Photoinitiated charge transfer in tailor-made molecules studied with 100 kilohertz two-dimensional white-Light spectroscopy

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In this project funded by the Chemical Structure Dynamics and Mechanism-A (CSDM-A) program of the Chemistry Division, Professor Martin T. Zanni of the University of Wisconsin-Madison is studying charge transfer in tailor-made molecules using a new technique called two-dimensional white-light (2D WL) spectroscopy. Charge transfer processes play a central role in chemistry, such as for delivering electrons to speed up chemical reactions. A detailed understanding of charge transfer is difficult, because one must know the structures of the molecules, how much energy they contain, and how fast the electrons transfer from one place to another. Professor Zanni is addressing this challenge by studying a series of tailor-made molecules with well-defined and systematically varied structures. This series of molecules is being synthesized by the research group of Professor Michael Wasielewski at Northwestern University. Since these molecules absorb light of different colors, Professor Zanni uses white light spectroscopy to study the energy levels of the molecules and the rates of charge transfer. White light is a combination of all other colors of visible light. Absorption spectra are graphic descriptions of how atoms or molecules absorb light of different colors (wavelengths). By exciting the molecules with brief light pulses separated from each other by a few hundred femtoseconds (a femtosecond is one quadrillionth of a second), Professor Zanni is able to generate two-dimensional spectra, which look like topographical maps, and provide clues about how charge and energy are moving within or between molecules. The insights provided by this research may advance our understanding of charge transfer processes in living systems, and may have implications for the design of materials relevant to solar energy technologies. Professor Zanni and his group are involved in various activities focused on the public science education. They give presentations at the Wingra Elementary School Science Night and host an Research Experiences for Undergraduates (REU) high school teacher as part of their research dissemination plan. Professor Zanni and his group have also designed and prepared a televised lecture for Wisconsin Public Television on transitioning technologies from university research into the commercial sphere.A series of custom-made molecules are being studied by the Zanni research group using a 2D WL spectroscopy technique developed by this group. The series of molecules, prepared by the Wasielewski group, is designed to vary the electronic couplings and charge-transfer character in a systematic way. One set of compounds are caged complexes designed to capture and deliver multiple charges for use in multielectron redox reactions. Another set are bridged dimers of terrylene and perylene diimide, held in specific geometries. The bridged dimer systems have systematically controlled electronic couplings relevant to charge transfer in singlet fission. Two dimensional WL spectroscopy uses continuum generation for the pulse sequence, creating spectra that span nearly the entire visible and near-IR region of the spectrum. White-light is much weaker than the output of an optical parametric amplifier (OPA), and so the spectrometer is paired to a 100 kHz laser source and a shot-to-shot pulse shaper. With a broad spectral range and high signal-to-noise, this 2D WL spectrometer is well-suited for studying Professor Wasielewski's compounds. Initial 2D WL spectra contain cross peaks that reveal charge-transfer transitions that were previously unobserved in absorption spectra, uncovering the photophysical pathway for charge transfer in these compounds. The sensitivity of this 100 kHz spectrometer is so high that the final aim of this research is to implement it as a microscopy tool to measure spatially resolved 2D WL spectra. This research may extend our understanding of charge transfer and to develop new optical technologies. The experiments made possible by the combination of these tailor-made molecules and new 2D WL spectroscopy may overcome existing challenges and extend the understanding of charge transfer in chemistry.