Thèse Commutation de Fluorescence Exaltée Utilisant des Composés Photochromes Négatifs H/F - Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Physique École doctorale : Ondes et Matière Laboratoire de recherche : CEA/SPEC - Service de Physique de l'Etat Condensé Direction de la thèse : Celine FIORINI ORCID 0000000309609878 Début de la thèse : 2026-10-01 Date limite de candidature : 2026-07-31T23:59:59 Ce projet vise à développer une nouvelle classe de systèmes supramoléculaires commutables par la lumière visible, permettant une modulation réversible de la fluorescence avec un contraste élevé. En combinant des unités moléculaires photochromiques négatives et des fluorophores, nous espérons obtenir à la fois des propriétés de fluorescence activable et une amplification géante de la commutation photosensible de la fluorescence. Le projet consiste à créer des assemblages moléculaires hautement efficaces qui réagissent collectivement à la lumière. Le doctorant doit avoir une solide formation en photophysique/optique et être disposé à travailler à l'interface physique/chimie au SPEC/LEPO (France) et au laboratoire du professeur Abe (Japon), en étroite collaboration avec le laboratoire PPSM (France), qui apporte une expertise complémentaire. Le projet permettra de faire progresser la compréhension fondamentale des processus de transfert d'énergie et fournira des matériaux polyvalents contrôlés par la lumière pour des applications en nanophotonique, en bio-imagerie et dans les dispositifs optiques.
Fluorescent and photoswitchable systems are highly attractive due to their various potential applications.[1] They rely on photochromic materials, fully controlled by light, switching reversibly between two isomeric states with distinct optical properties. [2] Such compounds are ideal to design photoswitchable fluorescent systems.
In most reported materials designs, photochromic molecules are either intrinsically fluorescent in one of their states or covalently coupled to a fluorophore. In these molecular systems, one state is emissive while the other is quenched via energy transfer. Numerous materials have been described based on this principle, and the vast majority operate in turn-off fluorescence mode, where the initial form (stable) is emissive and the second form (metastable) is quenched. [3] Such turn-off materials cannot reach a completely dark state and suffer from a limited optical contrast between the two states.
To overcome this limitation, the Giant Amplification effect of Fluorescence Photoswitching (GAFP) was reported [4]. This effect emerges when multiple fluorophores are confined within a small volume so that a single photochromic molecule can quench all emitters located within its Förster sphere. This collective energy transfer enables an efficient fluorescence quenching, reaching a total dark state with only a few absorbed photons.
However, turn-on fluorescent systems, where light triggers emission from a dark initial state, offer a much higher contrast; single fluorescent molecules detection being moreover easier in a dark background. In this context, negative photochromic compounds, for which the initial state can play the role of quencher, have emerged recently and represent promising molecular switches to design turn-on fluorescence. A 1st proof of concept and noteworthy illustration of this phenomenon was presented by Prof. Abe, who synthesized a new molecular family of turn-on fluorescent switches. [5] The photochromic-fluorescent molecule is non-emissive in its initial state, and turns fluorescent upon visible light exposure. Subsequently, the system undergoes a spontaneous return to its initial state in the dark.
The objective of this project is to study and determine the conditions by which these new turn-on fluorescence systems exhibit GAFP with both a high off-on contrast and minimal light input. In contrast to the majority of previous studies that have concentrated on covalently linked molecules, this project focus on non-covalent systems in which photochromic and fluorescent components are assembled in dense molecular architectures through supramolecular forces. The objective will be to define the best assemblies to favor efficient multiple energy transfer processes and collective responses while avoiding complex covalent syntheses. This supramolecular strategy offers greater flexibility in materials design, easier processing, and the possibility to tune optical properties by adjusting the local organization The project aims to design and develop visible-light-responsive supramolecular fluorescence photoswitching systems with high contrast and fast response. The main objectives are:
- Construction of supramolecular assemblies enabling collective and reversible fluorescence modulation.
- Investigation of optical and photophysical properties, including photochromism, fluorescence turn-on behavior, switching contrast, speed, and amplification, using advanced spectroscopic and microscopic techniques.
- Design and synthesis of optimized photochromic and fluorescent molecular building blocks.
- Establishment of structure-property relationships to understand and control the mechanisms governing energy transfer and photoswitching.
- Exploration of potential applications in bioimaging, nanophotonics, and optical devices.
The project will combine molecular design, supramolecular assembly engineering, and optical characterization to achieve reversible turn-on fluorescence switching with GAFP. Preliminary studies will rely on a first generation of promising compounds (bi-naphthyl-bridged imidazole dimers (Bn-Imd)) already available, which will serve as model systems before moving toward the design of new molecular architectures. The approach will include:
- Characterization of individual components: The optical and photochromic properties will be thoroughly characterized using spectroscopy to evaluate the intrinsic performance of the synthesized individual molecules.
- Formation of supramolecular and dense assemblies: Photochromic and fluorescent molecules will be co-assembled through non-covalent interactions (H-bonding, - stacking, ...), or confined within dense matrices such as 2D assemblies, nanoparticles, or hybrid materials.
- Characterization of the fluorescence photoswitching: These architectures are expected to promote the emergence of GAFP. Previous results have already demonstrated the feasibility of fluorescence photoswitching in polymer matrices using negative photochromic compounds and fluorophores. These findings will serve as the starting point to achieve the GAFP. LEPO's expertise in local probe microscopy, combined with light-excitation and spectroscopic measurements, will be used to monitor in situ the evolution of fluorescence photoswitching under illumination, down to the single nano-object level.
- Investigation of energy transfer dynamics: The mechanisms of fluorescence quenching and recovery will be studied through kinetic and spectroscopic analyses. By varying molecular organization and chromophore density, the characterization of the GAFP in the turn-on mode will be performed. Moreover, the Bn-Imd compounds exhibit a third isomer that may induce specific transient photophysical effects. Particular attention will be devoted to the role of this specific isomer, as it could lead to new fluorescence photoswitching properties and open additional switching pathways.
- Establishing structure-property relationships: The influence of molecular arrangement and local chromophore density on the amplification phenomena will be systematically investigated. This study will clarify how supramolecular organization governs the efficiency and dynamics of fluorescence photoswitching, providing a foundation for rational material design.
- Molecular design and synthesis: New negative photochromic molecules and fluorophores with complementary spectral properties will be selected and synthesized to enable efficient visible-light control of the fluorescence. Structural tuning will ensure proper spectral overlap and photostability compatible with energy transfer-based modulation. Both components will be designed or functionalized to promote supramolecular self-assembly and controlled organization within dense molecular systems. For the photochromic units Bn-Imd will be employed, as Prof. Abe is a leading expert in this field.

Solides connaissances en photophysique
Solides connaissances en spectroscopie optique
Solides connaissances en optique et microscopie optique
Connaissances en conception moléculaire et chimie physique
Connaissances en matériaux supramoléculaires