| Single molecule imaging has emerged as a powerful tool in a range of applications, but the field is limited by a lack of fluorescent probes with sufficient brightness and photostability for many demanding applications such as tracking of single biomolecules in cells and tissues at video rate with 1 nm spatial resolution. Conjugated polymers hold great promise as a solution to these issues, with a high density of pi electrons and variety of chemistries, allowing efficient and tunable absorption of light. This dissertation describes the development and characterization of a novel type of nanoparticle composed of conjugated polymer called CPdots. CPdots retain the high brightness of conjugated polymers in solution and in films, but can be dispersed in water, making them suitable for many biological applications. These CPdots have been shown to have one-photon absorptivities that range from 106 -- 107 M-1 cm-1 (2-3 orders of magnitude higher than most other fluorescent dyes), and two-photon cross sections as high as 2 x 105 G.M. units (the highest reported value to date for a nanoparticle). A variety of complex photophysical phenomena were observed in CPdots, including complex photobleaching kinetics, reversible photobleaching, complex picosecond fluorescence kinetics, and collective excitation effects in single nanoparticles. A novel theoretical model describing the interactions between excitons and polarons in CPdots was developed. The model results predict complex photobleaching kinetics and complex picosecond fluorescence kinetics, in close agreement with experimental data. The model is also in qualitative agreement with many phenomena observed in fluorescence experiments performed on single nanoparticles. Gelation thermodynamics and kinetics of the conjugated polymer poly(2,5-dinonylparaphenylene ethynylene), which are important in film casting techniques, were investigated allowing the design of film casting methods that will yield specific energy transfer efficiencies. These investigations provide a thorough understanding of CPdot photophysics, necessary for the rational design of improved fluorescent probes. It is also hoped that the results of these investigations could help in understanding key processes that could limit efficiency of organic optoelectronic devices such as polymer-based light-emitting displays and polymer-based photovoltaic devices, and thus help in the development of strategies aimed at improving device efficiencies. |
