Date of Award

Spring 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Engineering and Applied Science

First Advisor

Van Tassel, Paul

Abstract

This thesis focuses on the development of a single-molecule spectroscopy technique to study the mechanical and photophysical properties of conjugated polymers in a dual-trap optical tweezers system. It involves the design, synthesis, and implementation of a novel optical tweezers platform and the preparation of tailor-made conjugated polymers. Novel SiO2-coated zinc oxide microspheres (ZnO@SiO2) with tunable size were developed as materials for dual-trap optical tweezers in organic solvents. The microspheres can be used as stable traps in both aqueous and various organic solvents ranging in polarity and refractive index. Specifically, we demonstrated the use of the microspheres to study the single-molecule behavior of DNA in aqueous solution and poly(methyl-methacrylate) (PMMA) in organic solvents. Poly(3-hexyl thiophene) is one of the most widely studied conjugated polymers because it can be readily used in opto-electronic devices, such as organic solar cells and light-emitting diodes. We aimed to develop methods to utilize our dual-trap optical tweezers system to study the photophysical properties of poly(3-hexyl thiophene) with well-controlled conformations under piconewton force. Two different tailor-made polythiophenes were prepared. Hetero-functionalized poly(3-hexyl thiophene) was synthesized to study the mechanical properties of poly(3-hexyl thiophene) in an aqueous solution. In this case, no highly-ordered conformation was observed in P3HT in force spectra, which implies that P3HT was amorphous. A polythiophene with an amphiphilic side chain and azide groups was then synthesized to facilitate single-molecule fluorescence measurements. The ensemble properties (aggregation behavior) of discrete oligo(3-hexyl thiophene) (Đ = 1.0) was studied by UV-vis absorption spectroscopy as a supplement to our proposed single-molecule experiments. These experiments provide a basis for understanding the nucleation process of P3HT in thin-film devices. The UV-vis spectra of a series of oligo(3-hexyl thiophenes) were collected in different binary solvent mixtures. The isosbestic point in UV-vis spectra demonstrated the co-existence of two conformational states (planarized, aggregated state and coiled state) in the presence of a “poor” solvent, such as methanol. The fraction of aggregation analysis illustrates that the nucleation process of oligo(3-hexyl thiophene) is sensitive to molecular weight even at one repeat unit level. The invention of core-shell beads and molecular designs of P3HT make the single-molecule force and fluorescence measurements of P3HT possible in both aqueous and non-aqueous solution. Understanding photophysical properties of P3HT in single-molecule level with high precision could potentially improve the performance of opto-electronic devices.

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