Measuring Electronic Coherence at the Single-Molecule Level with Nonlinear Coherent Spectroscopy

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With support from the Chemical Measurement and Imaging (CMI) Program in the Division of Chemistry, Dr. Elad Harel and his research group at Michigan State University are developing new methods to measure the quantum mechanical behavior of single molecules. In quantum mechanics, the behavior of a molecule often depends on a delicate relationship between the different states of the system. This relationship, called coherence, is easily disrupted through interactions with the surrounding environment, and often requires experimentalists to make difficult measurements at low temperatures or in highly isolated environments in order to probe the quantum mechanical properties of a complex system. However, it is becoming increasingly clear that even under ambient conditions quantum mechanics plays an important role in the behavior of molecular systems, including photosynthetic proteins that convert sunlight into chemical energy. While coherence has been measured for collections of molecules, knowing the behavior of single molecules is important for understanding how molecular structure affects the function of complex systems. Therefore, Professor Harel’s team is developing tools to measure coherence at the single-molecule level by employing extremely sensitive detection methods. The research project also provides advanced technical training for students, as well as outreach activities targeting a wide audience of young people from a range of economic and social backgrounds.The development of nonlinear spectroscopy methods at the single-molecule level holds the promise of revealing important information that is not currently available from ensemble methods. In this project, Dr. Harel and his group are developing new methods to enable single-molecule nonlinear spectroscopy measurements at room temperature. The approach uses two-dimensional electronic spectroscopy to probe electronic coherences and electronic-vibrational coupling at the single-molecule level. These measurements reveal important structure-function-dynamics relationships in complex systems, including pigment-protein complexes and quantum-confined nanocrystals under ambient conditions. Understanding the true electronic coherence time and its physical origin free of inhomogeneous broadening are critically important for comparing experimental measurements with theoretical predictions, and for developing a deeper fundamental understanding of molecular interactions. The new approach being developed by the research team has the potential to impact understanding of molecular mechanisms that govern a wide range of complex chemical systems. In addition to enabling important new measurements of molecular systems, the research goals of the project are closely integrated with student training at the graduate and undergraduate levels, including a program in which graduate students develop five-week summer tutorial courses based on their research.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.