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In recent decades, fluorescent conjugated polymers have been widely studied due to their ability to produce low-cost and lightweight organic electronics. However, given that their solid state packing arrangements are difficult to control or predict, maintaining the desired emissive properties of the materials is challenging. As the emission wavelengths and quantum yields of these materials are highly sensitive to their solid state packing arrangements, pre-aggregation in solution can assist in maintaining properties through the transition between solution and solid state. Here we use poly(3- hexylthiophene) (P3HT), a well-studied organic semi-conductor, to explore the impact of aggregation on the photophysical properties of the material. We have employed various bulk and single molecule fluorescence-based methods to better understand the effect of aggregation on the emission properties of the polymer system. The addition of a highly-polar solvents to induce aggregation leads to strong emission quenching, but no change in the lifetime in solution. However, in the solid state the aggregates exhibit enhanced emission intensity combined with shorter lifetimes as demonstrated by fluorescence lifetime imaging microscopy (FLIM). To better understand the differences of the aggregate properties in the solution and solid state, fluorescence anti-bunching was used to probe the degree of electronic coupling of the polymer chains. Surprisingly, fluorescence anti-bunching measurements reveal the highly collective nature of the P3HT aggregate emission, which likely accounts for its strong fluorescence intensity in the solid state. Studying the aggregated system at both the bulk and single-molecule level will yield a better understanding of the aggregate properties which will lead to better control and higher performance of organic semiconductors in device applications.
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