CAREER: Unraveling Excitation-Energy Transfer Processes in Excitonic Light-Harvesting Systems

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The light-harvesting complex found in plants is a complex assembly of molecules that absorbs sunlight and funnels its energy to a central location where it is converted into useable fuels. The efficiency of this solar antenna is remarkable, and it is perhaps one of Nature's most spectacular molecular architectures. While the light-harvesting complex has been studied extensively, the origin of its high efficiency has remained a mystery. The problem is challenging. The light-harvesting complex not only consists of many individual molecules, but the structure is not rigid, and the molecular components are continually moving. The role that this motion plays in facilitating (or impeding) energy transport is unclear. Through support from the Macromolecular, Supramolecular and Nanochemistry Program of the NSF Division of Chemistry, Professor Eisele at The City College of New York (CCNY) is studying bio-inspired nanomaterials to elucidate Nature?s secret to efficient energy transport. Working together, Professor Eisele and her students are synthesizing new nanostructured molecular assemblies that mimic the features of the Nature's light-harvesting complex. They then watch the flow of energy through the assembly using sophisticated near-field scanning optical microscopies with the goal of understanding how structural fluctuations affect energy transport. The project could profoundly impact our understanding of natural photosynthetic systems, and could pave the way towards the design of efficient artificial systems for solar energy conversion applications. In addition, the project is training the next generation of scientists in a broad range fields including nanoscience, spectroscopy and optical microscopy. Through outreach activities, Professor Eisele and her students are working with middle- and high-school students, many from historically underrepresented minority groups, to produce short videos that highlight the challenges and opportunities of nanoscience, from everyday applications to the latest research in super-high resolution nanoimaging.Conceptually, achieving a better understanding of excitation energy transfer (EET) processes in bio-inspired nanomaterials requires model systems that allow thorough testing of current theoretical models of energy transport phenomena. To this end, Professor Eisele is synthesizing well-defined model systems based on monomers such as amphiphilic cyanine dyes and porphyrin dyes. In solution the monomers self-assemble into supramolecular nanotubes that show collective optical properties (excitons) similar to the optical properties found in natural light-harvesting complexes. The nanotube assemblies are then encapsulated in transparent (non-excitonic) materials to mimic the surrounding proteins of the natural light-harvesting complex. The structural and optical properties of individual nanotubes are studied using near field scanning optical microscopy (NSOM) techniques, and the EET process is followed using femtosecond pump-probe techniques. The project is also integrating the pump-probe spectroscopies with NSOM methods, to enable direct observation of energy transport in an individual structure. The experimental results are complemented with Monte Carlo computer simulations of the supramolecular structure.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.