2025 Theses Doctoral

Visible-Light-Activated Photopolymerization and Dynamic Covalent Chemistry for Adaptive Materials

Wei, Shixuan

Photochemistry represents one of the most powerful tools for controlling chemical reactions in materials science, offering unparalleled advantages including remote activation, precise spatiotemporal control, and operation under ambient conditions. Among various photochemical processes, photopolymerization enables the fabrication of macromolecules from monomers, providing toolboxes to engineer materials with vastly different properties from their small-molecule precursors. Photoresponsive dynamic covalent chemistry, on the other hand, provides pathways to create adaptive materials that can reversibly reconfigure their molecular architecture in response to optical stimuli.

These light-driven processes have revolutionized fields ranging from 3D printing to the development of smart materials capable of self-healing and shape-changing in response to environmental changes. However, traditional photoresponsive systems rely predominantly on high-energy ultraviolet (UV) light activation, which presents fundamental limitations including poor light penetration, photodamage, and restricted applicability in opaque materials. This dissertation addresses these challenges by systematically developing visible-light-activated photopolymerization and dynamic covalent chemistry systems for adaptive materials with enhanced control and functionality.

The first approach involves molecular engineering of dynamic covalent bonds to shift their photoactivation from UV to visible light. Carbazole-based thiuram disulfides were designed with extended conjugated systems that significantly enhance visible light absorptivity and reactivity compared to conventional alkyl thiuram disulfides. These engineered dynamic linkages enable fast photoinduced reshuffling and demonstrate controlled photopolymerization of alkyl acrylates as iniferters (chemicals that simultaneously act as initiator, transfer agent, and terminator), providing both dynamic exchange and polymerization capabilities based on a single dynamic covalent bond.

Taking advantage of these dynamic covalent bonds, we successfully developed a visible-light-induced living polymer network by incorporating these dynamic linkages into polymer backbones. This unique network showcases notable photoresponsive characteristics that facilitate fast reconfiguration, enabling photoinduced shape-shifting and multiple rounds of healing with retained mechanical properties. Additionally, utilizing the dynamic linkages in the backbone as visible-light iniferters, the polymer network can undergo chain growth, providing a robust platform for on-demand modulation of mechanical properties. Notably, due to visible-light activation and the radical exchange nature of thiuram disulfide reshuffling, photochemical control was achieved even in opaque materials.

Beyond successful modification of the singlet excited state, we discovered that extended conjugation in thiuram disulfides also significantly lowers their triplet energy levels, enabling efficient triplet energy transfer from appropriate photosensitizers. This insight led to the development of triplet-sensitized dynamic covalent chemistry using red light for activation under ambient conditions. The approach achieves fast reshuffling of thiuram disulfides with minimal sensitizer loading under red light irradiation at modest intensities without deoxygenation, with reaction rates comparable to direct blue light excitation. Most significantly, due to selective sensitization of carbazole thiuram disulfides, orthogonal equilibrium control was achieved for reshuffling between alkyl and carbazole thiuram disulfides. Thereby different photostationary states can be accessed through wavelength-selective activation, providing unprecedented control over covalent bond distributions.

Finally, triplet-triplet annihilation upconversion was employed as a general strategy to extend traditional blue-light-activated photochemistry to red light activation. This method converts low-energy red photons into higher-energy blue light to initiate free radical polymerizations within opaque objects, enabling photocuring of composite materials that cannot be processed with conventional UV-based approaches.

These advances collectively expand the accessible spectral window for photoresponsive materials from UV to red light, providing superior penetration depth, reduced photodamage, and enhanced temporal control. The resulting materials establish fundamental principles for next-generation photoresponsive materials with diverse applications including reconfiguration of polymer networks, precise control of mechanical properties, and fabrication of advanced composite materials, opening new possibilities for applications requiring deep light penetration.