Probing Structure Function Relationships in Novel Silicon Containing Macromolecules with Nonlinear Optical and Ultra-Fast Spectroscopy.

Abstract

Organic photovoltaics (OPVs) as a relatively novel technology have drawn significant attentions to many scientists during the past few years. Understanding the photophysical properties and energy transfer processes of the potential candidates is crucial to improving overall OPV device efficiencies and guiding new research of designing novel OPV materials. In this dissertation, nonlinear optical and time resolved methods such as two-photon absorption, time-resolved transient absorption, and fluorescence emission are used to study the nature of charge transfer character, energy transfer processes and charge transfer mechanisms in OPV materials. Two groups of organic macromolecules were investigated: 1) Three sets of chromophore substituted silsesquioxane derivatives were investigated to determine structure function relationships on a molecule basis. Exceptional red shift in emission and large two-photon cross-section found in [NH2vinylStilbeneSiO1.5]8 suggest that charge transfer character could be dramatically enhanced by introducing strong electron donating group to the substituted chromophores. Both steady state photophysical and two-photon absorption study of polyfunctional phenylsilsesquioxanes ([o-RPhSiO1.5]8, [2,5-R2PhSiO1.5]8 and [R3PhSiO1.5]8) indicate that adding additional chromophores in a nanostructure could strengthen the electronic coupling among substituted chromophores and enhance charge transfer character of the entire molecule. Time resolved absorption and emission spectroscopy reveal the excited state dynamics of corner and half ([p-Me2NStilSi(OSiMe)3], [p-Me2NStilSi(OSiMe)]4), as well as cube ([p-Me2NStil8SiO1.5]8). 2) A series of novel oligothiophene-perylene bisimide hybrid (DOTPBI) dendrimers (G0, G1, and G2) were investigated. Results revealed the ability of these molecules to undergo intramolecular fluorescence resonance energy transfer (FRET) from the dendritic oligothiophenes (DOT) to the perylene bismide (PBI) moiety. The delocalization length and the photoinduced electron transfer (PET) rates were investigated as a function of dendrimer generation. An ultra-fast (~200 fs) energy transfer process from the DOT dendron to the PBI core was observed. In the case of the G2 dendrimer, with relatively larger oligothiophene dendrons attached to the bay area of the perylene bisimide, the PBI core is highly twisted and thus loses its electron trapping ability. As a result, among the three generations studied, G1, which has the best two-photon cross section and the most efficient energy transfer, is the best light harvesting material among three samples.