Energy and electron transfer in photovoltaic materials
This thesis reports on excited state relaxation, electron transfer, and energy transfer dynamics in materials used in the manufacture of dye sensitized and organic photovoltaics and organic electronic applications. Materials studied include terthiophenes substituted with electron donating and withdrawing end groups, size selected colloidal ZnO nanocrystals attached to thiophene and porphyrin dye sensitizers, and phenyl core spiral thiophene dendrimers. Excited state dynamics, measured via femtosecond resolution fluorescence and pump-probe spectroscopy techniques, are interpreted in order to assign radiative and non-radiative decay rates of the substituted terthiophenes, electron transfer kinetics from attached dye sensitizers to the ZnO nanocrystals, and intramolecular energy transfer rates between arms of the thiophene dendrimers. Charge transfer excited states are demonstrated in some of the substituted terthiophenes, allowing for the ability to select the singlet decay mechanism via solvent polarity. Electron transfer kinetics in the dispersed ZnO systems are found to be relatively independent of dye coverage and preparation conditions and are even found to be tunable by controlling the size of the ZnO nanocrystals to change the relative density of acceptor states available to the dyes via quantum confinement. Finally, intramolecular energy transfer between arms in thiophene dendrimers, measured via ultrafast fluorescence anisotropy techniques, are found to be independent of the relative orientation of the arms around the phenyl core. These dynamics are compared to theoretical through-space transfer rates calculated via Forster point-dipole and transition density methods.
The remainder reports on photolytic bond cleavage and recombination dynamics in a molybdenum complex used in industrial dinitrogen cleavage reactions. Results of pump-probe measurements are analyzed to demonstrate sub-100 fs bond cleavage followed by two fast recombination events, one of which is related to a coherent oscillation found in the pump-probe results. Taken together, these experiments demonstrate the versatility of femtosecond spectroscopy in measuring complex excited state dynamics, the understanding of which is vitally important to the design of materials for photovoltaic, electronic, and other applications.