Design of Photo-Responsive Molecules and Study of Loops in Dynamic Covalent Networks

Wang, Xiaodi

doi:https://doi.org/10.21985/n2-8yby-8392

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Design of Photo-Responsive Molecules and Study of Loops in Dynamic Covalent Networks

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Part I: Design of Photo-Responsive Molecules towards Biomedical ApplicationsThe use of light to control systems provides numerous advantages such as spatiotemporal precision, non-invasive penetration, and precise energy input. Specifically, molecules that undergo photoinduced cleavage, photoremovable protecting groups (PPGs) have emerged as an active area of research due to their broad potential in applications ranging from drug delivery to material sciences. While the application of PPGs in chemical and biological systems is now abundant, most require initiation in the UV range. Therefore, poor penetration depth and cytotoxicity are common challenges in the field. Consequently, the development of long-wavelength PPGs is critical to expand the applicability of these molecules in biological settings. In this project, I designed and synthesized benzoquinone-based photoremovable groups with high aqueous efficiency. These PPGs exhibit controlled release of substrate upon irradiation at 626 nm. The best-performing PPG demonstrated a quantum yield of 5.7% in 30:70 v/v water/acetonitrile and approximately 80% release of benzoic acid using 626 nm LEDs. Additionally, the design enabled photo-orthogonality between this PPG and a UV-activated photouncaging group, providing high selectivity for activation depending on the light source used. Overall, this project provides insights on the development of molecules that can be activated by longer-wavelength light and their potential applications via multicolor release. Part II: Modulating Loops in Dynamic Covalent NetworksWhen multifunctional molecules are coupled, at sufficiently high conversion, infinite networks are formed. In real networks, some of these couplings will occur intramolecularly, forming topological defects known as loops. Because these intramolecular linkages are chemically identical to intermolecular ones, loops are under-explored compared to their counterparts such as dangling ends and entanglements. Recently, the Johnson and Olsen groups at MIT pioneered techniques to quantify loop content in networks, showing how synthetic conditions can be used to manipulate loops and their effect on network properties. Their techniques have been applied to networks based on permanent covalent bonds. Motivated by the recent advance in the field of loops, I designed and worked on two projects. In the first project, I sought to modulate polymer properties, such as their mechanical behavior, using a bottom-up approach based on molecular photoswitches. Polymer networks that respond to external stimuli enable on-demand tunability. Developments in the ability to understand the polymer physics of loops have provided new avenues to control network topology through molecular design, which can in turn translate into the network’s bulk properties. For example, a recent report by Johnson and coworkers demonstrated photocontrol of topological states in a polymer network via conformational changes in metal-organic cages. To complement this supramolecular approach, we designed an alternative method based on reversibly photocontrolling topology by incorporating an indigo photoswitch into dynamic covalent networks. We envisioned that conformational changes via photoswitch isomerization will enable reversible topological changes, which can result in reversibly controlled mechanics. An indigo photoswitch was designed and synthesized to accomplish the above-mentioned goals. Upon completion of its synthesis, analysis of its photophysical properties revealed that rapid relaxation occurs when the photoswitch is exposed to the aliphatic amine that participate in dynamic covalent bonds. NMR studies were performed to further elucidate the observed phenomenon. Alternative design of the photoswitch is, thus, required to accomplish the original goals. In the second project, I aimed to investigate the fundamental effect of loop fraction on the mechanical properties of covalent adaptable networks. Vitrimers are an emerging class of covalent adaptable networks that utilize associative exchange reactions. While these systems behave like thermosets at lower temperatures, the dynamic nature of the crosslinks enables reprocessing and repair without loss of network integrity at higher temperatures. The conditions at which recycling occurs can be greatly affected by a parameter called stress relaxation, which describes the rate at which the material dissipates applied stress. Theoretical work has suggested topological defects such as loops are important for the stress relaxation process. However, the quantitative relationship between loop fraction and bulk mechanical properties in vitrimers has yet to be explored experimentally. Recently, it was observed in our laboratory that despite similar molecular weight and monomer composition, statistical and block copolymer-derived acrylic vitrimers demonstrate drastically different mechanical properties. We hypothesized that property differences can be attributed to a higher loop fraction in the block vitrimers due to the close proximity of crosslinking functional groups. Taking advantage of a technique called network disassembly spectrometry (NDS) recently developed by the Johnson and Olsen groups, I have designed, and synthesized monomers that could accommodate the NDS technique. Once the monomer synthesis is completed, respective block and statistical pre-polymers will be synthesized and cross-linked to undergo network formation. Further analysis using NDS will be then be performed on the networks in collaboration with the Johnson lab. This study provides the first attempt to experimentally study the influence of network defects on the stress relaxation behavior of vitrimers.

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