Non-covalent interactions
The fine control of intermolecular interactions has a crucial importance for controlling and optimizing the physical properties of molecular materials in the solid state.
Beside traditional non-covalent interactions such as bonding hydrogen or interactions π-π, the study of σ-hole interactions is a recent area to which our team has brought major contributions.
We have thus highlighted for the first time a neutral-ionic charge conversion by transfer of halogen in adducts linked by halogen bond, by analogy with acid-base systems associated by hydrogen bonding (Cryst. Growth Of. 2015).
The phenomenon of anticooperativity of the halogen bond has also been demonstrated for the first time (Cryst. Growth Des. 2016), with a ditopic and conjugated halogen binding donor.
Once coordinated by the most activated iodine atom, this donor shows a strong deactivation of the second iodine atom.
This understanding of σ-hole interactions is not limited to halogens but has been extended in recent years in the case of chalcogens (Se, Te).
We have successfully demonstrated that in organic selenocyanates, one of the two σ-holes of selenium is activated in the extension of the Se-CN bond to provide a strong directionality of the chalcogen bond (Chem. Commun. 2017).
These molecules are shown to be able, by strong chalcogen binding, to self-assemble with a recurring 1D motif ••• NC (R) –Se ••• NC (R) –Se ••• (New J. Chem. 2018), but also to form co-crystals with Lewis polydentate bases (4,4'-bipyridine, pyrazine, etc.), or with halide anions (Cryst. Growth Des. 2019).
σ-hole interactions are efficiently implemented in crystalline molecular conductors and/or magnetic compounds. Thus, the activation of iodine in iodinated tetrathiafulvalene cation radicals (TTF) has been shown in solution (Phys. Chem. Chem. Phys. 2016) but also in solid state (Chem. Commun. 2016).
Networking of anions based on the halogen bond, has recently been taken advantage to organize cationic Fe (III) complexes with spin conversion (SCO). Thus a complete inversion of magnetic behavior has been demonstrated between Cl− or I− anions salts alone, or organized in a 2D or 3D network linked by halogen bond with the tritopic halogen bond donor sym-triiodo-trifluoro-benzene (2 × Chem. Commun. 2017). This type of intermolecular interactions has also been implemented with compounds immobilized in self-assembled monolayers in particular for the detection of halogenated anions (Chem. Commun. 2019).
Functional surfaces
Surface modification is also a strong theme of the MaCSE team to achieve functional interfaces, concerning not only their development but also the characterization and implementation of their specific properties. The work carried out in this area is a fundamental approach towards applicative aims. The team has in particular a very strong expertise in the covalent grafting of organic monolayers on various types of materials (carbon, gold, hydrogenated silicon, nanoparticles, etc).
The covalent functionalization of hydrogenated silicon surfaces by organic monolayers with specific groups (electroactive, molecular receptors, catalysts, ...) makes it possible to obtain hybrid robust interfaces. This strategy is currently perfectly known in the team both in composition (mono- or bimolecular chains) and structure (homogeneity, organization and density). This allowed for example the development of photoswitchable molecular micromemories based on silicon functionalized by a redox layer (Chem. Rev. 2016)
The (electro-) chemical reduction of aryldiazonium salts is also a functionalization method widely used in the team allowing the formation of very robust interfaces. This simple, fast and flexible simple, fast and flexible method to implement is based on the responsiveness of aryl radicals produced from diazonium cations, able to bond to all types of surfaces, conductive, semiconductor and even insulating. The work carried out contributed to enhance the control of formation, organization and structuring of monolayers (Curr. Opin. Electrochem. 2018)
To this end, new strategies have been developed. For example, we have highlighted the impact of isomerism position of fluorene or spirobifluorene end groups on the structuring, organization and permeation properties of monolayers (J. Phys. Chem C 2017).
Taking advantage of the pre-organized structure of macrocycles, we have developed an original strategy for grafting dense and compact monolayers from calix [4] arene-tetradiazonium on all types of surfaces, including polymers (PP, PS, PET, J. Phys. Chem C 2016).
Monolayers of calixarenes are versatile platforms allowing the introduction of chemical functions or objects with very fine spatial control thanks to pendant groups carried by the small neck, for example funnel molecules for the selective recognition of primary amines (J. Am. Chem. Soc. 2016).
This concept is now extended to nanomaterials (metallic nanoparticles, nanotubes of carbon). In particular, we obtained gold nanoparticles decorated with a monolayer of calix [4] arenes, easily post-functionalizable and exhibiting an exceptional colloidal stability in aqueous medium (Chem. Commun. 2016).
The organization of functional molecules on surfaces is an essential aspect to exploit their functions in devices. However, immobilization on a surface can cause them to loose their most interesting properties. Thus, it is not uncommon that electronic coupling with a gold surface inhibits the response of phosphors or chromophores grafted via self-assembled monolayers of thiols.
One solution is to use spacers long enough to move the active center away from the gold surface. This strategy has been used successfully to immobilize photoelectrostimulable organometallic switches (Chem. Eur. J. 2017).
In contrast, recent work has shown that clusters of molybdenum immobilized in very dense monolayers on the gold surface via ligands thiocyanate retain their luminescence, despite the short length of the spacer. This remarkable result is due to the thiocyanate ligands used both for anchoring and protection that preserve the optical properties of the heart molybdenum (Mater. Horiz. 2019). It opens up perspectives in the design of lighting devices, including at nanometric scales, for bio-imaging, photovoltaics or telecommunications.
These works on surface modification have also allowed the development of pH-sensitive electrodes (quinone groups grafted on the surface and possibly modified by biomimetic membranes) used to probe the charge transfer properties (electron and / or ion transfer) of different proteins, sometimes membranes (Electrochim. Acta 2019, ACS Omega 2018).
Finally, this know-how makes it possible to offer innovative solutions to develop sensitive and selective sensors , for example for the detection of nitrated pollutants using an immobilized titanium catalyst (Anal. Chem. 2019), or in the field of pollution control.
Porous electrode surfaces modified by organic redox catalysts (J. Hazard. Mat. 2017) and metallic nanoparticles (ChemElectroChem 2016) have thus led to very efficient results for the degradation of biorecalcitrant compounds.
Rather than aiming for total mineralization, the work carried out in the team uses the redox catalysis or indirect electrolysis to increase the biodegradability of pollutants. This strategy is completely original compared to advanced oxidation processes usually employed.
Selective and well controlled, it optimizes the amount of energy needed to make the pollutant biodegradable (cost reduction) while avoiding the formation of toxic by-products by uncontrolled processes.
