CAREER: Ultrafast Nanoscopy of Energy Transport in Molecular Assemblies

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The Chemical Structure, Dynamics and Mechanisms Part A (CSDMA) Program in the Chemistry Division of the NSF supports Professor Libai Huang at Purdue University to develop innovative microscopy techniques that provide 'movies' of how light energy absorbed by molecules moves in space and in time. The molecules in this research absorb light strongly and are potentially useful for devices such as solar cells. In order to use them in solar energy conversion devices, the molecules have to be closely packed together and the packing greatly influences how energy is transferred from one molecule to the other. These factors determine the efficiency of the solar cell. By tracking how energy moves in molecular assemblies with different packing, this work is resolving mechanisms that control the speed and distance of energy migration and providing guidelines for designing structures for efficient solar energy harvesting. To achieve these goals, microscopy techniques are being developed to record fast energy transfer events with a resolution of 100 femtoseconds (a femtosecond is one quadrillionth of a second) and to image energy migration distance with a resolution of 20 nanometers (a nanometer is one billionth of a meter). Dr. Huang is developing a new instructional model that integrates active learning with scientific writing aimed at better teaching abstract concepts in physical chemistry. In particular, solar energy applications in this award are being developed into demonstrations to illustrate key concepts in quantum mechanics. Writing assignments of these demonstrations are being utilized to promote high-level cognitive understanding of abstract concepts. The researchers are also developing a scientific outreach program to bring state-of-the-art solar energy research to high schools in Northwest Indiana. The ultrafast nanoscopy methods being developed in this project directly image exciton transport across multiple length and time scales to elucidate coherent and incoherent energy transfer pathways in molecular assemblies. Exciton populations and dynamics following photoexcitation are being mapped with simultaneous 100 fs temporal resolution and ~20 nm spatial precision. These measurements provide first-of-a-kind visualization of the spatial extent of coherent transport. Two model molecular aggregates are employed to systematically probe energy transfer in the intermediate coupling regimes. Linear H aggregates serve as model 1D excitonic quantum wires in which long-range wavelike coherent transport is expected. Tubular aggregates are employed to elucidate exciton transport as a function of dimensionality. Building on results from the model systems, the long-term goal of controlling transport in supramolecular assemblies is achieved by modulating both short- and long-range intermolecular coupling. The research and educational activities are integrated to educate the next generation of solar energy researchers at the K-12, undergraduate, and graduate levels.