Excitonic Coupling in Molecular and Polymeric Aggregates: Beyond Conventional J- and H-aggregation

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NONTECHNICAL SUMMARYOrganic molecules and polymers continue to receive attention as semiconducting materials in a wide array of practical applications, including paper electronics, solid-state lighting, solar cells and scaffolds for tissue growth. Most devices benefit from rapid energy transport, which is dictated by how fast an electronic excitation on one molecule can be transferred to a more distant molecule. However, despite the more than five decades of intensive experimental and theoretical research, there are still many questions regarding the nature of the electronic excitations in organic molecular aggregates, thin films and crystals. The main objective of the proposed research is to develop a theory of electronic coupling between molecules which takes into account the organization of molecules relative to each other. This theory will provide information about the rate of energy transfer and will reveal important information about the properties of the electronic excitations, information that can be used to design more efficient devices. The PI's group will also explore the use of optical microcavities for controlling material properties. Microcavities consist of two mirrors separated by a very short distance of the order of one millionth of a meter. The interaction between light and matter is greatly amplified for materials inserted between such mirrors. The proposed research will benefit from collaborations with the experimental groups at the University of Massachusetts and the University of Montreal. The broader impact of the proposed research will be in an enhanced understanding of the photophysical and transport properties of a technologically important class of materials. The commercial impact of organic electronic devices is expected to dramatically increase over the next decade through products like flexible displays, electronic labels, solid-state lighting and solar cells. The wide array of photophysical and transport behaviors afforded by molecular aggregates studied in this proposal could provide the basis for novel design paradigms for efficient solar absorbers and light emitting materials.TECHNICAL SUMMARYThe main goal of the proposed research is to develop a theory for the photophysical response of molecular aggregates which accounts for the simultaneous presence of long-range Coulombic interactions and short-range charge-transfer (CT) interactions between the constituent molecules. The theory will consider the electronic coupling, the electronic-nuclear coupling, and diagonal and off-diagonal disorder on equal footing and will extend the conventional J- and H-aggregate model of Kasha to systems such as molecular pi-stacks, in which the very close proximity between neighboring molecules allows for significant overlap between molecular orbitals. Investigations are planned to understand the photophysical and transport properties of different aggregate types and how such aggregates evolve into conventional H- and J-aggregates as the adiabatic CT exciton is tuned away from the parent Frenkel exciton. Studies will also be undertaken to determine how aggregate properties are altered by immersion into a tuned optical microcavity. The analyses will be based on Holstein-like Hamiltonians represented in a one- and two-particle basis set. Fundamental excitations and their spectral signatures will be evaluated using numerical matrix techniques. Specific applications will be made to rylene pi-stacks which have been studied as dye pigments and electron-transporting materials, as well as poly(3-hexylthiophene) pi-stacks which make excellent exciton transporting materials in photovoltaic devices. The proposed research will benefit from collaborations with the experimental groups at the University of Massachusetts and the University of Montreal. The broader impact of the proposed research will be in an enhanced understanding of the photophysical and transport properties of a technologically important class of materials. The commercial impact of organic electronic devices is expected to dramatically increase over the next decade through products like flexible displays, electronic labels, solid-state lighting and solar cells. The wide array of photophysical and transport behaviors afforded by molecular aggregates studied in this proposal could provide the basis for novel design paradigms for efficient solar absorbers and light emitting materials.