Research in photochromic molecules is considered as one of the possible routes to the post-silicium era in opto-electronics, a major challenge at the 2020 horizon. Photochromic molecules have the specificity to be stable under two isomeric structures, the absorption spectra of which do not overlap in a wide range. Moreover, these molecules can switch from one isomer to the other by interaction with a photon at the maximum of absorption, or thermally. This intrinsic property of photochromic molecules has its origin in the electronic structure of the two isomeric forms: the electron delocalization is very different in both isomers. Indeed, one isomer has usually a delocalized electronic cloud and absorbs the visible light, whereas no such complete delocalization exists in the other one. There, the electron clouds are shared between two areas that are poorly connected, thus pushing the absorption to the UV. Thus, this property makes the photochromic molecules really good candidate for the use as optical memories or electro-optic switches. For this use, photochromic molecules must be coupled to macroscopic systems through a series of adaptators, those objects of smaller scale being other molecules or nano-objects linked to the photochromic molecule. The goal of this project is to provide a conceptual and semi-quantitative information on the coupling between selected photochromic molecules and several types of environment (gas phase, deposited on rare gas cluster, solvated with a given number of molecules, interaction with a nanoparticule) for a direct use by electro-optics devices designers. This project deals with the full characterization of the electronic structure dynamics and reactivity of some photochromic molecules in various environment previously mentioned. A concerted approach will involve spectroscopic and dynamics experiments complemented by molecular dynamics calculations and TD-DFT calculations. We will focus on 3 main mechanisms, which are involved in Spiropyrans, Diarylethane and Anyls. These three classes of molecules were chosen for the following reasons: i) Spiropyrans show a mechanism involving a zwitterionic state, as observed in gas phase by our group. This molecule is also sensitive to the molecular environment. ii) Diarylethene is the most promising molecule for electro-optical purposes, because of its high efficiency coupled to its rigidity. Here the molecular motion is small in the isomerisation process which can be accomplished as well in a crystal, as in a single molecule. Furthermore, the large electronic clouds make the molecule sensitive to the proximity of a metallic particle. iii) Anyls have a fundamental interest, as their property are induced by the shift of an H atom, like in Acetylacetone and fully examined in our laboratory. Protic solvents can also perturb strongly their dynamics. Moreover Anlys are used are non linear optical devices. This project will benefit from the GDRI PHENICS network for the development of collaboration between France, Japan, China and Russia in the field of photochromic molecules. This project will link 4 young scientists (<36 year old) very closely connected geographically and scientifically, each of them leading a specific and different task, directed at a better understanding of the interaction between light - with the photochromic molecules - molecular environment. This project intends to initiate the formation of a new team dedicated to the elaboration of fundamental concepts in molecular chemical-physics which are directly linked to a valorization process through collaborations already established. The creation of this new team is fully supported by the head of the Laboratoire Francis Perrin.

An increasing number of situations in our everyday life rely on the mechanical response of interfaces. The characterization of the mechanical response of interfacial layers having eventually gradients of mechanical properties, and the understanding of the correlations between the molecular structure of the layers and their mechanical response, represents crucial steps for the design of innovative materials and solutions in the domain of friction, lubrication, mechanical reinforcement in composite and nano-composite materials, surface coatings, bio-compatible materials, integrated micro-electro-mechanical or fluidic devices. We propose a program based on the use of three complementary techniques: the Surface Force Apparatus in the dynamic and elasto-hydrodynamic mode (DSFA), the instrumented JKR friction test, and Near Field Laser Velocimetry (NFLV), to tackle the direct determination of interfacial mechanical properties at the sub-micrometric scale. We will ascertain our approach on controlled model systems, such as self assembled monolayers and various grafted polymer films specifically designed to exhibit increasing complexity in their mechanical properties (purely elastic or viscoelastic) in order to directly address the question of the correlation between the molecular organization and the interfacial mechanics. Contrary to previous experiments performed with a surface force apparatus, we shall not directly put two layers into contact to indent or shear them, but we shall use a liquid flow as a probe to mechanically solicit the layers. In order to control the boundary condition for the flow at the interface between the layer and the liquid, we shall use the near field velocimetry technique available in one of the associated teams to directly access this velocity on similar interfaces. There are several difficulties to overcome and challenges to address: i) Sensing the mechanical properties of nanometric interfacial layers is intrinsically a difficult task, ii) in order to be able to relate the observed mechanical response of the surface layers to the molecular mechanisms responsible for the stress transmission through the interface, one needs to compare the mechanical behavior of series of surface layers with growing complexity, keeping the molecular parameters under control.. This project is at the interface between the physical chemistry of surfaces, low Reynolds number hydrodynamics in the lubrication regime, adhesion and friction science. This project involves a strong knowledge in the control of physical chemistry of surfaces including grafting of polymers and polyelectrolytes, self-assembled monolayers. It also needs to improve and develop new well-controlled surface with a defined geometry combined with very precise measurements of the mechanical properties at the nanoscale. Finally this project will allow us to construct a new surface forces apparatus with better mechanical properties (stiffer) and a controlled environment.

