| Novel materials must take into account both the generation and extraction of photogenerated charges in order to be effective for use in the active layer of organic photovoltaics. A common architecture employed in active layers is the bulk heterojunction design, where electron donor and acceptor materials are randomly intermixed. Although processing steps may improve the degree of order in such systems, these layers are inherently disordered and introduce loss pathways that lower the efficiency of the photovoltaic cell. Small molecule donor-acceptor dyad or triad systems which have well-controlled intermolecular interactions are a promising approach toward increasing cell efficiency. The work in this thesis is aimed at studying the photophysical and assembly behavior of systems designed with the generation of charge transport conduits in mind. Systems were designed to include perylene-3,4:9,10-bis(dicarboximide) (PDI) molecules, which can be induced to form ordered pi-stacks under certain solvation conditions.;A linear charge separation triad system with terminal PDI groups formed a variety of poorly-defined aggregate structures, and aggregation was primarily controlled by the central 1,6-diaminopyrene secondary donor group. The variety of aggregate structures directly impacted the photophysical behavior of the system, ranging from turning off charge separation to producing a range of charge recombination lifetimes. Attempts to generate a second generation system with better-controlled aggregation behavior were unsuccessful. An alternate approach toward designing application-relevant materials utilized recently-developed chemistry to functionalize the 2,5,8,11-positions of PDI and study charge transfer reactions through these positions. In the first study, donor groups were appended to the so-called "headland" positions and the rate of charge separation and recombination measured, demonstrating a high degree of electronic coupling between the PDI core and donor substituents. In the second study, naphthalene-3,4:7,8-bis(dicarboximide) (NDI) terminal acceptors were appended in the headland position of a PDI-phenothiazine charge transfer dyad. Electron paramagnetic resonance experiments detected the presence of a radical pair with the electron residing on the NDI units although transient absorption measurements show the yield of this charge shift from PDI to NDI is minimal. |
