Electronic Coupling and Polymorphic Heterogeneity in Singlet Fission Microcrystals Studied with 2D White-Light Microscopy

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

In this project, funded by the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program of the Division of Chemistry, Professor Martin Zanni’s laboratory at the University of Wisconsin explores a novel property of some materials that can convert light into electricity. In these materials, when one photon, or unit of light, hits the material, multiple electrons can carry away its electrical energy, a process known as singlet fission, whereas in most materials only one electron is excited by one photon. Systems undergoing singlet fission can benefit the design of photovoltaic devices like solar cells and photodetectors. Although singlet fission has been observed in crystals, the detailed mechanism by which it occurs is not established yet. Theoretical and experimental studies show that the arrangement of molecules in a crystal plays an important role in determining electronic and charge transfer couplings that drive singlet fission. Hence, different crystal polymorphs (i.e. different crystal packing/arrangement of the same molecule), often exhibit different singlet fission efficiencies. By studying two model systems, this project is providing better understanding of how molecular structure, its packing into polymorphs, and its charge generation are related. These experiments are contributing to a better understanding of how singlet fission might be utilized for more efficient charge generation. Professor Zanni and his students also continue their involvement in educational activities, including mentoring undergraduate research students, participating in a program to provide peer-mentoring and other support for minority, low-income and first-generation graduate students, and presenting a science show and chemistry worksheet activities to local elementary schools. Professor Zanni and his students are studying singlet fission in two molecular systems, triisopropylsilylethynyl (TIPS)-pentacene and rubrene, with the specific goal of identifying and understanding the impact of non-equilibrium structures on singlet fission. TIPS-pentacene is a solution processable organic semiconductor capable of undergoing rapid and efficient singlet fission. In previous NSF-funded work, Professor Zanni’s group revealed that TIPS-pentacene adopts different packing structure depending on the film formation procedure. Even small changes to the crystal structure alter the interactions between two neighboring molecules, changing the electronic couplings of the singlet, triplet, and charge transfer states. It was found that TIPS-pentacene microcrystals contain small populations of slip-stacked geometries that improve triplet state generation and also enable equilibration between the singlet and triplet states. In this project, Professor Zanni and his group aim to extend the understanding of how spatial variation in electronic couplings through defects and polymorphs impact singlet fission and other energy transfer processes. The slip-stacked structures are being studied by varying growth conditions, annealing, imaging crystal growth, and other experiments. Rubrene is another material that undergoes singlet fission and crystallizes into several different polymorphs. Singlet fission kinetics are being studied in each polymorph, in detail, to understand the relation between molecular geometry and charge transfer dynamics. State of the art two dimensional white light (2D WL) spectroscopy and broadband transient absorption microscopy (TAM) are used for the studies. A second-generation 2D WL microscope is being built with pulses centered in the blue part of the spectrum. The spectral range of this new instrument allows for study of rubrene and many other important materials which absorb at shorter wavelengths than TIPS-pentacene. This microscope is expected to enable the imaging of exciton diffusion by independently scanning the position of the pump and probe pulses. Crystal packing and spatial heterogeneity of electronic couplings can strongly impact exciton and charge transport processes. In addition, Professor Zanni and his group assist members of the scientific community and industry in collecting and simulating 2D IR and 2D WL data, as well as participate in the educational and outreach activities discussed above. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.