An unusual plank-shaped nematogen with a graphene nanoribbon core

A [12]phenacene exclusively decorated with four lateral hexylester substituents self-assembles into a nematic liquid crystal glass on cooling after melting at high temperature. This uniaxial nematic organization of a plank-shaped nanographene is unprecedented and in strong contrast to the common design rules for liquid crystals. Highly birefringent samples emitting polarized fluorescence can be obtained in homogeneously planar or twisted waveguiding configurations that are stable against crystallization at and below room temperature and up to 100 °C

Autour de nouveaux chélates:les 8,8'-diphényle-3,3'-biisoquinolines (Machines moléculaires rapides et nouvelles topologies)

Résumé français : notice = 7Ko MAXIMUM : résumé trop long empêche la validation : longueur = 1700 caractéresRésumé anglais : idemSTRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Synthèse de composés aromatiques polycycliques distordus par réaction de Scholl vers des nanorubans de carbone courbés

Les nanorubans de carbone présentent aujourd hui un grand intérêt en tant que segments de graphène aux propriétés électroniques modulables. Alors que des techniques de synthèse destructives top down donnent des rubans de très grande taille, d autres techniques constructives bottom up, par synthèse organique, pourraient former des nanorubans bien définis de géométries contrôlées. Dans cette optique, la réaction de Scholl est un outil chimique précieux car elle permet la graphénisation de longs précurseurs flexibles de type polyphénylène.Etonnamment, des structures distordues peuvent être obtenues majoritairement même si des isomères plans moins encombrés sont a priori favorisés. Nous avons ainsi montré que contre toute attente même un encombrement stérique important n a aucun effet notoire sur la régiosélectivité et que des composés aromatiques polycycliques courbés sont préférentiellement formés. Ainsi, des structures particulièrement tordues, tel que l hexabenzotriphénylène (HBTP) peuvent être facilement obtenues à partir de précurseurs de type polyphénylène.Après avoir découvert cette régiosélectivité inattendue, nous en avons tiré parti pour former des composés de type polyhélicène. Plusieurs tentatives de formation de l hexaphénanthrotriphénylène (HPTP) furent infructueuses à cause de problèmes de réactivité lors des synthèses des précurseurs flexibles correspondants. En mettant au point une stratégie de synthèse versatile fondée sur un précurseur commun, plusieurs substrats flexibles de symétrie C3 ont été synthétisés puis soumis à la réaction de Scholl. Des produits de réarrangement ont cependant été obtenus au détriment des [6]hélicènes attendus. Toutefois, un HBTP fonctionnalisé par des groupements TMS a pu être efficacement préparé, ainsi qu un hexabenzocoronène (HBC) dont l exceptionnelle solubilité est due à la distorsion du coeur aromatique sous l effet des groupements encombrants situés dans les régions baie.Ce fragment [5]hélicène favorisé a enfin été incorporé dans la formulation de nanorubans de carbone tordus, alors composés d une succession de ce motif. En tant que réactions test, les synthèses du monomère et du dimère correspondants ont été effectuées avec d excellents rendements et les deux composés entièrement caractérisés. Leurs structures ont été déterminée par diffraction de rayons X sur monocristaux et ont fourni d intéressantes informations complémentaires quant à leurs configurations. Une stratégie plus générale a enfin été développée et optimisée pour la synthèse systématique d oligomères plus longs de nanorubans de carbone tordus. En utilisant cette technique les trimère et tétramère correspondants ont été synthétisés et caractérisés par spectrométrie de masse.Carbon nanoribbons are today of great interest as graphene segments with modulable electronic properties. Whilst top down techniques give giant ribbons, bottom-up organic synthesis may lead to exactly designed nanoribbons of controlled geometries. The Scholl reaction is a precious chemical tool for that purpose since it yields efficiently to the graphitization of long and flexible polyphenylene precursors.Surprisingly, twisted structures may be obtained preferentially even if less crowded isomers are also feasible. It has been shown that, against all expectation, even a strong steric hindrance has no marked effect on regioselectivity and highly twisted polycyclic aromatic hydrocarbons are sometimes preferentially formed, whereas their flat and more symmetrical isomers are only obtained in minority. Highly twisted structures such as hexabenzotriphenylene (HBTP) may then be obtained very easily from flexible polyphenylene precursors.After discovering this unexpected regioselectivity, we used it on purpose to form polyhelicenic species. Attempts to prepared hexaphenanthrotriphenylene (HPTP) were unsuccessful due to reactivity issues when synthesizing the corresponding flexible precursors. By using a new versatile strategy leading to an advanced common precursor, several C3-symmetrical flexible substrates have been synthesized and submitted to Scholl reaction. The expected [6]helicenes were not obtained and rearranged products were formed instead, but TMS-bearing HBTP could be prepared, as well as a hexabenzocoronene (HBC) which exceptionnal solubility is due to the distortion of the aromatic core under the effect of bulky tert-butyl substituents in bay regions.The easily formed [5]helicene fragment has been incorporated in the design of twisted carbon nanoribbons that would be composed of a succession of such motifs. As a test reaction, the corresponding monomer and dimer have been synthesized with an excellent yield and fully characterized. Their X-ray structures have even been determined, giving interesting information about their configuration. A more general strategy has then been developed and optimized for the systematic synthesis of longer oligomers of twisted nanoribbons. Using this technique, the trimer and tetramer have been synthesized and characterized by mass spectrometry.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Dipyreno- and diperyleno-anthracenes from glyoxylic Perkin reactions

