Date of Degree
Adam B. Braunschweig
Organic semiconductors have received substantial attention as active components in optoelectronic devices because of their processability and customizable electronic properties. Tailoring the organic active layer in these devices to exhibit desirable optoelectronic properties requires understanding the complex and often subtle structure-property relationships governing their photophysical response to light. Both structural organization and frontier molecular orbitals (FMO) play pivotal roles in energy relaxation processes, and complex interplay between organization and orbital energies are difficult to anticipate based upon the molecular structure of the components alone, especially in systems comprised of multiple components. In pursuit of design rules, there is a need to explore multicomponent systems combinatorially to access larger data sets, which can be facilitated when error correcting, noncovalent assembly is employed to achieve long-range order. Another challenge that should be addressed to derive structure-activity relationships is the need to determine the relative organization of different components within active layers with molecular-scale precision. This thesis will first review the use of supramolecular v chemistry to study combinatorial, hierarchical organic systems with emergent optoelectronic properties (Chapter 1). Specifically, previously reported systems that undergo deactivation by charge transfer (CT), singlet fission (SF), and Förster resonance energy transfer (FRET) following supramolecular assembly will be described. In doing so, we show the value of adopting combinatorial, supramolecular assembly to study emergent photophysics promises, which can rapidly accelerate progress in this important research field. In Chapter 2, it will be shown how two diketopyrrolopyrroles (DPP) and three rylenes (NDI, dPyr PDI, and dEO PDI) were combined to form six hierarchical superstructures that assemble as a result of orthogonal H-bonding and π•••π stacking (Chapter 2). The individual components and the DPP—NDI as well as DPP—PDI pairs were cast into films, and their superstructures were interrogated by electron microscopy and advanced spectroscopy. All six superstructures feature different mesoscale geometries as a result of subtle changes in the solid-state packing of the DPPs. Changes in DPP stacking, occurring because of interactions with adjacent rylenes, impact the excited state dynamics and SF. These superstructures afford triplet quantum yields as high as 65% for a correlated pair of triplets and 15% for an uncorrelated pair of triplets. Our studies demonstrate the benefits of combinatorial supramolecular assembly for exploring the impact of structure on advanced light management in the form of SF. An ongoing challenge in the use of devices containing organic semiconductors is determining their film structures. To address this challenge, microcrystal electron diffraction (MicroED) was used to determine structures of three organic semiconductors and show that these structures can be used along with grazing-incidence wide-angle X-ray scattering (GIWAXS) to understand crystal packing and orientation in thin films (Chapter 3). Together these complimentary techniques provide unique structural insights into organic semiconductor thin films, a class of vi materials whose device properties and electronic behavior are sensitively dependent on solid-state order. MicroED, GIWAXS, and UV-Vis spectroscopy were used to determine the unit cell structure and the relative composition of dimethylated diketopyrrolopyrrole (MeDPP) H- and J-polymorphs within thin films subjected to vapor solvent annealing (VSA) (Chapter 4). Electronic structure and excited state deactivation pathways of the different polymorphs were examined by transient absorption spectroscopy, conductive probe atomic force microscopy, and molecular modeling. We find VSA initially converts amorphous films into mixtures of H- and J-polymorphs and promotes further con-version from H to J with longer VSA times. Though both polymorphs exhibit efficient SF to form coupled triplets, free triplet yields are higher in J-polymorph films compared to mixed films because coupling in J-aggregates is lower, and, in turn, more favor-able for triplet decoupling. The work described herein offers guidance for the supramolecular and photophysics communities by providing experimental strategies and design principles for creating systems containing organic semiconductors and that display emergent optoelectronic properties. Specifically, the examples here provide methods and techniques for designing molecules with functionality that simultaneously tailors FMO and programmed molecular packing, while also describing appropriate supramolecular and optoelectronic characterization methods. We learn from these studies the subtle but profound impact of changing aggregate structure on SF lifetimes and yields, and this understanding can be applied towards energy harvesting, sensing, or photocatalytic applications.
Levine, Andrew, "Emergent Photophysics In Diketopyrrolopyrrole Superstructures" (2021). CUNY Academic Works.