Charles L. Braun
Professor of Chemistry Emeritus
Professor Braun received his B.S. degree in Chemistry in 1959 from the South Dakota School of Mines and Technology and his Ph.D. in physical chemistry from the University of Minnesota in 1963. He joined the faculty of the Department of Chemistry in 1965 and served as department Chair from 1982 to 1985. His chief research interest for many years has been in the processes which follow the photoionization of a molecule in liquids and solids. In 1987 he received the Distinguished Teaching Award by vote of the Dartmouth Senior class; in 1991 he received the Catalyst Award of the Chemical Manufacturers Association. He became emeritus in 2005.
Research Interests
Professor Braun studies the properties of photoexcited molecules in the liquid and solid phases. Such molecules may fluoresce, react chemically, or transfer an electron to a nearby molecule or into the surrounding medium. Photoinduced electron transfer is of particular interest in that light can thus be used to produce an electrical conductor from an insulating solid or liquid. This may eventually allow the construction of molecule-based solar batteries. Light-induced-conductivity is already the basis for electrophotography (xerography), a copying process of obvious importance.
In photosynthesis and in electrophotography, electron transfer between a photoexcited electron donor and an acceptor is the critical first step in forming high-energy, cation-anion pairs. In a broad class of organic molecular donor-acceptor complexes, photoexcitation in the charge-transfer (CT) absorption band of the complex produces a nearest neighbor cation-anion pair. Such ion pairs rarely dissociate except in highly polar solvents More importantly, distant (1 nm or more) electron donors and acceptors weakly absorb a photon, thereby becoming a partially separated ion pair which can dissociate thermally. Eventually, we hope to mimic nature (photosynthesis) by providing a low energy pathway for high efficiency dissociation of the CT excited states.
We have learned how to use laser-excited photocurrents to measure the change in charge distribution (dipole moment) upon photoexcitation of a molecule. This new technique has proved useful in studying a number of examples of intra- and intermolecular electron transfer. We can watch newly excited dipoles rotate on the subnanosecond time scale. We can measure the extent of charge transfer in excited state complexes. Excitation of CT complexes in solution allows observation of the separtation of anion and cation into free charges. We measure the quantum yield of that separation as a function of solution dielectric constatn and are thus gaining insight into intraionic potentials at small ion separtation. It is fascinating to directly watch ion pair dipoles both decay and separate with our time-resolved photocurrent experiments. We also find that some symmetrical molecules like bianthryl play some involved tricks after photoexcitation as they decide where their dipole moment will point. We hope to apply the techniques we are developing to problems in photosynthesis and in vision. Additional details are on the electron transfer website.