Electron-Electron Interactions and the Photophysics of Semiconducting Conjugated Polymers and Single-Walled Carbon Nanotubes

University Of Arizona, Tucson AZ

Investigators

Abstract

This award supports theoretical research on semiconducting conjugated polymers and single-walled carbon nanotubes. The award is supported by the Theoretical and Computational Chemistry Program and the Materials Theory Program. The project has four distinct goals. (1) We intend to develop a comprehensive theory of spin-dependent exciton formation in the charge-transfer (CT) reaction between oppositely charged polarons in organic light emitting diodes (OLED's). We will go beyond our previous work, which was focused on the relative yields of the lowest singlet and triplet excitons, to determine the overall yields in each spin channel for a wide range of Coulomb parameters, molecular structures, and relative orientations between molecular components. (2) We will develop a microscopic theory of bimolecular triplet-triplet annihilation (TTA). (3) We will obtain correlated electron descriptions of pi-conjugated polymers with optical gaps in the infrared. (4) Finally, we plan to construct a theory of excitonic electronic structure and optical nonlinearity of semiconducting single-walled carbon nanotubes (SWNT's). The investigations will consist of many-body calculations within the Pariser-Parr-Pople and extended Hubbard Hamiltonians for pi-conjugated molecules and polymers. The approaches taken will include exact diagonalizations, singles and multiple-reference singles and doubles configuration interaction (SCI and MRSDCI), and the Density Matrix Renormalization Group (DMRG). The relative yields of competing CT reactions that occur in OLED's will be determined within a time-dependent Schroedinger formulation. Our emphasis is on developing conceptual frameworks, and not on quantitative calculations of materials parameters. Thus the apparent neglect of electron-phonon interactions is based on the argument that their qualitative effects can be grafted on once the effects of electron-electron interactions are understood. Understanding the photophysics of pi-conjugated systems is difficult, because of the moderately strong electron-electron interactions in these systems. The present grant poses timely and important questions, in two different classes of carbon-based semiconductors. We will use state of the art techniques to investigate these questions. Our exact approach to the study of CT reactions in OLED's treats intermolecular interactions and many-electron interactions on equal footing. The research on TTA will be the first attempt to construct a microscopic theory of this phenomenon. The DMRG calculations on the small optical gap polymers will give valuable information on the feasibility of utilizing these and related polymers in emissive or photovoltaic devices. The research on the SWNT's will give new qualitative insight on their excitonic electronic structure and their nonlinear optical properties. During the past twenty years pi-conjugated polymers and molecules have evolved from laboratory curiosities to key new optical materials. Research done here will have strong impact on a broad range of investigations being pursued by experimentalists, extending from purely fundamental science to applied aspects. We hope to answer several questions being probed by experimental colleagues, as well as influence them to venture out and try new materials synthesis and new experiments. One key theme here is electron-electron interaction effects. This is a vital broad area of research. We expect that the knowledge base created in our studies will impact our understanding of strongly correlated electron systems in general. As merely one example, we draw attention to our work on the nonlinear optics of cuprates, whose origin can be traced directly to our work on optical nonlinearities in pi-conjugated polymers. Finally, the research offers an excellent opportunity to train students and postdoctoral associates in the scientifically and technologically vital areas of materials physics, chemistry and advanced computational physics of organic materials and strongly correlated systems. %%% This award supports theoretical research on semiconducting conjugated polymers and single-walled carbon nanotubes. The award is supported by the Theoretical and Computational Chemistry Program and the Materials Theory Program. During the past twenty years pi-conjugated polymers and molecules have evolved from laboratory curiosities to key new optical materials. Research done here will have strong impact on a broad range of investigations being pursued by experimentalists, extending from purely fundamental science to applied aspects. We hope to answer several questions being probed by experimental colleagues, as well as influence them to venture out and try new materials synthesis and new experiments. The research offers an excellent opportunity to train students and postdoctoral associates in the scientifically and technologically vital areas of materials physics, chemistry and advanced computational physics of organic materials and strongly correlated systems. ***