Molecular Vibrations in Chiral Europium Complexes Revealed by Near-Infrared Raman Optical Activity

Raman optical activity (ROA) is commonly measured with green light (532 nm) excitation. At this wavelength, however, Raman scattering of europium complexes is masked by circularly polarized luminescence (CPL). This can be avoided using near-infrared (near-IR, 785 nm) laser excitation, as demonstrated here by Raman and ROA spectra of three chiral europium complexes derived from camphor. Since luminescence is strongly suppressed, many vibrational bands can be detected. They carry a wealth of structural information about the ligand and the metal core, and can be interpreted based on density functional theory (DFT) simulations of the spectra. For example, jointly with ROA experimental data, the simulations make it possible to determine absolute configuration of chiral lanthanide compounds in solution.

Molecular Properties of 3d and 4f Coordination Compounds Deciphered by Raman Optical Activity Spectroscopy

Molecular properties of coordination compounds can be efficiently studied by vibrational spectroscopy. The scope of Raman spectroscopy has been greatly enhanced by the introduction of Raman optical activity (ROA) sensitive to chirality. The present review describes some of its recent applications to study the coordination compounds. 3d and 4f metal complexes often absorb the excitation light, or exhibit luminescence. Therefore, effects caused in ROA spectra by electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) must be taken into consideration.In 3d metal complexes ECD and circularly-polarized Raman scattering compete with the resonance ROA (RROA) signal. Pure RROA spectrum can thus be obtained by subtracting the so-called ECD-Raman component. CPL is frequently encountered in 4f systems. While it can mask the ROA spectra, it is useful to study molecular structure. These electronic effects can be reduced by using near-infrared excitation although vibrational ROA signal is much weaker compared to the usual green laser excitation scenario. The ROA methodology is thus complex, but capable of providing unique information about the molecules of interests and their interaction with light.

Detection and analysis of chiral molecules as disease biomarkers

The chirality of small metabolic molecules is important in controlling physiological processes and indicating the health status of humans. Abnormal enantiomeric ratios of chiral molecules in biofluids and tissues occur in many diseases, including cancers and kidney and brain diseases. Thus, chiral small molecules are promising biomarkers for disease diagnosis, prognosis, adverse drug-effect monitoring, pharmacodynamic studies and personalized medicine. However, it remains difficult to achieve cost-effective and reliable analysis of small chiral molecules in clinical procedures, in part owing to their large variety and low concentration. In this Review, we describe current and emerging techniques that detect and quantify small-molecule enantiomers and their biological importance.

A Raman optical activity spectrometer can sensitively detect lanthanide circularly polarized luminescence

Recently, many studies have appeared in which the Raman optical activity (ROA) instrument was found to be convenient for measuring circularly polarized luminescence (CPL). Typically, weak lanthanide luminescence including circular polarization could be detected. The new detection scheme is referred to as ROA-CPL spectroscopy. It is particularly useful when also the vibrational (ROA) itself is detectable as the molecule structure can be examined more reliably. In this review, development of this chiroptical approach and its applications in structural studies of biomolecules are summarized.

Measurement and Theory of Resonance Raman Optical Activity for Gases, Liquids, and Aggregates. What It Tells about Molecules

Resonance Raman Optical Activity (RROA) appeared as a natural extension of the nonresonance branch. It combines the structural sensitivity of chiroptical spectroscopy with the signal enhancement coming from the resonance of molecular electronic transitions with the excitation laser light. However, the idea has been hampered by many technical and theoretical problems that are being clarified only in recent years. We provide the theoretical basis and several examples documenting the problems, achievements, and potential of RROA, in particular in biomolecular studies.

Theory and algorithms for chiroptical properties and spectroscopies of aqueous systems

Chiroptical properties and spectroscopies are valuable tools to study chiral molecules and assign absolute configurations. The spectra that result from chiroptical measurements may be very rich and complex, and hide much of their information content. For this reason, the interplay between experiments and calculations is especially useful, provided that all relevant physico-chemical interactions that are present in the experimental sample are accurately modelled. The inherent difficulty associated to the calculation of chiral signals of systems in aqueous solutions requires the development of specific tools, able to account for the peculiarities of water-solute interactions, and especially its ability to form hydrogen bonds. In this perspective we discuss a multiscale approach, which we have developed and challenged to model the most used chiroptical techniques.

