Chiroptical Spectroscopy Research Articles

Measurement of Raman Optical Activity with High-Frequency Polarization Modulation.

Many chiroptical spectroscopic techniques have been developed to detect chirality in molecular species and probe its role in biological processes. Raman optical activity (ROA) should be one of the most powerful methods, as ROA yields vibrational and chirality information simultaneously and can measure analytes in aqueous and biologically relevant solvents. However, despite its promise, the use of ROA has been limited, largely due to challenges in instrumentation. Here, we report a new approach to ROA that exploits high-frequency polarization modulation. High-frequency polarization modulation, usually implemented with a photoelastic modulator (PEM), has long been the standard technique in other chiroptical spectroscopies. Unfortunately, the need for simultaneous spectral and polarization resolution has precluded the use of PEMs in ROA instruments. We combine a specialized camera system (the Zurich imaging polarimeter, or ZIMPOL) with PEM modulation to perform ROA measurements. We demonstrate performance similar to the current standard in ROA instrumentation while reducing complexity and polarization artifacts. This development should aid researchers in exploiting the full potential of ROA for chemical and biological analysis.

Symmetry and Chirality: Where Physics Shakes Hands with Chemistry and Biology

AbstractIn reflecting the author's personal perspective, this review demonstrates how chirality is a unifying theme in modern science. Using the discrete symmetries of parity P, time reversal T and charge conjugation C to characterize chiral systems, the concepts of true and false chirality that emerge provide an extension of Lord Kelvin's definition to dynamic situations This clarifies the nature of physical influences able to induce absolute enantioselection, emphasizing the distinct behaviour of processes under thermodynamic and kinetic control. Parity violation and CP violation are suggested to be manifestations of true and false cosmic chirality, respectively, with possible roles in abiotic enantioselection. Truly chiral systems support time‐even pseudoscalar observables which endows chiroptical spectroscopies with a special sensitivity to absolute stereochemistry, exemplified by the application of vibrational Raman optical activity to small chiral organic molecules and intact viruses.

Chiral sensing with achiral anisotropic metasurfaces

Recently, we proposed a metasurface design for chiral sensing that (i) results in enhanced chiroptical signals by more than two orders of magnitude for ultrathin, subwavelength, chiral samples over a uniform and accessible area, (ii) allows for complete measurements of the total chirality (magnitude and sign of both its real and imaginary part), and (iii) offers the possibility for a crucial signal reversal (excitation with reversed polarization) that enables chirality measurements in an absolute manner, i.e., without the need for sample removal. Our design is based on the anisotropic response of the metasurface, rather than the superchirality of the generated near-fields, as in most contemporary nanophotonic-based chiral sensing approaches. Here, we derive analytically, and verify numerically, simple formulas that provide insight to the sensing mechanism and explain how anisotropic metasurfaces, in general, offer additional degrees of freedom with respect to their isotropic counterparts. We provide a detailed discussion of the key functionalities and benefits of our proposed design and we demonstrate practical measurement schemes for the unambiguous determination of an unknown chirality. Last, we provide the design principles towards broadband operation - from near-infrared to near-ultraviolet frequencies - opening the way for highly sensitive nanoscale chiroptical spectroscopy.

Mixed Multimetallic tris(2‐pyridylmethyl)amine Based Complexes: Synthesis and Chiroptical Properties

AbstractContinuous intensification of reaction screening methodologies has required the development of novel high‐throughput analytical technologies. This urge involves also chirality related fields where nowadays novel methods for reliable, fast and effective analysis are developed. Within this field, chiroptical spectroscopies have emerged as promising techniques, in particular when they are combined with stereodynamic probes for optical signal amplification. We report here the synthesis and characterisation of a series of molecular architectures based on a modified tris(2‐pyridylmethyl)amine (TPMA) complex which are able to amplify the electronic CD signal of phenylalanine. The study reports the synthesis of six novel TPMA based supramolecular architectures characterised by the presence of two different metals within the same molecule. Besides the synthesis, the study highlights how mixing metals influences the chiroptical properties of the formed compounds.

