Turinese Stereochemistry: Eligio Perucca's Enantioselectivity and Primo Levi's Asymmetry

In situ study of chiral symmetry breaking in sodium chlorate solutions by Mueller matrix polarimetry

This work presents a novel approach for investigating symmetry-breaking processes during crystallization using Mueller matrix polarimetry. By applying this method to the cooling process of NaClO3 solutions, we demonstrate its ability to capture not only the initial and final stages of crystallization but also the intermediate steps and dynamics of the process. This technique provides more comprehensive information and insights into the symmetry-breaking mechanisms involved in crystal formation. Overall, this study highlights the potential of Mueller matrix polarimetry for in situ statistical measurements of the optical rotation and for monitoring the evolution of enantiomeric excesses.

Primo Levi: A frustrated stereochemist?-Insight in hindsight

Primo Levi (1919-1987) has become an iconic figure at the intersection of chemistry and culture. Levi has long been praised for his autobiographical account as survivor in Auschwitz and by his literary masterpiece "The Periodic Table." Little is however known beyond such facts, especially his academic period and early research on stereochemistry at the University of Turin, which were abruptly truncated by the racial laws and WWII. Even if, later on, Primo Levi succeeded as industrial chemist, he had a vivid interest in molecular asymmetry that lasted through his entire life. This concise paper highlights a little known academic period in the midst of social and political upheaval. Levi left us his humanity in an otherwise tortuous life, and his literature took inspiration from chemistry; perhaps as a metaphor connecting the physical world and people's life.

Unveiling controlled breaking of the mirror symmetry of Eu(fod)3 with α-/β-pinene and BINAP by circularly polarised luminescence (CPL), CPL excitation, and 19F-/31P{1H}-NMR spectra and Mulliken charges

The first CPL signals at f–f transitions of Eu(fod)₃ in α- and β-pinene as chiral solvents and with BINAP (1 : 1) in chloroform were detected.

Chiral Imprinting in the Gas Phase

Undoing the twist: Recent successful attempts to change the relative populations of two otherwise identical enantiomers of a large gas-phase molecule using resonant microwave fields are highlighted. Specifically, the population of a specific enantiomer of a chiral terpene could be enhanced relative to the other enantiomer by the application of a sequence of microwave pulses in a phase- and polarization-controlled manner.

Environmental dependence of artifact CD peaks of chiral Schiff base 3d-4f complexes in soft mater PMMA matrix

Four chiral Schiff base binuclear 3d-4f complexes (NdNi, NdCu, GdNi, and GdCu) have been prepared and characterized by means of electronic and CD spectra, IR spectra, magnetic measurements, and X-ray crystallography (NdNi). A so-called artifact peak of solid state CD spectra, which was characteristic of oriented molecules without free molecular rotation, appeared at about 470 nm. Magnetic data of the complexes in the solid state (powder) and in PMMA cast films or solutions indicated that only GdCu preserved molecular structures in various matrixes of soft maters. For the first time, we have used the changes of intensity of artifact CD peaks to detect properties of environmental (media solid state (KBr pellets), PMMA cast films, concentration dependence of PMMA in acetone solutions, and pure acetone solution) for chiral 3d-4f complexes (GdCu). Rigid matrix keeping anisotropic orientation exhibited a decrease in the intensity of the artifact CD peak toward negative values. The present results suggest that solid state artifact CD peaks can be affected by environmental viscosity of a soft mater matrix.

Circular dichroism tensor of a triarylmethyl propeller in sodium chlorate crystals

