitation wavelengths. Furthermore, the ROA technique enables fluorescence identification by measuring the degree of circularity. The CPL dissymmetry ratio was measured as the ROA circular intensity difference of 0.71, the largest one ever reported. The alternative CPL measurement enhances applications of lanthanides in analytical chemistry and chemical imaging of biological objects.

Detection of Circularly Polarized Luminescence of a Cs-Eu(III) Complex in Raman Optical Activity Experiments.

Circularly polarized luminescence (CPL) spectra are extremely sensitive to molecular structure. However, conventional CPL measurements are difficult and require expensive instrumentation. As an alternative, we explore CPL using Raman scattering and Raman optical activity (ROA) spectroscopy. The cesium tetrakis(3-heptafluoro-butylryl-(+)-camphorato) europium(III) complex was chosen as a model as it is known to exhibit very large CPL dissymmetry ratio. The fluorescent bands could be discriminated from true Raman signals by comparison of spectra acquired with different laser excitation wavelengths. Furthermore, the ROA technique enables fluorescence identification by measuring the degree of circularity. The CPL dissymmetry ratio was measured as the ROA circular intensity difference of 0.71, the largest one ever reported. The alternative CPL measurement enhances applications of lanthanides in analytical chemistry and chemical imaging of biological objects.

ChemInform Abstract: Comparison of Experimental and Calculated Chiroptical Spectra for Chiral Molecular Structure Determination

AbstractReview: 40 refs.

Comparison of the electronic and vibrational optical activity of a europium(III) complex.

The geometry and the electronic structure of chiral lanthanide(III) complexes are traditionally probed by electronic methods, such as circularly polarised luminescence (CPL) and electronic circular dichroism (ECD) spectroscopy. The vibrational phenomena are much weaker. In the present study, however, significant enhancements of vibrational circular dichroism (VCD) and Raman optical activity (ROA) spectral intensities were observed during the formation of a chiral bipyridine-Eu(III) complex. The ten-fold enhancement of the vibrational absorption and VCD intensities was explained by a charge-transfer process and the dominant effect of the nitrate ion on the spectra. A much larger enhancement of the ROA and Raman intensities and a hundred-fold increase of the circular intensity difference (CID) ratio were explained by the resonance of the λ = 532 nm laser light with the (7)F0 → (5)D0 transitions. This phenomenon is combined with a chirality transfer, and mixing of the Raman and luminescence effects involving low-energy (7)F states of europium. The results thus indicate that the vibrational optical activity (VOA) may be a very sensitive tool for chirality detection and probing of the electronic structure of Eu(III) and other coordination compounds.