Thèse Polyoxométallates Hybrides des Processus de Transfert d'Électron Photo-Induit aux Machines Moléculaires Activables par des Stimuli Lumineux. H/F - Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Chimie École doctorale : Sciences Chimiques : Molécules, Matériaux, Instrumentation et Biosystèmes Laboratoire de recherche : Institut Lavoisier de Versailles Direction de la thèse : Emmanuel ALLARD ORCID 0000000278959954 Début de la thèse : 2026-10-01 Date limite de candidature : 2026-07-31T23:59:59 Les rotaxanes, des assemblages constitués d'au moins un macrocycle traversé par une entité moléculaire en forme d'haltère, suscitent un vif intérêt chez les chimistes grâce à leurs propriétés dynamiques. Ces caractéristiques les rendent particulièrement prometteurs pour des applications en nanosciences, catalyse et transport ionique. La fonctionnalité de ces systèmes repose non seulement sur la nature des sous-unités qui les composent, mais également sur le contrôle précis de leurs mouvements relatifs. L'objectif ambitieux de ce projet de thèse est de concevoir la première machine moléculaire intégrant un polyoxométallate, et plus particulièrement un polyoxovanadate (POV). Cette machinerie moléculaire permettra par exemple de réguler l'activité catalytique mais aussi biologique du POV grâce à la lumière. Ce concept repose d'une part sur l'utilisation de systèmes moléculaires donneur(s)-accepteur(s), et d'autre part sur les propriétés super-chaotropes des polyoxométallates. De ce fait, la première partie du projet doctoral s'articulera autour de la conception et l'étude des propriétés de transfert d'électron photo-induit au sein d'hybrides organique-inorganique en forme d'haltère, constitués de photosensibilisateurs organiques (dérivés de Porphyrines ou de BODIPYs) agissant également comme donneurs d'électrons et de POVs agissant comme accepteur et réservoir d'électron(s). Ces architectures moléculaires complexes seront obtenues à partir de briques organiques et inorganiques, qui seront combinées via des réactions de chimie clic ou des réactions de couplage croisé au palladium. Différents ponts reliant les partenaires photo- et électro-actifs seront considérés dans ce projet pour assurer des processus efficaces de séparation de charge. Ces architectures moléculaires hybrides devraient être capables de conduire à des transferts électroniques sous l'action de la lumière en tirant profit des propriétés des photosensibilisateurs organiques : i) Forte absorption de lumière et propriétés de donneur d'électrons, modulables grâce à une fonctionnalisation appropriée des photosensibilisateurs ; à celles des entités inorganiques, ii) transfert de charge vers le POV compte-tenu de son caractère accepteur et iii) réservoir d'électrons permettant l'accumulation possible de deux charges sur l'entité inorganique. Les mécanismes de transfert de charge photo-induits et d'accumulation de charges au sein de ces hybrides seront étudiés en utilisant des analyses de spectroscopies stationnaires (absorption et fluorescence) et résolues en temps (fluorescence, absorption transitoire à l'échelle nanoseconde et femtoseconde). La conception et l'étude photophysique de ces systèmes seront également assistées par des calculs théoriques.
La seconde partie du projet doctoral visera à exploiter les propriétés de transfert d'électrons photo-induit pour concevoir des machines moléculaires activables par des stimuli lumineux. Pour atteindre cet objectif, nous tirerons profit des propriétés d'association des POVs hybrides avec les cyclodextrines, qui dépendent directement de l'état redox du POV. À l'état non réduit, le POV sera entièrement encapsulé, le rendant inactif. En revanche, la photo-réduction du POV induira un déplacement des macrocycles le long de l'axe du rotaxane, exposant ainsi le POV à son environnement et le rendant actif. Étant donné que le POV sélectionné pour la conception de nos machines moléculaires de type [3]-rotaxanes possède des propriétés biologiques (e.g., antibactériennes) et catalytiques, les systèmes développés dans le cadre du projet doctoral devraient aboutir à des dispositifs multifonctionnels activables par la lumière.
Rotaxanes, assemblies composed of at least one macrocycle threaded onto a dumbbell-shaped entity, have garnered significant interest among chemists due to their dynamic properties.[4] These characteristics make them particularly promising for applications in nanosciences, catalysis, and ion transport. The functionality of these systems depends not only on the nature of their constituent subunits but also on precise control of their relative motions. This PhD project aims to design the first molecular machine incorporating a polyoxometalate, specifically a polyoxovanadate (POV). This molecular machine will, for instance, enable the regulation of the catalytic and biological activities of the POV using light. The concept is based on the use of donor-acceptor molecular systems exhibiting photo-induced electron-transfer properties combined with the super-chaotropic properties of polyoxometalates allowing their encapsulation.