Water-Mediated Folding Behaviors and Chiroptical Inversion of Ferrocene-Conjugated Dipeptides.

The hydration effect on the folding behavior of oligopeptides is of vital importance both in the structure basis of biomolecules and in the rational design of peptide-based materials, which however has rarely been addressed. Here we present the hydration impact on the spontaneous folding of dipeptides conjugated by the ferrocene spacer. In organic phase, the ferrocene-glycine-phenylalanine dipeptide formed a parallel β-sheet structure and Herrick's conformation, which underwent conformational transformation encountering aqueous media, by significantly switching dipeptide arm angles around the ferrocene axis up to 72°. The conformational transformation behavior aroused inversion of the chiroptical activity. Solid X-ray structures, proton nuclear magnetic resonance, chiroptical spectroscopy, and the density functional theory calculation were employed to unveil the hydration effect in the secondary structure transition, in which the rearrangement of hydrogen bonds played the vital role. This work deepens the understanding of water functioning in the structure modulation of biomolecules and also provides an alternative protocol in designing novel chiroptical switches and adaptive peptide-based biomaterials.

Structural analysis of MDMA in solution by methods of chiroptical spectroscopy supported by DFT calculations

The synthetic drug 3,4-methylenedioxy-N-methylamphetamine (MDMA), also known as ecstasy, is one of the most abused dance drugs spread around the world. The number of seizures of ecstasy tablets increases every year. As MDMA is a chiral substance with one chiral center, it may occur as two enantiomers. These enantiomers have different physiological effects, therefore the reliable detection of individual enantiomers and a detailed structural description are highly desirable. In this work, the 3D structure of MDMA in solution was investigated by vibrational circular dichroism and electronic circular dichroism supplemented by conventional infrared and ultraviolet absorption spectroscopies. The obtained results were supported by density functional theory (DFT) calculations using different levels of theory. Initially, sixty-four starting geometries were optimized at the B3LYP/6-31G(d) level including the solvent effects. Four stable conformers were further reoptimized at higher levels of theory and the spectra were also simulated. The experimental and the calculated spectra were compared via the similarity index. The spectra comparison showed very good agreement, which enabled us to describe the molecular structure of MDMA in detail.

A theoretical study to the loliolide molecule and its isomers: a study by circular dichroism, QTAIM, and NMR theoretical methods.

The determination of an absolute configuration is a challenge in the structure elucidation of chiral natural products. With advancements in computational chemistry of chiroptical spectroscopy, the time-dependent density functional theory (TDDFT) calculation has emerged as a very promising tool. This paper attempts to illustrate the applicability of computational approaches in comparison with experimental data to understand the conformation, interaction, and stabilization of the loliolide's isomers. The quantum chemical calculations were used from optimized geometries of the (6R,7aS)-, (6S,7aR)-, (6R,7aR)-, and (6S,7aS)-6-hydroxy-4,4,7a-trimethyl-6,7-dihydro-5H-1-benzofuran-2-one. The spectroscopic values were obtained for 13C NMR isotropic shielding by GIAO method in mPW1PW91/cc-pVTZ level, in TDDFT at the ωB97X-D/cc-pVTZ level to the circular dichroism, and in theoretical analyses of non-covalent interaction to study the isomer's stability. The TDDFT calculation of circular dichroism can be used to quantify the individual isomers and the nature of excitation in the molecule. The (6R,7aS) and (6R,7aR) isomers present a higher stability due to electronegativity associated at the hydroxyl group.

