The redox reactivity of aromatic molecules, synthesized in the team, upon electrochemical oxidation or reduction is scrutinized in the absence (dimerization/polymerization processes) or in the presence of nucleophiles (such as pyridine, imidazoles, phosphines, nitrites, halides…) or electrophiles (aryl-halides, CO2…). Currently, our efforts are mainly focused on the electrochemically-driven pi-extension of aromatic molecules (Chem. Commun. 2018). These electrochemical transformations lead to new functional molecules/materials. As much as possible, a particular emphasis is devoted to simplify the electrochemical cell setup (one compartment/two electrode cell, working with constant current) and to work with "greener" experimental conditions (high concentration, "green" solvents, room temperature, atmospheric pressure). Three representative examples are briefly described below:
-The dimerization (Dalton Trans. 2012, 2014) and functionalization (J. Org. Chem. 2014, Chem. Eur. J. 2015) of porphyrins, including porphine (Chem Commun. 2011), have been successfully performed.
-Efficient and "green" experimental conditions were found for the functionalization of pyrene with azolium salts (Green Chem. 2015).
-Mild electrosynthesis of imidazolium carboxylates by electroreduction of imidazolium salts (=> formation of carbene species) under a CO2 atmosphere was performed (Org. Lett. 2013).
Organometallic and coordination complexes
Thanks to the fine tuning of the potential, the oxidation state of metals in organometallic and coordination complexes can be accurately modified. Thus, important intermediates involved in catalytic reactions can be isolated and characterized leading to a better understanding of reaction mechanisms (Chem.-Eur. J. 2011; Inorg. Chem. 2013).
It is also possible to purify one compound from a mixture of several redox active compounds thanks to their different redox potential. As an illustrating example, 1,1'-dibromoferrocene derivative (3) was efficiently purified on a multigram scale from a mixture of 1-monobromoferrocene (2) and ferrocene (1) thanks to the exhaustive electrochemical oxidation of (1) and (2) (Chem. Commun. 2017).
B) Electrofunctionalization of surfaces
This part is strongly correlated to Theme 3 (see below) since these new functional materials are used for (or as part of) sensors and organic electronic devices.
Oxidation of different types of monomers (aniline and pyrrole-based derivatives, porphyrins such as diarylporphyrins and porphine, the fully unsubstituted porphyrin) leads to original functional materials. Optimization of the experimental electrodeposition conditions (concentration of the monomer, solvent, temperature, potentiostatic/potentiodynamic modes, potential values, additives such as bases…) is essential to obtain a perfect control of the electropolymerization process (Electrochim. Acta 2010, 2011, 2014, 2016).
The chemical, physical and thermodynamic properties of original polymers (for example, the poly(2,3,5,6-tetrafluoroaniline)) are described (ACS Appl. Mater. Interfaces, 2018).
Electroreduction of aryl-diazonium derivatives
This alternative approach of electrode functionalization lies on the electroreduction of the diazonium function grafted on aromatic compounds (phenyl-based, porphyrins…). The electrogenerated aryl-radicals react with the electrode surface and theoretically formed covalently-bonded monolayers. As for electropolymerization, numerous experimental parameters have to be controlled to assure the good reproducibility of the modified electrodes.
We managed to directly graft on the electrode substrate, for the first time, porphyrins via their meso-position, starting with meso-diazonium porphyrins and post-functionalize these materials with insertion of metals in the porphyrin cavity (Chem. Eur. J. 2015, ChemElectroChem 2016, 2018).
SAM's of thioalkanes on gold electrodes are generally produced by passive adsorption. We found that potential-assisted deposition is much more selective and faster (Electrochim. Acta, 2010) than the passive adsorption route (see Theme 3 below). We are interested in structure-property relationships involving redox molecules confined within a monolayer. This may be the study of the nonspecific interactions between a redox molecule and a monolayer; methylene blue (MB) partitioned in an alkanethiol self-assembled monolayer is an appropriate system to examine this phenomenon (Electrochim. Acta, 2012). We demonstrated that MB is trapped reversibly into hydrophobic pockets and presents an adsorption behavior that fits well a Langmuir isotherm model with independent and non-cooperative hydrophobic pockets. The amount of MB trapped in the monolayer can be calculated for each MB concentration in solution. This method is easily transposed to the determination of the amount of drugs entrapped in gold nanocarriers.