The covalent association of polyoxometalates (POMs) and photosensitizers (PS) in dyads and polyads has recently attracted increasing interest in the field of solar energy conversion for artificial photosynthesis.[5] In these systems, POMs which are metal-oxo clusters built from early transition metals (mainly MoVI, WVI and VV), constitute interesting electro-active candidates due to their ability to act as an electron reservoir. Their covalent functionalization with photosensitizers (e.g. Ru or Ir complexes, porphyrins and BODIPY), which also act as electron donors, has been mainly considered for Mo- or W-based POMs.[5-6] This association led to hybrid assemblies in which intramolecular electron transfer from the organic PS to the POM has been demonstrated. This ET occurs through space or bonds, depending on the POM structure/composition and its environment,[6] with the accumulation of up to two electrons per POM being reported in only a few systems.[6a-b, 6e]. Surprisingly, polyoxovanadates (POVs) are underutilized in photo- and electro-active systems, especially given that this class of POM accepts electrons more easily than their Mo and W-based congeners,[7] which should in principle facilitate the electron transfer rate. This is certainly due to: i) the lack of post-functionalization strategies for POVs and ii) the intricate redox behaviour of POVs that involves concerted or stepwise proton-coupled electron transfer processes. Both challenges have been partially solved very recently,[8] opening an avenue for the rational design of photo- and electro-active hybrids based on polyoxovanadates (POVs), which is one of the cornerstones of this project. Thus, this project aims to first design a new class of organic-inorganic hybrids based on photosensitizer-donor (porphyrin and BODIPY derivatives) and POV entities linked covalently, and to study the processes of photo-induced electron transfer and accumulation of at least two charges within these hybrids. In addition, these hybrids, having a dumbbell-like shape, will be used to develop the first light-responsive molecular machines built from a polymetallic unit. The functioning of such [3]-rotaxane hybrids will be based on the association dynamics of hybrid POVs with cyclodextrins,[1] which are intrinsically linked to the POV's redox state. In its oxidized form, the POV will remain fully encapsulated and inactive. Upon photo-reduction, however, the macrocycle will shift along the rotaxane axis, exposing the POV to its surroundings and triggering its activation.
This project aims to firstly design a new class of organic-inorganic hybrids based on photosensitizer-donor (porphyrin or BODIPY derivatives) and POVs entities linked covalently and to study the processes of photo-induced electron transfer and charge accumulation. Then these hybrids, having a dumbbell-like shape, will be used to develop the first light-responsive molecular machines built from a polymetallic unit. The functioning of such [3]-rotaxane hybrids will be based on the association dynamics of hybrid POVs with cyclodextrins,[1] which are intrinsically linked to the POV's redox state. In its oxidized form, the POV will remain fully encapsulated and inactive. Upon photo-reduction, however, the macrocycle will shift along the rotaxane axis, exposing the POV to its surroundings and triggering its activation.

To address the challenges in synthesizing firstly these dumbbell-shaped entities, we will employ covalent strategies such as click chemistry and palladium-catalyzed cross-coupling reactions. The design centers on hybrid trans-{V6} platforms adorned with two alkoxide-bridged ligands, serving as the core motif of the dumbbell-shaped structure. These platforms will be functionalized by attaching two photosensitizer/donor conjugates to the {V6} core, acting as stoppers. These photosensitizer-donors will also be functionalized with polar solubilizing appendages to facilitate not only the characterization and photophysical studies of the hybrids in solution, especially in polar media, but also the preparation of the [3]-rotaxanes. In addition, the spacer connecting the electron donor and acceptor will be a critical factor in achieving an efficient charge-separated (CS) state,[2] thus different strategies will be considered using rigid connectors, as these linkers should allow control of not only the orientation/distance between the photo and electro-active partners, but also the rate of electron transfer (ET). In this context, covalent linkers such as triazole-phenyl or ethynyl-phenyl groups will be considered to connect the two entities. These linkers represent excellent bridges for rapid and efficient ET as demonstrated in a series of donor-acceptor conjugates.[3] The photo-induced electron transfer properties and charge accumulation behavior of the dumbbell-shaped hybrids will be investigated using a comprehensive set of photophysical techniques. These methods include steady-state and time-resolved absorption and fluorescence, and spectro-electrochemistry. This evaluation will help identify and guide the selection of the most suitable dumbbell-shaped hybrids for the development of our molecular machines. As an initial hypothesis, we propose that hybrids demonstrating superior photo-reducibility will emerge as the most promising candidates. We aim to develop systems capable of photo-accumulating up to two charges on the POV unit, facilitating the creation of molecular machines with a well-defined on/off switch triggered by light irradiation.
The design of rotaxanes requires the insertion of macrocycles into the dumbbell-shaped entities. This step must be completed before grafting the photoactive stoppers onto the hybrid trans-{V6} platforms. Research by the Versailles team has demonstrated that the encapsulation of hybrid POVs by cyclodextrins is facilitated by the ability of the CD host to dehydrate the hybrid POV.[1] Consequently, the coupling reactions between the preformed host-guest complexes and the photoactive stoppers must be conducted in an aqueous solution. While this step may present challenges, the development of such coupling methods is highly advantageous as it aligns with environmentally sustainable practices. The structural characterization of these molecular machines will be performed using ESI-mass spectrometry, multinuclear NMR, and X-ray diffraction. Additionally, the molecular motion within our supramolecular architectures, triggered by light irradiation, will be investigated through in-situ NMR and voltammetry studies. If all steps are successful, our molecular machines will be employed to develop light-switchable catalytic systems.

Le candidat devra posséder un bon cursus universitaire avec un profil de chimiste organicien. Des connaissances dans le domaine des composés hybrides organiques/inorganiques ou dans les mesures photophysiques (absorption/fluorescence) seraient un plus.