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Author ORCID Identifier

https://orcid.org/0000-0001-8453-3020

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Chemistry

Year Degree Awarded

2019

Month Degree Awarded

September

First Advisor

Micheal D. Barnes

Subject Categories

Materials Chemistry | Physical Chemistry

Abstract

My thesis focused on understanding the structural changes producing different spectral signatures seen in aggregates of 7,8,15,16- tetrazaterrylene (TAT). Recent work from our group showed crystallographically selective directional charge-separation within isolated extended TAT crystals without the need of an interface. Aggregates of different size not only exhibited different exciton recombination kinetics, but different spectral signatures. The motivation for understanding the change in the structural properties producing the unique spectral signatures is elucidating the mechanism of this directional charge-separation, intrinsic or extrinsic. In this case, an intrinsic mechanism means it is caused by molecular design and packing, and extrinsic mechanism means it is caused by grain boundaries or faults in stacking during the crystallization process. In pursuit of this question, two fundamental questions were investigated (1) What are the different stages of TAT crystal growth, and what is the dominant coupling at each one? (2) What is the underlying crystal structure of the small and large aggregates? (1) Investigation of different stages of TAT crystal growth and spectral signatures: Our initial hypothesis was TAT formed pristine nanoscale structures that had the same dominant intermolecular coupling. We based our theory on TAT's molecular structure lacking substituents that could alter the molecular packing during solution-phase self-assembly. Our hypothesis was tested by sampling assemblies of different size aggregates and comparing their photoluminescence images and spectral signatures. Similar spectral signatures would indicate similar molecular packing and final exciton recombination pathway supporting an intrinsic mechanism. Different spectral signatures would suggest a different molecular packing and exciton recombination pathway. We found TAT could be isolated into assemblies of three different sizes and spectral signatures: small-clusters (>250 nm, J-type), large-clusters (500 nm, HJ- type), and extended-crystals (microns, H-type). The dominant coupling was assigned based on a comparison of the 00/01 peak intensity ratio from the spectral signatures of the isolated monomer and assemblies. The spectrally resolved images of the large clusters showed the spectral signatures varied horizontally through the crystal, ranging from a peak intensity ratio of 1.4 to 0.5. The peak intensity ratio of 1.4 is similar to the single-molecule (1.43), and 0.5 is similar to the extended crystal. The transition in the observed spectral signatures from J > monomer < H as a result of transitioning from a small-cluster- large-cluster – extended-crystal indicated a naturally occurring exciton band inversion upon the assembly process. Typically, an exciton band inversion is caused by the manipulation of side chains to alter the molecular packing of the structure. Although the structural reason behind this naturally occurring exciton band inversion was unknown, this observation provided the opportunity during self-assembly to select for specific optical properties such as exciton recombination dynamics. (2) The underlying crystal structure for the small and large aggregates: To elucidate the structure producing the change in the spectral signatures, the aggregates growth process needed to be controlled to ensure the spectral signatures and photophysical properties could be correlated. Solvent vapor annealing was used to control the aggregation process. This method is widely used to induce aggregation by plasticizing the polymer matrix allowing for aggregation of the analyte to occur. Based on the concentrations used in the above study, we found we can isolate films of small-and large-cluster and extended-crystals. These aggregates have similar spectral signatures as that of the solution-grown aggregates but have higher polarization contrast parameter, M. The three-dimensional structure was probed using polarization anisotropy, defocused imaging, and TEM. The defocused imaging and polarization anisotropy showed the small-clusters were highly aligned linear dipoles. DFT calculations show cofacial dimer geometries that would have linear dipoles could range from a slip of 0.5-1.5 along the chromophore axis. TEM of the small-clusters and solution-grown crystals showed the same unit cell but grown along different crystallographic axis [011] vs. [010]. These findings suggest TAT initially forms in one crystallographic direction driven by N-H bonding and then along p-stacking direction when the -interactions become more dominant. Our hypothesis is the preservation of the unit cell at different stages of growth shows the J- to H-transition in TAT has little to do with the Coulombic coupling but is dependent on the charge-transfer interaction. Our findings also show for an unfunctionalized molecule, solvent vapor annealing is controlling more than just order of the chromophores, but the interference between the neighboring cofacial molecular orbitals. Our goal was to create design principles based on the effect molecular architecture has on chromophore coupling and resulting spectral signatures in TAT aggregates and nanowires. These paradigms would be applied for the advancement of using semiconductor nanowires as a route for directional control over energy and/or charge-transport. We probed isolated aggregates of TAT using photoluminescence spectroscopy and microscopy to understand the different interchromophore interactions present at different stages of growth. TEM, defocused imaging, and polarization anisotropy probed chromophore and crystalline structure within the aggregates. From my results three key findings can be made (1) exciton band inversion does not always need to be controlled by side chains (2) solvent vapor annealing can be used to control aggregate size and indirectly interchromophore interactions (3) Charge transfer interactions can experimentally be observed to have a profound impact on the spectral signatures of HJ aggregates. Ultimately, our vision is that these principles will be used to design new molecular systems that can be engineered to undergo singlet fission and incorporated into polarization control optical properties.

Available for download on Tuesday, September 01, 2020

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