Twofold Perkin condensation of 2,5-dibromophenylene-1,4-diacetic acid with arylglyoxylic acids followed by cyclo-dehydrobromination leads to dipyreno- and diperyleno-anthracene tetraesters and diimides. The imides show surprisingly large absorption shifts versus the esters, illustrating that electron-withdrawing substituents at the anthracene unit efficiently impart long wavelength absorption in such electrondeficient graphene nanoribbon fragments

Fused Helicene Chains: Towards Twisted Graphene Nanoribbons

International audienceBy taking advantage of an unexpected regioselectivity of intramolecular Scholl reactions on pentaphenylenecompounds that favors distorted [5]helicenes over their flat counterparts, a new synthetic approach to twisted graphenenanoribbons has been designed based on side-fused di-tertbutyl-[5]helicene fragments. Syntheses of both small monomers and dimers have been achieved and their structures have been studied. An iterative synthetic strategy has been developed for the formation of longer flexible precursors, which relies on the step-by-step elongation of mono-functionalized oligomeric chains. The flexible trimer and tetramer have, thus, been synthesized and submitted to intramolecular Scholl reactions, which revealed important purification and characterization issues

Highly Twisted Arenes by Scholl Cyclizations with Unexpected Regioselectivity

The Scholl reaction with quinquephenyl derivatives has been shown to have an unexpectedly strong preference for forming twisted, helicene aromatic polycycles, instead of their flat counterparts. This tendency is so strong that it will overcome even severe steric hindrance, and the procedure can be used in the efficient synthesis of hexa-tert-butylhexabenzotriphenylene from a simple biaryl starting material

Twisted Polycyclic Arenes by Intramolecular Scholl Reactions of C3- Symmetric Precursors

With the aim of opening an efficient access to large and sterically crowded polycyclic arenes as well as improving insight into the geometrical preferences of the Scholl reaction, a versatile synthesis strategy has been developed to form a family of flexible yet strongly crowded substrates for multiple dehydrocyclizations. Their intramolecular Scholl reactions lead with high selectivity either to considerably twisted species where the initial C3 symmetry is maintained, or to strongly rearranged products where the formation of multiple [6]helicene fragments is avoided by the formation of unusual hexa[7]circulene moieties under loss of the C3 symmetry

Tetraazaarenes by the ceramidonine approach

The synthesis of extended heteroarenes via the acid-promoted dehydrocyclisation of arylamino-anthraquinones is examined as an approach to highly conjugated electron-acceptor materials and eventually to heterographene nanoribbons. Whilst the latter perspective is found to remain challenging, the former is exemplified by the synthesis of extended tetraazaheterocycles bearing solubilising alkyl substituents

Synthesis of a [20]phenacene dodeca-ester by controlled condensation of seven naphthalene-based building blocks

International audienceComplementary protecting techniques for the Perkin reaction between arylacetic acids and arylglyoxylic acids efficiently allowed the multistep condensation of seven naphthalene fragments, linked by maleate bridges. Photocyclisation of this heptameric flexible precursor has led to a [20]phenacene dodeca-ester, the longest known phenacene to date

European J Organic Chem

An efficient and versatile synthetic approach has been developed for the synthesis of large carboxy-substituted polycyclic arenes. This strategy relies on Perkin reactions between arylacetic and arylglyoxylic acids for the formation of flexible precursors. A subsequent rigidification step, by catalyst- or light-induced intramolecular cyclization reactions, then yields fully condensed polycyclic aromatic compounds. Protection and deprotection techniques have been developed to allow the controlled assembly of a large number of building blocks. This method can now be applied to the formation of planar or non-planar, linear or macrocyclic species of various sizes.Nanobagues aromatiquesNano-rubans cycliques aromatique