Twisted paddlewheel rhodium complexes: Contribution of central and axial chirality to ECD, VCD, and NMR spectra

Dirhodium complexes bearing N-substituted chiral amino acid ligands are investigated. These complexes have an unusual twisted paddlewheel structure, showing inherent chirality. We would like to demonstrate that parallel application of chiroptical spectroscopic methods (ECD and VCD) and NMR spectroscopy combined with quantum chemical calculations constitutes a powerful tool to determine the configuration of the complexes unequivocally. Two chiroptical methods are needed to determine the absolute configuration: ECD for the coordinated nitrogen atom and VCD for the rhodium core. A quick to use NMR method is also presented: Upon the coordination of small molecules in the axial position, the relative configuration of both the rhodium core and the nitrogen atom can be determined simultaneously by studying spatial proximities provided by 1D NOE spectra.

Circularly Polarized Luminescence of Aluminum Complexes for Chiral Sensing of Amino Acid and Amino Alcohol

Determination of the absolute configuration (AC) of chiral molecules is a key issue in many fields related to chirality such as drug development, the asymmetric reaction screening, and the structure determination of natural compounds. Although various methods, such as X-ray crystallography and NMR spectroscopy, are used to determine the AC, a simple and cheap alternative method is always anticipated. So far, electronic circular dichroism (ECD) spectroscopy has been widely used to ascertain the AC and enantiomeric excess (ee) values by applying appropriate organic probes. Here, circularly polarized luminescence (CPL) spectroscopy was applied to determine the AC and ee values of a series of amino acid and amino alcohol. The measurements were conducted by mixing the amino acids or amino alcohols with an achiral 1-hydroxy-2-naphthaldehyde. Upon in situ formation of the Schiff base complexes, the system showed emission enhancement and CPL in the presence of Al³⁺ , whose intensity and sign can be used to assign the chiral sense of the amino acids and amino alcohols. The authenticity of the method was further compared with the established CD spectroscopy, revealing that CPL spectra of formed Al³⁺ complex were effective to determine the AC of chiral species.

Chiroptical methods in a wide wavelength range for obtaining Ln3+ complexes with circularly polarized luminescence of practical interest

We studied enantiopure chiral trivalent lanthanide (Ln3+ = La3+, Sm3+, Eu3+, Gd3+, Tm3+, and Yb3+) complexes with two fluorinated achiral tris(β-diketonate) ligands (HFA = hexafluoroacetylacetonate and TTA = 2-thenoyltrifluoroacetonate), incorporating a chiral bis(oxazolinyl)pyridine (PyBox) unit as a neutral ancillary ligand, by the combined use of optical and chiroptical methods, ranging from UV to IR both in absorption and circular dichroism (CD), and including circularly polarized luminescence (CPL). Ultimately, all the spectroscopic information is integrated into a total and a chiroptical super-spectrum, which allows one to characterize a multidimensional chemical space, spanned by the different Ln3+ ions, the acidity and steric demand of the diketone and the chirality of the PyBox ligand. In all cases, the Ln3+ ions endow the systems with peculiar chiroptical properties, either allied to f-f transitions or induced by the metal onto the ligand. In more detail, we found that Sm3+ complexes display interesting CPL features, which partly superimpose and partly integrate the more common Eu3+ properties. Especially, in the context of security tags, the pair Sm/Eu may be a winning choice for chiroptical barcoding.

Pre-resonance enhancement of exceptional intensity in Aggregation-Induced Raman Optical Activity (AIROA) spectra of lutein derivatives

Recently reported new phenomenon of Aggregation-Induced Raman Optical Activity is demonstrated here for the first time in the pre-resonance conditions for lutein diacetate and 3'-epi-lutein supramolecular self-assembles. We demonstrate that minor alterations in the lutein structure (e.g. acetylation of hydroxyl groups or different configuration at one of the chiral center) can lead to definitely different spectral profiles and optical properties due to formation of aggregates of different structure and type. Lutein forms only H-aggregates, lutein diacetate only J-aggregates, while 3'-epi-lutein can occur in both forms simultaneously. Variety of aggregates' structures is so large that not only the type of aggregation is different, but also their chirality. It is remarkable that even in the pre-resonance conditions, aggregation of lutein derivatives can lead to the intense ROA signal, and moreover, 3'-epi-lutein demonstrated the highest resonance ROA CID ratio that has ever been reported.

