Synthesis and photophysical characterization of conjugated molecules for potential solar cell uses

• Main Author: Chudomel, John Matthew
• Language: ENG
• Published: ScholarWorks@UMass Amherst 2012
• Subjects: Organic chemistry|Polymer chemistry|Materials science
• Online Access: https://scholarworks.umass.edu/dissertations/AAI3545911

Three new strategies were successfully pursued for the synthesis of defined length oligomers of p-phenylene-vinylene. These strategies are interchangeable and allow the fast and efficient synthesis of a wide variety of oligomers with a number of different substituents. An assortment of new molecules and oligomers were synthesized and characterized during this study to prove the effectiveness of each strategy. The new strategies were compared to previous methodology for making similar oligomers. A large, nonplanar, conjugated chromophore 9BrH was synthesized based on an adaptation of previous work. 9BrH and its synthetic precursor, pre9BrH, were characterized using X-Ray crystallography. The experimentally determined conformation and bond lengths of 9BrH were compared to previous theoretical studies and confirmed much of what was predicted. The 9BrH chromophore was stockpiled for use in additional studies. Three highly twisted triarylamines were synthesized and investigated for internal charge transfer behavior. Using a large chromophore as one aryl group forced the triarylamines into twisted, propeller-like conformations. The chromophore anthracene was utilized to induce the twist in the triarylamines 9DAAA and 910BAA. The previously synthesized 9BrH was utilized to induce a twisted conformation for the triarylamine 9DAAH. Theoretical predictions indicated that electron density should be delocalized in the ground state and localized on the large chromophore in the excited state, behavior consistent with molecular internal charge transfer. 9DAAA and 910BAA were characterized by X-Ray crystallography which confirmed the desired twisted conformation of the triarylamines in the solid state. UV-Vis absorption spectra for all three triarylamines had long wavelength, broad absorption peaks characteristic of internal charge transfer. Solution fluorescence of each triarylamine demonstrated a large dependence on the surrounding environment; when solvent polarity was increased, fluorescence intensity decreased and red shifted. This behavior was also attributed to interactions between the strong dipole of the triarylamines in the excited state with the dipole of solvent molecules in the surrounding environment. Fluorescence lifetime studies allowed the derivation of a model in which the triarylamines had two different, competing decay pathways from the ground state to the excited state. The aggregation properties of 9DAAA were studied using binary solvent mixtures which forced dissolved 9DAAA from the solution. Suspensions of aggregates of 9DAAA were found to have enhanced emission properties up to 700% more intense than solutions of 9DAAA. This behavior was attributed to the change in surrounding environment of 9DAAA when switching from dissolved to aggregated. The polar solvent surrounding fully dissolved 9DAAA suppresses fluorescence while the solid-state environment of 9DAAA upon aggregation allows decay from the excited state to the ground state to occur via fluorescence. 9DAAA and 910BAA were characterized by cyclic voltammetry which indicated that their energy levels fell between two commonly used components of dye sensitized solar cells. Studies were done to evaluate the effect of triarylamine additives to the redox couple solution of dye sensitized solar cells. Both triarylamines improved the performance of the cells to which they were added; 9DAAA improved the cells' VOC parameter while 910BAA improved the cells' JSC parameter. These studies suggest that 910BAA actively participates in electron shuttling between a cell's redox couple and dye.