Modulation of chemical reactivity using photoresponsive dithienylethenes

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(Thesis) Ph.D.
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The development of molecular systems that show reversible changes in their structures and functions is an active area of research as it has potential applications in molecular devices. One of the most promising photoresponsive frameworks is found in the 1,2-dithienylethenes (DTEs) because they can be toggled between two thermally stable and structurally unique forms when irradiated with the appropriate wavelengths of light and provide a practical means to regulate the chemical and physical properties of molecular devices. However, there are only a few examples that take advantage of these reversible structural and electronic differences between the two forms of DTE derivatives to influence chemical reactivity. The DTE backbone is well-suited to modulate chemical reactivity since the electronic differences between the ring-open and the ring-closed isomers can be used as the “on” and “off” functions to start and stop reactions using light as the external stimulus and has potential applications in the design of light-activated reagents, catalysts, and biochemical reagents. The research presented in this thesis focuses on integrating photoswitching and nucleophilicity using a pyridine-functionalized DTE to demonstrate the concept of photomodulating reactivity and catalysis. The first approach uses axial coordination to a metalloporphyrin as a probe to assess the differences in Lewis basicity between the two DTEs. The results show that the ring-open isomer is more effective as a ligand than the ring-closed form. This difference in coordination ability indicates that the pyridine is less Lewis basic in the ring-closed form of the DTE system. The second approach consists of evaluating this concept in chemical synthesis using pseudo-first-order kinetics to probe the differences in the apparent rates of alkylation between the two DTE isomers. The results show that the ring-open form reacts faster than the ring-closed form indicating that the pyridine is less nucleophilic in the ring-closed form of the DTE system. The application of the DTE system as a potential nucleophilic catalyst is also investigated and the results show that the ring-open isomer gives a higher yield of product than the ring-closed isomer indicating that the ring-open isomer is acting as the more efficient catalyst for the reaction.
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