Taking advantage of Co(ii) induced enhanced VCD for the fast and sensitive determination of enantiomeric excess

Large Co(ii)-induced VCD signals provide a method for determining the enantiomeric excess of α-amino acids. This strategy can be followed for building new VCD protocols.

Can the Resonance Raman Optical Activity Spectrum Display Sign Alternation?

The monosignate character of resonance Raman optical activity (RROA) spectra has been often taken as granted in experimental and computational approaches, on the basis of basic theoretical approximations only considering resonance with a single electronic state of the molecule and the scattering process to be governed by the Franck-Condon mechanism. We show in this letter for the first time that, by resorting to a fully quantum mechanical (QM) methodology able to take into account all terms entering the general definition of RROA, and which considers excited state interference and Herzberg-Teller effects, sign alternation and at the same time intensity enhancement in RROA spectra is obtained. Such features constitute an important milestone toward the exploration of RROA of a wide range of chiral biological molecules.

Aggregation-Induced Resonance Raman Optical Activity (AIRROA) and Time-Dependent Helicity Switching of Astaxanthin Supramolecular Assemblies

New methods for enhancing the Raman optical activity (ROA) signal are desirable due to the low efficiency of ROA, demanding otherwise high sample concentrations, high laser powers, and/or long acquisition times. Previously, we have demonstrated a new phenomenon, aggregation-induced resonance ROA (AIRROA), that produces significant enhancement of the ROA signal provided that the excitation wavelength coincides with the absorption of the measured species and that the electronic circular dichroism (ECD) signal in the range of this absorption is nonzero. In this work, analyzing three very different supramolecular astaxanthin aggregates (H1, H2, and J), we confirm the phenomenon and demonstrate that aggregation itself is not enough to enhance the ROA signal and that the above-mentioned conditions are necessary for induction of the resonance ROA effect. Additionally, by analyzing the changes in the ECD spectra of the H1 assembly, we demonstrate that the supramolecular helicity sign switches with time, which is dependent on the prevalence of kinetic or thermodynamic stabilization of the obtained aggregates.

Intense chirality induction in nitrile solvents by a helquat dye monitored by near resonance Raman scattering

Chirality induction phenomena attract attention because of their relevance to intermolecular interactions encountered in living matter. Usually, such effects are weak. However, enantiomers of a [6]helquat dye were found to induce exceptionally strong chirality in several achiral solvents containing nitrile groups. This effect was observable as an intense Raman optical activity (ROA) induced in acetonitrile, acetonitrile-d3, and liquid hydrogen cyanide solvents. The observation was verified by measurement of both helquat enantiomers which provided mirror image ROA spectra. Theoretical analysis indicated that the 532 nm laser excitation light was in a near resonance with electronic transitions of the dye, which made the effect observable in very dilute solutions (1 : 200 000 helquat to nitrile ratio) and thus the phenomenon can be generally useful in analytical chemistry.

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.

Simulation of Raman optical activity of multi-component monosaccharide samples

Determination of the saccharide structure in solution is a laborious process that can be significantly enhanced by chiral optical spectroscopies.

Chiral sensing of amino acids and proteins chelating with EuIII complexes by Raman optical activity spectroscopy

Chiroptical spectroscopy of lanthanides sensitively reflects their environment and finds various applications including probing protein structures.

Transition polarizability model of induced resonance Raman optical activity

Induced resonance Raman optical activity (IRROA) proved to be a very sensitive method to detect molecular chirality. It is exhibited, for example, by complexes of lanthanides with chiral alcohols or ketones. So far, the phenomenon has not been understood at a quantitative level. To elucidate its mechanisms and to correctly relate the spectra to the structure, a transition polarizability model (TPM) is developed and applied to a camphor-europium complex. The model well reproduces the high ROA/Raman intensity ratio of the IRROA observed experimentally. The results additionally indicate a fundamental role of the nonchiral fod ligand in the Eu(fod)3 compound for the chirality enhancement. The TPM model thus serves as a guidance for both experimental and theoretical studies to come.