UC San Diego

The identification and characterization of excited states in conjugated organic molecules have been active areas of fundamental research for decades. Recent interest has been spurred by emerging applications in organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). Among the most important fundamental questions are the diffusion rates of excited states in organic molecular materials, as well as the size (spatial extent) of excited states. Triplet excited states have been a particular focus, in part because of the long lifetimes and transport distances of these species, as well as the fact that they are the products of singlet exciton fission. In this dissertation, the structures and dynamics of excited states were probed in two important families of organic molecules: perylene-3, 4,9,10-tetracarboxylic diimide (PDI), and oligothiophene. The vibrational spectrum of a PDI triplet excited state is reported for the first time. Several enhanced bands in the 1300--1600 cm⁻¹ region are readily distinguished from bands of the ground state, and the triplet spectrum is expected to be a useful reference for further studies of these excitons in PDI materials. Additionally, the diffusion constant and diffusion length (25 nm) for a singlet excited state in a PDI aggregate are derived from an analysis of exciton-exciton annihilation kinetics recorded by transient absorption spectroscopy. The triplet excited states for a series of solvated oligothiophenes with 3 to 9 rings are probed with transient resonance Raman spectroscopy. The spectra evolve significantly up to a chain length of 7 units. This length is inferred to be the natural extent of the triplet excited state for long thiophene oligomers or polythiophene at room temperature in solution. The effect of substituents was also elucidated in this study