In 1919, Perucca reported anomalous optical rotatory dispersion from chiral NaClO(3) crystals that were colored by having been grown from a solution containing an equilibrium racemic mixture of a triarylmethane dye (Perucca, E. Nuovo Cimento 1919, 18, 112-154). Perucca's chiroptical observations are apparently consistent with a resolution of the propeller-shaped dye molecules by NaClO(3) crystals. This implies that Perucca achieved the first enantioselective adsorption of a racemic mixture on an inorganic crystal, providing evidence of the resolution of a triarylmethyl propeller compound lacking bulky ortho substituents. Following the earlier report, NaClO(3) crystals dyed with aniline blue are described herein. The rich linear optical properties of (001), (110), and (111) sections of these mixed crystals are described via their absorbance spectra in polarized light as well as images related to linear dichroism, linear birefringence, circular dichroism, and anomalous circular extinction. The linear dichroism fixes the transition electric dipole moments in the aromatic plane with respect to the growth faces of the NaClO(3) cubes. Likewise, circular dichroism measurements of four orientations of aniline blue in NaClO(3) fix a bisignate tensor with respect to the crystal growth faces. Electronic transition moments and circular dichroism tensors were computed ab initio for aniline blue. These calculations, in conjunction with the crystal-optical properties, establish a consistent mixed-crystal model. The nature of the circular extinction depends upon the crystallographic direction along which the crystals are examined. Along 100, the crystals evidence circular dichroism. Along 110, the crystals evidence mainly anomalous circular extinction. These two properties, while measured by the differential transmission of left and right circularly polarized light, are easily distinguished in their transformation properties with respect to reorientations of the sample plates. Circular dichroism is symmetric with respect to the wave vector, whereas anomalous circular extinction is antisymmetric. Analysis of Perucca's raw data reveals that he was observing a convolution of linear and circular optical properties. The relatively large circular dichroism should in principle establish the absolute configuration of the propeller-shaped molecules associated with d- or l-NaClO(3) crystals. However, this determination was not as straightforward as it appeared at the outset. In the solid state, unlike in solution, a strong chiroptical response is not in and of itself evidence of enantiomeric resolution. It is shown how it is possible to have a poor resolution-even an equal population of P and M propellers-within a given chiral NaClO(3) crystal and still have a large circular dichroism.

Crystals in light

We have made images of crystals illuminated with polarized light for almost two decades. Early on, we abandoned photosensitive chemicals in favor of digital electrophotometry with all of the attendant advantages of quantitative intensity data. Accurate intensities are a boon because they can be used to analytically discriminate small effects in the presence of larger ones. The change in the form of our data followed camera technology that transformed picture taking the world over. Ironically, exposures in early photographs were presumed to correlate simply with light intensity, raising the hope that photography would replace sensorial interpretation with mechanical objectivity and supplant the art of visual photometry. This was only true in part. Quantitative imaging accurate enough to render the separation of crystalloptical quantities had to await the invention of the solid-state camera. Many pioneers in crystal optics were also major figures in the early history of photography. We draw out the union of optical crystallography and photography because the tree that connects the inventors of photography is a structure unmatched for organizing our work during the past 20 years, not to mention that silver halide crystallites used in chemical photography are among the most consequential "crystals in light", underscoring our title. We emphasize crystals that have acquired optical properties such as linear birefringence, linear dichroism, circular birefringence, and circular dichroism, during growth from solution. Other crystalloptical effects were discovered that are unique to curiously dissymmetric crystals containing embedded oscillators. In the aggregate, dyed crystals constitute a generalization of single crystal matrix isolation. Simple crystals provided kinetic stability to include guests such as proteins or molecules in excited states. Molecular lifetimes were extended for the preparation of laser gain media and for the study of the photodynamics of single molecules. Luminophores were used as guests in crystals to reveal aspects of growth mechanisms by labeling surface structures such as steps and kinks. New methods were adopted for measuring and imaging the optical rotatory power of crystals. Chiroptical anisotropies can now be compared with the results of quantum chemical calculations that have emerged in the past 10 years. The rapid determination of the optical rotation and circular dichroism tensors of molecules in crystals, and the interpretation of these anisotropies, remains a subject of future research. Polycrystalline patterns that form far from equilibrium challenged the quantitative interpretation of micrographs when heterogeneities along the optical path and obliquely angled interfaces played large roles. Resulting "artifacts" were nevertheless incisive probes of polycrystalline texture and mesoscale chemistry in simple substances grown far from equilibrium or in biopathological crystals such as Alzheimer's amyloid plaques.