Synthesis and Structure-Photophysical Property Relationships of T8, T10, T12 and Oligomeric Organic Functionalized Silsesquioxanes.

Abstract

Silsesquioxanes with conjugated organic tethers (chromophores) offer high orders of functionality (> 8 tethers), unusual enhanced absorption, emission and charge separation over free chromophores, excited state electron delocalization, and high thermal stability. This dissertation presents the synthesis and characterization of organic functionalized T10 and T12 [RSiO1.5]10,12 molecules, with emphasis on their synthesis by fluoride catalyzed rearrangement from [RSiO1.5]n and an understanding of their unique photophysical properties targeting components in optoelectronic devices. Initial discussion focuses on the synthesis of silsesquioxanes from silica via conversion of rice hull ash (RHA) silica to spirosiloxanes [i.e. Si(2-methyl-2,4-pentane-diolato)2] by reaction with 2-Me-2,4-pentanediol and catalytic NaOH. The resulting spirosiloxane reacts with selected arylLi reagents to form mono-aryl-spirosiloxane, suggesting a pentacoordinate silicon based mechanism. These aryl-spirosiloxanes are then converted through fluoride catalysis to novel aryl-silsesquioxanes [RSiO1.5]8,10,12. Thereafter we detail the development of [RSiO1.5]10,12 materials by fluoride catalyzed rearrangement and its mechanisms. F--catalyzed rearrangement of polymeric and octameric SQs is indispensable to the synthesis of [RSiO1.5]10/12, and mixed [R1R2SiO1.5]10,12 molecules in up to 95% yield. [PhSiO1.5]10 is synthesized in the highest reported yield to date (~50%), and is used as a model system for mechanism studies. The likely mechanistic paths taken to form T10 and T12 SQs are analyzed by experiment with MALDI/NMR to identify intermediates and computational modeling for the most likely pathways. The most favorable pathway to T10 from T8 involves coincidental participation of fluoride and water with a net enthalpy of ~-24 kcal/mol. We also explore in detail the photophysical properties of [StilbenevinylSiO1.5]8,10,12, which show similar absorption and emission in solution, but decreasing fluorescence quantum efficiencies with increasing cage size, suggesting more chromophore interactions and non-radiative decay. [StilbenevinylSiO1.5]10 shows the highest two-photon absorption cross-section of this series (5.7 GM/chromophore), offering the best polarization and charge transfer character. Fluorescence upconversion fluorescence lifetime studies on [StilbenevinylSiO1.5]8,10,12 find ultrafast charge transfer dynamics (<1 ps) indicative of chromophore-chromophore interactions in the excited state, unobserved for stilbenvinylSi(OEt)3, suggesting excited state charge delocalization.