Controlling the architectures and optical properties of conjugated polymer aggregates and films

Abstract

The semiconducting properties of conjugated polymers are finding use in various optoelectronic applications, including chemical sensors and light-emitting diodes. In this thesis, we investigate aggregation in conjugated polymers and how it affects the optical properties of these organic materials. We discuss how aggregation enhances exciton transport properties in fluorescent polymers, thereby increasing the probability of excitons reaching low-energy sites in the polymer. A consequence of this aggregation-enhanced exciton migration is that low-energy defect sites in a conjugated polymer can dramatically alter the polymer's fluorescence properties when it is in an aggregated state. In a poly(pphenylene ethynylene) (PPE) that was previously proposed to form green-emitting excimers, we found that a small concentration of anthryl defects in the polymer emitted green fluorescence that was only noticeable when the polymer was in an aggregated state (otherwise the polymer was fluorescent blue). After elucidating the origin of the green fluorescence, we purposely added more emissive anthryl units into the polymer to enhance the blue-to-green fluorescence color change that accompanied polymer aggregation.

(cont.) Using this anthryl-doped conjugated polymer, we developed aggregationbased chemical sensors that exhibited a visually noticeable fluorescence color change upon addition of poor solvents or biologically relevant, nonquenching, multicationic analytes (e.g., polyamines, neomycin) to the polymer solution. We also studied the effects of aggregation on the optical properties of a chiral poly(p-phenylene vinylene) (PPV) derivative in solutions and in films. We found that the organizations and functional properties existing in aggregated polymer solutions can be transferred to the film state by controlling the processing conditions. Using the same polymer, we were able to obtain films with different architectures and luminescence properties simply by adjusting the spin-casting solvent and film annealing conditions. Controlling the organizations and functional properties of conjugated polymer films is important in the fabrication of conjugated polymer-based optoelectronic devices.