Multielectron dynamics of singlet fission in the condensed phase

Abstract

Elementary energy and electron transfer processes are ubiquitous in the renewable energy science of the last half of the 20th century. As global energy demands increase, researchers are inclined to explore chemical physics that is outside the scope of the single electron paradigm using new theoretical concepts and methods. This thesis advances theories of two specific condensed phase phenomena: singlet fission, and energy transfer in photosynthetic light harvesting complexes. Some photoactive organic molecules relax through a multielectron process known as singlet fission, where a photon excites a chromophore that can down-convert the energy of a singlet excitation by relaxing to two triplet excitations. Singlet fission may lead to unprecedented solar power conversion efficiencies, but its many-body chemical physics can be challenging to model. We explore the fundamental role of thermal energy in singlet fission in liquids and solids over multiple timescales. Using quantum master equations and diabatic representations of the single and double electronic excitations, we study the scope of the Markovian approximation for the chemical environment's response to singlet fission. To better understand how singlet delocalization and triplet localization impact quantum yields in molecular crystals, we develop a theory for delocalized singlets interacting with a dense band of two triplet excitations that includes biexciton interactions. We use the Bethe Ansatz for the two triplets and calculate an entanglement for indistinguishable bipartite systems to analyze the triplet-triplet entanglement born out of singlet fission.