Circular differential scattering of polarized light by a chiral random medium

International audienceDifferential scattering of left- and right-handed circularly polarized waves is a general property of scattering media lacking space-inversion symmetry. We show in this paper that this scattering difference has remarkable properties in the case of an almost transparent chiral random medium. By tuning the refractive indices of the materials composing the medium, this scattering difference can be varied and even inverted. Within the limit of a dense scattering material, the difference of scattering cross sections between polarizations of opposite handedness can be largely adjusted. These properties are illustrated by measurement of the difference between the transmission of right- and left-handed polarized waves through a chiral Christiansen filter

Helicene-based chiroptical switches

International audienceHelicenes are mols. with ortho-fused arom. rings that adopt an inherently chiral helical shape. This helical topol. combined with the extended π-conjugated system provides them with excellent chiroptical and photophys. properties. These properties and peculiarities make helicenes important candidates for the conception of new chiroptical switches. Several examples of light-, redox- and pH-triggered helicene-based switches have recently appeared in the literature including examples from our group. Some of them are multi-input and/or multi-output systems in which the changes can be triggered by different stimuli and/or read-out by different techniques, an attractive advantage for functional materials

Fresnel vu par les chimistes : la biréfringence circulaire dans les milieux optiquement actifs

À partir de la biréfringence connue dans la calcite, et la polarisation de la lumière mise en évidence par Malus en 1807, Fresnel fait une expérience sur le parcours de la lumière dans certaines solutions. Il montre qu’en décomposant une onde polarisée rectilignement en la somme de deux ondes polarisées circulairement droite et gauche, celles-ci ne se propagent pas à la même vitesse dans un milieu chiral (composé de deux énantiomères) : cette découverte des propriétés des milieux chiraux est très importante dans la chimie des médicaments. Beaucoup de médicaments possèdent des propriétés thérapeutiques différentes selon leur forme énantiomère (ainsi le drame de la Thalidomide dans les années 1950 s’explique par la méconnaissance du danger d’un des deux énantiomères d’une substance administrée).Using birefringence known in calcite, and the polarisation of light revealed by Malus in 1807, Fresnel experimented on the course of light in certain solutions. He has shown that while decomposing a rectilinearly polarised wave into the sum of two circularly polarised waves from right to left, these do not propagate at the same speed in a chiral medium (consisting of two enantiomers). This discovery of the properties of chiral media is very important in pharmaceutical chemistry. Many drugs have different therapeutic properties depending on their enantiomeric form (and the Thalidomide disaster in the 1960s reflects the lack of awareness of the dangers of one of the two enantiomers of a given substance)

Helicene-based transition metal complexes: synthesis, properties and applications

International audienceThe coordination and/or organometallic chemistry of π-helicenic ligands appears as a powerfultool to generate multifunctional molecules displaying optimized chiroptical properties combinedwith new properties furnished by the metallic center. In this review, we relate the differentexamples that have been described in this field of research up to now

Conformational Kinetics in Chiral Poly(diphenylacetylene)s: A Dynamic P/M Memory Effect

Dynamic P/M (plus/minus) helical memory in chiral dissymmetric poly(diphenylacetylene)s (PDPA) is shown using a PDPA that bears the benzamide of (L)-alanine methyl ester as pendant. For a single chiral polymer, it is possible to obtain either P or M helical structures in a specific solvent without the presence of any chiral external stimuli. To do that, it is necessary to combine the conformational control at the pendant group with a high steric hindrance at the backbone. In this case, by thermal annealing in low-polar solvents, an anti-conformer is stabilized at the pendant which commands a P helix in the PDPA. Next, solvent removal followed by addition of a polar solvent such as dimethyl sulfoxide (DMSO), results in the kinetic conformationally trapped P helix. However, in this medium, the preferred handedness and the thermodynamic macromolecular helix for poly-(L)-1 is M. This process also occurs in the opposite way. Electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) studies show that the dynamic memory effect is present both in ground and excited statesWe thank financial support from AEI (PID2019-109733GB-I00) and Juan de la Cierva Incorporación contract (IJC2020-042689-I, R. R.). Xunta de Galicia (ED431C 2022/21, Centro Singular de Investigación de Galicia acreditación 2019–2022, ED431G 2019/03 and the European Regional Development Fund (ERDF) are gratefully acknowledged. J. J. T. thanks MICINN for an FPU contractS

