Date of Award

December 2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Committee Member

Jason McNeill

Committee Member

Jeffrey Anker

Committee Member

Leah Casabianca

Committee Member

Steven Stuart

Abstract

Charge carrier dynamics in conjugated polymers are of fundamental interest as they directly affect the performance of conjugated polymer based devices. Through efficient fluorescence quenching by hole polarons, dynamics of individual charge carriers, including generation, recombination, and transport, can be observed through single particle fluorescence study of conjugated polymers nanoparticles (CPNs). In this dissertation, the hopping dynamics of hole polarons in CPNs were studied using superresolution microscopy. Due to quenching of the local fluorescence, a hole polaron can form a “dark spot” that moves with the polaron and results in displacements in the fluorescence centroid. These position fluctuations were used to track the nanoscale motion of hole polarons. In long trajectories, we observed random walk-like behavior consistent with multiple trap sites, whereas in some short segments, repeated hopping between two traps was observed. The hopping times range from a few ms to seconds, following a power law distribution, while the hopping distances range from 2-5 nm, following an exponential distribution. From the hopping time distribution, we estimated the energy barrier height for polaron hopping in CPNs to be from 430 to 570 meV, indicating the presence of deep traps with nearest-neighbor distances of 2-5 nm, consistent with a low or moderate density of structural or chemical defects dominating charge transport at low carrier densities.

Based on polaron generation and recombination dynamics, we developed a new class of photoswitchable nanoparticles for superresolution imaging. By doping nanoparticles of conjugated polymer PFBT with fullerene derivative PCBM, a large population of polarons can be efficiently generated in CPNs, sufficient to nearly completely quench the nanoparticle fluorescence. However, fluctuations in the number of quenchers lead to occasional bursts of fluorescence. 3-5×104 photons were detected during each burst event (1-2 orders of magnitude brighter than photoswitchable dyes), resulting in a localization precision of 0.6 nm, ~4 times better than the typical resolution obtained by localization of dye molecules. In addition, since polaron generation is a photo-driven process, we demonstrated that the blinking duty cycle of PCBM doped CPNs can be controlled by excitation intensity as well as by PCBM fraction. The unprecedented brightness and tunable spontaneous photoswitching properties of PCBM doped PFBT CPNs make them a promising class of superresolution probes, which provide clear advantages for imaging of various biological systems.

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