Supramolecular chemistry concerns the manner in which molecular building blocks associate via non-covalent interactions and form aggregates. The particular building block in this research is a photoresponsive molecule 4,4’-azobenzene dicarboxylic acid (ADA), a molecule that isomerizes reversibly around an N-N double bond upon irradiation with different wavelengths of light. The large structural changes in the molecule that result from isomerization have the potential to modulate the properties of a supramolecular aggregate. ADA was studied under a variety of environmental conditions for the purpose of understanding aggregation behavior and geometries. A UV-Vis and Circular Dichroism (CD) spectroscopic study of ADA showed that it is soluble only at very basic pH (above 10.8) and that a decrease in pH gives rise to the formation of an aggregate. Further studies of the self-aggregation process at low pH showed that this process is under hierarchical control. Four procedures for the formation of homo- or hetero-aggregates of ADA were characterized. Each gave rise to the formation of a different structure showing the importance of the pathway undertaken during the aggregation process. CD spectra of the ADA aggregates showed that, typically, they have a preferential asymmetric geometry despite the symmetry of the constituent molecules. Moreover, the use of a chiral template macromolecule (poly-glutamate) during the formation of the aggregate can affect the structure of the supramolecular species and direct its chirality. The study of the photoisomerization of ADA in the aggregated form showed that the highly packed aggregates of the trans isomer were unable to photoisomerize to the cis isomer upon UV (365 nm) irradiation. On the contrary the aggregates containing the cis isomer of ADA were able to retain their ability to photoisomerize to the aggregates of the trans form.
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Chemistry | Physical Sciences and Mathematics
Fitz, Julie A., "Design and Characterization of Photoresponsive Supramolecular Aggregates" (2014). Honors Theses. 24.