ChEMS Seminar: Light-Driven Processes in Self-ordering Molecular Materials

Mike Wasielewski
McDonnell Douglas Engineering Auditorium (MDEA)
Michael R. Wasielewski
Department of Chemistry and Institute for Sustainability and Energy
Northwestern University
Evanston, Illinois 
Abstract: It is well-known that self-assembly of small electron donor-acceptor (D-A) molecules into discrete and monodisperse nanostructures provides geometrically defined platforms to emulate the photo-induced electron transfer processes in photosynthetic systems. Organization of molecules by this thermodynamically-driven method can result in architectures with unique inter-chromophore relationships that are otherwise difficult to realize by conventional covalent synthesis. In particular, p-stacked D-A dyads and triads can afford ordered and segregated D/A domains through which photo-generated holes and electrons can be further separated and rapidly transported to electrodes in photovoltaics or to catalysts for solar fuels formation. The design of these self-ordering molecular assemblies must ensure that charge hopping of the separated holes and electrons between the non-covalent donors and acceptors within their respective segregated conduits must be significantly faster than charge recombination (Figure 1). We will present results demonstrating that photo-induced charge separation can take place within covalent chromophoric redox partners followed by transport of photo-generated holes and electrons independently through well-ordered, segregated molecular charge conduits.

We are also developing new materials that undergo singlet fission (SF), the spontaneous down-conversion of a singlet exciton to two triplet excitons (Figure 2), using guidance from electronic structure calculations to assure the requisite relationships between molecular singlet and triplet exciton energies. We are preparing hierarchical assemblies from these chromophores, starting from covalent dimers and trimers, then developing supramolecular assemblies, and engineered crystalline materials to investigate SF in bulk, ordered materials. We are using femtosecond transient optical spectroscopy to characterize the SF mechanism and the factors that determine its efficiency. We will present new results on the role of charge transfer states in enabling SF in molecular materials. 

Biography: Professor Wasielewski received his Ph.D. in 1975 from the University of Chicago. Following his graduate work, he was a postdoctoral fellow at Columbia University. He then joined the Argonne National Laboratory, eventually becoming Group Leader of the Molecular Photonics Group. In 1994, he joined the faculty of Northwestern University, where he is currently the Clare Hamilton Hall Professor of Chemistry.  He served as chair of the Department of Chemistry at Northwestern from 2001-2004.  He is currently Executive Director of the Institute for Sustainability and Energy at Northwestern (ISEN), Director of the Argonne-Northwestern Solar Energy Research (ANSER) Center, which is a US-DOE Energy Frontier Research Center, and the Solar Fuels Institute, a global consortium of research centers. Professor Wasielewski's research focuses on light-driven charge generation and transport in molecules and supramolecular materials, artificial photosynthesis, molecular systems for solar fuels and electricity, molecular electronics, spin dynamics, spintronics, and time-resolved optical and EPR spectroscopy. His research has resulted in over 500 publications. Professor Wasielewski’s recent awards include the 2016 American Institute of Chemists Chemical Pioneer Award, the 2013 Royal Society of Chemistry Environment Prize, a 2013 Humboldt Research Award, the 2012 Arthur C. Cope Scholar Award of the American Chemical Society, the 2008 Porter Medal for Photochemistry, the 2006 James Flack Norris Award in Physical Organic Chemistry of the American Chemical Society, and the 2004 Photochemistry Research Award of the Inter-American Photochemical Society.