Evolution of the Electronic Properties of Conjugated Materials from Isolated Molecules to Aggregates

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In this project, funded by the Chemical Structure, Dynamics and Mechanisms Program of the Chemistry Division, Professor Linda Peteanu and her students will work towards a better understanding of how the chemical and physical processing steps required to turn a polymer into a working device affect its charge transport and light emitting capabilities. This knowledge is required for the successful design of the most efficient lightweight, flexible, polymer-based materials for next-generation photovoltaic devices and low power consumption lighting. The Peteanu group focuses on how these critical properties are impacted by aggregation or the interactions between polymer chains that inevitably occur during processing to form solid state films. This proposal addresses these issues by optical spectroscopy on the bulk and single molecule level of aggregates formed from short-chain model oligomers of the electroluminescent polymers MEH-PPV, CN-PPV and polyfluorene by solvent poisoning. These oligomer aggregates are excellent model systems for the spectroscopy and dynamics of the more complex polymers. Moreover, they fortunately lack the structural defects that make interpreting the polymer data more difficult. The light emitting properties and energy transfer dynamics are measured using photon correlation in single aggregates and ultrafast spectroscopy in bulk samples. Ultrafast spectroscopy and Stark spectroscopy are used to probe charge separation and transport in these materials as well. Electronic structure calculations in collaboration with David Yaron (CMU) then enable obtaining a unified picture of the structure, spectroscopy, and dynamics in these systems. These efforts contribute not only to the development of energy saving devices but to training of a work force that can capitalize on their commercial potential.