Thèse Contrôle du Transfert d'Énergie dans les Plexcitons par Hétérofonctionnalisation de Particules Plasmoniques. H/F

Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Chimie École doctorale : Sciences Chimiques : Molécules, Matériaux, Instrumentation et Biosystèmes Laboratoire de recherche : Service de Bioénergétique, Biologie Structurale et Mécanismes Direction de la thèse : Manuel LLANSOLA-PORTOLES ORCID 0000000280659459 Début de la thèse : 2026-10-01 Date limite de candidature : 2026-10-01T23:59:59 Les plexcitons sont des états hybrides lumière-matière émergeant du couplage fort entre des excitons moléculaires et des modes plasmoniques localisés dans des nanostructures métal-liques. À la différence des polaritons formés dans des cavités de type Fabry-Pérot, les plexci-tons sont des systèmes intrinsèquement interfaciaux, où les émetteurs moléculaires se trouvent en contact direct avec la surface métallique. Cette proximité ouvre des canaux supplémentaires de transfert de charge, de déclin non radiatif et de génération d'électrons chauds, mais limite en même temps le degré de contrôle structurel et dynamique pouvant être atteint.
Malgré l'intense effort expérimental et théorique consacré à ces systèmes au cours de la der-nière décennie, les plexcitons n'ont pas encore convergé vers une application clairement défi-nie. L'une des principales limitations réside dans le faible contrôle de l'organisation moléculaire à l'interface métal-molécule et dans la compétition entre le couplage dipolaire cohérent et les mécanismes d'extinction (quenching) médiés par la surface. Des études structurelles récentes combinant la RMN, le Raman, le THz-Raman et la théorie de la fonctionnelle de la densité (DFT) ont montré que même dans des systèmes modèles, tels que les plexcitons formés par des agrégats de TDBC sur des nanoparticules d'argent, la couche moléculaire présente un de-gré significatif de désordre, ainsi que la coexistence d'espèces monomères et agrégées à la sur-face.
Ce projet vise à surmonter ces limitations en introduisant de nouveaux degrés de contrôle structurel dans les systèmes plexcitoniques et en établissant des relations claires entre structure et dynamique grâce à l'utilisation de la spectroscopie ultra-rapide.
Plexcitons are hybrid light-matter states that arise when a molecular exciton, an electronically excited state delocalised over a dye molecule or aggregate, enters the strong-coupling regime with a localised surface plasmon supported by a metallic nanostructure. In this regime, energy is exchanged between the excitonic transition and the plasmonic mode faster than either can dissipate it, leading to new eigenstates with mixed excitonic and plasmonic character. Experi-mentally, this hybridisation is typically manifested by a characteristic mode splitting in the optical response and by modified excited-state dynamics compared with the uncoupled com-ponents. Unlike polaritons in Fabry-Pérot cavities, plexcitons are inherently interfacial: the molecular emitters sit in close proximity to, or in direct contact with, the metal surface. This geometry enables additional pathways, including charge transfer, non-radiative decay, and hot-electron generation, but it also constrains how precisely one can control structure and ex-cited-state dynamics. Despite the fact that the study and characterisation of plexcitons is still in its infancy, they are already promising because they provide a direct, nanoscale handle on light harvesting, energy routing, and interfacial dissipation. By combining strong field con-finement with hybrid light-matter character, plexcitonic platforms could enable tunable nano-emitters, ultrasensitive sensing schemes, and photochemical control strategies in which cou-pling is used to bias competing excited-state pathways. Their intrinsically interfacial nature is also appealing for optoelectronic and photocatalytic concepts that influence charge-transfer character or hot-carrier processes, provided that structural disorder and surface-mediated quenching can be brought under reliable control.
Hence, a key parameter to develop application of plexcitons is the control of molecular organi-sation at the metal-molecule interface. The fine tuning of the supramolecular assemblies de-termines the competition between coherent dipolar coupling and surface-induced quenching, and hence the fate of the excited states. For example, cyanine dye J-aggregates can adopt multiple packing motifs such as staircase, ladder, and brickwork geometries, which strongly influence exciton delocalisation and energy transport. We have shown recently by structural studies using NMR, Raman spectroscopy, and DFT that even nominally simple two-dimensional model systems, such as TDBC aggregates on silver nanoparticles, can exhibit complex substructures, substantial disorder, and coexistence of monomeric and aggregated species. Ultimately, the control over molecular structure in plexcitons, and consequently its excited state dynamics will allow the tailoring of hybrid systems for solar energy and photo-catalytic applications. A thorough understanding and control of the dissipative pathways will open new technologies that rely on the long-distance energy transfer enabled by the plasmon excitation between chromophores and molecular catalytic sites, as well as bolster the catalytic effect of metallic hot electrons with molecular long-lived states.
In this project, we will investigate how structural polymorphism at the surface of plasmonic nanoparticles controls plexcitonic behaviour and energy transfer. We will build a systematic series of nanoparticle-chromophore assemblies spanning distinct aggregation states (J-, H-, and related motifs), characterise how these chromophores organise at the nanoparticle inter-face, and track how excited-state dynamics evolve from isolated chromophores to aggregates and finally to plexcitons using fsTA and femtosecond stimulated resonant Raman spectrosco-py. The overarching aim is to establish quantitative links between well-defined structural mo-tifs and ultrafast excited-state dynamics.
The goal is to establish control of excited state dynamics in functional plexcitonic systems by coupling supramolecular hetero-functionalisation with surface engineering. The experimental strategy is built around the design of plexcitonic architectures in which ex-cited state landscapes can be tuned to bias energy flow toward a chosen acceptor. The meth-odology is organised in four phases: (i) plasmonic nanoparticles synthesis and controlled as-sembly, (ii) structural characterization, (iii) and determination of the dynamics of excited states and their structure, and (iv) interpretation through excitonic and polaritonic modelling. A central objective is the student's progressive training across all three phases, from materials preparation to time-resolved measurements and mechanistic interpretation.

