Collaborative Research: Excited-state Dynamics in Organic Charge-Transfer Compounds: An Experimental and Theoretical Study

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Non-technical description: This project is a study of crystals made up of carbon-based organic molecules that conduct electricity in a way similar to the behavior of the silicon that makes up computer chips. The project aims to develop a fuller understanding of how these organic materials respond to light, and how the flow of electricity through them is affected by the vibrations of the molecules. Experiments that measure light absorbed and emitted by the crystals are compared to calculations of the various energies of the electrons in the crystal. From these experiments and calculations it is now possible to determine how the vibrations of the molecules affect the electrons that are responsible for the flow of electricity, thus gaining fundamental understanding of the electronic properties of these materials. The findings of this project help determine the best materials to use to make more efficient and cheaper electronic devices such as displays for flat-screen TVs and other media, photosensors, and solar cells. Graduate students and undergraduates participating in the project are developing valuable skills in experimentation and computation while contributing to fulfilling a national and global societal need for more efficient and sustainable technology.Technical description: Organic semiconductors are of significant interest due to their potential for opto-electronic applications such as solar cells and photosensors. Charge transfer compounds, which are made of two or more different organic molecules in which one species acts as a donor of electric charge and the other as an acceptor, could provide new properties or improved performance to increase the range of application of organic semiconductors. The goals of this project are to elucidate the excited-state dynamics of selected charge transfer compounds and develop a deep understanding of electronic couplings and electron-phonon couplings in them. Charge transport and excited-state dynamical processes are critical to applications of these materials in opto-electronic devices, and depend on a subtle interplay between electronic and electron-phonon interactions. Transient absorption and fluorescence lifetime measurements, when interpreted in light of computational evaluation of the rates of various electron-transfer processes, allow the decay mode of excitons in these materials to be determined. Resonant Raman experiments are used to extract relaxation energies and transfer integrals. These experimental findings are interpreted in light of site energies, electron-phonon and electronic couplings computed using a variety of methods, including density functional theory calculations of large molecular clusters and those based on periodic boundary conditions, semi-empirical approaches, and tight-binding models, and molecular dynamics simulations. This strongly-coupled series of experimental investigations and theoretical modeling opens a large range of functionalities not manifest in monomolecular solids. The findings ultimately contribute to fulfilling a national and global societal need for more efficient and sustainable technology.