Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

Jao van de Lagemaat

Second Advisor

David M. Jonas

Third Advisor

Garry Rumbles

Abstract

Model conjugated organic semiconductors such as pentacene and thiophene-based dendrimers have photophysical properties that could potentially improve the performance of organic photovoltaic devices. Spectroscopic and time-resolved measurements of these materials are used to create kinetic schemes of their excited states. This thesis first presents time-resolved spectra of pentacene on nanostructured films. The excited states of pentacene on these films are found to exhibit an enhanced absorption of visible light due to surface plasmon interactions. Furthermore, the triplet state kinetics are altered in the presence of a surface plasmon active layer, and a strong increase in the yield of triplets is observed. These results indicate that surface plasmon coupling can be used to direct the excited state of such molecular systems to a desired outcome and that potentially such control can be used to improve the efficiency of solar energy conversion. A detailed kinetics analysis is presented of the other conjugated system, thiophene-based dendrimers, first in the experimentally simpler solution form. From femtosecond-resolution transient absorption data, we create a kinetic scheme that includes two cooling processes to convert the initially generated, hot singlet into the geometrically relaxed, first excited singlet. Subsequently, relaxation of this singlet occurs via one of three pathways: radiative decay to yield the ground state, internal conversion to yield the ground state, or intersystem crossing to produce the triplet excited state. Flash photolysis measurements allow the longer time delay necessary to study this long-lived triplet state. The decay time of the triplet decreases with an increase in the arm length of the dendrimer. Finally, steady-state and time-resolved absorption measurements of dendrimer films are presented. These studies follow from those of the dendrimers in solution and provide excited state kinetic information that is more relevant to the application of dendrimers in photovoltaic devices. Steady-state absorption measurements show that spun cast and drop cast dendrimer films follow the same trend as the dendrimers in solution: a red-shift of the absorption with increasing number of thiophene units. Flash photolysis data reveal that the triplet is quenched by oxygen; films in an inert atmosphere have triplet lifetimes on the same order (tens of microseconds) as in solution.

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