Dartmouth Events

Bio: Prof. Dr. Hermann A. Wegner studied chemistry in Göttingen, Boston and Stanford. He received his PhD under supervision of Prof. Dr. Armin de Meijere before pursuing postdoctoral studies in the group of Sir Prof. Dr. Jack Baldwin at Oxford, where he was also appointed as stipendiary lecturer at Merton College. He started his independent career at Basel University and moved in 2013 to his current position as Professor of Organic Chemistry at the Justus-Liebig-University, Giessen, where he is currently the Executive Director of the Institute of Organic Chemistry and member of the board of directors of the Center for Materials. His research interests are in the area of physical organic chemistry for molecular materials, ranging from bidentate Lewis acid catalysis, novel boron-nitrogen materials, molecular switches, unusual aromatics and their supramolecular chemistry, organic on-surface synthesis and energy management with organic molecules.

Abstract: Efficient photosynthetic energy conversion requires quantitative, light-driven formation of high-energy, charge-separated states.  However, energies of high-lying excited states are rarely extracted, in part because the congested density of states in the excited-state manifold leads to rapid deactivation.  Conventional photosystem designs promote electron transfer (ET) by polarizing excited donor electron density toward the acceptor (“one-way” ET), a form of positive design.  Curiously, negative design strategies that explicitly avoid unwanted side reactions have been under-explored.  This talk describes how the electronic polarization of a highly conjugated molecular chromophore – a supermolecule – can be used as both a positive and negative design element in a light-driven reaction.  Intriguingly, appropriate engineering of polarized excited states can steer a “U-turn” electron transfer—where the excited electron density of the donor is initially pushed away from the acceptor—to outcompete a conventional one-way ET scheme.  Direct comparison of one-way vs. U-turn ET strategies is achieved in a linked donor-acceptor (DA) assembly in which selective optical excitation produces donor excited states polarized either toward or away from the acceptor.  Ultrafast spectroscopy of DA pinpoints the importance of realizing donor singlet and triplet excited states that have opposite electronic polarizations to shut down intersystem crossing.  These results demonstrate that oppositely polarized electronically excited states can be employed to steer photo-excited states toward useful, high-energy products by routing these excited states away from states that are photosynthetic dead-ends.

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