Thèse Stockage d'Énergie Solaire et Conversion en Énergie Thermique Grâce à des Cocktails de Molécules Photochromes H/F - 75

Établissement : Université Paris-Saclay GS Chimie
École doctorale : Sciences Chimiques : Molécules, Matériaux, Instrumentation et Biosystèmes
Laboratoire de recherche : PPSM - Photophysique et Photochimie Supramoléculaires et Macromoléculaires
Direction de la thèse : Rémi METIVIER ORCID 0000000156128327
Début de la thèse : 2026-10-01
Date limite de candidature : 2026-03-31T23:59:59

En Europe, le chauffage et la climatisation représentent près de 50 % de la consommation d'énergie, principalement à base de combustibles fossiles. L'énergie solaire peut être exploitée pour une production d'énergie durable, à condition de pouvoir la stocker puis l'utiliser à la demande sous forme de chaleur en absence de soleil. Le projet de thèse consiste à exploiter de nouvelles familles de molécules photochromes comme systèmes innovants de stockage d'énergie solaire thermique moléculaires (MOST). En effet, les photochromes peuvent subir, sous l'effet de la lumière, une isomérisation réversible vers un état métastable à plus haute énergie, stockant ainsi l'énergie capturée sous forme chimique. Cette énergie peut ensuite être libérée à la demande sous forme de chaleur en déclenchant le retour à l'isomère de départ.
Dans le cadre de ce projet de thèse, nous proposons d'étudier plusieurs familles de photochromes, les terarylènes (TAs) et les Donor-Acceptor Stenhouse Adducts (DASAs), comme candidats MOST. Ces systèmes présentent en effet des caractéristiques à la fois innovantes et prometteuses pour répondre à plusieurs conditions requises pour les MOST. Le travail de thèse consiste à concevoir des molécules photochromes, puis explorer leur propriétés spectroscopiques, thermodynamiques et cinétiques, de manière à identifier les meilleurs candidats sur la base de leur correspondance spectrale avec le soleil, de leur efficacité de photoisomérisation, de la quantité d'énergie stockable, de leur cyclabilité, ainsi que la possibilité de déclencher la libération de chaleur grâce à un stimulus acide externe. Ensuite, plusieurs molécules différentes seront combinées sous forme de 'cocktails photochromes' pour couvrir au mieux le spectre solaire et tirer profit de leurs propriétés complémentaires. Enfin, les systèmes moléculaires optimisés seront mis en oeuvre sous forme de dispositifs applicatifs pratiques MOST de photochimie en flux (micro- ou milli-fluidique) ou à l'état solide (polymères ou matériaux à changement de phase dopés par les molécules photochromes).