Orientation averaging of optical chirality near nanoparticles and aggregates

Artificial nanostructures enable fine control of electromagnetic fields at the nanoscale, a possibility that has recently been extended to the interaction between polarised light and chiral matter. The theoretical description of such interactions, and its application to the design of optimised structures for chiroptical spectroscopies, brings new challenges to the common set of tools used in nano-optics. In particular, chiroptical effects often depend crucially on the relative orientation of the scatterer and the incident light, but many experiments are performed with randomly oriented scatterers, dispersed in a solution. We derive new expressions for the orientation-averaged local degree of optical chirality of the electromagnetic field in the presence of a nanoparticle aggregate. This is achieved using the superposition $T$-matrix framework, ideally suited for the derivation of efficient orientation-averaging formulas in light scattering problems. Our results are applied to a few model examples and illustrate several nonintuitive aspects in the distribution of orientation-averaged degree of chirality around nanostructures. The results will be of significant interest for the study of nanoparticle assemblies designed to enhance chiroptical spectroscopies, and where the numerically efficient computation of the averaged degree of optical chirality enables a more comprehensive exploration of the many possible nanostructures.

Traceability and Development of Optical Activity and Chiroptical Spectroscopy

Traceability and Development of Optical Activity and Chiroptical Spectroscopy

Absolute Configuration Reassignment of Natural Products: An Overview of the Last Decade

The assignment of absolute configuration (AC) is a crucial step in the structural characterization of natural products, especially for those subjected to biological assays. Methods such as X-ray crystallography, stereocontrolled organic synthesis, nuclear magnetic resonance (NMR), and chiroptical spectroscopies are commonly used to determine the AC of chiral natural compounds. Even with these well-established techniques, however, unambiguous stereochemical assignments of natural products remain a challenge, resulting in an increasing number of structural misassignments being reported every year. Herein, we will present the main techniques that have been used in AC reassignments of natural products over the last 10 years, along with some selected examples. Special attention will be paid to the strengths and weaknesses of each approach. With this, we expect to provide the readers with critical information to help them to choose the appropriate methods for correct AC determinations.

Fluorescence detected circular dichroism (FDCD) for supramolecular host-guest complexes.

Fluorescence-detected circular dichroism (FDCD) spectroscopy is applied for the first time to supramolecular host–guest and host–protein systems and compared to the more known electronic circular dichroism (ECD). We find that FDCD can be an excellent choice for common supramolecular applications, e.g. for the detection and chirality sensing of chiral organic analytes, as well as for reaction monitoring. Our comprehensive investigations demonstrate that FDCD can be conducted in favorable circumstances at much lower concentrations than ECD measurements, even in chromophoric and auto-emissive biofluids such as blood serum, overcoming the sensitivity limitation of absorbance-based chiroptical spectroscopy. Besides, the combined use of FDCD and ECD can provide additional valuable information about the system, e.g. the chemical identity of an analyte or hidden aggregation phenomena. We believe that simultaneous FDCD- and ECD-based chiroptical characterization of emissive supramolecular systems will be of general benefit for characterizing fluorescent, chiral supramolecular systems due to the higher information content obtained by their combined use.

Single‐Layered Reflective Metasurface Achieving Simultaneous Spin‐Selective Perfect Absorption and Efficient Wavefront Manipulation

AbstractAs the 2D counterpart of metamaterials, metasurfaces have drawn wide attentions in recent years due to their unique electromagnetic (EM) properties and subwavelength footprints. Although great efforts have been made in metasurface‐based full vectorial manipulation of EM waves, the realization of multifunctional devices with high compactness is still challenging. Here, by employing the mutual coupling between the meta‐atoms to induce spin‐selective magnetic resonance, a generalized design principle based on single‐layered metasurface is proposed that can achieve giant chiral absorption with average circular dichroism larger than 80%. In addition, arbitrary wavefront manipulation for reflected waves can also be realized with operation efficiency larger than 88%. As a proof of concept, three kinds of chiral devices including beam deflector, vortex beam generator, and meta‐hologram are numerically investigated in infrared region with high performance. It is believed that these multifunctional devices with simple geometry have great potentials in the field of chiral imaging, chiroptical spectroscopy, and EM communication.

