Studies of singlet exciton fission in perylenediimide films and triplet exciton transfer at organic:inorganic interfaces

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2021-01-27

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Bender, Jon Alexander

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Abstract

Herein, I showcase three studies that have formed the backbone of my work at UT Austin. First, we studied singlet exciton fission (SF) in a common perylenediimide (PDI) derivative often used as a molecular organic semiconductor, C8-PDI, using pump-probe spectroscopy to model SF and singlet-singlet annihilation (SSA). Therein, we were surprised to report a SF rate orders of magnitude slower than predicted by computational studies of PDI dimers. In Chapter 3, we study a suite of six PDI derivatives that adopt different crystal structures to further assess the nature of SF in polycrystalline films grown on sapphire and fused-quartz. We developed a kinetic model across the PDI series based on the analysis of time-resolved photoluminescence, free from SSA contributions to kinetics that must otherwise be carefully modelled for TA data of these films. We confirm that Redfield theoretic approximations of the SF rate in these materials better captures the trend in kinetics implying the important role the charge-transfer character in these excitons play in mediating the process. The variation in intermolecular organization and associated changes in the Coulombic and exchange coupling between nearest-neighbor molecules correctly captures a qualitative trend in the observed SF rate, though the observed rates are an order of magnitude smaller than expected. We propose this discrepancy arises because the PDI dimer model we use for our predictive model for polycrystalline PDI thin films neglects changes in the excited state character/energetics that become important in strongly interacting molecular solids. The contents of Chapter 4 are then a study of colloidal suspensions of PbS nanocrystals (NC) decorated with a TIPS-pentacene ligand, 2-CP. We set out to search for evidence of the formation of spin-triplet excitons on the 2-CP ligands after photoexcitation of the PbS NC. Triplet exciton formation is observed with no clear observation of an intermediate charge separated species. However, an intermediary state is observed and carefully assigned to a surface associated state on the PbS NC. This hypothesis is further supported by the presence of a multitude of triplet excited states found in constrained DFT computations and fluence dependent pump-probe data providing evidence for the photopassivation the intermediate surface state. In totality, my studies have elucidated excited state dynamics in singlet fission capable polycrystalline films of perylenediimide molecules and contributed to our growing understanding of triplet exciton transfer between PbS NCs and molecular ligands.

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