Singlet Fission and Exciton Coupling Design Principles for Efficient Photon Harvesting

Doctoral thesis, 2024

Extensive research efforts have been dedicated to unravelling the diverse array of processes triggered by the interaction of light with matter. Harnessing and comprehending some of these phenomena holds immense potential in humanity's transition towards renewable energy sources. At the heart of this transition lies the Sun, which provides the Earth with an abundance of energy in the form of light. In addition to generating electricity through solar cells, sunlight can also drive photochemical reactions to produce fuels such as hydrogen gas. Despite significant strides in utilizing solar energy, there remain numerous processes that are not fully understood, presenting opportunities for refinement and enhancement. This thesis embarks on an exploration of the photophysical processes of singlet fission (SF) and exciton coupling, aimed at maximizing the efficient utilization of the energy of light, particularly from the Sun.

Both SF and exciton coupling rely heavily on the relative orientation and distance between the interacting molecules. Therefore, the essence of the research presented in this thesis revolves around the complex interplay between molecular structure and photophysical properties. This thesis underscores how various configurations of identical molecules can result in diverse photophysical outcomes. Furthermore, it showcases the importance of considering both the interconnectivity of molecules from a Lewis structure and optimal energy configuration standpoint, as well as their capacity to assume different conformations dynamically.

SF has been explored in three systems. The initial investigation, focused on pentacene derivatives, aimed at elucidating the impact of rotational conformations in intramolecular SF systems. The second study aimed to expand the limited library of photostable, intramolecularly capable SF molecules by investigating an anthracene derivative. However, this study revealed additional processes that hindered SF efficiency, underscoring the delicate balance required between energetics, molecular interconnectivity, and solvent polarity for efficient SF. In the final SF study, strides were made towards integrating SF into dye-sensitized solar cells, utilizing a derivative of diphenylisobenzofuran integrated with semiconductor thin films. The study highlights the importance of substrate energetics and solvent polarity in dictating the dominant photophysical processes on the surface, with highly polar solvents impeding SF by stabilizing charge-separated states. In the exciton coupling related study, strategic alignment of molecular systems comprising boron dipyrromethene and anthracene in a covalent J-aggregate-like configuration demonstrated a novel method of selectively modulating the energy of the singlet excited state while leaving the triplet excited state energy unaffected. This work thus demonstrates how photophysical properties can be tuned via molecular design, with potential applications in various fields of optoelectronics.