Photoinduced Charge-Shifting and Self-Assembly of Photochromic Polyelectrolytes

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There are many ways to control the properties of matter through chemical reactions. Light is an especially unique tool for causing chemical reactions that has many advantages: it can pass through barriers such as glass, travel long distances, and be switched on and off easily. To control matter with light, a particular type of light-sensitive chemical called a photochrome is especially useful. Photochromes are molecules that change structure and color when irradiated with light. Photochromes also changes back to their original form when the color of light is changed or the light is removed. Examples of photochromic matter are glasses that become sunglasses outdoors, but are regular glasses indoors. The research group of Professor Samuel Thomas at Tufts University investigates new forms of photochromic matter: plastics that not only change color, but also dissolve in water when irradiated with light. Because light causes photochromic molecules to switch reversibly between two structures, these polymers can then be separated from water when the light is removed, allowing for "on-demand" control over the water solubility of these plastics. This fundamental research has the potential to benefit society by opening the door to a new class of aqueous nanomaterials that harnesses the power of light to remotely control their properties for eventual utility in encapsulation and release of drugs in biomedical applications, membranes with switchable permeability, and reversible adhesives. Beyond the hands-on training that this research provides to graduate students and postdoctoral scholars, this project also provides college students from a variety of racial and ethnic backgrounds with their first experiences in original scientific research. Professor Thomas continues his successful program of recruiting students from BunkerHill Community College, as well as his partnerships in the Tufts Visiting and Early Research Scholars Experiences (VERSE) program, which recruits summer research students from Historically Black Colleges and Universities. Jointly supported by the Macromolecular, Supramolecular, and Nanochemistry Program of the NSF Division of Chemistry and the Polymers Program of the Division of Materials Research, this project develops a novel approach to control the properties of polyelectrolytes and enable the formation of new, self-assembled nanostructures and coacervates that undergo reversible, photoinduced phase changes as a result of charge shifting. The first phase of this project is to establish how the structures of both spiropyran pendants on polymers and hydrophilic co-monomers influence the key properties of the photochromic units. These properties include the acidity of protonated merocyanines, the quantum yields of merocyanine ring closures, and the rates of spiropyran openings. Measurements of photoinduced changes in solution pH and zeta potentials serve to correlate changes in optical properties to changes in polyelectrolyte charge. The second phase of this project harnesses these systematically-derived structure-property relationships to develop reversible, photoinduced formation of polyelectrolyte nanostructures and coacervates through the corresponding changes in polymer charge at near-neutral pH. Further extension of these principles to photochromic block copolymers is aimed at reversible self-complexation and self-coacervation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.