Using Amphiphilic Semiconducting Polymers to Control Structure and Exited State Dynamic in Conjugated Organic Assemblies

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With this award, the Macromolecular, Supramolecular, and Nanochemistry Program in the Chemistry Division is supporting Professor Sarah H. Tolbert and her team at the University of California, Los Angeles (UCLA) to develop artificial photosynthesis systems to efficiently use light energy. The understanding gained from this research may allow the future production of inexpensive solar cells or the fabrication of devices that use sunlight to perform important chemical reactions. In photosynthesis in plants, light falling on a plant leaf causes an electron to be excited in a specific fashion, launching a chemical reaction sequence that the plant utilizes to capture carbon dioxide (CO2) reductively and convert CO2-moieties into the carbon atoms of the glucose. This project uses the same ideas to create a variety of artificial photosynthesis systems to efficiently harvest and utilize energy from visible light. The basic problem with artificial photosynthesis is that even though it is relatively easy to use light to energetically excite an electron, it is difficult to maintain the required state; electrons often return to their original starting point without performing useful chemistry. This work uses specially designed molecular systems that self-assemble into predetermined structures in water. Those structures control the motion of electrons after they are photo-excited to ensure that the desired chemical state is achieved. In addition to the potential scientific advances, this multidisciplinary project provides training for undergraduate and graduate students in areas of national need. The project also helps bring experiments on related topics of energy harvesting from sunlight and nanoscale materials to Los Angeles area high schools.Professor Tolbert’s team is examining the assembly of organic systems with intrinsic anisotropy, minimized charge recombination, and high light absorption. The project is a three-way collaboration among researchers who have complementary expertise in synthesizing the appropriate molecules (Yves Rubin - UCLA), in understanding the structure and self-assembly of these molecular systems (Sarah Tolbert - UCLA), and in monitoring the photodynamics and photochemistry of these assemblies (Benjamin Schwartz-UCLA). In biological photosynthetic systems, absorption of light leads to the requisite charge-separated state with nearly 100% quantum yield, thanks to evolutionary tuning of electron transfer cascades that spatially separate charges, preventing recombination. This project examines the creation of analogous artificial organic assemblies with intrinsic anisotropy, that are designed to minimize charge recombination while optimizing light absorption. One specific aim of this work is to synthesize a large family of amphiphilic electron-donating semiconducting polymers and amphiphilic small-molecule electron-acceptors that spontaneously form cylindrical micelles in aqueous solution. These molecules are made with a range of sizes, charges, electronic structures, and steric constraints, allowing them to self-assemble in a predictable manner according to specific hydrophobic and steric interactions. The next aim is to characterize the assemblies structurally and optically to understand the geometries that are created. The final aim is to study the assemblies spectroscopically and in functional opto-electronic devices to determine what design changes can further enhance the efficiency of charge-separation. Overall, these studies on the design and tuning of self-assembling light-harvesting polymers are expected to contribute to the development of artificial photosynthetic systems. In the longer term, advancements made through these foundational studies are expected to provide useful knowledge and insight for the fields of photovoltaics and photocatalysis.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.