Investigation on the vibrational and structural properties of a self-structured bridged silsesquioxane.

The crystalline structure of ureidopyrimidinone-based silane (UPY) has been determined. The local and long range order structuring of the bridged silsesquioxane (MUPY) resulting from the sol-gel hydrolysis-condensation of the former precursor has been investigated by MFTIR (Mid Fourier Transform InfraRed) combined with DFT (Density Functional Theory) and XRD (X-ray diffraction) studies. These studies showed that a long range structuring exists within the organic fragments with the transcription of the DDAA (Donor-Donor-Acceptor-Acceptor) H-bonding array from UPY to MUPY whereas a disordered siloxane network was revealed in the hybrid material.

Nanostructuration of phenylenevinylenediimide-bridged silsesquioxane: from electroluminescent molecular J-aggregates to photoresponsive polymeric H-aggregates.

A new approach to control molecular aggregation of pi-conjugated chromophores in the solid state has been investigated. Our strategy was to use a modifiable bulky fragment which should induce a J-aggregation and offer the possibility to reach an H-aggregation upon its chemical modification by lateral slip of pi-conjugated molecules. The chosen fragment for that purpose was the hydrolyzable triethoxysilane function (Si(OEt)3). Our objective was to design and synthesize electroluminescent or solar cell hybrid organic-inorganic materials by the sol-gel process applied to a bifunctionalized silane. With this intention, the synthesis of the sol-gel processable phenylenevinylenediimide silsesquioxane 6 was accomplished and the study of spin-coated thin films of the pure silane precursor subjected or not to the sol-gel process has been carried out. Optical properties of 6 are consistent with the formation of J-aggregates in the solid state due to the steric hindrance introduced by the triethoxysilane units. Conversely, the spectroscopic behavior observed for the hybrid film 6F is attributed to an H-aggregation corresponding to a "card pack" orientation of the distyrylbenzeneimide chromophores in the compressed silicate network. Morevover, 6 and 6F also exhibited different electronic behaviors: light-emitting diodes exhibited high brightness with the native precursor 6 and almost no light output with the sol-gel processed silsesquioxane 6F. Photovoltaic cells showed the opposite behavior with low photocurrent generation in the precursor case and higher photocurrents with the sol-gel processed layers. These results provide a deeper understanding of the present self-assembly process that is strongly governed by the molecular packing of the oligosiloxane precursor.

Relevant synthesis parameters for the sequential catalytic growth of carbon nanotubes.

Sequential catalytic growth provides an efficient tool for the synthesis of carbon nanotubes periodically inserted with catalyst nanoparticles. Several synthesis parameters were found crucial in order to induce this particular growth mechanism. The presence of phosphorus is required to form metal phosphide particles active for the formation of carbon nanotubes with a matchstick morphology. The metal composition (Ni/Fe ratio) and the carbon supply have no influence on the nanofilament type but strongly affect the nanotube yield. The synthesis temperature induces important changes on both the nanofilament type and yield, which are correlated with important transformations of the catalyst layer in terms of composition, particle size, and physical state.