1. Material synthesis and plexciton assembly (work to be carried out at UNAM)
The student will be trained in the preparation and reproducible assembly of three clas-ses of supramolecular aggregates. First, homo-aggregates will serve as references: J-aggregates derived from TDBC will be synthesised using a previously reported four-step sequential route, benefiting from established know-how within the collaboration between the two co-supervisors (see ref). Second, hetero-aggregates will be produced by inducing co-assembly of two chromophoric species through modulation of Cou-lombic interactions and the use of specific counter-ions. In practice, synthesised TDBC derivatives will be combined with commercial dyes (FEW Chemicals). The dye set is chosen to span different frontier orbital energies (LUMO), with the aim of creating en-ergetic gradients that bias energy flow. Third, hybrid systems integrating acceptors will be assembled by forming J-aggregate layers in the presence of discrete molecular ac-ceptors anchored at the surface. In parallel, the student will learn the preparation of plasmonic building blocks by synthesising colloidal silver nanoparticles (AgNPs) fol-lowing established protocols to obtain a plasmon resonance centred around 600 nm. Plexcitonic states will then be formed by controlled addition of molecular solutions to the colloid, promoting adsorption and exciton-plasmon hybridisation. To limit interfa-cial quenching and tune coupling strength, the student will implement two existing dis-tance-control levers: cyanine derivatives with variable alkyl chain lengths and surface functionalisation with controlled insulating layers.

2. Structural characterisation (work to be carried out at UNAM)
The student will learn to validate supramolecular assembling, and interface organisa-tion using NOESY NMR and Terahertz-Raman (THz-Raman) spectroscopy. Those studies will be completed by resonance Raman and EM at JOLIOT.

3. Dynamics and structure of excited states (work to be carried out at JOLIOT)
The student will then complete advanced training at the LBMS laboratory, with hands-on formation in ultrafast spectroscopy (fsTA and FSRRS). The student will ap-ply these tools across the different plexcitonic constructs to connect structure to dy-namics and to establish reliable workflows for data acquisition and interpretation. Ex-cited state dynamics will be quantified using femtosecond stimulated Raman spectros-copy (FSRS) to isolate excited state vibrational signatures with molecular specificity, enabling mechanistic assignment of energy transfer between aggregates and discrimi-nation of contributions from dark states and metal-related excitations.

4. Modelling and theoretical analysis (work to be carried out at UNAM)
The student will be involved in the interpretation of the experimental observables with-in excitonic and polaritonic frameworks parameterised by the structural information obtained above.

References
Banos-Gutierrez, J.; Bercy, R.; Jomaso, Y. G.; Balci, S.; Pirruccio, G.; Stenlid, J. H.; Llansola-Portoles, M. J.; Finkelstein-Shapiro, D., Molecular structure, binding, and disorder in TDBC-Ag plexcitonic assemblies. arXiv:2601.22022 2026.
Finkelstein-Shapiro, D.; Mante, P.-A.; Sarisozen, S.; Wittenbecher, L.; Minda, I.; Balci, S.; Pullerits, T.; Zigmantas, D., Understanding radiative transitions and relaxation pathways in plexcitons. Chem 2021, 7 (4), 1092-1107.