Beads on a Chain (BoC) Fluorescent Oligomeric Materials: Interactions of Conjugated Organic Cross-Linkers with Silsesquioxane Cages

Organic electronic materials have advantages over inorganics in terms of versatility, cost and processability. Recent advancements in organic materials for light emitting diodes (OLED), field effect transistors (OFET), and photovoltaics have engendered extensive innovation potential on this field. In this research, we focus on synthesizing SQ (silsesquioxane) based oligomers cross- linked by di-bromo-aromatic linkers and explore how the cross-linker and oligomer length influence their photophysical properties. Bis-tri-alkoxy silyl (linker) model compounds were synthesized to compare non-cage photophysical properties with the oligomers. Several techniques such as UV/Vis, fluorescence, FTIR, thermal gravimetric analysis (TGA) have been used to characterize the systems. Time-resolved fluorescence and femtosecond transient absorption spectroscopy are used to understand the excited state dynamics of these materials. Studies are carried out to understand the differences between monomers and oligomers and potential energy transfer and charge transfer between the cages and cross-linking chromophores. Transient absorption showed lower energy absorption from the excited states, suggesting short range communication between moieties. Single photon counting studies have shown distinct lifetime differences between most linkers and cages showing possible excitation energy transfer through these materials. Transient absorption anisotropy measurements have shown signatures for excitation energy transfer between linker chromophores for oligomeric compounds. The silsesquioxane (SQ) backbone of the oligomers gives substantial thermal stability as well as solution processability, giving better flexibility for achieving energy transfer between linking chromophores.