Singlet fission in organic chromophores, in which an absorbed photon of visible light yields a pair of triplet excitons, has the potential to enhance the efficiency of solar energy harvesting. Endothermic singlet fission is particularly valuable, as high energy triplet excitons can be accessed through this process and can then be used to sensitize efficient photovoltaic materials such as silicon or perovskites. To attain efficient endothermic singlet fission materials, it is crucial to understand the mechanism and develop molecular design criteria that lead to high endothermic singlet fission efficiencies. We employed the photochemically stable and highly absorbing perylene chromophores and designed a series of covalently coupled oligomers that provide insights into the role of exciton delocalization, size of the system, chromophore coupling, and transition dipole moment orientations on the rate and yield of endothermic intramolecular singlet fission.<br/>In this presentation the excited state dynamics of the oligomers (ranging from dimer to tetramer) obtained by transient absorption and time resolved fluorescence spectroscopies will be described. All compounds were observed to undergo sub-nanosecond intramolecular endothermic singlet fission, however the rate and yield varied with molecular geometries. While the transition dipole moment orientation, chromophore coupling, and endothermicity of singlet fission are intertwined, we were able to glean the effects of these through subtle changes in covalent coupling. We found molecular geometries which minimize radiative rate, thereby making endothermic singlet fission more competitive. Moreover, we found that ability of triplet excitons formed by singlet fission to become electronically independent is predominantly controlled by the size of the molecular system. The mechanistic understanding of endothermic singlet fission obtained from the insights of our studies and molecular design guidelines for maximizing this process will be discussed.

Effects of transition dipole moment coupling on endothermic singlet fission in perylene oligomers

2019 Fall National Meeting

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