Research in our lab focuses on the development of new chemical tools for biomedical applications, as well as the synthesis of dynamically responsive materials. Philosophically, we take the view that many problems in biology and materials science are at heart problems in mechanistic physical organic chemistry. Using a joint theoretical/experimental approach, we design and synthesize new materials that have applications in biomedicine and materials science.
Photocages are light-sensitive chemical protecting groups that mask a substrate through a covalent linkage that renders the substrate inert. Upon irradiation, light is absorbed by the photocage and the substrate is released, restoring the substrate’s biological activity. These chemical tools are extremely valuable in biological settings because they allow investigators to control when, where, and how much of a bioactive substrate is released using targeted pulses of focused light. Released cargos can include drugs, dyes, ions, signaling agents, neurotransmitters, nucleotides, or bioactive small molecules.
A drawback of most known photocages is that they absorb ultraviolet light. Irradiation with UV light is a problem for biological studies because UV light is phototoxic and does not penetrate far into tissue. The problem we are working on is developing chemical protecting groups that can release bioactive molecules using visible light, particularly with light wavelengths in the biological window where light penetration into tissue is maximal to allow photorelease in living systems (see graph above). Additionally, visible light is not phototoxic like UV light.
Air Stable Radicals for Stimuli Responsive Materials
Interest in the synthesis of stimuli-responsive soft materials has led to a search for new strategies for achieving large property changes in organic materials in response to mild external cues. Our approach to this problem is to achieve stimuli-responsive changes in organic material properties by switching the spin state of stable organic radical-derived materials. Such spin crossover materials that change spin configuration upon a stimulus (e.g. heating) are established for inorganic molecules and transition metal complexes, but are essentially unprecedented for organic structures. Inorganic spin crossover materials find applications in thermochromic paints, sensors, displays, and mechanical actuators, and exploit the large changes in properties that occur upon switching spin states of a metal.
Our key innovation has been to adapt the idea of spin crossover that has thus far been restricted to inorganic materials and bring this idea to organic materials. Specifically, we have identified organic free radicals based on viologen cation radicals that can switch between diamagnetic spin-paired forms and paramagnetic spin-unpaired forms upon different stimuli (e.g. temperature change, non-covalent binding, etc), leading to large changes in optical and magnetic properties. The key to our approach is to covalently link two stable organic radicals that form a weak pi bond (~2 kcal/mol) that leads to spin pairing of the two radicals. A stimulus that disrupts this very weak bonding interaction between the two radicals then leads to a spin-unpaired paramagnetic species, with large accompanying changes in optical and magnetic properties. These novel materials will lead to new classes of stimuli-responsive soft polymers and turn-on MRI contrast reagents that provide contrast upon activation by a biomolecule or other stimulus (pH, enzymatic activity, binding event, etc).