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

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

Regioselective addition of DDQ on a quinoid ring: an entry into chiral zwitterionic bridging ligands

International audienceThe regioselective insertion of DDQ into a C-H bond of the 6π + 6π electron zwitterionic benzoquinonemonoimines 4a-c results in the formation of the novel chiral C-substituted quinoid ligands 11a-c. These Michael adducts feature a preserved zwitterionic form and a quaternary stereogenic carbon center as evidenced by the single crystal X-ray structure of the derivative 11a. The ECD spectra, optical rotations and racemization barriers of the two enantiomers of 11a were measured subsequently to their separation by a preparative chiral HPLC. Because the racemization of 11a is stopped in basic media, compounds 11a-c give a new entry in chiral zwitterionic bridging ligands

Synthesis and chiral recognition ability of helical polyacetylenes bearing helicene pendants

International audienceNovel polyacetylenes bearing an optically active or racemic [6]helicene unit as the pendant groups directly bonded to the main-chain (poly-1s) were prepared by the polymerization of the corresponding acetylenes (1-rac, 1-P, and 1-M) with a rhodium catalyst. The optically active polyacetylenes (poly-1-P and poly-1-M) formed a preferred-handed helical conformation biased by the optically active helicene pendants, resulting in the induced circular dichroism (ICD) in their π-conjugated polymer backbone regions. The optically active helical polymers, when employed as an enantioselective adsorbent, showed a high chiral recognition ability towards racemates, such as the monomeric [6]helicene and 1,1’-binaphthyl analogues, and enantioselectively adsorbed one of the enantiomers

Ruthenium-Grafted Vinylhelicenes: Chiroptical Properties and Redox Switching

International audienceThe properties of mono- and bis-Ru-vinyl[6]helicene complexes (2 a and 2 b, respectively), recently synthesized by using molecular engineering of helicenes based on the grafting of lateral organometallic substituents on the π-helical backbone through a vinyl bridge, are presented. These helicene derivatives are thoroughly characterized, with special attention given to their chiroptical properties and redox switching activity. The UV/Vis and electronic circular dichroism (ECD) spectra of P and M enantiopure species, both in the neutral and oxidized states ([2 a](·+), [2 b](·+), and [2 b](2+)), are analyzed with the aid of quantum-chemical calculations. The extended π-conjugation facilitated by the vinyl moiety, clearly visible in the electronic structures of 2 a,b, introduces new active bands in the ECD spectra that consequently lead to a significant increase in optical rotation of Ru-vinylhelicenes compared with the organic precursors. The vibrational circular dichroism (VCD) spectra were measured and calculated for both the organic and organometallic species and constitute the first examples of VCD for metal-based helicene derivatives. Finally, the redox-triggered chiroptical switching activity of 2 a,b is examined in detail by using ECD spectroscopy. The modifications of the ECD spectra in the UV/Vis and NIR region are well reproduced and rationalized by calculations

Rhenium complexes bearing phosphole-pyridine chelates: simple molecules with large chiroptical properties.

International audienceThe synthesis and chiroptical properties of chiral rhenium complexes bearing mono- or di-topic phosphole ligands are described. [on SciFinder(R)

Combining chirality and hydrogen bonding in methylated ethylenedithio- tetrathiafulvalene primary diamide precursors and radical cation salts

Methyl- and dimethyl-ethylenedithio-tetrathiafulvalene ortho-diamide donors Me-EDT-TTF(CONH2)2 (1a) and DM-EDT- TTF(CONH2)2 (1b) have been prepared by the direct reaction of the corresponding diester precursors with aqueous ammonia solutions. The neutral (rac)-1a, (R)-1a, and (S,S)-1b donors have been characterized by single crystal X-ray diffraction. In the three compounds, which crystallized in the non-centrosymmetric monoclinic space group P21, the amide groups are disordered, yet they form the classical intra-molecular hydrogen bond for such an ortho-diamide motif. Electro- crystallization experiments afforded the mixed valence radical cation salts[(S,S)-1b]2XO4 and [(R,R)-1b]2XO4 (X = Cl, Re) containing four independent donors in the asymmetric unit, with the positive charge localized essentially on two donors, while the two others are neutral. The topology of the organic layer is of β′-type. Single crystal resistivity measurements show semiconducting behavior for [(S,S)-1b]2ClO4 and [(R,R)-1b]2ReO4, with a room temperature conductivity of 5 × 10−5 S cm−1 and activation energies Ea ≈ 3000 K. Tight-binding band structure calculations of extended Hückel type in combination with density functional theory calculations are in agreement with the semiconducting behavior and suggest a localized Mott type semiconductor rather than a band gap semiconductor

Conducting chiral nickel(II) bis(dithiolene) complexes: structural and electron transport modulation with the charge and the number of stereogenic centres

Nickel(II) bis(dithiolene) complexes can provide crystalline conducting materials either in their monoanionic or neutral forms. Here we show that the use of chiral dithiolene ligands with one or two stereogenic centres, together with variation of the counter-ion in the anionic complexes, represents a powerful strategy to modulate the conducting properties of such molecular materials. The chiral ligands 5-methyl-5,6-dihydro-1,4-dithiin-2,3-dithiolate (me-dddt) and 5,6-dimethyl-5,6-dihydro-1,4-dithiin-2,3- dithiolate (dm-dddt) have been generated from the thione precursors 1 and 2 which have been structurally and chiroptically characterized. Anionic Ni(II) complexes of these two ligands with tetrabutyl- ammonium (TBA) and tetramethyl-ammonium (TMA) have been prepared and structurally characterized, suggesting that it is the nature of the counter-ion which mostly influences the solid state organization of the complexes. Both TBA and TMA radical anion salts are Mott insulators with antiferromagnetic ground state, as suggested by spin polarized DFT band structure calculations. However, the TMA salts are one order of magnitude more conducting than the TBA counterparts. The neutral materials [Ni(me-dddt)2] and [Ni(dm-dddt)2] are direct band gap semiconductors, as determined by DFT and extended Hu ̈ckel band structure calculations, with their conductivity drastically increased up to 0.05-3.3 S cm1 under the highest applied pressures of 10-11 GPa. At equivalent applied pressures the dm- dddt materials are more conducting than the me-dddt ones, in agreement with the lower calculated activation energy and larger bands dispersion for the former. This trend follows the structural change when going from one to two methyl substituents, since the packing and intermolecular interactions are completely different between [Ni(dm-dddt)2] and [Ni(me-dddt)2], the packing of the latter being related to the one of the achiral parent [Ni(dddt)2]. Subtle differences of conductivity are also observed within both series of neutral complexes between the enantiopure and racemic forms. This represents the first series of chiral nickel bis(dithiolene) complexes which shows modulation of the conducting properties with the number of stereogenic centres, the conductivity, measured on single crystals, strongly increasing upon applying hydrostatic pressure

Modulation of chiroptical and photophysical properties in helicenic rhenium(I) systems: the use of an N‐(aza[6]helicenyl)‐NHC ligand

The photophysical and chiroptical properties of a novel, chiral helicene-NHC−Re(I) complex bearing an N-(aza[6]helicenyl)-benzimidazolylidene ligand are described, showing its ability to emit yellow circularly polarized luminescence. A comparative analysis of this new system with other helicene-Re(I) complexes reported to date illustrates the impact of structural modifications on the emissive and absorptive properties