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

Induced Chirality in Canthaxanthin Aggregates Reveals Multiple Levels of Supramolecular Organization

Carotenoids tend to form supramolecular aggregates via non-covalent interactions where the chirality of individual molecules is amplified to the macroscopic level. We show that this can also be achieved for non-chiral carotenoid monomers interacting with polysaccharides. The chirality induction in canthaxanthin (CAX), caused by heparin (HP) and hyaluronic acid (HA), was monitored by chiroptical spectroscopy. Electronic circular dichroism (ECD) and Raman optical activity (ROA) spectra indicated the presence of multiple carotenoid formations, such as H- and J-type aggregates. This is consistent with molecular dynamics (MD) and density functional theory (DFT) simulations of the supramolecular structures and their spectroscopic response.

Why Does One Measure Resonance Raman Optical Activity? A Unique Case of Measurements under Strong Resonance versus Far-from-Resonance Conditions

Raman optical activity (ROA) spectroscopy exhibits significant potential in the study of (bio)molecules as it encodes information on their molecular structure, chirality, and conformations. Furthermore, the method reveals details on excited electronic states when applied under resonance conditions. Here, we present a combined study of the far from resonance (FFR)-ROA and resonance ROA (RROA) of a single relatively small molecular system. Notably, this study is the first to employ the density functional theory (DFT) analysis of both FFR-ROA and RROA spectra. This is illustrated for cobalamin derivatives using near-infrared and visible light excitation. Although the commonly observed monosignate RROA spectra lose additional information visible in bisignate nonresonance ROA spectra, the RROA technique acts as a complement to nonresonance ROA spectroscopy. In particular, the combination of these methods integrated with DFT calculations can reveal a complete spectral picture of the structural and conformational differences between tested compounds.

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.

Combination of Resonance and Non-Resonance Chiral Raman Scattering in a Cobalt(III) Complex

Resonance Raman optical activity (RROA) spectra with high sensitivity reveal details on molecular structure, chirality, and excited electronic properties. Despite the difficulty of the measurements, the recorded data for the Co(III) complex with S,S-N,N-ethylenediaminedisuccinic acid are of exceptional quality and, coupled with the theory, spectacularly document the molecular behavior in resonance. This includes a huge enhancement of the chiral scattering, contribution of the antisymmetric polarizabilities to the signal, and the Herzberg-Teller effect significantly shaping the spectra. The chiral component is by about one order of magnitude bigger than for an analogous aluminum complex. The band assignment and intensity profile were confirmed by simulations based on density functional and vibronic theories. The resonance was attributed to the S0 →S3 transition, with the strongest signal enhancement of Raman and ROA spectral bands below about 800 cm-1 . For higher wavenumbers, other excited electronic states contribute to the scattering in a less resonant way. RROA spectroscopy thus appears as a unique tool to study the structure and electronic states of absorbing molecules in analytical chemistry, biology, and material science.

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.

Broadband Mueller ellipsometer as an all-in-one tool for spectral and temporal analysis of mutarotation kinetics

Spectroscopic Mueller matrix ellipsometry is becoming increasingly routine across physical branches of science, even outside optics. The highly sensitive tracking of the polarization-related physical properties offers a reliable and non-destructive analysis of virtually any sample at hand. If coupled with a physical model, it is impeccable in performance and irreplaceable in versatility. Nonetheless, this method is rarely adopted interdisciplinarily, and when it is, it often plays a supporting role, which does not take benefit of its full potential. To bridge this gap, we present Mueller matrix ellipsometry in the context of chiroptical spectroscopy. In this work, we utilize a commercial broadband Mueller ellipsometer to analyze the optical activity of a saccharides solution. We verify the correctness of the method in the first place by studying the well-known rotatory power of glucose, fructose, and sucrose. By employing a physically meaningful dispersion model, we obtain 2π-unwrapped absolute specific rotations. Besides that, we demonstrate the capability of tracing the glucose mutarotation kinetics from just one set of measurements. Coupling the Mueller matrix ellipsometry with the proposed dispersion model ultimately leads to the precisely determined mutarotation rate constants and spectrally and temporally resolved gyration tensor of individual glucose anomers. In this view, Mueller matrix ellipsometry may stand as an offbeat yet equal technique to those considered classical chiroptical spectroscopy techniques, which may help open new opportunities for broader polarimetric applications in biomedicine and chemistry.