Seeking Design Principles for Control of Ultrafast Energy Transfer in Synthetic Light Harvesting Scaffolds

Using solar energy as an efficient renewable energy source is one of the grand challenges for science in the 21st century. Chemistry has succeeded in synthesizing a vast library of tailor-made light harvesting molecules, but tethering these molecules in a useful way that creates a system more than the sum of its parts has proven to be a challenge. Photobiology, on the other hand, has succeeded at using only a few chromophores to create efficient light harvesting networks due to their protein scaffolds, which hold and modulate the photoactive components with unmatched precision. Synthetic light harvesting seeks to mimic this approach, with dyes on double stranded DNA becoming a standard means of studying fundamental energy transfer properties as well as having applications in many bio-applied sciences. While creating new synthetic machinery and divulging energy transfer properties through spectroscopy and process of elimination is valuable, it is an indirect means of characterizing an exciton, while an ultrafast microscopy would enable the direct viewing of excitons and charge carriers. Coupled together, this research seeks a fundamental understanding of the photophysics driving light harvesting and manipulation. In dye-DNA work, the existence of an ultrafast nonradiative trap state in a molecular photonic wire composed of Cy5 dimers was determined to be linked to macroscale dynamics of the molecular photonic wire as a whole, where both the trap state and the overall dynamics were tied to fluence and temperature. In complementary work, a novel technique called ORI was theorized, which could lead to the direct imaging of ultrafast energy transfer through excitons. Finally, preliminary work on a more sophisticated system, the four-arm star, is presented, in which incipient fluence dependence is seen.