UC Santa Barbara

In part one of this thesis, I develop a novel analytical model that can be used to measure specific properties of materials that exhibit thermally activated delayed fluorescence (TADF). TADF materials are promising candidates for use in organic light-emitting diodes, because they are able to harness the energy of triplet excitons without the use of expensive heavy metal atoms, such as Pt and Ir. Key to their success is the phenomenon of reverse intersystem crossing, a property that is challenging to experimentally measure. Besides being able to determine this photophysical rate, my analytical model is also able to determine the singlet-triplet splitting energy and diffusion coefficients of singlet and triplet excitons. After applying the model to a small library of molecules in order to elucidate structure-property relationships, a particular brominated derivative is found to be able to interconvert between the singlet and triplet excited state approximately 36 times during one lifetime, and whose exciton diffusion length exceeds 40 nm, both highly unique properties.

In part two of this thesis, I investigate how the addition of particular Lewis acids to Lewis basic semiconducting polymers affects their optical and electrical properties. In some cases, Lewis acids have been shown to modify the optical properties of organic semiconductors, making them a useful tool for post-synthesis tuning of a material’s optical bandgap. In other cases, Lewis acids have been shown to act as efficient p-type dopants, dramatically increasing a material’s conductivity. However, a unified theory that accounts for both of these behaviors has been missing, and neither process has been well understood in general. Using a variety of spectroscopic techniques on a range of Lewis acids and Lewis basic semiconducting polymers, I propose a unified theory in which both phenomena are accounted for. In particular, I propose that p-type doping is mediated by trace amounts of water, which ultimately protonate the polymer backbone and, subsequently, lead to electron transfer events. For polymers with strongly Lewis basic atoms, the Lewis acids tend to form adducts with the polymer, leading to a reduction of the polymer bandgap and precluding the doping process.