This award by the Inorganic, Bioinorganic, and Organometallic Chemistry Program supports the work of Professor Dennis L. Lichtenberger and Dr. Nadine E. Gruhn at the University of Arizona to build an understanding of structure features that determine the electronic properties of separated molecular functional units that communicate through molecular linkers or bridges. The connections between these molecular functional units will be examined in the gas and condensed phases using photoelectron spectroscopy (PES). The proposed work focuses on the application of organometallic metal-linker-metal systems (with a variety of saturated and unsaturated linkers of differing rigidity levels) as well as thiol modified surfaces with concentrated and dilute metal centers. The researchers have unique access to the only U.S.-based facility for PES. Drs. Lichtenberger and Gruhn have extensive, successful collaborations throughout the chemical community and regularly host high school teachers and minority high school students in a summer outreach program.
This research project explored the nature of electrons and electronic effects spread over extended distances on the atomic scale. Electrons are best known as the "glue" that bonds atoms together and determines the geometric structure of molecules and materials. Electron bonds in the small distances between atoms have been major subjects of scientific research over the last century, and are relatively well understood. However, in many important situations electrons are better envisioned as expanded electron clouds extending over large numbers of atoms and molecules, and large volumes of materials. These extended electron clouds are much less well understood, but they are central to many important properties, such as the conductor and semiconductor performance of digital electronics, the light emitting diodes (LEDs) and other display technology of televisions and smartphones, the conversion of solar energy to electrical energy in solar cells, and even the biological electron transfer that allows enzymes in our bodies to function or allows plants to grow. These extended electron clouds cannot be observed by traditional techniques, and so one purpose of this project was to develop new instrumentation and methods that allow us to directly measure the extended electronic effects. A major outcome of this project is the ability to measure in ways not available previously the energy and structure consequences of these extended electron clouds. Our approach centers on a technique called photoelectron spectroscopy, which as the name implies involves the interaction of light (photons) with electrons. The principle of this technique was first explained by Einstein (Figure 1). The excitation and movement of electrons by light is the basis of solar energy conversion, and conversely the movement of electrons that emits light is the basis of light-emitting diodes. Thus the energies measured by photoelectron spectroscopy are directly related to the fundamental principles of solar cells and LEDs. Two specific intellectual outcomes of this project development are (1) demonstration (with the new instrument we constructed) of the ability of high-energy X-ray probes of molecules in the gas phase to help reveal electron interactions not previously seen (Figure 2), and (2) the extension of these detailed observations of molecules in the gas phase to properties in solution (electrochemistry) and solid materials (electron transport), often with surprising results that are the reverse of traditional expectations (Figure 3). The facility we have developed at the University of Arizona is the only source in the United States for this gas-phase photoelectron spectroscopy of large neutral molecules. We have developed the laboratory as a multi-user facility available to the scientific community. Because of the importance of experimental electronic structure information and the uniqueness of our capabilities, the facility has had broad impacts on the research of a large number of other scientists who have been attracted to our facility. As such this facility is a major contributor to the scientific infrastructure for research and education in this country. Among the intellectural outcomes, the research has (1) revealed through-space electron interactions in metal nitrides related to conversion of nitrogen gas to ammonia needed for fertilizers, (2) measured electron transfer parameters related to the function of LED’s in display technologies, (3) probed the electronic properties of iron-sulfur catalysts for the production of hydrogen from water related to sustainable energy and clean solar fuels, and (4) determined structure-electron energy relationships in molybdenum-containing enzymes that have important functions in the human body. The fundamental importance of these explorations also has had broad impacts to other scientific and enginieering disciplines outside of chemistry and beyond. Examples in materials science and engineering in which our studies have a direct impact include optoelectronic devices such as field-effect transistors (FETs), light-emitting diodes (LEDs), photovoltaic and solar cells, nonlinear optical materials, quantum dots, and molecular conductors and semiconductors. For the health related fields our studies of electron transfer processes include systems that model the active sites of metalloenzymes. In energy science we are investigating the mechanisms of conversion of electrical energy to chemical energy. In a broader sense the studies are relevant to commercial technology, environmental protection, public health, clean and sustainable energy, and welfare needs of society. Additional outcomes are in human resource development and resources for education. Because of the diverse applications and numerous collaborations that have resulted from the development of these research capabilities, 34 undergraduate students, 16 Ph.D.-seeking students, and 9 postdoctoral students who participated in this project have had the opportunity to interact with many other scientists in this country and around the world, learn a wide range of science, and develop the ability to work with others in team projects. In addition, this project made possible the offering of a summer program in energy research to a diverse group of 4 high school teachers and 28 high school students over the course of this project.