The use of solar energy is a rapidly expanding field, and various technologies have been developed to date, including photovoltaic panels, that convert light into electricity, and solar thermal systems, that transfer solar radiation to heat transfer fluids. However, none of these devices can store the generated energy. Molecular Solar Thermal (MOST) energy storage systems are a promising green solution based on organic photochromic molecules that undergo reversible changes, switching from a thermodynamically stable form to a metastable photoisomer at a higher energy level through light absorption. This process stores solar energy as chemical energy at room temperature and releases it as heat on demand.
Photochromic molecules are chemical compounds that can reversibly convert under light, from one isomer to another, characterized by different absorption spectra. [1] In most cases, the transition from a colorless species to a colored species is observed, a phenomenon defined as 'positive' photochromism. Conversely, if the thermodynamically stable molecule is colored and light-induced discoloration due to isomerization is observed, the phenomenon is called 'negative' photochromism.
MOSTs can be applied in the form of flow devices, where the MOST solution circulates in a solar-irradiated circuit, undergoing photoconversion. The resulting energy-charged liquid can be easily stored long-term at room temperature in a tank. When heat is needed, the solution is injected into a fixed catalytic bed, which triggers the conversion back to the original molecule and releases the energy on demand. This process can be repeated multiple times. [2] For solid-state applications, MOST films are an innovative way to integrate solar energy collection and storage into transparent coatings. This paves the way for applications such as windows that can release heat to compensate for heat loss in the absence of sunlight, thanks to the precise adjustment of the thermal stability of the photoisomer. However, MOST systems must be developed to offer energy storage and conversion efficiencies comparable to other technologies, using appropriate stimuli to trigger heat release, without the need of expensive metal catalysts and in a green solvent.
The main photochromic molecules used to date as MOST, are azobenzenes, norbornadienes, and dihydroazulenes. However, these molecules have several drawbacks: azobenzenes have limited storage life, norbornadienes absorb mainly in the UV range, and dihydroazulenes have moderate energy storage. In most cases, an expensive or polluting metal catalyst is needed to trigger heat release. Recently, we demonstrated in the laboratory that two other families of photochromes, terarylenes (TAs) [3,4] and Donor-Acceptor Stenhouse Adducts (DASAs), [5,6] have remarkable capabilities. TAs are positive photochromes that can release stored heat through simple acid catalysis. DASAs are negative photochromes that can absorb in the visible range of the solar spectrum. These unique properties, beyond their particularly effective photochromism, open up a wide field of exploration for developing original, ultra-high-performance MOSTs.

The first objective of the Ph.D. project is to identify and synthesize photochromic molecules of the terarylene (TAs) and Donor-Acceptor Stenhouse Adducts (DASAs) types. These molecules must have substituents adapted to the context of MOSTs, i.e. capable of reacting to light and acid addition.

The second objective is to characterize these photochromic molecules from a spectroscopic (absorption spectra), photochemical (photochromic reaction), kinetic (conversion rate in light and darkness), and thermodynamic (amount of energy stored and then released) points of view.

The third objective relates specifically to the effect of acidity on photochromic properties. The MOST system must be sufficiently slow in a neutral environment to store solar energy over a long period of time and then undergo an accelerated, finely controllable reverse reaction in an acidic environment to release thermal energy at the desired moment.

The fourth objective is to combine different families of photochromic molecules to extend the active spectral range and take advantage of complementary properties in the form of 'photochrome cocktails' with enhanced performance.

The final objective relates to the practical application of solar energy storage and conversion. This involves creating flow or solid matrix devices, then characterizing them optically and thermally by quantifying their energy yields.

Based on the host team's experience in the field of photochromism and the preliminary results obtained in recent months in the laboratory, the Ph.D. project will use the following methods and tools to achieve the targeted objectives:
- Multi-step organic synthesis to obtain a variety of photochromic terarylenes (TAs) and Stenhouse Adduct Donor-Acceptors (DASAs), according to protocols established in the laboratory or through a long-standing collaboration with Dr. Pei Yu (ICMMO, Univ. Paris-Saclay).
- Theoretical chemistry to predict the geometry, energy, and spectroscopic properties of the molecular structures to be synthesized.
- Various spectroscopic techniques, enabling real-time, high-speed spectral monitoring of photochromic molecules, to characterize and quantify the efficiency of photoisomerization (kinetics in the presence or absence of light, with or without acid).
- Thermodynamic methods to measure the amount of heat released when the photochrome returns from its metastable state to its most stable form, either by spontaneous reaction or by acid-catalyzed effect, in collaboration with Dr. Vincent Goetz (PROMES, Univ. Perpignan).
- Flow photochemistry, in order to set up a closed-loop milli-fluidic MOST device composed of several elements: a solar photoreactor, a reservoir, a fixed acid catalytic bed, a heat exchanger, and a basic regenerator, in collaboration with Dr. Karine Loubière (LGC, Univ. Toulouse).
- Materials chemistry, to integrate photochromic molecules into a polymer (or gel) matrix, or even a phase-change material, to amplify the heat storage and release of the MOST, for applications on fixed supports.