Clapham, Margaret2023-11-282023-11-282023-05https://hdl.handle.net/11299/258717University of Minnesota Ph.D. dissertation. May 2023. Major: Chemistry. Advisors: Renee Frontiera, Christopher Douglas. 1 computer file (PDF); xxiii, 187 pages.Rubrene (5,6,11,12-tetraphenyltetracene) is an organic semiconducting material with extremely high charge mobility, making it useful for organic devices from solar cells to OLEDs. The electronic properties of rubrene heavily rely on the specific packing of molecules within the crystal lattice. Three different crystalline forms, or polymorphs, of rubrene exist with one of the forms displaying a charge mobility an order of magnitude larger than the others. Polymorphism, however, is vastly unpredictable. My thesis is focused on understanding polymorphism in rubrene and derivatives. By trying to gain a deeper understanding of the interactions that direct polymorphism, I have also gained insight into how these interactions influence the superior properties of the ideal rubrene crystal structure. To fully understand polymorphism, we must be able to characterize it. Current standards to assess crystal structure typically involve inefficient or destructive methods. My early work focused on using Raman microscopy to non-destructively quantify a polymorph ratio on a large scale. Raman spectroscopy can measure low-frequency vibrations, or phonons, which are indicative of different crystalline arrangements. The integration of microscopy further allows for efficient, non-destructive polymorph assignment based on these low-frequency vibrations. I further showed the usefulness of polymorph characterization with Raman microscopy through crystal engineering of mixed rubrenes. While previous synthetic and theoretical studies predicted which interactions may be the most crucial in dictating crystal structure, we lacked strong experimental evidence of the importance of these interactions. I selected a series of rubrenes known to form non-ideal structures individually, and found when crystallized together, they had a similar pattern of phonons as seen in ideal rubrene derivatives. Further analysis revealed these mixed crystals formed the ideal crystal arrangement. The advent of mixed crystallization provided strong experimental evidence for specific intermolecular interactions dictating the ideal crystal structure. I later engineered a new series of rubrene derivatives, all sharing similar crystal structures and phonon spectra. To determine the effects these phonons have on excited state properties, I probed each of these phonons individually and monitored their excited state dynamics using dual-excitation femtosecond stimulated Raman spectroscopy. These studies revealed that specific amplification of one of the phonons leads to suppression of excited state dynamics. This provides some of the first experimental evidence in rubrene derivatives of the detrimental effects an individual phonon can have. Together, these studies of synthesis, crystal engineering, and Raman spectroscopy demonstrate how interdisciplinary approaches to the study of intermolecular interactions in organic semiconducting materials can lead to significant advances in the understanding of material structure-property relationships.enThesis or Dissertation