Nanostructuration of Phenylenevinylenediimide-Bridged Silsesquioxane: From Electroluminescent Molecular J-Aggregates to Photoresponsive Polymeric H-Aggregates

A new approach to control molecular aggregation of π-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 π-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.