Guest-Induced Electrical Conductivity and Photovoltaic Activity of Metal-Organic Frameworks

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Non-Technical Abstract:The prospect of advancing modern technologies in a clean, safe, and sustainable way depends on constant innovation and supply of new functional materials that can produce and store energy and perform myriad other complex functions. Self-assembled from metal ion clusters and organic linkers, crystalline porous materials known as metal-organic frameworks (MOFs) exhibit remarkable synthetic simplicity, structural diversity, and size-selective guest uptake capacity, which make them attractive platforms for a variety of advanced applications. Although their practical applications are well documented, electronic and photonic MOFs are still at their infancy and evolving rapidly. Supported by the Solid State and Materials Chemistry program of the National Science Foundation, this project focuses on developing stimuli-responsive MOFs that not only capture complementary guest molecules inside their cavities, but also engage them in specific electronic interactions to stimulate new properties and functions, particularly, charge conduction and light to electrical energy conversion. Precisely oriented films of electro- and photoactive MOFs grown on functionalized electrodes are integrated into prototype devices to exploit the preorganized porous architectures filled with complementary guests to promote directional charge movement in electrical and photovolatic devices. Systematic structure-property relationship studies demonstrate how stimuli-responsive MOFs adapt to different physical and chemical inputs and how their tunable electronic and optical properties can be exploited. In addition to providing graduate students and postdoctoral researchers a cutting-edge research and learning environment, this interdisciplinary project is enabling the PI to engage undergraduate and high school students in research, motivate younger generation to pursue higher education in science and technology through outreach activities at local schools and science museum, and raise the public awareness on the importance of basic research by discussing scientific breakthroughs in public forums. Technical Abstract:The electrical conductivity and photovoltaic activity are among the most coveted but remote properties of metal-organic frameworks. The lack of intrinsic charge carrier density and the light-harvesting capability, however, deprives many existing porous frameworks of tunable electronic and optical properties that are key to their abilities to serve as viable semiconductors and photovoltaic materials. To address these issues and transform MOFs into viable electronic and photonic materials, in this project, the PI and his team are (1) constructing stimuli-responsive MOFs based on redox- and photoactive ligands that can interact with and respond to various guests, applied electric field, and light to create new non-native properties, (2) growing robust, oriented MOF-films in a bottom-up fashion on functionalized electrode surfaces that can promote directional charge movement, and (3) doping these stimuli-responsive MOF films with complementary redox-active guests that can produce mobile charge carriers by interacting with the ligands and facilitate charge dispersion through the frameworks by forming extended ð-stacks. The current-voltage profiles of MOF films measured before and after guest infiltration and in the presence and absence of light reveal their stimuli-responsive nature and the impact of the encapsulated electroactive guests on their electronic and optical properties. Since guest intercalation is a ubiquitous and non-destructive process, it can be adopted broadly to engineer non-native electrical and photonic behaviors in any adaptive MOFs. The prototype MOF-based solar cells developed in the PI's lab are also used for outreach activities to demonstrate light to electrical energy conversion to K-12 students and motivate them to pursue higher education in science, technology, engineering, and math.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.