Singlet Exciton Fission: A Discussion of the Mechanism and What It Means for
Public DepositedThis thesis investigates singlet exciton fission, a physical process that converts one singlet exciton to a pair of triplet excitons. Singlet fission was discovered nearly forty years ago, but the mechanism for this process is still not well understood. Recent work has suggested that singlet fission may be capable of enhancing the performance of dye-sensitized solar cells. This thesis proposes the first complete mechanism for singlet fission and examines what type of molecules will undergo efficient singlet fission. We begin by reviewing the literature of singlet fission and by examining individual chromophores that have been identified as promising for singlet fission. Electronic structure methods, such as density functional theory, are used to examine how the properties of individual chromophores are affected by combining them to create a coupled chromophore pair (CCP). We focus on how the energy levels and electronic coupling depend on the specific geometry of these CCPs, and how those parameters are expected to affect singlet fission. We propose a mechanism for singlet fission whereby the initial singlet excitation undergoes one electron transfer event to reach a charge transfer intermediate, followed by a second electron transfer to produce the triplet pair state. A ten state model is developed in order to analyze the dynamics and efficacy of singlet fission in molecular systems. We examine the dynamics of several molecules, both real and simulated, within this model in two different regimes of electron transfer. Density matrix formalism is used first, to examine how singlet fission proceeds when the electron transfer steps are fast and coherent. We then use Marcus theory and classical kinetics to investigate singlet fission in the regime of slow electron transfer steps, where molecular rearrangement and vibrations play a critical role in electron transfer. We use each set of simulations to predict principles of molecular design for ideal CCPs for singlet fission.
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