Modeling Molecular Aggregate Photophysics in Free Space and in Optical Microcavities

Temple University, Philadelphia PA

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports theoretical and computational research and education on how light is absorbed or emitted from semiconductor materials made of organic molecules. The most familiar semiconductor is probably silicon which is used in microelectronics and the chips inside modern computers. However,semiconductors based on organic molecules continue to make inroads into commercial devices, such as organic light-emitting diodes or OLEDs. Thin organic films can be driven electrically to emit light or can be used to convert solar energy into electrical energy. The PI and his research team will investigate the fundamental processes in organic crystals or aggregates when they absorb and emit light. The PI will also investigate how absorbed energy is transported between molecules which is similar to how plants transport energy during the process of photosynthesis. The research team will conduct a theoretical investigation by solving equations based on quantum mechanics which describe how organic molecules respond to light. The equations will be solved using sophisticated computer algorithms. The PI will also investigate the effect of enclosing a thin organic film in a very small "micro" cavity formed by two reflecting mirrors separated by a tiny distance equal to about a wave length of light. A microcavity enhances the interaction between light and the enclosed molecules and can dramatically alter the behavior of the enclosed organic film, allowing one to better control its optical properties. The proposed activities will also enhance research infrastructure through domestic and international collaborations involving Professor Libai Huang at Purdue University, who will employ state-or-the-art experimental techniques to probe energy transport in organic films, and Dr. Felipe Herrera at the University of Santiago, Chile, who will assist in the theoretical investigations of organic microcavities. Overall, this research effort should contribute to a blueprint for the next generation of electronic devices based on organic materials. TECHNICAL SUMMARY This award supports theoretical and computational research and education on how light is absorbed or emitted from semiconductor materials made of organic molecules. Solid phases of pi-conjugated molecules and polymers continue to receive widespread attention as semiconducting materials in field effect transistors, light emitting diodes, and solar cells. However, despite the more than five decades of intensive experimental and theoretical research following Kasha's pioneering work on H- and J-aggregates, there remain important questions regarding the nature of the photo-excitations in molecular aggregates and how the optical response is related to crystal packing and morphology. The PI and his research team have recently extended Kasha's model, which is predicated entirely on long-range Coulombic coupling, to include short-range (super-exchange) coupling arising from intermolecular charge-transfer in packing arrangements hosting close intermolecular contacts. Although the model can predict with quantitative accuracy details of the absorption spectral line shape, it is limited in its ability to describe energy transport, as it does not account for excimers, which are commonly encountered in many dye aggregates and crystals. Excimers, which can trap energy and limit transport, arise when an optically-excited state couples strongly to an intermolecular coordinate. Hence, a primary goal of the PI's research activity is to expand the post-Kasha model to include excimers. The approach is based on a multi-particle representation of a Holstein-style Hamiltonian which is superior to most others in that it treats all the important physical processes including exciton coupling, the mixing between Frenkel and charge-transfer excitons, exciton-vibrational coupling, and exciton-photon coupling, on equal footing. The essentially exact treatment of physical observables within a large phase space enhances the likelihood for discovering new and potentially useful physical phenomena. With quantitative reproductions of both the absorption and photoluminescence spectra in hand, predictions of the efficiency of exciton transport will be made. The Huang Group at Purdue will provide the experimental validation by conducting femtosecond-resolved transport measurements of several perylene diimide (PDI) derivatives with varying degrees of excimer emission. In another thrust, the PI will investigate the behavior of organic materials inside optical microcavities, where a strong cavity field can be used to control the mixing between material eigenstates. Of particular interest is the possibility of modulating the mixing between Frenkel and charge-transfer excitons, thereby controlling the formation of excimers. In addition, the fundamental photophysical properties of the recently discovered "dark" polaritons - composite quasiparticles consisting of a mixture of electronic, photonic and vibrational degrees of freedom - will be explored in collaboration with Felipe Herrera at the University of Santiago, Chile. The analyses will be based on Holstein-style Hamiltonians for free-space and cavity-confined molecular aggregates represented in a multi-particle basis set sufficient for obtaining highly accurate spectral and transport observables. Overall, the project has the potential to significantly advance our understanding of i) the relationship between molecular aggregate properties, particularly photophysics and transport, and packing morphology; ii) the way microcavity coupling can be exploited as a means for controlling super-exchange coupling and excimer formation in pi-stacks and iii) novel types of polaritons involving all three degrees of freedom, electronic, vibrational, and photonic. This award is jointly supported through the Condensed Matter and Materials Theory Program in the Division of Materials Research and the Chemical Theory, Models and Computational Methods Program in the Chemistry Division. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.