Titre de la dissertation: Designing original photoswitches based on photoresponsive azobenzene-containing peptoids for molecular solar thermal energy storage.

Promoteurs de thèse: Monsieur Lazzaroni et Monsieur Dimitri Stanicki

Résumé de la dissertation: The transition from fossil fuels to renewable energy sources, particularly solar energy, is a critical scientific challenge today. Efficient storage solutions are essential for the widespread adoption of solar energy. In recent decades, chemical storage using Molecular Solar Thermal (MOST) systems appears promising despite being challenging. These systems operate through iterative closed cycles of photoisomerization and back-isomerization between a parent organic compound and its metastable isomer. Energy is stored in the metastable isomer, which has a certain half-life, and thermal energy is released during the thermal back-isomerization process. Among MOST systems, azobenzene, with its E ® Z photoisomerization, has been studied extensively. However, improvements are needed in characteristics such as solar photon absorption, storage enthalpy, and half-life time. Chemical modifications of the chromophore have been explored but often result in trade-offs between these properties. Consequently, grafting azobenzenes onto a template support has emerged as a promising approach, leveraging potential interactions between chromophores on a rigid support.This work explores the use of peptoids as an innovative template support. Peptoids offer several advantages, including thermal and chemical stability, minimal contribution to the overall energy density of the system, and the adoption of specific secondary structures, such as helices, in solution. The objective was to utilize the peptoid secondary structure to induce interactions between azobenzenes and modify MOST properties.The first part of this work focused on peptoid synthesis and purification. The solid-phase synthesis protocol developed by Zuckermann was optimized, including the workup process. Sixteen peptoids containing both (S)-1-phenylethyl as the helix inducer and azobenzene as the photo-responsive side chains were produced and characterized, primarily using advanced mass spectrometry methods.For a model 3-unit peptoid, we developed a comprehensive protocol to measure the MOST properties of the synthesized molecules. In addition to the commonly used UV-vis spectroscopy and Differential Scanning Calorimetry, we demonstrated that liquid chromatography coupled with mass spectrometry can be advantageously used to determine photostationary distributions and measure the kinetics of the thermal retro isomerization reaction. Our measurements revealed significant site selectivity of the peptoid backbone, as the positioning of the azobenzene chromophore along the backbone strongly impacts MOST properties, particularly storage time and stored energy.We also investigated the secondary structure of longer 6-unit peptoids using circular dichroism spectroscopy. We demonstrated that the helical structures are modified upon chromophore photoisomerization. Additionally, in peptoids with at least two azobenzene residues, we observed interactions between adjacent chromophores, leading to enhanced MOST properties—one of the most promising findings of this thesis. Increasing the number of azobenzenes on the backbone was also shown to enhance energy density over periods ranging from 12 hours to more than 15 days.This work establishes the potential of peptoids as a novel support in the MOST field, offering the ability to tune key MOST properties by appropriately choosing the backbone anchoring position. Future investigations of numerous isomers using the developed methodology could further advance this promising avenue.