Dissymmetrical U-Shaped π-Stacked Supramolecular Assemblies by Using a Dinuclear CuI Clip with Organophosphorus Ligands and Monotopic Fully π-Conjugated Ligands

International audienceReactions between the U-shape binuclear CuI complex A bearing short metal-metal distances and the cyano-capped monotopic π-conjugated ligands 1-5 carrying gradually bulkier polyaromatic terminal fragments lead to the formation of π-stacked supramolecular assemblies 6-10 respectively in 50-80 % yields. These derivatives have been characterized by multinuclear NMR spectroscopy and X-ray diffraction studies. Their solid state structures show the selective formation of U-shaped supramolecular assemblies in which two monotopic π-conjugated systems present large (6,7,9) or medium (8,10) intramolecular π-overlap revealing π-π interactions. These assemblies self-organize into head-to-tail π-stacked dimers that in turn self-assemble affording infinite columnar π-stacks. The nature, the extentand the complexity of the intermolecular contacts within the head-to-tail π-stacked dimer depends on the nature of the terminal polyaromatic fragment carried by the cyano-capped monotopic ligand but it does not alter the result of the self-assembling process. These results demonstrate that the dinuclear molecular clip A bearing short metal-metal distance allows selective supramolecular assembling processes driven by the formation of intra- and intermolecular short π-π interactions in the resulting self-assembled structures demonstrating that their shape is not only dictated by the symmetry of the building blocks. This approach opens perspectives toward the formation of extented π-stacked columns based on dissymmetrical and functionnal π-conjugated systems

Ethylenedithio--Tetrathiafulvalene--Helicenes: Electroactive Helical Precursors with Switchable Chiroptical Properties

International audienceElectroactive fused ethylenedithio-tetrathiafulvalene-[4]helicene and -[6]helicenes have been synthesized through a strategy involving the preparation of 2,3-dibromo-helicene derivatives as intermediates. The dihedral angles between the terminal helicenes, as determined by single crystal X-ray analyses, amount to 22.7° and 50.7° for the [4]helicene and [6]helicene, respectively. The solid state architectures show interplay between S···S and ··· intermolecular interactions. The chiroptical properties of the enantiopure EDT-TTF-[6]helicene derivatives have been investigated and supported by TD DFT calculations. Remarkable redox switching of the circular dichroism signal between the neutral and radical cation species has been achieved

3D Coumarin Systems Based on [2.2]Paracyclophane Synthesis, Spectroscopic Characterization, and Chiroptical Properties

International audienceIn this article, we report the preparation of a series of [2.2]paracyclophane-fused coumarin systems through a simple and general procedure involving a transition-metal-catalyzed cyclization of aryl alkynoates as the key step. We also highlight the influence of the [2.2]paracyclophane (pCp) motif and its "phane" interactions on the spectroscopic properties of the newly synthesized fluorophores, which emit in the blue-green region of the visible spectrum (lambda(em), up to 560 nm) and show extremely large Stokes shifts (up to 230 nm). Finally, we demonstrate that our straightforward approach can easily be used to access optically active planar chiral 3D coumarins. Compared to previously described fluorescent paracyclophanes and other organic dyes, our compact heteroaromatic derivatives show promising chiroptical properties, both in term of circular dichroism (g(abs) similar to 8 x 10(-3)) and circularly polarized luminescence (g(lum) similar to 5 x 10(-3)), thus demonstrating a practical application of our synthetic method

Helically chiral NHC‐gold(I) complexes: synthesis, chiroptical properties and electronic features of the [5]helicene‐imidazolylidene ligand

The golden twist: Monodentate [5]helicene-imidazolylidene gold(I) complexes were prepared in good yields and structurally characterized. Benefiting from the presence of the configurationally stable [5]helicenic unit, they exhibit appealing chiroptical features, such as strong circular dichroism, moderate circularly polarized phosphorescence and dual emission with phosphorescence lifetimes up to the millisecond range