Plants are excellent systems to study epigenetic mechanisms, such as DNA methylation, small RNA regulators and chromatin remodeling. Indeed very few chromatin regulators are essential for plant viability allowing genetic approaches and plant studies can be conducted both in context of multicellular organisms or in cell cultures. Plant epigenetic studies have therefore significant advantage for elucidating in vivo roles of these regulators and consequently, significant impact on similar researches in other organisms. Furthermore, plant development shows a remarkable developmental plasticity suggesting flexibility and specificity of plant epigenetic mechanisms. Polycomb Repressive Complexes (PRC1 and PRC2) are key chromatin complexes in gene silencing in eukaryotes. While PRC2 complex is conserved in plants and has important functions in seed development, flowering time regulation, vernalization response or cell fate determination, the existence of a plant PRC1 complex currently remains controversial due to the fact that no homologue to the PRC1 subunits could be identified by sequence homology. Recent studies have shown that in Arabidopsis, PRC1-like complexes might function as the animal PRC1. One PRC1-like core component is likely to be LIKE HETEROCHROMATIN PROTEIN 1 (LHP1) as it recognizes the H3K27me3 epigenetic mark as does Polycomb, a subunit of PRC1, allowing the establishment of specific repressive chromatin states. These data address several questions: have PRC1 complexes evolved slightly differently in plants? Do plant PRC1 complexes have specific function? Are these properties in relation to the remarkable plant plasticity? The present proposal aims to study the functions and the composition of LHP1/PRC1-like complexes in A. thaliana in order to understand the mechanism of the plant Polycomb-mediated repression. This study will focus on the characterization of newly identified LHP1-binding proteins. In a first part, we will study RNA binding proteins. We will search for the RNA ligands, study their localization and the mechanism of interaction. These data will allow a better understanding of intriguing links between plant PRC1-like complexes and the RNA world. In a second part, we will study two proteins with RING domains (LOC1, LOC2). LOC sequence homologies and physical interactions between LOC1 and LHP1 suggest that the LOC proteins are involved in histone H2A ubiquitylation. We will test this hypothesis and study histone H2A ubiquitylation in Arabidopsis and the putative role of this epigenetic mark in gene silencing mediated by LHP1. In the third part, we will characterize the interplay of histone code with PRC1-like silencing on genomic targets of LHP1/PRC1-like complexes, which show different modes of gene regulation. We will compare epigenomic profiles with chromatin accessibility and chromatin profiling of LHP1/LOC or GCN5 (an acetyltransferase with an antagonistic effect on trimethylation of H3K27. This study will help to understand the dynamics of silent state establishment in relation to transcription regulation at these loci. This research program, based on complementary approaches (genetics, genomics, cytology, biochemistry, bioinformatics), will add a novel dimension to the current knowledge of LHP1-mediated gene silencing. Conserved as well as plant-specific aspects of epigenetic regulation will be elucidated.