Synthesis and Photophysical Properties of Silsesquioxane Based Molecules and Polymers

Abstract

Completely condensed polyhedral silsesquioxanes (SQs), [RSiO1.5]8,10,12, appended with organic conjugated chromophores can offer high symmetry, high thermal stability, highly red-shifted emissions and charge separation compared with the free chromophores, indicating unconventional conjugation in the excited state and involving the SQ cages. This dissertation presents the synthesis and characterization of unsymmetrical and incomplete SQ cages with organic chromophores as well as the SQ-based organic-inorganic hybrid polymers/oligomers with emphasis on the mapping structure-property relationships and understanding their unique photophysical properties for potential uses in optoelectrical devices. First, this dissertation describes the synthesis of corner-modified, corner-missing, double decker closed and open SQs via silylation of the respective silanol phenyl-SQ precursors. Subsequent iodination/bromination of the phenyl groups on the cages and then Heck coupling of the halogenated derivatives provides stilbene-functionalized SQs where stilbene is a model for conjugated chromophore. Open SQ cages with ~7 stilbene groups display essentially the same fluoresce behavior as found for symmetric closed octa-stilbene functionalized SQs, with absorption ~300 nm and emission ~410 nm. The red-shifts of ~60 nm in emission relative to the free chromophores demonstrate excited-state electronic communication between the π^* orbitals of the stilbenes and through the cages, which indicates removing a corner or breaking two opposing bridges has little effect on LUMO formation. Interestingly, when the cages have only 2 stilbenes attached, they show similar absorption and emission to stilbene itself; however, with quite high luminescence quantum yields (≥0.7 vs. ≈ 0.07) proving that there is a threshold for the excited state conjugation to occur but also demonstrating that the cage does affect photophysical properties nonetheless. Organic-inorganic hybrid polymers with SQs in the main chain are prepared via Heck cross coupling of di-vinyl functionalized double decker vinylMeSiO[PhSiO1.5]8OSiMevinyl and ladder vinylMeSiO[PhSiO1.5]4OSiMevinyl SQs with dibromo-aromatic tethers. Double decker SQ-based polymers/oligomers show red-shifted emission from model bis-dimethoxysilyldivinyl-aryl analogs in the range of 50-120 nm, again supporting the electronic communication along the polymer chain through the SQ cage in the excited state, even with two -(O)2Si siloxane bridges. Coincidentally, the non-cage ladder SQs derived polymers/oligomers exhibit even further red-shifted emission beyond analogous double decker polymers in the range of 20-50 nm, even with shorter chain lengths, suggesting more efficient electronic interaction in ladders. In both systems, copolymers with biphenyl, terphenyl and stilbene display high photoluminescent quantum yields up to 0.8, suggesting potential emitting components in optoelectronic devices. Double decker derived terpolymers are synthesized with alternating biphenyl and thiophene linkers via stepwise Heck cross-coupling with the goal of shifting emission wavelengths to towards the visible coincident with high quantum yields. The resulting terpolymers display emissions at 430 nm, intermediate between those of the respective copolymers rather than emission from both units as would be expected from physical mixtures. This again provides further evidence for excited-state conjugation along polymer chains and through cages via disiloxane conjugated linkers. In addition, the quantum yields of the terpolymers DD-thiophene-DD terphenyl and DD-thiophene-DD-stilbene improve from 0.09 for the DD-co-thiophene copolymer to 0.20 and 0.24 respectively, providing successful examples of novel combination of improved quantum yields as well as longer-wavelength emission around 480 nm and new opportunities to tailor photophysical properties by modifying structures. Lastly, ‘hairy polymers’ are prepared via further functionalization of halogenated phenyl groups to stilbenes on the SQ cages in the main chain of hybrid polymers to explore possible 3-dimentional conjugation