Caractérisation de matériaux moléculaires amorphes pour optimiser leur préparation et leurs applications

Amorphous molecular materials, also known as molecular glasses, are small organic molecules capable of being organized in a disordered manner. In addition to sharing some of the useful properties of polymers, they offer additional advantages because they are isomolecular species for which synthesis, purification and processing are facilitated by a relatively low viscosity. However, the usually demanding preparation conditions of these materials and their limited functional lifetime due to their tendency to relax to the crystalline state remain obstacles to their use for certain applications, e.g. opto-electronics, nanolithography, pharmaceuticals. The development of strategies to facilitate the preparation of the vitreous phase and avoid its crystallization is therefore essential for the design of functional amorphous molecular materials. The main objective of this thesis is to establish relationships between the molecular structure of molecular glasses and their properties. To achieve it, various libraries of model compounds, analogues of triazine derivatives that have demonstrated excellent glass-forming ability, are used to i) determine the influence of the nature and the position of the groups on the triazine; ii) explore the influence of hydrogen (H) bonds on the properties of glasses when their structure includes functional groups known to facilitate crystallization and when their preparation conditions are similar to those used in industry; and iii) exploit the amorphous phase in order to study the photoresponsiveness of azobenzenes (azo) in order to optimize their use in different applications. The influence of the various substituent groups on the triazine (headgroup, ancillary and linkers) on the glass-forming ability (GFA), the kinetic glass stability (GS) and the glass transition temperature (Tg) of the compounds is first studied. A classification system based on differential scanning calorimetry (DSC) and variable temperature infrared spectroscopy (IR) measurements combined to chemometrics analyses facilitate the rationalization of the roles played by each group. The impact of the H-bonds, the energy of the rotation barrier, and the steric hindrance of the groups is determined, leading to the conclusion that the headgroup is the most influential group and that the presence of H-bonds is not essential to the GFA, but important to obtain a high Tg. The influence of the H-bonds on the properties of glasses approaching those exploited in industry is then explored. Variable temperature IR spectroscopy measurements, DSC studies, and single crystal structure resolution have led to the conclusion that H-bonds impede the crystallization of the compounds even though they are simultaneously functionalized with moieties that promote crystallization (π-π stacking between fluorinated and non-fluorinated stilbene groups). In addition, three similar compounds functionalized with a headgroup presenting a decreasing capability to establish H-bonds (donor, acceptor, none) were vapor-deposited (PVD), a technique used, among others, in the opto-electronic industry, to evaluate their capability to form ultrastable glasses. These PVD glasses all show properties that are similar to those previously reported for ultrastable glasses, including higher density and anisotropy, and are all more kinetically stable than glasses prepared by cooling from the viscous state. However, the PVD glasses prepared with a H-bond donor headgroup are less stable than the others by at least an order of magnitude, suggesting that H-bonds limit the level of kinetic stability achievable by PVD. Finally, a triazine molecular glass functionalized with an azo group is used to study, from a molecular point of view, the perturbations caused by the photoisomerization of the azo. A new IR spectroscopy method was developed to observe a molecular environment gradient along the molecule during photoisomerization, making it possible to support certain hypotheses concerning the resulting macroscopic transport of the material. Triazine-based molecular glass blends are also used as an ideal platform for decoupling the influence of Tg and azo content on the azo photo-orientation, but also on the inscription kinetics and the diffraction efficiency of surface relief gratings (SRGs). This work enables the determination of an optimal Tg range for the inscription of SRGs. Altogether, these new insights will lead to a more rational design of new molecular glasses, which can extend to other amorphous molecular materials.

Keyphrases: Molecular glass, Glass transition, Glass-forming ability, Crystallization, Hydrogen bonding, Infrared spectroscopy, Chemometrics, Physical vapor deposition, Azobenzene