Quadrupolar carbo-chromophores and alpha-graphyne – Carbo-ChromMat

Most of the work has consisted in the design and synthesis of the targeted carbochromophores, through the use of classical organic chemistry methods. These multi-step syntheses, well mastered in the group, are adapted to the preparation of each particular target, and all the synthesis intermediates are characterized. The optical and electronic properties of the obtained carbo-mers were studied through the development of external
collaborations with experts of each domain. 2PA NLO properties measurements were performed at León in Mexico in collaboration with Professor José-Luis Maldonado team, and the measurements of charge transport properties (single molecule conducitivity) were realized in Philadelphia (Temple University) in collaboration with Professor Eric Borguet.
The experimental results have been compared with the theoretical calculations performed in parallel.

Several series of carbo-meric molecules have been prepared and fully characterized, among which quadrupolar carbo-benzenes, and carbo-para-quinoids, envisaged in the initial project, but also series of novel molecules such as carbo-oligoacetylenes, and carbo-cyclohexadienes. The two-photon absorption properties of two carbo-benzenes and a carbo-quinoid bearing the same types of substituents have been compared, evidencing the effect of the p-conjugation extent, and of the character aromatic or not of the latter, on the two-photon absorption efficiency.
The single molecule conductance property of carbo-benzenes, which study was not initially envisaged in the project, was evidenced by Professor Eric Borguet’s team (Philadelphia,USA) who showed that these molecules exhibit a conductance 10 times higher than that of
comparable molecules.
The synthesis of a carbo-naphthalene, first fragment of carbo-graphene ever described, has also been performed.

The synthesis of a carbo-naphthalene opens the way towards the corresponding bidimensional material with promising theoretical properties.

The work realized with the context of this project has given rise to the publishing of 18 articles between 2012 and 2015 among which 2 review articles. Most of the articles were published in high impact journals (2 Chem. Commun., 3 Chem. Eur. J., 1 Angew. Chem. Int. Ed., 1 Chem. Sci., 1 Nature Commun., 1 Chem. Soc. Rev.). Several articles remain to be
published and should appear in 2016. The results have also been presented in numerous conferences, at both national and international levels.

Within the popular family of the so-called “carbon-rich” compounds and in the sub-family of “carbon-enriched” compounds (namely the “carbo-mers”), the main protagonists of the project belong to the “carbo-benzene” genus. The aim is a new development of the carbo-mer chemistry by changing the focus from concepts to applications, from molecules to materials, from synthesis to production. This is envisioned through the study of two selected types of targets.
1. Quadrupolar carbo-chromophores with Two Photon Absorption (TPA) properties. The search for molecules with a high TPA efficiency is motivated by promising outlooks in various technical domains (optical data storage, photodynamic therapy…). Extensively conjugated quadrupolar systems being prototypes of TPA chromophores, the C18 carbo-benzenic core deserves a particular attention. As the aromaticity of either the ground or excited states of the chromophores was recognized to play a determining role, two sub-types will be envisioned: (i) strongly aromatic carbo-benzenes bearing two identical acceptor or donor substituents at C1 and C10 positions, and (ii) the non-aromatic carbo-p-benzoquinoid counterparts bearing four similar substituents at the exo-C1 and exo-C10 double bonds.
2.Alpha-Graphyne. This carbon allotrope (the carbo-mer of graphene) has long been proposed as a challenging target, in particular for its promising conducting properties. To the best of our knowledge however, no attempt at experimental studies of alpha-graphyne has ever been reported. Since it can be regarded as an assembly of fused carbo-benzene bricks, it is here envisioned as a possible metathesis polymerization product of hexapropynyl-carbo-benzene.
Regarding both targets, two main aspects will be investigated.
1. Synthesis. This point is the essential prerequisite, and will thus represent the major part of the work. Beyond the intrinsic value of the targets, improved methods are required to overcome the limited amounts of carbo-benzenes available by “classical” synthetic strategies. Three main tools can thus be envisioned as complementary or alternative approaches for the formation of the C18 macrocycle: (i) double addition of a C8 dinucleophile to a C10 dicarbonyl dielectrophile. This multi-step strategy passes through a [6]pericyclynedione, which already proved to be a versatile precursor of few p-disubstituted carbo-benzenes. (ii) Sequential reductive coupling of bis(dihalovinylidene) precursors for the direct formation of the butatriene edges of the carbo-benzenic rings. (iii) Sequential catalytic metathesis of 1,7-di- or tetra-ynes to form the C18 ring in a three-step-one-pot process. The photophysical properties, and more particularly the optical properties, of the targeted carbo-chromophores (p-disubstituted carbo-benzenes and carbo-p-benzoquinoid derivatives) will be investigated.
2. Computational studies. Both types of targets will be addressed. (i) The quadrupolar carbo-chromophores will be investigated in both their ground state and excited states in order to estimate their TPA efficiency. Exploratory studies will be performed in order to design optimal synthetic targets. Valence Bond modeling and linear response time-dependent density functional theory will allow for a detailed analysis of TPA properties by reference to experimental data. (ii) The alpha-graphyne sheet will be computationally built up by considering substructures of the carbo-benzenoid type of increasing size. Electron delocalization and aromaticity of the rings will be studied. The redox properties and TPA efficiency of alpha-graphyne and fragments thereof (carbo-fusenes, starting from carbo-naphthalene) will also be investigated.