Molecular Engineering and Controlled Assembly of Nano Objects built on porphyrins – MECANO

Two approaches were explored to graft porphyrin derivatives on carbon nanotubes. The first was the attachment of porphyrin derivatives to the CNTs by covalent chemical bonds and the second was the attachment by weak interactions, in particular, hydrophobic interactions between large aromatic surfaces and alkyl chains wrapped around the CNTs. The grafting of porphyrin derivatives to CNTs was confirmed by spectrophotometry in the UV-visible (UV-vis) range and atomic force
microscopy (AFM), whereas the reactivity was monitored by electrochemistry techniques. Electrochemistry was also used to estimate the catalytic performance of the CNT/porphyrin combination. Investigation and optimization of self-assembly processes were studied both in solution, by nuclear magnetic resonance (NMR) and UV-vis, and on surfaces by AFM in the case of surface-directed self-assembly. These studies were expected to lead to a better understanding of the
parameters controlling the growth of nano-objects in solution and on surfaces, thus facilitating incorporation of these objects in devices.

This project has led to several proofs of concept that will contribute to the design of new strategies in the development of nanotechnologies at the molecular scale. A reconstructed protein bearing a clickable azide function was grafted on a CNT by a cyclo-addition reaction, demonstrating the feasibility of protein engineering on the CNT surface. In the case of electrocatalysts, comparison of the two grafting methods showed that covalent grafting seemed insufficient for significant catalysis. On the contrary, grafting based on weak interactions between CNTs and porphyrins increased the electrocatalysis in comparison to both compounds studied separately. The growth of porphyrin nanostructures on surfaces is now fully understood and a comprehension of growth in solution should follow soon through new international and national collaborative programs initiated during the MECANO project. Significant results were also obtained for the growth of nanowires from seeds on CNTs and the possibility of mechanical linkage to ensure the stability of the scaffolds.

The project and results are essentially scientific and conceptual. We showed that CNTs can be used as an electroactive support to convey electrons to catalysts grafted on the CNT surface. These grafted systems perform biologically and economically relevant chemical reactions such as the reduction of oxygen. The self-assembly process of porphyrin nanowires induced by surfaces was both demonstrated and deciphered and strategies to mechanically lock the final architectures are being investigated.

A total of 22 publications have been published over the period and 6 have been co-authored by Japanese/French researchers. The results will be detailed in the scientific report and the topics concerned by these publications are the following:
-Control of self-assembly: Chem. Comm. (3), Chem. Eur. J., Chem. Asian J.
- Refinement of protein and catalyst functions: J. Am. Chem. Soc., Inorg. Chem., ChemPlusChem., New J. Chem., J. Porph. Phthalocyanines.
-Surface grafting: RSC Adv., Chem. Lett.
-Surface reactivity: RSC Adv., PhysChem ChemPhys, Inorg. Chem., Langmuir, Chem. Select.
In addition, two new topics have emerged from collaborative work and have led to two additional publications (Chem. Sci. and Chem. Eur. J.) co-authored by Japanese/French partners. Three more papers on one of the new topics are submitted or in preparation.
Together with other laureates of the ANR/JST program (V. Artero and M. Holzinger) the 1st Workshop on molecular technology dedicated to energy and electron transfers in molecularly engineered materials was organized in Strasbourg in June 2017.

The MECANO project focusses on the development of new concepts inspired by the efficiency of Nature in the production, storage and use of energy. Two specific natural processes are of particular interest and concerned by the MECANO project. The first process is the oxygen reduction reaction (ORR) that is of utmost importance in the fuel-cell functioning and the replacement of platinum in fuel-cells. The second process is the light harvesting in vegetal and bacterial photosynthesis. In both processes, porphyrin derivatives, hemes for the ORR, chlorophylls and bacterio-chlorophylls for the photosynthesis, are involved. Porphyrins are the link between all members of the consortium, through protein engineering, enzyme models, metalloporphyrins in electrocatalysis, or self-assembly of porphyrins on surfaces, especially carbon surfaces.
In the MECANO project, reconstructed proteins and strapped porphyrins, as hemoproteins models, will be modified (Task1) to further associate with carbon electrodes (Task2) and generate modified electrodes which properties will be investigated (Task3). The modification of electrodes will be achieved covalently and non-covalently in order to determine which anchoring mode leads to the best properties, depending on the type of electrode material used (nanotubes, graphene, HOPG). The graftable structures will also be used as anchored initiators for the growth of self-assembled wires, in order to form chromophore brushes on the surface of electrodes. The brushes will reproduce the light harvesting that takes place in the photosynthesis. Ultimately, functional nano-objects will be produced and integrated into prototypal devices of fuel-cells and solar cells.
During the project, care will be taken to promote multidisciplinary training of students and post-docs through the practice of mutagenesis, synthesis, scanning probe microscopy and electrocatalysis.