A Photophysical Study of Rylene Family Members in the Search for Singlet Fission Candidates Using Optical Spectroscopy

As human’s global energy consumption increases, reaching 17.5 TW annually in 2012, and continues to grow, the need for improved efficiencies of current renewable energy sources, such as solar energy, becomes vital. A majority of solar photons that are absorbed by photovoltaic cells are higher in energy than the bandgap material and lose that excess energy as heat. Singlet fission is a multi-electron process which fissions one excited singlet state into two separate triplet states. Incorporating singlet fission materials into a photovoltaic cell is one route to improving the efficiency of photovoltaic cells. An ideal singlet fission molecule for use in a photovoltaic cell requires three distinct improvements over current candidates: (1) increased photostability given the nature of photovoltaics and their constant exposure to sunlight, (2) increased triplet energy level so as to facilitate energy transfer at the silicon bandgap and (3) high singlet fission efficiencies. For these reasons, the rylene family with its reputation for photostability and multiple candidates with suitable energetics for singlet fission is explored in this work. Ultrafast time resolved optical spectroscopy methods are used to study two rylene candidates: peropyrene and diindenoperylene. The crystal structure of solid peropyrene consists of a herringbone arrangement of -stacked molecular pairs and diindenoperylene’s crystal structure consists of edge-to-face herringbone arrangement. We find no evidence for rapid singlet fission in the peropyrene crystals, due to the large shift of the singlet state to lower energy where it no longer fulfills the energy condition for singlet fission. Additionally, no evidence for singlet fission is found in diindenoperylene for which singlet fission would be 2,000 cm-1 uphill. These results demonstrate how both energetics and crystal packing influence the ability of a molecule to function as a singlet fission material and offer tools for surveying in the future.A high pressure absorption cell is also built and tested in this work. The cell manipulates Beer’s Law and increases the transition dipole moment between a molecule’s S0 state and its T1 state, offering the unique ability to directly measure a molecules lowest triplet state energy