Understanding Excimers in Molecular J- and H-aggregates: A Holstein-Peierls Approach

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NONTECHNICAL SUMMARY This award supports theoretical/computational research and education on how light interacts with semiconductor materials made of organic molecules. The more familiar semiconductors like silicon, which is widely used in microelectronics and computer chips, are inorganic. However, semiconductors based on organic molecules offer advantages such as less expensive materials processing and more favorable mechanical properties. Organic materials continue to make inroads into commercial devices, such as organic light-emitting diodes or OLEDs used in displays (the “OLED” TV), solar cells, which convert sunlight into electrical energy, and even “wearable” electronic devices and sensors. The PI and his research team will investigate fundamental processes such as light absorption and light emission in organic crystals and aggregates, as well as how the energy from light absorption is transported between molecules - similar to what occurs in plants 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. Of particular interest are certain electronic excited states known as excimers, which emit light of longer wavelengths and convert electronic energy into light energy less efficiently. The PI and his team will study how such excimers form and ultimately how to design molecular aggregates which avoid excimer formation. The proposed activities will also enhance research infrastructure through collaborations with experimentalists such as Professor Libai Huang at Purdue University, who will employ state-or-the-art experimental techniques to probe energy transport in organic films. 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 molecular H- and J-aggregates, important questions remain regarding the fate of photo-excitations and how their spectral signatures depend on crystal packing and morphology. The PI’s group has 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, as well as local coupling to the vinyl-stretching mode responsible for pronounced vibronic progressions in the UV-Vis spectra of a great many conjugated molecules. Although the model can predict with quantitative accuracy details of the absorption spectral line shape and correlate spectral features to the nature of the underlying excitons, it is limited in its ability to describe photoluminescence and energy transport, processes which often require the inclusion of excimers. Excimers, common in pi-conjugated molecules, arise when an electronic excited state relaxes along a “slow” intermolecular (phonon) coordinate resulting in featureless, red-shifted emission. In this project, the next generation of molecular H- and J-aggregate models will be developed which account for excimer formation and emission. The approach is based on a Holstein-Peierls Hamiltonian which includes electronic coupling along the slow-coordinate, is particularly strong in closely packed systems like π-stacks, where the intermolecular electron and hole transfer integrals are hypersensitive to small, sub-Angstrom, changes in the relative orientation of neighboring chromophores. The Holstein-Peierls approach enables all of the important physical processes to be treated on equal footing and fully quantum-mechanically. The model will be employed to account for the absorption and photoluminescence spectral line shapes in excimer-forming perylene diimide dimer complexes and larger π-stacks. The ability of excimers to function as energy traps will be investigated through analysis of the density matrix equations of motion. Libai Huang at Purdue University will collaborate and provide experimental validation by conducting spectroscopic measurements and femtosecond-resolved transport measurements of several perylene diimide derivatives which display varying degrees of excimer emission. The PI’s approach may enhance the likelihood for discovering new and potentially useful physical phenomena, as well as design strategies for controlling excimer formation for device applications.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.