Chiroptical spectroscopy of a freely diffusing single nanoparticle

Chiral plasmonic nanoparticles can exhibit strong chiroptical signals compared to the corresponding molecular response. Observations are, however, generally restricted to measurements on stationary single particles with a fixed orientation, which complicates the spectral analysis. Here, we report the spectroscopic observation of a freely diffusing single chiral nanoparticle in solution. By acquiring time-resolved circular differential scattering signals we show that the spectral interpretation is significantly simplified. We experimentally demonstrate the equivalence between time-averaged chiral spectra observed for an individual nanostructure and the corresponding ensemble spectra, and thereby demonstrate the ergodic principle for chiroptical spectroscopy. We also show how it is possible for an achiral particle to yield an instantaneous chiroptical response, whereas the time-averaged signals are an unequivocal measure of chirality. Time-resolved chiroptical spectroscopy on a freely moving chiral nanoparticle advances the field of single-particle spectroscopy, and is a means to obtain the true signature of the nanoparticle’s chirality.

Chiroptical Properties in Thin Films of π-Conjugated Systems.

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (gabs and glum) and on the application of ECD and CPL to derive structural information on aggregated states.

Chiroptical Spectroscopy: Toward Chiral Sensing and Spectroscopy Enabled by All‐Dielectric Integrated Photonic Waveguides (Laser Photonics Rev. 14(9)/2020)

Chiroptical Spectroscopy: Toward Chiral Sensing and Spectroscopy Enabled by All‐Dielectric Integrated Photonic Waveguides (Laser Photonics Rev. 14(9)/2020)

Recent research progress on surface-enhanced spectra ofchiral molecules

Chiral molecules are ubiquitous in natural world. Molecular chirality determines their physical and chemical properties. Detection and characterization of these chiral enantiomers are of considerable importance for biochemical and pharmaceutical industries. Conventional chiroptical spectroscopy techniques nowadays can not meet the needs in practical applications, with the disadvantages of small signal and large noise, etc. It is necessary to solve these problems and characterize chiral enantiomers fast and sensitively. Surface-enhanced spectroscopic techniques delivered through specifically designed nanostructures have been proved to be highly successful for ultrasensitive chiral detection, with dramatic enhanced interactions between light and chiral molecules induced by nanostructures. There are considerable researches on surface-enhanced spectroscopic techniques in recent years, and the enhancement effect on the circular dichroism (CD), vibrational circular dichroism (VCD) and Raman optical activity (ROA) spectrum is excellent. In this review, we present a survey of the recent work about surface-enhanced spectroscopic techniques in the two aspects of metal and dielectric nanostructures, respectively. Plasmonic nanostructures based on metallic elements can provide highly enhanced electric fields, which ensures the generation of strong chiral near fields, showing promise for the enhancement of detection sensitivities of chiral spectroscopies. The dielectric nanostructures exhibit strong enhancement of the electric near-field while simultaneously providing intense magnetic resonances, which can generate uniform-sign superchiral field, ensuring the untrasensitive chiral sensing. Besides, dielectric nanostructures show extremely low optical losses, which is very beneficial for the detection of enhanced chiroptical spectroscopy. With surface-enhanced spectroscopic method, it is promising to solve the problems in the chirality characterization techniques nowadays and lead to unprecedented levels of chiroptical sensitivity.

Chiral Functionalization of an Atomically Precise Noble Metal Cluster: Insights into the Origin of Chirality and Photoluminescence.

