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Spectroscopic studies of singlet fission and triplet excited states in assemblies of conjugated organic molecules

Abstract

Triplet states of organic molecules are excited-state species that are important on a fundamental level, as well as for emerging organic electronic devices. The singlet fission mechanism, which generates as many as two triplet excitons per absorbed photon, may improve the efficiency of new OPVs. Singlet fission is a primary focus of this dissertation, particularly the relationship between interchromophore coupling and the yield of triplet excited states. Zeaxanthin, a carotenoid, is a model molecule for this study because it forms aggregates with different interchromophore couplings. Three types of aggregates, as well as zeaxanthin multimers in vesicles, were prepared and probed. The spectroscopic techniques included transient absorption and time-resolved resonance Raman spectroscopy. The coupling strengths between chromophores spanned the range from weak to strong. High yields of triplets by singlet fission were found in all systems with exciton coupling. The most triplets were produced in assemblies with the greatest coupling strength. The results disprove earlier suggestions in the literature that the singlet fission in carotenoids is intramolecular. Our findings also reveal that an optically-allowed S2 state is the parent state for fission, rather than the lowest excited singlet state (S1) as concluded in earlier theoretical work. The first resolved resonance Raman spectra of oligothiophenes in their triplet excited state are also reported and analyzed in this dissertation

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