Synthesis and Mechanisms of Light Harvesting Photoreagents

Porphyrins and their analogues demonstrate great applications in various scientific fields such as catalysis, supramolecular chemistry, biomimetic models for photosynthesis, and medical applications such as O2-independent phototherapeutic agents for photodynamic therapy. By incorporating intensely absorbing functionalities into reactive molecules, reactive species can be locally generated in vivo in order to target specific locations within the body. The discovery of photochemical Bergman cyclization as a novel approach to drive enediyne reactivity in a controlled manner prompted us to design enediyne molecules with strongly absorbing chromophores and low thermal barriers to cyclization. Vicinal alkyne substitution at the periphery of these porphyrins modulates their already unusual electronic structure and makes them susceptible to Bergman cyclization in the presence of heat or light to generate a novel class of highly conjugated picenoporphyrins. Additionally, when photo-excited, diazo- containing compounds lose N2 and generate reactive carbenes as intermediates. In both cases, the radicals generated at the periphery can serve as reagents for biological substrates, or from a synthetic perspective, serve to generate some of the most unique porphyroid structures known via internal cyclization events. This latter theme lies at the foundations of generating new chromophores that could be used in photochemical applications.

In order to study photoreactive molecules and their excited states, we utilize transient spectroscopic methods. These experiments reveal information about the electronic structure and reactive intermediates of these molecules by allowing investigation of their excited states. Our transient spectroscopic systems allow us to measure the UV-visible absorption and emission spectra of excited state species by exciting a molecule with a short pulse of laser light and observing the resulting spectroscopic changes.