Dennis Lichtenberger

Before he was old enough to enter school, Dennis Lichtenberger was curious about looking into nature at a scale smaller than the eye can see. By the time he was eight years old his tools for looking deeper into nature had progressed from a magnifying glass to a microscope to a chemistry bench. His interest in tools for exploring matter at the molecular level led naturally to his later development of photoelectron spectroscopy for probing molecular electronic structure. He obtained his B. S. degree in chemistry in 1969 from Indiana University with an interest in physical and inorganic chemistry and a Ph.D. degree in Physical Chemistry in 1974 from the University of Wisconsin under the guidance of Professor Richard F. Fenske. After a two-year postdoctoral appointment at the University of Illinois with Professor Theodore L. Brown he joined The University of Arizona as an assistant professor. He was drawn to physical inorganic and organometallic chemistry by the multitude of fascinating structures and properties of matter composed of elements throughout the periodic table, with a focus on the electronic structures that determine these properties. His explorations of electronic structure and bonding initially centered on theory and computations and later expanded to experimentally probing electronic structure via photoelectron spectroscopy. In many ways he was in the right place at the right time. First, as a graduate student he shared an office with Michael B. Hall while Hall was in the midst of programming an approximate method for molecular orbital calculations that later came to be known as the Fenske-Hall method (named by Larry Dahl). Second, while he was at Wisconsin the Department obtained one of the first instruments for what was called ESCA at the time (Electron Spectroscopy for Chemical Analysis, now XPS). He developed the instrument for the study of molecules in the gas phase rather than as solids and with UV sources rather than X-ray sources (UPS), and was able to pioneer the acquisition of  detailed quantitative information on the valence electronic structures of organometallic molecules. With photoelectron spectroscopy he showed how to experimentally ‘observe’ the metal d electron configurations, orbital overlap interactions, and electronic symmetry at the metal centers. He obtained energy measures of fundamental electronic bonding and backbonding interactions of metal carbonyls, thiocarbonyls, dinitrogen, ammonia, cyclopentadienyls, and related ligands. The research also emphasized the value of collaborative interactions in projects with Larry Dahl, Tom Whitesides, Bob Angelici, Dieter Sellmann, Steve Nelson, and Chuck Casey, to name a few. Such collaborations have been a continuing characteristic of his career, and have led to many exciting discoveries at the forefronts of chemistry. Postdoctoral Research. Following his Ph.D. research Lichtenberger sought to broaden his experience in other methods and in particular to gain experience in preparing new molecules with properties to advance the understanding of inorganic and organometallic electronic structure and behavior. He was the first to observe by NMR the slowing of the Berry pseudorotation process in five-coordinate d8 metal carbonyl complexes, and showed that steric factors were dominant in determining the rate of fluxionality. In a related study he was able to show that the cis labilization of carbonyl substitution, which was counter to expectations based on electron richness at the metal center, followed from stabilization of the transition states and intermediates by π donor ligands. He also explained the stability of different structures of dicobalt octacarbonyl observed in the IR. Interestingly, these studies of dimetal carbonyl structure, fluxionality, and reaction mechanisms with donor ligands all relate to his recent investigations of the mechanisms of electrocatalytic production of hydrogen with mimics of the active sites of [FeFe]-hydrogenase enzymes. The University of Arizona. Up to this time publications of photoelectron spectra were generally accompanied by electronic structure computations to assign and interpret the ionizations. Lichtenberger felt that for the technique to be truly valuable it needed to provide chemically useful information independent of computations. He took two approaches toward this goal. The first approach was to develop the instrumentation and the photoelectron experiment so that (a) the technique could obtain higher resolution and precision in the measure of the ionization energies, (b) stable photon sources of different energy could be built to take advantage of the variable ionization cross-sections for assigning the ionizations, (c) advanced data analysis procedures could be developed based on the fundamental physics of the photo-ionization process to extract chemical information, and (d) data could be obtained on large and reactive molecules. For many of the systems Lichtenberger has investigated there is no other instrument capable of making the measurements. The second approach to developing photoelectron spectroscopy was to build the relationships of the ionization energies themselves to thermodynamic cycles of bond energies, protonation energies, and other reaction processes and physical properties. In one sense, all chemical behavior may be viewed as the movement of electrons. An obvious example is oxidation and reduction processes, but so too is the selective making and breaking of bonds in catalysis, the transport of electrons in molecular wires, and the interactions of molecules with light. Most recently Lichtenberger has bridged the detailed gas-phase energy measures of photoelectron spectroscopy to the energies in the Marcus theory of electron transfer, to the electronic energies in solid-state assemblies such as light-emitting diodes, and to the redox free energies in solution related to the photoelectrocatalytic production of clean and sustainable solar fuels