The negligible spin orbit coupling in many organic molecules creates opportunities to alter the energy of excited electrons by manipulating their spin. In particular, molecules with a large exchange splitting have garnered interest due to their potential to undergo singlet fission (SF), a process where a molecule in a high-energy spin-singlet state shares its energy with a neighbor, placing both in a low-energy spin-triplet state. When incorporated into photovoltaic and photocatalytic systems, SF can offset losses from carrier thermalization, which account for ~50% of the energy dissipated by these technologies. Likewise, compounds that undergo SF’s inverse, triplet fusion (TF), can be paired with infrared absorbers to create hybrid structures that upconvert infrared into visible light. However, integrating materials that undergo SF or TF with existing electronics remains challenging as the efficacy of these processes depends strongly on how molecules order in the solid state. I will summarize work aimed at identifying critical structure-function relationships that guide SF within perylenediimide (PDI) films. By adding functional groups at key locations along the PDI backbone, we can force these molecules to adopt different structures in the solid state. Guided by electronic structure calculations, we have used this approach to optimize the electronic coupling between PDIs such that they undergo SF with near quantitative efficiency. In addition, I will describe recent work whose goal is to understanding energy transfer pathways that operate in hybrid organic:inorganic structures that use TF to upconvert infrared light into the visible range.