We probe the origin of photoluminescence of an atomically precise noble metal cluster, Ag24Au1(DMBT)18 (DMBT = 2,4-dimethylbenzenethiolate), and the origin of chirality in its chirally functionalized derivatives, Ag24Au1(R/S-BINAS)x(DMBT)18-2x, with x = 1-7 (R/S-BINAS = R/S-1,1'-[binaphthalene]-2,2'-dithiol), using chiroptical spectroscopic measurements and density functional theory (DFT) calculations. Combination of chiroptical and luminescence spectroscopies to understand the nature of electronic transitions has not been applied to such molecule-like metal clusters. In order to impart chirality to the achiral Ag24Au1(DMBT)18 cluster, the chiral ligand, R/S-BINAS, was incorporated into it. A series of clusters, Ag24Au1(R/S-BINAS)x(DMBT)18-2x, with x = 1-7, were synthesized. We demonstrate that the low-energy electronic transitions undergo an unexpected achiral to chiral and back to achiral transition from pure Ag24Au1(DMBT)18 to Ag24Au1(R/S-BINAS)x(DMBT)18-2x, by increasing the number of BINAS ligands. The UV/vis, luminescence, circular dichroism, and circularly polarized luminescence spectroscopic measurements, in conjunction with DFT calculations, suggest that the photoluminescence in Ag24Au1(DMBT)18 and its chirally functionalized derivatives originates from the transitions involving the whole Ag24Au1S18 framework and not merely from the icosahedral Ag12Au1 core. These results suggest that the chiroptical signatures and photoluminescence in these cluster systems cannot be solely attributed to any one of the structural components, that is, the metal core or the protecting metal-ligand oligomeric units, but rather to their interaction and that the ligand shell plays a crucial role. Our work demonstrates that chiroptical spectroscopic techniques such as circular dichroism and circularly polarized luminescence represent useful tools to understand the nature of electronic transitions in ligand-protected metal clusters and that this approach can be utilized for gaining deeper insights into the structure-property relationships of the electronic transitions of such molecule-like clusters.

Resonance Raman Optical Activity Shows Unusual Structural Sensitivity for Systems in Resonance with Multiple Excited States: Vitamin B12 Case.

In this work, cobalamins with different upper axial substituents and a cobalamin derivative with a ring modification were studied using chiroptical spectroscopies, in particular resonance Raman optical activity (RROA), to shed light on the influence of structural modifications on RROA spectra in these strongly chiral systems in resonance with multiple excited states at 532 nm excitation. We have demonstrated that for these unique systems RROA possesses augmented structural specificity, surpassing resonance Raman spectroscopy and enabling at the same time measurement of cobalamins at fairy low concentrations of ∼10–5 mol dm–3. The enhanced structural specificity of RROA is a result of bisignate spectra due to resonance via more than one electronic state. The observation of increased structural capability of RROA for cobalamins opens a new perspective for studying chiral properties of other biological systems incorporating d-metal ions.

Chiral Triptycenes in Supramolecular and Materials Chemistry.

Triptycenes are an intriguing class of organic molecules with several unusual characteristics, such as a propeller‐like shape, saddle‐like cavities around a symmetrical scaffold, a rigid π‐framework. They have been extensively studied and proposed as key synthons for a variety of applications in supramolecular chemistry and materials science. When decorated with an appropriate substitution pattern, triptycenes can be chiral, and, similarly to other popular chiral π‐extended synthons, can express chirality robustly, efficiently, and with relevance to chiroptical spectroscopies. This minireview highlights and encompasses recent advances in the synthesis of chiral triptycenes and in their introduction as molecular scaffolds for the assembly of functional supramolecular materials.

Thin Films of Homochiral Metal–Organic Frameworks for Chiroptical Spectroscopy and Enantiomer Separation

Chiral nanoporous solids are a fascinating class of materials, allowing efficient enantiomer separation. Here, we review the status, applications, and potential of thin films of homochiral metal–organic frameworks (MOFs). Combining the advantages of MOFs, whose well-defined, crystalline structures can be rationally tuned, with the benefits of thin films enables new opportunities for the characterization of the enantioselectivity, e.g., via chiroptical spectroscopy and straightforward molecular uptake quantifications. By incorporating photoresponsive molecules in the chiral MOF films, the enantioselectivity of the material can be dynamically remote-controlled. The most promising application of MOF films is their use as membranes, where the enantioselective separation of chiral molecules is demonstrated and parameters for further improvements are discussed.